Sunday, 14 September 2014

The structure of a dicotyledonous root in relation to the pathway of water from root hairs through the cortex and endodermis to the xylem. Apoplastic and symplastic pathways. Transpiration and the effects of light, temperature, humidity and air movement. The roles of root pressure and cohesion-tension in moving water through the xylem.

The structure of a dicotyledonous root in terms of the up take of water.

The first cell is a root hair cell. This is an epidermal cell with a long extention which provides a large surface area for the diffusion of water.

There is a high water potential in the soil, because there are not many ions dissolved in it, and there is a low water potential in the cell, because the vacuole contains cell sap which has many ions dissolved in it, water moves by osmosis from the soil into the root hair cell.

The second cell is a parenchyma (packing) cell of the cortex, water moves between these cells in two ways.
  1. The symplastic pathway- cell walls have spaces in which water can move along
  2. The apoplastic pathway- there are strands of cytoplasm called plasmodesma which link the cytoplasm of different cells, water moves through these along the concentration gradient by osmosis.

The third cell is an endodermal cell. Water in the apoplastic pathway arrives in the ‘protoplast’ of the cell through the plamodesma, but water in the symplastic pathway has to be forced out of the cell wall by a waterproof strip called the Casparian strip.

Root pressure

Endodermal cells actively transport ions into the xylem, this means the xylem has loads of ions in it and so has a really low water potential- so water will move into the xylem as there is a concentration gradient. This helps move water through the plant and is known as ‘root pressure’.

Water moved through a leaf

Water moves from the roots to the leaves through the xylem. When it reaches the leaf it is moved into the mysophyll cells through the apoplastic and symplastic pathways. Water evaporates from the mysophyll cells into the air spaces in the leaf, it then leaves through the stomata- this is called transpiration.

Cohesion-tension theory

Water molecules stick together due to hydrogen bonds formed between them, this is called ‘cohesion’. When water molecules are moved through the leaf they pull other molecules up behind them- this means that as water molecules evaporate from the mysophyll they pull more molecules into the cell behind them, in turn this pulls molecules up in the xylem. In this way there is a pull on the water in the xylem which moves water in the stem.

Factors affecting transpiration

this evaporates more water (by increasing kinetic energy and so the space between molecules making them a gas.) It also decreases the humidity of the air.

humid air has many water molecules in and so it has a low water potential- this means less water can diffuse into it.

Air movement (wind speed):
the more air movement, the more quickly water vapour gets taken away from the stomata- this means that the air can be cleared of vapour and have a higher water potential and more water will diffuse out of the leaf into it.

photosynthesis happens when there is light, so the more light, the more photosynthesis, the more gas exchange is needed to happen. This means that when it is light the stomata will open and therefore water vapour will escape.

Friday, 23 May 2014

An index of diversity describes the relationship between the number of species and the number of individuals in a community. Calculation of an index of diversity from the formula. Candidates should be able to • calculate the index of diversity from suitable data • interpret data relating to the effects of human activity on species diversity and be able to evaluate associated benefits and risks • discuss the ways in which society uses science to inform the making of decisions relating to biodiversity.

The index of diversity is a way of quantifying species diversity.

The formula is as follows (this will be given in an exam):
where N= total number of organisms of all species (community)
and n= total number of organisms of each species

This means that, if we take a quadrant from a rain forest and we see one tree, five birds, twenty ants and two snakes the equation will be as follows:


Courtship behaviour as a necessary precursor to successful mating. The role of courtship in species recognition.

Species all display different behaviour, this helps them to recognise each other.

This is beneficial to a species as they want to breed with each other so that they produce fertile offspring and pass on their genes.

There are other advantages of specific behaviour displayed during courtship:

  • Show that they are capable of breeding: to optimise the chance of producing offspring.
  • Form a pair bond: to successfully raise the offspring.
  • Mate at an appropriate time: optimise the chance of fertilisation.

The principles and importance of taxonomy. Classification systems consist of a hierarchy in which groups are contained within larger composite groups and there is no overlap. The phylogenetic groups are based on patterns of evolutionary history. A species may be defined in terms of observable similarities and the ability to produce fertile offspring. One hierarchy comprises Kingdom, Phylum, Class, Order, Family, Genus, Species. Candidates should be able to appreciate the difficulties of defining species and the tentative nature of classifying organisms as distinct species.

Taxonomy is the system used to group organisms, it is important to scientist to establish the relationships between different species.

All organisms originated from the same organism but through evolution have ended up with significant amounts of variation.

Phylogenetic groups are how recently two species have been related (how recently they evolved apart into different species).

Sometimes organisms are organised due to observable physical characteristics which do not necessarily mean they are closely genetically related.

Classification is not yet perfected due to the sheer number and the complexity of life on earth.

The most common system is:


The definition of a species is a group of organisms which can breed to create fertile offspring.

The cells of multicellular organisms may differentiate and become adapted for specific functions. Tissues as aggregations of similar cells, and organs as aggregations of tissues performing specific physiological functions. Organs are organised into systems.

Each cell in the body starts off the same but then they specialise by expressing certain genes.

This is beneficial as different cells are better off at carrying out certain jobs.

Aggregation is grouping together.

A groups of similar cells function together as a tissue (are aggregated).

Tissues are then aggregated into organs. These have several different types of tissue which function together as a system.

Similarities and differences between individuals within a species may be the result of genetic factors, differences in environmental factors, or a combination of both. Candidates should appreciate the tentative nature of any conclusions that can be drawn relating to the causes of variation.

Both genetics and environment can have an effect on the things that make up an individual. Key examples include genes on eye colour, environment on how far you reach your growth potential and environment and genes on skin colour.

It is often difficult to tell weather variation is environmental or genetic. In almost all cases it is a mixture of both. The prime display of this being that two twins with exactly the same genes have the potential to end up very different.

There are three things that cause genetic variation:

  • The random fusion of gametes: the specific sperm and egg that happened to meet at the specific moment that a offspring is created are completely random and have a massive effect on the genes inherited.
  • Mutations: DNA can be changed by mistakes in copying or caused by things like radiation.
  • Meiosis: this form of cell division that creates gametes has crossing over (genetic recombination) and independent segregation which both vary the genes inherited.

The concept of normal distribution about a mean. Understanding mean and standard deviation as measures of variation within a sample. Candidates will not be required to calculate standard deviation in questions on written papers. Candidates should be able to analyse and interpret data relating to interspecific and intraspecific variation.

When figures get larger towards the mean with no bias left or right, the graph will have this shape:
The shape is known as a bell curve and the distribution of values is called 'normal distribution'.
If a graph has normal distribution, then 68% of the results will be within 1 standard deviation and 95% of results will be within 2 standard deviations:

Standard deviations are a measure of accurate data is; the scattering of data around the mean.

The more centrally (around the mean) scattered they are then the more accurate the data is. If data is really spread out and there is only a small proportion of the data that is near the mean then it will have a large standard deviation, and will not be good data to draw conclusions from.

To work out standard deviation you add together all the deviations and then divide this by the number of values minus one.

To work out a deviation, first find the mean then subtract the it from a value. This answer needs to be squared to eliminate negative numbers. Do this for each value and then add them up. Divide this number by the number of values minus one. Find the square root.

You don't actually need to be able to do this according to the mark scheme, but it would seem wise to learn it.

For example, if I have 1, 2 and 3 and I want to work out the standard deviation I first find the mean
This is done by adding up all the values and dividing by the number of values you have (3):
1+2+3= 6
Next you need to subtract the mean from each value and square the answer:
1-2= -1
-1 squared= 1
2-2= 0
0 squared= 0
3-2= 1
1 squared= 1
Then add together all these answers:
1+0+1= 2
Now divide this by the number of values (3) minus one:
2/2= 0
Now find the square root of this:
the square root of 0= 0
So here one standard deviation is zero.

Variation exists between members of a species. The need for random sampling, and the importance of chance in contributing to differences between samples.

Members of a species differ from each other (intra-specific variation).

We can use random sampling to analyse these differences.

This involves taking a small number of individuals at random from a population, analysing them, gaining statistics and then making the assumption that these statistics are the same for the whole population.

Normally this is carried out by having a grid of an area and using a random number generator to pick squares in the grid to analyse.

The benefit of random sampling is that there are multiple values put in so you get an average, reducing anomalies and making the result more representative, however because the figure is taken from just a sample, it means that you do not have to attempt to find a figure for every single individual.

Unfortunately there is an element of chance involved meaning that the statistics gained may be inaccurate as a non-representative sample was chosen. However, by increasing the sample size you can decrease the element of chance.

Friday, 25 April 2014

DNA is the genetic material in bacteria as well as in most other organisms. Mutations are changes in DNA and result in different characteristics. Mutations in bacteria may result in resistance to antibiotics. Resistance to antibiotics may be passed to subsequent generations by vertical gene transmission. Resistance may also be passed from one species to another when DNA is transferred during conjugation. This is horizontal gene transmission. Antibiotic resistance in terms of the difficulty of treating tuberculosis and MRSA.Candidates should be able to • apply the concepts of adaptation and selection to other examples • evaluate methodology, evidence and data relating to antibiotic resistance • discuss ethical issues associated with the use of antibiotics • discuss the ways in which society uses scientific knowledge relating to antibiotic resistance to inform decision-making.

Bacteria carry DNA both as circular chromosome and as plasmids (little rings of DNA).

Mutations are random changes that occur to the DNA sequence. A change in DNA results in different protein being produced, this changes the characteristics of an organism.

Variation is caused by two things: mutations; recombination of existing DNA.

Bacteria do not sexually reproduce, so until recently it was unknown how bacteria were so diverse without recombination of existing DNA, however, now we know that there is recombination of existing DNA but not from two parents to an offspring, it happens from one existing bacteria to another: plasmids are transferred between them in a process known as conjugation:

  • One bacteria starts to grow a tube called a pilus or conjugation tube
  • It also starts to replicate a plasmid
  • The tube reaches another bacteria and the DNA starts to move through it as a line
  • It enters the new bacteria and forms a ring (the new plasmid)
  • The tube is broken down
This process takes a matter of seconds and allows the transfer of genetic material from one existing bacteria to another- this is known as horizontal gene transmission (because it occurs across a generation). In this way a mutation could be passed between the same type of bacteria, to a different strain, or even to a different species. Like this resistance to an antibiotic could be gained by a species.

Bacteria also pass DNA on by asexual reproduction, this is known as vertical gene transmission as DNA is passed down from one generation to another.

Conjugation discovery
In 1946 Lederberg and Tatum designed an experiment to prove that DNA was transferred horizontally between bacteria:
  • Take two strains of E. coli, one that can synthesise everything but methionine and biotin and one that could synthesise everything but threonine and leucine
  • This meant that when they were placed in a medium without the nutrients neither strain could grow
  • They mixed the two together and left them for several hours
  • They put the mixture back onto a medium with no nutrients
  • Some bacteria were able to survive
  • This meant that there must have been DNA exchanged so that the strains had the ability to synthesise the nutrients they needed to survive
Antibiotic resistance
Most of the time mutations are not beneficial to an organism, they stop it from being able to function properly, however, occasionally a mutation happens that can increase the success of an organism.

In bacteria mutations can occur that make them resistant to antibiotics, this means that if they are in an infected person who is taking antibiotics then all the other, non mutated bacteria will die off, the one with the mutation will survive and replicate passing on the resistant gene and making the resistant bacteria more common.

This will only occur because of the presence of antibiotics giving the resistant strains an advantage, therefore the more antibiotics are used, the more resistance happens.

An example of this is the mutation of an enzyme in a bacteria that changed an enzyme to make one which could break down penicillin (an antibiotic) before it could harm the bacteria. This meant that it was not destroyed by the antibiotic and lived on to reproduce successfully. The gene was then passed on to future generations of the bacteria to make a resistant population, but it was also transferred horizontally so that other species were also resistant.

Discovering resistance
A bacteria is placed in a Petri dish with an antibiotic. If the bacteria is not resistant then it will not be able to grow near the antibiotic, and it will be visible everywhere apart from near the antibiotic. It the bacteria is resistant it will grow even in the area of the antibiotic and therefore will be visible over the whole dish.

To treat the bacterial infection of tuberculosis (TB) you have to take a 6-9 month course of antibiotics. Often people start to feel better a little way into the course because a lot of bacteria has been killed off so they stop taking the antibiotic. However, even though a lot of bacteria had been killed off, some would not yet have died- this is the more resistant stuff that is harder to kill. As a consequence the resistant bacteria are left in the body, free to multiply and spread. This is known as selection pressure.

As a result, TB is resistant to most known antibiotics- the definition of a super bug.

Methicillin-resistant staphylococcus aureus (MRSA) is also a super bug. It is found in hospitals where: people tend to be ill anyway, making them more susceptible to infection; people are in close proximity; there is a lot of contact (doctors patients); there are a lot of antibiotics being taken (which make resistant strains more successful.

Antibiotics may be used to treat bacterial disease. One way in which antibiotics function is by preventing the formation of bacterial cell walls, resulting in osmotic lysis.

Bacteria in the body may cause disease, this can be treated by the use of antibiotics.

These work in a variety of ways to kill off bacteria. One way in which this can be one to cause osmotic lysis:
  • Antibiotics can prevent the formation of the proteins for peptide cross linkages in cell walls
  • This means that the cell wall which regulates water movement cannot be formed
  • As a result, water moves by osmosis into the cell
  • Eventually the cell will become too full and burst, this is called osmotic lysis

Thursday, 24 April 2014

Diversity may relate to the number of species present in a community. The influence of deforestation and the impact of agriculture on species diversity

Community- all the different species living in an area.

Diversity is the variation in species living in an area: it is important to look at the amount of different species but it is also important to look at the size of each of the species.

This is because there could be many species, but some could be very unsuccessful or endangered and others could be successful and dominant.

An example of where this is important is in an agricultural setting where a certain crop is cultivated. For example in a wheat field, there will be some insects and mammals so potentially lots of species, but they will all have a relatively small number of members compared to the wheat which will have a huge number of individuals. Here, even though there are many species the biodiversity is low.

Deforestation causes disruption to the relationships in a community by removing on species (trees) and decreasing the biodiversity. Important things such as food and habitat are lost which can cause a decrease in the population of other species.

The structure of arteries, arterioles and veins in relation to their function. The structure of capillaries and their importance in metabolic exchange. The formation of tissue fluid and its return to the circulatory system.

All the blood vessels have a lining or endothelial layer which prevents friction.

  • Carry blood away from the heart
  • Blood inside is high pressure to push blood all the way round the body
  • Thick muscle layer so the amount of blood going through can be controlled
  • Thick elastic lining to maintain pressure
  • Having thick walls also helps prevent it from bursting
  • Small lumen keeps the pressure high
  • Are in between arteries and veins to decrease the pressure before the blood reaches the capillaries so they don't burst
  • Thick muscle layer so that it can control the blood flow into the capillaries
  • Thinner elastic layer as pressure is not so high
  • Carry blood back to the heart
  • Doesn't need a thick elastic layer because the blood pressure is low (no danger of bursting)
  • Thin elastic and muscle layers make it easy to compress so that blood can be pushed through
  • Valves make sure blood doesn't flow in the wrong direction
  • Thin lining layer and no muscle or elastic layer so there is a small diffusion distance
  • It is also small so that it can get in between tissues so cells are close, decreasing the diffusion distance again
  • Spaces in lining to let white blood cells through
  • There are lots of them and they are small to increase the surface area
Large surface area and small diffusion distance are key to aiding fast diffusion, this is key because it is through the capillaries that crucial metabolic substances like oxygen are delivered to the cells of the body

Tissue fluid
At the arteriole end of capillaries there is a lot of blood pressure (hydrostatic pressure), this forces substances out by pressure filtration. Water and some other substances are pushed out of the capillaries into what is know as the tissue fluid (however large molecules like proteins cannot fit through the gaps); this surrounds cells and delivers substances to them like glucose, amino acids and oxygen.

Because a lot of water moved out of the capillary but the proteins remained, at the venous ends of the capillary, the water potential is very low: this causes water in the tissue fluid move back into the blood by osmosis. Some of the liquid enters the lymphatic system, but most then renters the blood at the neck.

Similarities and differences between organisms may be defined in terms of variation in DNA. Differences in DNA lead to genetic diversity. The influence of the following on genetic diversity • selection for high-yielding breeds of domesticated animals and strains of plants • the founder effect • genetic bottlenecks. Candidates should be able to discuss ethical issues involved in the selection of domesticated animals.

Genetic diversity is the level of variation between organisms, the more differences in DNA (nucleotides, genes of genomes) the larger genetic diversity. It can also be defined as the number of different allels present in a population.

Genetic variation is seen as a good thing because it reduces the inheritance of genetic diseases from recessive alleles. It also means that if a disaster occurred (e.g. a change of atmosphere or the introduction of a new predator) some of the species may be able to survive because of their differences.

Selective breeding
This is where humans choose animals or plants with desired characteristics and breed and nurture them. The selected ones will be more successful at reproducing as they are encouraged by the humans and therefore be a successful species.

A prime example is the breeding of dogs, which was occurring even in the Roman times when wolves with the ability to read human behaviour were selected to breed and keep as pets.

Another species that this can be seen in is the pig, where wild boar were captured and farmed. Characteristics such as being fat (more food) and having small legs (can't run away) meant the animals would be selected to have offspring to pass on the traits. Years of selective breeding have left us with the domestic pig, too fat and disproportionate to walk properly- it raises ethical issues about weather it is fair to manipulate the DNA of animals for domestic qualities as it does not benefit a species and can reduce their chances of survival in the wild.

The founder effect
When some of a population moves to start a new colony, only some of the alleles will be taken. This means that in the new community there a smaller variety of different DNA so genetic diversity is decreased. This can cause problems, for example in the Amish community, people suffer from many genetic disorders because they are more likely to have two parents with the same genotype who are both carriers.

Bottle necks
A similar thing can happen when the size of a population suddenly drops- some alleles may be completely lost (either by chance or because their characteristic was vulnerable), decreasing the genetic diversity. When cheetahs died in the ice age, the population sank to around just fifty, of the survivors a very high proportion had fertility issues, this meant the characteristic was passed on to a high proportion of the offspring and that even as the population began to expand again the percentage with fertility issues remained high.

Wednesday, 23 April 2014

Comparisons of amino acid sequences in specific proteins can be used to elucidate relationships between organisms. Immunological comparisons may be used to compare variations in specific proteins. Candidates should be able to interpret data relating to similarities and differences in base sequences in DNA and in amino acid sequences in proteins to suggest relationships between different organisms.

Genes code for amino acids to make proteins. So if we look at a protein in two different species and examine its amino acids, we can see how similar or different their genes are.

For example, (arbitrary data used) if we took heamaglobin from a gorilla and a human and looked at the amino acids and they looked like this:
Human: Ser; Val; Ser; Glu; Ile; Gln; Leu; Met; His; Asn
Gorilla: Ser; Val; Val; Ser; Ile; Gln Leu; Met; His; Asn
then we could see they are relatively genetically similar with  7/10 amino acids are the same.

One way to compare proteins is by comparing antigens on the body cells of a specie. This is immunological comparison:

  • Extract blood serum from a species
  • Put it in a second species
  • This species will produce antibodies that are complimentary to the antigens of the first species
  • Extract the antibodies
  • Mix them with the blood serum of a third species
  • A precipitate will form if the antibodies respond to the antigens of that specie
If the antibodies that were created for the antigens of the first specie can respond to antigens of the third then that means it must be complimentary to both. That means that both antigens had a similar tertiary structure, which means they had a similar primary structure, which means they had a similar sequence of amino acids which means that had a similar base sequence which means they are genetically similar. Therefore antibody can respond to both= more precipitate= more genetically similar.

If there is not very much precipitate its because the antibody can respond to the antigens in the third species because it is not sufficiently similar to the first, this shows us that they are genetically dissimilar.

Comparison of DNA base sequences is used to elucidate relationships between organisms. These comparisons have led to new classification systems in plants. Similarities in DNA may be determined by DNA hybridisation.

If two species have recently shared a common ancestor, then they are closely related. If this is the case, then they will have much of the same DNA because when one species gives rise to another they initially have similar DNA.

Over time mutations will render the DNA very different through the accumulation of mutations. This means that species that have not shared a common ancestor recently will have very different DNA.

So by looking at DNA scientists can determine what other species a species is related to and therefore what type it should be classified as.

A recent example of where this has been useful is in the classification of plants. Before genetic comparison plants had been categorised due to physical characteristics, however when the DNA was examined, scientists found that some plants that looked very different had more similar genes than ones that looked similar. The whole plant kingdom had to be re-classified.

DNA hybridisation is one method of comparing the base sequence of DNA:

  • Heat the DNA of two species, this will break the hydrogen bonds holding together the two sides of the chromosome
  • Leave them to cool and reform hydrogen bonds between complimentary base pairs
  • Some of the DNA will have formed with one strand from each species
  • This 'hybrid' DNA is then heated in stages to see at what temperature it separates into single strands
A higher temperature means more energy was required because there are more hydrogen bonds, this means that there were many complimentary base pairs made, and that means that the DNA from both species was similar.

If the DNA was not similar there would not be very many complimentary base pairs, so the hybrid DNA would have few hydrogen bonds and not take very much energy to break them all. This means that hybrids strands that separate at a low temperature come from two species that are not genetically similar.

Genetic comparisons can be made between different species by direct examination of their DNA or of the proteins encoded by this DNA.

Genetic comparisons are made to find out the classification of a living thing, i.e. what its ancestors are.

Comparing DNA will show what characteristics two species have in common, and what mutations have been made to make them different.

This can also be done by looking at proteins because DNA codes for amino acids which make up proteins. Therefore looking at one protein from two species and seeing the similarities and differences in amino acids equates to seeing similarities and differences in their genetics.

Monday, 21 April 2014

The basic structure and functions of starch, glycogen and cellulose and the relationship of structure to function of these substances in animals and plants.

Starch is the form of carbohydrate which plants store energy as: in small grains especially in the seeds and storage organs.
It is a polysaccharide made up of α-glucose to make a long straight chain which then winds up tight (unbranched helical chain).
Being wound up so tight means you can fit a lot of it in a small space and therefore a convenient way to store energy.
It is also a positive that it is made up of α-glucose, because this means when it is hydrolysed (broken down) that will be the molecule produced and it is easy to transport and use in respiration reactions.
Starch is also insoluble, this is good for two reasons: one, it does not tend to diffuse out of cells; two, it doesn't tend to draw water into cells by osmosis.

Glycogen is the form of carbohydrate which animals store energy as: in small granules especially in the muscles and liver.
It is a polysaccharide made up of α-glucose to make a short and very branched chain which winds up tight (branched helical chain).
Like starch it is insoluble, fits a lot of energy into a small space and makes α-glucose when hydrolysed, but because it is shorter is is hydrolysed more quickly.

Cellulose is found in plant cell walls.
It is a polysaccharide made up of β-glucose. If you have two β-glucose molecules and perform a condensation reaction, one of the molecules will have to turn up side down; this is because the order of the OH and the H is reversed on one side, so to match up it has to be turned round. This fact means that in a chain of β-glucose the 'CH2OH' group will alternate between being at the top and the bottom of the chain. The importance of this is that it can't coil up.
So, the chain is straight and unbranched which means several chains can lie next to each other; hydrogen bonds will form between these chains creating a strong 'microfibril' (what fibres are made of).

Beta glucose forming a glycosidic bond
Chain of cellulose

Sunday, 20 April 2014

The structure of b-glucose as... b-glucose and the linking of b-glucose by glycosidic bonds formed by condensation to form cellulose.

β-glucose has the same chemical formula as α-glucose, and the other hexose sugars (C6H12O6), but a different structure:

In a condensation reaction, the following will be formed:

Water is formed from the OH of the one glucose and the H of the others OH, the left over O forms a glycosidic (between sugars) bond.

Many β-glucose joined together in this way a polysaccharide called cellulose will be formed. 

Thursday, 17 April 2014

There are fundamental differences between plant cells and animal cells. The structure of a palisade cell from a leaf as seen with an optical microscope. The appearance, ultrastructure and function of • cell wall • chloroplasts. Candidates should be able to apply their knowledge of these and other eukaryotic features in explaining adaptations of other plant cells.

Plant cells and animal cells share some of the same organelles, but there are some that differ. Cell walls and chloroplasts are examples of things that plant cells have which animal cells don't. Bother can have vacuoles but plants almost always have large central ones and animals rarely have them, and when they do they are small and scattered. Another difference is that plants store glucose as starch and animals store it as glycogen.

A palisade cell is traps sunlight with chlorophyll, it is found in the leaf.
Cell wall
This is needed in a plant cell to offer structural support, stop cells bursting and provide the symplastic pathway for water movement. They are made up of polysaccharides, like cellulose, and have a middle lamella which holds adjacent cells together.

These are needed in plant cells as they need the suns energy to carry out photosynthesis.
They are made up of three parts:

  • The chloroplast envelope- double plasma membrane to control the movement of substances in and out of the cell.
  • The grana- stacks of disks (thylakoids) which contain chlorophyll and have a large surface area for the first stage of photosynthesis. Tubular extensions can link them.
  • The stoma- matrix with the enzymes needed for the second stage of photosynthesis
  • DNA and ribosomes to manufacture proteins for photosynthesis
Root hair cells adaptations

  • Carrier proteins for active transport
  • Lots of mitochondria to produce ATP for active transport
  • Long thin shape to increase surface area for diffusion
  • Large vacuole containing a high proportion of ions to decrease the water potential and encourage osmosis into the cell
Xylem vessel adaptations
  • Thick walls to cope with the negative pressure of transpiration
  • Thickening happens in a spiral so the plant is still flexible
  • They are hollow and elongated so that water can move up them
  • They are dead so that water does not need to diffuse through anything and can go quickly through
  • Have a substance called lignin in their walls to offer strength and make it water proof (so water doesn't move out by osmosis)

Wednesday, 16 April 2014

The general pattern of blood circulation in a mammal. Names are required only of the coronary arteries and of blood vessels entering and leaving the heart, liver and kidneys.

Mammals have a double circulatory system, this means that there are two different sides of the heart pumping to two different places (the lungs and the rest of the body.)

We need a system like this because mammals are so large and so things need to be transported long distances and we are have a fairly high metabolic rate so we need a lot of oxygen transported.

The vein that brings blood into the heart from the body is called the vena cava.
The artery that takes blood out of the heart and to the lungs is the pulmonary artery.
The vein that brings blood into the heart from the lungs is the pulmonary vein.
The artery that takes blood out of the heart is called the aorta.

The hepatic artery brings blood in.
The hepatic vein takes blood out.
The vein that brings blood to the liver from the stomach is the hepatic portal vein.

The renal artery brings blood in.
The renal vein takes blood out.

The haemoglobins are a group of chemically similar molecules found in many different organisms. Haemoglobin is a protein with a quaternary structure. The role of haemoglobin in the transport of oxygen. The loading, transport and unloading of oxygen in relation to the oxygen dissociation curve. The effects of carbon dioxide concentration.Candidates should be aware that different organisms possess different types of haemoglobin with different oxygen transporting properties. They should be able to relate these to the environment and way of life of the organism concerned.

Haemoglobins are molecules consisting of four polypeptide chains and four haem groups. Different ones are found in different organisms.

There is a primary level of four polypeptide chains, two alpha and two beta, which turn into helices at the secondary level and fold up at the tertiary level much like any other protein. The difference is an additional quaternary structure in which the four chains are bonded to each other and four haem groups (iron).

Because the haem groups can form temporary bonds with oxygen, this means that the haemoglobin is able to carry four molecules of oxygen.

The dissociation curve shows the concentrations of oxygen at which the haem groups bind to oxygen.
On this graph we see the four points at which oxygen binds to the haemoglobin molecule.


The curve can be moved left or right depending on the type of haemoglobin and conditions.

Moving right means it has a lower affinity to oxygen and means it will give up oxygen more easily. High concentrations of CO2 will move the curve to the right. This means that near the muscles where there is a high concentration of CO2 (because it is a waste product of respiration that happens there) the molecule will easily give oxygen to the muscles that need it.

Moving to the left will mean the molecule has a greater affinity for oxygen and give it up less easily. In the lungs there are low amounts of CO2 because it is being removed by the lungs, this means the curve is moved to the left and haemoglobin easily picks up the oxygen it needs to.

This is called the Bohr effect. The movement happens because the shape of the molecule effects how it binds to oxygen, PH effects the shape and so CO2 which has a low PH will cause these changes.

Different organisms need different affinities of their haemoglobin because of the environment they live in and their metabolic rates.

By moving the curve to the left, an organism in an environment with a low partial pressure of oxygen can still fully load its haemoglobin.

An organism with a high metabolic rate needs oxygen frequently to respire and provide ATP (energy) to cells, this is achieved by moving the curve to the right so that oxygen dissociates easily even at higher partial pressures.

Monday, 14 April 2014

The use of monoclonal antibodies in enabling the targeting of specific substances and cells. • evaluate methodology, evidence and data relating to the use of vaccines and monoclonal antibodies • discuss ethical issues associated with the use of vaccines and monoclonal antibodies.

So... what are monoclonal antibodies?
Monoclonal just means a group of the same thing, so when we talk about monoclonal antibodies we mean a whole load of antibodies that are exactly the same. This is significant because inside the body, you find many different types of antibodies: this is because B cells which produce antibodies are induced to clone by antigens on the surface of a pathogen, there are many different antigens on a single cell, so many different B cells are produced and hence many different antibodies (polyclonal).

How are they made?
There now several ways to produce monoclonal antibodies, the one below is Milstein and Kohlers method (1975):

  1. Introduce a pathogen (with complimentary antigens to the antibody you want) to a mammal.
  2. The antigen will induce the cloning of the B cell that produces the right antigen. However, it will also produce others, giving B cells that will produce polyclonal antibodies.
  3. The B cells are taken out of the body and fused with tumour cells. The result is a hydribomas which is a cells that produces antibodies but can live for a longer time and divide outside the body.
  4. Different hybridoma are separated off and left to divide until it forms a group (a clone).
  5. Each clone is screened for the antibody that is needed- if it is being produced then it is grown on an industrial scale.
  6. Antibodies are extracted from the clone.
What are they used for?
Monoclonal antibodies are useful in the treatment of illness and the application of science because they allow the targeting of specific cells due to the fact that they will only bind with one antigen.
  • Separation techniques.
  • Immunoassay: when you take a pregnancy, drugs or HIV test, there are complimentary monoclonal antibodies in the test which will form an antigen-antibody complex with the protein that is being looked for which triggers a colour change.
  • Cancer therapies: trigger the immune system to attack cancer cells; carry radiation to the cancer cells; block signals that tell the cancer cells to divide.
  • Preventing rejection of transplanted organs: by targeting the T cells involved with destroying the foreign tissue.
  • Diabetes treatment.
What are the associated ethical issues?
  • You have to give cancer and illness to mammals (mice).
  • You have to make transgenic mammals (giving the properties of humans to make the antibodies suitable to use in the treatment of people).
  • Testing on humans caused organ failure.
  • Some people with MS have died as a cause.

Wednesday, 19 March 2014

Gas exchange • across the body surface of a single-celled organism • in the tracheal system of an insect (tracheae and spiracles) • across the gills of a fish (gill lamellae and filaments including the countercurrent principle) • by leaves of dicotyledonous plants (mesophyll and stomata). Candidates should be able to use their knowledge and understanding of the principles of diffusion to explain the adaptations of gas exchange surfaces. Structural and functional compromises between the opposing needs for efficient gas exchange and the limitation of water loss shown by terrestrial insects and xerophytic plants.

Single-celled organism
High surface area to volume ratio means that things can diffuse across the organism quickly. This is why they do not need systems, but can just rely on diffusion through them.
Low surface area to volume ratio and waterproof coverings (to stop water loss) mean that insects cannot rely on diffusion over their body.
They have developed a tracheal system: air enters via spiracles (controlled by valves) into trachea and then into tracheoles.
Tracheoles go throughout the body meaning that gasses diffuse through them into the all of the insect.
Sometimes insects pulse or move muscles to ventilate the system.
Reducing water loss; water proof outer layer; reduced surface area to volume ratio (small); they can close spiracles.
Low surface area to volume ratio and non-permeable outer means they need a specialised system for gas exchange: gills.
Gills consist of gill filaments (which are like strands of a feather) and gill lamellae (which are like discs strung along the filaments). Blood is running through the gills.
A fish pumps water over the gills by opening an closing its mouth- this pushes water out of the mouth, over the gills and then out the sides of the fish.

The water and the blood are flowing in opposite directions- this is called countercurrent flow and is explained in this post:

Leaves have a large surface area to volume ratio so they do not need a gas exchange system.
However there are adaptations to make diffusion easier, for example the flat shape meaning there is a short diffusion distance and spaces in-between meysophyll cells for gasses to diffuse through easily.
Leaves have stomata on them (which are basically just holes) so they can control diffusion. This means that when they are at risk of loosing too much water the guard cells (that control the stomata) can close the hole to prevent water from evaporating from the leaf- this is an example of a compromise made between the need to exchange gasses and preserve water.

Xerophytic plant adaptations
These plants have developed adaptations to limit the water loss due to the fact that transpiration exceeds root uptake of water.

  • Spines for leaves mean that there is a reduced surface area to volume ratio so less surface area from water to be lost from; but they have a thick stem to provide a large surface area for photosynthesis. (cacti)
  • Having hairy leaves traps moist air around the plant reducing the water potential gradient.
  • Stomata that are sunken into groves also will have moist air trapped around them.
  • Leaves roll up to trap moist air around the stomata as well. (marram grass)
  • A thick cuticle (waxy layer) which is waterproof stops water being evaporated through the top of the leaf.

The countercurrent principle.

Countercurrent flow is when two things are flowing in the opposite direction to increase the exchange of products between them.

This in the gas exchange system of a fish- blood is flowing the opposite way to water so that more CO2 will leave the blood and enter the water, and more O will leave the water and enter the blood.

If blood and water were flowing in the same direction then at the beginning (nearest the mouth) there would be a lot more O in the water than in the blood so O would move into the blood. By the end of the gill, the O level in the blood would have risen, but that in the water has been lowered so there is a small, or non existent, concentration gradient. In this case O would only be exchanged at the start of the gills and along the rest of the length, diffusion would not take place.

If the blood is running in the opposite direction to the water, then at the beginning of the gill the water would have a high O concentration and the blood would also have a high O concentration (because it has already been past water on its way along the gill) but it would still have a lower concentration than the water because the waters concentration is so high when it has just entered so diffusion would occur. At the end of the gills when the O in the water had depleted, the blood has a very low oxygen content (as it has just been round the body) so there would still be diffusion. In this way the gills can extract O from water along the full length of the gills.

The diagram shows how even though the O concentration in the water goes down, it is still higher than the blood at all times so diffusion occurs.

This video is helpful:

Tuesday, 18 March 2014

Over large distances, efficient supply of materials is provided by mass transport

Mass transport is a system which transports necessary materials around an organism.

It consists of a transport medium, vessels, a means of moving the transport medium and something that controls the direction of flow.

In mammals this corresponds to blood, blood vessels, the heart and valves.

A mass transport system is needed if an organism is too large (low surface area to volume ratio) to get the materials it needs from diffusion.

The relationship between the size of an organism or structure and surface area to volume ratio. Changes to body shape and the development of systems in larger organisms as adaptations that facilitate exchange as the ratio reduces. Candidates should be able to explain the significance of the relationship between size and surface area to volume ratio for the exchange of substances and of heat.

The bigger the surface area (SA) in comparison to the volume, the faster material can diffuse across an object or organism.

So it is crucial in living things that rely on diffusion to deliver oxygen for respiration, that the SA is large in proportion to size.

In bigger organisms the ratio reduces so that the SA is not big enough to allow for diffusion to supply the whole organism with oxygen. In addition to this the diffusion distance is larger and so it would take a long time for gas exchange to take place via diffusion.

To overcome this large organisms develop systems for gas exchange, for example lungs and gills. These systems will have a large surface area allowing for quick diffusion.

Other animals change their shape to increase their surface area, for example Flat-worms have a flat shape.

The graph displays the fact that as size increases, the SA gets smaller (in proportion to the volume.) As a consequence of this, diffusion will be slower in larger objects meaning heat and substances will take longer to diffuse to the middle of an object.

Monday, 17 March 2014

The use of vaccines to provide protection for individuals and populations against disease. • evaluate methodology, evidence and data relating to the use of vaccines and monoclonal antibodies • discuss ethical issues associated with the use of vaccines and monoclonal antibodies • explain the role of the scientific community in validating new knowledge about vaccines and monoclonal antibodies, thus ensuring integrity • discuss the ways in which society uses scientific knowledge relating to vaccines and monoclonal antibodies to inform decision-making.

Vaccines involve injecting a weak or inactive form of a pathogen into the body.

The antigens stimulate an immune response from white blood cells.

The cells destroy the pathogen, but more importantly, they also produce memory cells.

This means that if the real pathogen enters the body, memory cells will produce large amounts of plasma cells very quickly to combat the pathogen- so it is destroyed before it can harm the body.

This is often carried out throughout whole populations so that everyone is protected against a pathogen and it can be eradicated.

Using a weak or inactive form of the pathogen means that there is no risk of the pathogen from the vaccine harming the body.

The MMR vaccine
A vaccine that protects against Measles, Mumps and Rubella is given to all children in the UK to prevent them getting these potentially disabling diseases. Andrew Wakefield published a study on the vaccine in 1998 which appeared to show that it increased the risk of children getting autism.

The claims are now believed to be completely unfounded in light of: new research showing no link; the small sample size he used; his vested interest to prove the link for the Legal Aid Board. However, at the time there was a big following of this idea and many people decided not to vaccinate their children. As a result the cases of all three diseases rose.

Ethical issues

  • Testing on animals
  • Potentially harmful testing on humans
  • Possible side effects
  • The fact that it might breach peoples rights to make vaccines compulsory

The effects of antigenic variability in the influenza virus and other pathogens on immunity.

Some pathogens have many different strains.

Influenza (common flue) is an example of a pathogen with multiple strains.

The different strains have different antigens- this is known as antigenic variability.

Memory cells will recognise antigens they have seen before and tackle a pathogen before symptoms arise- this is why you can only get chicken pox once.

However, if the antigen is different, the memory cell will not recognise it and be able to destory it.

This means that it is down to the slower and less effective primary response to kill the pathogen, allowing time for the pathogen to harm the body and cause symptoms- this is why you can get influenza multiple times.

The essential difference between humoral and cellular responses as shown by B cells and T cells. The role of plasma cells and memory cells in producing a secondary response.

Lymphocytes are white blood cells. They are created as stem cells in the bone marrow. They have defences that are specific to the pathogen they are attacking (unlike phagocytes which do the same for everything) which makes response slower, but more effective long term.

B cells
  • mature in the bone marrow
  • respond to antigens in the bodies fluids: tissue fluid; blood (humoral response)
  • produce antibodies
  • produce memory cells
  1. ingest pathogen and present antigens on the surface
  2. these are recognised by helper T cells, which stimulate mitosis
  3. plasma cells and a Memory cells are produced
  4. plasma cells secrete antibodies which attach to antigens on a pathogen to destroy it (primary response)
  5. memory cells stay in the blood stream for many years, if they encounter the same pathogen again, they can divide rapidly and with greater intensity to make plasma cells which will make antibodies (secondary response)
The secondary response provides long term protection as they memory cells stay alive for many years. They produce many more plasma cells and are much faster at doing so than the primary response, this means that the pathogen can be fought before it causes harm to the body.

T cells
  • mature in the thymus glands
  • recognise antigens if presented on the surface of other cells (cell-mediated response)
  • stimulate b cells and phagocytes
  • kill infected cells
  • produce memory cells
  1. Phagocytes, infected cells and cancer cells all display antigens on their surface
  2. A specific helper T cell will have receptors that fit exactly with the antigens- when they meet, the helper T cell stimulates other T cells to form appropriate clones by mitosis
  3. These T cells can: stimulate B cells; stimulate phagocytes; develop into memory cells; kill cells
  4. They kill cells by producing a protein which breaks cell-surface membranes.

Thursday, 13 March 2014

Antibody structure and the formation of an antigen-antibody complex.

Antibodies are often compared to a Y shape because of their one receptor binding site and two pathogen binding sites.

Antibodies are made of two different polypeptide chains, a light chain and a heavy chain. They are attached to each other, but can move in the pathogen binding site to help bind to the pathogen.

The variable region is different on different types of antibody because it needs to be specific to the antigen it is targeting. The constant region is the same in all antibodies.

The variable region has a tertiary structure that is complimentary to (fits with) that of the antigen it is aiming to destroy- this is so that the two can bind and form what is known as an antigen-antibody complex.


Wednesday, 12 March 2014

Definition of antigen and antibody.

An antigen is a 'marker' on a cell that is foreign to the body that identifies it as non-self.

An antibody is a protein produced by the body to destroy pathogens.

Phagocytosis and the role of lysosomes and lysosomal enzymes in the subsequent destruction of ingested pathogens.

Phagocytes are white blood cells. They destroy bacteria by engulfing them and breaking them down- this process is called phagocytosis.

The phagocyte recognises a pathogen because of its chemical products and so moves towards it.

It then binds with the pathogen and begins to engulf (wrap around) it- by doing this it forms a vesicle (sac) with the phagocyte inside it know as a phagosome.

Lysosomes (vesicles with enzymes inside) release digestive enzymes into the phagosome, this means that it can be broken down. Useful products are absorbed by the cell and others are excreted.

Risk factors associated with coronary heart disease: diet, blood cholesterol, cigarette smoking and high blood pressure. Candidates should be able to describe and explain data relating to the relationship between specific risk factors and the incidence of coronary heart disease

A number of factors can increase the risk of coronary heart disease:


  • Salt raises blood pressure.
  • Saturated fat increases blood cholesterol.

Blood cholesterol

  • Low-density lipoproteins associate with white blood cells to cause atheromas.
  • High-density lipoproteins help lower cholesterol.


  • Nicotine stimulates the production of adrenalin, this causes a quicker heart rate and therefore raises the blood pressure.
  • Nicotine makes platelets stick together, so thrombosis is more likely.
  • Carbon monoxide combines with heamaglobin so less oxygen can be carried in the blood. The heart has to pump more quickly to deliver the same amount of oxygen, so blood pressure is raised. The heart muscles may not get enough oxygen leading to a heart attack or angina (chest pain).

High blood pressure

  • Arteries are put under more pressure so will form hard walls to resist the pressure- these thicker walls constrict blood flow.
  • The pressure can burst open the arteries (haemorrhage).

Monday, 10 March 2014

Atheroma as the presence of fatty material within the walls of arteries. The link between atheroma and the increased risk of aneurysm and thrombosis. Myocardial infarction and its cause in terms of an interruption to the blood flow to heart muscle.

Atheroma is the name given to fatty build ups in artery walls: consisting of cholesterol, fibres, dead cells and white blood cells attached to fats.

This is when an atheroma weakens an artery wall and it swells with blood making an aneurysm. If it bursts (haemorrhage) blood is lost- this can be fatal. In the brain this is what we know as a stroke.

When an atheroma bursts the lining of an artery (endothelium) and obstructs blood flow. A clot (thrombus) can form here and block the vessel, or be carried around the body and block another vessel. The blocked off area doesn't receive oxygen so will die.

Myocardial infraction
This is a heart attack. A blockage stops blood getting to the heart tissue; the tissue doesn't receive enough oxygen and so gets damaged. The damage prevents the heart from pumping properly.


The effects of fibrosis, asthma and emphysema on lung function.

Scars in the lung tissue thicken the alveoli increasing the diffusion distance, decrease the volume in the lungs and reduce elasticity. This means that less air can be taken in, and oxygen diffuses more slowly decreasing the bodies supply.

  • Shortness of breath- attempt to increase oxygen supply
  • Cough- reflex to obstruction of the scars
  • Pain- increased pressure
  • Fatigue- lack of oxygen for respiration so less ATP (energy) is produced
An allergic reaction causes white blood cells to release histamine, a chemical which inflames breathing pathways and constricts them (by contracting muscles)  and increases mucus.
  • Difficulty breathing- constriction, inflammation and mucus
  • Wheezing- constriction
  • Tight feeling- lungs can't ventilate because of airway constriction
  • Coughing- reflex to obstruction
Elastic tissue in the lungs looses its elastic properties due to smoking. The lungs can't recoil to push air out effectively, and so old air is not replenished and there is a reduced diffusion gradient. Alveoli break down so surface area and the walls thicken so the diffusion distance is increased. Due to these factors less oxygen can diffuse into the blood.
  • Shortness of breath- attempt to increase oxygen supply
  • Cough- reflex to obstruction of damaged tissue
  • Blueish skin- low oxygen levels in the blood

The course of infection, symptoms and transmission of pulmonary tuberculosis.


Mycobacterium tuberculosis and Mycobacterium bovis are bacteria that causes tuberculosis; it transmitted in aerosol (air born droplets) so can be transmitted by coughing, sneezing and spitting- providing some cells are breathed in. It can also be contracted from cows milk.

Risk of infection is increased by:
  • Over crowded conditions
  • Weak immune system
  • Repeated contact with infect people
Course of infection

Primary infection:
  • Bacteria enter the lungs and begin to multiply
  • White blood cells go to attack them (engulfed by phagocytes) (encased in tubercle by macrophages)
  • This inflames lymph nodes so they can't drain the lungs (causing cough)
  • Often at this stage most bacteria are destroyed, but some may remain (dormant)
Post-primary infection:
  • Years later the remaining bacteria cause a second infection (if immunosuppressed)
  • They attack the epithelial cells- causing the infected person to cough it up damaged lung tissue
  • The damage allows bacteria to enter the blood and spread to other organs
  • The damage decreases the available surface area for gas exchange and makes the diffusion distance further, meaning sufferers will have reduced gas exchange
  • Cough
  • Fever
  • Fatigue

In eukaryotes, much of the nuclear DNA does not code for polypeptides. There are, for example, introns within genes and multiple repeats between genes.

Introns are sections of DNA that do not code for anything.

Sometimes they are in the middle of genes and sometimes they come between different genes.

Due to these introns very little of DNA has a code that actually makes proteins.

Wednesday, 26 February 2014

The importance of meiosis in producing cells which are genetically different. Within this unit, meiosis should be studied only in sufficient detail to show • the formation of haploid cells • independent segregation of homologous chromosomes. Gametes are genetically different as a result of different combinations of maternal and paternal chromosomes • genetic recombination by crossing over.

Meiosis is the process by which sex cells (gametes) are produced. They have half the genetic information of a normal (somatic) cell.

The 'normal' amount of chromosomes is called the diploid number (in humans it is 46 chromosomes/23 pairs of chromosomes) and gametes have the haploid number of chromosomes (23 in humans).

It is important that gametes have half the genetic information so that when two gametes meet the child gets a mixture of genes from its parents and therefore is very different. This is called variation and is what has enabled evolution to happen.

Haploid cells are made in two stages: meiosis 1; and meiosis 2:

Prophase 1: chromatin condenses to form chromosomes. Homologous pairs (the same chromosomes, one from your mum and one from your dad) come together, here some segments of DNA can transfer from one into another, so alleles will swap- this is called crossing over and increases variation. Nuclear envelope and nucleolus dissolve. Spindle fibres start to form and chromosomes move to the poles.

Metaphase 1: homologous pairs line up in the middle of the cell, some of the paternal chromosomes are on the right and some on the left, the same for maternal chromosomes.

Anaphase 1: the pairs are pulled apart towards the opposite poles by spindle fibres.

Telophase1 : the nuclear envelope and nucleus reform (and cytokineses happens) so two new cells have been made. Because some of paternal and maternal DNA was in a random order, the cells have different DNA, this is called independent segregation.

Meiosis 2 happens in both of the cells formed by meiosis 1, it is the same as mitosis:

Prophase 2: nuclear envelope and nucleolus dissolve. Spindle fibres start to form. Spindle fibres start to form and chromosomes move to the poles.

Metaphase 2: chromosomes move into the middle.

Anaphase 2: spindle fibres pull sister chromatids apart in towards opposite poles.

Telophase 2: nuclear envelope and nucleolus reform (and cytokineses happens) so that each cell has become two, resulting in four cells, each with half the number of chromosomes than a somatic cell.


Candidates should be able to analyse, interpret and evaluate data concerning early experimental work relating to the role and importance of DNA.

One experiment performed when investigating the nature of DNA was one to find out if its replication is conservative or semi conservative.

Conservative means there would be one completely new piece of DNA made and one completely old one left.
Semi-conservative means there are two pieces of DNA that are both made up of half old and half new DNA.

DNA contains nitrogen.
There are two isotopes of nitrogen 14N and 15N.
15N is heavier and 14N lighter.
If DNA contains 15N it would sink towards the bottom of a tube when centrifuged.
If DNA contains 14N it would be towards the top of a tube when centrifuged.

Scientists grew bacteria on 14N, they then removed the DNA and centrifuged it- it came out at the top of the tube, showing that the DNA of the bacteria would have in it the DNA it grew on.

Scientists then grew bacteria on 15N. This DNA came out at the bottom of the tube.

Scientists moved the bacteria from the 15N to the 14N and left them long enough that the DNA would replicate once.

They found that DNA from these bacteria came out in the middle of the tube. This was because they had one light strand, and one heavy strand. This showed that the DNA included one old strand of DNA from when it was on 14N and one new strand of DNA from when it was on 15N.

So they could conclude that DNA replicated semi-conservatively, making two strands of half-new-half-old DNA.

Tuesday, 25 February 2014

DNA and chromosomes resources

Mitosis resources

I've decided to put up resources that I use to create posts and revise, so enjoy:

During mitosis, the parent cell divides to produce two daughter cells, each containing an exact copy of the DNA of the parent cell. Mitosis increases the cell number in this way in growth and tissue repair. Candidates should be able to name and explain the events occurring during each stage of mitosis. They should be able to recognise the stages from drawings and photographs.

We always need more cells (to replace old ones and) for growth and tissue repair.

In mitosis, one cell divides into two new cells: the first cell is the parent cell and those produced are daughter cells. The daughter cells are identical to the parent cell because they have the exact same DNA.

There are four phases in mitosis:

DNA that was in chromatin form to condenses into chromosome form: that means it associates with proteins and winds itself into a structure. Each chromosome consists of two chromatids (identicle pairs of DNA) which are joined in the middle by centromeres.
Centrosomes move towards the poles (opposite ends) of the cell and begin to form spindle fibers (micro tubules that act like rope).
The nuclear envelope and nucleolus dissolve.

Chromosomes line up in the middle of the cell.
Spindle fibres which attach to the centromere of chromosomes.

Spindle fibres pull one half of each chromosome in each direction, at the same time the centrosomes are moving further apart towards the poles of the cell.

Chromosomes reach poles and turn back into chromatin.
The nuclear envelope and nucleolus reform.

Candidates should be able to relate their understanding of the cell cycle to cancer and its treatment.

If there is a change in the DNA that controls the cell cycle, there can be an increase in the speed that cells grow at.

One cell with this mutation will rapidly divide into to, and this will continue to create a large group of cells, called a tumour.

These tumours are potentially disruptive to the functions of organs, as we know cancer takes many lives.

Chemotherapy is the use of chemicals to inhibit the cell cycle. They effect parts of the cell cycle for example: stopping spindle formation in the metaphase; stopping DNA replication during synthesis.

These chemicals effect the cycles of all cells in the body, but they will have a bigger effect the faster the cycle. This means that cancer cells are significantly slowed, but so are other quickly replicating cells are too, like hair cells.

Mitosis and the cell cycle. DNA is replicated and this takes place during interphase.

After a cell is created it goes through a several stages:

  1. G1: the first phase of growing, when proteins and organelles are being made
  2. S: synthesis, when DNA is being replicated
  3. G2: the second phase of growth when organelles and energy supplies are increased
  4. Prophase: chromosomes become visible, nuclear envelope and nucleolus disintegrate
  5. Metaphase: chromosomes line up in the middle of the cell; spindle fibres form
  6. Anaphase: Spindle fibres contract pulling chromosomes to the poles
  7. Telophase: nuclear envelopes and nucleoli form around both sets of DNA
  8. Cytokinesis: the cells cytoplasm divides into two
1, 2, and 3 are all part of the 'interphase' when the cell is not in the process of replicating.
4, 5, 6 and 7 are all part of mitosis, which is explained in greater detail in the following post:

This diagram represents the cell cycle:
(ignore the check point markings)

In eukaryotes, DNA is linear and associated with proteins. In prokaryotes, DNA molecules are smaller, circular and are not associated with proteins.

Eukaryote DNA

  • Long chains of DNA
  • Associated with proteins (wrapped up with)
  • 3.2bn nucleotides long

Prokaryote DNA

  • Circular chromosome
  • Not associated with proteins
  • 4.6mn nucleotides long

Thursday, 20 February 2014

The semi-conservative replication of DNA in terms of • breaking of hydrogen bonds between polynucleotide strands • attraction of new DNA nucleotides to exposed bases and base pairing • role of DNA helicase and of DNA polymerase.

The replication of DNA is called semi-conservative because each new piece of DNA is half old DNA and half new.

  1. DNA helicase breaks the hydrogen bonds between the base pairs
  2. The two strands of the DNA separate, leaving 2x polynucleotide strands
  3. Free activated nucleotides are attracted to the complimentary nucleotides on the polynucleotide strand
  4. They are then joined together by DNA polymerase
Good to visualise with a animation:

Differences in base sequences of alleles of a single gene may result in non-functional proteins, including non-functional enzymes.

Bases in DNA can change (mutate) during replication.

The amino acid that the base pair coded will change, this will result in a different protein being made.

This different protein is called an allele, because it is a different form of a gene. (E.g blue or brown eyes.)

Sometimes the changes in the base pairs can make a polypeptide chain that makes a non-functional protein.

If a base is changed, one amino acid will be different which will change the protein a bit.

If a base is added in or taken away, it will change every amino acid in the chain- because it is read as in triplets so the whole sequence will move along one- which is likely to make a non-functional protein.

Enzymes are proteins, they can be made non-functional by changes in bases.

A gene occupies a fixed position, called a locus, on a particular strand of DNA. Genes are sections of DNA that contain coded information as a specific sequence of bases. Genes code for polypeptides that determine the nature and development of organisms. The base sequence of a gene can change as a result of a mutation, producing one or more alleles of the same gene. A sequence of three bases, called a triplet, codes for a specific amino acid. The base sequence of a gene determines the amino acid sequence in a polypeptide.

Three base pairs code for a amino acid. This is called the triplet code.

E.g. a thymine followed by a guanine followed by a thymine is the code for the amino acid cysteine.

A sequence of base pairs can, therefore, make a polypeptide chain (chain of amino acids that makes up a protein.)

A section of DNA that codes for a specific protein is called a gene.

Organisms are made of and controlled by proteins, so genes determine what an organism is like.

The base sequence of a gene can change as a result of a mutation,  producing one or more alleles of the same gene

An allele of a gene is a different protein produced for the same purpose.

E.g the proteins in the iris can be brown or blue (or many other colours.) So brown and blue are different alleles for the same gene.

Different alleles occur when there is a change (mutation) in the base pairs, so the amino acids are made differently resulting in a different protein.


The locus is the place on the DNA where a gene is.

The double-helix structure of DNA, enabling it to act as a stable information-carrying molecule, in terms of • the components of DNA nucleotides: deoxyribose, phosphate and the bases adenine, cytosine, guanine and thymine • two sugar-phosphate back bones held together by hydrogen bonds between base pairs • specific base pairing

DNA is often compared to a ladder that has been twisted, this is because it consists of two back bones bonded together and then twisted into a double helix.

The back bone is made up of a sugar, deoxyribose, and a phosphate group.

These two molecules are bonded together along with an organic base; together they are called a nucleotide.

Organic bases are the molecules that make up the code of DNA.

There are four different bases: cytosine (C); thymine (T); adenine (A); guanine (G).

C and T are single ring-bases. A and G are double-ring bases (so they are twice as long.)

Single rings only ever join with double rings so that the 'rungs of the ladder' are always three rings long.

The pairing goes: C with G; A with T.

The bases are joined together by hydrogen bonds. Two for A to T; three for C to G.

  • Bases code for genes
  • Hydrogen bonds can be easily broken for when DNA needs to replicate
  • The back bone protects the bases
  • The twisting makes it smaller so more information can fit
  • Covalent bonds between the phosphate and deoxyribose make the back bone strong
NB: often deoxyribose is drawn as a pentagon.