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.