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.