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.

lillianwaller

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:
https://www.youtube.com/watch?v=cVFqME-NW9s

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.

jochemnet

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.

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.

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.

wikipedia