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AS level Biology

Aqa as level biology
by

emsy Thomson

on 12 September 2012

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Transcript of AS level Biology

AS Biology Unit 1 Disease A pathogen is any organism that causes disease.
Pathogens are mostly microorganisms, but there are some larger, like tape worms.
All viruses are pathogens, some bacteria and some fungi. What a pathogen is and how it enters the body. Pathogens penetrate an organisms interface with the environment;
-either via the gas exchange system and through the alveoli;
-the skin, through breaks in it (cuts);
-and the digestive system, where they invade gut wall cells. Your body defends each location from pathogens however;
-In the gas exchange system, by trapping pathogens in mucus lining the lung epithelium, which is then moved up out the trachea by cilia.
-In the digestive system, the acid in the stomach will kill most pathogens.
-The skin, by blood clotting at the point of entry. The main ways pathogens cause disease are by;
-The production of Toxins.
-Cell damage. How Pathogens cause disease -Toxins are harmful molecules and do various things, like blocking the function of certain never cells, like in tetanus, causing muscle spasms. Pathogens can physically damage host cells by;
-Rupturing them to release nutrients
-using a host cells nutrients for itself, starving the cell
-bursting the cell by replicating themselves multiple times inside the host cell (some viruses do this) There are other ways in which people get diseases,
many attributed to one's own lifestyle;
-Coronary heart disease (CHD) is a disease that affects your heart.
Some factors that increase the risk of developing this disease are;
- Poor diet, high in saturated fats or salts.
- Smoking, lack of exercise, and excessive alcohol intake all lead to high blood pressure, which can damage the heart and increase the risk of CHD.
-Cancer is the result of uncontrolled cell division.
Factors that increase it's risk are;
- Smoking - the main cause of mouth, throat and lung cancer.
- Excessive exposure to sunlight or sunbeds can cause skin cancer.
- excessive alcohol intake increase the risk of many types of cancer, mainly liver cancer. Lifestyle The Immune
System Antigens are molecules found on the surface of cells. A pathogen's antigens are recognized by the body as foreign which activates cells in the immune system. There are four stages; Immune System Primary Response 1. Phagocytes engulf Pathogens
2. Phagocytes activate T-cells
3. T-cells activate B-cells, which divide into plasma cells
4. Plasma cells make more antibodies for specific antigens A phagocyte is a type of white blood cell that carries out phagocytosis. They are found in the blood and in the tissues. They are the first cell to respond to a pathogen inside the body. Stage 1 - A phagocyte recognizes the antigens on a pathogen.
- The cytoplasm of the phagiocyte moves round the pathogen, engulfing it.
- This contains the pathogen in a phagocytic vacuole in the phagocyte's cytoplasm.
- A lysosome fuses with the vacuole. Enzymes within the lysosome (lysosomal enzymes) break down the pathogen.
- The phagocyte then puts the pathogens antigens on it's own surface, activating other immune system cells. A T-cell is another type of white blood cell. The T- cell is activated once it binds to the antigens attached to the surface of the phagocyte. It binds via proteins on it's own surface. Different T-cells respond in different ways, either;
- By releasing substances to activate B-cells.
- or by attaching to the antigens on the pathogens surface, killing the pathogen. Stage 2 B-cells are also a type of white blood cell. They are covered with antibodies, which are proteins that bind to antigens, to form an antigen-antibody complex. Each B-cell has a different shaped antibody on it's membranem so different ones bind to different shaped antigens.
- An antibody on the surface of a B-cell binds to a complementary shaped antigen when it meets it.
- Along with substances released from T-cells, this activates the B-cell.
- The B-cell then divides into plasma cells. Stage 3 Plasma cells are identical to B-cells, they are clones of them, and they secret loads of the antibody specific to the antigen of the pathogen. Antibodies;
- Coat the pathogen making it easier for a phagocyte to engulf
- Coating it to prevent it from entering host cells
- Binding to toxins, produced by the pathogen, to neutralise them Stage 4 Antibodies are proteins - they're made up of chains of amino acid monomers linked by peptide bonds. The specificity of an antibody depends on it's variable regions. Each antibody has a different shaped variable region (due to different amino acid sequences) that's complementary to one specific antigen. The constant regions are the same in all antibodies. The immune response is often split into two;
- Cellular response: T-cells and other immune system cells they interact with, eg, phagocytes.
- Humoral response: B-cells and the production of antibodies. Cellular and Humoral A pathogen which has previously entered the body and had been removed by the immune system can re-enter the body a second time.
But this time the required antibodies for dealing with this pathogen are already in the body, as T-cells and B-cells have produced memory T and B-cells.
Memory B-cells divide into plasma cells which produce the right antibody to the antigen.
Memory T-cells divide into the correct type of T-cells to kill the cell carrying the recognised antigen.
The secondary response is must faster at removing pathogens. Secondary Immune Response After the primary response is over and the pathogen removed, the person can be considered immune to the pathogen. Vaccines
and Antibodies
in Medicine - Vaccines contain antigens that cause your body to produce memory cells against a particular pathogen without having to get ill. You become immune without getting the disease's symptoms.
- Individuals become immune to the disease, but this also leads to herd immunity, where people who haven't been vaccinated are protected, since less people have the disease and so they are less likely to catch it themselves.
- The antigens in the vaccine may be free or attached to a dead or attenuated (weakened) pathogen.
- Vaccines can be taken orally, however this means it could be broken down by enzymes in the gut, or the molecules could be too large to absorb into the blood. And so are usually injected.
- Booster vaccines can be given after several years to make sure that memory cells are produced. Vaccines Protect Individuals
and Populations Some pathogens can change their surface antigens in order to avoid the immune system; A pathogen that has already invaded once and is fought off can invade again as if it was never there previously as the immune system doesn't have the correct memory cells to combat it, since it's antigens have changed.
For this reason vaccines can be very difficult to manufacture against these pathogens.
Some pathogens that show antigenic variation are; HIV, S.pneumoniae bacteria, and the influenza virus.
How it works in the influenza virus: Antigenic Variation Monoclonal Antibodies Targeting cells - Cancer Targeting Substances -
Pregnancy Tests - The influenze virus causes influenze (flu)
- Proteins (neuraminidase and haemagglutinin) on the surface of the influenza act as antigens, triggering the immune system
- These antigens change regularly, forming new strains of the virus
- Memory cells from the first infection will not recognise it on the second infection
- A primary response is triggered everytime you're infected with a new strain, so you can get the flu more than once
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