Immune System

Ten million of people die from infectious diseases, many survive or appear not to be affected.

Any infection is an interaction between the pathogen and the body's defence mechanism. Sometimes the pathogen overwhelms the defence and the person dies. Sometimes the defence overwhelms the pathogen and the person recovers.

Having overwhelmed the pathogen the body's defence seems to be better prepared for the second infection from the same pathogen and can kill it before it can cause harm. This is know and immunity and the main reson why some people are unaffected by certain pathogens

There is range of intermediates between the stages but they depend on the state of health of the person. A fit, healthy adult rarely die from an infection. Those not healthy, young and elderly are more vulnerable

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Defence Mechanisms.

Human body has a range of defences to protect itself from pathogens. Some are general and immediate defences like skin form a barrier to entry of pathogens and phagocytosis. Others are more specific, less rapid but long lasting

Responses involve a type of white blood cell called lymphocyte and take two forms

  • Cell-mediated responses involing T lymphocytes
  • Humoral responses involving B lymphocytes
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Recognising your own cells

To defend the body frominvasion by foreign material, lymphocytes must be able to distinguish the body's own cells and molecules (self) and foreign cells (non-self). If they can't do this they would destroy the organism own tissues.

Each cell self or non-self has specific molecules on its surface that identify it. These molecules can be a variety of types, proteins are most important because have specific tertiary structure/3D structure that distinguishes one cell from another. They allow the immune system to identify: Pathogens (like HIV), non-self material (cells from organism same species), toxins (produced by certain pathogens like bacteria that causes cholers), abnormal body cells (cancer cells).They are all potetially harmful first stage is removing them.

Implication - people who have tissure or organ transplants, sometimes there immune system recognises them as non-self even though from same species it attempts to destroy. To minimise this they try to get a close match from people genetically similar and can use immunosuppressant drugs.

High probability you have protein with complementary proteins to pathogen that enters the body, if do then will stimulate for it to divide to increase numbers so can effectively destroy it called clonal selection and explains lag time between exposure and bringing under control

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How lymphocytes recognise cells

  • Ten million different lymphocytes each capable of recognising a different chemical shape
  • In the fetus, lymphocytes are constantly colliding with other cells
  • Infection in the fetus is rare because it is protected from the outside by mother and placenta
  • Lymphocytes will collide almost the body's own material (self)
  • Some lymphocytes will have receptors that exactly fit those of body's cells
  • Lymphocytes either die or are suppressed
  • Remaining lymphocytes might fit foreign material (non-self) and respond to foreign material
  • In adults lymphocytes produced in the bone marrow initially only encounter self-antigens
  • Any lymphocytes that show immune response to self-antigens undergo programmed cell death (apoptosis) before they can differentiate into mature lymphocytes
  • No clones of these anti-self lymphocytes will be in the blood, leaving only non-self antigens
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Two types of white blood cells: Phagocytes (ingest and destroy the pathogen by phagocytosis) and Lymphocytes (Involved in immune response)


Large particles like types of bacteria can be engulfed by cells in the vesicles fromed from cell-surface membranes. Phagocytes provide important defence against the pathogens that manage to enter the body, some travel in the blood but can move out into tissues

  • Chemical products of pathoogens or dead, damaged and abnormals cells act as attractants causing phagocytes to move towards the pathogen
  • Phagocytes have several receptors on their cell-surface membrane that recognise and attach to chemicals on the surface of the pathogen
  • Phagocytes engulf the pathogen to from a vesicle called phagosome
  • Lysosomes move towards the vesicles and fuse with it
  • Enzymes called lysozymes present within lysomsone which destroy ingested bacteria by hydrolysis
  • Soluble products from breakdown of pathogen are absorbed into the cytoplasm of the phagocytes
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Is any part if an organism or substance that is recognised as non-self (foreign) by the immune system and stimulate an immune response.

They are usually proteins that are part of the cell-surface membranes or cell wall of invading cell such as microorganism, abnormal body cells or cancer cesll

Presence of an antigen triggers production of antibodies

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There are two types of responses non-specific (Phagocytosis which occurs for any infection) and specific (rescts to specific antigents)

Specific response is slower at start but can provide long-term immunity and depends on type of white blood cell (lymphocyte) there are two types B lymphocytes (B cells) and T lymphocytes (T cells)

B lymphocytes (B cells)

  • They mature in the bone marrow and are associated with humoral immunity which involved antibodies what are present in body fluids or humor such as blood plasm

T lymphocytes (T cells)

  • They mature in the thymus glands and associated with cell-mediated immuntiy involving body cells
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Cell mediated immunity

Lymphocytes respond to an organism's own cells that have been infected by non-self material from different species and respond to cells from other individuals as are genetically different therefore have different antigens on membrane that their own cells

T Lymphocytes distinguish as invader because: Phagocytes (engluf and hydrolyse pathogen and present pathogens on cell surface), body cells invade (viruses present some antigens on their surface), transplant cells (same species different antigens on surface), caner cells (different from normals body cells and different antigens presented)

Cells that display foreign antibodies called antigen-presenting cells. T lymphocytes (T cells) respond to antigens on body cells are receptors on T cells respons to single antigen

  • Pathogen invades call or taken in by phagocytosis, and phagocytes places antigens from pathogen on its cells membrane
  • Receptors on helper T cells fit onto these anigens and activate T cells to divide by mitosis and form a clone of genetically identical cells
  • Clone T cells then either: develop into memory cells (help with respone in the future to same pathogen), stimutate phagocytes (for phagocytes), stimular B cells(to divide and secrete antibodies), activate cytotoxic T cells
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How cytotoxic t cells kill infected cells

They kill abnormals cells and body cell infected by pathogens by producing a perforin which is a protein that makes in holes in the cell-surface membrane.

The holes mean that the membrane becomes permeable to all substances and the cell dies. This is why the cell-surface membrane is important for survival

T cells are most effective against viruses becuase viruses replicate inside cells. As they use living cells to replicate by killing off some the body cells it stops the viruses from multiplying and infecting more cells

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Humoral immunity

Involves antibodies which are soluble in blood and tissue fluid. There are 10 million B cells each produced specifc antibody in response to a specifc antigen.When antigen on surface of pathogen, foreign cell, toxic or damaged or abnormal cell enters the blood or tissue fluid a B cell with that has the antibody and complementary shape attached to antigen and pulls into the B cell by endocytosis and presents of antigens on its surface (processed). Helper T cells bind to the processed antigens and stimulate B cells to divide by mitosis to form a clone of B cells to produce antibodies specific to antigen (clonal selection) which helps for rapid response. Some pathogens produce toxins and each toxin acts as an antigen, therefore many different B cells are cloned to produce specific antibody (monoclonal antibodies)

Each clone develops into two types of cells

  • Plasma cells - secrete antibodies into blood plasma, only last few days, but make 2000 antibodies every second, lead to destruction of antigen, is primary immune response
  • Memory cells - secondary immune response, live longer, don't produce antibodies but circulate in blood and tissue fluid. When encounter same antigen divide rapidly and develop into plamsa cells (to produce antibodies and destroy pathogens ) and memory cells which circulate again and help with long-term immunity, increase amount of antibodies, release them faster and quicker response
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Role of B Cells in immunity

  • The surface antigen of an invading pathogen are taken up by a B cells
  • The B cell processes the antigens and presents them on its surface
  • Helper T cell attach to the processed antigens on the B cells and acitate the B cells
  • B cells now activated divide by mitosis to give a clone of plasm cell
  • Cloned plasma cells produce and secrete the specific antibody that fits the antigen on the pathogen's surface
  • The antibody attaches to antigens on the pathogen and destroys them
  • Some B cells develop into memory cells. These can respond to future infections by the same pathogen by dividing rapidly and develop into plasma cells that produce antibodies which is a secondary immune response
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Proteins with specific binding sites synthesised by B cells. Ehen body infected by non-self material a B cell produces a speficic antibody. This specific antibody reacts with an antigen on the surface of the non-self material by binding to them. Each antibody has two identical binding sites which are complementary to specific antigen. The variety is because they are made of proteins.

Antibodies made of four polypeptide chains. One pair are long and called heavy chains while shorter chains called light chains.

Each antibody has specific site that fits onto specific antigen form a antigen-antigen complex, binding site called variable reigion.

Each consists of a sequence of amino acids that from 3D shape that binds directly to specific antigen. Rest of anitbody called constant region which binds to receptors on cells such as B cells

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How the antibody leads to destruction of antigen

Different antibodies lead to the destruction of an antigen in different ways

When antigen is a bacterial cells antibodies assist in its destruction is two ways

  • Cause agglutination of bacterial cells, forming clumps of bacterial cells making it easier of phagoctes to locate them as they are less spread-out within the body
  • Are markers to stimulate phagocytes to engulf the bacterial cells
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Medication to cells to specific drug

Monoclonal antibodies can be used to target specific substances and specific cells. One type of cells thay can target is cancer cells. Monoclonal antibodies can be used to treat cancer in a number of ways by direct monoclonal antibody therapy.

  • Monoclonal anibodies are produced that are specific to antigens on cancer cells
  • These anitbodies are given to a patient and attach themselves to the receptors on the cancer cells
  • They attach to the surface on the cancer cells and block the chemical signals that stimulate their uncontrolled growth

Advantage of direct monoclonal antibody therapy is that since the antibodies are not toxic and are highly specific they lead to fewer side effects

Indirect monoclonal antibody therapy involved attaching a radioactive or cytotoxic drug that kills the cell to the monoclonal antibody. When the antibody attaches to the cancer cell it kills them

Monoclonal antibodies (magic bullets) are used in small doses and target specific sites. Smaller doses as cheaper and reduce side effects

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Medical Diagnosis and pregnancy tests

Medical Diagnosis

  • Monoclonals antibodies are used to diagnose infections and produce results quicker and are important in diagnosing certain cancers. For example men with prostate cancer oftern produce more prostate specific antigen (PSA) leading to high levels in the blood.
  • Monoclonal antibodies that interact with the antigen and you an then obtain a measure of PSA in the blood sample. Higher than normals PSA is not a diagnosis of the disease but give an early warning for possible needs for further teast

Pregnancy Testing

  • Important to know as soon as possible. Pregnancy kits that can be used at home rely on fact that placenta produces a homorne called human chorionic gonadatrophin (hCG) and found it mothers urine
  • Monoclonal antibodies present on the test ***** on pregnancy test are linked to coloured particels. If hCG present in the urine it binds to these antibodies and hCG-antibody-colour complex forms and moves along the ***** until is trapped by different type of antibody creating a coloured line
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Ethical monoclonal antibodies

Development of monoclonal antibodies provides society with power and opportunity to treat disease in different ways but it has raised some ethical issues

  • Production of monoclonal antibodies involved mice to produce antibodies and tumour cells. Production of tumour cells involved deliberately introducing cancer to mice. Guidelines have been drawn up to minimise any suffering
  • Monoclonal antibodies been used successfully to treat diseases like cancer and diabetes saving lives. But in treatment of multiple sclerosis there have been some deaths. It is therefore important for patients to have full knowledge of risks and benefits and must give informed constent
  • In March 2006 six healthy volunteer took part in trial of new monoclonal antibody but within minutes they suffered multiple organ failure probably as a result of T cells overproducing chemicals to stimulate immune response or attacking body tissue. They all survived but it raised issues about way drug trials are conducted

Must combine issues raised, scientific knowledge to make a decision about use. Must balance advantage and dangers to make an informed decision at individual, local, national and global level about ethical use of drugs

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mmunity is the ability of an organism to resist infections and takes two forms

Passive Immunity

  • Produced by intoduction of antibodies into individuals from outside source. No direct contract with pathogen or antigen. Antibodies not produced and replaces, no memory cells formed no lasting immuntiy. E.g anti-venom for snake bitres

Active Immunity

  • Produces by stimulating production of antibodies by own immune system. Direct contact with pathogen or antigen, immuntiy takes time and long lasting and has two types:
  • Natural active immunity - results from individual becoming infected with a disease under normal circumstances, body produces own antibodies and continues do do for many years
  • Artificial active immunity - Forms basis of vaccination and involved introducing immune response without them suffering symptoms of the disease.

Material introducted by the vaccination is called a vaccine

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successful vaccination program

Vaccination is used as a precautionary measure to prevent individuals contracting a disease. Some programmes of vaccination have success other don't.

Success depends of number of factors:

  • A suitable vaccine must be economically available in sufficient quantities to immunise most of the vulnerable population
  • There must be few side-effects from vaccination. Unpleasant side-effects may discourage individuals in the population from being vaccinated
  • Means of producting, storing and transporting the vaccine must be available. This usually involved technologically advanced equipment, hygienic conditions and refrigerated transport
  • There must be the means of administering the vaccine properly at the appropriate time, which involves training staff with appropriate skills at different centres throughout the population
  • Must be possible to vaccinate the vast majority of the vulnerable population to produce herd immunity
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Herd Immunity

When a large proportion of the population has been baccinated to make it difficult for a pathogen to spread within that population. Based on idea that pathogens are passed from induvidual to individual when in close contact. So in majority are immune then less likely come in contact with an infected person

It is important because it is never possible to vaccinate everyone in a large population

Percentage of population to achieve herd immunity is different for each disease. So to achieve herd immunity vaccination best carried out at one time, so for a certain time there are few people in the population with the disease and transmission of pathogen is interrupted

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Why vaccines may not eliminate disease

  • Fails to induce immunity to certain people E.g people with defective immune systems
  • People may develop the disease immediately after vaccination but before immunity levels are high enough to prevent it
  • Pathogen may mutate so antigens change suddenly therefore vaccines become ineffective because new antigen on pathogen not recognised by immune system and immune system doen't produce the antibodies to destroy the pathogen. (antigenic variability)
  • Many varieties of a particular pathogen and it is impossible to develope a vaccine that is effective against all of them
  • Certain pathogen hide from the body's immune system either by concealing themself inside cells or by living in places out of reach (inside intestines)
  • People may have objections to vaccination for religious ethical or medical reasons
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Structure of HIV

HIV causes the disease aquired immune deficiency syndrome (AIDS)

  • On the outside is a lipid envelipe embedded with attachment proteins.
  • Inside the envelop there is a protein layer called capsid that encolses 2 single strands of RNA and some enzyme. One of the enzymes is reverse transcriptase (catalyses production of DNA from RNA) reverse reaction carried out by transcriptase
  • By having reverse transcriptase and ability to make DNA from RNA means HIV is a virus called retrovirus
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Replication of HIV

  • Infection of HIV enters the bloodsream and circulate around the body
  • Protein of HIV binds with protein called CD4 which occurs of different cells but HIV frequently attached to helper T cells
  • Protein capsid fuses with cell-surface membrane. The RNA and enzyme of HIV enters the helper T cell
  • HIV reverse transcriptase converts the virus's RNA into DNA
  • Newly made DNA moves into the helper T cell's nucleus and inserts its DNA
  • HIV DNA creates mRNA using cell's enzymes. The mRNA contains instructions for making new viral proteins and the RNA to go into new HIV
  • mRNA passes out of the nucleus through a nuclear pore and uses cell's protein synthesis mechanism to make HIV particles
  • HIV particles brealk away fom helper T cells taking piece of cell-surface membrane which forms lipid envelope

Once infected with HIV person is HIV +. However replication of HIV often goes into dormancy and only leads to AIDS years later

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How HIV causes AIDS symptoms

HIV attacks helper T cells. HIV causes AIDS by killing or interfering with the normal functioning of helper T cells

An unifected person normally has between 800 and1200 helper T cells in each mm3 of blood.

A person suffereing from AIDS has 200 per mm3 of blood.

Without a sufficient number of helper T cells the immune system cannot stimulate B cells to produce antibodies or cytotoxic T cells that kill cells infected by pathogens

Memory cells may also become infected and destroyed therefore the body is unable to produce an immune response and becomes susceptible to other infections and cancers

Many AIDS sufferers develop infections of lung, intestine, brain, eyes as well as weight loss, diarrhoea and can cause death

HIV doesn't kill directly but it can cause ill health and eventually death

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ELISA test

Enzyme linked immunosorbant assay. It uses antibodies to detect presence of proteins and quantity. Very sensitive and can detect small amount of molecules


  • Apply sample of a surface (slide) which antigens will attach
  • Wash the surfacee several times to remove unattached antigens
  • Add the antibody that is specific to antigen you are trying to detect and leave to bind together
  • Wash the surface to remove excess antibodies
  • Add seconds antibody that will bind to first antibody, which has enzyme attached to it
  • Add colourless substrate of the enzyme which acts on the substrate to change it into a colour product
  • The amount of antigen present is relative to intensity of colour
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Why antibiotics are ineffective against viruses

Antibiotics prevent bacteria from making normal cell walls

In bacteria water constantly enters by osmosis, this would normally cause the cell to burst but due to cell wall it doesn't. The wall is made of murein (peptidoglyan) which doesn't easily stretch. As water enters the cell it expands and pushes aganist the cells wall. The cell wall resists expansion and further entry of water

Antibiotics like penicillin inhibit certain enzymes required for synthesis and assembly of peptide croos-linkages in cell walls. This weakens the walls making them unable to withstand pressure. As water enters naturally by osmosis the cell bursts and bacterium dies.

Viruses rely on host cell to carry out their metabolic activity and therefore lack their own metabolic pathways and cell structure. As a result antibiotics are ineffective becuase there is nothing for them to disrupt

Viruses also have a protein coat rather than a murein cell wall, so doesn't hae sites where antibodies can work. When viruses are within an organism's own cells antibiotics cannot reach them

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Surface Area to Volume ratio

Exchange takes place at surface of an organism, but materials absorbed are used by the cells that mostly make up its volume. For exchange to be effective, the exchange surface of the orrganism must be large compared with its volum

As organisms become larger, their volume increases at a faster rate then their SA. Because of this diffusion of substances across the outer surface can only meet the needs of relatively inactive organisms. Even if the outer surface could supply enough of a substance it would take too long for it to reach the middle of the organism if diffusion was method of transport

Organism have evolved

  • Flattened shape so that no cell is ever far from the surface
  • Specialised exchange surfaces with large areas to increase the SA:Vol
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Features of specialised exchange surfaces

For effective transfer of materials across specialised exchange surfaces by diffusion or active transport, surfaces show following characteristics

  • Large suface area relative to the volume of the organism which increase the rate of exchange
  • Very thin so that the diffusion distance is short and therefore materials cross the exchange surface rapidly
  • Selectively permeable to allow selected material to cross
  • Movement of the environmental medium E.g air to maintain diffusion gradient
  • A transport system to ensure the movement of interal medium E.g blood in order to maintain a diffusion gradient

Being thin, specialised exchange surfaces are easily damaged and dehydrateed. They are therefore often located inside an organism.  Where they are located inside the body, the organism need to have a means of moving the external medim over the surface E.g Means of ventilating the lungs in a mammal

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Gas Exchange in single-cell organism

They are small and therefore have a large surface area to volume ratio.

Oxygen is absorbed by diffusion across their body surface, which is covered by a cell-surface membrane.

Carbon dioxide from respiration diffuses out across their body surface

Where a living cell is surrounded by a cell wall this isn't an additional barrier to the diffusion of gases

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Insect Gas Exchange

Insects have evolved to conserve water. Increase in SA doesn't help to conserve water because it will evaporate from it. For gas exchange insects have enolved an internal network of tracheae which are supported by ring to stop them collapsing. They are divided into small deadend tubes called tracheoles which exten throughout the body tissue. Oxygen brough directly to respiring tissue and short pathway from tracheole to body cells

How gases move in and out of tracheal system

  • Along diffusion pathways - as oxygen is used up there is low concentration at end og tracheoles and there is a carbon dioxide in the opposite direction as it is produces as oxygen is used up
  • Mass transport - as muscles contrat they squeeze trachea enabling movement of air in and out 
  • Ends of tracheoles are filled with water - muscles respire by anaerobic respirateion which produces lactate which is soluble and lowers water potential. Water moves into the cells from the tracheoles by osmosis. Water in ends of tracheoles decreases in volume as air drawn up 

Gases enter and leave by tiny pores called spiracles on body surface that have valves

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Located in fish behind the head made up of gill filaments which are stacked in pilles like book pages. At right angles to filaments are gill lamellae, they increase SA. Water taken in through the mouth and forced over the gills and out through opening on each side of the body. Water flows over the gill lamellae and there is a flow of blood in the opposite direction this is called  (countercurrent flow). This ensures maximum gas exchange achieved

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Counter current exchange principle

Blood and water that flow over gill lamellae are in opposite directions. This means that

  • Blood is already well loaded with oxygen meets water, which has its maximum concentration of oxygen. Therefore diffusion of oxygen from water to blood takes plae
  • Blood with little oxygen in it meets water which has most of oxygen removed

As a result diffusion gradient maintained across gill lamellae. 80% of oxygen available in water is absorbed into the blood of the fish. If in same direction gradient only maintained across part of length so only 50% of oxygen absorbed into the blood

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Structure of plant leaf and gas exchange

Gas exchange in pplant similar to insects because:

  • No living cell is far from external air and therefore source of oxygen and carbon dioxide
  • Diffusion takes place in of air which makes it more rapid than if it were in water

Short, fast diffusion pathway. Also air spacess inside a leaf have larger SA compared to volume. There is no specific transport system for gases to move in a through the plant by diffusion. Most gas exchange occurs in the leaves as it is adapted for rapid diffusion

  • Many small pores (stomata) and no cell is far from a stoma and is short diffusion pathway
  • Lots of interconnecting air-spaces throughout mesophyll so gases can readily come in contact with mesophyll cells
  • Large SA of mesophyll cells for rapid diffusion
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Minute pores that occur mainly on the leaves especially the underside

Each stoma (singular) is surrounded by a pair of special cells (guard cells) which can open and close the stomatal pore, to control the rate of gas exchange

This is important because organisms lose water by evaportation. Plants have evolved to balance gas exchange and control of water loss. This is done by closing stomata when water loss is high

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Limiting water loss in insects

Insects have adapted to reduce water loss

  • Small SA:Vol ratio to minimise area which water is lost
  • Waterproof covering over their body to create a rigid outer skeleton of chitin covered with a waterproof cuticle
  • Spiracles which are openings of the tracheae at the body surface and these can be closed to reduce water loss. The need for oxygen occurs when the insect is at rest

These mean that insects cannot use their body surface to diffuse respiratory gases in the way a single0celled organism does. Instead they have an internal network of tubes called tracheae that carry air containing oxygen directly to the tissues

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Limiting water loss in plants

Certain plants have a restricted supply of water and have evolved to limit water loss through transpiration these are called xerophyted. They have modified their leaves

  • Thick cuticles - waxy cuticle on leaves form waterproof barrier. 10% water loss from leaves, thicker the cuticle the less water can escape
  • Rolling up of leaves - protects the lower epidermis from the outside trapps a region of air within the rolled leaf. This region becomes saturated with water vapour and has a high water potential. There is no water potential gradient between the inside and outside of the leaf so no water loss E.g Marram grass
  • Hairy Leaves - thick layer of hairs on leaves this trapps moist air next to the leaf surface, water potential gradient is reduced and less water is lost by evaportation
  • Stomata in pits or grooves - traps moist air next to the leaf and reduces water potential gradient
  • Reduce SA:Vol - smaller it is the slower the rate of diffusion and the rate of water loss can be reduced
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Are site of gas exchange in mammals and are inside the body because air is not dense enough to support and protect them also the body would lose water and dry out

They are supported by the ribcage. The ribs can be moved by the muscles between them, they are ventilated by tidal stream of air ensuring that the air within them is constantly replenished. The main parts of gas exchange system are:

  • Lungs - pair of lobed structures made up of highly branched tubles called bronchioles which end in tiny air sacs called alveoli
  • Trachea - flexible airway that is supported by rings of cartilage to prevent from collapsing when pressure falls. Walls are mafe up of muscle lined with ciliated epithelium and goblet cells
  • Bronchi - division of trachea, one for each lung and produce mucus to trap dirt particles and have cilia that move the mucus and dirt towards the throat
  • Bronchioles - brancing of bronchi, have muscles lined with epithelial cells, muscle allows them to constrict to control flow of air in and out of alveoli
  • Alveoli - air sacs diameter of 100-300μm, there is collagen and elastic fibres between them which allow them to stretch as they fill with air and spring back as expel CO2
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Breathing in is an active process and occurs by:

  • External intercostal muscles contract, while the internal intercostal muscles relax
  • The ribs are pulled upwards and outwards, increasing the volume of the thorax
  • The diaphragm muscle contracts, causing it to flattern, which increases the volumn of the thorax
  • The increased volume of the thorax results in reduction of pressure in the lungs
  • Atmospheric pressure is now greater then pulmonary pressure so air is forced into the lungs
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Breathing out is a passive process and occurs by:

  • The internal intercostal muscles contract, while the external intercostal muscles relax
  • The ribs move downwards and inwards, decreasing the volume of the thorax
  • The diaphragm muscles relax and it pushes up the contents of the abdomen that were compressed during inspiration. The volume of the thorax further decreases
  • The decreased volume of the thorax increases the pressure in the lungs
  • The pulmonary pressure is now greater than the atomspheric pressure so air is forced out of the lungs

During normal breathing the recoil of the elastic tissue in the lungs is the main cause of air being forced out. Only under more strenuous conditions such as exercise does the muscle play a major part

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Role of alveoli in gas exchange

300 million alveloi in each human lung there total surface is around 70m2 (half area of tennis court). Each alveolus is lined with epithelial cells 0.05μm – 0.3 μm thick. Around wach alveolus is a network of pulmonary capillaries that are 0.04μm – 0.2 μm thick

There is fast diffusion between alveoli and the blood because:

  • RBC are slowed as they pass through the pulmonary capillaries allowing more time for diffusion
  • Distance between the alveolar air and RBC is reduced as RBC are flattened against the capillary wall
  • Walls of alveoli and capillaries are very thin and is therefore short diffusion distance
  • Alveoli and pulmonary capillaries have larfe SA
  • Breathing movements constantly ventilate the lungs and the action of the hear constantly circulates blood around the alveoli. Together these ensure steep concentration gradient of exchange gases
  • Blood flow through the pulmonary capillaries maintains a concentration gradient
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Digestive system

  • Oesophagus - carries food from mouth to stomach
  • Stomach - muscular sac, inner layer produces enzymes, it stores and digests food especially protein (glands produces enzyme to digest protein)
  • ileum (small intestines) - long muscular tube, it further digests food by enzymes produced in its walls and by glands. Inner wall folded into villi and microvilli to increase SA helps with absorption of products of digestion into the blood
  • Large intestine - absorbs water, most of water from secretions of digestive glands
  • Rectum - final section, faeces are stored here before being removed via anus by egestion
  • Salivary glands - situated near the mouth, pass their secretions via a duct into the mouth. The secretions contain enzyme amylase which hydrolyses starch into maltose
  • Pancreas - large gland situated below the stomach, produces secretions called pancreatic juice which contain proteases to hydrolyses proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch
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Physical Breakdown

Large food is broken down into smaller pieces (e.g by teeth), provides a large SA for chemical digestion. Also another example is food churned by muscles in the stomach

Chemical Breakdown

Hydrolyses large, insoluble molecules into smaller soluble ones, carried out by enzymes. They split up molecules by adding water to the chemical bonds. Enzymes are specific so more than one enzyme need to hydrolyse large molecules. One to hydrolyse into large molecules into secretions which are hydrolyses into smaller molecules

  • Carbohydrases - hydrolyse carbohydrates to monosaccharides
  • Lipases - hydrolyse lipids (fats and oils) into glycerol and fatty acids
  • Proteases - hydrolyse proteins to amino acids
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Carbohydrate digestion

  • More that one enzymes used to hydrolyse large molecule (one hydrolyses the molecule into smaller sections, another hydrolyses them futher into their monomers)
  • Enzymes produced in different parts of digestion as important that enzymes added to the food in correct sequence
  • First amylase (produced in mouth + pancrease), it hydrolyses alternate glycosidic bonds of the starch molecule to produce a disaccharide (maltose)
  • Maltose is hydrolyse into monosaccharide (alpha glucose) by maltase (produced in lining of ileum)
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Carbohydrate digestion in humans

  • Saliva enters mouth from salivary glands and mixed with food during chewing
  • Saliva contains salivary amylase which starts hydrolysing any start in the food into maltose. It also contain mineral salts to help maintain neutral pH as optimum for the amylase
  • Food swallowed and enters stomach, acidic conditions which denatures amylase and prevents hydrolysis of starch
  • Then passes into small intestine and mixed with pancreatic juice which contains amylase which allows hydrolysis of starch to continue (alkaine salts also produced to maintain neutral pH)
  • Muscles in intestine wall push food along the ileum
  • Epithelial lining of the ileum produced maltase and is a membrane-bound disaccharidase so is not released into the lumen

Other membrane-bound disaccharides:

  • Sucrase - hydrolyses the single glycosidic bond in surose to produce its monosaccharides glucose and fructose
  • Lactase - hydrolyses the single glycosidic bond in lactose to produce its monosaccharides glucose and galactose
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Lipid digestion

  • lipids hydrolyses by lipases which are enzymes produced in the pancreas that hydrolyse the ester bond found in triglycerides to form fatty acids and monoglycerides
  • monoglyceride - is a glycerol molecule with single fatty acid molecule


  • lipids (fats and oils) split up into tiny droplets called micelles by bile salts (produced in liver) by emulsification which increases surface area fo the lipid speeding up action of lipases
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Protein digestion

  • Proteins are large, complex molecules that are hydrolyses by peptidases (proteases)

There are different types of peptidases

  • Endopeptidases - hydrolyse peptide bonds between amino acids in central region of a protein molecule forming a series of peptide molecules
  • Exopeptidases - hydrolyse peptide bonds on the terminal amino acids of peptide molecule formed by endopeptidases (they release dipeptides and single amino acids)
  • Dipeptidases - hydrolyse the bond between the 2 amino acids of a dipeptide. They are membrane-bound and part of cell-surface membrane of the epithelial cells lining the ileum
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structure of the ileum

  • Wall of ileum is folded and has finger-like projection about 1mm long called villi which are have thin walls with rich network of capillaries and have large surface area to increase rate of absorption
  • Villi are between the lumen of the intestines and the blood and other tissues and part of an exchange surface adapted to absorb produces of digestion

They increase efficiency of absorption by:

  • increasing the surface area for diffusion
  • very thin walls so reduced diffusion distance
  • contain muscle so can move which helps to maintain concentration gradient (because as they move it mixes contents of ileum)
  • well supplies with blood vessels so that blood can carry away absorbed molecules which also maintain diffusion gradient
  • villi have microvilli which further increase the surface area for absorption
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Absorption of amino acids and monosaccharides

  • Digestion of proteins produces amino acids
  • Digestion of carbs produce monosaccharides such as glucose, fructose and galactose etc.
  • Absorption of these products happens by diffusion and co-transport
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Absorption of triglycerides

  • Monoglycerides and fatty acids combine with bile salts to emulsify the lipid droplets forming micelles
  • Micelles are tiny around 4-7nm in diameter
  • Micelles will eventually come into contact with the epithelial lining and will break down releasing monoglycerides and fatty acids (as non-polar they diffuse across membrane into the epithelial cells)
  • Inside the epithelial cell monoglycerides and fatty acids are transported to the endoplasmic reticulum and are combined to form triglycerides
  • They continue to the Golgi apparatuse and combine with cholesterol and lipoproteins to form chylomicrons 
  • Chylomicrons are able to move out of the cell by exocytosis
  • They then enter the lymphatic capillaries called lacterals which are found at the centre of each villus (or villi)
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