EXAMINATION REVISION FOR UNIT 3 BIOLOGY
ASSESSMENT AND EXAMINATION ADVICE
Your level of success in the mid year exam is not determined by the number of facts you can
remember. The exam will assume that you recall facts, definitions and explain concepts and
principles and apply your understanding of concepts and processes to unfamiliar situations.
Take notes during class most importantly revision classes. This assists in memorising key
concepts. There is no point memorising a definition if you do not understand it. Avoid memorising
word for word. Ensure that you understand what you are learning, otherwise there is no point
learning it, as you will not know when to apply the information. At the start of each session revise
what you studied in the last session for that subject.
Practising answers to exam questions from previous exam papers and sample exam papers is a
necessity. You will get a feel for the types of questions that are asked and be exposed to a wide
variety of different sorts of questions.
THE MID YEAR EXAMINATION
The mid year examination is one and a half hours, (plus fifteen minutes reading time), in duration
and contributes to 33% of your final mark.
Use your reading time. You will have fifteen minutes to read the paper before writing time begins.
Read the paper thoroughly, including the front cover. If you finish reading the paper, start
answering the questions in your head, or find a diagram or graph and ensure you understand what
it shows.
You will have one hour and thirty minutes writing time for the mid year exam.
SECTION A: Multiple Choice questions
There will be a number of multiple choice questions on the mid year exam. You are required to
choose the correct response to a question when posed with (usually four) alternatives.
- Read the question carefully and, if possible, decide on your response BEFORE you look at
the alternatives available. Read ALL of the alternatives and see if your answer is there. If
your answer is there, check the alternatives you didn’t choose and ensure they are not
correct. Then mark in your response.
- If your response is not there, it does not mean the examiner has made a mistake. Re-read
the question and then the answers and work through each possibility in order to eliminate
the incorrect answer.
- NEVER leave a multiple choice answer space blank. If you are extremely unsure as to the
answer then leave it, answer the rest of the paper and then come back to it. Do not spend
too much time on any one question.
SECTION B: Short answer questions
- Use your writing time carefully. Don’t spend too long on one question. If you are having
problems then leave that question and come back to it later.
- The number of lines that you are given to write on is an indication, (but only as an
indication) of how much you are expected to write. Completing past exams will also assist
you in this area. Writing more than you need will result in you running out of time to
complete the exam paper.
- Check how many marks the question is worth. If you have written one word for a three
mark question, then you will not have received three marks.
- Read questions carefully, very carefully, and only provide information relevant to the
question you have been asked.
- Underline the key words in the question and use them in your answer.
- Write neatly. The marker must be able to read your answers.
- Think about what you are going to write before you write it.
- Re read your answer in order to ensure it makes sense and answers the question.
- Make sure you spell biological words correctly.
- If a question asks for a one word answer then only that word is required.
- Your explanations should be detailed and likely, rather than just possibilities.
- Use the information provided. If you are describing a graph, then refer, if appropriate, to
numbers given with the graph.
- Know the difference between ‘explain’ and ‘describe’. If a question asks you to explain, it is
asking you to address why something is occurring. If a question is asking you to describe
something, it is asking you to write what is happening but not why.
- If you are asked to make comparisons, you should refer to both ‘objects’ you are
comparing.
EXAM TIPS

When responding to sort answer questions in an exam, avoid the use of pronouns,
such as ‘it’ and ‘they’ at the start of your answer. If the examiner is not sure what you
are referring to, you are unlikely to gain any marks. Always start your answer with an
appropriate noun. Consider the question – If the active site of an enzyme was
denatured, what effect would this have on the enzyme’s activity? Explain (2 marks).
Note that there are two marks allocated to the question. This implies two points need
to be made in the answer, with each point gaining one mark. A good answer might
read: The active site of the enzyme would become chemically altered (1 mark). This
would result in the enzyme not being able to bind to its substrate, thereby reducing the
activity of the enzyme (1 mark).
 If asked in an exam question to describe the general trend shown by a graph, write
you answer in terms of what the X and Y axes represent.
 Often an exam question asks a student to draw a conclusion on the results of an
experiment. In such a case, re-read the question carefully and find out what the AIM of
the experiment was. In most cases the correct answer can be written by looking ar the
results of the experiment in terms of the original aim of the experiment.
SUMMARY NOTES FROM CHAPTERS
THESE NOTES ARE A GUIDE ONLY!!! It is important that you have a complete understanding of
the content of each chapter, particularly key points and questions. You should make your own
notes and make sure you tie the key points of the chapters into the Unit 3 outcomes.
CHAPTER 1
PROTEINS
Proteins are made up of different amino acids (20) that are joined together by peptide bonds, and
then folded into three dimensional structures. Enzymes and Antibodies are proteins.
Globular proteins are usually soluble in water. Examples of globular proteins are enzymes and
antibodies as well as haemoglobin (carrier protein), casein and albumin (storage proteins) and
insulin (hormone).
Fibrous proteins are a major group of proteins that are usually insoluble in water. Examples include
actin and myosin in muscles and fibrin, which is involved in blood clotting.
Many proteins when heated above 50 degrees cannot perform their normal biological functioning
because their secondary and tertiary structures are damaged.
Proteins have four distinct aspects to their structures: primary (long chains of amino acids), a coiled
helix (alpha helix) or a pleated sheat (beta sheat) is the secondary structure that are linked by
hydrogen bonds, tertiary structure are 3 dimensional shapes formed by coiled or pleated
polypeptides. Quaternary structure refers to the structural relationship between the polypeptides
that make up a particular protein.
LIPIDS
Lipids are not soluble in water and can be divided into two main groups. Complex lipids which
contain fatty acids, and include fats, oils, waxes and phospholipids and simple lipids, which do not
contain fatty acids, such as steroids.
Phospholipids form a component of the cell membrane, where the lipid bi-layer is arranged so that
the hydrophilic (polar and water loving) parts are on the outer surface of the membrane, while the
hydrophobic (non-polar and water hating) part of the lipid faces inwards. Smooth endoplasmic
reticulum is the site where phospholipids and fatty acids are made.
POLYSACCHARIDES
Large complex carbohydrates. Polysaccharides are joined together glycosidic linkages and form a
very large and branched polymer molecules, which are insoluble in water. Examples of
polysaccharides include starch (plants) and glycogen (animals).
NUCLEIC ACIDS
In eukaryotic cells the nucleus contains DNA, a double helix structure which controls the cell’s
functioning and is responsible for cell division (mitosis). The DNA found in the nucleus is a nucleic
acid built of nucleotides. Each nucleotide consists of a sugar molecule to which is bonded a
phosphate group and a specific base: Adenine (A), Guanine (G), Cytosine (C) and Thymine (T).
Adenine will only pair with Thymine, and Guanine will only pair with Cytosine.
Small amounts of DNA are found in chloroplasts and mitochondria and it enables these organelles
to control their own division.
The control the nucleus has over the cell’s functioning is achieved by certain sections of DNA being
copied into m-RNA. Messenger RNA is a single stranded nucleic acid with the base thymine
founding DNA replaced with the base uracil. Therefore if the instruction on one strand is the DNA
molecule is AATCGGTTC the RNA molecule will form UUAGCCAAG. The RNA molecule which
has formed is moved out of the nucleus into the cytoplasm where the instruction from the DNA is
carried out. Decoding of the message occurs in the cytoplasm in the ribosomes.
PROTEOMES AND PROTEOMICS
Proteome – Protein and Genome
A proteome is the total amount of all proteins found in the particular structure of an organism such
as cells. Example, all the proteins found in a muscle cell will be regarded as a cellular proteome.
Similarly for liver cells or any other types of cells founding an organism. However, the proteome in
a muscle cell will be different from a proteome in a liver cell, because muscle cells do not have the
same types of proteins that liver cells do and visa versa.
Each organism has a complete proteome. The complete proteome of an organism refers to the
complete set of proteins that are found in all the different cells that make up an individual organism.
Obviously different species will have different proteomes; however individual members of the same
species will also have different proteomes. The types of proteins differ.
By determining the structure of proteins and especially proteins that are involved in causing
diseases, scientists can design compounds and drugs to block these proteins stopping them from
functioning and therefore being able to treat the disease.
CHAPTER 2
Structure and Function within cells
Cells are the basic units of structure and function in all living organisms, and all new cells come
from pre existing cells.
In unicellular organisms one cell carries out all the functions necessary to survive, whereas in
multicellular organisms, cells are specialised in order to carry our particular functions; for example
xylem vessel cells are dead due to lignin stopping substances entering or leaving the cell.
Because of the size of the cells, you need a microscope to see them. The light microscope allows
us to view functioning living cells, as well as larger structures such as the nucleus and chloroplast.
However cells cannot be seen in a living state if stained or under an electron microscope.
Prokaryotic cells, for example bacteria, do not have a distinct nucleus and the nuclear material is
found spread throughout the cell. Prokaryotic cells are much smaller than eukaryotic cells and
prokaryotic cells do not have membrane bound organelles. In Eukaryotic cells, the membrane
bound organelles create separate environments within the cell so that specialised functions can
tack place efficiently within the organelles, for example the light reaction in photosynthesis takes
place on membranes called grana. They are covered with the pigment chlorophyll which absorbs
light energy.
Cell Specialisation
Eukaryotic cells vary a great deal in structure. Variations in structure are related to function. Cells
vary in the number of each organelle present and in their shape and life cycle.
FUNCTION
STRUCTURE
EXAMPLE
Absorption of
Materials
Large surface area created by folds of
membrane known as microvilli. Large
Number of mitochondria to provide
energy for active transport.
A cell lining the
intestine
Photosynthesis
Large number of chloroplasts, large
Vacuole for water storage.
A leaf cell
Communication
Elongated shape, large number of
Mitochondria for energy production.
A motor neuron
Transport
Cells of the phloem vessels in plants
Have thickened walls, elongated shape
and porous ends.
A sieve tube cell
Need to know the difference between prokaryotic and Eukaryotic cells.
Be able to draw the cells.
CELL ENVIRONMENTS AND CELL SIZE.
- exchange of materials with their environments.
- Surface area to volume ratio.
NAME, FUNCTION AND LOCATION OF CELL ORGANELLES.
- try and locate in a table outlining the Name and Location of the organelles, the description
and diagram, function of the organelle.
APOPTOSIS: is self-destruction by cells for the good of the whole organism. List three possible
death signals a cell might receive to initiate apoptosis.
Chapter 3
ENZYMES
A catalyst is any chemical which speeds up the rate of a chemical reaction withur itself being used
in the reaction. Biological catalysts are enzymes.
-Describe enzymes.
- What is the active site?
- What are enzymes sensitive to?
- Enzyme inhibition, include competitive inhibition and non-competitive inhibition.
- What is the effect of poisons on enzymes?
- What is a Co-enzyme.
CELL MEMBRANES
- All cells are surrounded by a cellular membrane. Cell membranes are selectively
permeable, allowing some substances to pass between the internal and external
environments, but not others.
- Describe the structure of the cell membrane.
- The phospholipid and protein components of the membrane allow the passage of different
molecules. Carbohydrate chains are involved in cell recognition.
-
Diffusion: movement of gases, liquids or solutes from a region of high concentration to low
concentration.
-
Osmosis: is the net movement if water across a differently permeable membrane, from an
area in which water is in high concentration to an area where water is in low concentration.
Isotonic solution: solute concentration the same as intracellular fluid.
Hypotonic solution: a lower concentration of solutes than the cell.
Hypertonic solution: a higher solute concentration than the cell.
-
Movement across membranes:
- Simple diffusion: High solute concentration to low solute concentration. These include
oxygen, water and urea. No energy is used.
- Facilitated diffusion: transport of substances such as glucose, amino acids through protein
channels provided by embedded protein molecules. High to low. NO energy used.
- Active transport: movement of molecules such as potassium across the membrane from
low concentration to high concentration through protein channels. Requires energy ATP.
- Endocytosis: (Pinocytosis is fluid movement, and Phagocytosis large particles and debris).
Parts of the membrane fold around the material. Requires energy.
- Exocytosis: cells secrete cell products and eliminate wastes. Vesicles leave by fusing with
the cell fuse with the cell membrane.
ENERGY IN CELLS
- Energy releasing reaction (exergonic) is oxidation. Oxidation involve the addition of oxygen
to a substance (removal of electrons). Also known as CATABOLIC REACTIONS.
- Reactions that involve the removal of oxygen from organic compounds are known as
endergonic reactions (energy requiring). Also known as ANABOLIC REACTIONS.
PHOTOSYNTHESIS
- Light Dependant phase – Water is split into Hydrogen ions and Oxygen (gas). Hydrogens
transported by carriers (taxis) NADP+. Occurs in the Grana of the Chloroplast.
- Light Independent (does not require light). Uses Carbon Dioxide to produce Glucose in the
Calvin Cycle. Gives off Water vapour. Occurs in the Stroma of the Chloroplast.
RESPIRATION
- Energy transfer from glucose to ATP.
- Glucose is broken down in the cytoplasm to two molecules of Pyruvate via Glycolysis.
- This is then further broken down in the Krebs Cycle. This occurs in the Mitochondria. 3
molecules of CO2 are formed and five Hydrogen molecules are loaded into four NAD+ and
FAD+ to the Electron Transport Chain.
- Electron Transport Chain uses the loaded acceptor molecules and converts them to ATP.
Occurs on the inner membrane of the mitochondria and uses compounds known as
cytochromes.
- Production of 36ATP (38 in heart, kidney and liver cells).
ANAEROBIC RESPIRATION
- Chemical breakdown of glucose without oxygen. The final product depends on the
enzymes present.
-
The total energy yield for anaerobic respiration is two ATP per glucose. The loaded
acceptor molecules (NADH) that are produced in glycolysis are needed to drive the
conversion of pyruvate to lactate.
Chapter 4
DESIGN OF DRUGS.
- Many drugs used in treatment of diseases are designed to bind with the target site or
receptor of proteins found on the cell membrane. If the drug binds to the target sire of the
receptor, it can block or stimulate the receptor. Such an action can block or stimulate
signal transduction in a cell and potentially bring about a therapeutic (healing) effect in a
patient. Designer drugs may target proteins on the cell memb4rane or they may target
proteins that are found in the cytoplasm of a cell.
- Example flu virus and neuraminidase.
MEDICAL TREATMENT
- Inherited diseases exist because of some defect in the DNA.
- Hypothyroidism: disorder caused by small or improperly functioning thyroid gland.
- Galactosaemia: inherited disorder where the lactose in milk is unable to be digested due to
an absence of galactase.
- Cystic Fibrosis: inherited disorder where the person secretes abnormal amounts of
secretions that have a serious affect on lungs and digestion.
-
What is PKU? How does it occur?
What treatment is given to babies with PKU?
-
Gene Therapy: is a procedure with the potential to correct some genetic defects.
Gene therapy involves inserting a functioning piece of DNA into the cells of an individual
with a genetic defect.
Virus vectors are commonly used to carry DNA into other cells.
Tests exist to distinguish functional from non-functional segments of DNA in an individual.
In these tests the DNA of one individual is often compared with the DNA of other family
members.
-
Understanding the mechanisms of an infective organism or agent, then we maybe able to
prevent it occurring.
Vaccinations contain antigens from disease causing organisms, that stimulate the immune
system to develop antibodies for future protection from the disease.
Some diseases caused by a deficiency of a particular protein can be treated with a
genetically engineered form of protein.
Chapter 5
- All cells in the body have an optimum intracellular and extracellular environment.
- If the body deviates too far form the normal steady state of a variable, death can occur.
- Homeostasis is the condition of a relatively stable internal environment.
- Homeostasis is essential for cell functioning.
- The hormone and nervous system are two systems that coordinate all other systems of the
body.
- Each control system involved in homeostasis has tow interrelated stages:
Stage1: Detecting change from a stable state
Stage 2: Counteracting change.
- This type of control is NEGATIVE FEEDBACK.
- All negative feedback systems involve sensors that monitor the state of a particular
variable and compare that state with optimal levels.
- All negative feedback systems involve effectors that respond to messages from sensors
and act to maintain a variable within its optimal limits.
- Although the majority of hormone systems operate by negative feedback, positive
feedback do exist.
-
A hormone can communicate signals to a cell only if the cell has receptors that enable it to
recognise the hormone.
Hormones can be hydrophilic or lipophilic. This affects the way in which hormones are
transported through the blood and how a signal is transmitted across a cell membrane.
Signal transduction occurs within the cytosol of a cell after a hormone signal has been
detected within a cell.
The detection of a hormone signal within a cell elicits an appropriate response from the
cell.
PHEREMONES are chemical signalling molecules secreted by animals, and are species
specific.
Often associated with reproduction.
Pheremones are used in various ways to reduce pest infestation.
PLANT HORMONES
The main types of plant hormones include:
1. AUXINS: - stimulate cell elongation in growing tips of plants.
- cause inhibition of lateral bud growth in stems but stimulates growth of lateral
roots.
- Causes phototropism and geotropiosm.
2.
-
GIBBERELLINS: - stimulate cell elongation and reproduction in stems.
promote seed germination
Hormone is produced in seeds and young leaves and other cells.
During germination gibberellins cause an enzyme amylase to be produced in the seed.
This converts the stored starch of the seed into simple sugars for respiration.
3. ABSCISIC ACID: - Inhibits growth of roots, induces bud and seed dormancy.
4. CYTOKININS: - promote cell division, if auxin is present, stimulating fruit development and
growth.
- Found in high concentrations in younger parts of a plant such a young leaves, fruit and
root tip.
5. ETHYLENE – Gas involved in ripening fruit. Can stimulate abscission of leaves and fruit.
Chapter 6
The Nervous System.
Composed of two main systems which operate together to work as an integrated unit:
1. CNS – Central Nervous System: The Brain and the Spinal Cord.
2. Peripheral nervous system consists of Voluntary nervous system and Involuntary nervous
system (autonomic).
The Autonomic Nervous System consists of sympathetic and parasympathetic motor neurons.
-
Sensory or afferent neurons – Relays messages from the receptor to CNS.
Connector or interneurone – Relays messages between other neurons; are only found
within the CNS; can link sensory and motor neurones.
Motor and efferent neurone – Relays message away from the CNS to the effector.
Neurones do not make physical contact with each other. Once a nerve impulse reachers
an axon terminal, neurotransmitters (eg acetylcholine) are released to diffuse across a gap
called the synapse. The message then continues via an impulse in the next cell.
Homeostasis generally involves elements of both the hormonal and nervous systems.
Homeostatic controls monitors variables.
Signalling molecules are used during homeostatic control.
Signalling molecules transmit messages from one part of a system to another part.
Receptor sites receive homeostatic signals.
Effectors respond to homeostatic signals.
Chapter 7
What is disease? A disease is any change that impairs the function of an individual in some
way. An infectious disease is one caused by a pathogen.
What is a pathogen? Non-cellular (prions or virus) or Cellular (bacteria
or fungi)
VIRUSES
Small particles made up of either DNA or RNA bur not both and in a
protein coat.
Virus reproduce by taking over a particular host cell. The nucleic acid of
the virus is injected into the cell. This takes over the nucleus and cellular
machinery to produce more virions.
PRIONS
Proteins that act like infectious molecules causing disease. Prions are
now believed to be associated with a number of degenerative diseases
in mammals central nervous system. Mad Cow disease (bovine spongioform encephalopathy)
has destroyed the British Beef industry. When a prion infects a cell with the normal protein the
prion catalyses the normal protein converting it to the prion.
BACTERIA
These organisms can be Gram +ve or Gram –ve prokaryotes.
Bacteria are classified on the basis of their physical and chemical characteristics.
Many bacteria produce toxins that destroy host cells. If given suitable conditions, bacteria
reproduce at a very high rate through binary fision.
Treatment of bacteria is often through antibiotics.
How do antibiotics kill/affect bacteria?
What is the difference between Gram –ve and Gram+ve bacteria.
FUNGI AND YEASTS
Most fungi infections are superficial infections because they infect the skin, nails or hair.
Unicellular or multicellular organisms.
Plant diseases include Dutch Elm disease. Animal Disease include Athlete’s foot.
PROTOZOA
Animal like protists which ingest food.
Ameobas: single celled organisms that ingest food by moving pseudopodium (false foot).
Apicomplexsans: spore-forming, parasitic single celled protozoa.
WORMS
Speicalised, multicellular worms.
Trichinosis an often fatal disease in which worms invade the muscle tissue.
How do pathogens cause disease?
How does it infect and effect the host?
1. Mode of transmission.
Discuss the use of vectors – how the host reaches the host.
2. It must be able to grow and REPRODUCE in the host.
3. It must cause damage to the host.
Chapter 8.
Plant defences: many plant diseases are spread by insects.
Defences occur in two ways:
1. Barriers to entry e.g. structural features such as hairs on surface of leaf, waxy cuticle.
2. Secondary defences e.g. chemical means such as enzymes, antibiotic chemicals, toxins.
Defence in Humans
- Non-Specific Immunity: includes physical barriers, chemical barriers.
- Specific Immunity: cell mediated immunity and humoral immunity. Recognises a
specific pathogen and targets attack to that pathogen.
White blood cells – LEUKOCYTES are:
- manufactured in the bone marrow
- possess a nucleus
- capable of independent movement.
Granular Leukocytes
Neutrophils (main phagocyte of blood). Ingest bacteria.
produce antibodies).
Eosinophils (Produce enzymes which detoxify foreign proteins.)
white blood. Develop into macrophage).
Basophils (Produce and release heparin and histamine.
Agranular Leukocytes
Lymphocytes ( Some
Monocytes (Largest
LINES OF DEFENCE
Animals have several lines of defence to assist the development of immunity. These defences
may be general, such as barriers to infection in the form of skin or scales. If the pathogen
progresses through several lines of defence the body may start to have more specific
responses and develop chemicals to fight infection and thereby provide immunity.
Non Specific defence mechanisms
First line of defence
Skin
Mucous membranes and their
secretions
Second line of defence
Phagocytic white blood cells
Anti-microbial proteins –
Compliment and interferons
The inflammatory response
Specific Defence Mechanisms
(immune system)
Third line of defence
Lymphocytes
Humoral (antibody mediated)
immune response
Cell-mediated immune
response.
Key Points:
 If non-specialised defences fail to prevent infection, specialised responses occur;
 All cells have protein markers on their surface;
 Non-self markers on cells entering a person are called antigens;
 A number of different kinds of cells are involved in specific immunity;
 The phenotype is the physical biochemical or physiological expression of the genotype;
 Some cells produce antibodies that circulate in body fluids and react with specific antigens.
B CELLS
B cells have immunoglobulins on their surfaces. Immunoglobulins are proteins that identify
antigens. Immunoglobulins are also called antibodies. When a B cell identifies an antigen, it
replicates rapidly to produce large numbers of special cells called plasma cells which produce
antibodies and release them into body fluids.
Immunity involving antibodies involving body fluids is called HUMORAL IMMUNITY.
T CELLS
When T cells mature in the thymus, many different kinds of T cells are produced which recognise
different antigens.
T cells DO NOT make antibodies. Immunity involving T cells and phagocytes is called cellular
immunity.
Helper T cells: Phagocytes that have ingested foreign material carry some of the foreign antigen on
their surface as well as their usual class 2 marker proteins. The T helper cells (Th) recognises
these antigens and stimulates B cells. B cells will not reproduce and form plasma cells without Th
cells.
Cytotoxic cells: Tc cells kill body cells that have been infected with a virus. Tc cells also kill cancer
cells.
Notes:
 Specific immunity can be acquired in different ways;
 In actively acquired immunity, the immune system of a person produces antibodies in
response to antigens;
 In passively acquired immunity, a person receives antibodies from an outside source;
 Both active and passive immunity can be acquired naturally or artificially.
Inflammatory Reaction
The inflammatory reaction is a local response to injury.
Damaged tissue releases bradykinin, which causes pain and stimulates mast cells to release
histamine.
Bradykinin and histamine produce vasodilation, ( increased blood vessel diameter) to increase
blood flow to the area.
Bradykinin and histamine also cause increased permeability (allows fluid to leak out). This brings
more defensive cells and chemicals to the area.
Neutrophils and monocytes are amoeboid white blood cells that squeeze out of the capillaries
and enter the damaged tissue.
Neutrophils phagocytize foreign material.
Monocytes are transformed into macrophages, which can phagocytize a large number of viruses
and bacteria.
Macrophages release white blood cell growth factor. This hormone stimulates the bone marrow to
produce leukocytes (white blood cells).
Basophils that have left the blood vessels to reside in the tissues become mast cells.
Pus is a large # of dead leukocytes that fought infection.
Antibody-Mediated Immunity
Antigens and Antibodies
Antibodies are proteins that protect against foreign invaders, either foreign molecules, viruses, or
cells. They are capable of recognizing specific particles due to their shape. Their ability to
recognize foreign shapes makes them useful in defending against foreign invaders.
Antigens are molecules that antibodies are capable of recognizing. They are usually a protein or
carbohydrate chain. The body can recognize bacteria and viruses as being foreign because they
have antigens on their surface which are different than the bodies "self" antigens.
Antibodies are Y-shaped molecules with a constant region and two binding sites that vary from one
antibody to the next.
Antibodies fit together with and bind with antigens like a lock and key.
The body does not produce antibodies that bind to its own (self) antigens. Therefore all particles
that are bound to antibodies are foreign.
Cells, particles, or molecules that are marked with antibodies:
1. may be phagocytized (engulfed) by neutrophils or macrophages.
2. may agglutinate (clump together) because each antibody is capable of binding to two antigens. If
the antigens are chemicals that are dissolved in the body fluids, the clumps of anibody-bound
particles will precipitate. Antigens attached to cells will cause the cells to clump together. The
clumps are then phagocytized.
3. may activate the compliment system (discussed below). The compliment system is a system of
blood proteins that enhances the elimination of foreign cells or particles.
During our life, we will encounter over 1 million different antigens, so we need at least 1 million
different antibodies, one for each kind of antigen.
There are 5 different classes of antibodies (IgA, IgD, IgG, IgH, IgM). One class contains
pentamers, another contains dimers.
A Lymphatic System
Functions of the Lymphatic System
1. take up excess tissue fluid and return it to the bloodstream
2. absorb fats at the intestinal villi and transport to the circulatory system
3. defend against disease
Lymphatic Vessels
Lymphatic vessels are similar to veins, including the presence of valves. They depend on the
movement of skeletal muscles to move the fluid inside.
The fluid they contain is called lymph.
They empty into the circulatory system via the thoracic duct and the right lymphatic duct. The
thoracic duct is much larger than the right lymphatic duct.
Lymph Nodes
Lymph nodes are small (1-25 mm), spherical or ovoid structures that are connected to lymphatic
vessels. They contain open spaces (sinuses), each with many lymphocytes and macrophages.
As lymph passes through, macrophages purify it of infectious organisms and particles.
The structures listed below are groups of nodules that also function to purify lymph:
tonsils - back of mouth
adenoids - back of mouth above the soft palate
Peyer’s patches - intestinal wall
Spleen
The spleen stores blood.
It helps purify blood that passes through it by removing bacteria and worn-out or damaged red
blood cells.
Thymus Gland
T lymphocytes mature in the thymus.
Bone Marrow
Macrophages and lymphocytes (B cells and T cells) are produced in the bone marrow. T cells
mature in the thymus gland, small intestine, and in the skin.
ntibodies are produced by B lymphocytes.
ADVERSE EVENTS ASSOCIATED WITH IMMUNITY
Mast cells are immune cells involved in the allergic responses. IgE binds to Mast cells where
antibodies are made against antigens such as dust, poll en and plant spores. If a person contains
IgE antibodies for a particular antigen then they are said to be sensitised to that antigen. If the
person is further exposed to the same antigen, cross links are formed between the antibody on the
mast cell and the antigen. These trigger the release of histamine.
RHESUS INCOMPATIBILITY
Human blood cells can be classified as Rhesus +ve or Rhesus –ve depending of the presence or
absence of the Rhesus protein on the red blood cells. The blood group is genetically determined.
See figure 8.37 in text book.
MULTIPLE SCLEROSIS
Chronic disease of the central nervous system (CNS), is thought to be an example of autoimmune
disease which the body’s immune cells attack its own tissues. MS is the breakdown of the fatty
myelin sheath that surrounds the processes of the nerve cells. This sheath is produced by
specialised cells. Damage to the sheath can short circuit the communication between parts of the
CNS and the peripheral nerves. The cause is unknown however the is is suggested that exposure
to infectious agents such as viruses could be involved. This may set the auto-immune system to
target the myelin. It is said to be hereditary.
REJECTION OF TRANSPLANTS
Before an organ is transplanted from one person to another the donor and the recipient have their
tissues ‘typed’ for find out the major antigens that are present in each. Drugs are available to inhibit
the immune response. Often the immune system will react against the ‘non-self’. T cells are
important. Th cells recognise the foreign material and attack. The better the match the higher the
chance of success.
 Any part of the immune response can be faulty;
 Cells of the immune system are involved in allergic reactions;
 An immune system can lose the ability to distinguish ‘self’ from ‘non-self’. The action of the
immune system can be reduced by treatment with certain drugs.
 Most plants resist infection by mechanical and chemical means.