1) Only small soluble molecules can be absorbed into the cells lining the gut.

2) Large molecules such as proteins must be reduced to their component parts ('building blocks') to allow the organism to synthesise its own molecules.

Triglycerides->Fatty acids and glycerol

->ABSORPTION

Proteins->Polypeptides->Di/tripeptides

->Amino acids->ABSORPTION

Polysaccharides->Disaccharides->Monosaccharides

->ABSORPTION

1) Temporary storage of food.

2) Mechanical breakdown of food by churning (via muscle contraction).

3) Start of protein digestion via pepsin.

4) Killing of bacteria in food via highly acidic gastric juice.

1) The main region of digestion and absorption.

2) Duodenum (1st part) receives pancreatic enzymes and bile from the liver. All food groups are digested.

3) Ileum (2nd part) is where most absorption occurs.

1) Site of water absorption.

2) The many harmless bacteria found here protect the gut by out-competing with harmful bacteria.

3) Some bacteria synthesise vitamins K and H.

1) Very long with many villi - large surface area.

2) Single columnar epithelial layer - short diffusion distance.

3) Microvilli on epithelial cells - increase surface area further.

4) Goblet cells secrete mucus - provides lubrication and a moist environment which aids the diffusion of substances.

5) Columnar epithelial cells are tightly packed - prevents leakage of absorbed molecules.

6)Smooth muscle - permits movement of villi towards food molecules.

7) Circular/longitudinal muscles - enables peristalsis which moves food molecules along.

8) Many blood capillaries and lymph vessels (e.g. lacteals, which transport fatty acids and glycerol) - enables efficient transport of absorbed molecules.

In absorption, the soluble products of digestion are first taken up by various mechanisms into the epithelial cells that line the small intestine. These epithelial cells then transport the absorbed molecules into the blood stream.

Assimilation occurs when the molecules have been transported to various tissues by the bloodstream, and are then absorbed by the cells of these tissues fro their own use e.g. the synthesis of new molecules such as proteins from the absorbed amino acids.

1) Oxygenated blood from the lungs enters the left atrium via the pulmonary vein and deoxygenated blood from the rest of the body enters the right atrium via the vena cava.

2) The atria fill with blood and contract simultaneous, squeezing blood into the ventricles as the atrioventicular (AV) valves open.

3) The ventricles fill with blood and contract simultaneously, squeezing blood into the aorta (via the left venticle) and the pulmonary artery (via the right ventricle) as the semilunar valves open.

Systole: when cardiac muscle is contracting.

Diastole: when cardiac muscle is relaxing.

Atrioventricular (AV) valves prevent the backflow of blood from the ventricles into the atria.

Semilunar valves prevent the backflow of blood from the arteries into the ventricles.

Atrioventricular (AV) valves prevent the backflow of blood from the ventricles into the atria.

Semilunar valves prevent the backflow of blood from the arteries into the ventricles.

1) An electrical impulse originates in the SAN, the pacemaker.

2) The impulse spreads rapidly though the walls of both atria, causing them to contract. This impulse cannot spread directly to the ventricles as there is a band of non-conducting tissue between the atria and the ventricles.

3) The AVN in the lower part of the right atrium wall is stimulated.

4) The impulse passes from this node into the venticles along conducting fibres called Purkinje fibres. The point where these fibres are found in the septum of the heart is known as the bundle of His.

5)The impulse spreads through the ventricles, causing them to contract.

• The atria are relaxed. The left atrium receives oxygenated blood from the lungs via the pulmonary veins as the right atrium receives deoxygenated blood from the rest of the body via the vena cava.
• The AV valves are shut.
• The semilunar valves are shut.

Describe the events of atrial systole.

• The atria contract simultaneously. As the pressure in the atria is greater than that in the venticles, the AV valves open.
• Blood enters the ventricles.
• Semilunar valves remain shut.

• The venticles contract simulteneously. As the pressure in the ventricles is greater than that in the atria, the AV valves close.
• The venticles continue to contract and the pressure in the ventricles surpasses that in the arteries.
• The semilunar valves open and blood enters the aorta from the left ventricle and the pulmonary artery from the right venticle.

• Red blood cells (erythrocytes)
• White blood cells (leucocytes): phagocytes and lymphocytes
• Platelets (thrombocytes)
• Plasma

The skin acts as a barrier - it is a dead, waterproof layer. Glands in hair follicles produce an oily secretion which controls bacterial and fungal growth. The enzyme lysozyme is present on the skin's surface to break down pathogens.

Membranes in the air passages produce mucus which traps pathogens (and other particles). Cilia move the mucus upwards towards the back of the throat where it is swallowed.

They destroy pathogens by ingesting and digesting them. The phagocyte flows aroung the pathogen, enclosing it in a vacuole inside the cell. Lysosomes then fuse with the vacuole and release digestive enzymes into it.

Phagocytes can ingest pathogens in the blood itself or pass through capillary walls to ingest them in body tissues.

Oxygen, carbon dioxide, urea, nutrients, hormones, antibodies and heat.

• Unprotected vaginal or anal sexual intercouse
• Breast feeding
• Via the placenta
• Blood transfusion
• Contaminated needles

• Breakdown of family structure
• Huge drain on medical resources due to the expense of the drugs available to control HIV
• Loss of workforce, which can negatively effect a country's economy

• Promiscuous people (e.g. prostitutes)
• Intravenous drug users
• Those with poor access to education or contraception

• Abstinence/use of condoms
• Better education of transmission routes and risks
• Not sharing of needles

• The virus frequently mutates, so antigens continuously change
• There is a very long incubation period
• There are different strains in Europe, America, Asia and Africa

• The central nervous system (CNS)
• The peripheral nervous system, which consists of nerves lying outside the CNS
• The autonomic nervous system

• Sensory neurons - conduct impulses from the receptor to the CNS
• Relay neurons - conduct impulses between other neurons
• Motor neurons - conduct impulses from the CNS to the effector

The cell membrane of a resting nerve fibre (axon) is electrically polarised - the inside of the axon membrane is negatively charge with respect to the outside to a value of around

-70 millivolts (mV). This electrical difference is known as the resting potential.

1. A nerve impulse arrives agt the synapse, causing calcium ions to enter the synaptic button.
2. The influx of calcium ions causes vesicles to fuse with the presynaptic membrane, releasing neurotransmitter into the synpatic cleft.
3. The neurotransmitter diffuses across the cleft and binds to receptors on the post-synaptic membrane.
4. This causes depolarisation of the postsynaptic membrane (sodium channels open). If this depolarisation is sufficiently large, an action potential is generated.

• Blood pH
• Oxygen and carbon dioxide concentrations
• Blood glucose concentration
• Body temperature
• Water balance (solute concentration)

• A receptor to monitor the level of a variable
• A co-ordinating centre to regulate the level of said variable
• An effector/effectors to bring about the changes directed by the co-ordinating centre

• External themoreceptors in the skin
• The hypothalamus in the brain

Alpha cells are sensitive to a fall in blood glucose concentration and secrete glucagon.

Beta cells are sensitive to a rise in blood glucose concentration and secrete insulin.

It has two effects in the liver, both of which raise the blood glucose concentration:

1. Activates enzymes responsible for the conversion of glycogen to glucose.
2. Stimulates the formation of glucose from other molecules.

It has several effects, all of which lower the blood glucose concentration:

1. Activates enzymes in the liver and muscle cells responsible for the conversion of glucose to glycogen.
2. Increases the rate of cellular respiration.
3. Increases the uptake of glucose by muscle and fat cells.
4. Activates enzymes in fat cells responsible for converting glucose to fat.

100 mg/100 cm³ of blood

OR

4-7mmol/litre

• Energy source (sperm use fructose to produce energy for propulsion via the flagella).
• Protection for the sperm as urine is acidic.

• After menstruation the endometrium is thick and smooth.
• An immature follicle starts to mature and the endometrium begins to thicken again so that it will be ready for implantation if an ovum is fertilised.
• Around day 14, the middle of the cycle, ovulation occurs - an ovum is released from a mature follicle. The empty follicle becomes the corpus luteum.
• Eight to nine days later, the corpus luteum begins to degenerate. Several days later,  the endometrium starts to break down, resulting in menstruation.

• FSH and LH are secreted by the anterior part of the pituitary gland.
• Oestrogen is secreted by follicle cells in the ovary.
• Progesterone is secreted by the corpus luteum in the ovary.

• Stimulates follicle growth.
• Stimulates the ovary to secrete oestrogen (positive feedback).

• Stimulates growth and repair of the endometrium.
• Stimulates the pituitary gland to secrete LH in the middle of the cycle.
• Inhibits the secretion of FSH from the pituitary gland.

• Stimulates ovulation.
• Stimulates the empty follicle to form the corpus luteum.
• Stimulates the corpus luteum to secrete progesterone.

• Stimulates the final growth of the endometrium ready for implantation.
• Inhibits FSH secretion so that no more follicles mature and less oestrogen is secreted.
• Inhibits LH secretion (when levels are sufficiently high)

• Pre-natal development of male genitalia.
• Maintenance of sex drive.
• Secondary sexual chacteristics.

1. FSH is injected into the woman, causing several follicles to develop.
2. Eggs are removed from mature follicles by a tube inserted via the vagina.
3. Each egg is mixed with sperm in a shallow dish.
4. After a few days of incubation, the embryos are examined for signs of abnormality.
5. Two or three embryos are selected and placed in the uterus via a tube.
6. A pregnancy test is taken to see if any embryos have implanted.

• Some governments cover costs on the NHS.
• May allow infertile couples to have children - it is fundamental human right to be able to have children.
• Screening and selection prevents genetic abnormalities being passed on.
• Unused embryos can be stored and implanted later, donated or used as a source of stem cells.

• Religious issues as more embryos are produced than are used, so some must be destroyed.
• Extreme costs of private treatment.
• Increased chance of multiple births, which put the mother and children at a higher risk.
• Embryos may be chosen for desirable features.
• Takes money away from other fields e.g. cancer research.