6.1 Digestion
6.1.1 Explain why digestion of large food molecules is essential.
● The foods that humans eat are made by other organisms, and therefore usually not suitable for human use in its ingested form. They must therefore be broken down and reassembled for human use.
● Many molecules in food are too large to be absorbed by the villi of the small intestine. These large molecules need to be broken down before they can be absorbed by diffusion, facilitated diffusion, or active transport.
6.1.2 Explain the need for enzymes in digestion.
The need for increasing the rate of digestion at body temperature should be emphasized.
● Enzymes catalyze reactions by lowering the activation rate required for the reaction.
● This is advantageous in living organisms because less energy in the form of heat is required to undergo reactions of the metabolism.
● This means that the reactions of digestion can occur at body temperature, which would otherwise be much too low to initiate reactions without the help of enzymes.
6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase.
| | Salivary amylase | Pepsin (a protease) | Pancreatic lipase |
| Source | salivary glands | stomach cells | pancreas |
| Substrate | amylose | proteins (polypeptides) | lipids |
| Products | maltose and glucose | amino acids | fatty acids and glycerol |
| Optimum pH | 7 | 1.5 | 7 |
6.1.4 Draw and label a diagram of the digestive system.
The diagram should show the mouth, esophagus, stomach, small intestine, large intestine, anus, liver, pancreas and gall bladder. The diagram should clearly show the interconnections between these structures.
6.1.5 Outline the function of the stomach, small intestine and large intestine.
Stomach
● The function of the stomach is to begin chemical digestion of proteins with enzymes.
● Goblet cells in the stomach secrete gastric juices, which include hydrochloric acid (HCl) and protease enzymes.
● HCl establishes the acidic environment of the stomach. This serves to kill harmful bacteria in incoming foods, and also provides the ideal environment to shift protease enzymes like pepsin into an active state.
● Protease enzymes break down proteins in the foods.
● Goblet cells also secrete mucus, which protects the interior stomach lining and prevents autolysis.
● Mechanical digestion also occurs in the stomach by churning action.
● Ultimately food leaves the stomach in the form of chyme.
Small intestine
● In the small intestines, digestion of carbohydrates, lipids, and proteins is completed and the useful products of digestion are absorbed.
● Chyme enters the small intestine through the duodenum, where it is met with pancreatic juices from the pancreas and bile salts from the bile duct. They are churned together by a form of peristalsis called segmentation.
● Bile is strongly alkaline and neutralizes the acidity of chyme. Also it lowers the surface tension of fat globules, causing them to break down to smaller droplets in a process called emulsification. This allows lipase to digest lipids faster.
○ Note: Bile itself contains no enzymes.
● Pancreatic juices are made of many enzymes including amylase, lipase, and proteases.
● The products of digestion (monosaccharides, amino acids, fatty acids and glycerol, vitamins and mineral ions) are absorbed as they make contact iwth epithelial cells of the villi of the small intestine.
○ The process is efficient because villi result in a huge surface area, and it is conducted through active transport.
● Sugars and amino acids pass into the capillary network whereas lipids go to lacteal vessels. They eventually undergo assimilation.
Large intestines
● Undigested matter from the small intestine enters the large intestine.
● Here, water is absorbed which leaves solid feces.
● Mineral salts are also absorbed.
● Feces are eventually egested through the anus.
6.1.6 Distinguish between absorption and assimilation.
● Absorption: uptake into the body (blood circulation or lacteals) of the useful products of digestion from the small intestine.
● Assimilation: uptake of nutrients into cells and tissue.
6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.
| villi | - provides a huge surface area for absorption |
| epithelium cells | - consists of only one layer of cells and therefore provides a small distance that molecules must diffuse across |
| mucus from goblet cells in epithelium | - lubricates movement of digested foods among the villi and protects plasma membrane of epithelial cells |
| lacteal | - branch of the lymphatic system into which triglycerides pass for transport to body cells |
| mitochondria in epithelial cells | - single layer of small cells, packed with mitochondria - the source of ATP for active uptake across the plasma membrane |
| channels and pumps | - allow for rapid absorption of food by facilitated diffusion or active transport, respectively |
| microvilli | - protrusions of the exposed part of the membranes of the epithelium cells increase surface area for absorption even further |
6.2 The transport system
6.2.1 Draw and label a diagram of the heart showing the four chambers, associated blood vessels, valves and the route of blood through the heart.
Care should be taken to show the relative wall thickness of the four chambers. Neither the coronary vessels nor the conductive system are required.
6.2.2 State that the coronary arteries supply heart muscle with oxygen and nutrients.
6.2.3 Explain the action of the heart in terms of collecting blood, pumping blood, and opening and closing of valves.
A basic understanding is required, limited to the collection of blood by the atria, which is then pumped out by the ventricles into the arteries. The direction of flow is controlled by atrio-ventricular and semilunar valves.
● The atria are the collecting chambers and the ventricles are the pumping chambers. Valves control direction of blood flow.
● The walls of the atria contract, pushing blood from the atria to the ventricles through the AV valves, which are open. Semilunar valves are closed, so ventricles fill with blood.
● Walls of the ventricle contract powerfully and the blood pressure within them rapidly rises. The rise in pressure causes the AV valves to close, preventing back-flow into the atria, and then causes semilunar valves to open, allowed blood to pump into the arteries. At the same time, atria begin to refill.
● The ventricles stop contracting and pressure falls inside them. This causes semilunar valves to close which prevents backflow from the arteries into the ventricles.
● When ventricle pressure drops below atrial pressure, the AV valves open.
● Blood from the atrium entering from the veins now flows into the ventricle, and the process repeats again.
6.2.4 Outline the control of the heartbeat in terms of myogenic muscle contraction, the role of the
pacemaker, nerves, the medulla of the brain and epinephrine (adrenaline).
● Cardiac muscles have the ability to contract without any stimulation by nerves, therefore the heartbeat is a myogenic process.
● The region that initiates a heartbeat is called the pacemaker, which is located in the right atrium.
● The heartbeat is regulated by two antagonistic nerves which receive signals from the medulla of the brain.
● One nerve causes the heartbeat to slowdown, and it is triggered when pressure within the heart is too high.
● The other nerve causes the heart beat to speed up, and is triggered when blood pressure within the heart is too low.
● The hormone adrenaline also causes the heart beat to speed up.
6.2.5 Explain the relationship between the structure and function of arteries, capillaries and veins.
| artery |  | 1. thick wall to withstand high pressures 2. thick outer layer of longitudinal collagen and elastic fibres to avoid bulges and leaks. 3. thick inner layer of circular elastic and muscle fibres to help pump blood on after each heart beat. 4. narrow lumen to maintain high pressures. |
| vein |  | 1. thin wall to allow veins to be pressed flat by adjacent muscles, helping blood move along 2. thin outer layer of longitudinal collagen and elastic fibres because there is little danger of bursting 3. thin inner layer of circular elastic and muscle fibres because blood does not flow in pulses and vein cannot help it move along 4. wide lumen to accommodate slow-flowing blood |
| capillary |  | 1. wall consists of a single layer of cells so that the distance for diffusion in or out is small 2. pores between cells allow some of the plasma to leak out to form tissue fluid; phagocytes can also squeeze in 3. very narrow lumen, about 10 micrometres, so that capillaries can fit into small spaces; many small capillaries sum to make a large surface area |
6.2.6 State that blood is composed of plasma, erythrocytes, leucocytes (phagocytes and lymphocytes) and platelets.
6.2.7 State that the following are transported by the blood: nutrients, oxygen, carbon dioxide, hormones, antibodies, urea and heat.
6.3 Defence against infectious disease
6.3.1 Define pathogen.
● Pathogen: an organism or virus that causes a disease.
6.3.2 Explain why antibiotics are effective against bacteria but not against viruses.
● Antibiotics block specific metabolic pathways found in other organisms, such as the synthesis and laying down of new cell wall materials in bacteria. Antiobiotoics are therefore effective at disrupting bacterial metabolism can suppress populations of bacteria.
● Viruses are not living things and have no metabolism of their own. They reproduce using the host cell’s metabolic pathways, and therefore they are not affected by antibiotics.
6.3.3 Outline the role of skin and mucous membranes in defence against pathogens.
Skin
● The external skin is covered by keratinized protein of the dead cells of the epidermis. This acts as a tough physical barrier against most pathogens.
● Sebaceous glands in the skin secrete lactic and fatty acids, which make the surface of the skin acidic and prevent most bacterial growth.
Mucous membranes
● These are soft areas of skin kept moist with mucus, and found in the nose, trachea, vagina and urethra.
● Many bacteria are killed by lysozyme, an enzyme in mucus.
● Mucus traps bacteria and cilia push the bacteria out (such as in the trachea).
6.3.4 Outline how phagocytic leucocytes ingest pathogens in the blood and inbody tissues.
● Phagocytes are a type of leukocytes that can identify pathogens and ingest them by endocytosis. Pathogens are then killed and digested within the cell by lysosomes.
● Phagocytes can ingest pathogens in the blood, but can also squeeze through the walls of capillaries and move through tissues to act directly at the site of infection.
● Large numbers of phagocytes at the site of infection form pus.
6.3.5 Distinguish between antigens and antibodies.
● Antigen: A ‘non-self’ substance, for example proteins produced by invading organisms. They are foreign substances and stimulate the production of antibodies.
● Antibody: Proteins that recognize and bind to specific antigens. They defend the body by binding to antigens on the surface of pathogens and stimulating its destruction.
6.3.6 Explain antibody production.
Many different types of lymphocyte exist. Each type recognizes one specific antigen and responds by dividing to form a clone. This clone then secretes a specific antibody against the antigen. No other details are required.
More detailed response at 11.1.4.
6.3.7 Outline the effects of HIV on the immune system.
● There is a reduction in the number of active lymphocytes and a loss of the ability to produce antibodies.
The HIV virus binds to a host lymphocyte cell membrane and passes the RNA core of the virus into the host cell. Inside the host cell, RNA and enzymes of the virus, namely ‘reverse transcriptase’, are released. Reverse transcriptase catalyzes the copying of the genetic code on the virus RNA to produce a DNA double helix. This DNA enters the host nucleus and is ‘spliced’ into the host DNA of a chromosome. It is replicated every time the host cell divides. It remains latent.
6.3.8 Discuss the cause, transmission and social implications of AIDS.
cause:
acquired immunodeficiency syndrome;
caused by HIV/human immunodeficiency virus,
low number of T-helper cells;
less antibodies produced;
body vulnerable to pathogens;
transmission:
transmission through body fluids / does not live long outside the body;
through sexual intercourse;
sharing of needles;
across placenta;
blood transfusions;
blood products / factor VIII used to treat hemophiliacs;
social implications:
families and friends suffer grief;
great costs/reduction in workforce/economic implications;
discrimination;
increase in number of orphans / family structure / stability affected;
The answer must include at least one reference to cause, transmission and social
implications in order to receive full marks.
6.4 Gas exchange
6.4.1 Distinguish between ventilation, gas exchange and cell respiration.
● Ventilation is the process of bringing frensh air to the alveoli and removing stale air.
● Gas exchange is the exchange of gases between an organism and its surroundings, including the uptake of oxygen and the release of carbon dioxide in animals and plants.
● Cellular respiration is the controlled release of energy in the form of ATP from organic compounds in cells. It is a continuous process in all cells.
6.4.2 Explain the need for a ventilation system.
A ventilation system is needed to maintain high concentration gradients in the alveoli.
● A ventilation system is needed to maintain high concentration gradients in the alveoli.
● It is necessary in large multicellular animals because their cells tend to be too far away from the body surface to receive oxygen by surface diffusion.
● Large animals also often develop a tough external skin which prevents gaseous exchange by surface diffusion.
● Active animals - and particularly large active animals, such as mammals - have a greater demand for energy and require a specialized ventilation system to meet these demands, which are not met by surface diffusion.
6.4.3 Describe the features of alveoli that adapt them to gas exchange.
This should include a large total surface area, a wall consisting of a single layer of flattened cells, a film of moisture and a dense network of capillaries.
6.4.4 Draw and label a diagram of the ventilation system, including trachea,
lungs, bronchi, bronchioles and alveoli.
Students should draw the alveoli in an inset diagram at a higher magnification.
6.4.5 Explain the mechanism of ventilation of the lungs in terms of volume and pressure changes caused by the internal and external intercostal muscles, the diaphragm and abdominal muscles.
Inspiration
● external intercoastal muscles contract, moving the ribcage up and out.
● diaphragm contracts, causing it to become flat and move down.
● the volume of the thorax, and therefore the lungs, increases as a result
● this results in pressure to be reduced below atmospheric pressure, and this negative pressure causes air to enter the lungs.
Expiration
● internal intercoastal muscles contract, moving the ribcage down and in
● diaphragm relaxes, causing it to move up into a dome shape.
● there is a decrease in volume in the thorax and therefore the lungs as well
● this causes pressure to increase above atmospheric pressure, causing air to flow out of the lungs.
6.5 Nerves, hormones, homeostasis
6.5.1 State that the nervous system consists of the central nervous system (CNS) and peripheral nerves, and is composed of cells called neurons that can carry rapid electrical impulses.
No other structural or functional divisions of the nervous system are required.
6.5.2 Draw and label a diagram of the structure of a motor neuron.
Include dendrites, cell body with nucleus, axon, myelin sheath, nodes of Ranvier and motor end
plates.
6.5.3 State that nerve impulses are conducted from receptors to the CNS by sensory neurons, within the CNS by relay neurons, and from the CNS to effectors by motor neurons.
6.5.4 Define resting potential and action potential (depolarization and repolarization).
● Resting potential: the potential difference across a nerve cell membrane when it is not being stimulated. It is normally about -70mV.
● Action potential: the potential difference produced across the plasma membrane of a nerve cell when stimulated, reversing the resting potential from about -70mV to about +40mV.
6.5.5 Explain how a nerve impulse passes along a non-myelinated neuron.
Include the movement of Na+ and K+ ions to create a resting potential and an action potential.
Establishing the resting potential
1. Na+/K+ pump and active transport
● Active transport is used to pump 3 Na+ ions OUT and across the membrane while also pumping 2 K+ ions IN across the membrane. This builds up a concentration of Na+ outside and K+ inside, however it makes no change to the potential difference.
2. Na+ and K+ facilitated diffusion
● The nerve cell membrane is more permeable to K+ ions back out than it is to Na+ ions back in. As a result, a negative charge establishes across the membrane on the inside, so the resting neurone becomes polarized to reach the reasting potential (-70mV).
○ The cell membrane is x50 permeable to K+ and x1 permeable to Na+.
3. Negative ions
● There are negatively charged ions (eg. organic ions and chloride) that are permanently located in the cytoplasm of the axon, which contribute to the negative resting potential.
Action potential
● Triggered by a stimulus recieved at a receptor cell or sensitive nerve ending.
○ An action potential in one section of a neuron causes an action potential to develop in the next section of the neuron.
● The neuron is breifly depolarized.
● Change in potential causes Na+ channels to open. Na+ diffuse in, down an electrochemical gradient.
● As Na+ flow in, the section of nerve fibre quickly becomes positive (as charge reverses from -70mV to +40mV).
○ This action potential runs the length of the nerve fibre in 2 milliseconds
○ This is depolarization.
● Sodium channels close and potassium channels open. K+ ions rush out of the cell down their electrochemical gradient, and this makes the charge inside more negative again. Then the K+ channels close.
● Resting potential is re-established by the Na+/K+ pump.
6.5.6 Explain the principles of synaptic transmission.
Include the release, diffusion and binding of the neurotransmitter, initiation of an action potential
in the post-synaptic membrane, and subsequent removal of the neurotransmitter.
Steps of synapse transmission
1. An action potential arrives at the synaptic knob of the pre-synaptic cleft, which causes calcium ion channels to open in the pre-synaptic membrane. Calcium ions flow inside from the synaptic cleft.
2. The calcium ions cause vesicles of transmitter substance to fuse with the pre-synaptic membrane and release transmitter substance into the synaptic cleft.
3. The transmitter substance diffuse across the synaptic cleft and fuses with a receptor protein.
- There are specific receptors for each transmitter substance. They act as channel proteins and are opened/activated by the transmitter substances.
- E.g. when acetylcholine (ACh) binds to a receptor, a Na+ channel opens. Na+ flow into the post-synaptic neuron, causing depolarization of its membrane. The more ACh that bind, the more likely that depolarization will reach the threshold level which will then subsequently generate an action potential in the post-synaptic neuron. This process is called ‘facilitation’.
4. Transmitter substances are immediately inactivated by enzyme action, causing ion channels to close and so allowing the resting potential to be re-established.
- E.g. Cholinesterase hydrolyzes ACh to chlorine and ethanoic acid, which are inactive as transmitters.
5. The inactivated products of the transmitter re-enters the pre-synaptic knob, are re-synthesized into transmitter substance and packaged for re-use.
6.6 Reproduction
6.6.1 Draw and label diagrams of the adult male and female reproductive systems.
6.6.2 Outline the role of hormones in the menstrual cycle, including FSH (follicle stimulating hormone), LH (luteinizing hormone), estrogen and progesterone.
Gonadotrophins(anterior pituitary hormones) have a role in regulating levels of sex hormones, and both are responsible for controlling the menstrual cycle.
FSH, secreted by the anterior pituitary, stimulates follicle development and secretion of estrogen by the follicle. Only one follicle will mature into the ovarian follicle. Estrogen secreted by the follicle cells has two functions: firstly, its responsible for the build-up and development of the endometrium; and secondly, it inhibits further secretion of FSH by the anterior pituitary. Estrogen levels rise to a peak (at approximately 14 days), and this triggers the secretion of LH by the anterior pituitary. LH rises rapidly to a peak and triggers ovulation. Furthermore, LH stimulates the conversion of the vacant follicle into a temporary endocrine gland called the corpus luteum, and it also inhibits further secretion of estrogen by follicle cells. The corpus luteum secrets large amounts of progesterone, which is responsible for the maintenance of the endometrium in anticipation of pregnancy. Due to high levels of the ovarian hormones, secretion of LH and FSH in the pituitary is inhibited. This subsequently reduces the levels of progesterone and estrogen. The endometrium can no longer be maintained and breaks down as a result, and is lost through the vagina in the first days of a new cycle. The low levels of progesterone triggers the secretion of FSH again to begin a new cycle.
6.6.3. Annotate a graph showing hormone levels in the menstrual cycle, illustrating the relationship between changes in hormone levels and ovulation, menstruation and
thickening of the endometrium.
6.6.4 List three roles of testosterone in males.
Testosterone initiates prenatal development of genitalia, triggers the development and regulates secondary sexual characteristics, and maintains the sex drive in adults.
6.6.5 Outline the process of in vitro fertilization (IVF).
LH and FSH secretion is suppressed in the female using drug injections. Then super-ovulation is induced using synthetic FSH. The male provides a semen sample, which is concentrated for healthy sperm. Several eggs are removed from the female using the a laparoscope and the aid of ultrasound. The egg and sperm are mixed in shallow dishes, and fertilization is ensured using microscope examination. The zygotes are incubated for 2-3 days at body temperature. Then microscope examination is used to ensure that they are at the 4-8 cell stage. Up to three zygotes are implanted into the female uterus, with the expectation of one successful implantation.
6.6.6 Discuss the ethical issues associated with IVF.
Pros:
● Infertile parents can have children.
● Individuals with cancer can have children with gametes harvested prior to radiation treatment/chemotherapy.
● Screen for inherited diseases before implantation is possible.
Cons:
● Allows infertility due to inherited defects to pass on to offspring.
● Since excess embryos are produced to increase chance of successful implantation, some embryos must be destroyed.
● Multiple pregnancies is common, which increases the chance of premature birth and risk of conditions like cerebral palsy.
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