This pages gives outline details of the content of the topic together with essential questions and student skills and applications. Helpful for revision.

6.1 Digestion and absorption

• The outline structure of the digestive system. (skill - annotated diagram)
• Food is mixed with enzymes and moved along by the contraction of circular and longitudinal muscle of the small intestine.
• Enzymes (amylase, lipase and an endopeptidase) are secreted by the pancreas into the small intestine where they digest macromolecules ( starch, glycogen, lipids and nucleic acids) in food into monomers.
• Tissue layers in the intestines include longitudinal and circular muscles, mucosa and epithelium.
• The presence of Villi in the small intestines increases the surface area of epithelium and helps absorb the monomers formed by digestion and mineral ions and vitamins.
• Different nutrients are absorbed into epithelial cells by different membrane transport mechanisms.
• Guidance:
• The enzyme name trypsin and the method used to activate it are not required.
• Students should know that cellulose remains undigested.

6.2 Blood system

• Arteries carry blood at high pressure from the ventricles of the heart to the tissues of the body.
• Arteries have muscle cells and elastic fibres in their walls.which maintain blood pressure between ventricle contractions.
• Capillaries carry blood through tissues and have permeable walls to allow exchange between tissue cells and the blood.
• Veins carry blood at low pressure from the tissues to the atria of the heart.
• Valves in veins and the heart prevent back-flow.
• There is a separate circulation for the lungs.
• The sinoatrial node initiates heart beat
• The SA node is made of specialized muscle cells in the right atrium which act as a pacemaker.
• The sinoatrial node sends out an electrical signal which is propagated through the walls of the atria and ventricles.
• The heart rate is controlled by impulses brought to the heart through two nerves from the medulla of the brain.
• Epinephrine (adrenalin) increases the heart rate.

6.3 Defence against disease

• The primary defence against pathogens are the skin and mucous membranes.
• Blood clotting seals cuts in the skin. (skin diagrams are not required.)
• Platelets release clotting factors.
• Fibrinogen is converted to fibrin by thrombin.
• Pathogens are ingested by phagocyte cells which gives non-specific immunity.
• Specific immunity is given by production of antibodies by lymphocytes in response to particular (antigens on) pathogens
• Antibiotics block prokaryotic cell processes but not processes in eukaryotic cells.
• Viruses lack a metabolism and cannot be treated with antibiotics. Some strains of bacteria have evolved antibiotic resistance and even multiple resistance.
• The effects of HIV on the immune system should be limited to a reduction in the number of active lymphocytes and a loss of the ability to produce antibodies, leading to the development of AIDS
• Subgroups of phagocyte and lymphocyte are not required but students should be aware that some lymphocytes act as memory cells and can quickly reproduce to form a clone of plasma cells if a pathogen carrying a specific antigen is re-encountered.

6.4 Gas Exchange

• Concentration gradients of oxygen and carbon dioxide between air in alveoli and blood flowing in adjacent capillaries are maintained by ventilation.
• Draw a diagram to show the structure of an alveolus and an adjacent capillary. Alveoli consist of two types of pneumocytes;
  • Type I are extremely thin and are adapted to carry out gas exchange.
  • Type II secrete a solution containing surfactant that creates a moist surface inside the alveoli. This reduces surface tension and prevents the sides of the alveolus sticking to each other.
• The trachea and bronchi carry air to the alveoli in bronchioles in the lungs.
• Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
• External and internal intercostal muscles, and diaphragm and abdominal muscles are antagonistic muscles, required for inspiration and expiration because muscles only work by contracting.

6.5 Neurones and synapses

• Electrical impulses are transmitted in Neurons. (details of neuron types not required)
• Sodium and potassium ions are pumped across neuron membranes to generate a resting potential.
• Action potentials are depolarization and repolarization of the neuron which are propagated along the axons of neurons creating nerve impulses. (oscilloscope traces showing resting potential & action potential.)
• Local currents cause each successive part of the axon to reach the threshold potential.
• Myelin sheath cells around the axons of nerve fibres allows for saltatory conduction.
• Synapses (only chemical synapses) join neurons and also join neurons to receptor or effector cells.
• Depolarized pre-synaptic neurons release a neurotransmitter into the synapse.
• A nerve impulse is only initiated if the threshold potential is reached.

6.6 Hormones and reproduction

• Hormones travel in the blood more slowly than nerve impulses but are more widely distributed
  • Insulin and glucagon control blood glucose (cells in islets of pancreas)
  • Thyroxin regulates metabolic rate & partly controls body temperature. (thyroid gland)
  • Leptin acts on the hypothalamus of the brain to inhibit appetite (secreted by cells in adipose tissue)
  • Melatonin controls circadian rhythms (jet lag) (secreted by pineal gland.)

• Hormones in human development & reproduction

• The sry gene of the Y chromosomes causes embryonic gonads to develop as testes and then they secrete testosterone which causes pre-natal development of male genitalia. In puberty it causes, development of male secondary sexual characteristics and sperm production.
• Estrogen and progesterone cause pre-natal development of female reproductive organs and in puberty the female secondary sexual characteristics.
• In the menstrual cycle FSH, LH, estrogen and progesterone have roles in negative and positive feedback mechanisms.
• IVF uses drugs to suspend the normal secretion of hormones, followed by the use of artificial doses of hormones to induce superovulation and establish a pregnancy.