Digestion

6.1.1 Explain why digestion of large food molecules is essential
Large macromolecules present in food are insoluble and need to be broken down into smaller molecules in order to pass through the walls of the villus in the small intestine, to be absorbed into the bloodstream.
It is easier for the body to reassemble the constituent parts of macromolecules into macromolecules that are required.
Large molecules, especially proteins, can stimulate an antibody response.

6.1.2 Explain the need for enzymes in digestion
Enzymes are needed to speed up the rate of digestion so that food molecules can be used or added to the body's reserve of lipids, amino acids and sugars.
Enzymes reduce the activation energy needed to hydrolyse large macromolecules into smaller soluble molecules.

enzymes speed up the digestive processes;
(chemical) break down of food/food particles/large molecules;
make soluble products/molecules small enough to be absorbed;

6.1.3 State the source, substrate, products and optimum pH conditions for one amylase, one protease and one lipase.

Enzyme	Source	Substrate	Products	Optimum pH
Amylase	Salivary glands	Starch	Maltose	6.5-7.5
Pepsin	Stomach wall	Protein	Peptides	2
Lipase	Pancreas	Lipid	Fatty acids & glycerol	8

6.1.4 Draw and label a diagram of the digestive system.
Hand-drawn image coming soon.

6.1.5 Outline the function of the stomach, small intestine and large intestine.

Stomach - Temporarily stores bolus (chewed food). Mixes half-digested food with gastric juice. Gastric juice containing HCl destroys any pathogens present in food. Gastric juice also activates enzymes required for protein synthesis, e.g. pepsinogen is activated and converted to pepsin.
Small intestine - Acidic bile from bile duct mixes with chyme (partly digested food mixed with gastric acid) and neutralizes it. Bile also emulsifies lipids into smaller droplets which speeds up the rate of digestion later on.
Pancreatic juice from the pancreas contains enzymes lipase, amylase and proteases that catalyze the hydrolysis of macromolecules into their constituent molecules.
Large intestine - Absorbs water, mineral salts (sodium and chlorine) and vitamins K and B12.
Temporarily stores faeces composed of fibre, bacteria, mucous and dead intestinal cells. Bile pigments colour the faeces. Sphincter muscles surrounding the anus discharge the faeces from the body.

Duodenum of small intestine

6.1.6 Distinguish between absorption and assimilation.

Absorption	Assimilation
Movement of nutrients into the body across a membrane	Uptake of absorbed nutrients from the intestine to cells and tissues
Nutrients are absorbed by the villi in the small intestine and are actively transported across the endothelial cells.	Sugars and amino acids are transported by the blood capillaries to the pool of reserves in the liver
	Fatty acids and glycerol are transported via the lacteal to the rest of the body.

6.1.7 Explain how the structure of the villus is related to its role in absorption and transport of the products of digestion.

Finger-like shape of villus	Increases the surface area of small intestine available for absorption
Microvilli on surface of endothelial cells	Increase the surface area of small intestine further, increasing absorption.
Small distance between capillary network and epithelial cells	To minimise the distance for diffusion and to quicken the process of diffusion.
Epithelial cells with mitochondria	Mitochondria produce ATP to provide energy for active transport across the endothelium membrane
Pump proteins in plasma membrane of endothelial cells	For active transport of nutrients across endothelial plasma membrane

The heart and blood

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.

Hand-drawn image coming soon.

6.2.2 State that the coronary arteries supply heart muscle with oxygen and nutrients.

If the coronary arteries are blocked by cholesterol and/or fatty acids, the heart will not receive sufficient amounts of oxygen and nutrients and will therefore not function properly. This can lead to heart attacks and coronary heart disease.

6.2.3 Explain the action of the heart in terms of collecting blood, pumping blood, and opening and closing of valves.

De-oxygenated blood enters the heart via the vena cava into the right atrium. The atrium muscles contract and force blood past the tricuspid valve into the right ventricle. Atrium muscles relax and the ventricle muscles contract causing blood pressure to close the tricuspid valve and open the semilunar valves, pushing blood through the semilunar valve and into the pulmonary artery to the lungs. Oxygenated blood from the lungs enters the heart again via the pulmonary veins, through the bicuspid valve into the left ventricle.

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).

Heartbeat is produced from myogenic muscle contractions from the pacemaker in the wall of the right atrium.

Pacemaker stimulates contraction of atria causing subsequent contraction of ventricles

Autonomic nerves from the medulla can either decrease or increase heart rate.

Secretion of adrenaline by adrenal glands can also increase the heart rate

Conversely, the secretion of acetylcholine decreases the heart rate

These hormones are carried by the blood to the heart in response to an external pressure, e.g. physical activity, perceived danger.

is myogenic;

pacemaker

stimulates atria to contract;

leading to contraction of ventricles;

autonomic nerves can alter the pace;

secretion of adrenaline

increase the pace;

adrenal glands release adrenaline;

(by secretion of) acetylcholine reduces the pace;

carried by blood to heart;

to increase pace;

6.2.5 Explain the relationship between the structure and function of arteries, capillaries and veins.

Arteries

Thick walls

Withstand high blood pressure required to pump blood to organs

Narrow lumen

Maintain high blood pressure required to pump blood to organs

Collagen fibres

Tough inelastic fibres that prevent walls from being overstretched.

Elastic fibres

Move and contract with pulse, maintaining blood flow.

Veins

Pocket valves

Prevent backflow of blood when blood pressure is low

Wide lumen

To keep blood at a low blood pressure

Elastic fibres

allow vein to be squished between muscle blocks, increasing blood flow

Capillaries

Single layer of endothelium cells

Allows rapid diffusion of substances dissolved in blood

Gaps between endothelium cells

More rapid diffusion because substances allow have to diffuse through the basement membrane.

Much branching and re-joining to form large capillary network

Transport of blood as close as possible to all living cells

Narrow lumen of 10 µm

High frictional resistance, restricting blood flow and reducing blood pressure. Allow capillaries to travel through tight places.

6.2.6 State that blood is composed of plasma, erythrocytes, leucocytes (phagocytes and lymphocytes) and platelets.

Plasma itself is composed of 90% water and 10% dissolved substances including nutrients, O₂, CO₂, hormones, antibodies, urea and heat.

6.2.7 State that the following are transported by the blood: nutrients, oxygen, carbon dioxide, hormones, antibodies, urea and heat.

Defence against infectious disease

6.3.1 Define pathogen.

Pathogen - 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 in bacteria, such as destroying the cell wall and preventing replication of bacteria. Viruses however use the metabolic pathways of the host cell. Antibiotics are designed not affect the host cell, thus viruses are not affected by antibiotics.

6.3.3 Outline the role of skin and mucous membranes in defence against pathogens.

Outer dead layer of skin is impervious and an effective barrier to most organisms unless the surface is broken, cut or deeply scratched. Pathogens are able to degrade the barrier if the skin is kept permanently moist, causing an infection known as athlete's foot.

Internal surfaces of the trachea, bronchi and bronchioles are protected from pathogens by mucous that traps pathogens and cilia that remove the mucous. Mucous is then taken to the throat and swallowed.

6.3.4 Outline how phagocytic leucocytes ingest pathogens in the blood and in body tissues.

Phagocytes destroy pathogens in blood and tissue fluid.

1. Phagocytic leucocytes detect the presence of an antigen, e.g. a bacterium. 2. Bacterium becomes attached to receptor on the plasma membrane of leucocyte. 3. Bacterium is engulfed into a food vacuole in the cytoplasm in a process called phagocytosis. 4. Lysosomes in phagocytic cytoplasm fuse with the vacuole and discharge hydrolytic enzymes (lysozyme and proteases) that kill and digest the pathogen.

6.3.5 Distinguish between antigens and antibodies

Antigens	Antibodies
Foreign substance that enters the body	Antibodies are not recognised as foreign substances
Antigen causes antibody production	Antibody does not cause antibody production because it
Antigens do not contain antigen-binding sites	Antibody contains antigen-binding sites

6.3.6 Explain antibody production.

Antigen is a foreign substance present on the surface of phagocytes.

An antigen in the body triggers an immune response, causing B-lymphocytes to produce antibodies specific to the invading antigen.

T-lymphocyte cells present the antigen to the B-lymphocyte, stimulating it to rapidly divide and form clones causing more antibodies to be produced.
These B-lymphocytes are produced in the bone marrow and are carried in the blood.

antigen causes an immune response to produce antibodies specific for that antigen;antibodies produced in B-lymphocytes;
B-lymphocytes produced in bone marrow and carried in blood;
helper T cell presents antigen to B cell stimulating it produce clones causing more antibodies to be produced.

6.3.7 Outline the effects of HIV on the immune system.

HIV is a retrovirus that infects T-helper cells, resulting in the inability to produce antibodies. This results in a compromised immune system that is more susceptible to disease.

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6.3.8 Discuss the cause, transmission and social implications of AIDS.

The cause of AIDS is the HIV retrovirus which lowers the number of active T-helper cells in the body, resulting in a loss of the ability to produce antibodies and therefore a compromised immune system. Eventually, individuals with HIV will contract several diseases such as Kaposi's sarcoma, resulting in AIDS.

Transmission of HIV occurs via bodily fluids including semen and vaginal fluids that a non-infected person can contract through sexual contact with an infected individual. The sharing of contaminated equipment such as needles and syringes between infected and non-infected people can transmit HIV, which is possible with drug users, tattoos and piercings.

Mothers with HIV/AIDS can infect their children via breastfeeding and cuts and abrasions as well as giving birth to a child that is already infected with HIV.

HIV/AIDS causes prejudices against people living with the virus, resulting in discriminationincluding the loss of social networks, violence, denial of hospital treatment and work and travel restrictions. Due to the social stigma attached with HIV/AIDS, HIV-infected people may feel worthless, depressed and may withdraw from society as a result of the association with HIV contraction and homosexuality, promiscuity, prostitution and intravenous drug use.

Young people living with HIV/AIDS may be unable to work, causing a major reduction in the workforce of a nation, resulting in a decreased economic output.

The loss of family members to AIDS can result in a loss of the primary income person, forcing the other parent or even children to resort to prostitution or other means of gaining income.

Gas exchange

6.4.1 Distinguish between ventilation, gas exchange and cell respiration

Ventilation	Gas exchange	Cell respiration
Air is drawn into and out of the thoracic cavity	Gases are exchanged between an organism and its surroundings	Controlled release of energy from organic compounds in cells to form ATP
Maintains high concentration gradients of oxygen in blood capillaries	Prevents build-up of unwanted gases in a cell	Produces ATP which is needed by the body for metabolism, synthesis & active transport
Occurs via a ventilation system	Occurs via diffusion	Occurs via enzymes and oxygen in aerobic respiration

6.4.2 Explain the need for a ventilation system

A ventilation system pumps fresh air or water to a gas exchange surface, to replace the air there and replenish oxygen in the blood capillaries after it has been uptaken by the cells. This is because when carbon dioxide is released into the blood and oxygen is taken into the cells, concentration gradient decreases due to the increased concentration of carbon dioxide in the blood. Thus carbon dioxide needs to be released from the body, and oxygen is required to be replaced in the blood capillaries in order to maintain high concentration gradients. High concentration gradients provide fast rates of diffusion that are needed during exercise to sustain cell respiration.

6.4.3 Describe the features of alveoli that adapt them to gas exchange.

High density of capillaries surrounding alveoli
Large surface area due to large number of alveoli
Alveoli walls one cell thick
Moist layer covering inner surface of alveoli

ventilation occurs within the lungs;
trachea divides to form two bronchi;
bronchi divide to form bronchioles;
several divisions of bronchioles;
alveoli connected to bronchioles;
trachea/bronchi/bronchioles/airways lined with cilia/ciliated epithelium;
diaphragm and intercostal muscles;
trachea/bronchi have rings/c-shaped pieces of cartilage;
alveolus is an (air) sac;
very small / diameter is (about) 100 μm;
many alveoli so large total surface area;
wall of alveolus is a single layer of cells;
cells in alveolus wall are very thin;
surrounded by a network of capillaries;
some (larger) cells in the wall secrete fluid/surfactant/natural detergent; 8 max
Award any of the above points if clearly drawn in a diagram.

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.

During inhalation, the internal intercostal muscles relax while the external intercostal muscles contract and the rib cage moves upwards and outwards. The radial muscles of the diaphragm also contract to lower the diaphragm. This increases the volume inside the thoracic cavity, decreasing the pressure below the atmospheric pressure, resulting in higher pressure air from the outside being forced into the thoracic cavity down the air pressure gradient.

During exhalation, the external intercostal muscles relax while the internal intercostal muscles contract and the rib cage move downwards and inwards. The radial muscles of the diaphragm relax while the abdominal muscles contract, moving the diaphragm upwards. This decreases the volume inside the thoracic cavity, resulting in pressure increasing above the atmospheric pressure, forcing air out of the lungs down the air pressure gradient.

REPRODUCTION

6.6.2 Outline the role of hormones in the menstrual cycle, including FSH (follicle stimulating hormone), LH (luteinizing hormone), estrogen and progesterone.

FSH causes:

Development of oocyte in follicle

Thickening of follicle wall

Secretion of follicular fluid and estrogen

LH causes:

Completion of meiosis in oocyte

Partial digestion of follicle wall allowing it to burst open (ovulation)

Growth of corpus luteum which secretes estrogen and progesterone

Estrogen causes:

Thickening of endometrium and blood vessel growth in endometrium

Increase in FSH receptors in follicle

Inhibition of FSH secretion and stimulation of LH secretion when estrogen levels are high

PROGESTERONE CAUSES:

Maintenance of thickening of the endometrium with many blood vessels

Inhibition of FSH and LH secretion

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.

Production of sperm.

Promotes growth and activity of male reproductive organs.

Promotes the development of secondary male sexual characteristics, e.g. (facial and chest hair, extra muscles and a deeper voice).

6.6.5 Outline the process of in vitro fertilization (IVF).

In vitro fertilization (IVF) involves the harvesting and fertilization of sperm and collected eggs in a petri dish. IVF can be used in cases where the woman has blocked fallopian tubes and/or the man suffers from impotency, preventing the couple from having a baby.

Drugs given to woman to down-regulate the menstrual cycle and stop the pituitary gland from producing FSH and LH.

FSH injected to stimulate follicle development

HCG injected to cause follicles to mature

Eggs are washed out of ovaries using a micropipette

Semen is collected and injected below a layer of sterile fluid

Swim-up test given whereby healthy sperm are selected that swim up over sterile fluid

Semen mixed with eggs in a petri dish

Incubated at body temperature to allow embryos develop sufficiently for implantation

Petri dish is examined and healthiest embryos are selected

Up to 4 embryos are inserted into uterus of woman using an embryo transfer catheter

Later pregnancy tests are conducted to establish whether the procedure had been successful.

drugs used to down-regulate the menstrual cycle;
FSH injected to stimulate many follicles to develop;
HCG injected to cause the follicles to mature;
eggs are harvested ovaries;
semen sample collected and injected below layer of sterile fluid
semen is processed to concentrate it whereby healthy sperm selected that swim up over sterile fluid
swim-up test given;
semen mixed with eggs in a dish
incubated at 37°C to allow embryos to develop sufficiently
for implantation;
dish examined to choose healthiest embryo;
embryos placed in uterus using a catheter
up to four embryos implanted;
pregnancy test used to see if procedure has been successful;

6.6.6 Discuss the ethical issues associated with IVF.

IVF (in vitro fertilization) is the fertilization of harvested sperm and harvested eggs outside the mother's womb in a laboratory dish.

Infertility can cause deep psychological suffering and IVF can be used by couples when a man suffers from impotency or the woman has blocked fallopian tubes preventing the birth of babies. However, the downside to this is that the infertility of the couple can be passed on to the children, causing them to suffer as well.

After fertilization extra embryos may be present which are either donated to other couples or destroyed. The destruction of embryos is controversial because many hold the view that life begins at conception, likening the destruction of embryos to the killing of a human life. However, at least 60% of embryos conceived naturally die in the first few days of life but, like embryos in IVF, they have no nervous system and do not suffer in any way.

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Topic 6: Human Health & Physiology