Topic 6 Human Physiology

Question 1

Outline the function of the mouth.

6.1

Answer 1

Voluntary control of eating and swallowing. Mechanical digestion of food by chewing and mixing with saliva, which contains lubricants and enzymes that start starch digestion

Skill: Production of an annotated diagram of the digestive system

Question 2

Outline the function of the stomach.

6.1

Answer 2

Churning and mixing with secreted water and acid which kills foreign bacteria and other pathogens in food, plus initial stages of protein digestion.

Skill: Production of an annotated diagram of the digestive system

Question 3

Outline the function of the small intestine.

6.1

Answer 3

Final stages of digestion of lipids, carbohydrates, proteins and nucleic acids, neutralizing stomach acid, plus absorption of nutrients

Skill: Production of an annotated diagram of the digestive system

Question 4

Outline the function of the pancreas.

6.1

Answer 4

Secretion of lipase, amylase and protease

Skill: Production of an annotated diagram of the digestive system

Question 5

Outline the function of the liver.

6.1

Answer 5

Secretion of surfactants in bile to break up lipid droplets

Skill: Production of an annotated diagram of the digestive system

Question 6

Outline the function of the gall bladder.

6.1

Answer 6

Storage and regulated release of bile

Skill: Production of an annotated diagram of the digestive system

Question 7

Outline the function of the large intestine.

6.1

Answer 7

Re-absorption of water, further digestion especially of carbohydrates by symbiotic bacteria, plus formation and storage of feces

Skill: Production of an annotated diagram of the digestive system

Question 8

List the tissue layers in the small intestine.

6.1

Answer 8

• serosa: an outer coat
• muscle layers: longitudinal muscle and inside it circular muscle
• sub-mucosa: a tissue layer containing blood and lymph vessels
• mucosa: the lining of the small intestine, with the epithelium that absorbs nutrients on its inner surface.

Skill: Identifcation of tissue layers in transverse sections of the small intestine viewed with a microscope or in a micrograph.

Question 9

Outline the process of food moving along the esophagus.

6.1

Answer 9

• Waves of muscle contraction, called peristalsis, pass along the intestine.
• Contraction of circular muscles behind the food constricts the gut to prevent it from being pushed back towards the mouth.
• Contraction of longitudinal muscle where the food is located moves it on along the gut.

Understanding: The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.

Question 10

Outline the main function of peristalsis

6.1

Answer 10

• The main function of peristalsis in the intestine is churning of the semi-digested food to mix it with enzymes and thus speed up the process of digestion.

Understanding: The contraction of circular and longitudinal muscle of the small intestine mixes the food with enzymes and moves it along the gut.

Question 11

Outline the enzymes that the pancreas secretes into the lumen of the small intestine.

6.1

Answer 11

Pancreatic juice contains enzymes that digest all the three main types of macromolecule found in ood:
* amylase to digest starch
* lipases to digest triglycerides, phospholipids
* proteases to digest proteins and peptides.

Understanding: The pancreas secretes enzymes into the lumen of the
small intestine.

Question 12

Outline the hydrolysis reactions carried out by the enzymes secreted by pancreas into the lumen of the small intestine.

6.1

Answer 12

• starch is digested to maltose by amylase
• triglycerides are digested to fatty acids and glycerol or fatty
  acids and monoglycerides by lipase
• phospholipids are digested to fatty acids, glycerol and phosphate by phospholipase
• proteins and polypeptides are digested to shorter peptides by protease.

Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine

Question 13

Outline the hydrolysis reactions carried out by the enzymes secreted by pancreas into the lumen of the small intestine.

6.1

Answer 13

• starch is digested to maltose by amylase
• triglycerides are digested to fatty acids and glycerol or fatty
  acids and monoglycerides by lipase
• phospholipids are digested to fatty acids, glycerol and phosphate by phospholipase
• proteins and polypeptides are digested to shorter peptides by protease.

Understanding: Enzymes digest most macromolecules in food into monomers in the small intestine

Question 14

Outline the features of villi.

6.1

Answer 14

Intestinal villi contain several key features which facilitate the absorption of digestive products:
* Microvilli – Ruffling of epithelial membrane further increases surface area
* Rich blood supply – Dense capillary network rapidly transports absorbed products
* Single layer epithelium – Minimises diffusion distance between lumen and blood
* Lacteals – Absorbs lipids from the intestine into the lymphatic system

Understanding: Villi increase the surface area of epithelium over which absorption is carried out

Question 15

List and outline the substances absorbed by the villi.

6.1

Answer 15

Villus cells absorb these products of digestion of macromolecules in food:
* glucose, fructose, galactose and other monosaccharides
* any of the twenty amino acids used to make proteins
* fatty acids, monoglycerides and glycerol
* bases from digestion of nucleotides.

They also absorb substances required by the body and present in foods but not needing digestion:
* mineral ions such as calcium, potassium and sodium
* vitamins such as ascorbic acid (vitamin C).

Understanding: Villi absorb monomers formed by digestion as well as mineral ions and vitamins

Question 16

Outline facilitated difusion as a method of membrane transport required to absorb different nutrients.

6.1

Answer 16

nutrients pass down the concentration gradient
through specific channel proteins in the membrane.

ex. hydrophilic nutrients such as fructose.

Understanding: Different methods of membrane transport are required to absorb different nutrients

Question 17

Outline simple difusion as a method of membrane transport required to absorb different nutrients.

6.1

Answer 17

nutrients pass down the concentration gradient between phospholipids in the membrane.

ex. hydrophobic nutrients such as fatty acids and monoglycerides

Understanding: Different methods of membrane transport are required to absorb different nutrients

Question 18

Outline active transport as a method of membrane transport required to absorb different nutrients.

6.1

Answer 18

nutrients are pumped through the membrane against
the concentration gradient by specific pump proteins.

ex. mineral ions such as sodium, calcium and iron.

Understanding: Different methods of membrane transport are required to absorb different nutrients

Question 19

Outline endocytosis (pinocytosis) as a method of membrane transport required to absorb different nutrients.

6.1

Answer 19

small droplets of the fluid are passed through the membrane by means of vesicles.

ex. triglycerides and cholesterol in lipoprotein particles.

Understanding: Different methods of membrane transport are required to absorb different nutrients

Question 20

Outline the sodium co-transporter protein to transport glucose.

6.1

Answer 20

1. sodium co-transporter proteins move a molecule of glucose together with a sodium ion across the membrane together into the epithelium cells.
2. The glucose can be moved against its concentration gradient because the sodium ion is moving down its concentration gradient.
3. The sodium gradient is generated by active transport of sodium out of the epithelium cell by a pump protein.

Understanding: Different methods of membrane transport are required to absorb different nutrients

Question 21

Outline the processes occuring in the small intestine that result in the digestion of starch.

6.1

Answer 21

• The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
• Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins
• Both maltose and dextrin are digested by enzymes (maltase) which are fixed to the epithelial lining of the small intestine
• The hydrolysis of maltose / dextrin results in the formation of glucose monomers

Application: Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the liver

Question 22

Outline the processes occuring in the small intestine that result in the digestion of starch.

6.1

Answer 22

• The digestion of starch is initiated by salivary amylase in the mouth and continued by pancreatic amylase in the intestines
• Amylase digests amylose into maltose subunits (disaccharide) and digests amylopectin into branched chains called dextrins
• Both maltose and dextrin are digested by enzymes (maltase) which are fixed to the epithelial lining of the small intestine
• The hydrolysis of maltose / dextrin results in the formation of glucose monomers

Application: Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the liver

Question 23

Outline howdigested glucose is absorbed and then transported to various body tissues.

6.1

Answer 23

1. Glucose is co-transported with sodium ions into the epithelial cells (of the villus).
2. Glucose moves by facilitated diffusion into the lumen of the villus.
3. Glucose then diffuses a short distance into the adjacent capillaries where it dissolves into the blood plasma.
4. Blood in the capillaries moves to to venules then to the hepatic portal vein which transports the glucose to the liver.
5. The liver absorbs excess glucose which it converts to glycogen for storage.

Application: Processes occurring in the small intestine that result in the digestion of starch and transport of the products of digestion to the liver

Question 24

Outline the results of a dialysis tubing experiment

6.1

Answer 24

Trial with starch and amylase
* Since amylase breaks down starch into monomers that can diffuse:
* Iodine test in the tube: dark yellow since some starch may not be fully digested
* Iodine test outside the tube: Yellow, no starch is present
* Benedict’s test: Should turn red/orange where maltose is present

**Trial with starch only: **
* Iodine test in the tube: Blue/black precipitate shows starch is present
* Iodine test outside the tube: Yellow, no starch is present
* Benedict’s test: Blue inside and outside the tube (no maltose present)

Application: Use of dialysis tubing to model absorption of digested food in
the intestine.

Question 25

Outline the Iodine test and Benedict’s test

6.1

Answer 25

Iodine test:
* turns black/blue in the presence of starch

Benedict’s test:
* Turns orange/red in the presence of maltose/reducing sugar

Application: Use of dialysis tubing to model absorption of digested food in
the intestine.

Question 26

Outline William Harvey’s discovery.

6.2

Answer 26

• Harvey demonstrated that blood flow through
  the larger vessels is unidirectional, with valves
  to prevent backfow.
• Arteries pumped blood from the heart (to the lungs and body tissues)
• Veins returned blood to the heart (from the lungs and body tissues)

Application: William Harvey’s discovery of the circulation of blood with the heart acting as a pump

Question 27

Outline the function of arteries.

6.2

Answer 27

The function of arteries is to convey blood at high pressure from the heart ventricles to the tissues of the body and lungs

Understanding: Arteries convey blood at high pressure from the ventricles to the tissues of the body

Question 28

Outline the structure of arteries.

6.2

Answer 28

• They have a narrow lumen to maintain a high blood pressure
• They have a thick wall containing an outer layer of collagen to prevent the artery from rupturing under the high pressure
• The arterial wall also contains an inner layer of muscle and elastic fibres to help maintain pulse flow (it can contract in vasoconstriction and stretch in vasodilation)

Understanding: Arteries convey blood at high pressure from the ventricles to the tissues of the body

Question 29

Outline the layers in the artery walls.

6.2

Answer 29

The wall of the artery is composed of several layers:
* tunica externa: a tough outer layer of connective tissue
* tunica media: a thick layer containing smooth muscle and elastic fibres made of the protein elastin
* tunica intima: a smooth endothelium forming the lining of the artery.

Understanding: Arteries have muscle cells and elastic fibres in their walls

Question 30

Outline the function and structure of capillaries.

6.2

Answer 30

• The function of capillaries is to exchange materials between the cells in tissues and blood travelling at low pressure
• Wall consists of a single layer of thin cells so the distance
  for diffusion in or out is small
• Massive number of capillaries: large surface area available for the exchange of substances

Understanding: Capillaries have permeable walls that allow exchange of material between cells in tissues and blood in capillaries

Question 31

Outline the structure and function of veins.

6.2

Answer 31

• Veins transport blood from capillary networks back to the atria of the heart.
• By now the blood is at much lower pressure than it was in the arteries.
• Veins do not therefore need to have as thick a wall as arteries and the wall contains far fewer muscle and elastic fibres.
• They can therefore dilate to become much wider and thus hold more blood than arteries.

Understanding: Veins collect blood at low pressure from the tissues of the body and return it to the atria of the heart

Question 32

Outline the function of valves in veins.

6.2

Answer 32

• Blood pressure in veins is sometimes so low that there is a danger of backfow towards the capillaries and insuffcient return of blood to the heart.

To maintain circulation, veins contain pocket valves, consisting of three cup-shaped flaps of tissue.
* If blood starts to fow backwards, it gets caught in the flaps of the pocket valve, which fill with blood, blocking the lumen of the vein.
* When blood flows towards the heart, it pushes the flaps to the sides of the vein. The pocket valve therefore opens and blood can flow freely.

Understanding: Valves in veins and the heart ensure circulation of blood by preventing backflow

Question 33

List the features for identification of blood vessels.

6.2

Answer 33

Artery:
* Larger than 10 µm
* Relatively thick wall and narrow lumen
* Three layers, tunica externa, media and intima.
* A lot of muscle and elastic fibres in the wall

Capillaries:
* Around 10 µm
* Extremely thin wall
* Only one layer (the tunica intima)
* No valves or muscle/elastic fibres

Vein:
* Much larger than 10 µm
* Relatively thin wall with variable but often wide lumen
* Three layers: tunica externa, media and intima
* Small amounts of muscle/elastic fibres
* Has valves

Skill: Recognition of the chambers and valves of the heart and the blood
vessels connected to it in dissected hearts or in diagrams of heart structure.

Question 34

Outline the process of double circulation.

6.2

Answer 34

Humans therefore have two separate circulations:
* the pulmonary circulation, to and from the lungs
* the systemic circulation, to and from all other organs, including the heart muscles.

• The pulmonary circulation receives deoxygenated blood that has returned from the systemic circulation
• the systemic circulation receives blood that has been oxygenated by the pulmonary circulation.

Understanding: There is a separate circulation for the lungs.

Question 35

Outline the chambers and valves of the heart and its blood vessels connected to it.

6.2

Answer 35

Chambers
* Two atria (singular = atrium) – smaller chambers near top of heart that collect blood from body and lungs
* Two ventricles – larger chambers near bottom of heart that pump blood to body and lungs

Heart Valves
* Atrioventricular valves (between atria and ventricles) – bicuspid valve on left side ; tricuspid valve on right side
* Semilunar valves (between ventricles and arteries) – aortic valve on left side ; pulmonary valve on right side

Blood Vessels
* Vena cava (inferior and superior) feeds into the right atrium and returns deoxygenated blood from the body
* Pulmonary artery connects to the right ventricle and sends deoxygenated blood to the lungs
* Pulmonary vein feeds into the left atrium and returns oxygenated blood from the lungs
* Aorta extends from the left ventricle and sends oxygenated blood around the body

Skill: Recognition of the chambers and valves of the heart and the blood
vessels connected to it in dissected hearts or in diagrams of heart structure.

Question 36

Outline Atherosclerosis.

6.2

Answer 36

• Low density lipoproteins (LDL) containing fats and cholesterol accumulate and phagocytes are then attracted by signalsfrom endothelium cells and smooth muscle.
• The phagocytes engulf the fats and cholesterol by endocytosis and grow very large.
• Smooth muscle cells migrate to form a tough cap over the atheroma.
• The artery wall bulges into the lumen narrowing it and thus impeding blood flow.
• atherosclerosis becomes much more advanced but often goes unnoticed until a major artery becomes so blocked that the tissues it supplies become compromised.

Applications: Causes and consequences of occlusion of the coronary arteries.

Question 37

Outline the cause and consequences of occulusion of the coronary arteries.

6.2

Answer 37

Coronary occlusion is a narrowing of the arteries that supply blood containing oxygen and nutrients to the heart muscle.

Causes:
* Age – Blood vessels become less flexible with advancing age
* Genetics – Having hypertension predispose individuals to developing CHD
* Obesity – Being overweight places an additional strain on the heart
* Diseases – Certain diseases increase the risk of CHD (e.g. diabetes)
* Diet – Diets rich in saturated fats, salts and alcohol increases the risk
* Exercise – Sedentary lifestyles increase the risk of developing CHD
* Sex – Males are at a greater risk due to lower oestrogen levels
* Smoking – Nicotine causes vasoconstriction, raising blood pressure

Consequences
* Lack of oxygen (anoxia) causes pain, known as angina, and impairs the muscle’s ability to contract.
* The fibrous cap covering atheromas sometimes ruptures, which stimulates the formation of blood clots that can block arteries supplying blood to the heart and cause acute heart problems.

Applications: Causes and consequences of occlusion of the coronary arteries.

Question 38

Define “myogenic”.

6.2

Answer 38

The heart is unique in the body as its muscles can contract without stimulation from motor neurons. The contraction is called myogenic, meaning that it is generated in the muscle itself.

Understanding: The heartbeat is initiated by a group o specialized muscle cells in the right atrium called the sinoatrial node.

Question 39

State where the sinoatrial node is located.

6.2

Answer 39

Within the wall of the right atrium are a specialised cluster of cardiomyocytes which direct the contraction of heart muscle tissue.

This cluster of cells are collectively called the sinoatrial node (SA node or SAN)

Understanding: The heartbeat is initiated by a group o specialized muscle cells in the right atrium called the sinoatrial node.

Question 40

Outline how the heartbeat is initiated.

6.2

Answer 40

Because the sinoatrial node initiates each heartbeat, it sets the pace for the beating of the heart and is often called the pacemaker.
* If it becomes defective, its output may be regulated or even replaced entirely by an artifcial pacemaker.
* This is an electronic device, placed under the skin with electrodes implanted in the wall of the heart that initiate each heartbeat in place of the sinoatrial node.

Understanding: The sinoatrial node acts as a pacemaker

Question 41

Outline how the sinoatrial node stimulates contraction of the atria and ventricular walls.

6.2

Answer 41

• The sinoatrial node initiates a heartbeat by contracting and simultaneously sends out an electrical signal that spreads throughout the walls of the atria.
• This can happen because there are interconnections between adjacent fbres across which the electrical signal can be propagated.
• Also the fbres are branched so each fbre passes the signal on to several others. It takes less than a tenth of a second for all cells in the atria to receive the signal.
• This propagation of the electrical signal causes the whole of both left and right atria to contract.
• After a time delay o about 0.1 seconds, the electrical signal is conveyed to the ventricles. The time delay allows time for the atria to pump the blood that they are holding into the ventricles.
• The signal is then propagated throughout the walls o the ventricles, stimulating them to contract and pump blood out into the arteries.

Understanding: The sinoatrial node sends out an electrical signal that stimulates contraction as it is propagated through the walls of the atria and then the walls of the ventricles

Question 42

Outline the cardiac cycle.

6.2

Answer 42

0.0 - 0.1 seconds
* The atria contract
* rapid but small pressure increase, which pumps blood from the atria to the ventricles, through the open atrioventricular valves.
* The semilunar valves are closed and blood pressure in the arteries gradually drops to its minimum as blood continues to fow along them but no more is pumped in.

0.1 - 0.15 seconds
* The ventricles contract
* rapid pressure build up that causes the atrioventricular valves to close.
* The semilunar valves remain closed.

0.15 - 0.4 seconds
* The pressure in the ventricles rises above the pressure in the arteries
* semilunar valves open and blood is pumped from the ventricles into the arteries (maximizing the arterial blood pressure)
* Pressure slowly rises in the atria as blood drains into them from the veins and they fill.

0.4 - 0.45 seconds
* The contraction of the ventricular muscles decreases
* pressure inside the ventricles rapidly drops below the pressure in the arteries
* semilunar valves close
* The atrioventricular valves remain closed

0.45 - 0.8 seconds
* Pressure in the ventricles drops below the pressure in the atria so the atrioventricular valves open.
* Blood from the veins drains into the atria and from there into the ventricles, causing a slow increase in pressure.

Application: Pressure changes in the left atrium, left ventricle and aorta during the cardiac cycle

Question 43

Outline how the medulla can increase and decrease heart rate.

6.2

Answer 43

• The sinoatrial node that sets the rhythm for the beating of the heart responds to signals from outside the heart.
• These include signals from branches of two nerves originating in a region in the medulla of the brain called the cardiovascular centre.
• The cardiovascular centre receives inputs from receptors that monitor blood pressure and its pH and oxygen concentration. The pH of the blood refects its carbon dioxide concentration.
• Low blood pressure, low oxygen concentration and low pH: heart rate needs to speed up, to increase the fow rate of blood to the tissues, deliver more oxygen and remove more carbon dioxide.
• High blood pressure, high oxygen concentration and high pH: the heart rate may need to slow down.

Understanding: The heart rate can be increased or decreased by impulses brought to the heart through two nerves from the medulla of the brain

Question 44

Outline the hormone epinephrine.

6.2

Answer 44

• The sinoatrial node also responds to epinephrine in the blood, by increasing the heart rate.
• This hormone is also sometimes called adrenalin and is produced by the adrenal glands.
• The secretion of epinephrine is controlled by the brain and rises when vigorous physical activity may be necessary because of a threat or opportunity.

Understanding: Epinephrine increases the heart rate to prepare for
vigorous physical activity.

Question 45

Define “pathogen”.

6.3

Answer 45

Microbes that cause disease are called pathogens.

Understanding: The skin and mucous membranes form a primary defence
against pathogens that cause infectious disease.

Question 46

Outline the human body’s primary defence.

6.3

Answer 46

• The primary defence of the body against pathogens is the skin. Its outermost layer is tough and provides a physical barrier against the entry of pathogens and protection against physical and chemical damage.
• Sebaceous glands are associated with hair follicles and they secrete a chemical called sebum, which maintains skin moisture and slightly lowers skin pH. The lower pH inhibits the growth of bacteria and fungi.
• Mucous membranes are a thinner and softer type of skin that is found in areas such as the nasal passages and other airways, the head of the penis and foreskin and the vagina. The mucus that these areas of skin secrete is a sticky solution of glycoproteins. Mucus acts as a physical barrier; pathogens and harmful particles are trapped in it and either swallowed or expelled. It also has antiseptic properties because of the presence of the anti-bacterial enzyme lysozyme.

Understanding: The skin and mucous membranes form a primary defence
against pathogens that cause infectious disease.

Question 47

Outline how cuts in the skin are sealed.

6.3

Answer 47

• When the skin is cut, blood vessels in it are severed and start to bleed.
• The bleeding usually stops after a short time because of a process called clotting.
• The blood emerging from a cut changes from being a liquid to a semi-solid gel. This seals up the wound and prevents further loss of blood and blood pressure.
• Clotting is also important because cuts breach the barrier to infection provided by the skin. Clots prevent entry of pathogens until new tissue has grown to heal the cut.

Understanding: Cuts in the skin are sealed by blood clotting.

Question 48

Outline the process of blood clotting.

6.3

Answer 48

Blood clotting involves a cascade of reactions, each of which produces a catalyst for the next reaction.
The process of clotting only occurs if platelets release clotting factors. Platelets are cellular fragments that circulate in the blood.
1. When a cut or other injury involving damage to blood vessels occurs, platelets aggregate at the site forming a temporary plug. They then release the clotting factors that trigger off the clotting process.
2. The cascade of reactions that occurs after the release of clotting factors from platelets quickly results in the production of an enzyme called thrombin.
3. Thrombin converts the soluble protein fbrinogen into the insoluble fibrin.
4. The fibrin forms a mesh in cuts that traps more platelets and also blood cells.
5. The resulting clot is initially a gel, but if exposed to the air it dries to form a hard scab.

Understanding: Clotting factors are released from platelets.

Question 49

Define “Coronary thrombosis”.

6.3

Answer 49

Coronary thrombosis is the formation of blood clots in the coronary arteries.

Application: Causes and consequences of blood clot formation in
coronary arteries.

Question 50

Outline the causes and consequences of coronary thrombosis.

6.3

Answer 50

Consequences:
* part of the heart is deprived of oxygen and nutrients
* Cardiac muscle cells are then unable to produce suffcient ATP by aerobic respiration
* contractions become irregular and uncoordinated and do not pump blood effectively (heart attacks and fibrillation).

Causes:
* Coronary occlusion, damage to the capillary epithelium, hardening of arteries and rupture of atheroma all increase the risk of coronary thrombosis.

Application: Causes and consequences of blood clot formation in
coronary arteries.

Question 51

Outline the function of phagocytes in the human body’s second line of defence.

6.3

Answer 51

• Phagocytic leukocytes circulate in the blood and move in response to infection
• Phagocytesingest pathogens by endocytosis.
• The pathogens are then killed and digested inside the cell by enzymes from lysosomes.

Understanding: Ingestion of pathogens by phagocytic white blood cells
gives non-specifc immunity to diseases.

Question 52

Define “antigen”.

6.3

Answer 52

Proteins on the surface of pathogens are recognized as foreign by the body and they stimulate a specifc immune response. Any chemical that stimulates an immune response is referred to as an antigen.

Understanding: Production of antibodies by lymphocytes in response to
particular pathogens gives specifc immunity.

Question 53

Distinguish between antigen and antibodies.

6.3

Answer 53

Antigen: An antigen is a substance that the body recognises as foreign and that will elicit an immune response

Antibody: An antibody is a protein produced by B lymphocytes that is specific to a given antigen

Understanding: Production of antibodies by lymphocytes in response to
particular pathogens gives specifc immunity.

Question 54

Outline the production of antibodies.

6.3

Answer 54

1. Antibodies are made by lymphocytes, one of the two main types of white blood cell. Antigens are foreign substances that stimulate the production ofantibodies.
2. A lymphocyte can only make one type of antibody so a huge number of different lymphocyte types is needed. Each lymphocyte puts some of the antibody that it can make into its cell surface membrane with the antigen combining site projecting outwards.
3. When a pathogen enters the body, its antigens bind to the antibodies in the cell surface membrane of one type of lymphocyte.
4. When antigens bind to the antibodies on the surface of a lymphocyte, this lymphocyte becomes active and divides by mitosis to produce a clone of many identical cells.
5. The cells produced by mitosis are plasma cells. They produce large quantities of the same antibody. The antibody binds to the antigens on the surface of the pathogen and stimulates its destruction.

Understanding: Production of antibodies by lymphocytes in response to
particular pathogens gives specifc immunity.

Question 55

Outline specific immunity.

6.3

Answer 55

• After an infection has been cleared from the body, **most of the lymphocytes used to produce the antibodies disappear, but some persist as memory cells. **
• These memory cells can quickly reproduce to form a clone of plasma cells if a pathogen carrying the same antigen is re-encountered.
• Immunity to an infectious disease involves either having antibodies against the pathogen, or memory cells that allow rapid production of the antibody.

Understanding: Production of antibodies by lymphocytes in response to
particular pathogens gives specifc immunity.

Question 56

List the methods of HIV transmission.

6.3

Answer 56

• sexual intercourse
• transfusion of infected blood
• sharing of hypodermic needles by intravenous drug users.

Application: Efects of HIV on the immune system and methods of transmission.

Question 57

List the methods of HIV transmission.

6.3

Answer 57

• sexual intercourse
• transfusion of infected blood
• sharing of hypodermic needles by intravenous drug users.

Application: Efects of HIV on the immune system and methods of transmission.

Question 58

Outline the effects of HIV.

6.3

Answer 58

• The human immunodefciency virus (HIV) invades and destroys helper T-cells.
• The consequence is a progressive loss of the capacity to produce antibodies. In the early stages of infection, the immune system makes antibodie’s against HIV.
• In most HIV-positive patients antibody production eventually becomes very ineffective (cannot easily fight defend itself)
• When the syndrome of conditions due to HIV is present, the person is said to have acquired immune defciency syndrome (AIDS). AIDS spreads by HIV infection.
• A reduction in the number of active lymphocytes and a loss of the ability to produce antibodies, leading to the development of AIDS.

Application: Efects of HIV on the immune system and methods of transmission.

Question 59

Outline the effect of antibiotics.

6.3

Answer 59

• An antibiotic is a chemical that inhibits the growth of microorganisms. Most antibiotics are antibacterial.
• They block processes that occur in prokaryotes but not in eukaryotes and can therefore be used to kill bacteria inside the body without causing harm to human cells.
• The processes targeted by antibiotics are bacterial DNA replication, transcription, translation, ribosome function and cell wall formation.

Understanding: Antibiotics block processes that occur in prokaryotic cells
but not in eukaryotic cells.

Question 60

Outline Florey and Chain’s experiments.

6.3

Answer 60

• Howard Florey and Ernst Chain investigated the use of chemical substances to control bacterial infections.
• The most promising of these was penicillin, discovered by Alexander Fleming in 1928.
• Florey and Chain’s team developed a method of growing the fungus Penicillium in liquid culture in conditions that stimulated it to secrete penicillin.

Application: Florey and Chain’s experiments to test penicillin on
bacterial infections in mice.

Question 61

Explain why antibiotics is ineffective against viruses.

6.3

Answer 61

• Viruses are non-living and can only reproduce when they are inside living cells.
• They use the chemical processes of a living host cell, instead of having a metabolism of their own.
• All of the commonly used antibiotics are not eective against viruses.

Understanding: Viral diseases cannot be treated using antibiotics because
they lack a metabolism.

Question 62

Explain how antibiotic resistance in bacteria can be avoided.

6.3

Answer 62

Antibiotic resistance is an avoidable problem. These measures are required:
* doctors prescribing antibiotics only for serious bacterial infections
* patients completing courses of antibiotics to eliminate infections completely
* hospital stalls maintaining high standards of hygiene to prevent cross- infection
* farmers not using antibiotics in animal feeds to stimulate growth
* pharmaceutical companies developing new types of antibiotics

Understanding: Some strains of bacteria have evolved with genes which
confer resistance to antibiotics and some strains of
bacteria have multiple resistance.

Question 63

Define ventilation, gas exchange and cell respiration

6.4

Answer 63

• Ventilation: The exchange of air between the atmosphere and the lungs – achieved by the physical act of breathing
• Gas Exchange: The exchange of oxygen and carbon dioxide between the alveoli and bloodstream (via passive diffusion)
• Cell Respiration: The release of energy (ATP) from organic molecules – it is enhanced by the presence of oxygen (aerobic)

Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood fowing in adjacent capillaries.

Question 64

Explain the importance of gas exchange and ventilation.

6.4

Answer 64

• This process of swapping one gas for another is called gas exchange.
• It happens by difusion in the alveoli of human lungs, so it depends on concentration gradients of oxygen and carbon dioxide between the air in the alveoli and blood flowing in the adjacent capillaries.
• To maintain these concentration gradients, the air in the alveoli must be refreshed frequently.
• The process of bringing fresh air to the alveoli and removing stale air is called ventilation

Understanding: Ventilation maintains concentration gradients of oxygen and carbon dioxide between air in alveoli and blood fowing in adjacent capillaries.

Question 65

Outline the function and structure of Type I pneumocytes.

6.4

Answer 65

• Extremely thin and permeable alveolar cells that are adapted to carry out gas exchange.
• Most of the wall of the alveolus consists of a single layer of these thin cells.
• Gases only have to diffuse a very short distance to pass through them.

Understand: Type I pneumocytes are extremely thin alveolar cells that
are adapted to carry out gas exchange.

Question 66

Outline the function and structure of Type II pneumocytes.

6.4

Answer 66

• Cells in the alveolus wall that secrete a fluid to keep the inner surface of the alveolus moist and allow gases to dissolve.
• The fluid also contains a natural detergent (surfactant) , to prevent the sides ofthe alveoli from sticking together by reducing surface tension.

Understanding: Type II pneumocytes secrete a solution containing surfactant that creates a moist surface inside the alveoli to prevent the sides of the alveolus adhering to each other by reducing surface tension.

Question 67

Outline how air travels to the alveoli for gas exchange.

6.4

Answer 67

1. Air enters through the nose or mouth
2. passes down the trachea
3. The trachea divides to form two bronchi (one bronchus leads to each lung)
4. Inside the lungs the bronchi divide repeatedly to form a tree-like structure of narrower airways, called bronchioles.
5. At the end of the narrowest bronchioles are groups of alveoli, where gas exchange occurs.

Understanding: Air is carried to the lungs in the trachea and bronchi and then to the alveoli in bronchioles.

Question 68

Explain how air moves in and out of the lungs due to muscle contractions.

6.4

Answer 68

• Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.
• Gases will move from a region of high pressure to a region of lower pressure (similar to movement via concentration gradient)
• When the more volume and low pressure in chest, air will move into the lungs (inspiration)
• When the low volume and high pressure in chest, air will move out of the lungs (expiration)

Understanding: Muscle contractions cause the pressure changes inside the thorax that force air in and out of the lungs to ventilate them.

Question 69

Explain why different muscles are required for inspiration and expiration.

6.4

Answer 69

• Muscles can be in two states: contracting and relaxing.
• Muscles therefore can only cause movement in one direction.
• If movement in opposite directions is needed at different times, at least two muscles will be required.
• Inspiration and expiration involve opposite movements, so different muscles are required, working as antagonistic pairs.

Understanding: Different muscles are required for inspiration and expiration because muscles only do work when they contract.

Question 70

Outline examples of antagonistic muscle action.

6.4

Answer 70

**Inhaling: **
* The external intercostal muscles contract, moving the ribcage up and out
* The diaphragm contracts, becoming flatter and moving down
* These muscle movements increase the volume of the thorax
* Air flows into the lungs from outside the body until the pressure inside the lungs rises to atmospheric pressure

Exhaling:
* The internal intercostal muscles contract, moving the ribcage down and in
* The abdominal muscles contract, pushing the diaphragm up into a dome shape
* These muscle movements decrease the volume of the thorax
* Air flows out from the lungs to outside the body until the pressure inside the lungs falls to atmospheric pressure

Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

Question 71

Outline examples of antagonistic muscle action.

6.4

Answer 71

**Inhaling: **
* The external intercostal muscles contract, moving the ribcage up and out
* The diaphragm contracts, becoming flatter and moving down
* These muscle movements increase the volume of the thorax
* Air flows into the lungs from outside the body until the pressure inside the lungs rises to atmospheric pressure

Exhaling:
* The internal intercostal muscles contract, moving the ribcage down and in
* The abdominal muscles contract, pushing the diaphragm up into a dome shape
* These muscle movements decrease the volume of the thorax
* Air flows out from the lungs to outside the body until the pressure inside the lungs falls to atmospheric pressure

Application: External and internal intercostal muscles, and diaphragm and abdominal muscles as examples of antagonistic muscle action.

Question 72

List the causes and consequences of lung cancer.

6.4

Answer 72

• Lung cancer describes the uncontrolled proliferation of lung cells, leading to the abnormal growth of lung tissue (tumour)

Consequnces:
* death
* tumors
* coughing up blood
* wheezing
* respiratory distress
* weight loss

Causes:
* smoking
* asbestos
* air pollution
* certain infections and genetic predispositions

Application: Causes and consequences of lung cancer.

Question 73

Outline the cause of Emphysema.

6.4

Answer 73

• The main causes of emphysema are smoking and air
  pollution.
• Cilia that line the airways and expel mucus are damaged and cease to function, so mucus builds up in the lungs, causing infections.
• Toxins in cigarette smoke and polluted aircause inflammation and damage to the white blood cells that fight infections in the lungs.
• A protease (trypsin) is released from inflamed cells and damaged white blood cells.
• This enzyme digests elastic fibres in the lungs and eventually causes complete breakdown of alveolus walls.

Application: Causes and consequences of emphysema.

Question 74

Outline the consequences Emphysema.

6.4

Answer 74

• Emphysema is a chronic and progressive disease with serious
  consequences.
• The surface area for gas exchange reduces so the oxygen saturation of the blood falls and exercise is more and more difficult.
• The lungs lose their elasticity, making it increasingly difcult to exhale (shortness of breath).
• Mucus in the lungs causes coughing and wheezing.

Application: Causes and consequences of emphysema.

Question 75

Distingush between the two systems of the body are used for internal communication.

6.5

Answer 75

• Two systems of the body are used for internal communication: the endocrine system and the nervous system.
• The endocrine system consists of glands that release hormones.
• The nervous system consists of nerve cells called neurons.

Understanding: Neurons transmit electrical impulses.

Question 76

Define “neuron”.

6.5

Answer 76

• Neurons help with internal communication by transmitting nerve impulses.
• A nerve impulse is an electrical signal.

Understanding: Neurons transmit electrical impulses.

Question 77

Outline the structures of the neuron.

6.5

Answer 77

• Neurons have a cell body with cytoplasm and a nucleus
• Dendrites are short branched nerve fibres
• Axons are very elongated nerve fibres

Understanding: Neurons transmit electrical impulses.

Question 78

Define “myelin”.

6.5

Answer 78

• Some nerve fbres are coated along most of their length by a material called myelin.
• It consists of many layers of phospholipid bilayer.
• Special cells called Schwann cells deposit the myelin by growing round and round the nerve fibre.

Understanding: The myelination of nerve fbres allows for saltatory
conduction.

Question 79

Outline saltatory conduction.

6.5

Answer 79

• There is a gap between the myelin deposited by adjacent Schwann cells.
• The gap is called a node of Ranvier.
• In myelinated nerve fibres the nerve impulse can jump from one node of Ranvier to the next. This is called saltatory conduction.
• It is much quicker than continuous transmission along a nerve fibre so myelinated nerve fibres transmit nerve impulses much more rapidly than unmyelinated nerve fibres.

Understanding: The myelination of nerve fbres allows for saltatory
conduction.

Question 80

Define “resting potential”.

6.5

Answer 80

• A neuron that is not transmitting a signal has a potential difference or voltage across its membrane that is called the resting potential.
• This potential is due to an imbalance of positive and negative charges across the membrane.

Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential

Question 81

Outline how neurons pump sodium and potassium ions across their membranes to generate a resting potential.

6.5

Answer 81

• Sodium potassium pumps transfer sodium (Na+) and potassium (K+) ions across the membrane. 3 Na+ ions are pumped out and 2 K+ ions are pumped in.
• Also the membrane is about 50 times more permeable to K ions than Na ions, so K+ ions leak back across the membrane faster than Na+ ions.
• These factors together give the neuron a resting membrane potential of about -70 mV.

Understanding: Neurons pump sodium and potassium ions across their membranes to generate a resting potential

Question 82

Outline the two phases of action potential.

6.5

Answer 82

• An action potential is a rapid change in membrane potential, consisting of two phases:
• depolarization: a change from negative to positive
• repolarization: a change back from positive to negative.

Understanding: An action potential consists of depolarization and
repolarization of the neuron.

Question 83

Explain how an action potential occurs.

6.5

Answer 83

• An action potential is the depolarization and repolarization
  of a neuron, due to facilitated difusion of ions across of the membrane through voltage-gated ion channels.
• If the potential across the membrane rises from -70 to -50 mV, voltage-gated sodium channels open and sodium ions difuse in down the concentration gradient.

Understanding: An action potential consists of depolarization and
repolarization of the neuron.

Question 84

Outline the process of depolarization.

6.5

Answer 84

• Depolarization is due to the opening of sodium channels in the membrane, allowing Na+ ions to diffuse into the neuron down the concentration gradient.
• The entry of Na+ ions reverses the charge imbalance across the membrane
• The inside of the neuron to develops a net positive charge compared to the outside
• This raises the membrane potential to a positive value of about +30 mV.

Understanding: An action potential consists of depolarization and
repolarization of the neuron.

Question 85

Outline the process of repolarization.

6.5

Answer 85

• Repolarization happens rapidly after depolarization and is due to the closing of the sodium channels and opening of potassium channels in the membrane.
• This allows potassium ions to diffuse out of the neuron, down their concentration gradient
• causes the inside of the neuron to develop a net negative charge again compared with the outside
• The potassium channels remain open until the membrane has fallen to a potential close to -70 mV.
• The diffusion of potassium repolarizes the neuron, but it does not restore the resting potential as the concentration gradients of sodium and potassium ions have not yet been re-established. This takes a few milliseconds and the neuron can then transmit another nerve impulse.

Understanding: An action potential consists of depolarization and
repolarization of the neuron.

Question 86

Outline how nerve impulses are propagated along neurons.

6.5

Answer 86

• A nerve impulse is an action potential that starts at one end of a neuron and is then propagated along the axon to the other end of the neuron.
• The propagation of the action potential happens because the ion movements that depolarize one part of the neuron trigger depolarization in the neighbouring part of the neuron.
• Nerve impulses always move in one direction along neurons.
• there is a refractive period after a depolarization that prevents propagation of an action potential backwards along an axon.

Understanding: Nerve impulses are action potentials propagated along
the axons of neurons.

Question 87

Explain the role of local currents in the propagation of nerve impulses.

6.5

Answer 87

• There is an action potential whenever a part of the axon reaches the threshold potential of -50mV.
• An action potential in one part of the axon triggers an action potential in the next part. This is called the propagation of the nerve impulse.
• It is due to difusion of sodium ions between a region with an action potential and the next region that is still at the resting potential.
• The difusion of sodium ions along the axon, both inside and outside the membrane, is called local currents.

Understanding: Propagation of nerve impulses is the result of local currents that cause each successive part of the axon to reach the threshold potential.

Question 88

Define “synpase”.

6.5

Answer 88

Synapses are junctions between cells in the nervous system.

Understanding: Synapses are junctions between neurons and between
neurons and receptor or efector cells.

Question 89

Outline how signals are sent across synapses.

6.5

Answer 89

• Chemicals called neurotransmitters are used to send signals across synapses.
• This system is used at all synapses where the pre-synaptic and post-synaptic cells are separated by a fuid-flled gap, so electrical impulses cannot pass across.
• This gap is called the synaptic cleft

Understanding: Synapses are junctions between neurons and between
neurons and receptor or efector cells.

Question 90

Outline the proces of synaptic transmission.

6.5

Answer 90

1. A nerve impulse is propagated along the pre-synaptic neuron until it reaches the end of the neuron and the pre-synaptic membrane.
2. Depolarization of the pre-synaptic membrane causes calcium ions (Ca2+) to diffuse through channels in the membrane into the neuron.
3. Infux of calcium causes vesicles containing neurotransmitter to move to the pre-synaptic membrane and fuse with it.
4. Neurotransmitter is released into the synaptic cleft by exocytosis.
5. The neurotransmitter diffuses across the synaptic cleft and binds to receptors on the post-synaptic membrane.
6. The binding of the neurotransmitter to the receptors causes adjacent sodium ion channels to open.
7. Sodium ions diffuse down their concentration gradient into the post-synaptic neuron, causing the post- synaptic membrane to reach the threshold potential.
8. An action potential is triggered in the post-synaptic membrane and is propagated on along the neuron.
9. The neurotransmitter is rapidly broken down and removed from the synaptic cleft.

Understanding: When pre-synaptic neurons are depolarized they release a neurotransmitter into the synapse.

Question 91

Define “acetylcholine”.

6.5

Answer 91

Acetylcholine is used as the neurotransmitter in many synapses, including synapses between neurons and muscle fbres.

Understanding: Secretion and reabsorption of acetylcholine by neurons
at synapses.

Question 92

Explain how acetylcholine is secreted and reabsorbed by neurons at synapses.

6.5

Answer 92

• It is produced in the pre-synaptic neuron by combining choline, with an acetyl group
• The acetylcholine is loaded into vesicles and then released into the synaptic cleft during synaptic transmission.
• The acetylcholine present in the synaptic cleft rapidly breaks down into choline and acetate.
• The choline is reabsorbed into the pre-synaptic neuron, where it is converted back into active neurotransmitter by recombining it with an acetyl group.

Understanding: Secretion and reabsorption of acetylcholine by neurons
at synapses.

Question 93

Outline the effect of neonicotinoids on insects.

6.5

Answer 93

• Neonicotinoids are synthetic compounds similar to nicotine.
• They bind to the acetylcholine receptor in cholinergic synapses in the central nervous system of insects.
• Acetylcholinesterase does not break down neonicotinoids, so the binding is irreversible.
• The receptors are blocked, so acetylcholine is unable to bind and synaptic transmission is prevented.
• The consequence in insects is paralysis and death.

Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

Question 94

List the advantage and disadvantage of the use of neonicotinoids as intsecticides.

6.5

Answer 94

• Advantage: are not highly toxic to humans and other mammals
• Disadvantage: effects of these insecticides on honeybees and other benefcial insects

Application: Blocking of synaptic transmission at cholinergic synapses in insects by binding of neonicotinoid pesticides to acetylcholine receptors.

Question 95

Outline what is required for a nerve impulse to be initiated.

6.5

Answer 95

An action potential is only initiated if the threshold potential is reached, because only at this potential do voltage-gated sodium channels start to open, causing depolarization.

Understanding: A nerve impulse is only initiated if the threshold
potential is reached.

Question 96

Outline what happens if the threshold potential is not reached.

6.5

Answer 96

• At a synapse, the amount of neurotransmitter secreted following depolarization of the pre-synaptic membrane may not be enough to cause the threshold potential to be reached in the post-synaptic membrane.
• The post-synaptic membrane does not then depolarize.
• The sodium ions that have entered the post-synaptic neuron are pumped out by sodium potassium pumps and the post-synaptic membrane returns to the resting potential.

Understanding: A nerve impulse is only initiated if the threshold
potential is reached.

Question 97

Outline how the pancreas controls blood glucose concentration.

6.6

Answer 97

• Cells in the pancreas respond to changes in blood glucose levels.
• If the glucose concentration deviates substantially from the set point, homeostatic mechanisms mediated by the pancreatic hormones insulin and glucagon are initiated.
• There are small regions of endocrine tissue called islets of Langerhans dotted through the pancreas that secrete hormones directly into the blood stream.

Understanding: Insulin and glucagon are secreted by alpha and beta cells in the pancreas to control blood glucose concentration.

Question 98

Outline the functions of both cells types in the islets of Langerhans.

6.6

Answer 98

• Alpha cells synthesize and secrete glucagon if the blood glucose level falls below the set point.
• This hormone stimulates breakdown of glycogen into glucose in liver cells and its release into the blood, increasing the concentration.
• Beta cells synthesize insulin and secrete it when the blood glucose concentration rises above the set point.
• This hormone stimulates uptake of glucose by various tissues, particularly skeletal muscle and liver, in which it also stimulates the conversion of glucose to glycogen.
• Insulin therefore reduces blood glucose concentration.

Understanding: Insulin and glucagon are secreted by alpha and beta cells in the pancreas to control blood glucose concentration.

Question 99

Outline the condition diabetes.

6.6

Answer 99

• Diabetes is the condition where a person has consistently elevated blood glucose levels

Application: Causes and treatment of type I and type II diabetes.

Question 100

Outline the type I diabetes.

6.6

Answer 100

• Type I diabetes, or early-onset diabetes, is characterized by an inability to produce suffcient quantities of insulin. It is an autoimmune disease arising from the destruction of beta cells in the islets of Langerhans by the body’s own immune system.

Application: Causes and treatment of type I and type II diabetes.

Question 101

Outline the type II diabetes.

6.6

Answer 101

• Type II diabetes, sometimes called late-onset diabetes, is characterized by an inability to process or respond to insulin because of a defciency of insulin receptors or glucose transporters on target cells.
• The causes of this form of diabetes are not well understood but the main risk factors are sugary, fatty diets, prolonged obesity due to habitual overeating and lack of exercise, together with genetic factors.

Application: Causes and treatment of type I and type II diabetes.

Question 102

Outline how type I and type II diabtetes is treated.

6.6

Answer 102

• Type I diabetes is treated by testing the blood glucose concentration regularly and injecting insulin when it is too high or likely to become too high.
• Type II diabetes is treated by adjusting the
  diet to reduce the peaks and troughs of blood glucose.

Application: Causes and treatment of type I and type II diabetes.

Question 103

Outline the hormone thyroxin.

6.6

Answer 103

• Thyroxin regulates the metabolic rate and also helps to control body’s temperature.
• Higher metabolic rate supports more protein synthesis and growth and it increases the generation of body heat.
• Prolonged defciency of iodine in the diet prevents the synthesis of thyroxin.

Lack of thyroxin leads to:
* lack of energy and feeling tired all the time
* forgetfulness and depression

Understanding: Thyroxin is secreted by the thyroid gland to regulate the
metabolic rate and help control body temperature.

Question 104

Outline the hormone leptin.

6.6

Answer 104

• Leptin is a protein hormone secreted by adipose cells (fat storage cells).
• The concentration of leptin in the blood is controlled by blood intake and the amount of adipose tissue in the body.
• The target of this hormone is groups of cells in the hypothalamus of the brain that contribute to the control of appetite
• If adipose tissue increases, blood leptin concentrations rise, causing long-term appetite inhibition and reduced food intake.

Understanding; Leptin is secreted by cells in adipose tissue and acts on
the hypothalamus of the brain to inhibit appetite.

Question 105

Outline experiment using leptin done on mice.

6.6

Answer 105

Discovered that obesity in mice could be caused by a lack of leptin and cured by leptin injections.
* When ob/ob mice were injected with leptin their appetite declined, energy expenditure increased and body mass dropped by 30% in a month.

Application: Testing of leptin on patients with clinical obesity and reasons for the failure to control the disease.

Question 106

List the failures in using leptin as a treatment in humans.

6.6

Answer 106

• target cells in the hypothalamus may have become resistant
  to leptin so fail to respond to it
• leptin is a short-lived protein and has to be injected several times a day
• leptin has been shown to affect the development and functioning of the reproductive system, so injections are not suitable in children and young adults.

Application: Testing of leptin on patients with clinical obesity and reasons for the failure to control the disease.

Question 107

Outline the circadian rhythm.

6.6

Answer 107

• Humans are adapted to live in a 24-hour cycle and have rhythms in behaviour that fit this cycle.
• These are known as circadian rhythms.
• They can continue even if a person is placed experimentally in continuous light or darkness because an internal system is used to control the rhythm.

Understanding: Melatonin is secreted by the pineal gland to control
circadian rhythms.

Question 108

Outline the hormone melatonin.

6.6

Answer 108

• Melatonin is secretted by the pineal gland in the brain.
• Melatonin secretion increases in the evening and drops to a low level at dawn and as the hormone is rapidly removed from the blood by the liver, blood concentrations rise and fall rapidly in response to these changes in secretion.
• High melatonin levels cause feelings of drowsiness and promote sleep through the night.
• Falling melatonin levels encourage waking at the end of the night.
• Experiments have shown that melatonin contributes to the night- time drop in core body temperature
• Melatonin receptors have been discovered in the kidney, suggesting that decreased urine production at night may be another effect of this hormone.

Understanding: Melatonin is secreted by the pineal gland to control
circadian rhythms.

Question 109

Outline the effects of jet lag.

6.6

Answer 109

• Jet lag is a common experience for someone who has crossed three or more time zones during air travel.
• The symptoms are diffculty in remaining awake during daylight hours and diffculty sleeping through the night, fatigue, irritability, headaches and indigestion.

Understanding: Causes of jet lag and use of melatonin to alleviate it.

Question 110

Outline the causes of jet lag and how it is alleviated.

6.6

Answer 110

• The causes are easy to understand: the pineal gland are continuing to set a circadian rhythm to suit the timing of day and night at the point of departure rather than the destination.
• To adjust to the new regime: impulses are sent by ganglion cells in the retina: effective at promoting sleep and helping to reduce jet lag

Understanding: Causes of jet lag and use of melatonin to alleviate it.

Question 111

Outline how male sex is determined.

6.6

Answer 111

• If the gene SRY is present, the embryonic gonads develop into testes.
• This gene is located on the Y chromosome, so is only present in 50% of embryos.
• SRY codes for a DNA-binding protein called TDF (testis determining factor).
• TDF stimulates the expression of other genes that cause testis development.

Undderstanding: A gene on the Y chromosome causes embryonic gonads to
develop as testes and secrete testosterone.

Question 112

Outline the primary and secondary sexual characteristics of males during puberty.

6.6

Answer 112

• Testosterone causes male genitalia to develop
• At puberty the secretion of testosterone increases.
• This stimulates sperm production in the testes, which is the primary sexual characteristic of males.
• Testosterone also causes the development of secondary sexual characteristics during puberty such as enlargement of the penis, growth of pubic hair and deepening of the voice due to growth of the larynx.

Understanding: Testosterone causes prenatal development of male genitalia and both sperm production and development of male secondary sexual characteristics during puberty.

Question 113

Outline how estrogen and progesterone cause prenatal development of female reproductive organs and female secondary sexual characteristics during puberty.

6.6

Answer 113

• If the gene SRY is not present in an embryo because there is no Y chromosome, the embryonic gonads develop as ovaries.
• estrogen and progesterone, are always present in pregnancy.
• At frst they are secreted by the mother’s ovaries and later by the placenta.
• During puberty the secretion of estrogen and progesterone increases, causing the development of female secondary sexual characteristics.
• These include enlargement of the breasts and growth of pubic and underarm hair.

Understanding: Estrogen and progesterone cause prenatal development of female reproductive organs and female secondary sexual characteristics during puberty.

Question 114

Outline the male reproductive system.

6.6

Answer 114

• Testis: Produce sperm and testosterone
• Scrotum: Hold testes at lower than core body temperature
• Epididymis: Store sperm until ejaculation
• Sperm duct: Transfer sperm during ejaculation
• Seminal vesicle and prostate gland: Secrete fuid containing alkali, proteins and fructose that is added to sperm to make semen
• Urethra: Transfer semen during ejaculation and urine during urination
• Penis: Penetrate the vagina or ejaculation o semen near the cervix

Skill: Annotate diagrams of the male and female reproductive system to show names of structures and their functions.

Question 115

Outline the female reproductive system.

6.6

Answer 115

• Ovary: Produce eggs, estrogen and progesterone
• Oviduct: Collect eggs at ovulation, provide a site for fertilization then move the embryo to the uterus
• Uterus: Provide for the needs of the embryo and then fetus during pregnancy
• Cervix: Protect the fetus during pregnancy and then dilate to provide a birth canal
• Vagina: Stimulate penis to cause ejaculation and provide a birth canal
• Vulva: Protect internal parts of the female reproductive system

Skill: Annotate diagrams of the male and female reproductive system to show names of structures and their functions.

Question 116

Outline the parts of the mentrual cycle.

6.6

Answer 116

1. Follicular phase
   * follicles are developing in the ovary with an egg that is stimulated to grow.
   * lining of the uterus (endometrium) is repaired and starts to thicken.
   * The most developed follicle breaks open, releasing its egg into the oviduct. Other follicles degenerate.
2. Luteal phase
   * wall of the follicle that released an egg becomes a body called the corpus luteum
   * Continued development of the endometrium prepares it for the implantation of an embryo.

• If fertilization does not occur the corpus luteum in the ovary breaks down. The thickening of the endometrium in the uterus also breaks down and is shed during menstruation.

Understanding: The menstrual cycle is controlled by negative and positive eedback mechanisms involving ovarian and pituitary hormones.

Question 117

List the four hormones involvedin the menstrual cycle.

6.6

Answer 117

• FSH and LH are protein hormones produced by the pituitary gland that bind to FSH and LH receptors in the membranes of follicle cells.
• Estrogen and progesterone are ovarian hormones, produced by the wall of the follicle and corpus

Understanding: The menstrual cycle is controlled by negative and positive eedback mechanisms involving ovarian and pituitary hormones.

Question 118

Outline the hormone FSH in the mentrual cycle.

6.6

Answer 118

• FSH rises to a peak towards the end of the menstrual cycle and stimulates the development of follicles, each containing an oocyte and follicular fuid. FSH also stimulates secretion o estrogen by the follicle wall.

Understanding: The menstrual cycle is controlled by negative and positive eedback mechanisms involving ovarian and pituitary hormones.

Question 119

Outline the hormone estrogen in the mentrual cycle.

6.6

Answer 119

• Estrogen rises to a peak towards the end of the follicular phase.
• It stimulates the repair and thickening of the endometrium after menstruation and an increase in FSH receptors that make the follicles more receptive to FSH, boosting estrogen production (positive feedback). When it reaches high levels estrogen inhibits the secretion of FSH (negative feedback) and stimulates LH secretion.

Understanding: The menstrual cycle is controlled by negative and positive eedback mechanisms involving ovarian and pituitary hormones.

Question 120

Outline the hormone LH in the mentrual cycle.

6.6

Answer 120

• LH rises to a sudden and sharp peak towards the end of the follicular phase.
• It stimulates the completion of meiosis in the oocyte and partial digestion of the follicle wall allowing it to burst open at ovulation.
• LH also promotes the development of the
  wall of the follicle after ovulation into the corpus luteum which secretes estrogen (positive feedback) and progesterone.

Understanding: The menstrual cycle is controlled by negative and positive eedback mechanisms involving ovarian and pituitary hormones.

Question 121

Outline the hormone progesterone in the mentrual cycle.

6.6

Answer 121

• Progesterone levels rise at the start of the luteal phase, reach a peak and then drop back to a low level by the end of this phase. Progesterone promotes the thickening and maintenance of the endometrium. It also inhibits FSH and LH secretion by the pituitary gland (negative eedback).

Understanding: The menstrual cycle is controlled by negative and positive feedback mechanisms involving ovarian and pituitary hormones.

Question 122

Outline the process of In vitro fertilization.

6.6

Answer 122

1.Down-regulation
* The woman takes a drug each day to stop her pituitary gland secreting FSH or LH.
* Secretion of estrogen and progesterone therefore also stops.

2.Artifcial doses of hormones
* Intramuscular injections of FSH and LH are then given daily to stimulate follicles to develop.
* Far more follicles develop than usual (called superovulation)

3.Egg retrieval and fertilization
* Eggs are washed out of the follicles.
* Each egg is mixed with sperm

4.Establishing a pregnancy
* If fertilization is successful then one or more embryos are placed in the uterus when they are about 48 hours old.
* If embryo grows, the rest follows a normal preganncy

Application: The use in IVF of drugs to suspend the normal secretion of hormones, Followed by the use of artifcial doses of hormones to induce superovulation and establish a pregnancy.

Question 123

Explain how William Harvey investigated sexual reproduction in deer.

6.6

Answer 123

• Deerare seasonal breeders and only become sexually active during the autumn.
• Harvey examined the uterus of female deer during the mating season by slaughtering and dissecting them.
• He expected to find eggs developing in the uterus immediately after mating (copulation), but only found signs of anything developing in females two or more months after the start of the mating season.

Application: William Harvey’s investigation of sexual reproduction
in deer.