Homeostasis and Response

Question 1

Homeostasis

Answer 1

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.
Homeostasis maintains optimal conditions for enzyme action and all cell functions.
In the human body, these include control of:
Blood glucose concentration
Body temperature
Water levels

Question 2

Enzyme functioning

Answer 2

Because humans are organisms that live in a changing environment, we must regulate our body’s internal conditions to make sure our enzymes and cells function well.
If conditions are not optimal, then our enzymes can denature (change shape).
This reduces their ability to catalyse (speed-up) metabolic reactions (chemical reactions in organisms).

Question 3

Regulating internal conditions

Answer 3

Humans must regulate their body’s internal conditions to make sure that enzymes and cells function well. The conditions that need to be regulated are:
Internal body temperature
Urea concentration in urine
Water levels
Blood sugar levels
Carbon dioxide levels

Question 4

Control system

Answer 4

Maintaining controlled conditions within the body is under involuntary (automatic) control.
This means that the brain stem (or non-conscious part of the brain) and the spinal cord are involved in maintaining homeostasis – you don’t consciously maintain your body temperature or blood glucose level.
These automatic control systems may involve nervous responses or chemical responses.
All control systems include:
Cells called receptors, which detect stimuli (changes in the environment).
Coordination centres (such as the brain, spinal cord and pancreas) that receive and process information from receptors.
Effectors (muscles or glands) which bring about responses which restore optimum levels.

Question 5

Positive feedback

Answer 5

Positive feedback is the opposite of negative feedback. It makes a small change even bigger. An example of positive feedback in the body is the release of oxytocin (hormone), which increases the number of contractions during childbirth.

Question 6

Human nervous system

Answer 6

The nervous system protects organisms from harm by responding to changes in the environment. It does this by coordinating communication between different parts of organisms.
The human nervous system consists of:
Central nervous system (CNS) – the brain and spinal cord
Peripheral nervous system (PNS) – all of the nerves in the body
The nervous system enables humans to react to their surroundings and to coordinate their behaviour
Information is sent through the nervous system as electrical impulses – electrical signals that pass along nerve cells known as neurones
A bundle of neurones is known as a nerve.

Question 7

Brain

Answer 7

The brain is a very complex organ that controls all conscious and unconscious thoughts in order to keep an organism alive.
The brain alongside the spinal cord is part of our central nervous system.
The brain is made of billions of interconnected neurones and is responsible for controlling complex behaviours.
Within the brain are different regions that carry out different functions.

Question 8

Spinal cord

Answer 8

The spinal cord is the other component (part) of the CNS. It is also important in coordinating the response of effectors to changes in the environment.

Question 9

Nerve cells

Answer 9

Neurones (nerve cells) carry electrical impulses (signals) between receptors, the central nervous system (CNS) and effectors.

Question 10

Synapses

Answer 10

Neurones never touch each other, they are separated by junctions (gaps) called synapses.
Synaptic junctions are incredibly small - around 10nm in size - and electrical impulses cannot cross them.
In a reflex arc, there are synapses between the sensory and relay neurones, and the relay and motor neurones
Chemicals called neurotransmitters (such as dopamine and serotonin) are released into the synaptic cleft and diffuse across it (down a concentration gradient).

Question 11

Reflex actions

Answer 11

An involuntary (or reflex) response does not involve the conscious part of the brain as the coordinator of the reaction.
Awareness of a response having happened occurs after the response has been carried out.
Responses are therefore automatic and rapid – this helps to minimise damage to the body.
Some examples of reflexes are: Blinking, sneezing.

Question 12

Stimulus

Answer 12

A stimulus can be any change in the environment to which the body needs to respond.
The stimulus is detected by a receptor.

Question 13

Receptors

Answer 13

Receptors are found all over the body.
They detect the change in the environment and initiate (start) a signalling process within the body.
The signal is picked up by a neurone (nerve cell).

Question 14

Sensory neurons

Answer 14

The sensory neurone, which carries the signal in the form of an electrical impulse to the central nervous system (CNS).
Sensory neurones are long and have a cell body branching off the middle of the axon.

Question 15

Effector

Answer 15

An effector is a muscle or gland that brings about an action in response to the change in the internal or external environment.

Question 16

Response

Answer 16

The response can be any action that helps the organism to avoid the harmful situation.

Question 17

Investigating the brain

Answer 17

The brain is an incredibly complex and delicate organ – this makes it extremely difficult for neuroscientists to study it to find out how it works.
Our understanding is limited because the brain is so complex and different regions can’t be studied in isolation.
Our limited understanding means that treating brain damage and disease is very difficult; in addition, any potential treatment carries risks of further damage occurring which can lead to increased problems.
Accidental damage could lead to speech or motor issues, or changes to personality which are permanent.

Question 18

Brain damage

Answer 18

The brain is delicate, complex, and not well understood.
Therefore, the treatment of brain damage and brain disease is difficult.
By studying patients with brain damage, where part of their brain doesn’t function, neuroscientists have been able to link particular regions of the brain to particular functions.

Question 19

Cerebellum

Answer 19

This is underneath the cerebral cortex and is responsible for balance, muscle coordination and movement.

Question 20

Cerebral cortex

Answer 20

This is the outer layer of the brain which is divided into two hemispheres. It’s highly folded and is responsible for higher-order processes such as intelligence, memory, consciousness and personality.

Question 21

Medulla

Answer 21

This part is responsible for unconscious activities (e.g. breathing and heartbeat).

Question 22

MRI scanners

Answer 22

MRI stands for Magnetic Resonance Imaging.
MRI scanners are very important diagnostic tools used to study the brain and other regions of the body using magnetic fields and the effect these have on protons in the water molecules of the body
Functional MRIs can produce images of different regions of the brain that are active during different activities like listening to music or recalling a memory (the scanners can detect changes in blood flow – more active regions of the brain have increased blood flow)
MRI scanners have allowed us to learn which areas of the brain are active during different activities, such as moving, speaking and listening.

Question 23

Electrical stimulation

Answer 23

Electrical stimulation has also allowed us to treat certain disorders of the brain.
Tiny electrodes can be pushed into different parts of the brain, tiny jolts of electricity stimulate these regions and the effects can be observed.
Because the nervous system communicates using electrical impulses, electrical stimulation is used to help treat conditions such as Parkinson’s disease (causes tremors).

Question 24

Retina scanning

Answer 24

Your retina is full of receptor cells, which are sensitive to both the brightness (light intensity) and the colour of light.
Retina scanning looks at the pattern of blood vessels in your retina to identify you.

Question 25

Iris

Answer 25

The iris controls pupil diameter and the quantity of light reaching the retina. If there isn’t much light then the iris will make our pupils dilate (get bigger).

Question 26

Adaptations of the nervous system

Answer 26

Neurones have a cell body (where the nucleus and main organelles are found) and cytoplasmic extensions from this body called axons and dendrites.
Some human neurones have axons over a metre in length (but only 1 - 4 micrometres wide).
This is far more efficient than having multiple neurones to convey information from the CNS to effectors – less time is wasted transferring electrical impulses from one cell to another.
The axon is insulated by a fatty myelin sheath with small uninsulated sections along it (called nodes) which the impulse jumps along.

Question 27

Structure of the nervous system

Answer 27

Information from receptors passes along cells (neurones) as electrical impulses to the central nervous system (CNS)
The receptors detect stimuli in the environment
The CNS is the brain and spinal cord
The CNS is the coordinator that coordinates the response of effectors which may be muscles contracting or glands secreting hormones
The pathway through the nervous system is:
Stimulus - receptor - coordinator - effector - response

Question 28

Reflex to being stabbed with a pin

Answer 28

The pin (the stimulus) is detected by a (pain/pressure/touch) receptor in the skin.
A sensory neurone sends electrical impulses to the spinal cord (the coordinator).
An electrical impulse is passed to a relay neurone in the spinal cord.
A relay neurone synapses with a motor neurone.
A motor neurone carries an impulse to a muscle in the leg (the effector).
The muscle will contract and pull the foot up and away from the sharp object (the response) when stimulated by the motor neurone.

Question 29

Relay neuron

Answer 29

Relay neurones are found inside the CNS and connect sensory and motor neurones.
Relay neurones are short and have a small cell body at one end with many dendrites branching off it.

Question 30

Motor neurons

Answer 30

Motor neurones carry impulses from the CNS to effectors (muscles or glands).
Motor neurones are long and have a large cell body at one end with long dendrites branching off it.

Question 31

How does synapse work

Answer 31

The electrical impulse travels along the first axon.

When an electrical impulse arrives at the end of the axon on the presynaptic neurone, chemical messengers called neurotransmitters are released from vesicles.

The neurotransmitters diffuse across the synaptic gap and bind with receptor molecules on the membrane of the second neurone (known as the postsynaptic membrane).

This stimulates the second neurone to generate an electrical impulse that travels down the second axon.

The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone which would cause repeated impulses to be sent.

Synapses ensure that impulses only travel in one direction, avoiding confusion within the nervous system if impulses were travelling in both directions.

As this is the only part of the nervous system where messages are chemical as opposed to electrical, it is the only place where drugs can act to affect the nervous system - eg this is where heroin works.

Question 32

Function of the eye

Answer 32

The eye is a sense organ containing receptor cells which are sensitive to light intensity and colour.
The purpose of the eye is to receive light and focus it onto the retina at the back of the eye.
There are two main functions of the eye:
Accommodation to focus on near or distant objects.
Adaptation to dim light.

Question 33

Retina

Answer 33

Controls the light receptor cells that detect light intensity and colour of light.

Question 34

Optic nerve

Answer 34

Sensory neurone that carries electrical impulses from the eye to the brain.

Question 35

Sclera

Answer 35

White outer layer which supports the structures inside the eye. It is strong to prevent damage to the eye.

Question 36

Cornea

Answer 36

Transparent covering of the front of the eye that refracts (bends) light.

Question 37

Ciliary muscles

Answer 37

Ring of muscles around the lens which relaxes and contracts to change the shape of the lens.

Question 38

Suspensory ligaments

Answer 38

Work with the ciliary muscles to change the shape of the lens.

Question 39

Lens

Answer 39

Transparent disc that changes shape to focus light onto the retina.

Question 40

Vitreous humour

Answer 40

Fluid behind the lens that helps maintain the shape of the eye and lens. It also keeps the retina against the wall of the eye.

Question 41

Aqueous humour

Answer 41

Fluid in front of the lens that helps maintain the shape of the eye and lens.

Question 42

Fovea

Answer 42

A region of the retina with the highest density of cones (colour detecting cells) where the eye sees particularly good detail.

Question 43

Adaptations of the eye

Answer 43

The eye can adapt its structures in response to light intensity.
This adaptation is a reflex action carried out to protect the retina from damage in bright light and protect us from not seeing objects in dim light.
The reflex action is controlled by two groups of muscle:
The radial muscles
The circular muscles
In dim light, the pupil dilates (widens) to allow as much light into the eye as possible
In bright light, the pupil constricts (narrows) to prevent too much light from entering the eye and damaging the retina

Question 44

Adaptation of the eye to dim light

Answer 44

Photoreceptors detect change in environment (dark).
Radial muscles contract.
Circular muscles relax.
Pupil dilates to allow more light to enter the eye.

Question 45

Adaptation of the eye to bright light

Answer 45

Photoreceptors detect change in environment (bright).
Radial muscles relax.
Circular muscles contract.
Pupil dilates to allow more light to enter the eye.

Question 46

Accommodation of the eye

Answer 46

Accommodation is the process of changing the shape of the lens to focus on near or distant objects.
The lens is elastic and its shape can be changed when the suspensory ligaments attached to it become tight or loose.
Changing the shape of the lens alters how much light is refracted .
This is important in making sure that light is focused on the retina of the eye rather than in front or behind it.
The contraction or relaxation of the ciliary muscles brings about the changes.

Question 47

Focusing on close objects

Answer 47

The ciliary muscles contract.
The suspensory ligaments loosen.
The lens is then thicker and refracts light rays more strongly.

Question 48

Focusing on distant objects

Answer 48

The ciliary muscles relax.
The suspensory ligaments are pulled tight.
The lens is then pulled thin and only slightly refracts light rays.

Question 49

Defects of the eye

Answer 49

Two common defects of the eyes are
myopia (short-sightedness).
hyperopia (long-sightedness).

In both defects rays of light do not focus on the retina
Generally, these defects are treated with spectacle lenses (glasses). Which refract the light rays so that they focus on the retina.

Question 50

Myopia

Answer 50

Myopia is short sightedness
The lens is too thick and curved and the eyeball is too elongated so the distance between the retina and lens is too great. This causes the image to be in focus before reaching the retina. A concave lens can be used to correct light rays so they focus on the retina.

Question 51

Hyperopia

Answer 51

Hyperopia is long sightedness
The eyeball is too short and the distance between the retina and the lens is too short. This causes the image to be in focus behind the retina. A convex lens can be used to refract light rays so they focus on the retina.

Question 52

Contact lenses

Answer 52

Hard and soft contact lenses:
These sit on the surface of the eye and are almost invisible, making them ideal for activities like sports.
Soft lenses are more comfortable but carry a higher infection risk.

Question 53

Laser eye surgery

Answer 53

Lasers can be used to change the shape of the cornea (changing how it refracts light onto the retina).
All surgical procedures have a risk of unexpected damage occurring during the procedure which could lead to worse vision or an infection.
For myopia: the cornea is slimmed down, reducing the refractive power.
For hyperopia: the cornea shape is changed so the refractive power is increased.

Question 54

Lens replacement surgery

Answer 54

This surgery completely replaces the lens of the eye with a plastic artificial lens (rather than changing the shape of the cornea during laser eye surgery).
The procedure is more invasive than laser surgery and carries a risk of damage occurring to the retina leading to complete sight loss.

Question 55

Monitoring body temperature.

Answer 55

The human body needs to maintain a temperature at which enzymes work best, around 37°C.
Processes such as respiration release energy as heat; and the body loses heat energy to its surroundings – the energy gained and lost must be regulated to maintain a constant core body temperature.
Body temperature is monitored and controlled by the thermoregulatory centre in the brain.
The thermoregulatory centre contains receptors sensitive to the temperature of the blood.
The skin contains temperature receptors and sends nervous impulses to the thermoregulatory centre.

Question 56

Increasing body temperature

Answer 56

If the body temperature is too low, blood vessels constrict (vasoconstriction), sweating stops and skeletal muscles contract (shiver).
These mechanisms reduce heat loss to the surroundings (with skeletal muscle contraction increasing heat released in the body).

Thermoreceptors in the hypothalamus and skin detect change. The body starts to shiver, vasconstriction occurs and skin hairs. are erected which trap air (an excellent insulator) which increases body temperature.

Question 57

Decreasing body temperature

Answer 57

If the body temperature is too high, blood vessels dilate (vasodilation) and sweat is produced from the sweat glands.
Both these mechanisms cause a transfer of energy from the skin to the environment, cooling the body down.

Thermoreceptors in the hypothalamus and skin detect change. This increases sweating, vasodilation occurs and hairs lie flat against the skin. This decreases the temperature when the sweat evaporates.

Question 58

Human endocrine system

Answer 58

The human endocrine system is composed of glands which secrete chemicals called hormones directly into the bloodstream.
The blood carries the hormone around the body, and when it reaches a target cell/organ it produces an effect.
Compared to the nervous system the effects of hormones are slower but they act for longer.

Question 59

Pituitary gland

Answer 59

The pituitary gland in the brain is a ‘master gland’ which secretes several hormones into the blood in response to body conditions.
These hormones in turn act on other glands to stimulate other hormones to be released to bring about effects.

Question 60

Thyroid gland

Answer 60

This gland is in our neck and releases thyroxine.
Thyroxine regulates our metabolism, heart rate and temperature.

Question 61

Adrenal gland

Answer 61

The two adrenal glands are situated above each kidney.
They produce adrenaline, which triggers the ‘fight or flight’ response when an organism is presented with a dangerous situation.

Question 62

Pancreas

Answer 62

The pancreas produces insulin and glucagon, both of which are involved in regulating blood glucose levels.

Question 63

Ovaries

Answer 63

The ovaries produce oestrogen, which controls puberty and is one of the main hormones in the regulation of the menstrual cycle.

Question 64

Testes

Answer 64

The testes produce testosterone, which controls puberty and sperm production.

Question 65

Thyroxine

Answer 65

Thyroxine is the main hormone secreted (released) by the thyroid gland. It increases the basal (resting) metabolic rate.
Thyroxine is responsible for regulating the growth and brain development of young animals.

Production is regulated by a negative feedback loop. This allows the body to maintain a fairly constant concentration of thyroxine in the blood.
When the concentration of thyroid hormones, such as thyroxine, in the blood is low, the production of thyroid-stimulating hormone (TSH) is increased. This stimulates the secretion of thyroxine by the thyroid gland.
When the concentration of thyroid hormones in the blood is high, TSH production is decreased.

Hyperthyroidism, caused by an overactive thyroid gland secreting too much thyroxine into the bloodstream which causes an increase in BMR and protein synthesis.
Hypothyroidism caused by an underactive thyroid gland secreting too little thyroxine into the bloodstream which can lead to heart and nerve problems, and death.

Question 66

Adrenaline

Answer 66

Adrenaline is a hormone secreted (discharged) by the adrenal glands. It has different functions:
Adrenaline increases heart rate and blood pressure, and dilates the pupils in our eyes.
Adrenaline enlarges the air passages of the lungs and alters metabolism in order to boost the delivery of oxygen and glucose to the brain and the muscles.
In times of fear or stress, adrenaline is part of the so-called ‘fight or flight’ response, preparing the body to move and think quickly in response to danger.

Question 67

Control of blood glucose

Answer 67

Blood glucose concentration is monitored and controlled by the pancreas.
The pancreas is an endocrine gland (making and secreting hormones into the bloodstream) and it also plays a vital (but separate) role in digestion (making and secreting enzymes into the digestive system).
Blood glucose concentration must be kept within a narrow range, so it’s another example of homeostasis (like temperature control).
Eating foods containing carbohydrate results in an increase of glucose into the bloodstream.
If the blood glucose concentration is too high, the pancreas produces the hormone insulin to bring it back down.
Too high a level of glucose in the blood can lead to cells of the body losing water by osmosis, which can be dangerous.
Insulin stimulates cells to take in glucose from the bloodstream (particularly liver and muscle cells)
In liver and muscle cells excess glucose is converted into glycogen (a polymer of glucose) for storage.

Question 68

Type 1 diabetes

Answer 68

Type 1 diabetes is a disorder in which the pancreas fails to produce sufficient insulin to control blood glucose levels.
Scientists think this is a result of a person’s own immune system destroying the cells of the pancreas that make insulin during development.
Type 1 diabetes is characterised by uncontrolled high blood glucose levels and is normally treated with insulin injections.

Question 69

Type 2 diabetes

Answer 69

In Type 2 diabetes the body cells no longer respond to insulin produced by the pancreas - the person still makes insulin but their cells are resistant to it and don’t respond as well as they should.
This can also lead to uncontrolled high blood glucose levels.
A carbohydrate-controlled diet and an exercise regime are common treatments for Type 2 diabetes.
Obesity is a big risk factor for Type 2 diabetes; probably because a person who is obese may consume a diet high in carbohydrates, and over-production of insulin results in resistance to it developing.

Question 70

Negative feedback of blood glucose

Answer 70

If the blood glucose concentration is too low, the pancreas produces the hormone glucagon that causes glycogen to be converted into glucose and released into the blood.
Glucagon and insulin interact as part of a negative feedback cycle to control blood glucose (sugar) levels in the body.

Question 71

Insulin

Answer 71

Insulin is produced when blood glucose rises and stimulates liver and muscle cells to convert excess glucose into glycogen to be stored – this reduces the blood glucose level.

Question 72

Glucagon

Answer 72

Glucagon is produced when blood glucose falls too low and stimulates liver and muscle cells to convert stored glycogen into glucose to be released into the bloodstream – this increases the blood glucose level.

Question 73

Maintaining water levels

Answer 73

Maintaining water levels in the body is vital to prevent harmful changes occurring to cells of the body as a result of osmosis.
If body cells lose or gain too much water by osmosis they do not function efficiently:
Too much water in the blood results in cells swelling as water moves into them, this has a diluting effect and can lead to cell lysis (bursting).
Too little water in the blood (or too high an ion concentration) and the cells lose water by osmosis, this has a dehydrating effect and can lead to cell death.

Question 74

Water loss in the body

Answer 74

There are two sources of water in the body: water produced as a result of aerobic respiration and water in the diet.
The cytoplasm of all cells is largely composed of water, as is the blood plasma.
Water is lost from the body in the following ways:
Water leaves the body via the lungs during exhalation (breathing out).
Water, ions and urea are lost from the skin in sweat.
However, the lungs and skin have no control over how much water, ion or urea is lost via exhalation or sweating.
Controlled loss of excess water, ions and urea is controlled by the kidneys when they filter the blood to produce urine.

Question 75

Deamination

Answer 75

The digestion of proteins from the diet results in excess amino acids which need to be excreted safely, as they cannot be stored by the body in the same way that excess glucose can.
Deamination is the process of breaking down excess protein and it predominantly occurs in the liver.
Enzymes in the liver split up amino acid molecules, with the part containing carbon turned into glycogen and the other part containing nitrogen (the amino part) turned into ammonia (this is why we say the amino acid has been deaminated).
Ammonia is toxic to cells and so it is immediately converted to urea which can be transported around the body via the blood safely for excretion by the kidneys.

Question 76

Structure and function of the kidney

Answer 76

The kidneys help to control the water content of the body and the concentrations of substances (such as sodium and potassium ions) dissolved in the fluids of the body.
The kidney contains highly branched capillary networks that form filters which contain pores with an average radium of about 3 nanometers.
When blood passes through the kidneys, the pressure it is under increases as it is pushed into the filters.
This high-pressure mass flow forces molecules that are small enough to pass through the pores out of the bloodstream – this is called filtration.
Substances forced out of the blood include glucose, urea and water with ions dissolved in it. The liquid formed is called filtrate.
Larger molecules (such as RBCs or proteins) are too big to pass out of the filter and so remain in the blood plasma
The kidneys then selectively reabsorb substances needed by the body back into the bloodstream (this is an active process).
In a healthy kidney, this includes all of the glucose and some ions from the filtrate.
Anything not reabsorbed forms urine, which is then stored in the bladder until it is excreted.
Urea, formed from the deamination of amino acids in the liver, is not selectively reabsorbed.
The concentration of urea in the urine is far higher than that of the blood plasma.
Reabsorption of water from the filtrate back into the bloodstream is why the concentration of urea in the filtrate is so much higher.

Question 77

Control of water levels

Answer 77

Water lost through the lungs or skin cannot be controlled, but the volume of water lost in the production of urine can be controlled by the kidneys.
The kidneys contain structures called tubules which filtrate passes through on its way to the bladder.
Water reabsorption occurs along these tubules; if the water content of the blood is too high then less water is reabsorbed, if it is too low then more water is reabsorbed.
The pituitary gland in the brain constantly releases a hormone called ADH; how much ADH is released depends on how much water the kidneys should reabsorb from the filtrate.
ADH, therefore, affects the permeability of the tubules to water.
If the water content of the blood is too high, the pituitary gland releases less ADH which leads to less water being reabsorbed in the tubules of the kidney (the tubules become less permeable to water).
If the water content of the blood is too low and the blood is too concentrated, the pituitary gland releases more ADH which leads to more water being reabsorbed in the tubules of the kidney (the tubules become more permeable to water).
The control of water reabsorption by the tubules is another example of negative feedback.

Question 78

Kidney failure

Answer 78

The kidneys might not work properly for several reasons, including accidents or disease.
Humans can survive with one functioning kidney, but if both are damaged then there will quickly be a build-up of toxic wastes in the body which will be fatal if not removed.

Question 79

Kidney dialysis

Answer 79

Dialysis is the usual treatment for someone with kidney failure.
Patients are connected to a dialysis machine which acts as an artificial kidney to remove most of the urea and restore/maintain the water and salt balance of the blood.
Unfiltered blood is taken from an artery in the arm, pumped into the dialysis machine and then returned to a vein in the arm.
Inside the machine the blood and dialysis fluid are separated by a partially permeable membrane – the blood flows in the opposite direction to dialysis fluid, allowing exchange to occur between the two where a concentration gradient exists.
Dialysis fluid contains:
A glucose concentration similar to a normal level in blood.
A concentration of salts similar to a normal level in blood.
No urea.

Question 80

Kidney transplant

Answer 80

Kidney transplants are a better long term solution to kidney failure than dialysis; however, there are several disadvantages to kidney transplants, including:
Donors won’t have the same antigens on cell surfaces so there will be some immune response to the new kidney (risk of rejection is reduced - but not removed – by ‘tissue typing’ the donor and the recipient first).
This has to be suppressed by taking immunosuppressant drugs for the rest of their lives – these can have long term side effects and leave the patient vulnerable to infections
There are not enough donors to cope with demand
However, if a healthy, close matched kidney is available, then the benefits of a transplant over dialysis include:
The patient has much more freedom as they are not tied to having dialysis several times a week in one place.
Their diets can be much less restrictive than they are when on dialysis.
Use of dialysis machines is very expensive and so this cost is removed.
A kidney transplant is a long term solution whereas dialysis will only work for a limited time.

Question 81

Puberty

Answer 81

During puberty, reproductive hormones cause secondary sex characteristics to develop.
The main male reproductive hormone is testosterone which is produced by the testes; testosterone stimulates sperm production.
The main female reproductive hormone is oestrogen which is produced by the ovaries.

Question 82

Stages of the menstrual cycle

Answer 82

During the menstrual cycle, the lining of the uterus builds up and ovulation occurs.
The average menstrual cycle is 28 days long and there are four overall stages:
Menstruation – loss of lining from the uterus, occurs at the start of the cycle if no fertilisation has occurred
The lining starts to thicken.
Ovulation occurs around the middle of the cycle (about day 14), the egg travels down the oviduct towards the uterus.
The lining is maintained ready to accept a fertilized egg.

Question 83

Hormonal control of menstruation cycle

Answer 83

Four hormones control the events that occur during the menstrual cycle.
Two of these hormones are produced by the pituitary gland in the brain:
Follicle-stimulating hormone (FSH) causes maturation of an egg in the ovary.
Luteinising hormone (LH) stimulates the release of the egg.
The other two hormones, oestrogen and progesterone are involved in maintaining the uterus lining with oestrogen being made by the ovaries and progesterone specifically by an empty egg follicle called the corpus luteum.

Question 84

Interaction of the hormones during the menstrual cycle

Answer 84

The pituitary gland produces FSH which stimulates the development of a follicle in the ovary.
An egg matures inside the follicle and the follicle produces the hormone oestrogen – so FSH stimulates the production of oestrogen.
Oestrogen causes growth and repair of the lining of the uterus wall and inhibits the production of FSH.
When oestrogen rises to a high enough level it stimulates the release of LH from the pituitary gland which causes ovulation (usually around day 14 of the cycle).
The follicle becomes a corpus luteum and starts producing progesterone.
Progesterone maintains the uterus lining (the thickness of the uterus wall).
If the egg is not fertilised, the corpus luteum breaks down and progesterone levels drop
This causes menstruation – commonly known as having a period.
If fertilisation does occur the corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down (breakdown of the lining would prevent a pregnancy).
Once the placenta has developed, it starts secreting progesterone and continues to do so throughout the pregnancy to maintain the lining.

Question 85

Changes in the levels of the pituitary hormones FSH and LH in the blood during the menstrual cycle

Answer 85

FSH (follicle-stimulating hormone) is released by the pituitary gland and causes an egg to start maturing in the ovary.
It also stimulates the ovaries to start releasing oestrogen
The pituitary gland is stimulated to release luteinising hormone (LH) when oestrogen levels have reached their peak.
LH causes ovulation to occur and also stimulates the ovary to produce progesterone.

Question 86

Changes in the levels of oestrogen and progesterone in the blood during the menstrual cycle

Answer 86

Oestrogen levels rise from day 1 to peak just before day 14.
This causes the uterine wall to start thickening and the egg to mature.
The peak in oestrogen occurs just before the egg is released.
Progesterone stays low from day 1 - 14 and starts to rise after ovulation.
The increasing levels cause the uterine lining to thicken further; a fall in progesterone levels causes the uterine lining to break down (menstruation).

Question 87

Control of fertility

Answer 87

Fertility can be controlled by a variety of hormonal and non-hormonal methods.
Contraceptive methods aim to prevent fertilisation and pregnancy, and include the use of hormones (oestrogen and progesterone) as well as non-hormonal methods (such as the use of barriers or surgery).
Hormones can also be used to increase the chance of pregnancy occurring when it previously might not have done.

Question 88

Surgical methods of sterilisation

Answer 88

In a female, the oviducts which connect the ovaries to the uterus (also called the fallopian tubes) can be cut and tied
In a male, the sperm ducts (the tube connecting the testes to the penis) can also be cut and tied in a procedure called a vasectomy.
Both methods are highly effective but there have been a small number of cases where tubes have rejoined.

Question 89

Barrier method of sterilisation

Answer 89

Barrier methods such as condoms and diaphragms prevent the sperm from reaching an egg.
Condoms are the only barrier method that can prevent the spread of sexually transmitted infections.
Spermicidal agents which kill or disable sperm – these are only 70 - 80% effective.

Question 90

Progesterone only pill

Answer 90

Stimulates the production of thick, sticky mucus, which is very difficult for any sperm to penetrate.
Inhibits the release of FSH, so that eggs don’t mature.
Just as effective as the combined pill but with fewer side effects.

Question 91

Combined pill

Answer 91

Contains both oestrogen and progesterone.
By taking it every day, enough oestrogen builds up in the female body to inhibit the production of FSH so that no eggs mature.

Question 92

Infertility

Answer 92

Infertility occurs when a couple find it difficult or are unable to conceive naturally.
This can be a result of insufficient or too low levels of reproductive hormones affecting the development of egg and sperm cells, or as a result of issues with the reproductive system of the female.

Question 93

Use of hormones to treat infertility

Answer 93

Artificial hormones are used as part of modern reproductive technologies to treat infertility, particularly when the female is not producing enough eggs, usually as a result of the pituitary gland not producing sufficient FSH to cause egg maturation.
The hormones FSH and LH are given as a ‘fertility drug’ to stimulate egg production.
An important social issue to consider with this is that several eggs can be released at once so this increases the chance of multiple births (twins or triplets etc).
It also doesn’t have a particularly high success rate and can be expensive.

Question 94

IVF treatment

Answer 94

An alternative treatment is for eggs to be fertilised by sperm outside of the body (‘in vitro’ means ‘in glass’) – this is used particularly when there are issues with both male and female fertility.
The process involves:
Giving a mother FSH and LH to stimulate the maturation of several eggs.
The eggs are collected from the mother and fertilised by sperm from the father in the laboratory.
The fertilised eggs develop into embryos.
At the stage when they are tiny balls of cells, one or two embryos are inserted into the mother’s uterus (womb)
The success rate of IVF is low (~30%) but there have been many improvements and advancements in medical technologies which are helping to increase the success rate.
These advancements include improvements in microscope techniques and micro-tools that enable single cells to be removed from an embryo for genetic testing to identify if the embryo is healthy or has genetic defaults the couple might want to consider.

Question 95

Issues with fertility treatment

Answer 95

Fertility treatments can give a couple the chance to have a baby of their own, which is a big positive.
Potential issues to consider include:
As several embryos are implanted, the risk of multiple births is quite high (which increases the risk of miscarriage or stillbirths).
The success rate is not very high (although it is increasing); IVF treatment failures can be very emotionally upsetting and physically stressful for couples.
Some women use IVF to get pregnant at a later age than they would be able to conceive naturally.
Some people are against IVF as more embryos can be produced than are used; the issue of who owns these embryos and whether they are used in research before eventually being destroyed is contentious (as embryos are a potential life).
The use of genetic testing is controversial as there is potential it could be misused in choosing characteristics of offspring (this is not allowed).

Question 96

Negative feedback

Answer 96

Negative feedback mechanisms in homeostasis help to maintain conditions in the body within an optimal narrow range; any movement away from ideal conditions results in changes occurring which bring them back.
This involves detecting that the level of a substance or a condition has gone above or below normal levels, which triggers a response to bring the level back to normal again.
Blood glucose level and core body temperature control are examples of negative feedback.

Question 97

Phototropism and gravitropism/geotropism

Answer 97

Plants need to be able to grow in response to light (phototropism) and gravity (gravitropism or geotropism).

The shoots must grow upwards, away from gravity and towards light, so that leaves are able to absorb sunlight – shoots show a positive phototropic response and a negative gravitropic response.
Roots need to grow downwards into the soil, away from light and towards gravity, in order to anchor the plant and absorb water and minerals from the soil particles so roots show a negative phototropic response and a positive gravitropic response.

Question 98

Gibberellins

Answer 98

Gibberellins can be used to:
End seed dormancy, as a high concentration of gibberellin promotes seed germination. Gibberellin levels naturally rise after a period of dormancy (exposure to cold and dry conditions) – usually, dormancy ends with an intake of water into the seed and warmer weather.
Promote flowering regardless of the weather conditions the plant is in.
Increase fruit size – higher levels of gibberellin promote the development and growth of fruit.

Question 99

Ethene

Answer 99

Ethene is used in the food industry to control ripening of fruit during storage and transport.
It is far more effective to transport unripe fruit, as ripe fruit is softer and therefore more easily damaged and spoiled.
The production of ethene can be inhibited to delay ripening of fruits in storage; this can either be achieved directly by adding chemicals that prevent ethene from being produced, or by reacting ethene in the air around fruit with substances that can remove it.
When ripening needs to be encouraged (eg. when fruit is in the supermarket), artificially produced ethene gas can be released to speed up the process.

Question 100

Uses of plant hormones

Answer 100

Plant hormones can be extracted or artificially made and used by gardeners and farmers in horticulture and agriculture to usefully control plant growth to obtain larger yields for example.
The use of auxins, ethene and gibberellins commercially has been very beneficial is helping producing food and plants for decoration.
However the everyday use of hormones as weed killers can have a negative effect on biodiversity; as the growth of unwanted but natural plants such as weeds is inhibited.
Many different species of plants are classed as weeds commercially, but to other organisms they are a food source and potential habitat, so destroying them can have negative effects on other organisms in the ecosystem.

Question 101

Uses of auxins

Answer 101

Auxins can be used as selective weed killers; negatively affecting the growth of broad-leaved plants which are weeds in comparison to the narrow-leaved grasses and cereals grown as crops for food production (which are desired).
They are also used as rooting powders (growth supplement) and to promote growth in tissue culture.

Question 102

Distribution of auxins

Answer 102

The distribution of auxin in the shoots is affected by light and gravity, whereas the distribution in the roots is primarily affected by gravity alone.
If a shoot or root is placed on its side, auxins will accumulate along the lower side as a result of gravity; so the uppermost side has a lower auxin concentration.
In the shoots, the lower side grows faster the upper side, so the shoot grows upwards.
In the roots, the lower side grows slower than the upper side (as auxin inhibits cell elongation and growth in the roots), so the root grows downwards.
Unequal distributions of auxin cause unequal growth rates in plant roots and shoots.