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Cambridge IGCSE0610

Coordination and response

Biology 0610 Chapter Notes

What this chapter covers

Coordination and responseSense organsHormonesHomeostasisTropic responses
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In this chapter

  1. 1The nervous system
  2. 2Neurones and the reflex arc
  3. 3Reflex action
  4. 4The synapse
  5. 5Structure of a synapse
  6. 6Events at a synapse
  7. 7One direction only
  8. 8Sense organs and stimuli
  9. 9Cornea, iris, pupil and lens
  10. 10Retina, optic nerve and blind spot
  11. 11The pupil reflex
  12. 12Muscles of the iris
  13. 13Accommodation
  14. 14Rods and cones
  15. 15The fovea
  16. 16What a hormone is
  17. 17Endocrine glands and their hormones
  18. 18Adrenaline and fight or flight
  19. 19Nervous and hormonal control compared
  20. 20Glucagon
  21. 21Adrenaline and metabolic activity
  22. 22Homeostasis
  23. 23Insulin and blood glucose
  24. 24Negative feedback and the set point
  25. 25Blood glucose: the liver, insulin and glucagon
  26. 26Treating Type 1 diabetes
  27. 27Structures in the skin
  28. 28Keeping warm and keeping cool
  29. 29Vasodilation and vasoconstriction
  30. 30Gravitropism
  31. 31Phototropism
  32. 32Investigating tropisms
  33. 33Chemical control of shoot growth
  34. 34The role of auxin

1. The nervous system

Electrical impulses travel along neurones.

The central nervous system (CNS) consists of the brain and the spinal cord. The peripheral nervous system (PNS) consists of the nerves outside of the brain and spinal cord.

The role of the nervous system is coordination and regulation of body functions.

Examiner insight

“Many candidates appreciated that the central nervous system consists of the brain and spinal cord, although a large majority of candidates incorrectly believed that it consists of the brain, spinal cord and peripheral nerves.” “Most answers were in terms of sensitivity or ability to respond to a stimulus, which are descriptions of parts of the nervous system rather than its function as a whole.”

Worked example 12 marks

State the names of the two parts of the central nervous system (CNS).

  1. 1

    brain [1]

  2. 2

    spinal cord [1]

2. Neurones and the reflex arc

A simple reflex arc has five parts, in this order: receptor, sensory neurone, relay neurone, motor neurone, effector. The impulse starts at the receptor and ends at the effector.

The sensory neurone transmits impulses. It does not detect the stimulus. The relay neurone transmits impulses from the sensory neurone to the motor neurone. The motor neurone passes impulses to the effector.

Examiner insight

“A common error was to reverse the order of the relay and the motor neurone.” “Some candidates described the sensory neurone as detecting stimuli as well as transmitting impulses, which is not correct.”

Worked example 12 marks

Table 4.1 shows structures that are involved in a reflex arc. The structures are not in the correct order. Identify the order that an electrical impulse travels in a reflex arc. Write the letters from Table 4.1 in the correct order.

ABCDE
effectormotor neuronereceptorrelay neuronesensory neurone
  1. 1

    C at the start and A at the end [1]

  2. 2

    E D B in this order, so the full order is C E D B A [1]

3. Reflex action

A reflex action is a means of automatically and rapidly integrating and coordinating stimuli with the responses of effectors (muscles and glands).

The brain is not involved in deciding what action to take in a reflex action.

Examiner insight

“Hardly any candidates cited the fact that reflexes are coordinated, which is an important facet of a reflex action.” “A common error was to mistake the cornea as the effector.”

Worked example 11 mark

In a pupil reflex, light stimulates receptor cells. The impulse then passes along P, then a relay neurone, then Q, then R (the effector), and the pupil diameter changes. Which structures are represented by P, Q and R?

PQR
Amotor neuronesensory neuronecornea
Bmotor neuronesensory neuroneiris
Csensory neuronemotor neuronecornea
Dsensory neuronemotor neuroneiris
  1. 1

    D: P is a sensory neurone, Q is a motor neurone and R is the iris. [1]

4. The synapse

A synapse is a junction between two neurones.

In a reflex arc there is a synapse between the sensory and relay neurones, and another between the relay and motor neurones.

Examiner insight

“Some candidates gave the word ‘neuro- transmitter’ and whilst this chemical would be present in the gap, it is not the name of the gap itself.” “A few gave synapsis as their answer which was not credited as it has a different biological meaning.”

Worked example 11 mark

State the name of the junction between two neurones.

  1. 1

    synapse [1]

5. Structure of a synapse

A synapse has vesicles containing neurotransmitter molecules. Between the two neurones is the synaptic gap.

The next neurone has receptor proteins.

Examiner insight

“A common misconception was to refer to the receptor protein as a receptor cell.” “Many did not read the question carefully and wrote about receptor cells rather than receptor molecules.”

Worked example 13 marks

Describe the structure of a synapse.

  1. 1

    vesicles containing neurotransmitter molecules [1]

  2. 2

    a synaptic gap between the two neurones [1]

  3. 3

    receptor proteins on the next neurone [1]

6. Events at a synapse

An impulse stimulates the release of neurotransmitter molecules from vesicles into the synaptic gap.

The neurotransmitter molecules diffuse across the gap.

They bind with receptor proteins on the next neurone. An impulse is then stimulated in the next neurone.

Examiner insight

“Some candidates stated that a nerve impulse stimulates the vesicles to produce neurotransmitters rather than to release them.” “Candidates should be reminded not to refer to the diffusion of neurotransmitters as impulses.”

Worked example 11 mark

What diffuses across a synapse?

A electrical signals

B neurotransmitters

C receptor molecules

D vesicles

  1. 1

    B: neurotransmitters. [1]

  2. 2

    Not A: neurotransmitter molecules diffuse across the gap, not impulses.

  3. 3

    Not C or D: receptor molecules and vesicles do not cross the gap.

7. One direction only

The vesicles are at the end of one neurone. The receptor proteins are only on the next neurone. Neurotransmitter molecules are released from the first neurone and bind with receptor proteins on the next.

Synapses ensure that impulses travel in one direction only.

Examiner insight

“Many stated that the one- way flow is caused solely by a diffusion gradient and did not refer to vesicles or receptors.”

Worked example 13 marks

A synapse lies between neurone X and neurone Y. Vesicles containing neurotransmitter molecules are found in X only. State which neurone has the receptor proteins. State the direction in which impulses pass at this synapse, and give a reason.

  1. 1

    Y has the receptor proteins. [1]

  2. 2

    Impulses pass in one direction only, from X to Y. [1]

  3. 3

    Neurotransmitter is released only from X and binds only with receptor proteins on Y. [1]

Recap

  • Electrical impulses travel along neurones.
  • CNS = brain and spinal cord. PNS = nerves outside the brain and spinal cord.
  • The nervous system coordinates and regulates body functions.
  • Reflex arc: receptor, sensory neurone, relay neurone, motor neurone, effector.
  • A reflex action is automatic and rapid, and the effectors are muscles and glands.
  • A synapse is a junction between two neurones.
  • Extended: a synapse has vesicles of neurotransmitter, a synaptic gap and receptor proteins.
  • Extended: events are release from vesicles, diffusion, binding with receptor proteins, then an impulse.
  • Extended: synapses ensure impulses travel in one direction only.

8. Sense organs and stimuli

A sense organ is a group of receptor cells responding to specific stimuli.

The stimuli are light, sound, touch, temperature and chemicals.

Give the stimulus, not the sense organ. Heat and cold are both the stimulus of temperature.

Examiner insight

“Very few candidates were able to correctly define a sense organ. Most did not mention receptor cells and simply defined an organ or gave examples of sense organs.” “Candidates were asked to state three other stimuli. Most candidates answered with the names of sense organs.”

Worked example 14 marks

Complete the sentences about sense organs. Each sense organ is a group of ........ cells. These cells respond to specific ........ . The cells can respond to light, ........ , temperature, ........ or touch.

  1. 1

    first gap: receptor [1]

  2. 2

    second gap: specific stimuli (or stimulus) [1]

  3. 3

    third gap, in either order: sound [1]

  4. 4

    fourth gap, in either order: chemicals [1]

9. Cornea, iris, pupil and lens

The cornea refracts light as it enters the eye. Write refracts. Bending is too vague, and reflects is wrong.

The iris controls how much light enters the pupil. The pupil is not a structure, so name the iris as the part that does this.

The lens focuses light on to the retina. The pupil does not focus light.

Examiner insight

“Many said that the function of the iris was to prevent too much light entering the pupil, whereas its function is to control the amount of light that enters.” “Refraction was commonly confused with reflection, as the term to describe what happens to light as it passes into the cornea.”

Worked example 13 marks

Name the part of the eye that

(a) refracts light as it enters the eye,

(b) controls how much light enters the pupil,

(c) focuses light on to the retina.

  1. 1

    (a) the cornea [1]

  2. 2

    (b) the iris [1]

  3. 3

    (c) the lens [1]

10. Retina, optic nerve and blind spot

The retina contains light receptors. Some are sensitive to light of different colours.

The optic nerve carries impulses to the brain. Write impulses, not messages or signals.

Learn where the blind spot is on a diagram of the eye. It is often marked wrongly at the fovea.

Examiner insight

“Only a minority of candidates appreciated that light receptors are found in the retina.” “Very few candidates were able to correctly indicate the position of the blind spot. The fovea was often incorrectly indicated.”

Worked example 13 marks

Describe the function of the retina and of the optic nerve.

  1. 1

    the retina contains light receptors [1]

  2. 2

    some receptors are sensitive to light of different colours [1]

  3. 3

    the optic nerve carries impulses to the brain [1]

11. The pupil reflex

The pupil reflex is the change in pupil diameter when light intensity changes.

In bright light the pupil gets smaller. In dim light the pupil gets larger. The iris changes the size of the pupil.

A smaller pupil reduces the amount of light entering the eye. Too much light would damage the retinal cells.

Examiner insight

“A large number of candidates gave an incomplete answer as they did not state that the amount of light entering the eye would be reduced.” “Some candidates used the word dilate but were unsure if this meant getting larger or smaller.”

Worked example 13 marks

A person walks from a dim room into bright sunlight. Describe the change in the pupil and explain why it happens.

  1. 1

    the pupil gets smaller (its diameter decreases) [1]

  2. 2

    so the amount of light entering the eye is reduced [1]

  3. 3

    this prevents too much light entering the eye, which would damage the retinal cells [1]

12. Muscles of the iris

The iris has circular muscles and radial muscles. They work against each other. This is antagonistic action.

In bright light the circular muscles contract and the radial muscles relax. The pupil gets smaller.

In dim light the circular muscles relax and the radial muscles contract. The pupil gets larger. The ciliary muscles do not control pupil size.

Examiner insight

“While many candidates correctly identified the states of the iris muscles in bright light, some candidates incorrectly believed that the radial muscles contracted and the circular muscles relaxed.” “The most common errors included reversing the roles of the circular and radial muscles or referring to ciliary muscles and suspensory ligaments as having a role in controlling the diameter of the pupil.”

Worked example 14 marks

Explain, in terms of the muscles of the iris, how the pupil gets smaller in bright light.

  1. 1

    the circular muscles contract [1]

  2. 2

    the radial muscles relax [1]

  3. 3

    so the diameter of the pupil decreases [1]

  4. 4

    the two sets of muscle work against each other: antagonistic action [1]

13. Accommodation

Accommodation is the ability of the eye to view near and distant objects. It changes the shape of the lens.

For a distant object the ciliary muscles relax. The suspensory ligaments are taut. The lens is pulled thinner. There is less refraction of light.

For a near object the ciliary muscles contract. The suspensory ligaments become slack. The lens becomes thicker. There is more refraction of light.

Examiner insight

“Some candidates incorrectly suggested that suspensory ligaments are antagonistic muscles, referring to them as contracting.” “The correct changes are that the ciliary muscles contract, suspensory ligaments become slack thereby allowing the lens to become thicker.”

Worked example 15 marks

(a) Explain the changes that occur to view a distant object.

(b) State the term that describes the ability of the eye to view near and distant objects.

  1. 1

    (a) the ciliary muscles relax [1]

  2. 2

    the suspensory ligaments are tensioned / taut [1]

  3. 3

    the lens is pulled flat / becomes thinner [1]

  4. 4

    less refraction of light, so light is focused on to the retina [1]

  5. 5

    (b) accommodation [1]

14. Rods and cones

The retina has two kinds of light receptor, rods and cones.

Rods have greater sensitivity, so they are used for night vision. They work in low light intensity. Rods are absent from the fovea and peak away from it.

There are three different kinds of cone. They absorb light of different colours, which gives colour vision. Cones peak at the fovea and are spread evenly outside it. There are no rods or cones at the blind spot.

Examiner insight

“Some candidates incorrectly thought that rods are found in the fovea.” “Most candidates knew the function of the rod and cone cells. Good responses referred to rod cells and low light intensity.”

Worked example 11 mark

The diagram shows the distribution of two types of light sensitive cells in the retina. What are the names of cell type X, cell type Y and region Z?

cell type Xcell type Yregion Z
Aconesrodsblind spot
Bconesrodsfovea
Crodsconesblind spot
Drodsconesfovea
  1. 1

    Cones peak at the fovea and rods are absent from it. In region Z the dashed line peaks and the solid line falls to zero. So Y is cones, X is rods and Z is the fovea.

  2. 2

    Z is not the blind spot. There are no rods or cones at the blind spot: that is the narrow gap to the right of Z.

  3. 3

    D: rods, cones, fovea [1]

15. The fovea

The fovea is a part of the retina. Yellow spot is also accepted as its name.

It has a high density of cones and no rods. There are three types of cone cell, and they absorb light of different colours. So the function of the fovea is colour vision.

The fovea is not the blind spot.

Examiner insight

“Many candidates incorrectly stated the presence of rod cells in the fovea.” “A few candidates described the role of cone cells in black and white vision rather than colour vision.”

Worked example 13 marks

Describe how the fovea is adapted for its function.

  1. 1

    it contains a high density of cones [1]

  2. 2

    there are three types of cone cell, which absorb light of different colours or wavelengths [1]

  3. 3

    so the fovea gives colour vision [1]

Recap

  • A sense organ is a group of receptor cells responding to specific stimuli: light, sound, touch, temperature and chemicals.
  • Cornea refracts light. Iris controls how much light enters. Lens focuses light on to the retina.
  • Retina contains light receptors. Optic nerve carries impulses to the brain.
  • Pupil reflex: bright light, smaller pupil, less light enters. Dim light, larger pupil.
  • Extended: in bright light the circular muscles contract and the radial muscles relax. They are antagonistic.
  • Extended: for a distant object the ciliary muscles relax, the ligaments are taut, the lens is thinner and there is less refraction. This is accommodation.
  • Extended: rods are more sensitive and give night vision. Three kinds of cone give colour vision. The fovea has a high density of cones and no rods.

16. What a hormone is

A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.

A gland that releases hormones is an endocrine gland. Hormones travel in the blood. They do not travel along nerves, and red blood cells do not carry them.

Examiner insight

“Definitions of the term hormone were sometimes incomplete and did not match the definition given in the syllabus.” “Common misconceptions were that hormones travelled along the nerves or that they were transported by the red blood cells.”

Worked example 11 mark

Which statement about human hormones is correct?

A A hormonal response lasts for a very short time. · B Hormones are transported by red blood cells. · C The heart is one of the target organs of adrenaline. · D The testes produce adrenaline.

  1. 1

    C: the heart is one of the target organs of adrenaline. [1]

17. Endocrine glands and their hormones

Each endocrine gland secretes its own hormone. The adrenal glands secrete adrenaline. The pancreas secretes insulin. The testes secrete testosterone. The ovaries secrete oestrogen.

Insulin acts to decrease blood glucose concentration. Saying that it ‘controls’ or ‘regulates’ the concentration is not precise enough.

Examiner insight

“Simply stating adrenaline gland was not accepted.” “Insulin acts to lower the blood glucose concentration and those candidates who stated that insulin ‘regulates’ or ‘controls’ did not gain credit as this answer is not precise enough.”

Worked example 11 mark

Insulin is a hormone produced in humans.

State the effect of insulin in the human body.

  1. 1

    decreases blood glucose (concentration) [1]

18. Adrenaline and fight or flight

Adrenaline is the hormone secreted in ‘fight or flight’ situations. The heart is one of its target organs.

It increases breathing rate, increases heart rate and increases pupil diameter.

Examiner insight

“Adrenaline increases the size of the pupils.” “Many candidates described how adrenaline makes a person feel, rather than how it affects the body.”

Worked example 11 mark

Which statement about adrenaline is correct?

A It decreases the heart rate. · B It decreases the diameter of the pupil. · C It increases the breathing rate. · D It is made in the pancreas.

  1. 1

    C: it increases the breathing rate. [1]

19. Nervous and hormonal control compared

Nervous control acts faster than hormonal control. Hormonal control acts more slowly.

Hormones have effects that last longer than nervous control.

Examiner insight

“Some candidates suggested nervous communication was fast without making a comparative statement with hormonal communication.”

Worked example 12 marks

Compare nervous and hormonal control in speed of action and duration of effect.

  1. 1

    nervous control acts faster than hormonal control [1]

  2. 2

    hormonal control has effects that last longer than nervous control [1]

20. Glucagon

Glucagon is secreted by the pancreas. It increases blood glucose concentration.

It stimulates the breakdown of glycogen to glucose in the liver. Glucagon is the hormone, not glycogen.

Examiner insight

“Glucagon is easily confused with glycogen, so accurate spelling is particularly important.” “While many candidates gave the correct response, some candidates incorrectly believed that glucagon is produced by the liver.”

Worked example 13 marks

Name the hormone from the pancreas that increases blood glucose concentration. Explain how it does this.

  1. 1

    glucagon [1]

  2. 2

    stimulates the breakdown of glycogen to glucose [1]

  3. 3

    in the liver (cells) [1]

21. Adrenaline and metabolic activity

Adrenaline increases blood glucose concentration. It does this by the breakdown of glycogen to glucose.

Adrenaline also increases heart rate.

The extra glucose is used in respiration to release energy for muscle contraction.

Examiner insight

“Few candidates could link the effect of adrenaline on blood glucose to the fight-or-flight reaction in sufficient detail to gain credit.” “Very few recognised that adrenaline causes the breakdown of glycogen to glucose but more stated that there was an increase in glucose concentration.”

Worked example 11 mark

Glucagon is a hormone that increases blood glucose concentration.

State the name of one other hormone that increases blood glucose concentration.

  1. 1

    adrenaline [1]

Recap

  • A hormone is a chemical substance, produced by a gland and carried by the blood, which alters the activity of one or more specific target organs.
  • Adrenal glands: adrenaline. Pancreas: insulin. Testes: testosterone. Ovaries: oestrogen.
  • Insulin decreases blood glucose concentration.
  • Adrenaline, in fight or flight: increases breathing rate, heart rate and pupil diameter.
  • Nervous control is faster. Hormonal effects last longer.
  • Extended: glucagon is secreted by the pancreas and increases blood glucose concentration.
  • Extended: adrenaline increases blood glucose concentration, by the breakdown of glycogen to glucose.

22. Homeostasis

Homeostasis is the maintenance of a constant internal environment.

Blood glucose concentration and internal body temperature are two things the body keeps steady. Both are examples of homeostatic control.

Examiner insight

“Many candidates misread the question and gave a detailed description of the mechanism of homeostasis instead of a definition.”

Worked example 11 mark

Give the term for the maintenance of a constant internal environment.

  1. 1

    homeostasis [1]

23. Insulin and blood glucose

Insulin is a hormone. It decreases blood glucose concentration.

Name the effect. Words such as ‘regulates’, ‘controls’ and ‘balances’ are not precise enough.

Examiner insight

“The role of insulin was often given as controlling blood glucose or regulating blood glucose. These answers were not accepted.”

Worked example 11 mark

Insulin is released into the blood after a meal. State the effect of insulin on blood glucose concentration.

  1. 1

    decreases blood glucose concentration [1]

24. Negative feedback and the set point

Homeostatic control works by negative feedback. The set point is the level the body keeps to.

If the level rises above the set point, a response brings it down. If it falls below, a response brings it up. Either way the level returns to the set point.

Examiner insight

“A number of candidates gave the response of homeostasis, which was already provided to them. Reference to negative feedback was the required response.”

Worked example 11 mark

The control of internal body temperature is an example of homeostatic control by negative feedback. State the name of one other example of homeostatic control.

  1. 1

    blood glucose concentration [1]

25. Blood glucose: the liver, insulin and glucagon

The pancreas monitors blood glucose concentration and releases the hormones. The liver stores and releases the glucose.

Insulin decreases blood glucose concentration. It stimulates liver cells to take up glucose and store it as glycogen.

Glucagon increases blood glucose concentration. It stimulates the breakdown of glycogen to glucose in the liver cells.

Examiner insight

“A common error was to state that the liver releases insulin and that the brain detects the rise in glucose concentration and instructs the pancreas to release insulin.” “Many candidates knew that glycogen was the product of glucose storage. Some gave incorrect spellings that were too similar to glucagon and so were not accepted.”

Worked example 11 mark

The table shows the blood glucose concentrations in the hepatic portal vein and the hepatic vein of a human.

Which statement explains the blood glucose concentrations in the table?

A Glucagon has been released by the pancreas to decrease blood glucose concentration.

B Glucagon has been released by the pancreas to increase blood glucose concentration.

C Insulin has been released by the pancreas to decrease blood glucose concentration.

D Insulin has been released by the pancreas to increase blood glucose concentration.

blood vesselblood glucose concentration / mg per dm³
hepatic portal vein70
hepatic vein100
  1. 1

    The hepatic vein has the higher concentration, so the blood glucose concentration has increased.

  2. 2

    B: glucagon has been released by the pancreas to increase blood glucose concentration. [1]

26. Treating Type 1 diabetes

People with diabetes cannot control their blood glucose concentration. After a meal, theirs is higher and returns to the starting level more slowly.

Treatment for Type 1 diabetes is insulin by injection, therapy, patch or pump. It also includes regular blood glucose tests, counting carbohydrates in the diet and matching food intake to activity.

Examiner insight

“The expected answer ‘regular blood glucose test’ was rarely seen. However, in answers where ‘glucose testing’ was mentioned the word ‘blood’ was very often omitted.” “Simple answers about healthy diet and exercise did not include the detail required.”

Worked example 13 marks

Outline the treatment for type 1 diabetes.

  1. 1

    insulin, injection / therapy / patch / pump [1]

  2. 2

    monitoring blood glucose / regular blood glucose tests [1]

  3. 3

    counting / monitoring, (named) carbohydrates (in diet) [1]

27. Structures in the skin

The skin has hairs, receptors, sensory neurones, blood vessels and fatty tissue. It also has hair erector muscles and sweat glands.

Learn three by their jobs. The hair erector muscle contracts to raise the hair. The sweat gland produces the liquid that cools the skin. Fatty tissue provides insulation.

Examiner insight

“Candidates were unfamiliar with the structures in mammalian skin, and few were awarded full marks.”

Worked example 13 marks

Name the part of the skin that:

contracts to raise the hair

produces liquid that cools the skin

provides insulation.

  1. 1

    hair erector muscle [1]

  2. 2

    sweat gland [1]

  3. 3

    fatty tissue [1]

28. Keeping warm and keeping cool

The brain monitors the temperature of the blood and coordinates the response.

In the cold, raised hairs trap a layer of insulating air. Fatty tissue also insulates. Shivering raises body temperature: muscle contraction generates heat.

In the heat, sweat glands produce sweat. Evaporation of the water needs heat energy. The body supplies this energy, so body temperature falls.

Examiner insight

“The main misconception was the idea that hair prevents cold air getting in, rather than prevents heat loss.” “Extremely few candidates said that evaporation of water required energy and that as this (heat) energy is supplied by the body, the body temperature is lowered.”

Worked example 11 mark

What is an example of a response that reduces body temperature?

A increased respiration · B shivering · C sweating · D vasoconstriction

  1. 1

    C: sweating. [1]

29. Vasodilation and vasoconstriction

Arterioles supply the capillaries at the skin surface. The arterioles change width. They do not move, and veins and capillaries do not change.

In the cold, vasoconstriction occurs. The arterioles constrict, so less blood flows to the skin surface capillaries.

In the heat, vasodilation occurs. More blood flows through the arterioles supplying the skin surface.

Examiner insight

“Some candidates referred to contraction of incorrect blood vessels such as veins or capillaries rather than constriction of arterioles.” “There were also some vague references to less blood flowing to the skin rather than less blood flow to the skin surface.”

Worked example 13 marks

Explain the changes that occur in the blood vessels in the skin in a cold environment.

  1. 1

    vasoconstriction [1]

  2. 2

    arterioles, constrict / become narrower [1]

  3. 3

    less blood flows to (skin) surface capillaries [1]

Recap

  • Homeostasis = the maintenance of a constant internal environment.
  • Insulin decreases blood glucose concentration.
  • Extended: homeostatic control works by negative feedback around a set point.
  • Extended: insulin decreases blood glucose concentration and glucagon increases it. The liver stores glucose as glycogen and breaks glycogen down to glucose.
  • Extended: Type 1 diabetes is treated with insulin, regular blood glucose tests and control of carbohydrate intake.
  • Extended: the brain monitors the temperature of the blood. Hair, fat, sweating and shivering help control body temperature.
  • Extended: vasoconstriction = arterioles constrict, less blood flows to the skin surface capillaries. Vasodilation = more blood flows to them.

30. Gravitropism

A tropism is the growth of parts of a plant towards or away from a stimulus.

Gravitropism is a response in which parts of a plant grow towards or away from gravity. Roots grow towards gravity. Shoots grow away from gravity.

Gravity acts on a plant in the dark as well, so a shoot under the soil still grows upwards.

Examiner insight

“Roots are positively gravitropic and grow downwards whereas shoots are negatively gravitropic and grow upwards even in total darkness.” “The term gravitropism was not widely known. Common incorrect answers included gravity, phototropism and sensitivity.”

Worked example 11 mark

A seed is planted under the soil and the shoot grows upwards.

What is the name of this response when the shoot is growing under the soil?

A gravitropism · B phototropism · C photosynthesis · D transpiration

  1. 1

    A: gravitropism. [1]

31. Phototropism

Phototropism is a response in which parts of a plant grow towards or away from the direction of the light source.

Shoots grow towards light. Roots grow away from light.

A plant in the dark has no light stimulus, so it cannot show phototropism.

Examiner insight

“Few candidates understood that roots respond to light by growing away from it.” “Some candidates incorrectly selected option A, all parts of a plant grow towards light.”

Worked example 11 mark

How do shoots and roots normally respond to light and gravity?

shoots: lightshoots: gravityroots: lightroots: gravity
Atowardsawayawaytowards
Bawaytowardstowardsaway
Ctowardsawaytowardsaway
Dawaytowardsawaytowards
  1. 1

    A: shoots grow towards light and away from gravity, roots grow away from light and towards gravity. [1]

32. Investigating tropisms

To show gravitropism, remove the light stimulus. A horizontal shoot grown in the dark and a shoot grown upright in the dark demonstrate it.

A light-proof box does this. Any curving of the root or shoot is then a response to gravity, not light.

The angle of the root and of the shoot can be measured at regular times. An increase in angle is found by subtraction, not as a percentage.

Examiner insight

“Phototropism was a common incorrect answer as candidates thought the plant was responding to light even though the diagram shows the two plants enclosed in a box that excludes light.” “A significant number of candidates would benefit from further practice in performing the appropriate calculation, as many of the calculations presented involved the procedure to achieve a percentage.”

Worked example 12 marks

A student places a maize seedling horizontally in a light-proof box. The student measures the angle of the root and shoot every 2 hours.

The graph shows the results of the investigation.

Use the information in the graph to calculate the increase in the angle of the root between 6 hours and 12 hours.

  1. 1

    read the root angle at 6 hours and at 12 hours: 46° and 60° [1]

  2. 2

    60 - 46 = 14^(°) [1]

33. Chemical control of shoot growth

Phototropism and gravitropism of a shoot are examples of the chemical control of plant growth. The chemical is auxin.

Auxin is unequally distributed across the shoot, in response to light and to gravity. The cells on the side with more auxin elongate more, so the shoot grows in a curve.

Examiner insight

“Some candidates incorrectly thought that auxin will move to the light side of the shoot, this would cause the shoot to bend away from the light.” “Some candidates stated that auxin is produced in the area in which the effect occurs, and others stated that auxin moves against gravity in roots.”

Worked example 14 marks

A shoot is lit from one side only. Explain how the shoot grows towards the light.

  1. 1

    auxin is made in the shoot tip [1]

  2. 2

    auxin diffuses down the shoot [1]

  3. 3

    more auxin on the side away from the light [1]

  4. 4

    auxin stimulates cell elongation on that side, so the shoot grows towards the light [1]

34. The role of auxin

Auxin is made in the shoot tip. It diffuses through the plant from the shoot tip.

Auxin is unequally distributed in response to light and gravity. Auxin stimulates cell elongation.

In a shoot on its side in the dark, auxin collects on the lower side. The cells there elongate more, so the shoot grows upwards.

Examiner insight

“A common error was to incorrectly identify the increased elongation was occurring on the upper side.” “Candidates often discussed elongation of roots or stems without stating explicitly that it was the cells that were elongating.”

Worked example 11 mark

A plant shoot was placed on its side in the dark.

Which row shows the expected changes as the shoot grows in these conditions?

side with increased concentration of auxinside showing increased cell elongation
Aupperlower
Bupperupper
Clowerlower
Dlowerupper
  1. 1

    C: auxin increases on the lower side, and the cells on the lower side elongate more. [1]

Recap

  • Tropism = growth of parts of a plant towards or away from a stimulus.
  • Gravitropism = growth towards or away from gravity. Roots grow towards it, shoots away.
  • Phototropism = growth towards or away from the direction of the light source. Shoots grow towards light, roots away.
  • A plant in the dark cannot show phototropism: name gravitropism.
  • Investigate gravitropism in a light-proof box, and measure the angle of the root and shoot over time.
  • Extended: auxin is made in the shoot tip and diffuses through the plant.
  • Extended: auxin is unequally distributed in response to light and gravity, and stimulates cell elongation.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. State what travels along neurones.1 mark
  2. A student says that the CNS consists of the brain, the spinal cord and the nerves outside them. Correct the student.2 marks
  3. State the role of the nervous system in the body.1 mark
  4. An impulse travels from a receptor along a neurone to a relay neurone. Name this type of neurone. Then name the type of neurone that carries impulses to an effector.2 marks
  5. Write these parts of a simple reflex arc in the order an impulse passes through them: effector, relay neurone, receptor, motor neurone, sensory neurone.2 marks
  6. A person touches a hot pan and pulls their hand away at once. The response is a reflex action. (a) Name the type of structure that is the effector. (b) State two features of a reflex action that this shows.3 marks
  7. A reflex arc has a receptor, one sensory neurone, one relay neurone, one motor neurone and an effector. State how many synapses there are between these neurones, and name the two neurones at each synapse.3 marks
  8. Name the structure that holds neurotransmitter molecules, the structure they cross, and the structure on the next neurone that they bind with.3 marks
  9. A student describes a synapse: ‘When an impulse arrives, the vesicles produce neurotransmitter. The impulse then diffuses across the gap and binds with receptor cells.’ (a) The description contains three mistakes. Correct each one. (b) Molecules of a drug bind with the receptor proteins at this synapse. The drug does not stimulate an impulse. Neurotransmitter molecules are still released as normal. Explain why fewer impulses are stimulated in the next neurone when the drug is present.6 marks
  10. A scientist studies the synapse between neurone P and neurone Q. In each test one neurone is stimulated so that an impulse travels along it to the synapse. The scientist records whether neurotransmitter appears in the synaptic gap, and whether an impulse then appears in the other neurone. The results are in the table below. Substance Z does not stop impulses travelling along P. (a) State which neurone has the vesicles. Use tests 1 and 2 to give a reason. (b) State what tests 1 and 2 tell you about the direction of impulses at this synapse. (c) Identify the first event at the synapse that substance Z prevents. Explain why there is no impulse in Q in test 3. | Test | Neurone stimulated | Neurotransmitter in the gap | Impulse in the other neurone | | --- | --- | --- | --- | | 1 | P | yes | yes | | 2 | Q | no | no | | 3 | P, after substance Z is added | no | no |6 marks
  11. Give the definition of a sense organ.2 marks
  12. Name the stimulus that each of these cells responds to. (a) receptor cells in the ear (b) receptor cells in the eye (c) receptor cells in the skin that respond when the air gets colder3 marks
  13. Name the part of the eye that (a) controls how much light enters the pupil, (b) refracts light as it enters the eye.2 marks
  14. Describe the function of the lens, of the retina and of the optic nerve.3 marks
  15. A student walks into a dark cinema. Describe the change in her pupils and its effect.2 marks
  16. A doctor shines a bright torch into a patient's eye. The pupil gets smaller. (a) State the stimulus. (b) Name the part of the eye that contains the receptors for this stimulus. (c) Name the part of the eye that changes the size of the pupil. (d) Explain why the response is useful.4 marks
  17. A person walks into a dark room. Explain how the muscles of the iris change the pupil.4 marks
  18. A person walks into a very dark room. She can see the shapes of the furniture but not its colours. Explain why, in terms of the light receptors in her retina.3 marks
  19. The lens of an old woman’s eye is stiff and stays thin. Her ciliary muscles and suspensory ligaments still work. She sees a distant hill clearly, but the words in a nearby book are blurred. (a) State how her ciliary muscles and suspensory ligaments change as she reads. (b) Explain why the words in the book are blurred. (c) A student writes: “Her suspensory ligaments are too weak to contract.” State the error.6 marks
  20. The graph shows the distribution of two types of light receptor, X and Y, across the retina of a human eye. R and S are two regions of the retina. (a) Name region R. Give the evidence from the graph. (b) Name region S and cell type X. Give the evidence from the graph. (c) An owl hunts at night. Almost all the light receptors in its retina are of one type, X or Y. State which type, explain how this helps the owl to hunt, and suggest one disadvantage.8 marks
  21. Name the type of gland that releases hormones into the blood.1 mark
  22. A student defines a hormone as ‘a chemical that is carried in the blood to a target organ’. State two things missing from this definition.2 marks
  23. Name the gland that secretes (a) testosterone and (b) oestrogen.2 marks
  24. Rewrite to earn two marks: ‘Insulin is secreted by the liver and controls blood glucose.’2 marks
  25. A cyclist sees a car pull out in front of her. Her body responds in a ‘fight or flight’ way. (a) Name the hormone secreted. (b) State two effects of this hormone on her body.3 marks
  26. A person is frightened by a loud bang and adrenaline is released. State what happens to the diameter of the pupils, and name the gland that releases adrenaline.2 marks
  27. A door slams. A man jumps at once. His heart rate rises and stays raised for several minutes. (a) State which of these two responses is caused by a hormone, and name the hormone. (b) Use the two responses to compare nervous control and hormonal control.4 marks
  28. A student writes: ‘Glycogen is a hormone secreted by the liver. It increases blood glucose concentration.’ The name of the hormone and the organ are both wrong. (a) State the correct name of the hormone and the organ that secretes it. (b) State how this hormone increases blood glucose concentration.3 marks
  29. A climber slips. The table gives measurements made before and five minutes after the slip. (a) Calculate the percentage increase in her blood glucose concentration. (b) Name the hormone responsible. State how it increases blood glucose concentration. (c) Explain how the extra glucose in her blood helps her muscles in a ‘fight or flight’ situation. | measurement | before | after | | --- | --- | --- | | heart rate / beats per minute | 70 | 120 | | blood glucose concentration / mg per 100 cm³ | 80 | 110 |6 marks
  30. In each of three tests, one hormone rises in a person’s blood: adrenaline, glucagon or insulin. The table gives measurements made before and after each rise. (a) Identify hormones P, Q and R. Give the evidence from the table for each. (b) Name the gland that secretes both P and Q, and the gland that secretes R. (c) State how Q and R cause their change in blood glucose concentration. | hormone | blood glucose before / mg per 100 cm³ | blood glucose after / mg per 100 cm³ | pupil diameter before / mm | pupil diameter after / mm | | --- | --- | --- | --- | --- | | P | 90 | 70 | 4 | 4 | | Q | 90 | 115 | 4 | 4 | | R | 90 | 110 | 4 | 7 |6 marks
  31. Define the term homeostasis.1 mark
  32. Insulin and glucagon are both hormones that act on blood glucose. State which of the two decreases blood glucose concentration.1 mark
  33. A student writes: ‘Insulin controls blood glucose.’ Explain why this does not earn the mark and write an answer that does.2 marks
  34. The control of internal body temperature is an example of homeostatic control. Name the type of feedback that brings the temperature back to the set point.1 mark
  35. After a meal, a person's blood glucose concentration rises. Name the hormone the pancreas releases, and describe what happens to glucose in the liver.3 marks
  36. A newly diagnosed patient has Type 1 diabetes. Outline three parts of the treatment.3 marks
  37. (a) Name the structure in the skin that contracts to raise the hair. (b) Explain how raised hair helps a person in cold weather.3 marks
  38. A runner's skin is flushed on a hot day. Explain the change in the blood vessels in her skin.3 marks
  39. A scientist measures the glucose concentration of the blood entering and leaving a person's liver. She does this 1 hour after a meal and again 7 hours after the meal. The person eats nothing in between. Her results are in the table. The set point for this person's blood glucose concentration is 90 mg per 100 cm³. (a) Explain the change in glucose concentration as the blood passes through the liver 1 hour after the meal. (b) Explain the change 7 hours after the meal. (c) Explain why the two changes together are an example of negative feedback. | time after the meal / hours | concentration entering the liver / mg per 100 cm³ | concentration leaving the liver / mg per 100 cm³ | | --- | --- | --- | | 1 | 140 | 110 | | 7 | 75 | 90 |8 marks
  40. A cyclist rides hard up a long hill. She wears a waterproof jacket that does not let water vapour out. At the top her internal body temperature is 38.2 °C and her skin is covered in liquid sweat. Her set point is 37.0 °C. (a) Describe how the rise in her internal body temperature is detected. (b) Explain why sweating has cooled her less than it would without the jacket. (c) She takes the jacket off and rests in a cold wind. Ten minutes later her internal body temperature is 36.6 °C. Explain the change that now occurs in the blood vessels in her skin.8 marks
  41. State what is meant by gravitropism.1 mark
  42. A seedling is lit from one side only. State the direction of growth of its shoot and of its root in response to the light.2 marks
  43. A bean seed germinates in the dark, buried in soil. State the direction in which its root grows, the direction in which its shoot grows, and name the response.3 marks
  44. A student places a bean seedling horizontally in a light-proof box for 24 hours. Explain why the box is light-proof, and name the response that takes place.2 marks
  45. A student places a maize seedling horizontally in a light-proof box and measures the angle of its root. Calculate the increase in the angle of the root between 3 hours and 9 hours. | time / hours | 0 | 3 | 6 | 9 | | --- | --- | --- | --- | --- | | angle of root / ° | 0 | 12 | 30 | 45 |2 marks
  46. State where auxin is made in a shoot, and how it moves through the shoot.2 marks
  47. A shoot is lit from one side only. A student writes: ‘Auxin moves to the light side of the shoot and makes the cells there divide.’ Correct the two mistakes in this statement.2 marks
  48. A shoot is placed on its side in the dark. State which side has more auxin, which side elongates more, and what happens to the direction of growth of the shoot.3 marks
  49. Two shoots, P and Q, are lit from the left side only. The tip of shoot Q is covered by a light-proof cap. After two days shoot P has grown towards the light. Shoot Q has grown straight up. Explain, in terms of auxin, the growth of each shoot.5 marks
  50. Two maize seedlings, R and S, are pinned horizontally inside a light-proof box. R is kept still. S is pinned to a disc that rotates constantly, so every side of its shoot receives the same pull of gravity. After seven days the shoot of R has curved upwards. The shoot of S has grown straight, without curving. (a) Name the response of the shoot of R. (b) Explain, in terms of auxin, why the shoot of R curves upwards. (c) Explain why the shoot of S does not curve.6 marks

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