Control & Coordination

Question 1

Why does the body need to pass information throughout?

Answer 1

It is essential that information can pass between these different parts, so that their activities are coordinated.
i.e. the control of blood glucose concentrations in mammals or to change the activity of some part of the organism in response to an external stimulus, such as moving away from something that may do harm.

Question 2

Types of information transfer used to coordinate the body’s activities

Answer 2

• nerves that transmit information in the form of electrical impulse
• hormones that are substances secreted into the blood

Question 3

Endocrine system

Answer 3

The endocrine system consists of all the ductless glands in the body. Examples are the pituitary gland, the islets of Langerhans in the pancreas, the adrenal glands, the testes and the ovaries.

Question 4

What are steroids and why are their receptors present inside of cells?

Answer 4

The testes and ovaries secrete the steroid hormones – testosterone, oestrogen and progesterone. Steroids are lipid-soluble, so they can pass through the phospholipid bilayer. Once they have crossed the cell surface membrane, they bind to receptor molecules inside the cytoplasm or the nucleus and activate processes such as
transcription

Question 5

Neurones

Answer 5

Nerve cells are also known as neurones, and they carry information directly to their target cells. Neurones coordinate the activities of sensory receptors (e.g. those in the eye), decision-making centres in the CNS, and effectors such as muscles and glands.

Question 6

Sensory neurones

Answer 6

A neurone that transmits nerve impulses from a receptor to the central nervous system (CNS)

Question 7

Intermediate neurones

Answer 7

Intermediate neurones (also known as relay or connector neurones) transmit impulses from sensory neurones to motor neurones.

Question 8

Motor neurones

Answer 8

Motor neurones transmit impulses from the CNS to effectors.

Question 9

Motor neurone structure

Answer 9

The cell body of a motor neurone lies within the spinal cord or brain. The nucleus of a neurone is always in its cell body. Often when viewed in the light microscope, dark specks can be seen in the cytoplasm. These are small regions of rough endoplasmic reticulum that synthesise proteins. Thin cytoplasmic processes extend from the cell body. Some are very short and often have many branches – these are dendrites. A motor neurone has many highly branched dendrites. These provide a large surface area for the axon terminals of other neurones. The axon is much longer and conducts impulses over long distances. Within the cytoplasm of an axon there are some organelles such as mitochondria. The ends of the branches of the axon have mitochondria, together with many vesicles containing chemicals called transmitter substances. These vesicles are involved in passing impulses to an effector cell such as a muscle cell or a gland.

Question 10

Sensory neurone structure

Answer 10

A sensory neurone has the same basic
structure as a motor neurone, but it has one long axon with a cell body that may be near the source of stimuli or in a swelling of
a spinal nerve known as a ganglion.

Question 11

Relay neurone location

Answer 11

Relay neurones are found entirely within the CNS.

Question 12

Myelin

Answer 12

Insulating material that spirals around, enclosing the axons of many neurones; myelin is made of layers of cell surface membranes formed by Schwann cells so that they are very rich in phospholipids and therefore impermeable to water and ions in tissue fluid. The sheath affects the speed of conduction of the nerve impulse.

Question 13

node of Ranvier

Answer 13

A very short gap between Schwann cells where myelinated axons are not covered in myelin so are exposed to tissue fluid.

Question 14

Positioning of node of Ranvier

Answer 14

They occur about every 1–3 mm in human neurones. The nodes themselves are very small, around 2–3 μm long.

Question 15

Example of coordination of neurones

Answer 15

Some reflex arcs have no intermediate neurone and the impulse passes directly from the sensory neurone to the motor neurone. An example is the knee-jerk (patellar) reflex. Within the CNS, the sensory neurone and the relay neurone have extensions that branch to connect with other neurones in the CNS. These connections allow the information from sensory neurones to be integrated so that complex forms of behaviour can be coordinated.

Question 16

Action potentials

Answer 16

The signals are very brief changes in the distribution of electrical charge across the cell surface membranes of neurones and muscle cells caused by the inward movement of sodium ions. These signals are called action potentials.

Question 17

Resting potential

Answer 17

The difference in electrical potential that is maintained across the cell surface membrane of a neurone when it is not transmitting an action potential; it is normally about –70 mV inside and is partly maintained by sodium–potassium pumps.

Question 18

All the factors that contribute to the resting potential of a neurone

Answer 18

• Sodium–potassium pumps in the cell surface membrane. These constantly move sodium ions (Na+) out of the axon, and potassium ions (K+) into the axon. The sodium–potassium pumps are membrane proteins that use energy from the hydrolysis of ATP to move both of these ions against their concentration gradients. Three sodium ions are removed from the axon for every two potassium ions brought in for every one molecule of ATP hydrolysed.
• The presence of many organic anions inside the cell, such as negatively charged proteins.
• The impermeability of the membrane to ions; sodium ions cannot diffuse through the axon membrane when the neurone is at rest.
• Channel proteins that respond to changes in the potential difference across the membrane are closed so sodium and potassium ions cannot diffuse through them. These are of two types, the ones that all the time and the ones that open/close depending on the p.d called Voltage-gated channel proteins. Open all the time: There are far more of these for potassium than for sodium and thus overall excess of negative ions are inside the membrane compared with outside (read 2nd para on pg 395). Voltage-gated channel proteins: Channel proteins that allow movement of sodium and potassium ions through cell membranes by opening or closing in response to changes in electrical potential across the membranes.

Question 19

Two factors that influence the inward movement of sodium ions during an action potential

Answer 19

there is a steep concentration gradient, and also the inside of the membrane is negatively charged, which attracts positively charged ions. A ‘double’ gradient like this is known as an electrochemical gradient.

Question 20

Events occuring to achieve threshold potential and depolarisation with reference to positive feedback

Answer 20

First, the electric current used to stimulate the axon causes the opening of the voltage-gated channels in the cell surface membrane. This allows sodium ions to pass through. Because there is a much greater concentration of sodium ions outside the axon than inside, sodium ions enter through the open channels. To begin with, only a few channels open. The inward movement of sodium ions changes the potential difference across the membrane, which becomes less negative on the inside. This is called depolarisation. It trigger some more channels to open so that more sodium ions enter. There is more depolarisation. If the potential difference reaches about –50 mV, then many more channels open and the inside reaches a potential of +30 mV compared with the outside. This is an example of a positive feedback because a small depolarisation leads to a greater and greater depolarisation. Action potentials are only generated if the potential difference reaches a value between –60 mV and –50 mV. This value is the threshold potential. If this value is not reached, an action potential does not occur.

Question 21

Events after depolarisation and towards repolarisation

Answer 21

After about 1 ms, all the sodium ion voltage-gated channels close, so sodium ions stop diffusing into the axon. At the same time, another set of voltagegated channel proteins open to allow the diffusion of potassium ions out of the axon, down their concentration gradient. The outward movement of potassium ions removes positive charge from inside the axon to the outside, thus returning the potential difference to normal (−70 mV). This is called repolarisation. In fact, the potential difference across the membrane briefly becomes even more negative than the normal resting potential (Figure 15.13). The potassium ion channel proteins (voltage-gated) then close and the sodium ion channel proteins become responsive to depolarisation again. The sodium–potassium pump continues pumping sodium ions out and potassium ions in all the time.

Question 22

Define depolarisation, repolarisation and threshold potential

Answer 22

Depolarisation: the reversal of the resting
potential across the cell surface membrane of a neurone or muscle cell, so that the inside becomes positively charged compared with the outside.
Repolarisation: returning the potential difference across the cell surface membrane of a neurone or muscle cell to normal following the depolarisation of an action potential.
Threshold potential: the critical potential difference across the cell surface membrane of a sensory receptor or neurone which must be reached before an action potential is initiated.

Question 23

Passage of action potential along a neurone

Answer 23

An action potential at any point in an axon’s cell surface membrane triggers the production of an action potential in the membrane on either side of that point. These local circuits depolarise the resting regions where voltage-gated sodium ion channel proteins open and the membrane potential reaches the threshold potential so that action potentials pass along the membrane It is a convention to show the direction of current flow with arrows pointing from positive to negative.

Question 24

Explain refractory period

Answer 24

This period of recovery, when the axon is unresponsive, is called the refractory period. This means:

• action potentials are discrete events; they do not merge into one another
• there is a minimum time between action potentials occurring at any one place on a neurone
• the length of the refractory period determines the maximum frequency at which impulses are transmitted along neurones; for many neurones this is between 200 and 300 impulses per second
• the impulse can only travel in one direction along the neurone.

Question 25

What determines the strength of a stimulus arising due to action potential?

Answer 25

The speed at which action potentials travel does not vary according to the size of the stimulus. In any one axon, the speed of impulse transmission is always the same. The frequency of action potentials arriving along the axon of a sensory neurone, and the number of neurones carrying action potentials, determines the strength of impulse detected by the CNS. Peak value of action potential is always +30 mV.

Question 26

Two factors determine the speed of conduction

Answer 26

• the presence or absence of myelin
• the diameter of axons. Thick axons transmit impulses faster than thin ones. This is because they have a greater surface area over which diffusion of ions can occur, which increases the rate of diffusion.

Question 27

Saltatory conduction

Answer 27

The local circuits exist from one node to the next. Thus action potentials ‘jump’ from one node to the next, a distance of 1–3 mm. This is called saltatory conduction. In a myelinated axon, saltatory conduction can increase the speed of transmission by up to 50 times that in an unmyelinated axon of the same diameter.

Question 28

Role of receptor cells

Answer 28

Receptor cells are transducers: they convert the energy of stimuli – such as light, heat or sound – into electrical impulses in neurones.

Question 29

Chemoreceptor

Answer 29

A receptor cell that responds to chemical stimuli; chemoreceptors are found in taste buds on the tongue, in the nose and in blood vessels where they detect changes in oxygen and carbon dioxide concentrations.

Question 30

Example of internal stimuli

Answer 30

Stretch receptors respond to changes inside the muscles.

Question 31

Chemoreceptors in taste buds

Answer 31

The tongue is covered in many small bumps or papillae. Each papilla has many taste buds over its surface. Within each taste bud are between 50 and 100 receptor cells that are sensitive to chemicals in the liquids that you drink or chemicals from your food that dissolve in saliva. Each chemoreceptor is covered with receptor proteins that detect these different chemicals. There are several types of receptor proteins, each detecting a different type of chemical and giving you a different sensation. There are five tastes: sweet, sour, salt, bitter and umami (savoury).

Question 32

Achieving receptor potential and passing of impulse to taste centre in CNS

Answer 32

Chemoreceptors in the taste buds that detect salt are directly influenced by sodium ions. These ions diffuse through highly selective channel proteins in the cell surface membrane of the microvilli leading to depolarisation of the membrane. The increase in positive charge inside the cell is the receptor potential. If there is sufficient stimulation by sodium ions in the mouth, the receptor potential becomes large enough to stimulate the opening of voltage-gated calcium ion channel proteins. Calcium ions enter the cytoplasm and lead to exocytosis of vesicles containing neurotransmitter from the basal membrane of the chemoreceptor. The neurotransmitter stimulates an action potential in the sensory neurone that transmits impulses to the taste centre in the cerebral cortex of the brain.

Question 33

All-or-none law

Answer 33

Neurones and muscle cells only transmit impulses if the initial stimulus is sufficient to increase the membrane potential above a
threshold potential.

Question 34

Normal case with threshold potentials as stimulus increases.

Answer 34

Threshold levels in receptors rarely stay constant. With continued stimulation, they often increase so that it requires a greater stimulus before receptors send impulses along sensory neurones.

Question 35

Summarised steps on stimulus passed on by taste buds

Answer 35

1. Entry of sodium ions depolarises the membrane in the sensory zone.
2. Depolarisation of the membrane stimulates calcium ion channel proteins to open.
3. Entry of calcium ions stimulates movement of vesicles and release of neurotransmitter by exocytosis.
4. If stimulation is above threshold, impulses travel to the brain along the sensory neurone.

Question 36

Synaptic cleft vs Synapse

Answer 36

Synaptic cleft: a very small gap between two neurones at a synapse; nerve impulses are transmitted across synaptic clefts by neurotransmitters.
Synapse: a point at which two neurones meet but do not touch; the synapse is made up of the end of the presynaptic neurone, the synaptic cleft and the end of the postsynaptic neurone.

Question 37

Outline of events occuring at a synapse

Answer 37

1. An action potential occurs at the cell surface membrane of the first neurone, or presynaptic neurone.
2. The action potential causes the release of molecules of transmitter substance into the cleft.
3. The molecules of transmitter substance diffuse across the cleft and bind temporarily to receptors on the postsynaptic neurone.
4. The postsynaptic neurone responds to all the impulses arriving at any one time by depolarising; if the overall depolarisation is above its threshold, then it will send impulses.

Question 38

Detail of events occuring at the Cholinergic synapse

Answer 38

In the part of the membrane of the presynaptic neurone that is next to the synaptic cleft, the arrival of the action potential also causes voltage-gated calcium ion channel proteins to open. Thus, the action potential causes not only sodium ions but also calcium ions to diffuse into the
cytoplasm of the presynaptic neurone. There are virtually no calcium ions in the cytoplasm but many in the tissue fluid surrounding the synapse. This means that there is a very steep electrochemical gradient for calcium ions. The influx of calcium ions stimulates vesicles containing
ACh to move to the presynaptic membrane and fuse with it, emptying their contents into the synaptic cleft. The attachment of vesicle to membrane is helped by SNARE proteins which are on the vesicles and the presynaptic membrane. Each action potential causes just a few vesicles to do this, and each vesicle contains up to 10 000 molecules of ACh. The ACh diffuses across the synaptic cleft, usually in less than 0.5 ms. The cell surface membrane of the postsynaptic neurone contains receptor proteins. Part of the receptor protein molecule has a complementary shape to part of the ACh molecule, so that the ACh molecules can temporarily bind with the receptors. The binding changes the shape of the protein, opening channels through which sodium ions can pass. Sodium ions diffuse into the cytoplasm of the postsynaptic neurone down the electrochemical. The choline is taken back into the presynaptic neurone, where it reacts with acetyl coenzyme A to form ACh once more. The ACh is then transported into the presynaptic vesicles, ready for the next action potential.

Question 39

Chances of increasing the effect of generating an impulse

Answer 39

The chance that an action potential is generated and an impulse sent in the postsynaptic neurone is increased if more than one presynaptic neurone releases ACh at the same time or over a short period of time.

Question 40

Neuromuscular junction

Answer 40

A synapse between a motor neurone and a muscle.

Question 41

Roles of synapses (detail)

Answer 41

Synapses ensure one-way transmission: Impulses can only pass in one direction at synapses. This is because neurotransmitter is released on one side
and its receptors are on the other. There is no way that chemical transmission can occur in the opposite direction.
Synapses allow the interconnection of nerve pathways: Synapses allow a wider range of behaviour than could be generated in a nervous system in which neurones were directly ‘wired up’ to each other.
Interconnection of many nerve pathways: This happens in two ways:
➊ Individual sensory and relay neurones have axons that branch to form synapses with many different neurones; this means that information from one neurone can spread out throughout the body to reach many motor neurones and many effectors as happens when you respond to dangerous situations.
➋ There are many neurones that terminate on each relay and motor neurone as they have many dendrites to give a large surface area for many synapses; this allows one neurone to integrate the information coming from many different parts of the body – something that is essential for decision-making in the brain.

Question 42

Types of muscle tissue and examples

Answer 42

Striated: This type of muscle tissue makes up the many muscles in the body that are attached to the skeleton. Striated muscle only contracts when it is stimulated to do so by impulses that arrive via motor neurones. Muscle tissue like this is described as being neurogenic.
Cardiac: Cardiac muscle in the heart is myogenic – it contracts and relaxes automatically, with no need for impulses arriving from neurones.
Smooth: It is found throughout the body in organs, such as in the gas exchange system, alimentary canal and in the walls of the arteries, arterioles and veins. Most smooth muscle only contracts when it receives impulses in motor neurones. However, smooth muscle in arteries also contracts when it is stretched by the pressure of blood surging through them. This happens without any input from the nervous system. This type of muscle is called smooth because, unlike the other two types of muscle tissue, it has no striations. Smooth muscle does not form smooth linings of tubular structures, such as the trachea and arteries; the lining of these structures is always formed by an epithelium.

Question 43

Structure of striated muscle

Answer 43

A muscle such as a biceps is made up of thousands of muscle fibres. Each muscle fibre is a highly specialised ‘cell’ with a highly organised arrangement of contractile proteins in the cytoplasm, surrounded by a cell surface membrane. The cell surface membrane is the sarcolemma, the cytoplasm is sarcoplasm and the endoplasmic reticulum is sarcoplasmic reticulum (SR). The sarcolemma has many deep infoldings into the interior of the muscle fibre, called transverse system tubules (also known as T-system tubules or T-tubules for short). These run close to the SR. The membranes of the SR have huge numbers of protein pumps that transport calcium ions into the lumen (cisternae) of the SR. The sarcoplasm often contains a large number of mitochondria, often packed tightly between the myofibrils. These carry out aerobic respiration, generating the ATP that is required for muscle contraction

Question 44

What are striation and how are they produced?

Answer 44

Parallel groups of thick filaments lie between groups of thin ones. Both thick and thin filaments are made up of protein. The thick filaments are made mostly of myosin, while the thin ones are made mostly of actin.
These are produced by a very regular arrangement of many myofibrils in the
sarcoplasm. Each myofibril is striped in exactly the same way, and is lined up precisely against the next one, so producing the pattern that is seen.

Question 45

Structure of a myofibril

Answer 45

The darker parts of the stripes, the A bands, correspond to the thick (myosin) filaments. The lighter parts, the I bands, are where there are no thick filaments, only thin (actin) filaments. The very darkest parts of the A band are produced by the overlap of thick and thin filaments, while the lighter area within the A band, known as the H band, represents the parts where only the thick filaments are present. A line known as the Z line provides an attachment for the actin filaments, and the M line does the same for the myosin filaments. The part of a myofibril between two Z lines is called a sarcomere. Myofibrils are cylindrical in shape, so the Z line is a disc separating one sarcomere from another and is also called the Z disc.

Question 46

Ultrastructure of thick and thin filaments

Answer 46

Thick filaments are composed of many molecules of myosin, which is a fibrous protein with a globular head. The fibrous portion helps to anchor the molecule into the thick filament. Within the thick filament, many myosin molecules all lie together in a bundle with their globular heads all pointing away from the M line. The main component of thin filaments, actin, is a globular protein. Many actin molecules are linked together to form a chain. Two of these chains are twisted together to form a thin filament. Also twisted around the actin chains is a fibrous protein called tropomyosin. Another protein, troponin, is attached to the actin chain at regular intervals.

Question 47

Muscle contraction (detail)

Answer 47

Muscles cause movement by contracting. The sarcomeres in each myofibril get shorter as the Z discs are pulled closer together. It is known as the sliding filament model of muscle contraction. The energy for the movement comes from ATP molecules that are attached to the myosin heads. Each
myosin head is an ATPase. When a muscle contracts, calcium ions are released from stores in the SR and bind to troponin. This stimulates troponin molecules to change shape. The troponin and tropomyosin proteins move to a different position on the thin filaments, so exposing parts of the actin molecules, which act as binding sites for myosin. The myosin heads bind with these sites, forming cross-bridges between the two types of filament. Next, the myosin heads move, pulling the actin filaments along towards the centre of the sarcomere.
The heads then hydrolyse ATP molecules, which provide enough energy to enable the heads to let go of the actin. The heads move back to their previous positions and bind again to the exposed sites on the actin. The thin filaments have moved as a result of the previous power stroke, so myosin heads now bind to actin further along the thin filaments closer to the Z disc. The myosin heads move again, pulling the actin filaments even further along, then hydrolyse more ATP molecules so that they can let go again. This continues for as long as the troponin and tropomyosin molecules are not blocking the binding sites, and the muscle has a supply of ATP.

Question 48

Stimulation of muscle contraction

Answer 48

Skeletal muscle contracts when it receives an impulse from a neurone. An impulse moves along the axon of a motor neurone and arrives at the presynaptic membrane. A neurotransmitter, generally ACh, diffuses across the neuromuscular junction and binds to receptor proteins on the postsynaptic membrane – which is the sarcolemma (the cell surface membrane of the muscle fibre). The binding of ACh stimulates the ion channels to open, so that sodium ions enter to depolarise the membrane and generate an action potential in the sarcolemma. Impulses pass along the sarcolemma and along the T-tubules towards the centre of the muscle fibre. The membranes of the SR are very close to the T-tubules. The arrival of the impulses causes voltage-gated calcium ion channel proteins in the membranes to open. Calcium ions diffuse out of the SR, down a very steep concentration gradient, into the sarcoplasm surrounding the myofibrils. The calcium ions bind with troponin molecules that are part of the thin filaments. This changes the shape of the troponin molecules, which causes the troponin and tropomyosin to move so exposing the binding sites for the myosin heads. The myosin heads attach to the binding sites on the thin filaments and form cross-bridges.

Question 49

Recovery of muscle length after contraction

Answer 49

When there is no longer any stimulation from the motor neurone, there are no impulses conducted along the T-tubules. Released from stimulation, the calcium ion channels in the SR close and carrier proteins pump calcium ions back into stores in the SR. As calcium ions leave their binding sites on troponin, tropomyosin moves back to cover the myosin-binding sites on the thin filaments. When there are no cross-bridges between thick and thin filaments, the muscle is in a relaxed state. There is nothing to hold the filaments together so any pulling force applied to the muscle will lengthen the sarcomeres so that they are ready to contract (and shorten) again. Each skeletal muscle in the body has an antagonist – a muscle that restores sarcomeres to their original lengths when it contracts. For example, the triceps is the antagonist of the biceps.

Question 50

Summary of events in a neuromuscular junction and in muscle fibre

Answer 50

1. An action potential arrives.
2. The action potential causes the diffusion of calcium ions into the neurone.
3. The calcium ions cause vesicles containing ACh to fuse with the presynaptic membrane.
4. ACh is released and diffuses across the synaptic cleft.
5. ACh molecules bind with receptors in the sarcolemma, causing them to open channel proteins for sodium ions.
6. Sodium ions diffuse in through the open channels in the sarcolemma. This depolarises the membrane and initiates an action potential which spreads along the membrane.
7. The depolarisation of the sarcolemma spreads down T-tubules.
8. Channel proteins for calcium ions open and calcium ions diffuse out of the sarcoplasmic reticulum.
9. Calcium ions bind to troponin. Tropomyosin
   moves to expose myosin-binding sites on the actin filaments. Myosin heads form cross-bridges with thin filaments and the
   sarcomere shortens.

Question 51

General coordination mechanism in plants

Answer 51

Most plant responses involve changing some aspect of their growth to respond to factors such as gravity, light and water availability. Plants can also respond fairly quickly to changes in carbon dioxide concentration, lack of water, grazing by animals and infection by fungi and bacteria. Some of these responses are brought about by quick changes in turgidity, as happens when stomata respond to changes in humidity, carbon dioxide concentration and water availability.

Question 52

Electrical communication in plants (overview)

Answer 52

Microelectrodes inserted into leaf cells detect changes in potential difference that are very similar to action potentials in animals. The depolarisation results not from the influx of positively charged sodium ions but from the outflow of negatively charged chloride ions. Repolarisation is achieved in the same way as in neurones, by the outflow of potassium ions. Plants do not have specific nerve cells, but many of their cells transmit waves of electrical activity that are very similar to those transmitted along the neurones of animals. The action potentials travel along the cell membranes of plant cells and from cell to cell through plasmodesmata that are lined by cell membrane. The action potentials generally last much longer and travel more slowly than in animal neurones.

Question 53

Venus fly trap structure

Answer 53

The specialised leaf is divided into two lobes either side of a midrib. The inside of each lobe is often red and has nectar-secreting glands around the edge to attract insects. Each lobe has three stiff sensory hairs that respond to being deflected. The outer edges of the lobes have stiff hairs that interlock to capture the insect inside. The surface of the lobes has many glands that secrete enzymes for the digestion of trapped insects. The touch of a fly or other insect on the sensory hairs on the inside of the folded leaves of the Venus fly trap stimulates action potentials that travel very fast across
the leaf causing it to fold over and capture the insect.

Question 54

Mechanism of impulse generation and closure of Venus fly trap

Answer 54

The deflection of a sensory hair activates calcium ion channels in cells at the base of the hair. These channels open so that calcium ions flow in to generate a receptor potential. If two of these hairs are stimulated within a period of 20−35 seconds, or one hair is touched twice within the same time interval, action potentials travel across the trap. When the second trigger takes too long to occur after the first, the trap will not close, but a new time interval starts again. If a hair is deflected a third time, the trap will still close. The time between stimulus and response is about 0.5 s. It takes the trap less than 0.3 s to close and capture the insect The lobes of the leaf bulge upwards when the trap is open. They are convex in shape. The lobes rapidly change into a concave shape, bending downwards so the trap snaps shut it is likely that the rapid change occurs as a result of a
release of elastic tension in the cell walls. However, the trap is not completely closed at this moment. To seal the trap, it requires ongoing activation of the trigger hairs by the captured prey. Unless the prey is able to escape, it will further stimulate the inner surface of the lobes, thereby triggering further action potentials. This forces the edges of the lobes together, sealing the trap so that the prey can be digested. Further deflections of the sensory hairs by the insect stimulate the entry of calcium ions into gland cells. Here, calcium ions stimulate the exocytosis of vesicles containing digestive enzymes in a similar way to their
role in synapses. The traps stay shut for up to a week for digestion to take place. Once the insect is digested, the cells on the upper surface of the midrib grow slowly so the leaf reopens and tension builds in the cell walls of the midrib and the trap is set again.

Question 55

Adaptations of Venus fly trap to avoid wastage of energy

Answer 55

First, the stimulation of a single hair does not trigger closure. This prevents the traps closing when it rains or when a piece of debris falls into the trap. Second, the gaps between the stiff hairs that form the ‘bars’ of the trap allow very small insects to crawl out. The plant would waste energy digesting a very small ‘meal’.

Question 56

Types of plant growth regulators

Answer 56

• Auxins, which influence many aspects of growth including elongation growth which determines the overall length of roots and shoots.
• Gibberellins, which are involved in seed germination and controlling stem elongation.

Question 57

What are plant growth regulators (plant hormones) ?

Answer 57

Any chemical produced in plants that influences their growth and development (e.g. auxins, gibberellins, cytokinins and ABA). They move in the plant either directly from cell to cell (by diffusion or active transport) or are carried in the phloem sap or xylem sap. Some may not move far from their site of synthesis and may have their effects on nearby cells. Plant hormones interact with receptors on the surface of cells or in the cytoplasm or nucleus. These receptors usually initiate a series of chemical or ionic signals that amplify and transmit the signal within the cell.

Question 58

Auxins (detail)

Answer 58

Plants make several chemicals known as auxins, of which the principal one is IAA. This is often simply referred to as ‘auxin’.
IAA is synthesised in the growing tips (meristems) of shoots and roots, where the cells are dividing. IAA is transported back down the shoot, or up the root, by active transport from cell to cell, and also to a lesser extent in phloem sap. Growth in plants occurs at meristems, such as those
at shoot tips and root tips. Growth occurs in three stages: cell division by mitosis, cell elongation by absorption of water, and cell differentiation. Auxin is involved in controlling growth by elongation. Auxin stimulates cells to pump hydrogen ions (protons) into the cell wall. The cell walls become acidified, which leads to a loosening of the bonds between cellulose microfibrils and the matrix that surrounds them. The cells absorb water by osmosis and the increase in the internal pressure causes the walls to stretch so that these cells elongate (become longer). Molecules of auxin bind to a receptor protein on the
cell surface membrane. The binding of auxin stimulates ATPase proton pumps to move hydrogen ions across the cell surface membrane from the cytoplasm into the cell wall. In the cell walls, proteins known as expansins are activated by the decrease in pH. The expansins loosen the linkages between cellulose microfibrils. Expansins disrupt hydrogen bonds between the cellulose microfibrils and surrounding substances, such as hemicelluloses, in the cell wall. This disruption occurs briefly so that microfibrils can move past each other allowing the cell to expand without losing much of the overall strength of the wall.
Potassium ion channels are also stimulated to open leading to an increase in potassium ion concentration in the cytoplasm. This decreases the water potential so water enters through aquaporins.

Question 59

Gibberellins (detail)

Answer 59

They are present in especially high concentrations in young leaves and in seeds, and are also found in stems, where they have an important role in determining their growth. Gibberellins promote cell extension in stems in a different way to auxins. Gibberellins stimulate enzymes known as XET in the cell walls of stems. XET breaks bonds within hemicellulose molecules so that cellulose microfibrils can move further apart, so allowing cell walls to expand. When the seed is shed from the parent plant, it is in a state of dormancy; that is, it contains very little water and is metabolically inactive. This is useful because it allows the seed to survive in adverse conditions, such as through a cold winter, only germinating when the temperature rises in spring. The seed contains an embryo, which will grow to form the new plant when the seed germinates. The embryo is surrounded by endosperm, which is an energy store containing the polysaccharide starch. On the outer edge of the endosperm is a protein-rich aleurone layer. The whole seed is covered by a tough, waterproof, protective layer. The absorption of water at the beginning of germination stimulates the embryo to produce gibberellins. These gibberellins diffuse to the aleurone layer and stimulate the cells to synthesise amylase. The amylase mobilises energy reserves by hydrolysing starch molecules in the endosperm, converting them to soluble maltose molecules. These maltose molecules are converted to glucose and transported to the embryo, providing a source of carbohydrate that can be respired to provide energy as the embryo begins to grow. Gibberellins cause these effects by regulating genes that are involved in the synthesis of amylase. In barley seeds, it has been shown that application of gibberellin causes an increase in the transcription of mRNA coding for amylase. It has this action by promoting the destruction of DELLA proteins that inhibit factors that promote
transcription.

Question 60

Define endosperm and aleurone layer

Answer 60

Endosperm: a tissue in some seeds, such as barley, that is a store of starch and other nutrients.
Aleurone layer: a layer of tissue around the endosperm that synthesises amylase during germination.

Question 61

Summary on germination of barley seed

Answer 61

1. Water uptake initiates germination.
2. Embryo synthesises gibberellin in response to water uptake.
3. Aleurone layer synthesises amylase in response to gibberellin.
4. Amylase hydrolyses starch to maltose, which is broken down to glucose and transferred to the embryo and respired.