Chapter 15: Coordination Flashcards
(124 cards)
1
Q
What forms part of the central nervous system?
A
Brain and spinal cord.
2
Q
What forms part of the peripheral nervous system?
A
Cranial and spinal nerves
3
Q
In what form is information transferred?
A
Nerve impulses/ electrical impulses
4
Q
What are nerve cells also called?
A
Neurones
5
Q
What are the three types of neurone?
A
Sensory neurone, intermediate neurones, motor neurones
6
Q
What does the motor neurone do?
A
It transmits impulses from the brain or spinal cord to a muscle or gland.
7
Q
Where is the nucleus of a neurone?
A
In its cell body
8
Q
Where is the cell body of a motor neurone?
A
In the spinal cord or the brain
9
Q
Give a brief overview of the structure of an unmyelinated neurone
A
Thin cytoplasmic processes extend from the cell body known as dendrites which give a large surface area. The axon is much longer and conducts impulses over long distances. There are small regions of RER which synthesise proteins present in the cytoplasm. The branches of the axon as well as the cytoplasm contain mitochondria.
10
Q
Where is the cell body of a sensory neurone found?
A
It may be found near the source of stimuli or in a swelling of a spinal nerve known as a ganglion.
11
Q
Where are relay neurones found?
A
They are found entirely within the central nervous system.
12
Q
Describe the myelin sheath of neurones.
A
Myelin is made when Schwann cells wrap themselves around the axon all along its length, enclosing the axon in many layers of its cell surface membrane. This enclosing sheath, called the myelin sheath, is made largely of lipid, together with some proteins.
13
Q
What does the myelin sheath affect?
A
It affects the speed of conduction of the nerve impulse.
14
Q
What are nodes of Ranvier?
A
The small, uncovered areas of axon between Schwann cells are called nodes of Ranvier.
15
Q
How often do nodes of Ranvier occur and how big are they?
A
They occur about every 1-3mm in human neurones and nodes are around 2-3um long.
16
Q
What is a reflex arc?
A
It is a pathway along which impulses are transmitted from a receptor to an effector without involving the ‘conscious’ regions of the brain.
17
Q
Can some reflex arcs not contain an intermediate neurone?
A
Yes
18
Q
What is a reflex action?
A
It is a response to a stimulus before there is any voluntary response involving the ‘conscious’ regions of the brain.
19
Q
In what part of the neurone is there the travel of electrical impulses?
A
Cell surface membrane of the neurone.
20
Q
In a resting axon, is the inside or the outside of the axon more negative?
A
The inside of the axon has a slightly negative electrical potential compared with the outside.
21
Q
What is potential difference?
A
It is the difference between the voltage.
22
Q
What is resting potential? ref. pd
A
The potential difference is often between -60mV and -70mV. In other words, the electrical potential of the inside of the axon is between 60 and 70mV lower than the outside. This difference is the resting potential.
23
Q
What is the resting potential maintained by?
A
The sodium-potassium pumps in the cell surface membrane of the neurones.
24
Q
Explain what happens at the resting potential.
A
The Na-K pumps constantly move 3Na ions out for every 2K ions that are brought in using the energy from the hydrolysis of ATP to move both of these ions against their concentration gradients. The membrane has protein channels for K and Na open all the time, however there are far more of these for the K than for Na. Therefore, K diffuses back out again much faster than sodium diffuses back in. In addition, there are many large, negatively charged molecules inside the cell that attract the K ions reducing the chance that they will diffuse out. The result of these effects is an overall excess of negative ions inside the membrane compared with the outside. The membrane is relatively impermeable to sodium.
25
What are two things that influence the inward movement of sodium ions during an action potential? ref. gradient
1. There is a steep concentration gradient.
2. The inside of the membrane is negatively charged which attracts positively charged ions.
A double gradient like this is known as an electrochemical gradient.
26
What is an action potential and what is it caused by?
An action potential is a potential difference across the membrane, caused by changes in the permeability of the cell surface membrane to Na and K ions.
27
What is a voltage gated channel?
As well as the channels open all the time, there are other channels in the membrane that open and close depending on the electric potential(or voltage) across the membrane, and therefore known as voltage gated channels.
28
When are the voltage gated channels closed?
At the resting potential
29
What happens in an action potential? *full
First, the electric current is used to stimulate the axon causes the opening of the voltage gated channels in the membrane which allow Na ions to pass through. As there is a much greater concentration of Na ions outside the axon than inside, Na ions enter through the open channels. This changes the potential difference across the membrane, making the inside less negative. This depolarisation triggers some more channels to open so that more sodium ions enter, creating more depolarisation. If the potential difference reaches about -50mV, then many more channels open and the inside reaches a potential of +30mV compared with the outside. This is an example of positive feedback because a small depolarisation leads to a greater and greater depolarisation.
After about 1ms, all the sodium voltage gated channels close, so the sodium ions stop diffusing into the axon. At the same time, the potassium ion channels open causing potassium ions to diffuse out of the axon, down their conc gradient. The outward movement of potassium ions removes positive charge from inside the axon to the outside, thus returning the potential difference back to normal(-70mV). This is called repolarisation.
In fact, the membrane briefly becomes even more negative than the normal resting potential. The K ion channels then close and the sodium ions channels become responsive to depolarisation. The Na-K pump continues to pump sodium ions out and K ions in and this helps to maintain the distribution of sodium ions and potassium ions across the membrane.
30
What determines whether an action potential is generated?
Action potentials are only generated if the potential difference reaches a value between -60mV and -50mV. This value is called the threshold potential. If it less than this, then an action potential does not occur.
31
What is the function of a neurone?
To transmit electrical impulses along itself.
32
Where does an action potential begin on an axon?
Anywhere along the axon's cell surface membrane.
33
Why can't action potentials be generated behind with reference to refractory period?
This is because the region behind it will still be recovering from the action potential it has just had and the sodium ion voltage gated channels are shut and cannot be stimulated to open. This period of recovery is the refractory period when the axon is unresponsive.
34
What are the differences between a strong and weak stimulus?
A strong stimulus produces a rapid succession of action potentials, therefore having a high frequency of action potentials. A weak stimulus results in fewer action potentials per second.
Moreover, a strong stimulus is more likely to stimulate more neurones than a weak stimulus.
35
What provides the brain with information about the strength of the stimulus being detected?
The brain can interpret the frequency of action potentials arriving along the axon of a sensory neurone and the number of neurones carrying action potentials, to get information about the strength of the stimulus being detected.
36
How is the nature of the stimulus deduced by the brain?
It is deduced from the position of the sensory neurone bringing the information.
37
How does myelin speed up the rate at which action potentials travel? Where do action potentials occur? \What is saltatory conduction?
It does this by insulating the axon membrane. Na and K ions cannot flow out of the membrane and so it is not possible for depolarisation or action potentials to occur in myelinated parts of the axon. Action potentials can only occur at nodes of Ranvier, where all the channel proteins and pump proteins are concentrated. The local circuits exist from one node to the next. Thus, action potentials jump from one node to the next, a distance of 1-3mm. This is called saltatory conduction. This can increase the speed of transmission.
38
How does diameter affect the speed of transmission?
Thick axons transmit impulses faster than thin ones, as their resistance is much less.
39
What is a receptor cell?
A receptor cell converts energy from one form into energy in an electrical impulse, responding to a stimulus by initiating an action potential.
40
Where are chemoreceptors found?
In the taste buds.
41
What is every chemoreceptor covered with?
It is covered with different receptor proteins that each detect different chemicals.
42
What are the five tastes?
Sweet, sour, salt, bitter and umami
43
What happens when the chemoreceptors detect salt?
When the chemoreceptors in the taste buds of papillae detect salt, the sodium ions diffuse through highly selective channel proteins in the cell surface membrane of the microvilli and this leads the the depolarisation of the membrane. The increase in positive charge inside the cell is the receptor potential. If there is sufficient stimulation by Na ions in the mouth then the receptor potential becomes large enough to stimulate the opening of voltage gated calcium ion channels. Calcium ions enter the cytoplasm and lead to the exocytosis of vesicles containing neurotransmitter from the basal membrane. The neurotransmitter stimulates an action potential in the sensory neurone that transmits impulses to the taste centre in the cerebral cortex of the brain.
44
What is the all or nothing law with action potentials?
When receptors are stimulated they are depolarised. If the receptor potential is below a certain potential, the stimulus only causes local depolarisation of the receptor cell and the sensory neurone is not stimulated to send impulses and vice versa. Neurones either transmit impulses or they do not.
45
What is a synaptic cleft?
Where two neurones meet, there is a gap called the synaptic cleft.
46
What is the average measurement of a synaptic cleft?
20nm
47
What is a synapse?
The parts of the two neurones near the cleft and the cleft itself make up a synapse.
48
What is the outline of the sequence of synaptic transmission?
■ An action potential occurs at the cell surface
membrane of the presynaptic neurone.
■ The action potential causes the release of molecules of transmitter substance into the cleft.
■ The molecules of transmitter substance diffuse across the cleft and bind temporarily to receptors on the postsynaptic neurone.
■ 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.
49
What are some examples of neurotransmitters in the nervous system?
Noradrenaline and acetylcholine(ACh)
50
What are cholinergic synapses?
Synapses that use acetylcholine as the transmitter substance are known as cholinergic synapses.
51
Explain the sequence of events that occur in a cholinergic synapse.
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 calcium ion voltage gated channels to open. Thus, the action potential causes sodium and 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, creating a 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, releasing its contents into the synaptic cleft. Each action potential causes just a few vesicles to do this. ACh diffuses across the synaptic cleft, usually in less than 0.5ms.
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. This changes the shape of the protein, opening channels through which sodium ions can pass. Sodium ions diffuse into the cytoplasm of the postsynaptic neurone and depolarise the membrane, starting an action potential.
52
Why are the channels on the postsynaptic neurone called chemically gated ion channels?
This is because the channels are stimulated by chemicals(neurotransmitters) and not by a voltage change.
53
What would happen if ACh were to remain bound to the receptor proteins on the postsynaptic neurone?
The sodium channels would remain open and the postsynaptic neurone would remain permanently depolarised.
54
How is ACh recycled?
The synaptic cleft contains an enzyme, acetylcholinesterase, which catalyses the hydrolysis of each ACh molecule into acetate and choline.
The choline is taken back into the presynaptic neurone, where it is combined with acetyl coenzyme A to form ACh once more. The ACh is then transported into the presynaptic vesicles, ready for the next action potential.
55
How much time does it take for the entire sequence of events, from the initial arrival of the action potential to the re-formation of ACh take?
5-10ms
56
What are the roles of synapses?
1. Synapses ensure one-way transmission.
2. They allow integration of impulses.
3. Synapses allow the interconnection of nerve pathways.
4. Synapses are involved in memory and learning
57
What is neurogenic?
It is when the muscle is stimulated to do something by impulses that arrive via motor neurones.
58
What are striated muscles?
They are muscles attached to the skeleton.
59
What is a syncytium?
They are cells with multiple nuclei.
60
What is each muscle fibre made of?
It is made of a specialised syncytium(multinucleate) with a highly organised arrangement of contractile proteins in the cytoplasm, surrounded by a cell surface membrane.
61
What is the cell surface membrane of a muscle fibre called?
Sarcolemma
62
What is the cytoplasm of a muscle fibre called?
Sarcoplasm
63
What is the endoplasmic reticulum of a muscle fibre called?
Sarcoplasmic reticulum
64
What are the deep infoldings of the muscle fibre called?
Transverse system tubules or T-tubules
65
What do the T tubules run close to?
To the sarcoplasmic reticulum.
66
What do the membranes of the sarcoplasmic reticulum have large numbers of and what does x do?
The membranes have huge numbers of protein pumps that transport calcium ions into the cisternae of the sarcoplasmic reticulum.
67
What does the sarcoplasm contain a large number of? Where is x found and what does it do?
The sarcoplasm contains a large number of mitochondria packed tightly between the myofibrils which carry out aerobic respiration, generating the ATP required for muscle contraction.
68
What are myofibrils made up of?
Myofibrils are made up of thick and thin filaments which are made of protein.
69
What are the thick filaments made up of?
Myosin
70
What are the thin filaments made up of?
Actin
71
What is the shape of the myofibril like?
Cylindrical in shape
72
What forms the A band?
The darkest parts of the A band are formed by the overlap of the 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.
73
What is the I band?
Here, there are only thin filaments and no thick filaments.
74
What is the M line?
It provides an attachment for the myosin filaments.
75
What is the Z line?
It provides an attachment for the actin filaments.
76
What is a sarcomere?
The part of a myofibril between two Z lines is called a sarcomere.
77
What separates one sarcomere from another?
The Z line is a disc that separates one sarcomere from another and is also called the Z disc.
78
What is myosin?
It is a fibrous protein with a globular head.
79
What is actin?
It is a globular protein.
80
What kind of protein is tropomyosin?
It is a fibrous protein.
81
What is the structure of the thin filament?
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 tropomyosin. Troponin, a protein is also attached to the chain at regular intervals.
82
What is the structure of the thick filament?
Many myosin molecules lie together in a bundle with their globular heads all pointing away from the M line.
83
Where does the energy required for the movement of muscles come from?
It comes from ATP molecules that are attached to myosin heads. Each myosin head is an ATPase.
84
Explain how muscles contract.
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 tilt, pulling the actin filaments along towards the centre of the sarcomere. The heads then hydrolyse ATP molecules, which provide enough energy to force the heads to let go of the actin filaments. The heads tip back to their previous positions and bind again to the exposed sites on the actin. the actin filaments have moved as a result of the previous contraction, so myosin heads now bind to actin further along the thin filaments closer to the Z disc. They tilt again, pulling the actin filament even further along, then hydrolyse more ATP molecules so that they can let go again. This goes on and on, until the troponin and tropomyosin molecules are not blocking the binding sites and as long as the muscle has a supply of ATP.
85
Explain how a muscle is stimulated.
A neurotransmitter, generally ACh, diffuses across the neuromuscular junction and binds to receptor proteins on the postsynaptic membrane, which is the sarcolemma. The binding of ACh stimulates ion channels to open, so that sodium ions can 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 sarcoplasmic reticulum are very close to the T tubules. The arrival of the impulses causes calcium ion channels in the membranes to open. Calcium ions diffuse out, down a very steep concentration gradient, into the sarcoplasm surrounding the myofibrils. The calcium ions combine with the troponin molecules, which causes the troponin and tropomyosin to move away and expose the binding sites for the myosin heads. |The myosin heads attach to the binding sites on the thin filaments and form cross-bridges.
86
What happens when there is no more stimulation from the motor neurone?
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 the calcium pumps move calcium ions back into stores in the sarcoplasmic reticulum. 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.
87
What is the other source of ATP apart from the supply in the muscle fibres?
Creatine phosphate.
88
Where is creatine phosphate stored in muscles?
In the sarcoplasm
89
Explain how creatine phosphate is used in the muscle fibres
A phosphate group can be used from creatine phosphate and combined with ADP to produce ATP
Creatine phosphate + ADP= Creatine + ATP
90
What happens to the creatine produced when the demand for energy has slowed down or stopped?
Creatine + ATP= creatine phosphate + ADP
91
What happens to the creatine if energy is still being demanded by muscles and there is no ATP to spare?
If there is no ATP to spare to regenerate the creatine phosphate, the creatine is converted to creatinine and excreted in urine.
92
Give some examples of hormones made in the endocrine gland.
Adrenaline, insulin, glucagon and ADH
93
What is a gland?
A gland is a group of cells that produces and secretes one or more substances.
94
Are hormone cell signalling molecules?
Yes
95
How many days are in the menstrual cycle?
28 days
96
What is the menstrual cycle coordinated by?
It is coordinated by glycoprotein hormones released by the anterior pituitary gland and by the ovaries.
97
What hormones does the anterior pituitary gland secrete and what activity do they control?
Follicle stimulating hormone and luteinising hormone control the activity of the ovaries.
98
What secretes oestrogen and when?
When follicles develop, they secrete oestrogen.
99
What secretes progesterone and when?
After the female gamete is released, from the ovary at ovulation, the remains of the follicle secretes progesterone.
100
What is considered to be the start of the menstrual cycle?
Menstruation which lasts 4 to 8 days.
101
Describe the process of the menstrual cycle.
During the time of menstruation, the anterior pituitary gland secretes LH and FSH and their concentration increase over the next few days.
In the ovary, one follicle becomes the dominant one. The presence of FSH and LH stimulates the secretion of oestrogen from the cells surrounding the follicle. The presence of oestrogen has a negative feedback effect on the production of LH and FSH, so the conc of the two hormones decrease. The oestrogen stimulates the lining of the endometrium to grow, thicken and develop numerous blood capillaries.
When the oestrogen conc in the blood has reached a level of around two to four times its level at the beginning of the cycle, it stimulates a surge in the secretion of LH and to a lesser extent, FSH. This surge of LH causes the dominant follicle to burst and to shed its gamete into the oviduct. This usually happens around 14-36 hours after the LH surge. The follicle then collapses to form the corpus luteum, which secretes progesterone and some oestrogen. Together these two hormones maintain the lining of the uterus, making it ready to receive the embryo if fertilisation occurs. Progesterone also inhibits the anterior pituitary gland from secreting FSH so no more follicles develop.
High levels of oestrogen and progesterone in the second half of the cycle inhibit the secretion of FSH and LH by the anterior pituitary. This means that there is less stimulation of the corpus luteum so that it begins to degenerate and secrete less oestrogen and progesterone. As the concentration of these two hormones decrease, the endometrium is not maintained and menstruation begins. The decrease also releases the anterior pituitary from inhibition, so FSH is secreted to begin another cycle.
102
What does the pill contain and why?
The pill contains steroid hormones that suppress ovulation.
103
Why are synthetic hormones used more than natural ones?
It is because synthetic hormones are not broken down so rapidly in the body and therefore act for longer.
104
What are combined oral contraceptives?
They are birth control pills that contain both progesterone and oestrogen.
105
How does the combined oral contraceptive pill work? *full
Both oestrogen and progesterone suppress the secretion of FSH and LH from the anterior pituitary gland, preventing their concentrations from reaching the levels that would stimulate ovulation. This essentially mimics the natural situation during the second half of the menstrual cycle.
Stopping taking the pill after 21 days allows the concentrations of oestrogen and progesterone to fall to the point at which the uterine lining is no longer maintained. Menstruation occurs, and this reassures the woman taking the pill that she is not pregnant.
106
How can the combination of oestrogen and progesterone be given?
By means of a skin patch from which the hormones are absorbed in the skin, by injection or by inserting an implant under the skin that is effective for several months.
107
How do contraceptives containing only progesterone work?
They seem to work as contraceptives by reducing the ability of sperm to fertilise the egg and by making the mucus secreted by the cervix more viscous and so less easily penetrated by the sperm.
108
What does the morning after pill contain and how does it work?
The pill contains a synthetic progesterone-like hormone. If taken early enough, it reduces the chances of a sperm reaching and fertilising an egg. However, in most cases, it probably prevents a pregnancy by stopping the embryo implanting into the uterus.
109
How do action potentials pass through plants?
The action potentials travel along the cell membranes of plant cells and from cell to cell through plasmodesmata that are lined by cell membrane.
110
What is the difference between action potentials in plants and animal neurones?
The action potentials in plants generally last much longer and travel more slowly than in animal neurones.
111
What is the structure of a venus fly trap?
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 trap the insect inside. The surface of the lobes has many glands that secrete enzymes for the digestion of trapped insects.
112
How does a venus fly trap work? *full
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 to 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 then the trap will still close. The time between stimulus and response is about 0.5s. It takes the trap less than 0.3s to close and trap the insect.
The lobes of the leaf bulge upwards when the trap is open. However, the trap is not completely closed at this moment. To seal the trap, it requires ongoing activation of the trigger hairs by the trapped 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 to form an external ʻstomachʼ in which prey digestion occurs. Further deflections of the sensory hairs by the trapped insect stimulate the entry of calcium ions into gland cells. Here, calcium ions stimulate the exocytosis of vesicles containing digestive enzymes. 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 so the trap is set again.
113
What chemicals are responsible for most communication in plants?
Plant hormones/plant growth regulators
114
How do plant hormones move in plants?
They move in plants either directly from cell to cell(diffusion or active transport) or are carried in the phloem sap or xylem sap.
115
What is the primary chemical of auxin?
IAA
116
What does auxin influence?
Auxins influence many aspects of growth including elongation growth which determines the overall length of roots and shoots.
117
Where is auxin synthesised?
Auxins are synthesised in the growing tips(meristems) of shoots and roots where the cells are dividing.
118
What are the three stages of growth? What stage is auxin involved in?
Growth occurs in three stages: cell division by mitosis, cell elongation by absorption water water and cell differentiation. Auxin is involved in controlling growth by elongation.
119
How does auxin carry out its function?
Molecules of auxin find 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 are proteins called expansins and are activated by the decrease in pH. The expansins loosen the linkages between cellulose microfibrils. This disruption occurs briefly as cells absorb water by osmosis and pressure potential causes the wall to stretch by the microfibrils that move past each other allowing the cell does expand without losing much of the overall strength of the wall.
120
How do gibberellins affect stem elongation?
The height of some plants is partly controlled by their genes. If the dominant allele, Le, is present, the plants can grow tall, but plants homozygous for the recessive allele, le, always remain short. The dominant allele of this gene regulates the synthesis of the last enzyme in a pathway that produces an active form of gibberellin, GA1. Active gibberellin stimulates cell division and cell elongation in the stem, so causing the plant to grow tall.
A substitution mutation in this gene gives rise to a change from alanine to threonine in the primary structure of the enzyme near its active site, producing a non-functional enzyme. This mutation has given rise to the recessive allele, le. Homozygous plants, lele, are genetically dwarf as they do not have the active form of gibberellin.
121
What can be applied to genetically short plants to stimulate growth?
Applying active gibberellin
122
What state are seeds in when they are shed from the parent plant and why?
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.
123
Describe the structure of seeds.
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
124
How does gibberellin stimulate seed germination?
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.
