Control and Coordination Flashcards
1
Q
hormone
A
~A chemical substance produced by an endocrine gland and carried by the blood
~They are chemicals that transmit information from one part of the organism to another and bring about a change
~They alter the activity of one or more specific target organs
2
Q
gland
A
a group of cells that produces and releases one or more substances (secretion)
3
Q
List the major endocrine glands in the body
A
1) pituitary gland: the ‘master gland’, situated at the base of the brain
2) thyroid gland: produces thyroxine
3) pancreas: produces hormones to regulate blood glucose level
4) adrenal glands: produces adrenaline
5) testes (in males): produces testosterone
6) ovaries (in females): produces oestrogen
4
Q
Why do the endocrine glands have a good supply of blood
A
so when they make hormones they can get them into the bloodstream (specifically the blood plasma) as soon as possible so they can travel around the body to the target organs to bring about a response
5
Q
Hormones only affect
A
~Cells with receptors that the hormone can bind to
~These are either found on the cell surface membrane, or inside cells
~Receptors have to be complementary to hormones for there to be an effect
6
Q
Hormones such as insulin, glucagon, and ADH are
A
~Peptides or small proteins
~They are water-soluble and so cannot cross the phospholipid bilayer of cell surface membranes
~These hormones bind to receptors on the cell surface membranes of their target cells, which activates second messengers to transfer the signal throughout the cytoplasm
7
Q
Hormones such as testosterone, oestrogen, and progesterone are
A
~Steroid hormones
~They are lipid-soluble and so can cross the phospholipid bilayer
~These hormones bind to receptors in the cytoplasm or nucleus of their target cells
8
Q
The human nervous system consists of
A
~Central nervous system
~Peripheral nervous system
9
Q
The central nervous system consists of
A
the brain and the spinal cord
10
Q
The peripheral nervous system
A
all of the nerves in the body
11
Q
Information is sent through the nervous system as
A
nerve impulses
12
Q
neurone
A
a nerve cell; a cell which is specialised for the conduction of nerve impulses
13
Q
nerve impulse
A
(usually shortened to impulse) a wave of electrical depolarisation that is transmitted along neurones
14
Q
A bundle of neurones is known as
A
a nerve
15
Q
Neurones coordinate the activities of
A
~sensory receptors (eg. those in the eye)
~decision-making centres in the central nervous system
~effectors such as muscles and glands
16
Q
Axon
A
The long fibre that neurones have
17
Q
What insulates an axon
A
a fatty sheath
18
Q
what are the small uninsulated sections of an axon called?
A
nodes of Ranvier
19
Q
The sheath of an axon is made of
A
myelin, a substance made by specialised cells known as Schwann cells
20
Q
How is myelin made
A
when Schwann cells wrap themselves around the axon along its length
21
Q
How does an electrical impulse travel down a neurone?
A
~the electrical impulse does not travel down the whole axon but jumps from one node to the next thus less time is wasted transferring the impulse from one cell to another
~Their cell bodies contain many extensions called dendrites
~This means they can connect to many other neurones and receive impulses from them, forming a network for easy communication
22
Q
Three main types of neurones
A
~sensory
~relay
~motor
23
Q
Sensory neurones
A
carry impulses from receptors to the CNS (brain or spinal cord)
24
Q
Relay (intermediate) neurones
A
are found entirely within the CNS and connect sensory and motor neurones
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Motor neurones
carry impulses from the CNS to effectors (muscles or glands)
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Sensory neurones have
the same basic structure as motor neurones but a cell body that branches off in the middle of the cell - it may be near the source of stimuli or in a swelling of a spinal nerve known as a ganglion
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Motor neurones have
~A large cell body at one end, that lies within the spinal cord or brain
~A nucleus that is always in its cell body
~Many highly-branched dendrites that extend from the cell body, providing a large surface area for the axon terminals of other neurones
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reflex arc
a pathway along which impulses are transmitted from a receptor to an effector without involving ‘conscious’ regions of the brain
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Examples of simple reflex actions
~Removing the hand rapidly from a sharp or hot object
~Blinking
~Focusing the eye on an object
~Controlling how much light enters the eye
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reflex pathway
stimulus→ receptor cells→ sensory neurone→ relay neurone→ motor neurone→ effector→ response
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receptor cells
a cell that responds to a stimulus
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How are receptor cells transducers
they convert energy in one form (such as light, heat or sound) into energy in an electrical impulse within a sensory neurone
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Some receptors, such as light receptors in the eye and chemoreceptors in the taste buds, are
specialised cells that detect a specific type of stimulus and influence the electrical activity of a sensory neurone
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Some receptors, such as some kinds of touch receptors, are
just the ends of the sensory neurones themselves
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When receptors cells are stimulated they are
depolarised
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depolarization
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
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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
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If the stimulus is very weak or below a certain threshold
the receptor cells are not sufficiently depolarised and the sensory neurone is not activated to send impulses
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If the stimulus is strong enough
it will increase the receptor potential above the threshold potential so the sensory neurone is activated and transmits impulses to the CNS
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all-or-nothing principle
An impulse is only transmitted if the initial stimulus is sufficient to increase the membrane potential above a threshold potential
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receptor potential
When a physical stimulus (eg. touch) acts on a sensory receptor cell that is designed to respond to that stimulus, the energy of the stimulus is transduced, or transformed, into an electrical response - this response is called receptor potential. Receptor potentials are produced by the movement of positively charged ions into the cell through ion channels in the cell membrane. If the receptor potential reaches a certain threshold, action potentials are generated.
42
threshold levels in receptors often increase with
continued stimulation, so that a greater stimulus is required before impulses are sent along sensory neurones
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action potential
a brief change in the potential difference from –70 mV to +30 mV across the cell surface membranes of neurones and muscle cells caused by the inward movement of sodium ions
44
potential difference
the difference in electrical potential between two points; in the nervous system, between the inside and the outside of a cell surface membrane such as the membrane that encloses an axon
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resting potential
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 (negative) and is partly maintained by sodium-potassium pumps
46
How resting potential is maintained
1) Sodium-potassium pumps in the axon membrane: These pumps move sodium (Na+) ions out of the axon and potassium (K+) ions into the axon. The pump proteins use energy from the hydrolysis of ATP to continue moving these ions against their concentration gradients
2) Many large, negatively charged molecules (anions) inside the axon: This attracts the potassium ions reducing the chance of them diffusing out
3) Impermeability of the axon membrane to ions: Sodium ions cannot diffuse through the axon membrane when the neurone is at rest
4) Closure of voltage-gated channel proteins (required for action potentials) in the axon membrane: Stops sodium and potassium ions diffusing through the axon memebrane
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papillae
the many small bumps that cover surface of the tongue
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the surface of each papilla is covered in
many taste buds
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chemoreceptors
~a receptor cell that responds to chemical stimuli
~they 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
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Each chemoreceptor is covered with
receptor proteins that detect different chemicals
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Chemoreceptors and the sequence of events that results in an action potential in a sensory neurone (stimulus is salt)
1) Chemoreceptors in the taste buds that detect salt (sodium chloride) respond directly to sodium ions. If salt is present in the food (dissolved in saliva) being eaten or the liquid being drunk than
2) Sodium ions diffuse through highly selective channel proteins in the cell surface membranes of the microvilli of the chemoreceptor cells
3) This leads to the depolarisation of the chemoreceptor cell membrane. The increase in positive charge inside the cell is known as the receptor potential
4) If there is sufficient stimulation by sodium ions and sufficient depolarisation of the membrane, the receptor potential becomes large enough to stimulate voltage-gated calcium ion channel proteins to open
5) As a result, calcium ions enter the cytoplasm of the chemoreceptor cell and stimulate exocytosis of vesicles containing neurotransmitters from the basal membrane of the chemoreceptor
6) The neurotransmitter stimulates an action potential in the sensory neurone
7) The sensory neurone then transmits an impulse to the brain
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Unlike normal electric currents, impulses are not
a flow of electrons
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action potentials occur via
very brief changes in the distribution of electrical charge across the cell surface membrane
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voltage-gated channel protein
a channel protein through a cell membrane that opens or closes in response to changes in electrical potential across the membrane
55
When an action potential is stimulated in a neurone
1) Sodium channel proteins in the axon membrane open allowing sodium ions to pass into the axon down the electrochemical gradient
2) This reduces the potential difference across the axon membrane as the inside of the axon becomes less negative – depolarization
3) If the potential difference reaches around -50mV (the threshold value), many more channels open and many more sodium ions enter causing the inside of the axon to reach a potential of around +30mV
4) This is an example of positive feedback (a small initial depolarization leads to greater and greater levels of depolarisation)
5) An action potential is generated
6) The depolarization of the membrane at the site of the first action potential causes current to flow to the next section of the axon membrane, depolarising it and causing sodium ion voltage-gated channel proteins to open
7) The 'flow' of current is caused by the diffusion of sodium ions along the axon from an area of high concentration to an area of low concentration
8) This triggers the production of another action potential in this section of the axon membrane and the process continues. In the body, this allows action potentials to begin at one end of an axon and then pass along the entire length of the axon membrane
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repolarisation
returning the potential difference across the cell surface membrane of a neurone or muscle cell to normal follow
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Repolarisation and the refractory period
1) Very shortly (about 1 ms) after an action potential in a section of axon membrane is generated, all the sodium ion voltage-gated channel proteins in this section close, stopping any further sodium ions from diffusing into the axon
2) Potassium ion voltage-gated channel proteins in this section of axon membrane now open, allowing the diffusion of potassium ions out of the axon, down their concentration gradient
3) This returns the potential difference to normal (about -70mV) – a process known as repolarisation (There is actually a short period of hyperpolarisation. This is when the potential difference across this section of the axon membrane briefly becomes more negative than the normal resting potential)
4) The potassium ion voltage-gated channel proteins then close and the sodium ion channel proteins in this section of the membrane become responsive to depolarisation again
5) Until this occurs, this section of the axon membrane is in a period of recovery and is unresponsive - this is known as the refractory period
58
Why is it important that a section of the axon is unresponsive during the refractory period
~It ensures that ‘new’ action potentials are generated ahead (I.e. further along the axon), rather than behind the original action potential
~This makes the action potentials discrete events and means the impulse can only travel in one direction. This is essential for the successful and efficient transmission of nerve impulses along neurones
~This also means there is a minimum time between action potentials occurring at any one place along a neurone
~The length of the refractory period is key in determining the maximum frequency at which impulses can be transmitted along neurones (between 500 and 1000 per second)
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speed of conduction of an impulse
how quickly the impulse is transmitted along a neurone
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How the speed of conduction is determined
by two main factors:
* the presence or absence of myelin (ie. whether or not the axon is insulated by a myelin sheath)
* the diameter of the axon
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Saltatory conduction
movement of an action potential along a myelinated axon, in which the action potential 'jumps' from one node of Ranvier to the next
62
How myelination affects the speed of conduction
-In unmyelinated neurones, the speed of conduction is very slow
-By insulating the axon membrane, the presence of myelin increases the speed at which action potentials can travel along the neurone:
1. In sections of the axon that are surrounded by a myelin sheath, depolarisation (and the action potentials that this would lead to) cannot occur, as the myelin sheath stops the diffusion of sodium ions and potassium ions so action potentials can only occur at the nodes of Ranvier
2. The local circuits of current that trigger depolarization in the next section of the axon membrane exist between the nodes of Ranvier
3. This means the action potentials ‘jump’ from one node to the next also known as saltatory conduction
4. This allows the impulse to travel much faster (up to 50 times faster) than in an unmyelinated axon of the same diameter
63
How diameter affects speed of conduction
-The speed of conduction of an impulse along neurones with thicker axons is greater than along those with thinner ones
-Thicker axons have an axon membrane with a greater surface area over which diffusion of ions can occur
-This increases the rate of diffusion of sodium ions and potassium ions, which in turn increases the rate at which depolarisation and action potentials can occur
64
synaptic cleft
a very small gap (about 20 nm) between two neurones at a synapse; nerve impulses are transmitted across synaptic clefts by neurotransmitters
65
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
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neurotransmitter
a chemical released at synapses to transmit impulses between neurones or between a motor neurone and a muscle fibre
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Synaptic transmission – primary mechanism
~Electrical impulses cannot ‘jump’ across synapses
1. When an electrical impulse arrives at the end of the axon on the presynaptic neurone, neurotransmitters are released from vesicles at the presynaptic membrane
2. The neurotransmitters diffuse across the synaptic cleft and temporarily bind with receptor molecules on the postsynaptic membrane
3. This stimulates the postsynaptic neurone to generate an electrical impulse that then travels down the axon of the postsynaptic neurone
4.The neurotransmitters are then destroyed or recycled to prevent continued stimulation of the second neurone, which could cause repeated impulses to be sent
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Amount of different known neurotransmitters
over 40
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Key neurotransmitters used throughout the body
acetylcholine (ACh) and noradrenaline
70
Synapses that use acetylcholine (ACh) are called
cholinergic synapses
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presynaptic neurone
a neurone ending at a synapse from which neurotransmitter is released when an action potential arrives
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postsynaptic neurone
the neurone on the opposite side of a synapse to the neurone in which action potential arrives
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acetylcholinesterase
an enzyme in the synaptic cleft and on the postsynaptic membrane that hydrolysis ACh to acetate and choline
74
Synaptic transmission – detailed mechanism
1. The arrival of an action potential at the presynaptic membrane causes depolarisation of the membrane
2. This stimulates voltage-gated calcium ion channel proteins to open allowing calcium ions to diffuse down an electrochemical gradient from the tissue fluid surrounding the synapse (high concentration of calcium ions) into the cytoplasm of the presynaptic neurone (low concentration of calcium ions)
3. This stimulates ACh-containing vesicles to fuse with the presynaptic membrane, releasing ACh molecules into the synaptic cleft
4. The ACh molecules diffuse across the synaptic cleft and temporarily bind to receptor proteins in the postsynaptic membrane
5. This causes a conformational change in the receptor proteins, which then open, allowing sodium ions to diffuse down an electrochemical gradient into the cytoplasm of the postsynaptic neurone
6. The sodium ions cause depolarisation of the postsynaptic membrane, re-starting the electrical impulse (that can now continue down the axon of the postsynaptic neurone)
7. To prevent the sodium ion channels from staying permanently open and to stop permanent depolarization of the postsynaptic membrane, the ACh molecules are broken down and recycled
8. The enzyme acetylcholinesterase catalyses the hydrolysis of the ACh molecules into acetate and choline
9. The choline is absorbed back into the presynaptic membrane and reacts with acetyl coenzyme A to form ACh, which is then packaged into presynaptic vesicles ready to be used when another action potential arrives
10. This entire sequence of events takes 5 – 10 ms
75
Why have synapses when it will be quicker to transmit along an unbroken neuronal pathway from receptor to effector
1. Synapses ensure one-way transmission. Impulses can only pass in one direction at synapses. This is because neurotransmitters are released on one side and their receptors are on the other. There is no way that chemical transmission can occur in the opposite direction.
2. 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. They do this by allowing the interconnection of many nerve pathways. This happens in two ways:
* 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.
* 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
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neuromuscular junction
a synapse between a motor neurone and a muscle
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striated muscle
type of muscle tissue in skeletal muscles, the muscle fibre have regular striations (any of the alternating light and dark crossbands or stripes present in certain muscles especially voluntary) that can be seen under the light microscope
striated muscles are also called neurogenic because they need nerve impulses in order to contract
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sarcolemma
the cell surface membrane of a muscle fibre
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sarcoplasm
the cytoplasm of muscle cells
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sarcoplasmic reticulum (SR)
the endoplasmic reticulum of a muscle fibre
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transverse system tubule (or T-system tubule or T-tubule)
infolding of the sarcolemma that goes deep into a muscle fibre and conducts impulses to the SR
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myofibril
one of many cylindrical bundles of thick (myosin) and thin (actin) filaments inside a muscle fibre
83
myosin
the protein that makes up the thick filaments in striated muscle; the globular heads of each molecule break down ATP (they act as an ATPase)
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actin
the protein that makes up the thin filaments in striated muscle
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sarcomere
the part of a myofibril between two Z discs
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tropomyosin
a fibrous protein that is part of the thin filaments in myofibrils in striated muscle; tropomyosin blocks the attachment site on the thin filament (actin) for myosin heads so preventing the formation of cross-bridges
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troponin
a calcium-binding protein that is part of the thin filaments (actin) in myofibrils in striated muscle
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Stimulating Contraction in Striated Muscle
1. Striated muscle contracts when it receives an impulse from a motor neurone via the neuromuscular junction
2. When an impulse travelling along the axon of a motor neurone arrives at the presynaptic membrane, the action potential causes calcium ions to diffuse into the neurone
3. This stimulates vesicles containing the neurotransmitter acetylcholine (ACh) to fuse with the presynaptic membrane
4. The ACh that is released diffuses across the neuromuscular junction and binds to receptor proteins on the sarcolemma
5. This stimulates ion channels in the sarcolemma to open, allowing sodium ions to diffuse in
6. This depolarises the sarcolemma, generating an action potential that passes down the T-tubules towards the centre of the muscle fibre
7. These action potentials cause voltage-gated calcium ion channel proteins in the membranes of the sarcoplasmic reticulum (which lie very close to the T-tubules) to open
8. Calcium ions diffuse out of the sarcoplasmic reticulum (SR) and into the sarcoplasm surrounding the myofibrils
9. Calcium ions bind to troponin molecules, stimulating them to change shape. This causes the troponin and tropomyosin proteins to change position on the thin (actin) filaments
10. The myosin-binding sites are exposed on the actin molecules. The process of muscle contraction (known as the sliding filament model) can now begin
89
Striated muscle is made up of
muscle fibres
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A muscle fibre is a
highly specialised cell-like unit
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Each muscle fibre contains
-an organised arrangement of contractile proteins in the cytoplasm
-a cell surface membrane that surrounds it
-many nuclei – this is why muscle fibres are not usually referred to as cells
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The sarcoplasm contains
mitochondria and myofibrils
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The membranes of the SR contain
protein pumps that transport calcium ions into the lumen of the SR
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H band
only thick myosin filaments present
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I band
only thin actin filaments present
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A band
contains areas where only myosin filaments are present and areas where myosin and actin overlap
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M line
attachment for myosin filaments
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Z line
attachment for actin filaments
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structure of myosin
-These are fibrous protein molecules with a globular head
-The fibrous part of the myosin molecule anchors the molecule into the thick filament
-In the thick filament, many myosin molecules lie next to each other with their globular heads all pointing away from the M line
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structure of actin
-These are globular protein molecules
-Many actin molecules link together to form a chain
-Two actin chains twist together to form one thin filament
-A fibrous protein known as tropomyosin is twisted around the two actin chains
-Another protein known as troponin is attached to the actin chains at regular intervals
101
How muscles contract – the sliding filament model
Muscles cause movement by contracting. During muscle contraction, sarcomeres within myofibrils shorten as the Z discs are pulled closer together. This is known as the sliding filament model of muscle contraction and occurs via the following process:
1. An action potential arrives at the neuromuscular junction
2. Calcium ions are released from the sarcoplasmic reticulum (SR)
3. Calcium ions bind to troponin molecules, stimulating them to change shape
4. This causes troponin and tropomyosin proteins to change position on the actin filaments
5. Myosin binding sites are exposed on the actin molecules so the globular heads of the myosin molecules bind with these sites, forming cross-bridges between the two types of filament
6. The myosin heads move and pull the actin filaments towards the centre of the sarcomere, causing the muscle to contract a very small distance
7. ATP hydrolysis occurs at the myosin heads, providing the energy required for the myosin heads to release the actin filaments
8. The myosin heads move back to their original positions and bind to new binding sites on the actin filaments, closer to the Z disc
9. The myosin heads move again, pulling the actin filaments even closer to the centre of the sarcomere, causing the sarcomere to shorten once more and pulling the Z discs closer together
10. The myosin heads hydrolyse ATP once more in order to detach again
11. As long as troponin and tropomyosin are not blocking the myosin-binding sites and the muscle has a supply of ATP, this process repeats until the muscle is fully contracted
102
Where does a venus flytrap get its supply of nitrogen compounds
The venus flytrap is a carnivores plant that gets its supply of nitrogen compounds by trapping and digesting small animals (mainly insects)
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Structure of a venus flytrap
-Has a specialised leaf that is divided into two lobes on either side of a midrib
-The inside of the lobes is red and has nectar-secreting glands on the edges to attract insects
-Each lobe has three stiff sensory hairs that respond to being touched
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What happens when an insect touches one of the hairs with enough force
-Action potentials are stimulated, which then travel very fast across the leaf
-These action potentials cause the two lobes to fold together along the midrib, capturing the insect
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How the closure of the trap is achieved
1. If one of the sensory hairs is touched with enough force, calcium ion channels in cells at the base of the hair are activated
2. When these channels open, calcium ions flow in and generate a receptor potential
3. If two of the sensory hairs are stimulated within a period of about 30 seconds, or one hair is stimulated twice within this period, action potentials will travel across the trap and cause it to close
4. When the trap is open the lobes of the leaf are convex in shape but when the trap is triggered, the lobes quickly become concave, bending downwards and causing the trap to shut – it is thought this occurs as a result of a release of elastic tension in the cell walls
5. Sealing the trap requires ongoing activation of the sensory hairs – the prey trapped inside provides this ongoing stimulation, generating further action potentials
6. Further stimulation of the sensory hairs stimulates calcium ions to enter gland cells where they stimulate the exocytosis of vesicles containing digestive enzymes
7. The trap then stays shut for up to a week to allow the prey to be digested and the nutrients from it to be absorbed by the plant
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Plant hormones (also known as plant growth regulators) are responsible for
most communication within plants
107
Auxins
a type of plant growth regulator that influences many aspects of growth, including cell elongation growth which determines the overall length of roots and shoots
108
The principle chemical in the group of auxins made by plants is
IAA (indole 3-acetic acid) also referred to as auxin
109
Auxin (IAA) is synthesised in
the growing tips of roots and shoots (ie. in the meristems, where cells are dividing)
110
The three stages of growth that occurs in meristems
-cell division by mitosis
-cell elongation by absorption of water
-cell differentiation
| Auxin (IAA) is involved in controlling growth by elongation
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Controlling growth by elongation
1. Auxin molecules bind to a receptor protein on the cell surface membrane stimulating ATPase proton pumps to pump hydrogen ions from the cytoplasm into the cell wall (across the cell surface membrane)
2. This acidifies the cell wall (lowers the pH of the cell wall)
3. This activates proteins known as expansins, which loosen the bonds between cellulose microfibrils
4. At the same time, potassium ion channels are stimulated to open
5. This leads to an increase in potassium ion concentration in the cytoplasm, which decreases the water potential of the cytoplasm. The cell then absorbs water by osmosis (water enters the cell through aquaporins)
6. This increases the internal pressure of the cell, causing the cell wall to stretch (made possible by expansin proteins) and the cell elongates
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Gibberellins
a type of plant growth regulator involved in controlling seed germination and stem elongation
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embryo
will grow into the new plant when the seed germinates
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endosperm
a starch-containing energy store surrounding the embryo
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An aleurone layer
a protein-rich layer on the outer edge of the endosperm
116
When a barley seed is shed from the parent plant, it is
-In a state of dormancy (contains very little water and is metabolically inactive)
-This allows the seed to survive harsh conditions until the conditions are right for successful germination (eg. the seed can survive a cold winter until temperatures rise again in spring)
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The Role of Gibberellin in the Germination of Barley
1. When the conditions are right, the barley seed starts to absorb water to begin the process of germination
2. This stimulates the embryo to produce gibberellins
3. Gibberellin molecules diffuse into the aleurone layer and stimulate the cells there to synthesise amylase
4. In barley seeds, it has been shown that gibberellin does this by regulating genes involved in the synthesis of amylase, causing an increase in the transcription of mRNA coding for amylase
5. The amylase hydrolyses starch molecules in the endosperm, producing soluble maltose molecules
6. The maltose is converted to glucose and transported to the embryo
7. This glucose can be respired by the embryo, providing the embryo with the energy needed for it to grow
