Module 5 Plant and Animal Responses

Question 1

Plant Responses

Answer 1

• Plants respond to their environment as this offers them a selective advantage. The environmental stimuli they respond to can be abiotic or biotic.
• Tropisms are growth movements in plants in response to directional stimuli. Movement towards a stimulus is positive and movement away from a stimulus is negative.
• Phototropism is a response to light, to maximise light possible for photosynthesis. Geotropism is a response to gravity, roots show positive, and shoots show negative, to ensure they grow in the right direction. Hydrotropism is a response to moisture; roots grow towards more moist soil to access more water. Thigmotropism is a response to touch and climbing plants use it to detect support and curl around it, seeking greater access to light. Chemotropism is a response to chemicals, as when pollen tubes grow towards ovules in plants.
• Mimosa pudica is a plant that responds to touch by its leaves folding rapidly inward. This is viewed as nastic movement instead of a tropism as the movement is not directional. This rapid movement is due to biochemical signals, to protect the plant from herbivorous insects.
• Repellent chemicals are used to avoid herbivory. Tannins are water soluble carbon compounds found in the vacuole of plant cells, that can be fatal to insects and provide a bitter taste for herbivores. Alkaloids are nitrogenous compounds that are bitter tasting and toxic to herbivores and insects. Pheromones are molecules that are released from one member of a species and affect the physiology or behaviour of another member of the species. They may trigger chemical defences in some plants.
• Abiotic stresses include freezing, drought, soil salinity and heavy metals. Plants respond to drought by closing their stomata and reducing water loss by transpiration. Plants prevent freezing by producing antifreeze chemicals that inhibit the formation of ice crystals.
• Plant shoots show positive phototropism. Practical investigations using coleoptiles have investigated the influence of IAA. Cutting off or covering the tip has prevented phototropism. Using gelatine has shown that the hormone can diffuse through. Using a barrier will prevent phototropism. The hormone is produced in meristems in the roots and shoots.
• Growth occurs by elongation. IAA is a specific growth factor and an auxin. IAA activates expansins which are proteins that loosen bonds between cellulose microfibrils, allowing cell elongation.
• Phototropism occurs as the concentration of IAA increases the rate of cell elongation. IAA moves to the shaded side of the shoot, so it bends towards light.
• In the roots IAA accumulates on the lower side, inhibiting cell elongation and causing the root to bend downwards. In the shoots IAA accumulates on the lower side, promoting cell elongation and causing the shoot to grow upwards.

Question 2

Plant hormones

Answer 2

• Deciduous plants lose leaves in hot and dry environments to reduce water loss, as well as during winter as photosynthesis is limited by low temperatures and light intensity. This is a response to shortening of day length. An abscission layer made from parenchyma cells with thin cell walls that are weak and easy to break forms at the base of the leaf stalk. Ethene stimulates the breakdown of these cell walls, causing the leaves to drop off. Auxins inhibit leaf loss in young leaves by making them insensitive to ethene. As the leaves mature, their auxin concentration decreases, and so leaf loss increases.
• Abscisic acid stimulates stomatal closure. ABA binds to ABA receptors on the cell surface membrane of guard cells. This inhibits protons pumps and stops the active transport of H+ out of cells, as well as causing calcium ions to move into the cells. The calcium ions act as secondary messengers. Channels for negative ions open, causing them to leave the cell. This causes potassium ion channels to open, and potassium ions leave the cell. Channels that allow potassium ions to enter the cell close. This loss of ions increases the water potential of the cell, and water leaves the cell by osmosis. The cells become flaccid, and the stomata closes.
• Gibberellins control seed germination and stem elongation. Seeds will remain dormant until the correct conditions are met. Seeds are made up of an embryo, an endosperm containing a starch energy store, and an aleurone protein rich outer edge.
• When the right conditions are met, water enters the seed, and the embryo produces gibberellins. The gibberellins diffuse to the aleurone and stimulates cells to produce amylase. The amylase catalyses the hydrolysis of starch into maltose. The maltose is then converted to glucose and transported to the embryo, where it is respired to provide energy for growth. ABA inhibits the production of amylase, so it maintains dormancy.

Question 3

Apical Dominance

Answer 3

• Auxins are produced in the growing tip at the apex of a shoot and prevent the formation of lateral buds. It is preferential for a plant to grow upwards to access maximum light.
• If the growing tip is removed, lateral buds will grow as there is no longer apical dominance. Lateral buds will eventually grow upwards towards the light.
• Experimentally, if the apical tip is removed, lateral buds will grow. If the apical bud is replaced by an agar bud containing auxin, no lateral buds will grow.

Question 4

Gibberellins

Answer 4

• Gibberellins stimulate cell division and elongation. Dwarf plants have low levels of gibberellins due to mutations. When treated with gibberellins, dwarf plants grow to normal height.
• Gibberellins also stimulate germination. If the seeds of mutant plants that contain no gibberellins are treated with them, they will germinate. Lettuce that requires light to germinate will germinate in the dark when gibberellins are applied.

Question 5

Practical Investigations

Answer 5

• The effect of plant hormones on growth can be investigated practically. IBA is an auxin typically used in rooting powders.
• Cuttings of genetically identical plants should be dipped in serial dilutions of IBA and a control with no IBA and left to grow for a week. Then, remove the roots from the cutting and weigh them.

Question 6

Commercial uses of plant hormones

Answer 6

• In high concentrations, auxins can be used as weed killers as they promote rapid growth of plant tissues, causing damage which allows pathogens to enter. Synthetic auxins are used at concentrations 100x greater than normal. They are used on cereal crops and grass lawns as grasses are less sensitive to the auxins than broadleaf weeds.
• Rooting powders contain auxins at low doses as they stimulate cuttings to grow new roots and shoots.
• Ethene stimulates ripening of fruit, which prevents damage during transport, as the fruit can be harvested unripen.
• Auxins and gibberellins stimulate the production of fruit in unpollinated flowers. This is used in the production of parthenocarpic plants, which produce seedless fruit. Auxins also prevent trees dropping fruit, which can make them bruised.

Question 7

Mamalian Nervous System

Answer 7

• Structurally, the nervous system is made up of the central nervous system and the peripheral nervous system. The central nervous system consists of the brain and spinal cord, while the peripheral nervous system consists of the nerves in the body. The nervous system controls and regulates body functions, and information is sent through neurones as nerve impulses.
• Functionally the nervous system is divided into the somatic nervous system and the autonomic nervous system.
• The somatic nervous system controls voluntary actions. It is made up of three types of nerves, sensory, motor, and spinal, which is made up of both types of neurones and found in the spinal cord.
• The autonomic nervous system controls involuntary processes such as heart rate, dilation of blood vessels and digestion. It is split into the sympathetic system that controls the flight or fight response through the release of adrenaline, and the parasympathetic system that controls the rest and digest system.

Question 8

The Brain

Answer 8

• The brain makes up part of the central nervous system, made up of interconnected neurones that control complex actions.
• The cerebrum is the largest part of the brain that is responsible for vision, thinking, speech, memory and hearing. It is made up of 5 lobes and separated into two hemispheres joined by a band of nerve fibres called the corpus callosum. The right hemisphere controls the left side of the body and vice versa. The cerebrum is surrounded by a thin outer layer called the cerebral cortex, made up of the cell body of neurones, highly folded to create a larger surface area. The inside of the cerebrum is made from the white matter of the axons of neurones.
• The hypothalamus is in the middle of the lower part of the brain, above the pituitary gland which it is connected to. The hypothalamus monitors blood flow, releases hormones and stimulates the pituitary gland. It regulates body temperature, osmoregulation, digestive activity and endocrine functions.
• The pituitary gland is found below the hypothalamus. It produces a range of hormones. The anterior pituitary gland produces and secretes hormones. The posterior pituitary gland stores and secretes hormones produced by the hypothalamus such as ADH and oxytocin.
• The cerebellum is found bellow the cerebrum and subconsciously monitors motor coordination and balance.
• The medulla oblongata is found at the base of the brain joining to the spinal cord. The cardiac centre controls heart rate, the vasometer centre controls blood pressure and the respiratory centre controls breathing rate.

Question 9

Reflex Actions

Answer 9

• A reflex action is an involuntary response to a stimulus that generally has a protective purpose or survival value.
• A stimulus is detected by a receptor cell which secretes a substance or transmits electrical activity. This travels to a coordinator in the central nervous system (generally the spinal cord or a subconscious area of the brain). An impulse is then conducted to the effector.
• The knee jerk action is used by doctors to test if the nervous system is working. A hammer is used to hit the ligament between the kneecap and tibia, and the leg involuntarily straightens. The stimulus is the pressure on the ligament. Stretch receptors in the quadricep muscle are the receptors. A signal is sent to the coordinator in the spinal cord which sends an impulse to the effector, the quadricep muscle, which contracts causing the leg to straighten. The impulse passes directly from a sensory neurone to a motor neurone, so no sensory neurones are involved. Only a single synapse is crossed, so the response is very rapid.
• Blinking is caused by the stimuli of an object travelling towards the eye, something contacting the cornea, and the cornea drying out. Impulses travel down the trigeminal sensory nerve to the medulla, where neurones connect to the effector muscles. A relay neurone is used in the connection to the lower eyelid. The superior levitator palpebrae muscle lowers the upper eyelid and the orbicularis oculi muscle brings the eyelids inwards.

Question 10

Coordination of responses

Answer 10

• The fight or flight response occurs in response to environmental stimuli that causes high levels of stress, fear or aggression. It is rapid and both the nervous and endocrine systems are involved.
• The sympathetic nervous system (part of the autonomic nervous system) controls the fight or flight response.
• Sensory neurones detect environmental stimuli and send impulses to the brain. The amygdala sends impulses to the hypothalamus. The hypothalamus sends impulses by sympathetic nerves to the adrenal glands, causing the medulla to secrete adrenaline. The hypothalamus also secretes a hormone that stimulates the anterior pituitary gland to release ACTH. ACTH travels in the bloodstream to the adrenal glands, where it triggers the cortex to release cortisol.
• Cortisol increases blood pressure and blood glucose concentration. Adrenaline causes pupils to dilate, a wider diameter of bronchioles by relaxing smooth muscle, vasoconstriction and more blood to travel to the brain and muscles.
• Adrenaline increases blood glucose concentration. It binds to receptors on liver cells causing confirmational change. This activates adenylyl cyclase which converts ATP to cyclic AMP. cAMP acts as the second messenger and activates protein kinase A enzymes. These enzymes activate phosphorylase kinase enzymes by adding a phosphate group, which activate glucose phosphorylase enzymes, that catalyse the hydrolysis of glycogen to glucose.

Question 11

Factors effecting heart rate

Answer 11

• Cardiac muscle is myogenic, so contractions are initiated by the muscle. The length of intervals between contractions can be regulated by the nervous and endocrine systems.
• The autonomic nervous system involves the cardio regulatory centre in the medulla. The acceleratory centre increases heart rate and the inhibitory centre decreases heart rate.
• When the acceleratory centre is activated, impulses are sent along sympathetic nerves to the SAN, stimulating it to secrete noradrenaline. Noradrenaline increases the frequency of electrical waves, increasing heart rate.
• When the inhibitory centre is activated, impulses are sent along parasympathetic nerves, stimulating the SAN to secrete acetylcholine. This reduces the frequency of electrical waves, decreasing heart rate. An increased concentration of carbon dioxide in the blood will increase heart rate.
• The adrenal glands secrete noradrenaline and adrenaline from the medulla, increasing heart rate. Thyroxine from the thyroid also increases heart rate.
• Heart rate can be measured by electrodes or a heart rate monitor. If individuals are being compared, they should be phenotypically similar.

Question 12

Structure of muscles

Answer 12

• Skeletal muscle is striated muscle that is connected to the skeleton, made up of muscle fibres, highly specialised cell like units. The muscle fibres have an arrangement of contractile proteins in the cytoplasm, surrounded by a cell surface membrane and contain many nuclei.
• The cell surface membrane is known as the sarcolemma, the cytoplasm the sarcoplasm, and the endoplasmic rectilium the sarcoplasmic rectilium.
• The sarcoplasm contains many mitochondria for aerobic respiration and microfibrils made from actin and myosin filaments, that slide to provide muscle contraction. The sarcoplasmic rectilium contains protein pumps that move calcium ions into the SR. The sarcolemma has T-tubules that run close to the SR.
• The microfibrils in the sarcoplasm are made up of thick myosin filaments and thin actin filaments. The H-band contains only thick myosin filaments, the I band contains only thin actin filaments, the A band contains areas of only myosin and areas where myosin and actin overlap. The M line is the attachment region for myosin and the Z line is the attachment region for actin. The sarcomere is the distance between two Z lines.
• Smooth muscle is under unconscious control and contains actin and myosin. It is made from elongated cells or fibres that contain one nucleus. It is found in blood vessels and many organs.
• Cardiac muscle is only found in the heart as is myogenic. It does not fatigue and is uninucleate. The fibres are connected by intercalated discs and form a network. Many mitochondria are present.

Question 13

Mechanism of muscles

Answer 13

• Striated muscles contract as a result of an impulse from a motor neurone. An action potential arrives at the presynaptic membrane of a neuromuscular junction, causing calcium ions to diffuse into the neurone. This causes vesicles containing ACh to fuse with the synaptic membrane, and ACh diffuses across the neuromuscular junction. ACh binds with receptors on the sarcolemma, causing sodium ion channels to open. Sodium ions move into the muscle fibre, causing depolarisation. An action potential travels along the T-tubules to the SR. This causes voltage gated calcium ion channels to open and calcium ions to move out of the SR into the sarcoplasm by facilitated diffusion. The calcium ions bind with troponin molecules.
• Acetylcholinesterase breaks down ACh, causing calcium ions to be actively transported back into the SR, terminating muscle contraction.
• Myosin filaments are thick and consist of fibrous proteins with globular heads. The fibrous section anchors the molecule. In thick filaments in the H band, myosin filaments lie next to each other, with their globular heads facing away from the M line.
• Actin filaments are thin and made from globular proteins, which link to form a chain. Two chains twist together to form a filament. Tropomyosin twists around the chain and troponin is attached to the filament at regular intervals.
• In the sliding filament model, sarcomeres shorten as the myosin and actin filaments move past each other.
• Calcium ions leave the SR by facilitated diffusion and bind to troponin molecules. This causes troponin and tropomyosin to change positions, exposing the myosin binding site. Myosin globular heads bind to the binding site forming cross bridges. The head bends and pulls the actin filament towards the M line. ATP then binds to the myosin head, causing it to detach from the actin filament.
• The myosin head converts ATP to ADP and Pi and uses the energy released to return to its original position, to bind to a new binding site. This process continues until the muscle is fully contracted.
• ATP is used in muscle contraction to allow myosin to detach from actin and return to its original position. It is also used to actively transport calcium ions back into the SR.
• Mitochondria provide energy for muscle contraction by aerobic respiration, but this process is slow. Phosphocreatine can be used to rapidly provide ATP. Phosphocreatine + ADP -> ATP + creatine. This allows muscles to contract continuously until mitochondria can supply ATP.
• Muscle fatigue occurs when muscles can no longer contract at the same rate, due to decreased availability of calcium ions or the production of lactate, which lowers pH and prevents muscle contraction.