StemUp: OCR A A level Biology 5.1.5 Plant and animal responses

Why do plants need to respond to stimuli? (1)

To increase their chance of survival by responding to changes in their environment, similar to animals

What is tropism? (1)

The response of a plant to a directional stimulus.

What is phototropism, and how do plants respond to light? (3)

- Phototropism is the growth of a plant in response to light

- Shoots are positively phototropic (grow toward light)

- Roots are negatively phototropic (grow away from light)

What is geotropism, and how do plants respond to gravity? (3)

- Geotropism is the growth of a plant in response to gravity

- Shoots are negatively geotropic (grow upward)

- Roots are positively geotropic (grow downward)

What is hydrotropism, and how do plants respond to water? (2)

- Hydrotropism is the growth of a plant in response to wate

- Roots are positively hydrotropic (grow towards water)

What is thermotropism? (1)

The growth of a plant in response to temperature

What is thigmotropism? (1)

The growth of a plant in response to contact with an object

What are alkaloids, and how do they defend plants against herbivory? (2)

- Bitter-tasting, nitrogenous compounds that deter or kill herbivores

- Example: Nicotine in tobacco plants is highly poisonous to insects

What are tannins, and how do they defend plants against herbivory? (2)

- Taste bitter and bind to proteins in the gut of herbivores

- Making plants hard to digest and deterring herbivores

What are pheromones, and how do plants use them? (3)

- Chemicals that affect the behaviour of other organisms

- Alarm pheromones are released when a plant is grazed on, triggering nearby plants to produce defences like tannins

- Corn plants release pheromones to attract parasitic wasps that kill caterpillars eating the plant

How does the Mimosa pudica plant respond to touch? (2)

- Folds up quickly when touched

- Possibly to scare off herbivores or knock off insects

How do plants respond to extreme cold? (4)

- Produce antifreeze proteins

- That bind to ice crystals and lower the freezing temperature

- Preventing more ice from forming and damaging the plant

- E.g. carrots

How do you set up a practical investigation to study phototropism in plants? (7)

1. Take 9 wheat shoots of equal height and plant them in individual pots with the same soil

2. Cover 3 shoots with a foil cap

3. Leave 3 shoots without foil

4. Wrap the base of 3 shoots with foil, leaving only the tip exposed

5. Place the shoots in front of a light source, ensuring each is the same distance from the light source

6. Control variables like temperature and water.

7. After 2 days, observe and record the amount and direction of growth

What results should you expect from the phototropism experiment? (3)

- Shoots with exposed tips should grow towards the light (positive phototropism)

- Shoots with the tip covered by foil will grow straight up (no phototropism)

- Shoots with the base covered but the tip exposed will grow towards the light

How do you set up a practical investigation to study geotropism in plants? (7)

1. Line 3 petri dishes with moist cotton wool, using the same amount of water and cotton wool in each

2. Place 10 cress seeds in each dish and press them into the cotton wool

3. Tape a lid onto each dish and cover them with foil to block light

4. Leave the dishes in a warm place

5. Prop one dish upright at 90°, another at 45°, and the third horizontally

6. Leave the seeds for 4 days

7. Measure the growth of the shoots and roots, and the angle of growth

What results should you expect from the geotropism experiment? (2)

- In all positions, the shoots will grow away from gravity (negative geotropism)

- The roots will grow towards gravity (positive geotropism)

What are plant hormones and their general function? (3)

- Chemicals produced in one region of the plant and transported through plant tissues

- To have an effect in another part of the plant

- They regulate growth and responses to environmental stimuli

What are the main functions of auxins in plants? (5)

- Control cell elongation

- Prevent leaf fall (abscission)

- Maintain apical dominance

- Stimulate the release of ethene

- Involved in fruit ripening and tropisms

What is indoleacetic acid (IAA), and how does it affect plant growth? (4)

- Important auxin produced in the tips of shoots and roots

- It stimulates cell elongation and moves by diffusion, active transport, or via the phloem, resulting in uneven growth

- High concentrations of IAA increase cell elongation in shoots (positive phototropism)

- High concentrations of IAA inhibit cell elongation in roots (negative phototropism)

How does phototropism occur in flowering plants? (4)

1. IAA is produced in the shoot tip and transported evenly down the shoot

2. Light causes IAA to move from the light side to the shaded side

3. IAA concentration is higher on the shaded side, causing faster elongation on that side

4. The shoot bends toward the light

How does gravitropism occur in flowering plants? (4)

1. IAA is produced in the root tip and transported evenly

2. Gravity causes IAA to accumulate on the lower side of the root

3. IAA concentration inhibits cell elongation on the lower side, while cells on the upper side elongate

4. The root bends downward, displaying positive gravitropism

What are the main functions of gibberellins in plants? (4)

- Cause stem elongation

- Trigger the mobilisation of food stores in seeds during germination

- Stimulate enzyme production for food breakdown

- Stimulate pollen tube growth in fertilisation

What are the main functions of abscisic acid (ABA) in plants? (3)

- Maintains dormancy in seeds and buds

- Stimulates cold-protective responses (e.g., antifreeze production)

- Triggers stomatal closing

What are the main functions of ethene in plants? (2)

- Causes fruit ripening

- Promotes abscission in deciduous trees

Why do deciduous plants lose their leaves in winter? (3)

- To conserve water during the coldest part of the year when soil water may be frozen

- Less light for photosynthesis

- So the plant conserves energy by dropping leaves when maintaining them is not energy-efficient

How do auxins and ethene regulate leaf loss in deciduous plants? (5)

- Auxins inhibit leaf loss by being produced in young leaves

- As leaves age, less auxin is produced, leading to leaf loss

- Ethene stimulates leaf loss by promoting the formation of the abscission layer

- Which separates the leaf from the plant

- Ethene causes cell expansion in the abscission layer, breaking cell walls and causing the leaf to fall off

What is the relationship between auxins and ethene in leaf loss? (2)

- Antagonistic

- Auxins inhibit leaf loss, while ethene stimulates it

How do guard cells regulate stomatal closing in plants? (2)

- When guard cells are turgid, stomata are open

- When they become flaccid, stomata close, reducing water loss by transpiration

How does abscisic acid (ABA) trigger stomatal closure? (5)

- ABA binds to receptors on guard cell membranes

- Causing ion channels to open and allowing calcium ions to enter the cytosol

- The increased calcium ion concentration triggers other ions (e.g., K⁺) to leave the guard cells

- Raising water potential and causing water to leave the guard cells by osmosis

- Making them flaccid and closing the stomata.

What is the difference between synergism and antagonism in plant hormones? (4)

- Synergism = Hormones work together to produce a greater effec

- E.g., auxins and gibberellins work together to promote tall plant growth

- Antagonism = Hormones have opposing effects

- E.g., gibberellins stimulate side shoot growth, while auxins inhibit it

What is apical dominance in plants? (3)

- When auxins stimulate the growth of the apical bud (the shoot tip)

- While inhibiting the growth of side shoots

- Preventing competition for light and energy

What happens to a plant when the apical bud is removed? (2)

- Removing the apical bud stops auxin production

- Allowing side shoots to grow by cell division and elongation

How does experimental evidence show that apical dominance is controlled by auxins? (4)

- If the apical bud is removed and replaced with an auxin source, side shoot growth is inhibited

- Showing that auxins regulate apical dominance

- Auxin concentration decreases as it moves away from the apical bud

- Allowing side shoots to grow near the bottom of tall plants

How do you set up an experiment to investigate the role of auxins in apical dominance? (7)

1. Plant 30 plants of similar age, height, and weight

2. Count and record the number of side shoots on each plant

3. For 10 plants, remove the tip and apply a paste without auxins to the stem

4. For another 10 plants, remove the tip and apply a paste containing auxins

5. Leave the remaining 10 plants untreated as controls

6. Let the plants grow for 6 days, controlling other variables (e.g., light, water)

7. After 6 days, count the number of side shoots on each plant

What should the results of the auxin apical dominance experiment show? (3)

- Removing the tip of the shoots causes extra side shoots to grow

- Removing the tip and applying auxins inhibits side shoot growth

- Indicating auxins are involved in apical dominance

What is the evidence that gibberellins are involved in seed germination? (4)

- Mutant seeds that lack gibberellins do not germinate

- But applying gibberellins externally allows normal germination

- Gibberellin inhibitors prevent germination

- But removing the inhibitor or applying gibberellins restores germination

How do you set up an experiment to investigate the role of gibberellins in stem elongation? (4)

1. Plant 40 plants of similar age, height, and mass

2. Leave 20 plants untreated as controls and water them normally

3. Water the other 20 plants with a dilute solution of gibberellin

4. Let the plants grow for 28 days, measuring the lengths of the stems weekly

What should the results of the gibberellin stem elongation experiment show? (2)

- The stems of the plants watered with gibberellin should grow more than the controls

- Suggesting that gibberellins stimulate stem elongation

How can you calculate the rate of growth of a plant in an experiment? (3)

- Rate of growth = (Growth in cm) ÷ (Number of days)

- Example: If a plant grows 10 cm in 28 days, the rate is 10 ÷ 28 = 0.36 cm/day

How is ethene used in the fruit industry? (2)

- Ethene stimulates enzymes that break down cell walls, chlorophyll, and convert starch to sugar, making the fruit soft and ripe

- Bananas are harvested unripe and exposed to ethene later to ensure they ripen simultaneously

How are auxins used commercially? (4)

- Selective weedkillers:

- Auxins make weeds grow long stems, causing them to grow too fast, leading to death due to lack of water and nutrients

- Rooting powders

- Auxins stimulate root growth in plant cuttings, allowing them to be planted and grow into new plants

Draw a diagram of the organisation of the mammalian nervous system (7)

Draw and label a diagram of the human brain (6)

What is the cerebrum, and what are its functions? (4)

- The largest part of the brain

- It is divided into two halves, called the cerebral hemispheres

- It has a thin, highly folded outer layer called the cerebral cortex

- The cerebrum is involved in vision, hearing, learning, and thinking

What is the cerebellum, and what is its role? (4)

- The cerebellum is located underneath the cerebrum and has a folded cortex

- It is important for controlling muscle coordination, posture, and balance

- It controls movement but does not initiate it

- Damage to the cerebellum causes jerky and uncoordinated movement

What is the medulla oblongata, and what does it control? (3)

- The medulla oblongata is located at the base of the brain, at the top of the spinal cord

- It contains regulatory centres for the autonomic nervous system

- It controls reflex activities such as breathing and heart rate

What is the hypothalamus, and what are its functions? (4)

- The hypothalamus is found just beneath the middle part of the brain

- It is the main controlling region for the autonomic nervous system

- It automatically maintains body temperature and monitors blood plasma composition

- It produces hormones that control the pituitary gland and regulates complex behaviours like sleeping, feeding, and aggression

What is the role of the pituitary gland? (2)

- The pituitary gland is located beneath the hypothalamus and is controlled by it

- It releases hormones that stimulate other glands (e.g., adrenal glands)

What is a reflex? (1)

An involuntary response to a sensory stimulus

Why are reflexes essential for survival? (3)

- Involuntary: The brain's decision-making regions are not involved, allowing the brain to focus on more complex responses

- Present at birth: Provides immediate protection

- Extremely fast: Involves 1 or 2 synapses, leading to rapid responses.

What is a reflex arc, and what are its components? (5)

- The pathway of neurones involved in a reflex action

- Receptor detects the stimulus and creates an action potential in the sensory neurone

- The sensory neurone carries the impulse to the spinal cord

- The relay neurone connects the sensory neurone to the motor neurone

- Motor neurone carries the impulse to the effector for the appropriate response

What is the function of the blinking reflex? (1)

Quickly closes the eyelids to prevent damage to the eye

How does the blinking reflex work? (5)

1. Sensory nerve endings in the cornea are stimulated by touch

2. The nerve impulse travels along the sensory neurone to a relay neurone in the CNS

3. The impulse is passed from the relay neurone to the motor neurones

4. The motor neurones send impulses to the orbicularis oculi muscles (eye muscles),

5. Causing them to contract and close the eyelids

How is the blinking reflex used in clinical assessment? (2)

- Indicates that the lower brain stem is functioning

- Helping to determine if a patient is brain-dead

What is the function of the knee-jerk reflex? (1)

Helps to maintain posture and balance

How does the knee-jerk reflex work? (4)

1. Stretching of the patellar tendon acts as a stimulus

2. This initiates a reflex arc that causes the extensor muscle on the top of the thigh to contract

3. A relay neurone inhibits the motor neurone of the flexor muscle, causing it to relax

4, This results in the leg kicking

What does the absence of the knee-jerk reflex indicate? (1)

Nervous system problems

What triggers the fight or flight response in organisms? (3)

1. When an organism is threatened

2. Sensory neurones send nerve impulses to the hypothalamus

3. Which activates both the hormonal system and the sympathetic nervous system

How is the hormonal system activated during the fight or flight response? (2)

1. The adrenal-cortical system is activated, and the pituitary gland releases ACTH

2. Which stimulates the adrenal cortex to release steroidal hormones

How does the sympathetic nervous system contribute to the fight or flight response? (3)

1. It triggers the release of adrenaline from the adrenal medulla

2. Which causes various physiological changes

3. Like increased heart rate, dilation of pupils, deeper breathing, and glycogen breakdown

What physiological changes occur during the fight or flight response due to adrenaline? (6)

1. Heart rate increases as blood is pumped faster

2. Pupils dilate to take in more light for better vision

3. Airways relax so breathing becomes deeper to allow more oxygen into the lungs

4. Glycogen is converted to glucose so that more glucose is available for muscle respiration

5. Blood is diverted from the skin and gut to the heart, lungs, and muscles for emergency functions

6. Erector pili muscles contract so hair stands up to make the animal appear larger

What is the role of first messengers and second messengers in cell signalling? (2)

- First messengers (e.g., hormones) = carry chemical messages from the endocrine gland to a receptor on the target cell

- Second messengers = relay signals within the cell and trigger changes in the cell's function, often via a cascade of reactions

How does adrenaline activate glycogen breakdown in liver cells? (4)

- Adrenaline binds to receptors on liver cells, activating adenylyl cyclase

- Which converts ATP to cyclic AMP (cAMP), the second messenger

- cAMP triggers a cascade of reactions

- That results in the breakdown of glycogen to glucose

How does the nervous system control heart rate? (2)

- The SAN generates electrical impulses that cause the heart muscles to contract

- The medulla unconsciously controls the rate at which the SAN fires

What types of receptors monitor blood pressure and chemical levels in the blood? (4)

- Baroreceptors (pressure receptors)

- Located in the aorta and vena cava, they detect high or low blood pressure

- Chemoreceptors:

- Located in the aorta, carotid artery, and medulla, they monitor oxygen levels, carbon dioxide, and pH levels in the blood

How do pressure and chemical receptors affect heart rate? (3)

1. Receptors send electrical impulses to the medulla via sensory neurones

2. The medulla processes the information and sends impulses to the SAN via motor neurone

3. To adjust heart rate

What is the response when baroreceptors detect high blood pressure? (7)

- Stimulus: High blood pressure

- Receptor: Baroreceptors detect high blood pressure

- Neurone: Impulses sent to the medulla.

- Medulla sends impulses along the vagus nerve

- Which secretes acetylcholine that binds to receptors on the SAN

- Effector: Cardiac muscles

- Response: Heart rate slows to reduce blood pressure back to normal.

What is the response when baroreceptors detect low blood pressure? (7)

- Stimulus: Low blood pressure

- Receptor: Baroreceptors detect low blood pressure

- Neurone: Impulses sent to the medulla

- Medulla sends impulses along the accelerator nerve

- Which secretes noradrenaline that binds to receptors on the SAN

- Effector: Cardiac muscles

- Response: Heart rate increases to raise blood pressure back to normal

What is the response when chemoreceptors detect high blood O₂, low CO₂, or high pH? (7)

- Stimulus: High O₂, low CO₂, or high pH in the blood

- Receptor: Chemoreceptors detect these chemical changes

- Neurone: Impulses sent to the medulla

- Medulla sends impulses along the vagus nerve

- Which secretes acetylcholine that binds to receptors on the SAN

- Effector: Cardiac muscles

- Response: Heart rate decreases to return O₂, CO₂, and pH levels to normal

What is the response when chemoreceptors detect low blood O₂, high CO₂, or low pH (e.g., during exercise)? (7)

- Stimulus: Low O₂, high CO₂, or low pH in the blood

- Receptor: Chemoreceptors detect these chemical changes

- Neurone: Impulses sent to the medulla

- Medulla sends impulses along the accelerator nerve

- Which secretes noradrenaline that binds to receptors on the SAN

Effector: Cardiac muscles.

- Response: Heart rate increases to return O₂, CO₂, and pH levels to normal.

How does the hormonal system help control heart rate? (4)

1. Adrenaline is released by the hormonal system

2. Binds to specific receptors in the heart

3. This causes the cardiac muscle to contract more frequently and with more force

4. Leading to an increase in heart rate and more blood being pumped by the heart

What is the fibre appearance of skeletal muscle? (1)

Striated

How is skeletal muscle controlled? (1)

Conscious (voluntary) control

How are skeletal muscle fibres arranged? (2)

- Regularly arranged

- So muscle contracts in one direction

What is the speed of contraction for skeletal muscle?

Rapid

What is the length of contraction for skeletal muscle?

Short

What is the structure of skeletal muscle fibres? (3)

- Made of many muscle fibres with many nuclei

- Muscle fibres can be centimetres long

- Cross-striations (striped patterns) are visible under a microscope

What is the fibre appearance of cardiac muscle? (1)

Specialised striated

How is cardiac muscle controlled? (2)

- Involuntary control

- Myogenic

How are cardiac muscle fibres arranged? (2)

- Cells branch and interconnect

- Resulting in simultaneous contraction

What is the speed of contraction for cardiac muscle? (!)

Intermediate

What is the length of contraction for cardiac muscle? (1)

Intermediate

What is the structure of cardiac muscle fibres? (3)

- Muscle fibres are connected by intercalated discs

- Low electrical resistance for easy passage of nerve impulses)

- Muscle fibres are branched and uninucleated, about 1mm long

What is the fibre appearance of smooth muscle? (1)

Non-striated

How is smooth muscle controlled? (1)

Involuntary control

How are smooth muscle fibres arranged? (2)

- No regular arrangement

- Allowing different cells to contract in different directions

What is the speed of contraction for smooth muscle?

Slow

What is the length of contraction for smooth muscle? (1)

Relatively long time

What is the structure of smooth muscle fibres? (3)

- Muscle fibres are uninucleated

- Spindle-shaped with pointed ends

- About 0.2mm long

What is skeletal muscle made of? (2)

- Large bundles of long cells

- Muscle fibres

What are muscle fibres made of? (1)

Bundles of myofibrils

What are myofibrils made of? (3)

- Made of bundles of myofilaments

- Actin (thin)

- Myosin (thick)

What is the sarcolemma? (1)

Cell membrane of muscle fibre cells

What is the sarcoplasm? (1)

The cytoplasm of a muscle cell

What is the function of the sarcoplasmic reticulum in muscle fibres? (2)

- Runs through the sarcoplasm

- Stores/releases calcium ions for muscle contraction

What are T-tubules, and what do they do? (3)

- Folds of the sarcolemma that stick into the sarcoplasm

- They help to spread electrical impulses through the sarcoplasm

- So they reach all parts of the muscle fibre

Why do muscle fibres have many mitochondria? (1)

To provide ATP for muscle contraction

What does it mean for muscle fibres to be multinucleate?

Contain multiple nuclei

What are myofibrils, and what do they contain? (3)

- Myofibrils are long, cylindrical organelles in muscle fibres

- They contain bundles of thick and thin myofilaments

- Thick myofilaments are made of myosin

What are thin myofilaments made of? (1)

Actin