Response to Stimuli (A Level only) (AQA A Level Biology): Flashcards

A tropism is a directional growth response to a stimulus.

Phototropism is a growth response to light.

Gravitropism is a growth response to gravity.

Chemicals released by plant cells that regulate growth in response to a stimulus.

They act in a similar way to hormones in animals.

They are usually produced in the growing parts of plants (e.g. the tips of roots and shoots).

They then move to other tissues, where they regulate growth.

A positive tropism causes the plant to grow towards a stimulus.

A negative tropism causes the plant to grow away from a stimulus.

Because different growth factors bring about different plant responses.

False.

You should use the A-level term growth factors rather than "plant hormones".

A directional growth response made by a plant is called a tropism.

Growth factors act in a similar way to hormones in animals.

IAA is a plant growth factor belonging to a group known as the auxins.

It controls the elongation of plant cells.

The elongation of plant cells.

The concentration of IAA determines the rate of cell elongation.

In shoots, IAA causes cell elongation.

In roots, IAA inhibits cell elongation.

IAA is produced in the shoot tip and transported through the shoot.

IAA moves to the shaded side of the shoot, setting up an IAA gradient.

IAA causes cell elongation, so cells on the shaded side elongate faster than those on the light side.

The shoot bends towards the light as it grows.

IAA is produced in the root tip and transported through the root.

IAA moves to the lower side of the root, setting up an IAA gradient.

IAA inhibits cell elongation, so cells on the lower side elongate more slowly than those on the upper side.

The root bends towards gravity as it grows.

From cell to cell by diffusion and active transport.

Over longer distances via the phloem.

False.

IAA influences plant growth by controlling cell elongation, not cell division.

In shoots, IAA accumulates on the shaded side, causing that side to elongate faster and the shoot to bend towards light.

The root bends away from the light (negative phototropism).

There is a greater concentration of IAA on the shaded side, which inhibits cell elongation there.

The illuminated side therefore grows at a faster rate.

IAA is synthesised in the root tips, so removing them means no IAA is produced.

With no IAA there is no inhibition of cell elongation, so growth is even.

In roots, a high concentration of IAA inhibits cell elongation.

A taxis is a directional response to a stimulus, involving movement towards or away from the stimulus.

A kinesis is a non-directional response to a stimulus, involving random movement.

A taxis is directional and involves movement towards or away from a stimulus.

A kinesis is non-directional and involves random movement.

They allow organisms to stay in a favourable environment.

In bright light they move faster and change direction more often.

This increases the chance of leaving the unfavourable conditions and avoiding predation.

Euglena swim towards a light source using a flagellum, allowing them to photosynthesise.

(Moths moving towards light also show positive phototaxis.)

Negative chemotaxis.

False.

A beetle moving away from light is showing a taxis, not a tropism. Tropisms are plant growth responses.

Kineses are non-directional responses that involve random movement.

A directional response involving movement towards or away from a stimulus is called a taxis.

Reflexes are rapid and automatic responses to stimuli.

Their purpose is to protect the organism from harm.

Sensory neurones

Relay neurones

Motor neurones

Sensory neurones carry impulses from receptors to the central nervous system (CNS).

Relay neurones connect sensory and motor neurones and are found within the CNS.

Motor neurones carry impulses from the CNS to effectors.

Because reflex arcs do not involve the conscious parts of the brain.

It prevents bright light from damaging the retina.

False.

Reflex arcs do not involve conscious parts of the brain, which is why reflexes are faster than other nervous responses.

The reflex arc pathway is: receptor → sensory neurone → relay neurone → motor neurone → effector.

The purpose of a simple reflex is to protect the organism from harm.

A choice chamber

A maze

The moist side is created by adding distilled water to the base of the chamber.

The dry side is created by adding a drying agent (e.g. anhydrous calcium chloride) to the base.

It keeps the woodlice at a safe distance from the water and the drying agent.

10 woodlice are placed in the centre of the chamber, where both conditions are accessible.

They are allowed to move freely for 10 minutes.

Then the number of woodlice in each section of the chamber is recorded.

Light intensity

Temperature

Surface texture

So that chemical traces left behind do not affect the results.

Observe the woodlice during the experiment and record observations of their movement.

True.

A paper maze can be used to investigate turning behaviour in woodlice under different conditions.

To investigate the effect of light instead of moisture, half of the choice chamber can be covered to make light the independent variable.

A receptor is a cell that detects a stimulus.

Receptors are specific, meaning each type detects a different type of stimulus.

Changes in pressure.

Deep in the skin, at the endings of sensory neurones.

Each type of receptor detects a different type of stimulus.

It is made of many layers of membrane (lamellae) separated by gel.

These layers surround the ending of a sensory neurone.

Lamellae (also referred to as connective tissue).

A generator potential – an initial change in the potential difference across the cell surface membrane.

False.

Receptors are specific, so each type detects a different type of stimulus and differs in structure and location.

Pacinian corpuscles are receptors found deep in the skin that respond to changes in pressure.

In a Pacinian corpuscle, the layers of membrane are separated by gel.

A generator potential is a change in the potential difference across the membrane of a nerve cell that occurs in response to a stimulus.

The size of the stimulus.

A larger stimulus produces a larger generator potential.

Pressure distorts the layers of membrane, causing stretch-mediated sodium channels in the axon membrane to open.

Sodium ions enter the axon via facilitated diffusion.

The influx of positive sodium ions makes the inside of the axon more positive; this is the generator potential.

Stretch-mediated sodium channels.

Facilitated diffusion.

A nerve impulse (an action potential).

Only at the site of a stimulus.

False.

Generator potentials differ in size depending on the size of the stimulus.

Pressure on a Pacinian corpuscle opens stretch-mediated sodium channels in the axon membrane.

As sodium ions enter the axon, the inside of the axon becomes more positive, producing a generator potential.

The sensitivity of temperature receptors

The habituation of touch receptors

The resolution of touch receptors

Habituation is when the response to a stimulus decreases after repeated or prolonged exposure.

The ability to distinguish between two different points of contact.

Areas with more touch receptors have higher resolution.

Temperature receptors can only detect temperature when it deviates significantly from body temperature.

When the thermometer is close to body temperature, only touch is felt.

To prevent sight of the stimulus from influencing the perceived touch.

By throwing a die – applying one point for an odd number and two points for an even number.

The fingers and soles of the feet.

They have a large number of touch receptors, so two very close points can still be identified separately.

They rely on humans relaying information about sensations, so results are subjective.

Only one subject is used, so the experiment should be repeated with several subjects.

False.

The fingertips have a higher resolution than the back of the hand, because they contain more touch receptors.

Areas of skin with a large number of touch receptors have a high resolution.

Rod cells

Cone cells

Light intensity only.

They are not sensitive to wavelength, so they generate black-and-white images.

Different wavelengths of visible light, allowing colour images to be generated.

Visual acuity is the ability to distinguish between two separate points when viewing an image.

Many rod cells connect to a single bipolar cell.

Many weak generator potentials can therefore combine (summation) to trigger a nerve impulse in the bipolar cell.

Because many rod cells synapse with a single bipolar cell.

Information from many neighbouring rods is detected as a single point, so the brain cannot tell which rod was stimulated.

A single cone cell synapses with a single bipolar cell.

Each cone sends a separate impulse, giving high acuity.

There is no summation, so a higher generator potential is needed to trigger an impulse, giving low sensitivity.

Light causes the chemical breakdown of optical pigments inside the cells.

This breakdown produces a generator potential.

If the generator potential is large enough, a nerve impulse is sent along a bipolar neurone to the optic nerve.

False.

Rod cells are not sensitive to wavelength, so they can only generate black-and-white images.

Cone cells are concentrated in the fovea of the retina.

Many rod cells connect to a single bipolar cell, allowing summation and giving high sensitivity to light.

The heart beats at a baseline rate without any input from the nervous system.

A group of cells in the wall of the right atrium, often called the pacemaker.

It initiates the wave of depolarisation.

It is conductive tissue that links the atria and ventricles, conducting electrical activity between them.

Conductive fibres that carry waves of depolarisation down the septum to the base of the ventricles.

Conductive fibres that carry waves of depolarisation from the base of the ventricles into the ventricle walls.

The SAN initiates a wave of depolarisation across the atria, causing them to contract.

The wave reaches non-conducting tissue before passing to the AVN, causing a delay that lets the atria empty into the ventricles.

Electrical activity passes from the bundle of His to the Purkyne tissue, causing the ventricles to contract from the base upwards.

It allows the atria to empty into the ventricles before ventricular contraction begins.

Because the Purkyne tissue carries the wave of depolarisation from the base of the ventricles into the ventricle walls.

False.

The SAN initiates the heartbeat. The AVN conducts electrical activity from the atria to the ventricles.

The SAN initiates a wave of depolarisation that spreads across the atria and causes them to contract.

Because the heart beats without input from the nervous system, its contraction is described as myogenic.

The cardioregulatory centre in the medulla, which is connected to the SAN by nerves of the autonomic nervous system.

The sympathetic branch ('fight or flight') increases heart rate.

The parasympathetic branch ('rest and digest') decreases heart rate.

Noradrenaline.

It binds to receptors on the SAN, which increases the frequency of heartbeat initiation.

Acetylcholine.

It binds to receptors on the SAN, which decreases the frequency of heartbeat initiation.

Chemoreceptors detect the carbon dioxide concentration (and pH) of the blood.

Pressure receptors detect blood pressure.

High blood CO₂ is detected by chemoreceptors.

Impulses are sent along sensory neurones to the medulla.

Impulses are sent along sympathetic neurones to the SAN.

Noradrenaline is released, and the SAN increases the frequency of heartbeat initiation.

The medulla controls heart rate through nerves of the autonomic nervous system, which controls unconscious activities.

False.

The sympathetic branch is involved in 'fight or flight'. The parasympathetic branch is involved in 'rest and digest'.

Cardiac output = heart rate × stroke volume

The same volume of water (e.g. 200 ml) instead of the caffeinated drink.

It is very difficult to control variables other than caffeine, e.g. differences between individuals in food consumed, fitness levels and responses to caffeine.

Taking part in the experiment may itself have unpredictable effects on heart rate.