6: Stimuli & Response for A-Level Biology (OCR B

Why do organisms respond to changes in their environment?

To increase survival.

Responses help organisms remain in favourable conditions and avoid harm.

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In flowering plants, where are the growth factors that control directional responses produced?

Growing regions

These growth factors are made in regions such as shoot and root tips, then move to other tissues.

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What substance controls cell elongation in tropic responses in flowering plants?

Indoleacetic acid

(IAA)

IAA is the main auxin named in the AQA specification.

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How does a shoot respond when exposed to light from one side?

It grows towards the light.

This directional growth response is phototropism.

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How does a root respond to gravity?

It grows towards gravity.

Roots are positively gravitropic, so they grow downwards.

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How does a shoot respond to gravity?

It grows away from gravity.

Shoots are negatively gravitropic, so they grow upwards.

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What happens to cells in plant shoots when IAA concentration increases?

They elongate more.

In shoots, IAA stimulates cell elongation, causing bending when distributed unevenly.

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What happens to cells in plant roots when IAA concentration increases above the optimum?

They elongate less.

In roots, higher IAA concentrations inhibit elongation, producing curvature during tropisms.

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Fill in the blank:

In phototropism, unequal distribution of IAA causes unequal cell ______ in the shoot.

elongation

Cells on one side grow faster than the other, so the shoot bends.

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Fill in the blank:

In gravitropism, the root bends because IAA causes less elongation on the ______ side.

lower

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

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What is a simple directional movement in which an organism moves towards or away from a stimulus?

Taxis

Taxis is directional, unlike kinesis, which depends on stimulus intensity rather than direction.

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What is a simple response in which an organism changes its rate of movement in response to stimulus intensity?

Kinesis

Kinesis helps organisms spend more time in favourable conditions without moving directly towards a stimulus.

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True or False:

Taxes and kineses can help keep a mobile organism in a favourable environment.

True

These responses improve the chance of remaining in suitable conditions.

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What type of response gives rapid protection from a harmful stimulus through an automatic pathway?

Reflex

Reflexes are quick, automatic and protective rather than conscious.

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How many neurones are involved in a simple reflex?

Three

The pathway includes a sensory neurone, a relay neurone and a motor neurone.

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Which type of neurone carries impulses from a receptor into a simple reflex pathway?

Sensory neurone

The sensory neurone transmits information from the receptor to the CNS.

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Which type of neurone passes impulses from the CNS to an effector in a simple reflex?

Motor neurone

The effector is usually a muscle or gland that carries out the response.

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Which type of neurone links the sensory neurone to the motor neurone in a simple reflex?

Relay neurone

The relay neurone is located within the CNS in a simple reflex arc.

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In required practical 10, what is investigated using a choice chamber or maze?

The effect of an environmental variable on animal movement.

The practical tests how a variable influences movement behaviour in a mobile organism.

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Name one apparatus AQA allows for required practical 10 on animal movement.

Choice chamber

A maze is the other permitted option named in the specification.

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What feature of sensory receptors ensures they respond only to one particular type of stimulus?

Specificity

Each receptor type is specialised to detect a particular stimulus such as pressure, light or chemicals.

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What electrical change is produced in a receptor when it is stimulated by the appropriate stimulus?

Generator potential

A generator potential is a local graded depolarisation produced in the receptor membrane.

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In sensory receptors, what name is given to the graded electrical potential produced directly by a stimulus?

Generator potential

If the generator potential reaches threshold, an action potential is triggered in the sensory neurone.

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What type of receptor structure in the skin detects pressure and vibration?

Pacinian corpuscle

Pacinian corpuscles are mechanoreceptors located deep in the skin and other tissues.

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What type of **stimulus** does a **Pacinian corpuscle** detect?

Mechanical pressure ## Footnote They respond to deformation caused by pressure or vibration.

What **ion channel type** is mechanically deformed when a **Pacinian corpuscle** is stimulated?

Stretch-mediated sodium ion channels ## Footnote Mechanical pressure changes the membrane shape, opening sodium channels.

What **ion** enters the neurone when **stretch-mediated channels** open in a Pacinian corpuscle?

Sodium ions ## Footnote Influx of Na⁺ depolarises the membrane and produces a generator potential.

# True or False: **Generator potentials** in receptors are all-or-nothing events like **action potentials**.

False ## Footnote Generator potentials are graded; their size depends on stimulus strength.

# Fill in the blank: The **Pacinian corpuscle** responds to mechanical deformation by opening \_\_\_\_\_\_ channels.

Stretch-mediated sodium ion ## Footnote These channels open when the lamellae are compressed.

What region of the **eye** contains the **photoreceptors** responsible for detecting light?

Retina ## Footnote The retina lines the back of the eye and contains rods and cones.

Which **photoreceptor type** is specialised for vision in **low light intensity**?

Rods ## Footnote Rods are highly sensitive to light but do not detect colour.

Which **photoreceptor type** allows **colour vision** in the human eye?

Cones ## Footnote Cones contain different pigments sensitive to different wavelengths.

Which **photoreceptor type** provides the **highest visual acuity**?

Cones ## Footnote Cones have a one-to-one connection with neurones, improving resolution.

Why do **rods** provide lower **visual acuity** than cones?

Many rods converge onto one neurone. ## Footnote Convergence increases sensitivity to light but reduces detail.

# True or False: **Rods** are responsible for **colour vision**.

False ## Footnote Rods detect light intensity only; cones detect colour.

What feature of **cones** allows them to detect different **colours of light**?

Different optical pigments ## Footnote Each cone type absorbs different wavelengths corresponding to colours.

# Fill in the blanks: In the **retina**, rods are highly sensitive to \_\_\_\_\_\_ \_\_\_\_\_\_ but do not detect colour.

light intensity ## Footnote This allows vision in dim conditions such as at night.

# True or False: Each **cone cell** usually connects to a single **neurone** in the optic nerve pathway.

True ## Footnote This one-to-one connection produces high visual acuity.

What structure carries **impulses** from the **retina** to the brain?

Optic nerve ## Footnote Axons of sensory neurones from the retina form the optic nerve.

What property of **rods** makes them particularly useful for **night vision**?

High sensitivity to light ## Footnote Rods can respond to very low levels of light but sacrifice resolution.

What does it mean that the **heart** is **myogenic**?

It generates its own electrical impulses. ## Footnote Cardiac muscle initiates contraction without needing direct nervous stimulation.

Which structure in the wall of the **right atrium** acts as the heart’s natural **pacemaker**?

Sinoatrial node | (SAN) ## Footnote The SAN initiates the wave of excitation that sets the basic heart rhythm.

Where is the **atrioventricular node** found?

Between the atria and ventricles ## Footnote The AVN lies in the septum between the atria, close to the atrioventricular valves.

Which specialised tissue carries the **wave of excitation** from the **AVN** down through the septum?

Bundle of His ## Footnote This tissue conducts the impulse toward the apex of the heart.

What fibres spread the **excitation** through the **ventricular walls** from the apex upwards?

Purkinje tissue ## Footnote Purkinje fibres ensure the ventricles contract from the bottom upwards for efficient ejection of blood.

Why is there a **delay** at the **atrioventricular node**?

To allow the atria to **finish contracting** before the ventricles contract. ## Footnote This ensures the ventricles fill fully before ventricular systole begins.

Why do the **ventricles** contract from the **apex** upwards?

To **push blood out** into the arteries. ## Footnote This helps force blood into the aorta and pulmonary artery efficiently.

# True or False: The **sinoatrial node** is located in the **left ventricle**.

False ## Footnote The SAN is in the wall of the right atrium.

# True or False: The **atrioventricular node** prevents the ventricles from contracting too soon.

True ## Footnote The AVN slows conduction briefly, creating a short delay.

# Fill in the blank: Cardiac output can be calculated using the equation CO = \_\_\_\_\_\_ \_\_\_\_\_\_ × stroke volume.

heart rate ## Footnote Cardiac output is the volume of blood pumped by the heart per minute.

What is **cardiac output**?

The **volume** of blood pumped by the heart per minute. ## Footnote It depends on both heart rate and stroke volume.

Which **receptors** detect changes in **blood carbon dioxide concentration** and blood pH?

Chemoreceptors ## Footnote These receptors help adjust heart rate when metabolic demand changes.

Where are **pressure receptors** found that monitor **blood pressure**?

In the aorta and carotid arteries ## Footnote These baroreceptors detect changes in stretch in artery walls.

What is another name for **pressure receptors** involved in **heart rate control**?

Baroreceptors ## Footnote They respond to changes in blood pressure by altering nerve impulses to the medulla.

Which part of the **nervous system** increases **heart rate**?

Sympathetic nervous system ## Footnote Sympathetic stimulation sends impulses to the SAN to make it fire more frequently.

Which part of the **nervous system** decreases **heart rate**?

Parasympathetic nervous system ## Footnote Parasympathetic stimulation, via the vagus nerve, slows the SAN.

What is the **effector** that changes the **heart rate** in response to autonomic input?

Cardiac muscle in the SAN ## Footnote The SAN changes the frequency of excitation, altering overall heart rate.

How do **chemoreceptors** help increase **heart rate** during exercise?

They detect raised carbon dioxide and lower pH, then stimulate centres that increase heart rate. ## Footnote This allows faster delivery of oxygen and faster removal of carbon dioxide.

# True or False: **Pressure receptors** can trigger a reduction in **heart rate** when blood pressure rises.

True ## Footnote Increased stretch increases impulses to the medulla, which can increase parasympathetic output.

# Fill in the blank: The **wave of excitation** passes from the SAN to the AVN and then along the bundle of \_\_\_\_\_\_.

His ## Footnote After this, Purkinje tissue carries the excitation through the ventricles.

Why would **heart rate** increase if **blood pressure** falls?

To help restore cardiac output and maintain blood flow. ## Footnote Reduced stimulation of baroreceptors leads to more sympathetic influence on the heart.

A person has a **heart rate** of 75 beats per minute and a **stroke volume** of 80 cm³ per beat. What is their cardiac output?

6000 cm³ min^-1 ## Footnote Cardiac output = 75 × 80 = 6000 cm³ min^-1, or 6.0 dm³ min^-1.

6: Stimuli & Response Flashcards

Describe how organisms detect stimuli and produce coordinated responses. (62 cards)