3.6 Organisms respond to changes in their internal + external environment

Question 1

What is a stimulus?

Answer 1

A detectable change in the environment that are detected by cells called receptors

Question 2

What is tropism?

Answer 2

Term when plants respond to stimuli through growth

Question 3

What kinds of tropisms are there?

Answer 3

Negative- meaning it grows away from the stimulus
Positive- meaning it grows towards the stimulus

Question 4

Two stimuli that plants respond to

Answer 4

Light (phototropism)
Gravity (gravitropism)

Question 5

What are tropisms controlled by?

Answer 5

Specific growth factors e.g. IAA

Question 6

What is IAA?

Answer 6

A type of auxin
Can control cell elongation in shoots
Inhibits growth of cells in roots
Made in tip of the roots and shoots but can diffuse to other cells

Question 7

Describe the steps of positive phototropism (4)

Answer 7

• Shoot tip cells produced IAA, causing cell elongation
• The IAA diffuses to other cells
• If there’s unilateral light, the IAA will diffuse towards the shaded side of the shoot resulting in a higher concentration of IAA there
• This causes the cells on the shaded side to elongate more & results in the plant bending towards the light source

Question 8

Describe negative phototropism (3)

Answer 8

•In the roots, a high concentration of IAA inhibits cell elongation
• This causes roots cells to elongate more on the lighter side and so the root bends away from the light
• This will help anchor the plants further into the ground and potentially reach more water sources

Question 9

What is a reflex?

Answer 9

A rapid, automatic response to protect you from danger

Question 10

What is a reflex action? (4 marks)

Answer 10

A reflex action is involuntary and innate . The stimulus always produces the same response, which is rapid, but short in duration.

Question 11

Define ‘taxis’

Answer 11

A directional movement towards or away from a stimulus (can either be positive or negative).

Question 12

Define ‘kinesis’

Answer 12

A non-directional response to a stimulus involving a change in the speed of movement or a change in the rate at which the organism changes direction.

Question 13

Describe the steps in a stimulus- response reflex arc (5 marks)

Answer 13

A receptor detects the stimulus. The receptor creates an impulse that is passed through a sensory neuron. The impulse moves along the sensory neurone through to a relay neurone. The impulse is then passed to a motor neurone, which carried the signal to an effector. The effector then brings an appropriate response to the stimulus.

Question 14

How do receptors turn into a response

Answer 14

Receptors detect a stimuli. Each receptor responds only to specific stimuli and this stimulation of a receptor leads to the establishment of a generator potential which can cause a response.

Question 15

Name three receptors (3)

Answer 15

Rods
Cones
Pacinian corpuscle

Question 16

What does the Pacinian corpuscle do?

Answer 16

Pacinian corpuscles transfer mechanical energy to electrical energy in the form of generator potentials.

Question 17

What makes the membrane of the Pacinian corpuscle special? (6 marks)

Answer 17

The membranes have stretch-mediated sodium channels
These open and allow sodium ions to enter the sensory neurone only when they are stretched and deformed.
When pressure is applied it deforms the neurone plasma membrane, stretches and Widnes the sodium ion channels so sodium ions diffuse in which lead to the establishment of a generator potential.

Question 18

Describe the role of sensory receptors in the nervous system (3 marks)

Answer 18

Sensory receptors detect physical stimuli. They are energy transducers, transforming environmental energy, such as light or heat, into electrical energy. This electrical energy can be passed to nerve cells/ sensory neurones, which take electrical signals into the nervous system.

Question 19

Explain how pressure on the Pacinian corpuscle produces changes in its membrane potential. (4 marks)

Answer 19

Pressure on the Pacinian corpuscle opens stretch mediated sodium ion channels, causing sodium ions to enter. Depolarisation occurs, leading to a generator potential. If enough sodium ions enter, an action potential is formed (reaches threshold/ all or nothing response). An increased pressure opens more channels, resulting in greater sodium entry.

Question 20

What two photoreceptors are in the retina?

Answer 20

Rods and cones