6 Organisms respond to changes in their internal and external environments (A-level only)

Question 1

6.2.1
Describe the all-or-nothing principle. (2 marks)

Answer 1

(1) An action potential is only generated when threshold is reached/A.P is not generated unless threshold is reached
(2) High pressure generates an action potential.

Question 2

6.1.2
Describe how the stimulation of a pacinian corpuscle produces a generator potential

Answer 2

• stretch-mediated sodium channels
• ^ pressure deforms (sensory neuron/axon) membrane
• Na+ ion channels open
• Na+ diffuse in
• Depolarisation ( leading to a generator potential )

Question 3

6.1.3
Explain how the SAN controls heart rate. (5 marks)

Answer 3

SAN sends wave of electrical activity / impulses (across atria) causing atrial contraction;
Accept excitation
2. Non-conducting tissue prevents immediate contraction of ventricles / prevents impulses reaching the ventricles;
3. AVN delays (impulse) whilst blood leaves atria / ventricles fill;
4. (AVN) sends wave of electrical activity / impulses down Bundle of His;
4. Allow Purkyne fibres / tissue
5. Causing ventricles to contract from base up;

Question 4

6.1.1

Give one similarity and one difference between taxis and tropism (2 marks)

Answer 4

Similarity - directional response ( to a stimulus) / movement towards/ away from a stimulus.
Difference - taxis (whole) organism moves and tropism a growth (response)