Year 13 - Survival and response, nervous coordination

Question 1

How does movement of IAA impact plant growth in the shoots and roots?

Answer 1

Shoot:
- Shoot tip produces IAA
- IAA moves by diffusion away from light
- More elongation of cells where there is a higher concentration of IAA
- Shoot grows towards light (positive phototropism)

Root:
- Root tip produces IAA
- IAA moves by diffusion to lower side of root
- Inhibition of elongation of cells where there is a higher concentration of IAA
- Results in more elongation of cells in lower conc of IAA so roots grow downwards (away from light)

Question 2

Name the different types of stimuli a receptor may respond to.

Answer 2

Light, pressure, touch, temperature, chemical, noise, smell

Question 3

Why do nerve impulses only travel in one direction?

Answer 3

• NT only made in and released from pre-synaptic neuron
• Complementary NT receptors are only on post-synaptic membrane
• NT packaged into vesicles that are in the pre-synaptic neuron

Question 4

Describe saltatory conduction and compare myelinated vs non-myelinated.

Answer 4

• Schwaan cells produce myelin that acts as an electrical insulator, causing saltatory conduction
• Hydrophilic Na+ and K+ ions can’t diffuse over this part of the membrane
• Depolarisation only occurs at nodes of ranvier so impulse jumps node to node
• Non-myelinated neurons transfer nerve impulse by depolarising next section of the membrane, occuring along the whole length of the neuron
• Therefore, nerve impulse is faster in myelinated neurons.

Question 5

Describe resting potential.

Answer 5

• Higher concentration of K+ inside and higher concentration of Na+ outside membrane
• Both K+ and Na+ voltage gated channels shut
• Axon membrane more permeable to K+ so some diffuse out
• Na+/K+ pump actively transports 3 Na+ out and 2 K+ in
• So resting potential is maintained at -70mv across axon membrane

Question 6

Describe depolarisation

Answer 6

• Threshold potential (-55mv) reached so maximal response due to the all or nothing principle
• Na+ voltage gated channels open increasing axon permeability to Na+
• Na+ enter by facilitated diffusion
• More Na+ channels open so positive feedback as potential becomes more positive
• Leading to depolarisation
• Membrane potential changes so it is positive (+40mv)