y2 key sequences

Question 1

Receptors:
What is the sequence to how a Pacinian corpuscle functions?

Answer 1

• in its normal (resting) state, the stretch-mediated sodium channels of the membrane around the neurone of a Pacinian corpuscle are too narrow to allow sodium ions to pass along them. In this state, the neurone of the Pacinian corpuscle has a resting state
• when pressure is applied to the Pacinian corpuscle, it is deformed and the membrane around its neurone becomes stretched
• this stretching widens the sodium channels in the membrane and sodium ions diffuse into the neurone
• the influx of sodium ions changes the potential of the membrane (i.e. it becomes depolarised), thereby producing a generator potential
• the generator potential in turn creates an action potential (nerve impulse) that passes along the neurone and then, via other neurones, to the central nervous system

Question 2

Survival and response:
What is the sequence for gravitropism in flowering plants?

Answer 2

The response of a horizontally growing root to gravity is as follows:
1. cells in the tip of the root produce IAA, which is then transported along the root
2. The IAA is initially transported to all sides of the root
3. gravity influences the movement of IAA from the upper side to the lower side of the root
4. a greater concentration of IAA builds up on the lower side of the root than on the upper side
5. as IAA inhibits the elongation of root cells and there is a greater concentration of IAA on the lower side, the cells on this side elongate less than those on the upper side
6. the relatively greater elongation of cells on the upper side compared to the lower side causes the root to bend downwards towards the force of gravity

Question 3

Survival and response:
what is the sequence for phototropism in flowering plants?

Answer 3

The response of shoots of flowering plants to unilateral light is due to the following sequence of events:
1. cells in the tip of the shoot produce IAA, which is then transported down the shoot
2. the IAA is initially transported evenly throughout all regions as it begins to move down the shoot
4. a greater concentration of IAA builds up on the shaded side of the shoot than on the light side
5. As IAA causes elongation of shoot cells and there is a greater concentration of IAA on the shaded side of the shoot, the cells on this side elongate more
6. the shaded side of the shoot elongates faster than the light side, causing the shoot tip to bend towards the light