Stimuli and Response Flashcards
Acid Growth Hypothesis
IAA increases the plasticity of cells by acid growth hypothesis. Hydrogen ions are actively transported from the cytoplasm into spaces in the cell wall. This activates expansions, loosening the cellulose causing it to become ‘more plastic’ and so allowing cells to elongate by expansion.
How Auxin moves through cells
The negatively charged auxin encounters the acidic environment of the cell wall, so picks up a Hydrogen ion. This means it is now a small, non-charged molecule which can diffuse directly through the plasma membrane. Inside the cell the Auxin is ionised, which temporarily traps the Auxin inside the cell. The cell is maintained at a pH of 7 through an ATP driven proton pump. Auxin exits the cell at the Basal end where there are specific carrier proteins in the plasma membrane.
Experiment to show that the light stimulus is detected by the tip of the root
Place a light proof cover on the intact tip of the shoot, there is no response.
Experiment to show that it is a chemical not electrical signal in plants
Tip removed, gelatine block inserted and then the tip is replaced. The tip bends towards the light, because gelatine allows chemicals to pass through it but not electrical signals.
Peripheral nervous system
made up of pairs of nerves that originate from either the brain or the spinal chord (can be divided into sensory and motor neurones).
Autonomic nervous system
carries nerve impulses to glands, smooth muscle and cardiac muscle. It is involuntary.
What is the spinal chord
A column of nervous tissue that runs along the back and lies inside the vertebral column for protection.
Importance of reflex arcs
They are involuntary and therefore do not require the decision making powers of the brain, so leaving it free to carry out more complex responses. This also means that the brain is not overloaded with situations where the response is always the same.
They are effective from birth and do not have to be learnt so protect the body from harm
They are fast because the neurone pathway is short and has few synapses. The absence of any decision making process also means that their action is rapid.
Cell Body
This contains all of the usual organelles including the nucleus and a large amount of rough ER (for producing proteins and neurotransmitters).
Schwann Cells
These surround the axon and provide protection and electrical insulation. They also carry out phagocytosis removing any cell debris, and they play an important role in nerve regeneration. They wrap themselves around the axon many times so that layers of membrane build up around it forming the myelin sheath.
Nodes of Ranvier
Gap between two adjacent Schwann cell
What is the position of the cell body in relation to the axon in a sensory neurone
cell body is at a 90 degree angle to the axon
Type of Sodium Channel in the Pacinian Corpuscle
Stretch mediated Sodium Channel
Generator potential
if the influx of sodium ions is too small to reach threshold value no action potential is generated. This is the all or nothing principle.
Resting Potential
- Sodium Potassium pump transports 2 Potassium ions into the axon for every 3 sodium ions that it pumps out. This means that it will be more positive outside the axon and more negative inside the axon.
- This creates a twofold gradient (of charge and concentration), so Sodium ions try to move into the axoplasm, however, the Sodium ion gated channels are closed so they can’t.
- So there is a potential difference across the membrane and so the axon is polarised.
Sodium potassium pump in the axon at resting potential
2 potassium ions into the axon for every 3 sodium ions out. More positive outside the axon and more negative inside.
Potential difference at resting potential
-65mV
Where are the rod cells found
on the retina, particularly at the periphery (none on the fovea)
Where are cone cells found
concentrated on the fovea
Rod cells key points
cannot distinguish between different wavelengths of light and therefore lead to black and white images.
They respond to low light intensity.
A number of rod cells are connected to a single bipolar cell.
Give low visual acuity
Only one type
there are more rod cells than cone cells.
what is the pigment in rod cells
Rhodopsin. There is enough energy in low light intensity light to break this down.
why are many rod cells connected to a single sensory neurone in the optic nerve
As a certain threshold has to be exceeded in order to create an action potential. Due to retinal convergence, there is a much greater chance that the threshold will be exceeded than If only a single rod cell were connected to each bipolar cell. This is due to Summation. However, it means that only a single impulse will be generated travelling to the brain regardless of how many of the neurones have been stimulated. This means that, in perception, the brain cannot distinguish between the separate sources of light that stimulated them, and so two dots close together cannot be resolved and will appear as a single blob. = low visual acuity.
What is retinal convergence
a number of rod cells are connected to a single bipolar cell
Cone cells main points
there are three different types each responding to a different range of wavelengths
Each have their own separate bipolar cell connected to a sensory neurone in the optic nerve.
Respond to high light intensity.
Concentrated in the fovea (this is where light is focused by the lens and so receives the highest concentration of light).
