| what are the two types of synapses? | excitatory and inhibitory |
| excitatory synapses | make the postsynaptic membrane more permeable to Na+
only one excitatory sysnapses is not enough to generate an acton potential |
| inhibitory synapses | make it less likely for an action potential to generate in the postsynaptic cell
Cl- ions move into the cell whilst K+ ions move out of the cell causing the membrane to be hyperpolarised
more exitatory neurones are needed to depolarise the membrane |
| what are the two types of summation? | spatial summation and temporal summation |
| what is summation? | the fact that each impulse adds to the effect of others |
| spatial summation | impulses are from different synapses usually from different neurons producing an action potential in the postsynaptic neurone |
| temporal summation | several impulses along the same neurone generate an action potential in the postsynaptic neurone |
| structue of a rod cell | outer segment
inner segment
synaptic region |
| outer segment of the rod cell | contains closely pack membrane disks containing rhodopsin |
| inner segment of the rod cell | contains cell organelles and the nucleus
also where lots of mitochondria are found |
| synaptic region of the rod cell | where the rod cell and the bipolar neurone synapse |
| what is rhodopsin? | light sensitive pigment
made up of opsin and retinal |
| what is retinal? | in cis form in the dark
in trans form in the light |
| the role of mitochondria in rod cells | produces ATP to provide energy for the ion pumps |
| what happens in our eyes in the dark? | Na+ flow into the outer segment of the rod cell
Na+ moves down the concentration gradient into the inner segment where pumps transport them back out the cell
the influx of Na+ produces a slight depolarisation of the rod cell
this triggers the release glutamate from the rod cells
glutamate binds to the bipolar cell stopping it from depolarising |
| what happens in our eyes in light? | light is absorbed by rhodopsin breaking it down into opsin and trans-retinal
opsin activate the hydrolysis of a cyclic molecule attached to the cation channel in the outer segment
the breakdown of the molecule results in the closure of the cation channel
so less Na+ enters the cell and the inner segment keeps pumping Na+ out
so the inside of the cell is more negative
the cell becomes hyperpolarised so the release of glutamate stops and the bipolar cell can depolarise |
| glutamate | neurotransmitter
released when there is no light present
inhibits the depolarisation of the bipolar cell so an impulse is not passed along the optic nerve to the brain |
| what is IAA? | a type of auxin
made in meristems
promotes cell growth and elongation
diffuses away from the cells in the light to cells in the shade
a plant hormone |
| define growth | an increase in plant volume or mass with or without the formation of new structures such as organs, tissues, cells or cell organelles |
| how is IAA transported around the plant? | long distances in the phloem
shorter distances between cells via specific carrier proteins |
| what is the effect of IAA on gene transcription? | auxins bind to protein receptors on their target cell
this activates intracellular second messenger signal molecules
these molecules activates transcription factors |
| define phototropism | the mechanism for plants growing towards the light |
| what is the effect of IAA on cell elongation? | process of phototropism
IAA attaches to specific receptor site in the cell membranes causing the active pumping of H+ into the cell wall
the influx of H+ into the cell wall causes the pH in the cell wall to fall
this provides an optimum acidic environment for expansins already present in the cell wall
ions move into the cell due increased permeability and water enters by osmosis
the cell swells as the vacuoles get larger so the wall grows to accommodate this
the cells in the shade grow whilst the cells in the light do not grow causing the bending of the shoot towards the unilateral light source
as the cell matures the auxin is destroyed and pH rises so the bonds reform between the cellulose microfibrils |
| role of expansins | break bonds between adjacent cellulose microfibrils and hemicelluloses causing the walls to become more flexible |
| why is auxin destroyed when the cell matures? | it is only found in growing tips |
| Darwin and Went's investigation into auxin and oat coleoptile growth | Darwin cut the tip of an oat coleoptile plant
he relplaced the tip back and the plant started growing towards the light again
Went cut the tip of the plant then placed it on an agar jelly block
he then put the block with tip on top on the rest of the plant
the plant started to grow again
Went then placed the agar on one side of the tip in the dark
the plant bent away from the side the agar was on |
| what does Darwin's and Went's investigation prove? | the presence of auxin in tips of plants
the importance of auxin in the growth of plants
the reason why plants grow towards the light
auxin is able to diffuse through agar jelly |
| define photoperiodism | the response of an organism(plants) to the changes in the length of days |
| define photomorphogenesis | developmental and morphological changes that are induced by light |
| define gravitropism | the important plant growth response to the environment that directs shoots upwards and roots downwards |
| define thigmotropism | oriented growth of an organism in response to mechanical contact e.g - plant coiling around its wooden support |
| what are phytochromes? | photoreceptors that detect direction quantity and wavelength of light
Pr and Pfr are photoreversible |
| Pr | phytochrome red
absorbs red light
is converted to Pfr in sunlight |
| Pfr | phytochrome far red
absorbs far red light
is converted into Pr in the dark or presence of far red light |
| how do phytochromes trigger germination? | the sun emits red light
red light is absorbed by Pr so it gets converted into Pfr
Pfr stimulates responses that lead to germination |
| how do plants know when to flower? | the ration of Pr to Pfr enables it to determine the length of day and night |
| long day plants | only flower when the day long and night short as they need Pfr to grow
e.g. strawberries |
| short day plants | flower when the period of uniterrupted darkness is more than 12 hours
they need to convert all the Pfr back to Pr
Pfr inhibits flowering in short day plants |
| how does Pfr work? | light shines and Pr converts into Pfr by changing its shape
Pfr can now bind to proteins
these proteins may act as transcription factors or activate transcription factors that bind to DNA
so light regulated genes are transcribed
the transcription and translation of proteins result in the plants response to light |
| greening | when the shoot has broken through soil into the sunlight the plant undergoes changes in form and biochemistry |
| nervous system | speed of transmission is fast
mode of transmission by neurones
made up of electrical impulses
influences muscles and glands |
| hormonal system | slow speed of transmission
mode of transmission is by glands
chemicals are released into the blood
alters the expression of genes
influences growth and development |
| similarities in plant and animal hormonal systems | involve chemical messages
chemicals are produced by cells
transported away from the site of production
influences gene expression
has a long term/permanent effect |
| hormonal system in animals | transported in the blood
slow action
doesnt just affect growth
responds is to stimuli like CO2 conc in blood |
| hormonal system in plants | diffuses through and between cells
slower action
affects only growth
responds to stimuli like light |
