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A2 Biology - Topic 8 - All You Need To Know - Part Three

Updated on June 19, 2012

Part 3!

This hub will cover the following topics in the following order!

  1. The Differences Between Nervous and Hormonal Control
  2. Coordination In Plants
  3. Parts Of The Eye
  4. The Retina and Nerve Impulses
  5. How Does Light Trigger Nerve Impulses?
  6. Phytochromes
  7. The Brain

Hormonal Control
Nervous Control
Chemical transmission through the blood
Electrical impulses through nerve impulses in neurones as well as chemical transmission at their synpases
Long-term changes like growth and maturation
Short term changes
Long term acting
Fast acting
Chemical signal taken to all cells but only certain ones (the target cells) will be affected by it
Signals sent to specific cells (which are connected to by neurones)
Response may be widespread (affecting a lot of different parts of the body e.g.all muscles)
Response is very local
Statements are made relative to the other

2. Coordination in Plants

  • Plants use chemicals called plant growth substances to control 1. growth, 2. development, and 3. responses to the environment.
  • Much like hormones, these plant growth substances are made in low concentrations.
  • Auxins are a class of plant growth substances
  • Plant stem tips exposed to unequal distribution of light will accumulate more auxins on their shaded sides leading that side to grow faster than the more lit side so that they grow towards the light.
  • Meristems are regions of a plant that are actively growing. Examples include: shoot tips, developing leaves, root tips and seeds.
  • Auxins work by binding to cell membrane receptors in target cells and subsequently causing a secondary messenger signal molecules within it, which changes gene expression in the cell. Transcription of enzymes then results in metabolic change and leads to increased growth.

Refer to this Diagram

The Wonderful Human Eye
The Wonderful Human Eye | Source

3. Parts Of The Eye

4 Main Parts:

  • Cornea - refracts light
  • Lens - focuses light on retina
  • Iris - controls amount of light entering the eye
  • Retina - contains the light sensitive rods and cones

Other Parts:

  • Ciliary Muscle - alters the thickness of the lens to change focus
  • Conjunctiva - protects the cornea
  • Sclera - protective layer around the eye
  • Fovea or 'Yellow Spot' - most sensitive part of the retina
  • Optic Nerve - group of neurones that lead to the CNS (brain)

4. The Retina and Photoreceptors

Rods and Cones - Definitions
In order to see we use two types of photoreceptors which are both located within the retina:

  1. Cones - these photoreceptors enable us to see in colour and do not work in low levels of light.
  2. Rods - these photoreceptors only enable us to see in black & white but do work in low levels of light.

Rods and Cones - Whereabouts

  • The very centre of the retina is made up entirely of cone cells.
  • In every other part of the retina, there are 20 times more rod cells than cone cells.

The Retina

  1. The retina is made up of three layers of cells.
  2. First is a rod or cone cell. This synapses with a bipolar neurone. This in turn synapses with a ganglion cell.
  3. Light must pass through all other layers of the retina before reaching the rod or cone cells at the back.

The three layers of the retina.
The three layers of the retina. | Source

5. How Does Light Trigger Nerve Impulses?


  • In both rods and cones there is a photochemical pigment that absorbs light and causes a chemical change.
  • In rods this photochemical pigment is called rhodopsin and it is a purplish pigment.
  • Rod cells are composed of an inner and outer segment and it is within the membranes of the many layers of flattened vesicles in these segments that rhodopsin is found.

When It Is Dark

  1. Sodium ions enter the outer segment of a rod or cone cell via non-specific cation channels.
  2. The sodium ions travel down to the inner segment via the concentration gradient (since less sodium ions are found in the inner segment due to 3.)
  3. Sodium ions are pumped out of the cell from the inner segment via pumps.
  4. The flow of sodium ions into the cell causes it to depolarise slightly, making it slightly more positive (-40mV) than a normal cell (-70v).
  5. The fact that the cone or rod cell is slightly depolarised results in a neurotransmitter (glutamate) being released which binds to bipolar cells and stops them from depolarising (which would have happened if not for the glutamate).

When It Is Light

  1. Light breaks the rhodopsin molecule down into opsin and retinal.
  2. Opsin activates a series of membrane-bound reactions which leads to the hydrolysis of molecules attached to the non-specific cation channels in the outer segment of the rod or cone cells. This leads to the closing of the non-specific cation channels.
  3. New sodium ions no longer enter the cell but existing ones are still pumped out, which leads to the cell becoming more negative (it becomes hyperpolarised).
  4. Therefore, the glutamate stops being produced and so bipolar cells depolarise.
  5. This leads to ganglion cells depolarising which create an action potential which is sent to the CNS and is processed as visual information.

Red to Far-red  light conversion diagram
Red to Far-red light conversion diagram | Source

6. Phytochromes

  • Phytochromes are plant photoreceptors.
  • A phytochrome is made up of 1. a protein component which is bound to 2. a non-protein photochemical pigment molecule.
  • The non-protein molecule can either be Pr (phytochrome red)or Pfr (phytochrome far red)
  • Pr absorbs red light (660nm)
  • Pfr absorbs far-red light (730nm)
  • When a Pr pigment absorbs red light it converts into a Pfr pigment.
  • When a Pfr pigment absorbs far-red light it converts into a Prpigment.

There is more red than far-red light in sunlight and therefore in the day, Pfr accumulates in a plant. Pfr slowly reverts back to Pr in the dark.

Phytochromes and Germination

  • For some plant species, a flash of red light triggers germination. In these species, a flash of far-red lightinhibits germination.
  • Remember that it is the presence of Pfr (which is not present in the dark since it reverts back to Pr) that is necessary to trigger the germination of these plants.

Phytochromes and Photoperiods

  • Photoperiodsare the relative time of day to night.
  • The differing ratio of the two phytochromes allow plants to gauge the length of day and night and germinate optimally.

Phytochromes and Greening

  • 'Greening' are the preliminary changes that occur after a plant shoot comes into contact with sunlight.
  • Phytochromes are responsible for 'greening' and initiate the development of primary leaves, leaf unrolling and the production of pigments. They also inhibit elongation of internodes.

How Do Phytochromes Cause Changes?

  • Phytochromes are activated by light which changes their shapes.
  • These new shapes determine which proteins they can bind to and so which proteins they can bind to and which protein complexes they can prevent from being created.
  • They act as signal proteins that act as or activate other transcription factors.
  • These transcription factors cause the transcription and translation of particular enzymes which result in the reaction to light that we see (e.g. the enzyme that controls chlorophyll production is made and greening of the shoot occurs).

Note Also:

  1. Gravity is also an environmental cue for plants and ensures that roots go down into the Earth (geotropism) and shoots head towards the light (phototropism).
  2. Touch and mechanical stress are also cues for some plants and

7. The Brain

For the purpose of keeping hubs at a reasonable length so as not to make them too daunting, this section has been covered in detail in a separate hub found here:

A2 Biology Revision: The Different Parts Of The Brain

You Should Now Be Able To

  1. Draw a table detailing the differences between hormonal and nervous control.
  2. Draw a labelled diagram of the eye, identifying key parts and explain what they do.
  3. Outline the differences of plant coordination.
  4. Describe what happens to rod and cone cells in the dark and in the light.
  5. Describe the differences between rods and cone cells as well as the structure of the retina.
  6. Describe the uses of phytochromes in certain plants.
  7. Draw and label a descriptive diagram of the brain.


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