Topic 8: Eyes, Habituation & Plant Hormones

In the Light (Inactive rod cells, signal sent)

• more light → more frequent action potentials

• parasympathetic nerves send signals to eye

• circular muscle contract, radial muscles relax → pupil constricts

• less light enters, protecting the retina

In the Dark (Active rod cells, no signal sent)

• less light → less frequent action potentials

• sympathetic nerves send signals to eye

• circular muscle relax, radial muscles contract → pupil dilates

• more light enters, improving vision

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Describe the role of rhodopsin in rod cells

rhodopsin is the photoreceptor

when light is absorbed cis retinal is converted to trans

which causes Na channels to close

membrane becomes hyperpolarised

Habituation

a diminishing of an innate response to a frequency repeated stimulus

Phytochrome

• Pr (inactive) → absorbs red light (660 nm)

• Pfr (active) → absorbs far red light (730 nm)

• long day plants → flower when the nights are short & it’s length is less than a critical threshold (high Pfr)

• short-day plants → flower when nights are long & uninterrupted which exceeds a critical length (low Pfr)

Florigen

when photoperiodism conditions are met, it’s produced in leaves and move to shoot tips to activate flowering genes

Phytocrhome & transcription

Pfr interacts with a transcription factor called PIF3 which stops it from inhibiting transcription

Apical dominance

Apical bud (tip of the shoot) produces IAA, inhibiting the growth of lateral buds

Phototropism

IAA accumulates on the shaded side → stimulates cell elongation, causing the shoot to bend towards the light; maximizing light absorption

Gravitropism

IAA accumulates on the lower side → inhibiting cell elongation → root bends downwards; maximizing water absorption & giving stability

Action of Auxin

1. IAA diffuses from shoot tip to zone of elongation

2. IAA binds to receptors, activating proton pumps

3. H+ pumped into cell wall, lowering pH

4. acidic conditions activate enzymes that break bonds between cellulose microfibrils

5. water enters cell → turgor pressure increases → cell elongates

Mechanism of Gibberellins

1. water uptake by seeds causes swelling & activates metabolism

2. embryo releases gibberellins, that diffuse into the aleurone layer

3. gibberellins activate a transcription factor that switches on the amylase gene

4. amylase is produced and breaks down starch → maltose + glucose (for energy)

Gibberellins & Transcription

1. gibberellins bind to receptors, activating enzymes that break down a repressor protein that prevents transcription

2. TF can bind to promoter region, triggering transcription of genes like amylase