Pearson Edexcel International A Level Biology: Sensory Reception, Plant Hormones and the Brain (YBI11) – Revision Notes

These notes condense statements 8.8 to 8.16 of the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2, including Core Practical 18. Topic 8 is International A2 (IA2) and there are no tiers. Read the study guide for full explanations and test yourself with the practice questions. The course hub, printable checklist and free diagnostics show what else to cover.

Key terms

Term	Meaning
Rhodopsin	Visual pigment of rods: opsin (protein) bound to retinal
Opsin	Protein part of rhodopsin; remains after bleaching
Retinal	Light-absorbing part of rhodopsin
Hyperpolarisation	Inside of the cell becomes more negative than at rest
Habituation	Reduced response to a repeated stimulus that is neither harmful nor rewarding
CNS / PNS	Brain and spinal cord / neurones outside them
Phytochrome	Plant pigment with Pr and Pfr forms
IAA	Indoleacetic acid, the main auxin
Gibberellin	Plant hormone that switches on amylase genes in cereal grains
Aleurone layer	Outer layer of a cereal endosperm that makes amylase

8.8 Rods: the sequence

In the dark: cation channels open, sodium ions enter, rod partly depolarised, neurotransmitter released continuously, bipolar neurone inhibited.

In the light, in steps:

1. Rhodopsin absorbs light and splits into opsin and retinal.
2. Cation channels close.
3. Sodium ions stop entering; pumping out continues.
4. Rod hyperpolarised.
5. Less neurotransmitter released.
6. Bipolar neurone depolarised and releases neurotransmitter.
7. Optic neurone reaches threshold and an action potential passes to the brain.

Remember: the light closes the channels. A rod is hyperpolarised in the light, not depolarised.

Rods: many connect to one bipolar neurone, giving high sensitivity and low detail.

8.9 Habituation

• Definition: a decrease in response to a stimulus repeated many times without a consequence.
• Value: saves energy and attention for new or dangerous stimuli.
• Not the same as: fatigue (a new stimulus gives a full response) or damage to a receptor.
• Usual mechanism: fewer calcium ions enter the presynaptic neurone, so less neurotransmitter is released.
• Practical: repeat the same stimulus at fixed intervals, record the response each time, and check that a new stimulus restores it.

8.10 Nervous system

CNS = brain and spinal cord. PNS = all neurones outside, including sensory neurones and motor neurones.

8.11 Plant hormones

Hormone	Made or acts	Effect	Transcription link
Phytochrome	Pr (red, about 660 nm) to Pfr (far-red, about 730 nm)	Light-sensitive germination, etiolation, flowering	Pfr enters the nucleus and affects transcription factors
Auxin (IAA)	Shoot tip, moves down	Elongation of shoot cells; inhibits elongation in roots at higher concentration; apical dominance	Repressor removed, transcription factor switches on elongation genes
Gibberellin	Embryo of seed	Stem elongation; amylase production in cereal grains	Repressor protein broken down, amylase gene transcribed

Method in steps for phototropism:

1. Light from one side.
2. IAA moves to the shaded side.
3. Wall pH falls on that side as proton pumps act.
4. Cells there elongate more.
5. Shoot bends towards the light.

Gravitropism of a root: IAA gathers on the lower side, inhibits elongation there, so the root grows downwards.

Flowering: Pfr promotes flowering in long-day plants and inhibits flowering in short-day plants.

8.12 Core Practical 18 summary

Item	Detail
Aim	Investigate amylase production in germinating cereal grains
Method	Grain halves (with and without embryo) on starch agar, with or without gibberellic acid
Detection	Remove grains, flood with iodine solution; the clear zone shows starch digestion
Measure	Clear zone diameter; area = πr²
Controls	Boiled grain halves; same temperature, time, grain type, agar
Hazards	Iodine staining; microorganism contamination of plates
Reliability	Repeat on several plates and calculate the mean

Worked reminder. A clear zone has a diameter of 14 mm.

• Radius = 7 mm.
• Area = π × 7² = 153.9 = 154 mm² (3 significant figures).
• A zone of diameter 7 mm has area 38.5 mm², a quarter of the area when the diameter is halved.

8.13 Nervous against hormonal

	Nervous	Hormonal
Pathway	Neurones	Blood
Speed	Fast	Slower
Effect	Short-lived, localised	Longer-lasting, widespread
Target	Effector cells	Cells with the right receptor

8.14 Brain regions

Region	Functions
Cerebral hemispheres	Conscious thought, memory, language, voluntary movement, interpreting senses
Hypothalamus	Temperature and water balance, hunger, thirst, controls pituitary
Pituitary gland	Hormone release, such as ADH, and control of other glands
Cerebellum	Coordinates movement and balance, posture
Medulla oblongata	Heart rate, ventilation rate, reflexes such as coughing and swallowing

8.15 Scans

Scan	Principle	Shows	Notes
MRI	Magnetic field and radio waves, hydrogen nuclei	Structure of soft tissue	No ionising radiation; cannot be used with some metal implants
fMRI	Blood oxygen changes	Active regions	Activity uses more oxygen
PET	Radioactive tracer, gamma rays	Metabolic activity	Tracer has a short half-life
CT	Rotating X-ray source	Cross-sectional structure	Ionising radiation; quick

Half-life reminder. Remaining fraction = (1/2) to the power of (time ÷ half-life).

• Tracer with a half-life of 110 minutes, after 220 minutes: (1/2)² = 1/4.
• Starting at 600 MBq, the activity is 600 ÷ 4 = 150 MBq.

8.16 Brain chemicals

Condition	Imbalance	Treatment idea
Parkinson’s disease	Loss of dopamine-releasing neurones, so less dopamine	L-DOPA, converted to dopamine in the brain
Depression	Low serotonin activity at synapses	SSRIs block uptake of serotonin from the synapse

Drug development: computer modelling and laboratory tests, animal tests, then clinical trials. Phase I: safety in healthy volunteers. Phase II: dose and effect in patients. Phase III: compared with a placebo or existing treatment, randomised and double-blind.

Interpreting data: a checklist

1. Read the axis titles and units before describing anything.
2. Describe the trend with figures from the table or graph, including the start and end values.
3. Calculate a percentage change as (new − old) ÷ old × 100.
4. Compare with the control, so that the effect can be attributed to the variable.
5. Explain using the biology: name the ion, hormone, structure or region.
6. Comment on reliability: repeats, means, and whether the ranges overlap.

Worked reminder: habituation. A habituating response falls steeply at first and then levels off. If a snail takes 40 s to re-extend its tentacles on trial 1 and 10 s on trial 12, the decrease is 30 s, which is 75%. A graph that levels off at a low value shows that the response has been largely lost, not that the animal has stopped being able to respond: a new stimulus should restore it.

Worked reminder: gibberellin and amylase. If amylase activity rises from 4 units to 20 units in the presence of gibberellin, the increase is 16 units, which is (20 − 4) ÷ 4 × 100 = 400%. Explain with transcription of the amylase gene in the aleurone layer, then hydrolysis of starch in the endosperm.

Must-know distinctions

• Depolarisation (inside less negative) and hyperpolarisation (inside more negative).
• Habituation and fatigue.
• Structural scans (MRI, CT) and functional scans (fMRI, PET).
• Hypothalamus (controls) and pituitary gland (releases hormones).
• Cerebellum (movement and balance) and medulla oblongata (heart rate and ventilation).
• IAA in shoots (promotes elongation) and in roots at a high concentration (inhibits it).
• Phytochrome (a pigment that senses light) and auxin (a hormone that moves).

Quick self-test

1. Name the two parts of rhodopsin.
2. In the dark, which ion enters the outer segment of a rod through open cation channels?
3. State what happens to the cation channels when rhodopsin splits.
4. A mouse is given 30 puffs of air and its startle response falls from 30 movements to 12 movements per minute. Calculate the percentage decrease.
5. Which form of phytochrome absorbs far-red light?
6. Give one response of a shoot to IAA and one response of a root to a high concentration of IAA.
7. Which tissue of a cereal grain makes amylase after gibberellin acts on it?
8. A clear zone on a starch plate has a diameter of 12 mm. Calculate its area to 3 significant figures.
9. State one structural and one functional scan technique.
10. A coleoptile grows from 18 mm to 27 mm. Calculate the percentage increase.
11. Name the brain region that controls the rate of ventilation.
12. Give the term for the process by which SSRIs increase serotonin in the synapse.

Answers

1. Opsin and retinal.
2. Sodium ions.
3. They close.
4. (30 − 12) ÷ 30 × 100 = 60%.
5. Pfr.
6. Shoot: cell elongation (bending towards light). Root: elongation is inhibited.
7. The aleurone layer.
8. Radius 6 mm; π × 36 = 113 mm².
9. Structural: MRI or CT. Functional: fMRI or PET.
10. (27 − 18) ÷ 18 × 100 = 50%.
11. The medulla oblongata.
12. Blocking reuptake (they inhibit serotonin uptake by the presynaptic neurone).

Where marks are usually lost

• Saying light opens the sodium channels in a rod.
• Leaving out the sodium ions when explaining hyperpolarisation.
• Describing habituation as a muscle getting tired.
• Giving only the shoot effect of IAA and omitting the root.
• Saying gibberellin “produces amylase” without mentioning the gene being transcribed.
• Calling the clear zone the part where starch is present.
• Forgetting a boiled-grain control, or not describing how contamination is prevented.
• Writing “the brain” instead of the named region.
• Saying fMRI uses radiation, or that CT shows function.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology (YBI11) specification, Issue 2, published by Pearson Education Limited.