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

This guide teaches sensory reception, plant hormones and the brain for the Pearson Edexcel International Advanced Subsidiary/Advanced Level Biology specification (YBI11), Issue 2. It covers statements 8.8 to 8.16 of Topic 8, including Core Practical 18. Topics 1 to 4 are International AS (IAS) and Topics 5 to 8 are International A2 (IA2), so this material is A2. There are no tiers. Use it with the revision notes and the practice questions. See the course hub, the printable checklist and the free diagnostics. For adrenaline, see skeletal muscle, cardiac control and adrenaline.

What this guide covers

Statement	What you must be able to do
8.8	Explain how rods detect light: rhodopsin, opsin, retinal, sodium ions, cation channels, hyperpolarisation, action potentials in optic neurones
8.9	Define habituation (and the recommended practical)
8.10	Know that the nervous system has a central and a peripheral part
8.11	Explain the actions of phytochrome, auxin (IAA) and gibberellins, including effects on transcription
8.12	Core Practical 18: amylase production in germinating cereal grains
8.13	Compare nervous and hormonal coordination in animals
8.14	Locate and state the functions of five brain regions
8.15	Explain how MRI, fMRI, PET and CT are used
8.16	Explain how imbalances of dopamine and serotonin affect health, and how new drugs are developed

Statements 8.1 to 8.7 (neurones, action potentials, synapses and drugs) are assumed here.

8.8 How rods detect light

A rod cell is a receptor cell in the retina of a mammal. Its outer segment holds a visual pigment called rhodopsin. Rhodopsin is made of a protein, opsin, bound to a light-absorbing molecule, retinal. Rods are used for vision in dim light.

In the dark. Cation channels in the surface membrane of the outer segment are open. Sodium ions enter, and the rod cell is partly depolarised. In this state it releases neurotransmitter continuously onto the bipolar neurone, which stops the bipolar neurone from firing.

In the light.

1. A photon is absorbed by retinal in rhodopsin, which changes shape.
2. Rhodopsin splits (bleaches) into opsin and retinal.
3. This leads to the closing of the cation channels, so sodium ions stop entering the outer segment. Sodium ions are still pumped out, so the inside becomes more negative. This is hyperpolarisation.
4. The hyperpolarised rod releases less neurotransmitter.
5. The bipolar neurone is no longer inhibited and is depolarised. It releases neurotransmitter onto a neurone of the optic nerve.
6. If the depolarisation of the optic neurone reaches the threshold, an action potential passes along the optic nerve to the brain.

Many rods connect to one bipolar neurone, so their effects add up. This gives sensitivity in dim light but poor detail.

Worked example. Explain why a rod cell is hyperpolarised when light falls on it.

• Light splits rhodopsin into opsin and retinal.
• The cation channels close, so sodium ions no longer enter the cell.
• Sodium ions are still removed by active transport, so the inside becomes more negative than in the dark. The rod is hyperpolarised.

8.9 Habituation

Habituation is a reduction in the response to a stimulus that is repeated many times and is not followed by anything harmful or rewarding. A new stimulus still produces the full response, so it is not muscle fatigue.

Its value is that an animal stops spending energy on harmless stimuli and stays ready for a new one that might signal a predator or food. In the usual model, fewer calcium ions enter the presynaptic neurone, so less neurotransmitter crosses the synapse.

Recommended additional practical. The specification recommends investigating habituation to a stimulus. A simple design is to tap the shell of a pond snail gently with the same force every 30 seconds and time how long it takes to re-extend its tentacles. Handle the animal gently.

Worked example. The snail takes 42 s to re-extend its tentacles after the first tap and 6 s after the fifteenth.

• Decrease = 42 − 6 = 36 s.
• Percentage decrease = 36 ÷ 42 × 100 = 85.7% (to 3 significant figures).
• A full response to a different stimulus, such as a shadow, shows the snail is not simply tired.

8.10 The nervous system

A mammal’s nervous system is divided into a central part (CNS), made of the brain with the spinal cord, and a peripheral part (PNS). The PNS includes every neurone outside the CNS: sensory neurones bring impulses in and motor neurones take impulses out to effectors.

8.11 Plant hormones and light

Plants coordinate responses with hormones, not nerves. A hormone binds to a receptor on or in a target cell. Often the effect is on transcription: a repressor protein that blocks a transcription factor is removed, so the factor can bind to DNA and switch genes on.

Phytochrome

Phytochrome is a pigment protein with two forms that convert into each other. Pr absorbs red light (about 660 nm) and is converted to Pfr. Pfr absorbs far-red light (about 730 nm) and is converted back to Pr. Pfr also reverts slowly in darkness. Pfr is the active form: it moves into the nucleus and affects transcription factors, so that genes are switched on or off.

Responses controlled by phytochrome include:

• germination of some small seeds that need light;
• reduced stem elongation and greening of seedlings in light (in darkness they are etiolated: long, thin and pale);
• flowering. Pfr promotes flowering in long-day plants and inhibits flowering in short-day plants.

Auxin (IAA)

Indoleacetic acid (IAA) is synthesised at the tip of the shoot and is transported downwards.

• In phototropism, IAA is carried to the shaded side of the shoot. More IAA there means more cell elongation on the shaded side, so the shoot bends towards the light.
• In the acid growth model, IAA causes proton pumps to move hydrogen ions into the cell wall. The lower pH activates wall-loosening proteins, so the cellulose microfibrils can slide. The cell takes in water by osmosis and elongates.
• IAA also affects transcription, switching on genes needed for elongation.
• Roots respond differently. In a root, a high IAA concentration inhibits elongation. In gravitropism, IAA accumulates on the lower side of a horizontal root, slows growth there, and the root bends downwards.
• IAA from the apical bud inhibits growth of lateral buds (apical dominance).

Worked example. Segments 20 mm long were placed in IAA solutions for 24 hours.

IAA (arbitrary units)	Stem final length (mm)	Root final length (mm)
0	21.0	21.0
1	23.0	20.2
10	26.0	19.0

• Stem change at 10 units = (26.0 − 20.0) ÷ 20.0 × 100 = +30.0%.
• Root change at 10 units = (19.0 − 20.0) ÷ 20.0 × 100 = −5.0%.
• IAA stimulates stem elongation more as concentration increases, but it inhibits roots at the higher concentration. The 0 units control (+5.0% for both) shows the change is due to IAA.

Gibberellins

Gibberellins are made in the embryo of a germinating seed. They also cause stem elongation. In cereal grains the embryo releases gibberellin, which diffuses to the aleurone layer. There, a repressor protein is broken down, a transcription factor becomes active, and transcription of the gene for amylase is switched on. Amylase is secreted into the endosperm and hydrolyses starch to maltose, which is converted to glucose for the embryo.

8.12 Core Practical 18: amylase in germinating cereal grains

Question. Does gibberellin increase amylase production by cereal grains?

Method.

1. Cut barley grains in half across the middle, so one half contains the embryo and the other half does not.
2. Place the halves, cut face down, on starch agar plates. Some plates contain gibberellic acid at different concentrations (for example 0, 1 and 10 units).
3. Include a control with boiled grain halves, which cannot make enzyme.
4. Incubate at a constant temperature for 24 to 48 hours.
5. Remove the grains and flood each plate with iodine solution in potassium iodide.
6. Measure the diameter of the clear zone, where starch has been digested, in two directions. Calculate the area from πr².

Variables. The independent variable is gibberellin concentration (or presence of the embryo). The dependent variable is the clear zone size. Keep constant the grain variety, temperature, time, agar volume and starch concentration. Sterilise the grain surface so that microorganisms do not add their own amylase.

Worked example. Mean clear zone diameters were: 0 units 3.33 mm, 1 unit 10.0 mm, 10 units 16.0 mm.

• Area at 1 unit = π × (10.0 ÷ 2)² = π × 25 = 78.5 mm².
• Area at 10 units = π × 8.0² = 201 mm².
• Area at 0 units = π × (3.33 ÷ 2)² = 8.7 mm².
• Area increase from 1 to 10 units = (201.1 − 78.5) ÷ 78.5 × 100 = 156%.
• Conclusion: more gibberellin gives more amylase.

Safety: iodine stains and irritates, so wear eye protection, and wash hands at the end.

8.13 Nervous and hormonal coordination

Feature	Nervous	Hormonal
Signal	Electrical impulse, then neurotransmitter	Hormone carried by the blood
Speed	Very fast	Slower
Duration	Short	Longer lasting
Spread	Specific effector	Any target cell with the receptor

The systems work together: the hypothalamus controls the pituitary gland (8.14), and adrenaline is released after nervous stimulation of the adrenal glands.

8.14 The human brain

Region	Location and main functions
Cerebral hemispheres	Two halves forming the largest part of the brain. Conscious thought, memory, language, interpretation of sensory input, voluntary movement. Each side mainly controls the opposite side of the body.
Hypothalamus	Below the hemispheres. Monitors blood temperature and water potential, controls hunger and thirst, and controls the pituitary gland.
Pituitary gland	Hangs beneath the hypothalamus. Releases hormones, for example antidiuretic hormone (ADH), and controls other glands.
Cerebellum	Below the back of the hemispheres. Coordinates movement and balance, and maintains posture.
Medulla oblongata	Joins the spinal cord. Controls heart rate and ventilation rate, and reflexes such as coughing and swallowing.

8.15 Brain imaging

Technique	What it shows and how
MRI	Magnetic fields and radio waves make hydrogen nuclei emit signals. Gives detailed images of soft tissue structure. No ionising radiation.
fMRI	Detects changes in blood oxygen. Active regions use more oxygen, so the image shows function.
PET	A radioactive tracer, such as glucose labelled with fluorine-18, is injected. Gamma rays show metabolically active regions.
CT	X-rays from a rotating source build cross-sectional images of structure. Quick, so suited to emergencies.

MRI and CT show structure, for example a tumour or bleeding after a head injury. fMRI and PET show activity. PET and CT involve ionising radiation. MRI cannot be used with some metal implants.

Worked example. Fluorine-18 has a half-life of about 110 minutes. What percentage of the activity remains after 330 minutes?

• 330 ÷ 110 = 3 half-lives.
• (1/2)³ = 1/8 = 12.5%.

8.16 Brain chemicals and drugs

In Parkinson’s disease, neurones that release dopamine die, so less dopamine is available to control movement. L-DOPA is converted into dopamine in the brain. In depression, low serotonin activity is linked to low mood. Selective serotonin reuptake inhibitors (SSRIs) block the uptake of serotonin from the synapse, so it stays active for longer. Say “can contribute”, because the causes are complex.

New drugs are developed through computer modelling and laboratory testing, then animal tests, then clinical trials. Phase I tests safety in healthy volunteers. Phase II tests dose and effect in patients. Phase III compares the drug with a placebo or an existing treatment, with random allocation and double-blinding.

Worked example. In a trial of 90 patients on a drug, 63 improved. In 90 on a placebo, 27 improved.

• Drug: 63 ÷ 90 × 100 = 70.0%; placebo: 27 ÷ 90 × 100 = 30.0%.
• Difference = 40.0 percentage points.

Common errors

• Writing that light “opens” sodium channels in rods. In the light the channels close.
• Calling hyperpolarisation a depolarisation.
• Describing habituation as fatigue or as a permanent loss of response.
• Giving one effect of IAA only. State stem and root effects.
• Forgetting that the clear zone in Core Practical 18 is where starch has been digested.

Official syllabus

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