B3 Synthesis: Homeostasis, Required Practicals and Exam Skills

You have now worked through the whole of Topic B3 of OCR Gateway Science A — the nervous system, reflexes and synapses, the brain and the eye, the endocrine system, the control of blood glucose, thermoregulation and the kidney, the hormones of reproduction, and plant responses. This final lesson pulls it all together. The big idea that links almost every part of B3 is homeostasis and the recurring receptor → coordination centre → effector pattern of negative feedback. This lesson shows how that one pattern appears again and again, recaps the nervous vs hormonal contrast, revisits the required practicals, and warns about the misconceptions that catch students out. Treat it as a revision and exam-technique session.

By the end of this lesson you should be able to apply the negative-feedback model across all of B3, compare nervous and hormonal control confidently, recall the required practicals and their variables, and avoid the most common B3 errors.

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The Big Idea: Homeostasis and Negative Feedback

The single most powerful idea in B3 is that the body keeps its internal conditions stable by negative feedback, using the same three-part control system every time:

flowchart LR
  A["Receptor<br/>detects a change"] --> B["Coordination centre<br/>processes the information"]
  B --> C["Effector<br/>brings about a response"]
  C -.->|"counteracts the change,<br/>returning to normal"| A

Whenever a condition moves away from its normal level, receptors detect the change, a coordination centre processes it, and effectors produce a response that counteracts the change and brings the level back to normal. The remarkable thing is how many different systems in B3 follow exactly this template:

Condition controlled	Receptor / detector	Coordination centre	Effector(s)	Correcting response
Blood glucose	Pancreas	Pancreas	Liver / body cells	Insulin stores glucose as glycogen (lowers); glucagon releases it (raises, Higher)
Body temperature	Skin & brain receptors	Thermoregulatory centre (brain)	Skin vessels, sweat glands, muscles	Vasodilation/sweating (cool); vasoconstriction/shivering (warm)
Water content (Higher)	Brain	Brain / pituitary	Kidney	ADH adjusts how much water is reabsorbed
Menstrual cycle	—	Pituitary & ovaries	Ovaries / uterus	Hormones build/maintain lining and trigger ovulation
Thyroxine level (Higher)	—	Pituitary	Thyroid	Negative feedback raises/lowers thyroxine

Seeing this shared pattern is exactly the kind of synoptic understanding that lifts an answer. If a question gives you an unfamiliar example of homeostasis, you can still answer it by applying receptor → coordination centre → effector → counteract the change → return to normal.

Exam Tip: When you meet any homeostasis question — even an unfamiliar one — reach for the same framework: name the receptor, the coordination centre, the effector and the corrective response, and state that the level returns to normal. This template earns marks across the whole topic.

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Nervous vs Hormonal Control: the Recurring Contrast

B3 has two coordinating systems, and the contrast between them runs through the whole topic. This table is worth knowing cold.

Feature	Nervous control	Hormonal control
Message carried by	Electrical impulses along neurones	Chemical hormones in the blood
Speed	Very fast	Slower
Duration	Short-lived	Long-lasting
Area affected	Precise/localised	Widespread (any target organ with receptors)
Example in B3	Reflex withdrawal; pupil reflex	Blood glucose control; the menstrual cycle

The two systems are not rivals — they work together. The fight-or-flight response is the perfect illustration: the nervous system reacts instantly, while adrenaline (hormonal) sustains the raised heart and breathing rate. Being able to say which system suits a given situation, and why (its speed and duration), is a recurring B3 skill.

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The B3 Required Practicals at a Glance

Practical	What you change / measure	Key technique	Top marks come from
Reaction time (ruler-drop test)	Catch a dropped ruler; measure the distance fallen	Drop without warning; convert distance to time	Keeping conditions the same; repeats and a mean; ignoring anomalies
Plant tropisms	Change the light direction (or orientation); measure growth direction	Germinating seedlings; equal seeds and water	Controlling temperature, water and seed type; explaining the auxin mechanism

Exam Tip: For both practicals, examiners reward identifying the independent variable (what you change), the dependent variable (what you measure) and the control variables (what you keep the same). For the ruler-drop test, "drop without warning" is a frequently rewarded detail.

Worked Example — Reaction-time data

A student does the ruler-drop test five times and records catch distances of $15, 14, 22, 13$15,14,22,13 and $16 \text{ cm}$16 cm. Identify any anomaly and calculate the mean of the remaining values.

Step 1 — spot the anomaly. The value $22 \text{ cm}$22 cm is much larger than the others, so it is an anomaly and should be excluded.

Step 2 — calculate the mean of the remaining four:

$\frac{15 + 14 + 13 + 16}{4} = \frac{58}{4} = 14.5 \text{ cm}$415+14+13+16​=458​=14.5 cm

Answer: the anomaly is $22 \text{ cm}$22 cm; the mean of the other readings is $14.5 \text{ cm}$14.5 cm. A shorter distance means a faster reaction, so this student's typical catch corresponds to a fast reaction time.

Common error: including the anomalous $22 \text{ cm}$22 cm in the mean, which would wrongly inflate it. Always identify and exclude anomalies first.

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Mastering the B3 Calculation: BMI

The main calculation in B3 is the body mass index, used when discussing the obesity link to Type 2 diabetes.

$\text{BMI} = \frac{\text{body mass (kg)}}{[\text{height (m)}]^{2}}$BMI=[height (m)]2body mass (kg)​

Worked example: A person has a mass of $72 \text{ kg}$72 kg and a height of $1.6 \text{ m}$1.6 m. Find their BMI.

Square the height first: $[1.6]^{2} = 2.56 \text{ m}^2$[1.6]2=2.56 m2. Then:

$\text{BMI} = \frac{72}{2.56} = 28.1 \text{ (to 3 s.f.)}$BMI=2.5672​=28.1 (to 3 s.f.)

Answer: a BMI of about $28$28, which falls in the overweight range (25.0–29.9).

Exam Tip: The classic BMI mistake is forgetting to square the height. Write the squaring as its own step, and remember the units are $\text{kg/m}^2$kg/m2.

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Command Words You Will Meet in B3

OCR uses specific command words that tell you what kind of answer to give. Reading them correctly is worth easy marks.

Command word	What it asks for
State / Name / Give	A short fact, no explanation (e.g. "Name the gland that releases insulin")
Describe	Say what happens, in order, with no need for reasons
Explain	Give reasons why — use "because", "so that", "this means that"
Compare	Give similarities and differences, ideally point-by-point
Calculate	Work out a number — show working and give a unit
Evaluate	Give points for and against and a brief conclusion

Exam Tip: The difference between describe and explain decides many marks. "Blood glucose falls after insulin is released" describes; "...because insulin makes the liver store glucose as glycogen" explains. If the command word is explain, give the reason.

Memory checklist — the facts to know cold

Cover the right-hand column and test yourself.