OCR GCSE Biology: Scaling Up (J247)

This guide covers Topic B2 Scaling Up, the second of six teaching topics in OCR GCSE (9-1) Biology A – Gateway Science (J247), for first assessment 2018. It builds directly on Topic B1 Cell Level Systems, moving from the workings of a single cell to why multicellular organisms need specialised exchange and transport systems at all.

Syllabus coverage

OCR GCSE (9-1) BIOLOGY A (GATEWAY SCIENCE) J247 – TOPIC B2 SCALING UP

• B2.1 Supplying the cell – explaining how substances (including water) are transported into and out of cells by diffusion, osmosis and active transport, using the terms concentration gradient and water potential; describing the process of mitosis in growth, including the cell cycle (cell growth, DNA replication, further cell growth, and movement of chromosomes); explaining the importance of cell differentiation, and giving examples of specialised cells; recalling that stem cells are found in embryonic and adult animals, and in meristems in plants; describing the functions of stem cells in growth, development and repair; and describing the difference between embryonic and adult stem cells.
• B2.2 The challenges of size – explaining why multicellular organisms need exchange surfaces and transport systems, in terms of surface area to volume ratio, including calculating surface area, volume and the ratio between them; describing which substances are transported into and out of organisms (oxygen, carbon dioxide, water, dissolved food molecules, mineral ions and urea); describing the human circulatory system, including the need for a double circulatory system in mammals; explaining how the structure of the heart and blood vessels is adapted to their function, including the cardiac muscle, valves, chambers and vessel walls; explaining how red blood cells and plasma are adapted to their transport role; explaining how water and mineral ions are taken up by plants, relating root hair cell structure to function; describing the processes of transpiration and translocation, including the structure of stomata; explaining how xylem and phloem are adapted to their functions; explaining the effect of light intensity, air movement and temperature on the rate of water uptake by a plant; and describing how a simple potometer is used to investigate factors affecting the rate of water uptake, including calculating rate and percentage change in mass.

Why size creates a biological problem

The specification’s own framing is worth taking seriously as an exam answer structure: as an organism gets larger, its volume increases faster than its surface area, so the surface area to volume ratio falls. A single-celled organism can rely on diffusion alone because its surface area is large relative to its volume; a large multicellular animal cannot, because diffusion alone would be far too slow to supply every cell with oxygen and nutrients or remove its waste in time. This single idea is the reason B2.2 exists at all – every exchange surface and transport system described in this subtopic (lungs, gut, capillaries, xylem, phloem, root hairs) is a biological solution to the same underlying maths problem, so an exam answer that opens with the surface-area-to-volume argument before describing the specific adaptation is answering the question the specification is actually asking.

Diffusion, osmosis and active transport – keeping them distinct

B2.1 expects all three transport mechanisms to be distinguished clearly, and exam questions routinely test this directly:

• Diffusion moves particles from a region of higher concentration to lower concentration, down a concentration gradient, and needs no energy input from the cell.
• Osmosis is the diffusion of water specifically, across a partially permeable membrane, from a region of higher water potential to lower water potential – also passive.
• Active transport moves substances against a concentration gradient (low to high concentration) and therefore requires energy, typically because a cell needs to accumulate a substance beyond what diffusion alone could achieve, such as mineral ion uptake by root hair cells when soil mineral concentration is lower than that inside the cell.

A common misconception addressed directly by the specification

OCR’s own specification flags that learners commonly believe blood flows slowly in capillaries because capillaries are narrow – the correct explanation is that the total cross-sectional area of all the capillaries in a tissue is around a thousand times greater than that of the aorta, so the blood moves more slowly (in linear speed) through the capillary bed even though the total volume flowing through per second is unchanged and individual capillaries are narrow. This matters biologically because slow flow through capillaries is precisely what allows time for diffusion of oxygen, nutrients and waste between blood and tissue – so the “problem” of slow capillary flow is in fact the adaptation that makes exchange possible.

Common mistakes

• Describing diffusion, osmosis and active transport interchangeably instead of identifying which one applies to a given scenario, especially confusing osmosis (water only) with diffusion (any particle).
• Explaining capillary blood flow by capillary width alone, rather than by total cross-sectional area, as above.
• Confusing xylem and phloem function – xylem transports water and dissolved mineral ions upward from roots (one-way, in dead cells); phloem transports dissolved food (sugars) in both directions (translocation, in living cells).
• Forgetting that stem cells are not only embryonic. Adult stem cells (in bone marrow, for example) and plant meristems are both valid, specification-required examples, and confusing “stem cell” with “embryonic stem cell” loses marks on definition questions.
• Miscalculating surface area to volume ratio by using the wrong units or forgetting the ratio decreases, not increases, as size increases – a frequent error given the mathematical demand (BM2.2i-iv) built into this subtopic.

How to approach it

Learn B2.1’s three transport mechanisms as a comparison table first – what moves, along or against what gradient, and whether energy is needed – because most exam questions test the ability to distinguish them rather than define any one in isolation. For B2.2, practise the surface-area-to-volume-ratio calculation itself (it is an explicit mathematical skill in this subtopic, not just background theory) before moving on to the specific exchange systems, since several exam questions ask you to calculate the ratio for a given shape and then use it to explain a biological consequence. Use the Practical Activity Group (PAG) investigations referenced in this topic – root hair cell examination, mitosis in root tip cells, and potometer investigations of transpiration rate – as genuine revision material, since OCR regularly examines the reliability and limitations of these specific methods rather than only the underlying biology.

Related resources

• Scaling Up revision notes
• Scaling Up practice questions

Official syllabus

OCR, GCSE (9-1) Biology A (Gateway Science) J247 Specification, version 4.0, Topic B2 Scaling Up, https://www.ocr.org.uk/images/234594-specification-accredited-gcse-gateway-science-suite-biology-a-j247.pdf, fetched and verified in full 2026-09-02.