OCR GCSE Biology A: Transport across membranes, mitosis and stem cells (J247)

This guide covers statements B2.1a to B2.1f (“Supplying the cell”) of the OCR GCSE (9-1) Biology A Gateway Science specification (J247), Version 4.0, for first assessment in 2018. None of these statements is printed in bold, so none is marked as Higher Tier only. Work through it with the revision notes and the practice questions. Course links: the OCR GCSE Biology hub, the printable checklist, and the free diagnostics. For the wider topic, see Scaling Up and Cell Level Systems.

What this unit covers

Statement	You must be able to
B2.1a	Explain how cells take in and lose substances through diffusion, osmosis and active transport, giving examples, direction of movement, concentration gradients and the term water potential
B2.1b	Describe mitosis in growth, including the cell cycle
B2.1c	Explain why cell differentiation matters, with examples of specialised cells
B2.1d	Recall where stem cells are found: embryonic and adult animals, and meristems in plants
B2.1e	Describe what stem cells and meristems do
B2.1f	Describe the difference between embryonic and adult stem cells in animals

The specification also lists a mathematical outcome: use percentiles and calculate percentage gain and loss of mass. Both appear below.

Moving substances across membranes (B2.1a)

A cell surface membrane is partially permeable. Small molecules such as oxygen and carbon dioxide pass through easily. Larger molecules and charged ions need help or energy. There are three routes.

Diffusion

Diffusion is the net movement of particles from where they are more concentrated to where they are less concentrated, down a concentration gradient. The particles move randomly, but more move down the gradient than up it. The cell does not supply energy, so diffusion is passive.

Examples to know:

• Oxygen crosses from the lung air sacs into the blood, and carbon dioxide crosses the other way.
• Glucose moves from the gut into the blood while the gut concentration is higher.
• Urea moves from body cells into the blood and is carried away.

A steeper gradient, a higher temperature and a larger surface all speed diffusion up.

Osmosis

Osmosis is the net movement of water across a partially permeable membrane from higher water potential to lower water potential. Water potential is a measure of how freely water can move. Pure water has the highest water potential. Adding dissolved solute lowers it. A dilute solution therefore has a higher water potential than a concentrated one, and water moves from dilute to concentrated. You do not need to use water potential in calculations.

Osmosis is passive. It only moves water. Solute particles are too large to follow.

• An animal cell in a solution more dilute than its cytoplasm gains water, swells and may burst.
• An animal cell in a more concentrated solution loses water and shrinks.
• A plant cell in dilute solution gains water and becomes firm (turgid) because the cell wall resists bursting. In a strongly concentrated solution it loses water and the membrane pulls away from the wall.

Active transport

Active transport moves substances up a concentration gradient, from where they are scarcer to where they are more plentiful. It needs energy from respiration, so it needs oxygen and glucose, and cells doing a lot of it have many mitochondria. Carrier proteins in the membrane do the moving.

Examples: root hair cells take up mineral ions from soil water when the soil concentration is lower than the concentration inside the cell. Cells in the gut lining can take up glucose when the gut concentration has become lower than the blood concentration.

Feature	Diffusion	Osmosis	Active transport
What moves	Any small particles	Water only	Ions and molecules
Direction	High to low concentration	High to low water potential	Low to high concentration
Energy from respiration	No	No	Yes
Needs a membrane	No	Yes, partially permeable	Yes, with carrier proteins

Worked example 1: swede cylinders in sucrose

Four swede cylinders, each of mass 3.00 g, were left in sucrose solutions. Final masses were measured after blotting.

Sucrose (mol/dm³)	Final mass (g)
0.0	3.36
0.2	3.15
0.4	2.94
0.6	2.70

Percentage change = (final mass − initial mass) ÷ initial mass × 100.

• 0.0: (3.36 − 3.00) ÷ 3.00 × 100 = +12.0%
• 0.2: (3.15 − 3.00) ÷ 3.00 × 100 = +5.0%
• 0.4: (2.94 − 3.00) ÷ 3.00 × 100 = −2.0%
• 0.6: (2.70 − 3.00) ÷ 3.00 × 100 = −10.0%

The change passes through zero between 0.2 and 0.4. Over that interval the change falls by 7.0 percentage points, and 5.0 of those are needed to reach zero. So the zero is about 0.2 + 0.2 × (5.0 ÷ 7.0) = 0.34 mol/dm³. At that concentration the solution and the cell contents have about the same water potential, so there is no net movement of water.

Explaining the 0.6 result: the solution had a lower water potential than the cell contents, so water moved out of the cells by osmosis, down the water potential gradient, and the mass fell.

Worked example 2: is it active transport?

A root hair cell takes up nitrate. Uptake is 5.0 units per hour in air. When the roots are placed in water bubbled with nitrogen instead of air, uptake falls to 1.0 unit per hour.

Percentage change = (1.0 − 5.0) ÷ 5.0 × 100 = −80.0%.

Conclusion: uptake depends on oxygen, so it depends on aerobic respiration releasing energy. That points to active transport, not diffusion. The remaining 1.0 unit could be diffusion, or respiration in the cells continuing without oxygen for a time.

Mitosis and the cell cycle (B2.1b)

Mitosis is the division of a cell nucleus that produces two nuclei with the same genetic information as the parent. In animals and plants it makes new cells for growth. It is one part of the cell cycle.

The cell cycle

OCR lists these stages:

1. Cell growth. The cell grows and increases the number of subcellular structures such as ribosomes and mitochondria.
2. DNA replication. Each chromosome is copied. Each now consists of two identical chromatids joined at one point.
3. More cell growth. The cell grows again and gets ready to divide.
4. Movement of chromosomes (mitosis). The chromosomes are pulled apart.

Finally the cytoplasm and cell membrane divide, giving two daughter cells.

What happens during mitosis

• The chromosomes shorten and thicken and line up across the centre of the cell.
• Spindle fibres pull the two chromatids of each chromosome to opposite ends.
• A new nucleus forms around each set, and the cell splits.

Each daughter cell has the same number of chromosomes as the parent and is genetically identical to it. A human body cell has 46 chromosomes, so each daughter cell has 46.

Worked example 3: counting chromosomes and DNA

A cell has 12 chromosomes and DNA content 8 units.

• After replication it still has 12 chromosomes, but each is two chromatids, so 24 chromatids. DNA content is 16 units.
• After mitosis and division each daughter cell has 12 chromosomes and 8 units of DNA.

Worked example 4: how many cells?

One cell divides every 8 hours. In 48 hours there are 48 ÷ 8 = 6 divisions. Cell number doubles each time, so 2⁶ = 64 cells.

Worked example 5: root tip slide

In a stained root tip, a student counts 250 cells in the field of view. 35 are in mitosis.

Percentage in mitosis = 35 ÷ 250 × 100 = 14.0%.

This is high because cells near the tip are dividing for growth. A section further from the tip would show a lower percentage.

Differentiation (B2.1c)

Differentiation is the process by which a cell becomes specialised for a particular job. It changes in structure so that it can do that job well. Specialisation lets an organism become more efficient, because different cells, tissues and organs can share the work instead of every cell doing everything.

Cell	Adaptation	Function
Sperm cell	Tail, many mitochondria, enzymes in the head	Swim to, and enter, an egg
Red blood cell	No nucleus, biconcave shape, haemoglobin	Carry oxygen
Root hair cell	Long projection, large surface area	Absorb water and mineral ions
Palisade cell	Many chloroplasts	Photosynthesis
Muscle cell	Many mitochondria, contractile fibres	Contract
Nerve cell	Long, with branches	Carry electrical impulses

Examples to be able to explain for each: structure first, then how it helps the function.

Stem cells (B2.1d to B2.1f)

A stem cell is an undifferentiated cell that can divide by mitosis and differentiate into one or more types of specialised cell.

Where they are found (B2.1d)

• Embryonic stem cells in animal embryos.
• Adult stem cells in animals, for example in bone marrow.
• Meristems in plants. These are regions at the tips of roots and shoots where cells divide.

What they do (B2.1e)

Stem cells divide to produce a range of cell types for development, growth and repair. Embryonic stem cells make every kind of cell in a developing embryo. Adult stem cells replace damaged and worn cells; bone marrow stem cells keep making blood cells. Meristem cells divide throughout the life of a plant. Their daughter cells elongate and differentiate into xylem, phloem and other tissues, which is how roots and shoots lengthen. A cutting or a cauliflower fragment can regrow into a whole plant because meristem cells can form every plant cell type.

Embryonic compared with adult (B2.1f)

Feature	Embryonic stem cell	Adult stem cell
Source	Early embryo	Tissues such as bone marrow
Range of cells it can become	Any cell type in the body	Limited range, often only cells of its own tissue
Role	Forming the embryo and its growth	Repair and replacement during life

Percentage gain and percentiles

Worked example 6: percentage gain

A child’s mass rises from 12.5 kg to 14.0 kg. Gain = 1.5 kg. Percentage gain = 1.5 ÷ 12.5 × 100 = 12.0%.

Worked example 7: percentile

A percentile shows the proportion of a group that falls below a value. In a sample of 20 seedlings, 15 are shorter than yours. Percentile = 15 ÷ 20 × 100 = 75th percentile. Growth charts use the same idea: a child on the 75th percentile for mass is heavier than 75% of children of that age.

Common errors

• Saying osmosis is the movement of “water from low concentration to high”. Use water potential, or say from a dilute to a more concentrated solution.
• Writing only that active transport “uses energy”, with no mention of respiration as the source.
• Forgetting the new cell growth stage before and after DNA replication.
• Writing that mitosis produces cells “similar” to the parent. They are genetically identical.
• Saying stem cells are all embryonic. Adult stem cells and plant meristems count.
• Using the final mass as the divisor in a percentage change.

Check your understanding

Try the practice questions, then use the revision notes to consolidate.

Official syllabus

OCR GCSE (9-1) Biology A (Gateway Science), specification J247, Version 4.0 (August 2026), for first assessment in 2018, published by OCR. Statements B2.1a to B2.1f and mathematical outcome BM2.1i.