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

These notes condense statements B2.1a to B2.1f of the OCR GCSE (9-1) Biology A Gateway Science specification (J247), Version 4.0, for first assessment in 2018. No statement here is printed in bold, so nothing is marked Higher Tier only. For full explanations see the study guide. Then test yourself with the practice questions. Other links: OCR GCSE Biology hub, printable checklist, free diagnostics, and Scaling Up.

Key definitions

Term	Meaning
Diffusion	Net movement of particles from more to less concentrated, down a concentration gradient. Passive.
Osmosis	Net movement of water across a partially permeable membrane from higher water potential to lower water potential. Passive.
Active transport	Movement of particles from lower to higher concentration, against the gradient, using energy from respiration.
Water potential	A measure of how freely water can move. Pure water is highest; more solute lowers it.
Mitosis	Nuclear division giving two genetically identical nuclei.
Cell cycle	Growth, DNA replication, more growth, mitosis, then division of the cytoplasm.
Differentiation	A cell becoming specialised for a job.
Stem cell	Undifferentiated cell that can divide and differentiate.
Meristem	Region in a plant, at root and shoot tips, where stem cells divide.

B2.1a Transport across membranes

Comparison

	Diffusion	Osmosis	Active transport
Moves	Small particles	Water	Ions, molecules
Down or against	Down concentration gradient	Down water potential gradient	Against concentration gradient
Energy	None	None	From respiration
Helped by	Steep gradient, warmth, large surface	Same	Carrier proteins, mitochondria, oxygen

Examples to learn

• Diffusion: oxygen into blood at the alveoli, carbon dioxide out; dissolved food into the blood from the gut; urea out of cells.
• Osmosis: water into root hair cells; water into or out of animal and plant cells placed in solutions.
• Active transport: mineral ions into root hair cells; glucose from the gut into the blood when the gut concentration is lower.

Cells in solutions

Solution compared with cell	Animal cell	Plant cell
More dilute (higher water potential)	Gains water, swells, may burst	Gains water, becomes turgid
Same	No net change	No net change
More concentrated (lower water potential)	Loses water, shrinks	Loses water, becomes flaccid, membrane pulls away from the wall

Method in steps: osmosis investigation

1. Cut identical cylinders of plant tissue (same diameter and length).
2. Blot, then measure each initial mass.
3. Place one in each solution of a different sucrose concentration. Include pure water.
4. Keep every chip in its solution for an equal time, at one temperature.
5. Blot and measure final mass.
6. Calculate percentage change for each.
7. Draw a graph of percentage change against concentration. The point where the line meets zero shows no net water movement.

Worked reminder: percentage change

Percentage change = (final − initial) ÷ initial × 100. Chip: 2.50 g to 2.75 g gives +10.0%. Chip: 2.50 g to 2.20 g gives −12.0%. A negative answer is a loss. Always divide by the initial value.

B2.1b Mitosis and the cell cycle

The cycle in order

1. Cell growth. More ribosomes, mitochondria and other subcellular structures.
2. DNA replication. Each chromosome becomes two identical chromatids.
3. More cell growth.
4. Movement of chromosomes (mitosis).
5. Cytoplasm and membrane divide: two daughter cells.

Mitosis in short

• Chromosomes condense and line up across the middle.
• Spindle fibres pull chromatids to opposite poles.
• Two nuclei form; the cell divides.

Numbers to remember

Stage	Chromosomes (parent has 12)	DNA (parent 8 units)
Before replication	12	8
After replication	12 (24 chromatids)	16
Each daughter cell	12	8

• Daughter cells are genetically identical to the parent and to each other.
• Number after n divisions from one cell = 2ⁿ. Five divisions give 32 cells.
• Mitosis happens in growth, and also in repair and replacement of cells.

Worked reminder: percentage in mitosis

200 cells counted, 18 in mitosis: 18 ÷ 200 × 100 = 9.0%. A high percentage means rapid growth in that tissue.

B2.1c Differentiation

• Differentiation produces specialised cells.
• Specialised cells let an organism be more efficient, because different cells do different jobs.

Cell	Key adaptations	Job
Sperm	Tail, mitochondria, enzymes in head	Reach and enter egg
Red blood cell	Biconcave, no nucleus, haemoglobin	Carry oxygen
Root hair cell	Hair-like extension	Take in water and ions
Palisade cell	Many chloroplasts	Photosynthesis
Muscle cell	Mitochondria, contractile fibres	Contract
Nerve cell	Long, branched	Carry impulses

Answer pattern: name the feature, then say how it helps.

B2.1d to B2.1f Stem cells

Where

• Embryonic stem cells: animal embryos.
• Adult stem cells: tissues such as bone marrow.
• Meristems: tips of roots and shoots in plants.

Functions

• Stem cells divide and differentiate to give cell types for development, growth and repair.
• Bone marrow stem cells keep making blood cells.
• Meristem cells divide throughout the life of a plant and differentiate to form tissues such as xylem and phloem.
• A cutting or a cauliflower fragment can grow into a whole plant, because meristem cells can become every plant cell type.

Embryonic compared with adult

	Embryonic	Adult
Where	Early embryo	Adult tissues such as bone marrow
Becomes	Any cell type	Limited range
Main role	Forming the embryo	Repair and replacement

Must-know distinctions

• Diffusion moves any small particle; osmosis moves only water.
• Diffusion and osmosis are passive; active transport uses energy.
• Mitosis is the division of the nucleus; the cell cycle includes the growth stages as well.
• A chromosome is copied into two chromatids; the chromatids then separate and become chromosomes in the daughter cells.
• Differentiation is the change into a specialised cell; a stem cell is one that has not yet differentiated.
• Embryonic stem cells can become any cell type; adult stem cells have a more limited range.

Model explanations

Use these as patterns for longer answers.

Why can a root hair cell take up nitrate when the soil has less nitrate than the cell? Diffusion would move nitrate out of the cell, down the gradient. The cell uses active transport instead, with carrier proteins, against the gradient. The energy comes from respiration, which is why the root hair cell contains many mitochondria and needs oxygen.

Why does a chip lose mass in a strong salt solution? The solution has a lower water potential than the cytoplasm of the cells. Water leaves the cells by osmosis, crossing their partially permeable membranes, and the chip becomes lighter.

Why does a growing root tip contain many cells in mitosis? Cells in the meristem are dividing to make new cells. Each division follows growth and DNA replication, so the tip has a high percentage of cells with visible chromosomes.

Why are stem cells useful for repair? They can divide to make more cells and then differentiate into the types needed to replace damaged ones.

Maths skills

• Percentage gain or loss in mass: (final − initial) ÷ initial × 100.
• Percentile: proportion of a group below a value. If 15 of 20 are lower, the value is at the 75th percentile.
• Use your own working. State units. Give the sign.

Quick self-test

1. Name the process that moves water across a partially permeable membrane.
2. Why does active transport need oxygen?
3. A cell is in a solution with a lower water potential than its cytoplasm. Which way does water move?
4. Put in order: DNA replication, cell growth, movement of chromosomes.
5. A cell has 8 chromosomes. How many does each daughter cell have after mitosis?
6. One cell divides 5 times. How many cells are there?
7. A potato chip goes from 4.00 g to 3.60 g. Calculate the percentage change.
8. Name the plant tissue where stem cells divide.
9. State one way an adult stem cell differs from an embryonic stem cell.
10. Give one reason differentiation helps a large organism.

Answers

1. Osmosis.
2. Oxygen is needed for aerobic respiration, which releases the energy.
3. Out of the cell.
4. Cell growth, DNA replication, (more cell growth), movement of chromosomes.
6. 2⁵ = 32 cells.
7. (3.60 − 4.00) ÷ 4.00 × 100 = −10.0%, a loss of 10.0%.
8. Meristem (at the tips of roots and shoots).
9. Adult stem cells form only a limited range of cell types, whereas embryonic stem cells can form any type.
10. Specialised cells do one job efficiently, so the organism as a whole works better.

Where marks are usually lost

• Writing “from high to low concentration” for osmosis without referring to water or water potential.
• Calling osmosis active, or saying water is “pumped”.
• Not naming respiration as the energy source for active transport.
• Using the final mass as the divisor.
• Leaving out the minus sign for a loss.
• Missing the second growth stage in the cell cycle, or placing replication after mitosis.
• Saying daughter cells are “the same” without genetically identical.
• Describing stem cells only as embryonic, and forgetting adult stem cells and meristems.
• Listing a cell’s features without linking them to its function.

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.