OCR GCSE Biology A: Gateway Science (J247)

B2.1 Supplying the cell

OCR Gateway B2.1 'Supplying the cell' ties together how substances enter and leave cells with the specialised cells that carry them. You need diffusion, osmosis and active transport (the three transport mechanisms) AND the adaptations of the five exam-favourite cells: red blood cells, alveoli, root hair cells, xylem and phloem. Gateway papers reliably ask 'name TWO adaptations and explain how each helps the function'. So we learn the mechanism (high-to-low? against gradient?) and then immediately tie it to the cell built to do that job — that pairing is what distinguishes a top-band OCR answer from a mid-band one.

From the J247 specification

Cells require a constant supply of substances (oxygen, glucose, water, mineral ions) and need to remove waste products. Substances move into and out of cells by diffusion (down concentration gradient, passive), osmosis (water across partially permeable membrane), and active transport (against concentration gradient, requires energy). Specialised cells have adaptations matched to their transport role: red blood cells (biconcave + no nucleus + haemoglobin); alveoli (large surface area + one-cell-thick walls + good blood supply); root hair cells (long extension + many mitochondria for active transport); xylem (dead hollow lignified tubes for water); phloem (living sieve tubes + companion cells for sucrose). Surface area to volume ratio falls as size increases — large organisms need transport systems.

Why this matters

Every cell is a partially-permeable bag that needs a steady supply of oxygen, glucose, water and mineral ions and a way to remove CO₂ and urea. Small uncharged molecules (oxygen, CO₂, water) cross the membrane unaided, driven by concentration differences. Larger or charged molecules (glucose, ions, amino acids) cross via specific transport proteins, and when the cell needs to move them against the natural flow it spends ATP from respiration. The OCR Gateway spin on this is to keep returning to a single question: which cell does this job, and how is it shaped to do it well? Once you can pair each transport process with its specialised cell — diffusion with the alveolus, osmosis with the root hair, active transport with the gut lining or root hair, mass flow with xylem and phloem — every B2.1 question becomes the same template.

How to learn this topic

Build on what you already know

B1.1: cell membrane controls what enters and leaves; mitochondria release energy by respiration.
KS3: particles in liquids and gases move randomly; substances spread from where they are concentrated to where they are not.
B2.2 (later): once substances are inside, transport systems (blood, xylem, phloem) move them around the body.

Start with diffusion — random movement of particles, net flow high to low. Anchor on the alveolus.
Then osmosis — special case of diffusion, water only, partially permeable membrane. Anchor on the root hair cell.
Then active transport — against the gradient, needs ATP. Anchor on the root hair cell (mineral ions) and gut lining (glucose).
Compare all three side by side so you never confuse them.
Then surface area to volume ratio — why bigger organisms need transport systems.
Finally the OCR cell-adaptation roll-call: red blood cell, alveolus, root hair cell, xylem, phloem — name each adaptation AND explain how it helps.

Key terms

diffusion: The net movement of particles from an area of higher concentration to an area of lower concentration. A passive process — no energy required. (Always include 'net movement' and 'higher to lower concentration'. Stating just 'particles move' loses the mark.)
osmosis: The movement of water across a partially permeable membrane from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration). (Three required elements: 'water', 'partially permeable membrane', 'dilute → concentrated'. Missing any one usually loses the mark.)
active transport: The movement of a substance against a concentration gradient (from low to high) using energy from respiration (ATP) and a carrier protein. (Examiners want THREE features: 'against the gradient', 'requires energy from respiration', and (usually) 'via a carrier protein'.)
concentration gradient: A difference in concentration of a substance between two regions — the driver of diffusion and (in reverse) of osmosis. (If asked 'in which direction does diffusion happen', the marking phrase is 'down the concentration gradient' (high to low).)
partially permeable membrane: A membrane that lets some substances through but not others — typically lets small molecules (like water) pass but blocks larger or charged solutes. Cell membranes are partially permeable.
turgid: The firm state of a plant cell when full of water — the cell membrane is pushed against the cell wall, creating turgor pressure that keeps the plant upright. ('Turgid' is the marking phrase for a healthy, water-filled plant cell. Don't write 'inflated' or 'puffed up'.)
plasmolysis: The state of a plant cell after losing water by osmosis — the cytoplasm shrinks away from the cell wall and the plant wilts. (Distinguish 'plasmolysed' (plant cell, has wall) from 'crenated' (animal cell, shrivelled).)
lysis: Bursting of an animal cell that has taken up too much water by osmosis. The cell membrane cannot withstand the pressure (no cell wall to support it). (Animal cells lyse in pure water. Plant cells don't — the cell wall prevents bursting.)
exchange surface: A region of an organism specially adapted for transferring substances between inside and outside (or between body compartments). Examples: alveoli, intestinal villi, root hairs, gills. (Three standard adaptations: large surface area, thin membrane, maintained concentration gradient (e.g. by blood supply).)
surface area to volume ratio (SA:V): The ratio of an object's surface area to its volume. Small objects have a high SA:V; large objects have a low SA:V. Determines whether diffusion alone is enough. (As size increases, volume grows proportionally faster than surface area, so SA:V decreases — that's the marking-phrase reason large organisms need transport systems.)
red blood cell: A specialised animal cell that carries oxygen from the lungs to body tissues. Biconcave disc shape, no nucleus, packed with haemoglobin. (OCR marking phrase: 'biconcave shape/disc | increases surface area | no nucleus | more space for haemoglobin'.)
haemoglobin: The iron-containing protein in red blood cells that binds oxygen reversibly — loading at the alveoli, releasing it at respiring tissues.
alveolus: A tiny air sac in the lung where gas exchange happens. Walls are one cell thick, total surface area is huge, and capillaries deliver and remove blood right next to the wall. (OCR marking phrase: 'large surface area for gaseous exchange | thin walls shorten diffusion distance | rich blood supply maintains a steep concentration gradient'.)
root hair cell: A specialised plant cell at the root tip with a long thin extension that increases surface area for absorbing water (by osmosis) and mineral ions (by active transport). (OCR marking phrase: 'large surface area | increases rate of absorption | water enters by osmosis | minerals absorbed by active transport'.)
xylem: A plant tissue made of dead, hollow, lignified cells joined end to end, forming continuous tubes that carry water and dissolved mineral ions from roots to leaves. (OCR marking phrase: 'hollow tubes | lignin provides strength and support | continuous columns/tubes | no energy required for transport'.)
phloem: A plant tissue made of living sieve tubes connected end-to-end, with companion cells alongside, that carries dissolved sucrose from photosynthesising leaves (source) to growing or storage tissues (sink). (OCR marking phrase: 'sieve tubes/cells have no nucleus | connected via cytoplasm | companion cells provide energy | sucrose moves from source to sink'.)
translocation: The transport of dissolved sucrose (and other organic substances) through phloem from source (leaves) to sink (growing tissues, roots, fruits).

Notes

Diffusion — high to low, no energy needed

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. Particles are moving randomly all the time — but if there are more of them on one side than the other, the net result is a flow from where there are more to where there are fewer.

Key features:

Happens to particles in a gas OR particles dissolved in a liquid.
Driven by the concentration gradient.
No energy from respiration needed — it is a passive process.
Continues until concentrations are equal (equilibrium).

Factors that increase rate: steeper concentration gradient, larger surface area, shorter diffusion distance, higher temperature.

Exam examples: oxygen diffuses from alveolar air into the blood; carbon dioxide diffuses the other way; digested glucose diffuses from the small intestine into the blood.

Osmosis — diffusion of water, with one rule

Osmosis is the movement of water molecules across a partially permeable membrane, from a dilute solution (high water concentration) to a more concentrated solution (lower water concentration). The membrane lets water through but not the dissolved solutes.

Three scenarios:

Animal cell in pure water — water rushes in, the cell swells and bursts (lysis). Red blood cells famously do this — they have no cell wall to stop them.
Animal cell in concentrated solution — water rushes out, the cell shrivels (crenation).
Plant cell in pure water — water rushes in, the cell wall stops the cell bursting, the cell becomes firm (turgid).
Plant cell in concentrated solution — water rushes out, cytoplasm shrinks from the wall (plasmolysis), the plant wilts.

Osmosis needs no energy from respiration.

Active transport — against the gradient, with energy

When a cell needs to move substances AGAINST their concentration gradient — from low to high — diffusion can't do it. The cell uses active transport:

Specific carrier proteins in the membrane bind the substance.
Energy from respiration (ATP, made by mitochondria) powers the carrier.
The substance is moved against the concentration gradient.

OCR-favourite examples:

Mineral ions (nitrates, phosphates) absorbed from soil into root hair cells — soil concentration is low, root hair concentration is high. Root hair cells have lots of mitochondria to make ATP for this.
Glucose absorption in the small intestine when blood glucose is already higher than gut glucose — active transport ensures every last molecule is taken up.

Comparing the three — the exam table

|---|---|---|---|

| Energy from respiration? | no | no | yes (ATP) |

The single most important exam question to ask: 'Is the substance moving high→low or low→high?' If high→low, it's diffusion (or osmosis if water). If low→high, it must be active transport.

Surface area to volume ratio — why size matters

A single bacterium has a huge surface area compared to its volume — every part of the cell is close to the outside, so diffusion alone supplies it. As an organism gets bigger, volume grows proportionally faster than surface area, and the SA:V ratio falls. Worked example: a 1 cm cube has SA = 6 cm², V = 1 cm³ → SA:V = 6:1. A 2 cm cube has SA = 24 cm², V = 8 cm³ → SA:V = 3:1.

So large organisms need specialised exchange surfaces (alveoli, villi, root hairs) AND transport systems (blood, xylem, phloem) — exactly the OCR theme.

Cell adaptations for transport — the OCR roll-call

This is the section OCR examiners reward most. For each cell, name the adaptation AND say how it helps.

Red blood cells carry oxygen from lungs to body. Adaptations: biconcave shape/disc — increases surface area for oxygen to diffuse across; no nucleus — more space for haemoglobin (the protein that binds oxygen); small and flexible — squeeze through narrow capillaries. Marking phrase: 'biconcave shape/disc | increases surface area | no nucleus | more space for haemoglobin'.

Alveoli (singular alveolus) are the air sacs of the lungs where gas exchange happens. Adaptations: large surface area for gaseous exchange — millions of tiny sacs; greater surface area means more diffusion can occur; thin walls (one cell thick) shorten diffusion distance; rich blood supply maintains a steep concentration gradient by carrying oxygenated blood away and bringing deoxygenated blood in.

Root hair cells absorb water and mineral ions from soil. Adaptations: long thin extension gives large surface area that increases rate of absorption; water enters by osmosis (soil is dilute, cell cytoplasm is concentrated); minerals absorbed by active transport — the cell already has more ions than the soil, so many mitochondria make ATP to power carrier proteins.

Xylem carries water and dissolved minerals from roots to leaves. Adaptations: hollow tubes with no end walls and no cytoplasm — water flows freely; lignin provides strength and support in the rings/spirals around the tube walls; continuous columns/tubes run from root to leaf; no energy required for transport — water is pulled up passively by the transpiration stream.

Phloem carries dissolved sucrose from photosynthesising leaves to growing or storage tissues (translocation). Adaptations: sieve tubes/cells have no nucleus and very little cytoplasm so the lumen is clear; connected via cytoplasm through sieve plates between cells; companion cells provide energy (ATP) for active loading of sucrose; sucrose moves from source to sink — leaves are the source, roots/fruits/growing tips are the sinks.

How exchange surfaces are adapted — the universal trio

Every exchange surface in biology uses at least two of:

Large surface area — folds, villi, alveoli, root hairs.
Thin membrane / short diffusion path — alveolar walls are one cell thick.
Maintained concentration gradient — good blood supply removes substances, breathing brings fresh air in.

If the question says 'explain two adaptations', pick the two most obviously relevant — and always finish the sentence with how that adaptation speeds up the transport process.

Exam tips

Always state the direction explicitly: diffusion is 'from a higher concentration to a lower concentration'. The full phrase is safer than just 'down the gradient'.
Osmosis questions: name the membrane as 'partially permeable' and the movement as 'water from dilute solution to more concentrated solution'. All three elements are required.
Active transport questions: name ALL three features — against gradient + needs energy from respiration + carrier protein.
Root hair cell questions: marking chain is 'many mitochondria → make ATP → for active transport of mineral ions against the gradient'. Skipping any step loses marks.
Animal cell in pure water → BURSTS (lysis). Plant cell in pure water → TURGID (firm). Don't muddle them.
For surface area to volume ratio, the marking phrase is 'volume increases proportionally faster than surface area' — that's the reason, not 'small things have more surface area'.
OCR cell-adaptation questions: never just NAME the adaptation. Always finish with 'which means…' — e.g. 'biconcave shape, which increases the surface area for oxygen diffusion'.
Xylem vs phloem: remember the four contrasts — dead vs living, water vs sucrose, one-way vs both-ways, passive vs active. State two contrasts for two marks.

Mark-scheme phrasing

Common misconceptions

Worked example

Question:

Answer:

Frequently asked questions

What is the difference between diffusion and osmosis?

Diffusion is the net movement of particles (gas or dissolved) from high to low concentration. Osmosis is specifically the movement of WATER molecules across a PARTIALLY PERMEABLE MEMBRANE from a dilute solution to a more concentrated one. Both are passive (no energy needed). Think of osmosis as a special case of diffusion that only applies to water and only when there's a membrane in the way.

Why does active transport need energy when diffusion does not?

Diffusion just lets particles wander randomly down a concentration gradient — the energy comes from the particles' own kinetic energy. Active transport works AGAINST the gradient: it pumps particles from low to high concentration, which is the opposite of what they would do naturally. Pushing something the 'wrong' way requires work, which costs energy. The cell pays that cost using ATP made by respiration in mitochondria — which is why cells doing lots of active transport (root hair cells, gut lining cells, kidney cells) are packed with mitochondria.

Why doesn't a plant cell burst in pure water like an animal cell does?

Both cells take up water by osmosis. The difference is the cell wall. Animal cells have only a flexible cell membrane, so when too much water enters, the membrane stretches and eventually ruptures (lysis). Plant cells have a tough cellulose cell wall outside the membrane. As water enters, the cell becomes turgid — the membrane presses against the cell wall, but the wall is rigid enough to hold its shape. This turgor pressure is exactly what keeps non-woody plants upright. Lose the water and the cells become flaccid → the plant wilts.

How is a red blood cell adapted for its function?

Three adaptations: (1) Biconcave disc shape — increases the surface area of the cell for oxygen to diffuse in and out across the membrane. (2) No nucleus — more space inside the cell for haemoglobin, so each cell can carry more oxygen. (3) Small and flexible — can squeeze through narrow capillaries to deliver oxygen to every tissue. The marking chain examiners want is 'no nucleus → more space → more haemoglobin → more oxygen carried'.

How is a root hair cell adapted for its function?

Three adaptations: (1) Long thin extension — increases the surface area in contact with the soil for water and mineral absorption. (2) Many mitochondria — provide ATP for active transport of mineral ions against the concentration gradient (the soil has fewer ions than the cell). (3) Thin cell wall — speeds up water uptake. The exam-marking chain is 'extension → surface area → faster absorption' and 'mitochondria → ATP → active transport → minerals taken up against gradient'.

What is the difference between xylem and phloem?

Xylem is made of DEAD, hollow, lignified cells stacked end-to-end with no end walls — water and dissolved minerals flow up from roots to leaves, passively, no energy needed. Phloem is made of LIVING sieve tube cells (with very little cytoplasm and no nucleus) connected through perforated sieve plates, with companion cells alongside that supply ATP for active loading. Phloem carries dissolved sucrose from source (leaves) to sink (growing tissues, fruits, roots) — and the direction can change with the seasons. Four key contrasts: dead vs living, water vs sucrose, one-way vs source-to-sink, passive vs active.

How does an alveolus achieve such efficient gas exchange?

Three classic adaptations. (1) Large surface area: each lung has hundreds of millions of alveoli, giving a total surface area of around 70 m². More surface area means more places for diffusion to happen simultaneously. (2) Thin walls (one cell thick): a very short diffusion path between alveolar air and the blood, so oxygen reaches haemoglobin almost instantly. (3) Rich blood supply (dense capillary network around every alveolus): blood arriving is deoxygenated and quickly carried away once oxygenated, which maintains a steep concentration gradient for oxygen to diffuse down. All three combine to make gas exchange as fast as the body needs.

Why does an organism's size affect how it exchanges gases?

Tiny organisms like amoebae or single-celled bacteria have a high surface area to volume ratio — their surface is large relative to the small amount of cytoplasm inside, so diffusion across the surface is fast enough to supply every part of the cell. As organisms get bigger, volume grows proportionally faster than surface area — so the SA:V ratio decreases. Eventually inner cells are too far from the surface for diffusion alone to deliver oxygen quickly enough. That's why bigger organisms have specialised exchange surfaces (lungs, gills) AND a transport system (blood) to carry substances over longer distances.