Relating a feature to the function of the cell
A specialised feature is something about a cell that fits it for a particular job. Finding the features is what the drawing is for; explaining them is what turns a set of drawings into an investigation.
What counts as a specialised feature
Four kinds, and you can see all four down a light microscope.
- The arrangement of cells within a tissue — packed in rows with no gaps, loosely packed with air spaces, or stacked upright side by side.
- The shape of a cell — column-like, brick-like, curved, drawn out into a long thin extension, or having no fixed shape at all.
- The presence or absence of an organelle — chloroplasts present in a mesophyll cell and absent from an onion bulb cell; a nucleus absent from a mature xylem vessel.
- The quantity or distribution of organelles — how many chloroplasts a cell holds, and whereabouts in the cell they sit.
The three parts of the explanation
Every feature is explained in the same three moves, in this order.
- Name the feature, exactly as you observed it. "The cells are packed with chloroplasts."
- State its function — what that structure does. "Chloroplasts absorb light and carry out photosynthesis."
- Give the reason — how or why the feature lets the cell do its job well. "The more chloroplasts a cell holds, the more of the light passing through it is absorbed rather than wasted, so the cell makes glucose faster."
The third move is the one people leave out. Naming a feature and stating a function only describes; the reason is what explains.
Turning a "what" into a "why"
If you are stuck on the third part, the reason almost always runs through one of these.
- Surface area — a larger area means faster exchange across it, because more of the substance can cross at once.
- Rate — more of the machinery means the reaction runs faster, so more product per minute.
- Distance — a shorter path means faster diffusion, because diffusion over a long distance is very slow.
- Position — being in the right place means the cell meets what it needs. Chloroplasts near the lit surface, root hairs in the soil.
- A physical property — a thickened wall resists a force; a flexible pellicle allows a change of shape; a hollow tube offers no resistance to flow.
Worked reasoning for four features
- Palisade cell packed with chloroplasts. Function: photosynthesis. Reason: light passes down through the cell, and each chloroplast in its path absorbs some of it, so a cell with many chloroplasts captures far more of the light arriving than one with few — and the rate of photosynthesis depends on how much light is absorbed.
- Palisade cells column-shaped and upright. Function: photosynthesis. Reason: standing on end lets many cells pack side by side under the same area of leaf surface, so more chloroplasts sit under every square millimetre of lit surface, and light travels down the length of each cell past more chloroplasts than it would crossing a flat cell.
- Xylem vessel hollow, with no cytoplasm or nucleus. Function: carrying water up the plant. Reason: the cell dies and its contents break down, leaving an open tube. Water meets nothing in its way, so it moves up far faster than it could through a column of living cells with membranes and cytoplasm to cross.
- Root hair cell drawn out into a long extension. Function: absorbing water and mineral ions. Reason: the extension adds a great deal of surface area in contact with the soil water, and absorption happens across that surface — so more area means more water and more ions absorbed per minute.
Worked ExampleExplaining a guard cell
Down the microscope you see a stoma in a leaf epidermis, enclosed by two curved guard cells. Unlike the other epidermal cells, the guard cells contain chloroplasts, and the wall on the side nearest the pore looks thicker than the wall on the outer side.
Identify two specialised features of the guard cells, and for each, state its function and give the reason it lets the cell do that job effectively.
Step 1 — Establish what the tissue does
Before explaining any feature, be clear what job it is serving. The stoma is a pore through the epidermis, and the guard cells' job is to open and close it, controlling the exchange of carbon dioxide and oxygen and the loss of water vapour.
Step 2 — Feature one: the unevenly thickened wall
Name it. The wall on the inner side of each guard cell, next to the pore, is thicker than the wall on the outer side.
State its function. The uneven thickening is what makes the pair of cells bend apart when they take in water, opening the pore.
Give the reason. When water enters by osmosis the cell becomes turgid and its wall is pushed outwards from the inside. The thin outer wall stretches easily; the thick inner wall hardly stretches at all.
Because one side lengthens and the other does not, the cell can only curve, bowing away from the pore. Two cells curving away from each other leave a gap between them, which is the open stoma. If the wall were the same thickness all round, the cell would simply swell into a fatter sausage and the pore would not open at all.
Step 3 — Feature two: chloroplasts present
Name it. Guard cells contain chloroplasts; the epidermal cells around them contain none.
State its function. The chloroplasts carry out photosynthesis inside the guard cell.
Give the reason. Opening a stoma is not passive — it needs solutes moved into the guard cell so that water follows by osmosis, and moving them costs energy.
Photosynthesis in the guard cell's own chloroplasts supplies sugars, and respiring those releases the energy for that work. It also links the opening of the stoma to light: the cell photosynthesises when light falls on the leaf, which is exactly when the leaf needs carbon dioxide coming in. So the stomata open by day, when photosynthesis can use the carbon dioxide, and close at night, when opening would lose water vapour for no gain.
Step 4 — Check both explanations went all three moves
Each one names an observed feature, states what it does, and then says why it does it well — the wall thickness by explaining what uneven stretching must produce, the chloroplasts by explaining what the energy is for and what it links opening to.