2.2: Cell structures

LEARNING GOALS

By the end of this lesson

Identify the major structures in typical animal and plant cells.
Interpret cell diagrams without assuming that shape, colour or position is fixed.
Distinguish cell structures from cells and tissues.

BIG QUESTION

How can a flat diagram represent a living cell that is three-dimensional, dynamic and mostly colourless?

Cell diagrams simplify shape and colour so structures can be identified, but the simplification must not be mistaken for the living cell itself.

A cell diagram is a model

Living cells are three-dimensional and their organelles move within the cytoplasm. Textbook diagrams use clear outlines, artificial colours and separated labels to make structures visible. The colours do not usually show the natural colour of the structures.

A section through a cell may miss an organelle or cut it in an unfamiliar way. Identification should therefore use several clues: boundary, relative position, shape, number and the type of cell shown.

Structures shared by typical animal and plant cells

Both typical animal and plant cells contain a cell membrane, cytoplasm, nucleus, mitochondria and ribosomes. These are eukaryotic structures, although not every specialised cell retains every structure—for example, a mature mammalian red blood cell has no nucleus.

The cell membrane forms the cell boundary. Cytoplasm fills much of the interior. The nucleus is usually one of the largest organelles, mitochondria are smaller and numerous, and ribosomes are too small to be resolved individually with a school light microscope.

Structures characteristic of many plant cells

A typical plant cell also has a outside its membrane and a large containing cell sap. Photosynthetic plant cells contain .

Do not turn “typical” into “all.” Root hair cells are plant cells but usually lack chloroplasts because roots receive little light. Some plant cells lose their living contents at maturity, as happens in xylem vessels.

Reading the two boundaries of a plant cell

The plant is the thicker outer boundary. The lies just inside it and controls exchange with the surroundings. They are separate structures with different properties.

The vacuole can occupy most of a mature plant cell, pushing the cytoplasm and nucleus towards the edge. A diagram with a large central space is therefore not showing an “empty” cell.

Labelled diagrams of a typical animal cell and a photosynthetic plant cell, showing their shared structures and the cell wall, chloroplasts and permanent vacuole found in the plant cell. — Typical animal and photosynthetic plant cells. Positions and colours are diagrammatic; cells vary in shape and organelle number.

Use position and boundaries as identification clues

No single clue should be used in isolation.

Visual clues in simplified cell diagrams
Structure	Useful visual clue	Caution
Cell membrane	Thin boundary around the cytoplasm.	In a plant cell it lies just inside the cell wall.
Nucleus	Often a large rounded organelle.	Position varies and some specialised cells lack one.
Mitochondrion	Small oval or bean-shaped organelle.	Usually many are present; not individually visible at low resolution.
Ribosome	Shown as tiny dots in diagrams.	Too small to resolve with a school light microscope.
Cell wall	Rigid outer boundary of a plant cell.	It is outside, and separate from, the membrane.
Permanent vacuole	Large fluid-filled compartment.	Its membrane is the tonoplast, usually beyond this specification.
Chloroplast	Oval organelle, often drawn green.	Found in photosynthetic cells, not every plant cell.

KEY TERMS

Language that earns precision

cell membrane: The selectively permeable boundary surrounding the cell contents.
cytoplasm: The cell material where many enzyme-controlled reactions occur.
nucleus: An organelle containing most of a eukaryotic cell’s genetic material.
mitochondrion: An organelle where aerobic respiration releases energy.
ribosome: A small structure where proteins are synthesised.
cell wall: A freely permeable cellulose layer that supports a plant cell.
permanent vacuole: A membrane-bound compartment containing cell sap in a mature plant cell.
chloroplast: An organelle containing chlorophyll where photosynthesis occurs.

Common misconceptions

Claim: The cell wall replaces the cell membrane in plants.

Correction: Plant cells have both: the membrane controls exchange and the wall provides support.

Claim: All plant cells contain chloroplasts.

Correction: Only photosynthetic plant cells require chloroplasts; most root cells do not contain them.

Claim: Cells and organelles naturally have textbook colours.

Correction: Most are colourless; stains and false colour increase contrast.

Claim: The vacuole is an empty gap.

Correction: It is a membrane-bound compartment filled with cell sap.

EXAM LANGUAGE

Match the answer to the command word

Identify

Give the structure’s name; use boundaries and position together rather than colour alone.

Label

Use a straight line that ends on the precise structure and does not cross another label line.

Describe

State visible features only; save functions for an explain question.

CHECK YOUR UNDERSTANDING

Recall, apply, then justify

It clearly communicates the main structures and their relative locations.
A single section cannot show the whole three-dimensional cell.
Plant cells differ by tissue and function, so not all contain chloroplasts or a large vacuole.
Colours, sizes, organelle numbers and positions are simplified.
A conclusion should state that the diagram is useful as a typical model but not a complete record of every plant cell.

END OF THE LESSON

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