Sub-cellular Structures and Specialised Cells

Last lesson you learned to sort cells into eukaryotic and prokaryotic families and to recognise the structures inside them. Now we go closer. First you will learn the job that each sub-cellular structure (organelle) does, so that you can explain why a cell needs it rather than just naming it. Then you will see how cells become specialised — taking on a particular shape and set of organelles so they can do one job extremely well. This is all part of Topic B1 of OCR Gateway Combined Science, and it links straight into the later topics on transport, respiration and the whole organism.

By the end of this lesson you should be able to state the function of the nucleus, cytoplasm, cell membrane, mitochondria, ribosomes, chloroplasts, cell wall and vacuole, and explain how named specialised cells — such as sperm, nerve, red blood, root hair and palisade cells — are adapted to their functions.

This lesson builds AO1 (knowledge of each organelle's function) and AO2 (applying it to explain how a specialised cell's structure suits its job).

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The Sub-cellular Structures and What They Do

A sub-cellular structure (organelle) is a part inside a cell that has a particular job. A useful way to picture it is as a factory: the cell is the whole factory, and each organelle is a different department.

Structure	Found in	Function
Nucleus	All eukaryotic cells	Contains the genetic material (DNA) as chromosomes; controls the cell's activities and which genes are used
Cytoplasm	All cells	Jelly-like fluid where most chemical reactions of the cell happen; holds the organelles
Cell membrane	All cells	Controls what enters and leaves the cell; it is partially permeable
Mitochondria	Most eukaryotic cells	Site of aerobic respiration, transferring energy from glucose
Ribosomes	All cells	Site of protein synthesis (making proteins from amino acids)
Chloroplasts	Plant and algal cells	Contain chlorophyll; site of photosynthesis
Cell wall	Plant, algal, fungal, bacterial cells	Strengthens and supports the cell (plant walls = cellulose)
Permanent vacuole	Plant cells	Filled with cell sap; keeps the cell firm (turgid) and stores substances

A closer look at each structure

Nucleus. This is the control centre of the cell. Its DNA carries the instructions (genes) for making every protein the cell needs. A nuclear membrane surrounds the nucleus and keeps the DNA separate from the rest of the cell — and it is exactly this feature that makes a cell eukaryotic.

Cytoplasm. The cytoplasm is not just empty filler. It is a watery gel packed with dissolved substances and enzymes, and most of the cell's reactions (including the first stage of respiration) take place in it.

Cell membrane. A thin, flexible layer that forms the edge of the cell. It is partially permeable, which means it lets some substances through — such as oxygen, water and glucose — while controlling or blocking others. In Topic B2 you will study how diffusion, osmosis and active transport move substances across it.

Mitochondria. These are often called the cell's "powerhouses", but you should never say they "make energy". They are the site of aerobic respiration, the process that transfers energy stored in glucose into a form the cell can use. Cells that need a lot of energy — muscle cells and sperm cells, for example — contain many mitochondria.

Ribosomes. Tiny structures, far too small to see with a light microscope. They are where protein synthesis happens: amino acids are joined together in the order set by the cell's DNA to build proteins such as enzymes.

Chloroplasts. Found only in the green parts of plants and in algae. They contain the green pigment chlorophyll, which absorbs the light energy needed for photosynthesis — the process that makes glucose. A leaf palisade cell is packed with them.

Cell wall. A rigid outer layer that sits outside the cell membrane. In plants and algae it is made of cellulose; it gives the cell strength and stops it bursting when it takes in water. (Fungi have walls of chitin; bacteria have walls of a different material again.)

Permanent vacuole. A large fluid-filled sac in the middle of a plant cell, containing cell sap (a solution of sugars and salts). When it is full it presses outwards on the cell wall and keeps the cell turgid (firm) — which is what holds non-woody plants upright.

Exam Tip: Match the number of an organelle to what the cell needs. If a cell "has many mitochondria", that tells you it needs a lot of energy; if it "has many chloroplasts", it does a lot of photosynthesis. Marks come from linking the structure to its function, not simply from naming the part.

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From General Cell to Specialised Cell

As an organism grows, its cells differentiate — they become specialised for a particular job by developing a specific shape and the right mix of organelles. Which job a cell takes on is controlled by which of its genes are switched on. The diagram below shows the idea.

flowchart TD
  A["Unspecialised cell<br/>(same genome)"] --> B["Differentiation<br/>(specific genes switched on)"]
  B --> C["Nerve cell<br/>long, with branches"]
  B --> D["Red blood cell<br/>biconcave, no nucleus"]
  B --> E["Sperm cell<br/>tail + many mitochondria"]
  B --> F["Root hair cell<br/>long extension"]
  B --> G["Palisade cell<br/>packed with chloroplasts"]

The key idea here is that every cell in an organism contains the same DNA, but a specialised cell switches on only the genes it actually needs. That is why a nerve cell and a red blood cell can look so completely different even though they share the same genome.

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Specialised Animal Cells

Cell	Adaptation	How it helps the function
Sperm cell	Long tail (flagellum)	Allows it to swim to the egg
	Many mitochondria in the mid-piece	Release energy for movement
	Acrosome (enzyme cap)	Digests the egg membrane so the sperm can enter
	Streamlined head	Reduces resistance while swimming
Nerve cell (neurone)	Very long	Carries electrical impulses over large distances
	Branched connections (dendrites)	Connect to many other cells
	Myelin sheath	Insulates and speeds up the impulse
Red blood cell	Biconcave disc shape	Large surface area for absorbing oxygen
	No nucleus	More room for haemoglobin to carry oxygen
	Packed with haemoglobin	Binds and transports oxygen
Muscle cell	Many mitochondria	Provide energy for contraction
	Contains protein fibres	Can shorten (contract) to produce movement

Exam Tip: The red blood cell comes up again and again. Two adaptations score the most marks: the biconcave disc shape (large surface area for gas exchange) and the absence of a nucleus (more space for haemoglobin). A frequent misconception is that all animal cells have a nucleus — the mature red blood cell is one of the rare exceptions, and it loses its nucleus precisely so that it can hold more haemoglobin.

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Specialised Plant Cells

Cell	Adaptation	How it helps the function
Root hair cell	Long hair-like extension	Large surface area to absorb water and mineral ions
	Thin cell wall	Short diffusion distance for water uptake
	Many mitochondria	Energy for active transport of mineral ions
Palisade mesophyll cell	Packed with chloroplasts	Maximises light absorption for photosynthesis
	Tall, column shape near the leaf surface	Catches the most light
Xylem cell	Hollow tubes, no end walls, no living contents	Allow water and minerals to flow up the plant; provide support (strengthened with lignin)
Phloem cell	Sieve plates between cells; few organelles	Allow sugars (dissolved food) to flow up and down the plant

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Worked Example — Reading Adaptations from a Description