Sub-cellular Structures and Specialised Cells

In the last lesson you sorted cells into eukaryotic and prokaryotic families and recognised the structures inside them. This lesson zooms in. First you will learn the function of each sub-cellular structure (organelle) so you can explain why a cell needs it, not just name it. Then you will see how cells become specialised — taking on a particular shape and set of organelles to do one job extremely well. This is part of Topic B1 of OCR Gateway Science A, and the ideas here connect directly to later topics on transport, respiration and the whole organism.

By the end you should be able to describe 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.

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The Sub-cellular Structures and What They Do

A sub-cellular structure (organelle) is a part inside a cell that carries out a specific job. Think of the cell as a factory and each organelle as a department.

Structure	Found in	Function
Nucleus	All eukaryotic cells	Contains the genetic material (DNA) as chromosomes; controls the cell's activities and gene expression
Cytoplasm	All cells	Jelly-like fluid where most chemical reactions of the cell occur; 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. The control centre. Its DNA carries the instructions (genes) for making every protein the cell needs. The nucleus is surrounded by a nuclear membrane that keeps the DNA separate from the rest of the cell — the very feature that defines a eukaryotic cell.

Cytoplasm. Far from being empty filler, the cytoplasm is a watery gel packed with dissolved substances and enzymes. Most of the cell's metabolic reactions (including the first stages of respiration) take place here.

Cell membrane. A thin, flexible layer that forms the boundary of the cell. It is partially permeable, meaning it lets some substances through (such as oxygen, water and glucose) while controlling or blocking others. You will meet diffusion, osmosis and active transport across this membrane in Topic B2.

Mitochondria. Often described as the cell's "powerhouses", though 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, sperm) contain many mitochondria.

Ribosomes. Tiny structures, far too small to see with a light microscope. They are the site of protein synthesis: amino acids are joined together in the order specified 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 light energy for photosynthesis — the process that makes glucose. A leaf palisade cell is packed with them.

Cell wall. A rigid outer layer outside the cell membrane. In plants and algae it is made of cellulose; it provides strength and stops the cell 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 full it pushes outward on the cell wall, keeping the cell turgid (firm) — this is what keeps non-woody plants upright.

Exam Tip: Match the number of an organelle to a cell's needs. "This cell has many mitochondria" is a clue that it needs a lot of energy; "this cell has many chloroplasts" tells you it does a lot of photosynthesis. Examiners reward linking structure to function, not just naming the part.

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From General Cell to Specialised Cell

When an organism grows, its cells differentiate — they become specialised for a particular job by developing a specific shape and the right mix of organelles. The process is controlled by which genes are switched on. The following diagram 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 is that every cell in an organism contains the same DNA, but specialised cells switch on only the genes they need. That is why a nerve cell and a red blood cell look so different despite sharing a 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 is a favourite. Two adaptations score most marks: the biconcave disc (large surface area for gas exchange) and the absence of a nucleus (more space for haemoglobin). Note it is a rare animal cell with no nucleus.

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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

A cell is described as long and thin, with a thin outer wall and many mitochondria, and it has a large surface area in contact with the soil. Name the cell and explain how two features suit its function.

Step 1 — identify the cell. A large surface area in contact with soil, a thin wall and many mitochondria point to a root hair cell.