Cells, Tissues and Organs: KS3 Biology Guide

All living organisms are built from cells — the smallest units of life capable of carrying out the functions needed to survive and reproduce. From a single cell, multicellular organisms like humans build tissues, organs, and organ systems through a process of growth and specialisation. This hierarchy of biological organisation is fundamental KS3 biology, typically taught in Year 7.

What is the hierarchy of biological organisation?

Life in multicellular organisms is organised into five levels:

Level | Definition | Example
Cell | Smallest unit of life | Red blood cell, muscle cell
Tissue | Group of similar cells working together | Muscle tissue, epithelial tissue
Organ | Group of different tissues working together | Heart, lung, stomach
Organ system | Group of organs working together for one function | Circulatory system, digestive system
Organism | A complete living thing made of all its organ systems | Human, oak tree

Each level builds on the one below. Understanding this hierarchy is the foundation for everything else in GCSE biology.

What are the structures of an animal cell and a plant cell?

Both types of cell share some structures but plant cells have additional components needed for photosynthesis and support.

Structure | Animal cell | Plant cell | Function
Cell membrane | Yes | Yes | Controls what enters and leaves the cell
Cytoplasm | Yes | Yes | Jelly-like fluid where chemical reactions occur
Nucleus | Yes | Yes | Contains DNA; controls cell activities
Mitochondria | Yes | Yes | Site of aerobic respiration; releases energy
Ribosomes | Yes | Yes | Site of protein synthesis
Cell wall | No | Yes | Rigid outer layer (made of cellulose); gives structural support
Chloroplasts | No | Yes (in green parts) | Contains chlorophyll; site of photosynthesis
Permanent vacuole | No | Yes | Fluid-filled space that maintains cell turgor (firmness)

Key point for exams: animal cells never have a cell wall, chloroplasts, or a permanent vacuole. If a diagram shows a cell with a regular rectangular shape and green dots, it is a plant cell. Animal cells have irregular shapes because they lack a rigid wall.

What is cell specialisation?

Most cells in a multicellular organism do not look like the generic diagrams above. During development, cells undergo differentiation — a process in which a cell takes on a specific structure and function, expressing only part of its DNA. The result is a specialised cell adapted precisely for its job. This happens very early in development from stem cells, which are unspecialised cells capable of dividing and producing many different cell types.

Key specialised cells at KS3

Red blood cell (erythrocyte)

No nucleus — maximises space for haemoglobin, the oxygen-carrying protein.

Biconcave disc shape — increases surface area for oxygen diffusion.

Flexible — can squeeze through narrow capillaries.

Sperm cell

Long tail (flagellum) — for swimming towards the egg.

Many mitochondria in the mid-section — provide energy (ATP) for the tail to beat.

Acrosome at the tip — contains enzymes to digest through the egg's outer membrane.

Muscle cell

Contains many mitochondria — muscles need large amounts of energy during contraction.

Contains protein fibres (actin and myosin) — slide past each other to produce movement.

Can contract (shorten) and relax repeatedly.

Root hair cell (plant)

Long, thin hair-like extension — greatly increases surface area for absorbing water and mineral ions from the soil.

No chloroplasts — roots are underground with no light for photosynthesis.

Large permanent vacuole — helps maintain water uptake by osmosis.

What are tissues, organs, and organ systems?

Tissues

A tissue is a group of cells of the same type that work together to perform a shared function. Examples in animals include:

Muscle tissue — cells that contract to produce movement.

Nervous tissue — cells that carry electrical impulses around the body.

Epithelial tissue — sheets of cells lining surfaces and cavities (e.g. inside the lungs or gut).

Organs

An organ is made of two or more different tissues working together. The heart, for example, is made of cardiac muscle tissue, epithelial tissue (lining the chambers), and nervous tissue (for the electrical conducting system). Other examples: the lung, liver, stomach, kidney, and brain.

Organ systems

An organ system is a group of organs that cooperate to carry out a major body function. The main organ systems covered at KS3 include:

Organ system | Key organs | Main function
Circulatory | Heart, blood vessels, blood | Transports oxygen, nutrients, and waste
Digestive | Stomach, intestines, liver, pancreas | Breaks down and absorbs food
Respiratory | Lungs, trachea, diaphragm | Gas exchange (O2 in, CO2 out)
Nervous | Brain, spinal cord, nerves | Receives stimuli and coordinates responses
Musculoskeletal | Bones, joints, muscles | Movement and support

The Department for Education's Science Programmes of Study for Key Stage 3 requires pupils to describe the structure and functions of cells, the hierarchical organisation of cells into tissues, organs and systems, and how cells become specialised. BBC Bitesize KS3 Biology covers cell structure, cell specialisation, and organ systems with diagrams and revision quizzes.

Frequently asked questions

What is the difference between a cell and a tissue?

A cell is the smallest unit of life — a single, membrane-bound compartment containing the machinery to carry out life processes. A tissue is a collection of many cells of the same type that work cooperatively to do a specific job. For example, a single muscle cell can contract a tiny amount; muscle tissue is millions of muscle cells contracting together to produce a movement large enough to move a limb. The distinction matters in biology because damage to a tissue (e.g. muscle tear) is very different from damage to a single cell.

Why do plant cells have a cell wall but animal cells do not?

Plant cells have a rigid cell wall made of cellulose because plants need structural support to grow tall without a skeleton. The cell wall also allows plant cells to build up internal pressure (turgor pressure) by absorbing water — this keeps non-woody plants firm and upright. Animal cells rely on the skeleton for support and need a flexible cell membrane (without a rigid wall) so they can change shape, move, and divide freely. An animal cell with a rigid wall could not, for example, send out the flexible projections that white blood cells use to engulf bacteria.

What does cell specialisation mean?

Cell specialisation (also called cell differentiation) is the process by which a cell changes during development to take on a specific structure suited to a particular function. All cells in an organism start with the same DNA, but during differentiation a cell switches on only the genes relevant to its job and switches others off. The result is a cell with a distinctive shape and contents — a red blood cell loses its nucleus to carry more haemoglobin; a root hair cell grows a long projection to increase its absorptive surface area. Specialisation is what allows multicellular organisms to be far more complex and efficient than single-celled organisms.

Name three organ systems in the human body.

Three important organ systems are: (1) the circulatory system (heart, blood vessels, blood), which transports oxygen, glucose, hormones, and waste products around the body; (2) the digestive system (mouth, oesophagus, stomach, small intestine, large intestine, liver, pancreas), which mechanically and chemically breaks down food and absorbs nutrients into the blood; (3) the nervous system (brain, spinal cord, peripheral nerves), which detects stimuli from the environment via sensory neurons and coordinates responses via motor neurons and effectors (muscles and glands).

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