Cell organisation: tissues, organs and organ systems

What you'll learn

This revision guide covers the hierarchical organisation of multicellular organisms, from individual cells through to complex organ systems. You'll understand how specialised cells group together to form tissues, how tissues combine to create organs, and how organs work together in coordinated systems. This topic is fundamental to understanding human biology and appears regularly in AQA GCSE Biology examinations.

Key terms and definitions

Cell — the basic structural and functional unit of all living organisms

Tissue — a group of similar cells working together to perform a specific function

Organ — a structure composed of different tissues working together to carry out a particular function

Organ system — a group of organs that work together to perform a specific physiological function

Specialised cell — a cell that has differentiated to perform a particular role, with adaptations suited to its function

Differentiation — the process by which cells become specialised for specific functions during development

Hierarchy — the levels of organisation in multicellular organisms, from cells to organ systems

Glandular tissue — tissue that produces and secretes substances such as enzymes or hormones

Core concepts

The hierarchy of organisation

Multicellular organisms are organised in a hierarchy of increasing complexity:

Cells → Tissues → Organs → Organ systems → Organism

This organisation allows for:

• Efficiency through division of labour
• Specialisation of function
• Coordination of complex processes
• Survival of large, complex organisms

Each level builds upon the previous one, with structures becoming more complex and capable of more sophisticated functions.

From cells to tissues

Tissues form when groups of similar specialised cells work together. In animals, the main tissue types include:

Epithelial tissue

• Covers body surfaces and lines organs
• Forms protective barriers
• Found in skin, gut lining, and inside blood vessels
• Cells are tightly packed with little intercellular space

Muscular tissue

• Capable of contraction
• Enables movement
• Three types: skeletal, smooth, and cardiac
• Contains protein fibres that can shorten

Glandular tissue

• Produces and releases substances
• Makes enzymes, hormones, or other secretions
• Found in digestive system, endocrine glands
• Cells contain extensive protein synthesis machinery

Nervous tissue

• Transmits electrical impulses
• Enables communication and coordination
• Found in brain, spinal cord, and nerves
• Contains neurons and supporting cells

From tissues to organs

Organs are structures formed from multiple tissue types working together. Each tissue contributes to the organ's overall function.

The stomach as an example organ:

The stomach contains:

• Muscular tissue — churns food to mix it with digestive juices
• Glandular tissue — produces digestive enzymes and hydrochloric acid
• Epithelial tissue — covers the outer surface and lines the inner cavity

This combination allows the stomach to:

• Store food temporarily
• Begin protein digestion
• Kill bacteria with acid
• Mix food into a liquid called chyme

The leaf as a plant organ:

Plant organs also contain different tissues:

• Epidermal tissue — covers and protects surfaces
• Palisade mesophyll — contains chloroplasts for photosynthesis
• Spongy mesophyll — has air spaces for gas exchange
• Xylem tissue — transports water and minerals
• Phloem tissue — transports dissolved sugars

Organ systems in animals

Organ systems consist of multiple organs working together to perform major body functions.

The digestive system

Organs involved:

• Mouth — mechanical digestion by teeth; chemical digestion by salivary enzymes
• Oesophagus — transports food to stomach via peristalsis
• Stomach — churns food; produces acid and protease enzymes
• Small intestine — completes digestion; absorbs nutrients into blood
• Large intestine — absorbs water; forms faeces
• Liver — produces bile to emulsify fats
• Pancreas — produces digestive enzymes (amylase, protease, lipase)

Function: Breaks down large insoluble food molecules into smaller soluble molecules that can be absorbed

The circulatory system

Components:

• Heart — pumps blood around the body
• Blood vessels — arteries, veins, and capillaries transport blood
• Blood — transports oxygen, nutrients, carbon dioxide, and waste products

Function: Delivers oxygen and nutrients to cells; removes waste products; distributes heat

The respiratory system

Organs:

• Trachea — carries air to and from lungs
• Bronchi — branch into each lung
• Bronchioles — smaller airways within lungs
• Alveoli — tiny air sacs where gas exchange occurs
• Diaphragm and intercostal muscles — enable breathing movements

Function: Enables gas exchange between air and blood

Other important systems:

• Nervous system — brain, spinal cord, nerves (coordination and control)
• Endocrine system — glands that produce hormones (chemical coordination)
• Excretory system — kidneys, bladder, ureter, urethra (removes waste)
• Reproductive system — produces gametes and enables reproduction
• Musculoskeletal system — bones and muscles (support and movement)

Organ systems in plants

Plants have organ systems adapted for their needs:

The shoot system

• Leaves — photosynthesis and gas exchange
• Stem — support and transport
• Flowers — reproduction

The root system

• Anchors plant in soil
• Absorbs water and mineral ions
• May store food (in some species)

Transport system

• Xylem — transports water and minerals from roots to leaves
• Phloem — transports dissolved sugars from leaves to other parts

Importance of organisation

This hierarchical organisation provides several advantages:

Efficiency

• Cells can specialise in one function
• Each level focuses on specific tasks
• Resources are used effectively

Size

• Enables organisms to grow large
• Maintains surface area to volume ratio through specialised exchange surfaces
• Allows complex body plans

Survival

• Redundancy — multiple cells perform each function
• Coordination — systems work together
• Adaptation — organs can be modified for different environments

Examples of coordination between systems:

The digestive and circulatory systems work together:

1. Digestive system breaks down food into small molecules
2. Small intestine absorbs these molecules
3. Circulatory system transports nutrients to all body cells

The respiratory and circulatory systems cooperate:

1. Respiratory system takes in oxygen at alveoli
2. Oxygen diffuses into blood
3. Circulatory system delivers oxygen to respiring cells
4. Carbon dioxide returns to lungs
5. Respiratory system removes carbon dioxide through exhalation

Worked examples

Example 1: Identifying levels of organisation

Question: The diagram shows the heart, which pumps blood around the body. The heart wall is made of cardiac muscle, which contracts regularly. Cardiac muscle cells contain many mitochondria.

(a) State what type of structure the heart is. (1 mark)

(b) Explain why cardiac muscle is described as a tissue. (2 marks)

(c) The heart is part of the circulatory system. Describe what is meant by an organ system. (2 marks)

Mark scheme answers:

(a) Organ (1 mark)

• Accept: "The heart is an organ"
• Do not accept: tissue, system, or organism

(b)

• Cardiac muscle is made of a group of similar/specialised cells (1 mark)
• That work together to perform a specific function/contract (1 mark)
• Accept: "cells with the same structure and function"

(c)

• A group of organs (1 mark)
• That work together to perform a (specific) function/system function (1 mark)
• Accept: "organs that coordinate their activities"
• Example: heart, blood vessels, and blood work together to transport substances

Example 2: The stomach

Question: The stomach is an organ in the digestive system.

(a) Name three types of tissue found in the stomach. (3 marks)

(b) The stomach contains glandular tissue. Describe the function of glandular tissue in the stomach. (2 marks)

Mark scheme answers:

(a) Any three from:

• Muscular tissue (1 mark)
• Glandular tissue (1 mark)
• Epithelial tissue (1 mark)
• Nervous tissue (1 mark — accept if offered, though less commonly mentioned)

(b)

• Produces/secretes enzymes (1 mark)
• Produces/secretes acid/hydrochloric acid (1 mark)
• Accept: "makes digestive juices"
• Accept: "secretes protease/pepsin"

Example 3: Plant organisation

Question: A leaf is a plant organ.

(a) State one function of a leaf. (1 mark)

(b) Name two tissues found in a leaf and describe their functions. (4 marks)

Mark scheme answers:

(a) Any one from:

• Photosynthesis (1 mark)
• Gas exchange (1 mark)
• Transpiration (1 mark — accept for GCSE, though primarily a consequence)

(b) Two tissues with functions, for example:

• Palisade mesophyll tissue (1 mark)
• Carries out photosynthesis/contains chloroplasts (1 mark)
• Xylem tissue (1 mark)
• Transports water and mineral ions (1 mark)

Alternative answers:

• Epidermal tissue — protects the leaf/covers surfaces
• Spongy mesophyll — allows gas exchange/has air spaces
• Phloem tissue — transports sugars/dissolved food

Common mistakes and how to avoid them

• Confusing the order of hierarchy — remember "Cells → Tissues → Organs → Organ systems" in that specific order. A tissue cannot contain organs; organs contain tissues. Use the mnemonic "Can Tigers Outrun Ostriches?" or create your own.

• Saying tissues are "made of different cells" — tissues are made of similar or the same type of cells, not different types. It's organs that are made of different tissues. Always specify "similar cells" or "cells of the same type" in your answers.

• Thinking muscle, nervous, epithelial, and glandular are organs — these are tissue types, not organs. The heart is an organ; cardiac muscle is a tissue. The brain is an organ; nervous tissue is what it's partly made from. Learn the distinction clearly.

• Forgetting that organs contain multiple tissue types — in exam questions, always mention at least two or three different tissues when describing an organ. For example, the stomach contains muscular, glandular, and epithelial tissues working together.

• Using vague language like "works properly" — be specific about functions. Don't write "glandular tissue helps the stomach work properly." Instead write "glandular tissue produces enzymes and acid for digestion."

• Not reading the question carefully — if asked for a tissue, give a tissue; if asked for an organ, give an organ. Many students lose marks by giving the wrong level of organisation. Circle or underline the key command word in the question.

Exam technique for "Cell organisation: tissues, organs and organ systems"

• Learn to recognise command words: "State" requires a simple answer without explanation (1 mark). "Describe" requires characteristics or features (usually 2-3 marks). "Explain" requires reasons or mechanisms with the word "because" or "so that" (2-4 marks). Match your answer length to the marks available.

• Use precise biological terminology: Write "muscular tissue contracts" not "muscle moves things." Write "glandular tissue secretes enzymes" not "glands make stuff." Examiners award marks for accurate scientific vocabulary, and it demonstrates your understanding clearly.

• Structure your answers in levels: When describing how systems work together, explicitly reference cells, then tissues, then organs, then systems. For example: "Epithelial cells in the small intestine form epithelial tissue that lines the organ. The small intestine works with other organs in the digestive system to absorb nutrients."

• Give examples when explaining organisation: If asked to explain what an organ system is, define it first then give an example: "An organ system is a group of organs working together to perform a function. For example, the digestive system includes the stomach, small intestine, and pancreas working together to digest food."

Quick revision summary

Multicellular organisms are organised hierarchically: cells form tissues, tissues form organs, and organs form organ systems. Tissues are groups of similar specialised cells (muscular, glandular, epithelial, nervous). Organs contain different tissues working together, like the stomach with its muscular, glandular, and epithelial tissues. Organ systems coordinate multiple organs—the digestive system includes mouth, stomach, intestines, liver, and pancreas. Plants also have organs (leaves, stems, roots) containing different tissues (xylem, phloem, mesophyll, epidermis). This organisation enables specialisation, efficiency, and survival of complex organisms.