B3.1 Tissues and Organs

A free GCSE Combined Science: Trilogy lesson for AQA (8464) from OBA School: how cells are organised into tissues, organs, organ systems and whole organisms. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Describe the levels of organisation from cells to whole organisms, with an example of each
• Explain what tissues and organs are
• Describe the tissues in the stomach wall and what each one does
• Explain how organ systems work together in the body
• Use magnification calculations with cells and tissues

Key words

• Cell: The basic building block of all living organisms.
• Specialised cell: A cell that has differentiated (changed) to carry out one particular job.
• Tissue: A group of cells with a similar structure and function.
• Organ: A group of different tissues working together to carry out a specific function.
• Organ system: A group of organs working together to carry out a job in the body.
• Organism: A whole living thing, made of organ systems working together.

Levels of organisation

A large multicellular organism, such as a human, is organised in levels. Specialised cells form tissues. Tissues form organs. Organs form organ systems. Organ systems together make the whole organism.

In an early embryo the cells are all similar. As it develops, cells differentiate: each gains the sub-cellular structures it needs for one job. Groups of these specialised cells then form tissues.

Key fact: Cell → tissue → organ → organ system → organism. Learn the order and one example of each.

Tissues

A tissue is a group of cells with a similar structure and function. Three animal tissues you should know:

• Muscular tissue; What it does Contracts (gets shorter) to move the organ or body part; Where it is found Stomach wall, heart, gut wall, arm muscles
• Glandular tissue; What it does Makes and releases (secretes) substances such as enzymes and hormones; Where it is found Salivary glands, pancreas, stomach lining
• Epithelial tissue; What it does Covers the outside and lines the inside of organs; Where it is found Lining of the gut, outer layer of skin

Nervous tissue (which carries electrical impulses) and blood are also tissues. Plants have tissues too, such as xylem and phloem; you will meet them in B4.

Organs: the stomach as an example

An organ is made of several different tissues that work together. The stomach holds food, churns it and starts the digestion of protein.

• Muscular tissue contracts to churn the food and mix it with digestive juices.
• Glandular tissue makes the digestive juices: protease enzymes and hydrochloric acid.
• Epithelial tissue covers the outside and lines the inside of the stomach.

Exam tip: To “explain why the stomach is an organ”, say that it is made of more than one type of tissue working together to carry out a function.

Organ systems working together

Organ systems depend on each other. The digestive system breaks food down into small soluble molecules. The circulatory system carries them, and oxygen, to every cell. The gas exchange (respiratory) system takes in the oxygen that cells use in respiration.

• Digestive; Main organs Stomach, small intestine, liver, pancreas; Main job Breaks down and absorbs food
• Circulatory; Main organs Heart, blood vessels; Main job Transports substances around the body
• Gas exchange; Main organs Trachea, lungs; Main job Takes in oxygen, removes carbon dioxide
• Nervous; Main organs Brain, spinal cord, nerves; Main job Detects changes and coordinates responses

Maths skill: sizes and magnification

Cells are tiny, so we measure them in micrometres (µm). 1 mm = 1000 µm. To find the real size of something in a micrograph: real size = image size ÷ magnification

Worked example: a cell in a tissue section

In a micrograph magnified × 500, a cell from the gut lining is 14 mm long. How long is the real cell in µm?
Real size = 14 mm ÷ 500 = 0.028 mm.
0.028 mm × 1000 = 28 µm.

Answer: 28 µm

Common mistakes

• Mixing up tissue and organ. A tissue is a group of similar cells; an organ is made of several different tissues.
• Saying glandular tissue “digests food”. It produces the enzymes; the enzymes do the digesting.
• Putting the levels in the wrong order, e.g. organ systems before organs.
• Forgetting to convert units: multiply mm by 1000 to get µm.

Revision questions

Try these, then check your answers below each one.

Question 1. Which list shows the levels of organisation from largest to smallest? (Choose from: organism → organ system → organ → tissue → cell, organism → organ → organ system → tissue → cell, organ system → organism → organ → cell → tissue, organism → organ system → tissue → organ → cell.)

Answer: organism → organ system → organ → tissue → cell

Working: An organism is made of organ systems; organ systems are made of organs; organs are made of tissues; tissues are made of cells.

Question 2. What name is given to a group of different tissues that work together to carry out a specific function?

Answer: organ

Working: An organ, such as the stomach or heart, is made of several tissues working together.

Question 3. True or false? The brain is a tissue.

Answer: False

Working: False. The brain is an organ: it contains nervous tissue, blood and other tissues working together.

Related lessons

• Previous lesson: B2.4 Stem Cell Dilemmas
• Next lesson: B3.2 The Human Digestive System
• In this chapter: B3.3 The Chemistry of Food
• In this chapter: B3.4 Catalysts and Enzymes
• In this chapter: B3.5 Factors Affecting Enzyme Action

Continue in the full course

This lesson is section B3.1 (AGCSC0301) of our free GCSE Combined Science course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Combined Science course.