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3.1 Tissues and Organs

GCSE Biology · AQA · Chapter 3: Organisation and the Digestive System

A free GCSE Biology lesson for AQA (8461) 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
  • Explain what tissues and organs are, with examples
  • Describe the tissues in the stomach and what each one does
  • Name some human organ systems and explain how they work together
  • Use size and magnification calculations with cells and tissues

Key words

  • Cell: The basic building block of all living things.
  • 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, working together.
  • Organ: A group of different tissues working together to carry out a particular function.
  • Organ system: A group of organs working together to do a job in the body.
  • Organism: A whole living thing, made of several organ systems working together.

Levels of organisation

A large multicellular organism, like you, is built in levels. Specialised cells form tissues. Tissues form organs. Organs form organ systems. Organ systems together make the whole organism.

In the early embryo, cells are unspecialised. As the organism develops, cells differentiate: they gain the sub-cellular structures they need 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 need to know:

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

Plants have tissues too, such as epidermal tissue, palisade mesophyll, xylem and phloem. You will meet these in the plant organisation chapter.

Organs: the stomach as an example

An organ contains several different tissues that work together. The stomach is an organ in the digestive system. Its job is to hold food, churn it and start 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: If a question asks you 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 work together to keep the whole organism alive. For example, the digestive system breaks food down into small soluble molecules. The circulatory system carries these molecules to every cell. The respiratory system brings in the oxygen that cells need to release energy from the food.

  • 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
  • Respiratory (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 and tissues are tiny, so we use micrometres (µm). 1 mm = 1000 µm. To find the real size of something in a micrograph, use: real size = image size ÷ magnification

Worked example: a cell in a tissue section

In a micrograph magnified × 400, a stomach lining cell is 12 mm long. How long is the real cell in µm?
Real size = 12 mm ÷ 400 = 0.03 mm.
0.03 mm × 1000 = 30 µm.

Answer: 30 µm

Common mistakes

  • Mixing up tissue and organ. A tissue is one type of cell; an organ is made of several tissues.
  • Saying glandular tissue “digests food”. It produces the enzymes; the enzymes do the digesting.
  • Putting the levels in the wrong order, e.g. putting 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 smallest to largest? (Choose from: cell → tissue → organ → organ system → organism, cell → organ → tissue → organ system → organism, tissue → cell → organ → organism → organ system, cell → tissue → organ system → organ → organism.)

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

Working: Similar cells form a tissue; tissues form an organ; organs form an organ system; organ systems form an organism.

Question 2. What name is given to a group of cells with a similar structure and function?

Answer: tissue

Working: A tissue is a group of cells with a similar structure and function.

Question 3. True or false? The stomach is an organ because it is made of several different tissues working together.

Answer: True

Working: True. An organ is a group of different tissues that work together to perform a function.

Related lessons

Continue in the full course

This lesson is section 3.1 (AGCBH0301) of our free GCSE Biology 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 Biology course.

 

 

 

 

 

 

 

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