Practise Principles of organisation. 11 exam-style questions plus unlimited generated ones on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How living things are built up from cells into tissues, organs, organ systems and whole organisms. It is usually tested with short recall questions (defining a tissue or an organ, putting the levels in order, classifying a structure) and as the opening part of longer questions on digestion, the heart or plants.
Grade by grade
What you need to be able to do, from the first marks up to the top grade.
3
Put the levels of organisation in orderFrom smallest to largest: cell → tissue → organ → organ system → organism.
4
Define tissue, organ and organ systemA tissue is a group of cells with a similar structure and function, an organ is a group of tissues working together, and an organ system is a group of organs working together.
5
Classify a structure by its levelFor example, a red blood cell is a cell, blood is a tissue, the heart is an organ and the circulatory system is an organ system.
6
Explain why a structure is an organSay that it is made of several different tissues that work together to carry out a specific function.
7
Explain why large organisms need organ systemsLarge organisms have a small surface area to volume ratio, so they need specialised exchange organs and a transport system to supply every cell.
Notes
The levels of organisation
Cells are the basic building blocks of all living organisms. In a multicellular organism most cells are specialised for one job.
A tissue is a group of cells with a similar structure and function, e.g. muscular tissue contracts to move things.
An organ is a group of different tissues that work together to perform a specific function, e.g. the stomach, the heart or a leaf.
An organ system is a group of organs that work together to perform a function, e.g. the digestive system digests and absorbs food.
Organ systems work together to form an organism.
Examples to learn
The stomach contains muscular tissue (contracts to churn the food), glandular tissue (makes digestive juices containing enzymes and acid) and epithelial tissue (covers the inside and outside of the stomach).
Blood is a tissue. The heart is an organ. The heart, blood vessels and blood together make up the circulatory system, an organ system.
In plants, a leaf is an organ. It contains epidermal tissue, palisade and spongy mesophyll, xylem and phloem.
The roots, stem and leaves together form a plant organ system that transports substances around the plant.
Why large organisms need organ systems grade 7+
A single-celled organism can get everything it needs by diffusion across its surface, because it has a large surface area to volume ratio.
A large multicellular organism has a small surface area to volume ratio, and many of its cells are far from the surface, so diffusion across the body surface alone is too slow.
It needs specialised exchange organs (such as the lungs and small intestine) and a transport system (heart, blood vessels and blood) to carry substances to and from every cell.
Specialisation lets each tissue or organ carry out one job efficiently, but the parts then depend on each other.
Cheatsheet
Order: cell → tissue → organ → organ system → organism
Tissue = a group of cells with a similar structure and function
Organ = a group of different tissues working together to perform a specific function
Organ system = a group of organs working together to perform a function
Blood = a tissue; heart = an organ; circulatory system = an organ system
Leaf = a plant organ; roots + stem + leaves = the plant transport organ system
How to answer each type of question
Recall: define a level or put the levels in order
1 mark each3
Learn the exact definitions: 'similar structure and function' for a tissue, 'different tissues working together' for an organ.
For ordering questions, always start with the cell and finish with the organism.
Example. (a) What is a tissue? (b) Put these in order from smallest to largest: organ, cell, organ system, tissue.
Show the model answer
(a) A group of cells with a similar structure and function (1) (b) cell → tissue → organ → organ system (1)
Classify structures by level of organisation
1 mark each5
Ask yourself: is it one cell, a group of similar cells, several different tissues, or several organs?
Watch for the traps: blood is a tissue, xylem is a tissue, a leaf is an organ.
Example. Give the level of organisation of each structure. Choose from: cell, tissue, organ, organ system. (a) the heart (b) xylem (c) a root hair cell (d) the stomach, liver, pancreas and intestines working together
Show the model answer
(a) organ (1) (b) tissue (1) (c) cell (1) (d) organ system (1)
Explain why a structure is an organ
2 marks6
Point 1: it is made of several different tissues (use the tissues named in the question).
Point 2: the tissues work together to carry out a specific function (say what the function is).
Example. The small intestine contains muscular tissue, glandular tissue and epithelial tissue. Explain why the small intestine is described as an organ.
Show the model answer
It is made of several different tissues (1) that work together to carry out a specific function, digesting and absorbing food (1).
Explain why large organisms need organ systems
3 marks7
Start with size: a large organism has a small surface area to volume ratio.
Say that diffusion across the body surface is too slow to supply cells deep inside the body.
Link to organ systems: exchange organs take substances in and a transport system carries them to every cell.
Example. An amoeba is a single-celled organism. A human is a large multicellular organism. Explain why a human needs a circulatory system but an amoeba does not.
Show the model answer
An amoeba has a large surface area to volume ratio, so diffusion across its membrane supplies the whole cell (1). A human has a small surface area to volume ratio / many cells are far from the body surface, so diffusion alone would be too slow (1). The circulatory system transports substances such as oxygen and glucose from the exchange organs to every cell (1).
Shortcuts and memory tricks
Order mnemonic: 'Clever Tigers Often Sleep Outside' = Cell, Tissue, Organ, organ System, Organism.
Each level is made of several of the level below: similar cells make a tissue, different tissues make an organ, different organs make a system.
Quick test: if it is a single named body part (heart, stomach, leaf, root), it is an organ.
Where marks are lost
Defining a tissue as 'a group of cells' without 'with a similar structure and function'.
Saying an organ is 'a group of cells'. It is a group of different tissues.
Calling blood an organ or a cell. Blood is a tissue.
Calling a leaf a tissue. A leaf is an organ; palisade mesophyll and xylem are tissues.
Writing 'they work together' without saying what function they carry out.
Exam technique
In 'explain why X is an organ' questions, name the tissues given in the question to show you have applied your knowledge.
This subtopic is often the first part of a longer question, so learn which organs belong to which organ system.
Use the key phrases examiners look for: similar structure and function, work together, specific function.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
Living organisms are organised into different levels. Name the organ system that the stomach is part of.
digestive system
The liver is part of the digestive system. Name the substance made in the liver that helps with the digestion of fats.
bile
Multicellular organisms are organised into cells, tissues, organs and organ systems. Name the organ system that pumps blood around the body.
circulatory system
Muscle cells need a supply of glucose and oxygen for respiration. Several organ systems work together to supply these substances and to remove waste. Name the organs that take oxygen into the body.
lungs
Complete the flow chart to show the levels of organisation in the plant transport system. xylem cell → ____________ (tissue) → ____________ (organ) → roots, stem and leaves (organ system)
xylem (tissue); stem / root / leaf (organ)
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
Living organisms are organised into different levels.
(a) Which shows the levels of organisation in the correct order, from smallest to largest? Tick (✓) one box.[1]
cell → tissue → organ → organ system → organism
tissue → cell → organ → organism → organ system
cell → organ → tissue → organ system → organism
organ → tissue → cell → organ system → organism
(b) What is a tissue?[1]
(c) Which of these is an organ? Tick (✓) one box.[1]
Digestive system
Heart
Muscle cell
Muscle tissue
(d) Name the organ system that the stomach is part of.[1]
Show the answer and mark scheme
(a)Answer: cell → tissue → organ → organ system → organism
(b)
a group of cells with a similar structure and function
(c)Answer: Heart
(d)Answer: digestive system
digestive (system)
Question 2Medium6 marks
Plants and animals are made of cells, tissues, organs and organ systems.
(a) Give the level of organisation of each of these structures. Choose from: cell, tissue, organ, organ system.
a leaf
palisade mesophyll
a guard cell
the roots, stem and leaves working together to transport substances
[4]
(b) The stomach contains muscular tissue, glandular tissue and epithelial tissue. Explain why the stomach is described as an organ.[2]
Show the answer and mark scheme
(a)Answer: leaf – organ; palisade mesophyll – tissue; guard cell – cell; roots, stem and leaves – organ system
leaf – organ
palisade mesophyll – tissue
guard cell – cell
roots, stem and leaves – organ system
(b)
it is made of several different tissues
which work together to carry out a specific function, e.g. digesting food
Question 3Hard6 marks
The skin is the largest organ of the human body. It contains epithelial tissue, glandular tissue (sweat glands), fat tissue and nervous tissue.
(a) Explain why the skin is described as an organ.[2]
(b) Suggest one function of the fat tissue in the skin.[1]
(c) A single-celled organism does not have tissues, organs or organ systems. Explain why large multicellular organisms need their cells to be organised into tissues, organs and organ systems.[3]
Show the answer and mark scheme
(a)
it is made of several different tissues
which work together to carry out specific functions, e.g. protection / controlling body temperature
(b)
insulation / reducing heat loss / an energy store
(c)
large organisms have a small surface area : volume ratio / many cells are far from the surface
so diffusion across the body surface is not enough to supply all the cells
they need specialised exchange organs, e.g. lungs / small intestine, and a transport system, e.g. the circulatory system
specialised tissues and organs can each carry out one function efficiently / organs work together in organ systems
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