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Unlock This CourseYour body is made of trillions of cells. They do not work alone. Living things are organised in levels, and each level is built from the one below it. Learn these levels in order.
Key terms:
Cells are the basic building blocks of all living organisms. Everything else in this lesson is made from them.
Read the list from the bottom up. Each level is made from the one before it.
The smallest living units. Similar cells are grouped together.
Groups of similar cells doing the same job, such as muscular tissue.
Different tissues combined to do one specific job, such as the stomach.
Then organs are organised into organ systems, and the organ systems work together to form the whole organism.
The order is: cell → tissue → organ → organ system → organism.
An organ is not made of just one tissue. It is a team of tissues. Take the stomach as an example. It contains three main tissues:
No single tissue could do the whole job. The organ works because the tissues cooperate.
Do not say that an organ is "a group of cells". That is a tissue. An organ is a group of tissues. Also, do not mix up the top two levels: an organ system is made of organs, and an organism is made of organ systems.
Each level is bigger than the one below it. Most animal and plant cells are very small, between about 10 and 100 micrometres (µm) across. That is far too small to see without a microscope. Tissues are groups of these cells, so they are bigger, but still often tiny. Organs are large enough to see and hold. Organ systems can run through the whole body.
You need to be able to compare sizes. Remember that 1 millimetre (mm) = 1000 micrometres (µm).
A cell is 20 µm wide. How many of these cells, side by side, would fit across 1 mm?
Step 1: change 1 mm into micrometres: 1 mm = 1000 µm.
Step 2: divide: 1000 ÷ 20 = 50 cells.
So a line of just 50 cells is only 1 mm long. An organ contains vast numbers of cells.
Comparing sizes by factors of ten helps too. A 20 µm cell and a 2 cm organ differ by a factor of 1000, because 2 cm = 20 000 µm and 20 000 ÷ 20 = 1000.
An organ system is a group of organs that work together to do a job for the whole body. Several organ systems are needed to keep you alive, and they depend on each other. For example, the digestive system provides food molecules, and the circulatory system carries them around the body to cells.
The digestive system is an example of an organ system in which several organs work together to digest and absorb food. This builds on what you learned in Key Stage 3.
Digest means to break down large food molecules into small ones. Absorb means to take those small molecules into the blood. Food starts as large molecules, which are too big to pass into the blood, so they must be broken down first.
The mouth chews food and mixes it with saliva from the salivary glands. The stomach churns food and adds digestive juices. The pancreas makes digestive juices. The liver and gall bladder also help digestion, as you will see in a later lesson.
The small intestine is where digestion is completed and where small food molecules are absorbed into the blood. The large intestine absorbs water from what is left. The oesophagus carries food from the mouth to the stomach.
Food passes through this long tube in order: mouth, oesophagus, stomach, small intestine and large intestine.
The digestive system shows the whole idea of this lesson. The stomach is an organ made of muscular, glandular and epithelial tissues. Those tissues are made of cells. The stomach joins the other organs to form one organ system. And that system works with others, such as the circulatory system, to keep the organism alive.
How digestive enzymes and bile break down food is covered in later lessons. Here you only need to know the organs and what each one does.
(a) Put these in order of size, starting with the smallest: organ, cell, organ system, tissue. [2 marks]
(b) The stomach is an organ. Explain why it is an organ and not a tissue. [2 marks]
(c) Name the organ system that digests and absorbs food. [1 mark]
(a) Cell, tissue, organ, organ system. (2 marks for all four in the right order; 1 mark if only one is out of place.)
(b) An organ is made of several different tissues (1), which work together to perform a specific function (1). The stomach has muscular, glandular and epithelial tissue.
(c) The digestive system (1).
For "explain why it is an organ", always say it is made of different tissues working together. Naming a real example from the stomach earns the mark.