Notes

Animal cells and plant cells
Animal cells usually have an irregular shape, and plant cells usually have a regular shape. Cells are made up of different parts. It is easier to describe these parts by using diagrams: Animal and plant cells both have a cell membrane, cytoplasm, and a nucleus. Plant cells also have chloroplasts, a vacuole and a cell wall. Animal cells and plant cells also contain tiny objects called mitochondria in their cytoplasm. Animal cells and plant cells both contain:
cell membrane, cytoplasm, nucleus and mitochondria. Plant cells also contain these parts, which are not found in animal cells:
cell wall, vacuole and chloroplasts.
Cell membrane Controls the movement of substances into and out of the cell Plant and animal cells
Cytoplasm Jelly-like substance, where chemical reactions happen Plant and animal cells
Nucleus Carries genetic information and controls what happens inside the cell Plant and animal cells
Mitochondria Where most respiration reactions happen Plant and animal cells
Vacuole Contains a liquid called cell sap, which keeps the cell firm Plant cells only
Cell wall Made of a tough substance called cellulose, which supports the cell Plant cells only
Cells and their functions
Humans are multicellular. That means we are made of lots of cells, not just one cell. The cells in many multicellular animals and plants are specialised, so that they can share out the processes of life. They work together like a team to support the different processes in an organism.
Specialised cells
The diagrams show examples of some specialised animal cells. Notice that they look very different from one another.
The variety of human cell types: epithelial cells, leucocyte, liver cell, nerve cell, fat cell, cartilage cell, striated muscle fibre, human cell, bone cell and smooth muscle cell.
Unicellular organisms

A unicellular organism is a living thing that is just one cell. There are different types of unicellular organism, including:
bacteria
protozoa
unicellular fungi
You might be tempted to think that these organisms are very simple, but in fact they can be very complex. They have adaptations that make them very well suited for life in their environment.
Bacteria

Bacteria are tiny. A typical bacterial cell is just a few micrometres across (a few thousandths of a millimetre). The structure of a bacterial cell is different to an animal or plant cell. For example, they do not have a nucleus but they may have a flagellum. This is a tail-like part of the cell that can spin, moving the cell along.
Protozoa

Protozoa are unicellular organisms that live in water or in damp places. The amoeba is an example of one. Although it is just one cell, it has adaptations that let it behave a bit like an animal:
it produces pseudopodia (“false feet”) that let it move about
its pseudopodia can surround food and take it inside the cell
contractile vacuoles appear inside the cell, then merge with the surface to remove waste
Yeast

You may be familiar with fungi from seeing mushrooms and toadstools. Yeast are unicellular fungi. They are used by brewers and wine-makers because they convert sugar into alcohol, and by bakers because they can produce carbon dioxide to make bread to rise.
Yeast have a cell wall, like plant cells, but no chloroplasts. This means they have to absorb sugars for their nutrition, rather than being able to make their own food by photosynthesis
Yeast can reproduce by producing a bud. The bud grows until it is large enough to split from the parent cell as a new yeast cell.
Cells, tissues, organs and systems

Multicellular organisms are organised into increasingly complex parts. In order, from least complex to most complex:
cells
tissues
organs
organ systems
organism
Tissues

Animal cells and plant cells can form tissues, such as muscle tissue in animals. A living tissue is made from a group of cells with a similar structure and function, which all work together to do a particular job. Here are some examples of tissues:
muscle
the lining of the intestine
the lining of the lungs
xylem (tubes that carry water in a plant)
Organs

An organ is made from a group of different tissues, which all work together to do a particular job. Here are some examples of organs:
heart
lung
stomach
brain
leaf
root
Organ systems

An organ system is made from a group of different organs, which all work together to do a particular job. Here are some examples of organ systems:
circulatory system
respiratory system
digestive system
nervous system
reproductive system
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Cells are organised into tissues, organs, systems and organisms