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The cell membrane
- Animal cells are surrounded by a cell membrane which controls what can go into and out of a cell – this means that they are selectively permeable.
- • An animal cell membrane is made from fatty acids (lipids) and is quite fluid.
- • Small molecules can pass through pores in the cell membrane.
- • Molecules that are soluble in lipids, such as oxygen and water, will dissolve in the cell membrane – this is called simple diffusion.
- • Some substances, such as glucose, enter the cell with the help of a carrier protein – this is called facilitated diffusion.
- • Larger molecules such as proteins and sodium, pass through the cell membrane using a process called active transport. This requires energy. • Some proteins can be embedded in the membrane to carry out specialised tasks – for example, some act as receptors to alert the cell to what is happening around it and antigens which alert the body to bacteria.
- • The cell membrane is able to repair itself if damaged.
Cilia and flagella
- • Some cells have cilia or flagella extending from the membrane.
- • Cilia are on the surface and create ‘waves’ that move fluid such as mucus and debris over the cell’s surface.
- • Flagella move the cell along – the only flagellum in mammals in mammals is the tail of a spermatozoon, this enabling sperm cells to swim.
- Organelles are the parts of the cell that help the cell to carry out its many functions. They:
- • control what goes in and out
- • store and protect our DNA
- • synthesize proteins
- • produce energy
- • clean up waste.
The nucleus – storing our genetic fingerprint
The nucleus is the heart of a cell and is the most visible organelle under the
- • The nucleus stores and protects our genes and DNA (in the form of
- chromatin or chromosomes).
- • Every cell in the body contains similar DNA. However, individual genes may be switched on or off to give the cell its unique characteristics. This is
- how a blood cell differs from a muscle cell.
- • The DNA controls the cell’s activities. It also allows the cell to divide.
- • Part of the nucleus, called the nucleolus produces ribosomes that are
- used in protein synthesis.
- • The nucleus is surrounded by a nuclear membrane.
The centrosome contains other organelles, called centrioles, which play a crucial role in cell division.
- • Two centrioles are contained within a centrosome.
- • They separate the chromosomes in the nucleus.
- • Before a cell divides, it creates a second centrosome.
A crucial part of a cell’s work is to synthesise proteins either for use within the cell’s own functions, or for sending out for use by other parts of the body. Several organelles are involved in this process:
- The endoplasmic reticulum (ER)
- There are two parts of the endoplasmic reticulum (ER).
- • The rough ER receives ribosomes from the nucleus and uses these to
- synthesise proteins.
- • It then transports or exports some of the proteins, such as digestive
- enzymes, outside the cell
- • The smooth ER helps to synthesise and transport other substances such
- as steroids. In liver cells it breaks down toxins.
- The Golgi apparatus
- The Golgi apparatus – sometimes called the Golgi complex – is a pile of
- flattened cells within the cytoplasm.
- • It takes the proteins synthesised by the ER and prepares them for use
- elsewhere in the cell and beyond.
- • It adds sugars to the proteins.
- • Then it sends proteins to the lysosome, to aid digestion.
- • And to the cell membrane, for example, to act as receptors and antigens.
- Some proteins are secreted from the cell to meet the needs of other cells
- and tissues.
- Basic cell structure 8
- Lysosomes – cleaning the cell
- • Lysosomes have the job of cleaning up the cell.
- • They contain proteins called enzymes that digest worn out cell
- components, bacteria or viruses and food particles.
- • As the enzymes are acidic, the lysosome has a membrane to stop the
- enzymes damaging the rest of the cell. • Waste products are expelled from the cell.
Mitochondria – producing energy
- • Mitochondria are the organelles where energy is extracted from food.
- • They produce energy by combining oxygen with sugar.
- • The energy is stored in a form the cell can use for all its activities –such as active transport, movement and division.
- • The energy produced is called ATP – Adenosine triphosphate.
- • There are large numbers of mitochondria in cells with high energy consumption, such as muscle cells.
Homeostasis – keeping the body in balance
Cells need the right levels of temperature, glucose, water and mineral salts
to function properly.
Scientists use the term homeostasis to describe how a body maintains a
stable internal environment by regulating:
- • temperature
- • glucose levels
- • the amount of water and mineral salts in blood.