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3

Cells and Cell Processes

BIOLOGY 2

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3Cells

All organisms (apart from viruses) are made up of one or more cells.

You contain billions of different cells. Here are some cells in living organisms:

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3How many not how big!

The cells in an ant and an elephant are about the same size but the elephant is a much larger animal because it has a lot more cells.

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3Important structures

Nucleus

This is the centre that controls all the cell’s activities. It contains chromosomes which are important in cell division. The nucleus is usually circular in shape

nucleus

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Cytoplasm

This structure is outside the nucleus. It is a clear jelly-like substance containing dissolved materials needed by the cell.

Cytoplasm

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Cell MembraneThis is the outer most layer of the cell which keeps the cytoplasm in and separates the cell from its environment. Substances can move through the cell membrane in a controlled way.

Cell membrane

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3Plant Cell

Plant cells contain some parts that are not present in animal cells. These are:

cell wall

vacuole

chloroplast

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3Cell wall

Cell wallThis is a thick and rigid layer made of cellulose. The cell wall surrounds the cytoplasm. Without this, a cell would not be able to maintain its shape.

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3Vacuole

VacuoleThis is a fluid filled space. It is surrounded by a plasma membrane. The fluid is called cell sap and it consists mainly of water, containing soluble substances such as minerals.

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3Chloroplasts

ChloroplastsThese are green discs containing the pigment chlorophyll. This is what allows a plant to absorb sunlight and is therefore needed for photosynthesis.

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3Onion Cell

Why aren’t there any chloroplasts in an onion cell? Clue: Where does the onion cell grow? Does it see the sun?

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3Patterns of Growth

When two cells divide we call this cell division. The type of cell division that allows an organism to grow is called mitosis. This happens in different places in plants compared to animals.

A cell dividing in two by mitosis to create two identical daughter cells.

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3Growth pattern in plants

We only see mitosis in the area where growth occurs in a plant – this is in the tips of the roots and the stem. These areas are called the meristem.

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3Growth pattern in plants

This cross section of a tree trunk shows obvious cycles of growth.

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3Growth pattern in

animals

Mitosis can take place in animal tissue at any stage in its development. Even though plants have the ability to grow new parts throughout their life cycle, on the whole animals are unable to do this.

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3Specialised cells

Cells adapt to perform different functions during their development.

e.g. red blood cells haven’t got a nucleus. This allows them to pack in the red pigment haemoglobin and can therefore they can carry a lot more oxygen around the body.

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3Undifferentiated Cells

Some cells are not specialised and can grow to form a range of potential tissues. We call these undifferentiated cells and they are referred to as stem cells.

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3Stem cells – why all the

controversy? Some scientists claim that stem cell research holds the key to curing many diseases.

Because these cells are taken from embryos, some believe that destroying them is immoral.

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3Use of Stem Cells

Stem cells can develop to become different tissues.

Here are two examples of their possible use in medicine:

Stem cells can develop to become different tissues.

Here are two examples of their possible use in medicine:

• Creating insulin-producing cells in the pancreas. This avoids the need to inject insulin.

• Replacing nerves destroyed after a spinal cord injury.

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3The Future

Scientists are now looking into using stem cells from adults so that embryos will not be involved.

In the year 2000, successful trials were held in Italy using adult stem cells to cure leukaemia in children.