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IGCSE BIOLOGY TOPIC 2

2.1) Cell structure and organisation

2.1) Cell structure and organisation

Cytoplasm contains ribosomes on rough endoplasmic reticulum and vesicles.

Almost all cells, except prokaryotes, have mitochondria and  rough endoplasmic reticulum.

Mitochondria are responsible for producing energy from food substances through the process of aerobic respiration.

Cells with high rates of metabolism require large numbers of mitochondria to provide sufficient energy

2.2) Levels of organisation

2.2) Levels of organisation

Most cells, when they have finished dividing and growing, become specialised.

  • They do one particular job
  • They develop a distinct shape
  • Special kinds of chemical change take place in their cytoplasm.

‘Division of labour’- the specialisation of cells to carry out particular functions in an organism.

Palisade mesophyll cells – photosynthesis

Nerve cells – conduction of impulses

Sperm and egg cells – reproduction

Tissue is a group of cells with similar structures, working together to perform a shared function.

Eg. Bone, nerve, muscle, epidermis, xylem

Organ is a structure made up of a group of tissues, working together to perform a specific function.

Eg. Stomach, heart, lungs, intestines, brain, eyes

Organ system is a group of organs with related functions, working together to perform a body function.

Eg. Nervous and circulatory system

2.3) Size of specimens

1000000 micrometres in a metre

10000 micrometres in a centimetre

1000 micrometres in a millimetre

MORE NOTES

Levels of organisation
Key terms:
● Tissue – a group of similar cells working together to carry out a particular process
● Organ – a group of tissues working together to carry out a specific function
● Organ system – a group of related organs working together to carry out functions in the
body
Specialised cells:
Cells and tissues are specialised to carry out their particular function. Examples of specialised
cells are:
● Ciliated cells – ciliated cells are found lining the trachea. They have hair-like projections
called cilia which move together to transport mucus, dust and bacteria upwards to the
throat.
● Root hair cells – are adapted to have a large surface area. This speeds up the rate of
osmosis and mineral ion uptake in plants.
● Xylem vessels – used to transport water through plants in transpiration. The xylem is
made from hollowed-out dead cells that have the ends removed to make a tube for
water to pass through. They have a thick cell wall to provide structural strength and are
thin to allow capillary action.
● Palisade mesophyll cells – this is where photosynthesis occurs. Mesophyll cells are tall
and closely packed to efficiently absorb light and contain lots of chloroplasts for
photosynthesis. They are also placed at the top of the leaf where most of the light hits
enabling them to absorb much light energy as possible.
● Nerve cells – nerve cells are adapted to rapidly transmit electrical impulses. Nerve cells
are myelinated, which insulates the cell and prevents the impulse weakening and
slowing down. They also contain lots of mitochondria to provide energy. Dendrites have
a large surface area and are branched to receive impulses from many other neurons.
● Red blood cells – red blood cells contain haemoglobin which allows them to carry
oxygen around the body. They have a biconcave shape which increases their surface
area, allowing for rapid diffusion. They also have thin cell membranes to decrease the
diffusion distance. They do not contain a nucleus, thus have more space for oxygen.

● Sperm cells – sperm cells are adapted by containing lots of mitochondria so that the cell
has enough energy to reach the egg cell. It has a tail to allow movement and contains
digestive enzymes to help penetrate the egg cell membrane.