Living things and their organizations
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All living things are made up of cells.
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Most cells are so small that you can only see them with a microscope.
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Cells have different components and each performs its own function within the cell.
What’s in a cell?
Cells are the basic building blocks of all animals and plants.
Inside cells are various structures that are specialised to carry out a particular function. Both
animal and plant cells have these components:
Cell membrane – this surrounds the cell and allows nutrients to enter and waste to leave it.
Nucleus – this controls what happens in the cell. It contains DNA, the genetic information that
cells need to grow and reproduce.
Cytoplasm – this is a jelly-like substance in which chemical reactions happen.
Mitochondria – these are the powerhouse of the cell. They are structures where respiration
takes place.
How are plant and animal cells different?
Plant cells have all the parts in the list above, plus a few extra structures:

Cell wall - this is an outer structure that surrounds the cell and gives it support.

Vacuole - this is a space within the cytoplasm of plant cells that contains sap.

Chloroplasts - these contain chlorophyll and are the site of photosynthesis.
How big are cells?
Cells can vary greatly in size. You need a microscope to see most human cells.
Red blood cells are some of the smallest cells in the human body. These have a diameter of
0.008 mm, meaning a line of 125 red blood cells is only 1 mm long.
The ovum (or egg cell) is one of the largest cells in the human body. It has a diameter of
roughly 0.1 mm, so you can see them without a microscope. A line of 10 egg cells is 1 mm long.
How do forensic scientists use cells?
Forensic scientists collect skin, blood and hair cells from a crime scene. They analyse DNA from
these cells and compare it to DNA samples already collected from thousands of people. This
helps them to identify suspects.
Specialised cells

Specialised cells have a specific role to perform.

Each specialised cell has a different job to do. They have special features that allow them to
do these jobs.

Muscle cells, for example, are held together in bundles, which pull together to make muscles
contract.
What are specialised cells?
Most cells share features such as having a nucleus, a cell membrane, cytoplasm and
mitochondria.
There are differences between cells, too. Each type of cell, has its own job to do. These cells
have special features that allow them to perform their functions effectively.
Here are some examples of specialised cells and the features they have to help them with their
role:
Red blood cells
Red blood cells carry oxygen around the body. They are well suited to this function because:

They contain haemoglobin, which carries oxygen molecules.

They don't have a nucleus, allowing more space to carry oxygen.

They are a flat disc shape (bi-concave) which gives them a large surface area, and the best
chance of absorbing as much oxygen as they can.
Nerve cells
Nerve cells transmit electrical signals. They are well suited to their function because:

They are thin, and can be more than 1 metre long. This means they can carry messages up
and down the body over large distances.

Nerve cells have branched connections at each end. These join to other nerve cells,
allowing them to pass messages around the body.

They have a fatty (myelin) sheath that surrounds them. The fatty sheath increases the
speed at which the message can travel.
Muscle cells
Muscle cells bring parts of the body closer together. They are well suited to this function
because:

Muscle cells are held together in bundles, which pull together to make muscles contract (get
shorter and fatter).
There are different types of muscle cell, each perfectly adapted to its function:

Cardiac (heart) muscle cells are branched, and they join together to make a net. Cardiac
muscle cells contract rhythmically, even outside the body. They never get tired.

Skeletal muscle is joined to bones. Its cells contract to make bones move and joints bend.

Smooth muscle cells make up thin sheets of muscle, such as the stomach lining. They can
also be arranged in bundles, or rings, like that in the anus.