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FIND OUT MORE. Evolution 74–75 • Microorganisms 85

Plasma membrane

is the thin barrier that

surrounds a cell

Cytoplasm is the area

of a living cell, outside

the nucleus, enclosed

by the cell membrane.

It contains organelles

(specialized structures

within a cell) that help

the cell to function

Cells

The tiniest living unit that exists is

a cell. Cells are the building blocks

of all organisms. Most cells have a

nucleus containing a set of building

blocks called. GENES.

GENES

The features of living things are controlled by their genes. The

genes inside an organism’s cells contain the instructions to make

proteins, which build that cell and control the way it works. Genes

are inherited by offspring from their parents.

ARE THERE DIFFERENT TYPES

OF CELL IN THE SAME ORGANISM?

Most organisms consist of many different types

of cell, each with a specific role to play. Cells with

a similar task, such as muscle cells in an animal,

are organized into a group. This group, called

a tissue, carries out a particular function,

such as bending a leg.

HOW DO PLANT

AND ANIMAL CELLS DIFFER?

Both plant and animal cells have

a nucleus and a plasma membrane

and contain cytoplasm. Plant cells,

however, have a fluid-filled vacuole

and green structures called chloroplasts.

Chloroplasts make food using sunlight

energy in a process called photosynthesis.

Animal cells must absorb food to survive.

ANIMAL CELL 3

Unlike plant cells, which have

a rigid outer wall, animal cells

are more fluid in shape. 1 ONION BULB CELLS

These magnified cells from an

onion bulb have been stained red.

Box-like cell walls have formed

around the mature cells.

1 CELL DIVISION IN AN ONION’S ROOT TIP
These onion cells (blue) are dividing in a process called mitosis. The cells’
16 chromosomes (black) also divide and separate, so that each new cell
gains a complete set. Mitosis allows an organism to grow or repair itself.

WHAT IS DNA?

Genes are made of a chemical substance called

deoxyribonucleic acid (DNA). It is stored in the

nucleus of all cells. DNA holds instructions for

making the proteins needed for the growth and

development of new organisms. It also passes

on genetic information to the next generation.

WHAT IS A CHROMOSOME?

Inside a cell’s nucleus, DNA is packaged into long,

thread-like structures called chromosomes. They are

visible only under a microscope when the cells divide.

During cell division, chromosomes shorten and

thicken, then split into identical halves, one for each

new cell. Chromosome numbers vary between species.

1 GENE DIFFERENCES

These two hamsters differ

because each has different

versions of the genes that control

fur pattern and colour. Otherwise

they look very similar because

most of their genes are identical.

Cell wall

Cytoplasm is

the jelly-like

liquid inside cell

Nucleus is the cell’s

control centre

Chloroplast (green)

captures sunlight

needed to make food

Vacuole

stores watery

cell sap that

gives cell

its shape

Plasma membrane

is pushed up against

the cell wall^1 PLANT CELL

This cross-section of a leaf cell

shows how the vacuole (blue) sap

pushes cytoplasm (yellow-green)

against the cell wall (brown).

This gives the cell its shape.

Nucleus is surrounded

by a double membrane

cells