BIOLOGY IN FOCUS

PATTERNS IN NATURE
cilla
cytoplasm
cytoplasm
pseudopodium
gullet
eye spot
chloroplast
nucleus
nucleus
nucleus
cell wall
Paramecium
Amoeba
Euglena
Figure 1.11 Common
unicellular organisms
that may be seen
in a drop of pond
water using a light
microscope
82
TR
Teaching analogy
Figure 1.12 Cells of
multicellular organisms
that can be seen using
a compound light
microscope:
(a) non-photosynthetic
plant cells (onion)
The structure of cells (as seen using a light microscope)
■ identify cell organelles seen with current light and
electron microscopes
The general contents of cells can be
studied using the light microscope, but
if more detailed information is required
an electron microscope must be used.
Cells vary greatly in shape, size,
structure and function. There is, in
reality, no such thing as a ‘typical’
cell. The majority of cells that form
the tissues and organs of an organism
become highly specialised for particular
functions, for example lung tissue and
blood tissue. To allow an understanding
of the general structure and functioning
of cells, a hypothetical or ‘typical’ cell
of plants and one of animals is often
studied, as shown in Figures 1.12
and 1.13 (light microscope) and 1.16
(electron microscope).
(a)
Cells that are found in plants and
animals have the same basic features,
with some variations. The cell parts
discussed below are those that are
visible with a light microscope:
■ It is in the protoplasm of cells
that the functions essential to life,
such as growth and respiration, are
carried out. The cytoplasm (that
part of the protoplasm outside of
the nucleus) consists of a liquidbased
background, the cytosol,
in which there are dissolved
chemical substances (e.g. ions
such as chloride ions), suspended
organelles and insoluble granules.
Approximately 90 per cent of the
cytoplasm is water—the medium
in which all cell chemicals are
dissolved or suspended.
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