Cambridge International A Level Biology Revision Guide

Cambridge International AS Level Biology
Prokaryotes
average diameter of cell is 0.5–5 μm
DNA is circular and lies free in the
cytoplasm
DNA is naked
slightly smaller (70S) ribosomes (about
20 nm diameter) than those of eukaryotes
no ER present
very few cell organelles – no separate
membrane-bound compartments unless
formed by infolding of the cell surface
membrane
cell wall present – wall contains murein,
a peptidoglycan (a polysaccharide
combined with amino acids)
Eukaryotes
cells commonly up to 40 μm diameter and commonly 1000–10 000 times the volume
of prokaryotic cells
DNA is not circular and is contained in a nucleus – the nucleus is surrounded by an
envelope of two membranes
DNA is associated with protein, forming structures called chromosomes
slightly larger (80S) ribosomes (about 25 nm diameter) than those of prokaryotes
ER present, to which ribosomes may be attached
many types of cell organelle present (extensive compartmentalisation and division
of labour):
■ some organelles are bounded by a single membrane, e.g. lysosomes, Golgi body,
vacuoles
■ some are bounded by two membranes (an envelope), e.g. nucleus,
mitochondrion, chloroplast
■ some have no membrane, e.g. ribosomes, centrioles, microtubules
cell wall sometimes present, e.g. in plants and fungi – contains cellulose or lignin in
plants, and chitin (a nitrogen-containing polysaccharide similar to cellulose) in fungi
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Table 1.2 A comparison of prokaryotic and eukaryotic cells.
Figure 1.31 shows the structure of a simple virus. It has
a very symmetrical shape. Its protein coat (or capsid) is
made up of separate protein molecules, each of which is
called a capsomere.
Viruses range in size from about 20–300 nm (about 50
times smaller on average than bacteria).
All viruses are parasitic because they can only
reproduce by infecting and taking over living cells. The
virus DNA or RNA takes over the protein synthesising
machinery of the host cell, which then helps to make new
virus particles.
capsid
protein
molecules
DNA or RNA
genetic code
Figure 1.31 The structure of a simple virus.
Summary
■■
The basic unit of life, the cell, can be seen clearly only
with the aid of microscopes. The light microscope uses
light as a source of radiation, whereas the electron
microscope uses electrons. The electron microscope has
greater resolution (allows more detail to be seen) than
the light microscope, because electrons have a shorter
wavelength than light.
■■
■■
With a light microscope, cells may be measured using
an eyepiece graticule and a stage micrometer. Using
I
the formula A = the actual size of an object (A) or its
M
magnification (M) can be found if its observed (image)
size (I) is measured and A or M, as appropriate, is known.
All cells are surrounded by a partially permeable cell
surface membrane that controls exchange between
the cell and its environment. All cells contain genetic
material in the form of DNA, and ribosomes for
protein synthesis.