CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
Sample answers have been written by the authors.
Coursebook answers
Chapter 1
Self-assessment questions
1
aStructures that animal and plant cells have
in common:
b
c
1
aactual diameter = 20 µm (see caption)
•
nucleus with nucleolus and chromatin
diameter on diagram = 58 mm =
58 000 µm
•
cytoplasm containing mitochondria,
Golgi apparatus and other small
structures
magnification, M = size of image,
I
=
actual size of specimen A
•
cell surface membrane.
=
Structures found only in plant cells:
•
chloroplasts
•
large, permanent central vacuole
•
cell wall with middle lamella and
plasmodesmata.
58 000
20
therefore magnification = ×2900
b
magnification = ×16 000 (see caption)
length on micrograph = 65 mm =
65 000 µm
size of specimen, A = size of image,
Structure found only in animal cells:
•
2
3
=
centriole.
I
magnification M
65 000
16 000
therefore actual size of chloroplast = 4.1 µm
=
•
Use a sharp pencil.
•
Do not use shading / do not draw the
nucleus as a solid blob.
•
Do not cross label lines.
•
Do not use arrowheads on label lines.
•
Use a ruler to draw label lines.
•
Make outline of cells less sketchy – lines
should be continuous, not broken.
•
Write labels horizontally, not at the same
angle as the label line.
•
Interpret what you see. For example, outlines
are not very accurate (too rounded).
4
The resolution of a microscope is limited by the
radiation used to view the specimen. Resolution
equals half the wavelength of the radiation used.
The shortest wavelength of light is 400 nm,
therefore the resolution of a light microscope
is 200 nm. The diameter of a ribosome is much
smaller than this, namely 25 nm.
Cambridge International AS & A Level Biology © Cambridge University Press 2020
CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
5
Feature
source of
radiation
wavelength
of radiation
used
maximum
resolution
lenses
specimen
Electron
microscope
electrons
400–700 nm
about 0.005 nm
stains
glass
living, nonliving or dead
coloured dyes
0.5 nm in
practice
electromagnets
non-living or
dead
heavy metals
image
coloured
black and white
6
7
200 nm
Detail in an animal cell seen with the electron
microscope but not apparent using a light
microscope:
•
In the nucleus, chromatin can be
distinguished.
•
The nucleus is seen to be surrounded by a
double membrane (envelope) with pores
in it.
•
Mitochondria have a surrounding double
membrane (envelope), the inner layer
forming finger-like folds pointing inwards.
•
Endoplasmic reticulum is extensive
throughout the cell, some with ribosomes
(rough) and some without (smooth).
•
Small structures seen under the light
microscope can be distinguished by the
electron microscope as lysosomes and
vesicles.
•
Free ribosomes are seen throughout
the cell.
•
The centriole is seen to be two separate
centrioles.
•
Finger-like extensions of the cell surface
membrane, known as microvilli, are seen.
•
Microtubules are visible in the cytoplasm.
Detail in a plant cell seen with the electron
microscope but not apparent using a light
microscope:
•
2
Light
microscope
light
In the nucleus, chromatin can be
distinguished.
•
The nuclear membrane can be seen as a
double structure (envelope), continuous
with the rough endoplasmic reticulum,
and with pores in it.
•
There is extensive rough and smooth
endoplasmic reticulum throughout the cell.
•
Free ribosomes are visible in the cytoplasm.
•
Microtubules are visible in the cytoplasm.
•
The mitochondria have a double
membrane (envelope), the inner layer
having folds into the matrix.
•
Chloroplasts have a double outer
membrane (envelope).
•
Grana can be seen in the chloroplast, as
stacks of sacs connected to other grana by
longer sacs (thylakoids).
8
adiameter of nucleus (I)
= 75 mm = 75 000 µm
magnification (M) = ×11 000
therefore actual diameter
75 000
of nucleus (A) =
= 6.8 µm
11 000
bThe nucleus may not have been sectioned
at its widest part. If you try cutting
a model sausage, the cut surface of a
transverse section will look like a circle.
The cut surface of a longitudinal section
will look like a sausage. Other sections
will be somewhere between circles and
sausages.
9
Cilia in the oviduct beat in order to move the
egg from the ovary to the uterus. (It may be
fertilised along the way.)
10 Cell surface membrane: essential because it
forms a partially permeable barrier between
the cell and its environment, regulating
movement of materials into and out of
the cell. This is necessary to maintain an
environment inside the cell which is different
from that outside the cell.
Cytoplasm: site of metabolic activity. It
contains biochemicals in solution.
Ribosomes: sites of protein synthesis, an
essential activity of all cells. (DNA controls
cells by controlling which proteins are
made.) Protein synthesis is a complex process
involving the interaction of many molecules;
Cambridge International AS & A Level Biology © Cambridge University Press 2020
CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
the ribosome provides a site where this can
happen in an organised way.
Comparing the success of
prokaryotic and eukaryotic cells
DNA: the genetic material. DNA contains the
information which controls the activities of
the cell. It can replicate itself, enabling new
cells to be formed.
Criteria could be:
Cell wall (absent in animal cells): prevents the
cell from bursting as a result of osmosis if it is
exposed to a solution of higher water potential.
Flagellum: needed for locomotion by some cells.
Reflection
What is a cell?
A cell is the basic unit of life. All living organisms
are made of cells. All cells have a cell surface
membrane, cytoplasm and genetic material in the
form of DNA. All cells have a protein synthesising
machinery which includes ribosomes.
Why are all living things made of cells?
The biochemistry of life must be separated from
the surrounding environment. This separation is
achieved by the cell surface membrane. The cell
surface membrane is partially permeable and
controls what enters and leaves the cell. Without it,
the chemicals of life would mix with the chemicals
of the environment and a separate chemistry of
life would be impossible.
3
•
complexity of cells
•
complexity of the whole organism
•
size
•
total biomass on the planet
•
range of habitats they can live in
•
ability to survive adverse conditions
•
ability to regulate their internal environments
•
number and variety of species.
Is this a meaningless question?
This question probably is a meaningless question,
although it doesn’t stop people discussing it. All
living things have evolved to be adapted to their
particular environments (ecological niches) and to
this extent are equally successful. Judgements can
be made in various ways. Eukaryotes are far more
complex than prokaryotes. Humans (probably
the most intelligent eukaryotes) have the greatest
control over their environments. Bacteria have the
greatest biomass and are the most versatile of cells.
They are probably the most likely cells to survive
a catastrophic change in the Earth’s climate/
environment.
Cambridge International AS & A Level Biology © Cambridge University Press 2020