CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
Coursebook answers
• the idea that increasing concentrations of
Chapter 1 carbon dioxide in the atmosphere are
causing global warming
Before you start • the controversy about whether the measles,
• Learners should be familiar with some of the mumps and rubella (MMR) vaccine can
structures seen in cells from work done at increase the risk of autism. Scientists are
IGCSE. Figures 1.4 and 1.5 will refresh their now agreed that there is no link between the
memory of cell structure. MMR vaccine and autism.
• The functions of the main structures are found
in the sections ‘Features that animal and plant Self-assessment questions
cells have in common’ and ‘Differences
between animal and plant cells’. 1 a Structures that animal and plant cells have
in common:
• As well as Figures 1.4 and 1.5, they will find
relevant information in the section ‘Differences • nucleus with nucleolus and chromatin
between animal and plant cells’. • cytoplasm containing mitochondria,
• Yes, there are organisms other than animals and Golgi apparatus and other small
plants. They are classified in different structures
kingdoms which learners will learn about later. • cell surface membrane.
Other types of organism include fungi, a group
b Structures found only in plant cells:
of mainly unicellular organisms called
protoctists, bacteria and viruses. • chloroplasts
• large, permanent central vacuole
Science in Context • cell wall with middle lamella and
• Two obvious examples are: plasmodesmata.
• Darwin and Wallace’s theory of evolution c Structure found only in animal cells:
by natural selection was highly controversial
• centriole.
because it appeared to be in conflict with the
religious belief that God created all species 2 • Use a sharp pencil.
of living things and that humans were a • Do not use shading / do not draw the
special creation. nucleus as a solid blob.
• Galileo was placed under house arrest for
• Do not cross label lines.
the rest of his life after putting forward the
idea that Earth and the other planets orbited • Do not use arrowheads on label lines.
the Sun rather than the Earth being at the • Use a ruler to draw label lines.
centre of the solar system. Again, this
• Make outline of cells less sketchy – lines
appeared to contradict the religious beliefs
should be continuous, not broken.
of the time. Other examples include:
• Write labels horizontally, not at the same
• Einstein’s theory of general relativity (which
angle as the label line.
challenged our ideas of the link between
space and time) • Interpret what you see. For example,
• Wegener’s theory of continental drift (which outlines are not very accurate (too rounded).
was controversial because scientists could 3 a a ctual diameter = 20 µm (see caption)
not explain how continents could move).
diameter on diagram = 58 mm =
• Modern scientific controversies include:
1
, CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
58 000 µm • The nucleus is seen to be surrounded by a
magnification, M = size of image, double membrane (envelope) with pores in
it.
= • Mitochondria have a surrounding double
actual size of specimen I A membrane (envelope), the inner layer
forming finger-like folds pointing inwards.
= • Endoplasmic reticulum is extensive
therefore magnification = ×2900 b throughout the cell, some with ribosomes
(rough) and some without (smooth).
magnification = ×16 000 (see caption)
• Small structures seen under the light
length on micrograph = 65 mm = microscope can be distinguished by the
65 000 µm electron microscope as lysosomes and
size of specimen, A = size of image, vesicles.
I • Free ribosomes are seen throughout the cell.
= • The centriole is seen to be two separate
magnification M
centrioles.
= • Finger-like extensions of the cell surface
therefore actual size of chloroplast = 4.1 µm membrane, known as microvilli, are seen.
4 The resolution of a microscope is limited by the • Microtubules are visible in the cytoplasm.
radiation used to view the specimen. Resolution
7 Detail in a plant cell seen with the electron
equals half the wavelength of the radiation used.
microscope but not apparent using a light
The shortest wavelength of light is 400 nm,
microscope:
therefore the resolution of a light microscope is
200 nm. The diameter of a ribosome is much • In the nucleus, chromatin can be
smaller than this, namely 25 nm. distinguished.
5 • The nuclear membrane can be seen as a
double structure (envelope), continuous with
Feature Light Electron
the rough endoplasmic reticulum, and with
microscope microscope pores in it.
source of light electrons • There is extensive rough and smooth
radiation endoplasmic reticulum throughout the cell.
wavelength 400–700 nm about 0.005 nm • Free ribosomes are visible in the cytoplasm.
of radiation
• Microtubules are visible in the cytoplasm.
used
• The mitochondria have a double membrane
maximum 200 nm 0.5 nm in (envelope), the inner layer having folds into
resolution practice the matrix.
lenses glass electromagnets • Chloroplasts have a double outer membrane
specimen living, nonliving non-living or (envelope).
or dead dead • Grana can be seen in the chloroplast, as
stains coloured dyes heavy metals stacks of sacs connected to other grana by
longer sacs (thylakoids).
image coloured black and white 8 a diameter of nucleus (I)
6 Detail in an animal cell seen with the electron = 75 mm = 75 000 µm
microscope but not apparent using a light magnification (M) = ×11 000 therefore
microscope:
• In the nucleus, chromatin can be actual diameter of nucleus (A) = =
distinguished. 6.8 µm
b The nucleus may not have been
sectioned at its widest part. If you try cutting
2
, CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
a model sausage, the cut surface of a Mitochondrion Site of ATP synthesis in
transverse section will look like a circle. The aerobic respiration
cut surface of a longitudinal section will
look like a sausage. Other sections will be Golgi apparatus Makes lysosomes
somewhere between circles and sausages.
Cilium Has a ‘9 + 2’ arrangement of
9 Cilia in the oviduct beat in order to move the microtubules
egg from the ovary to the uterus. (It may be
Lysosome Mainly contains digestive
fertilised along the way.)
enzymes
10 Cell surface membrane: essential because it
There are some obvious variations on this activity.
forms a partially permeable barrier between
Both the structures and the descriptions can be
the cell and its environment, regulating
varied. It is useful for students to choose their own
movement of materials into and out of the cell.
terms and descriptions because it reinforces learning.
This is necessary to maintain an environment
The number of cards can be varied to suit the number
inside the cell which is different from that
of students involved.
outside the cell.
Cytoplasm: site of metabolic activity. It contains
biochemicals in solution.
Reflection
Ribosomes: sites of protein synthesis, an
What is a cell?
essential activity of all cells. (DNA controls A cell is the basic unit of life. All living organisms
cells by controlling which proteins are made.) are made of cells. All cells have a cell surface
Protein synthesis is a complex process membrane, cytoplasm and genetic material in the
involving the interaction of many molecules; form of DNA. All cells have a protein synthesising
the ribosome provides a site where this can machinery which includes ribosomes.
happen in an organised way.
Why are all living things made of cells?
DNA: the genetic material. DNA contains the
The biochemistry of life must be separated from the
information which controls the activities of the
surrounding environment. This separation is achieved
cell. It can replicate itself, enabling new cells to
by the cell surface membrane. The cell surface
be formed.
membrane is partially permeable and controls what
Cell wall (absent in animal cells): prevents the enters and leaves the cell. Without it, the chemicals
cell from bursting as a result of osmosis if it is of life would mix with the chemicals of the
exposed to a solution of higher water potential. environment and a separate chemistry of life would
Flagellum: needed for locomotion by some cells. be impossible.
Practical activity Comparing the success of prokaryotic and
Here is the table with the terms and descriptions eukaryotic cells
correctly matched. Criteria could be:
Chloroplast Photosynthesis occurs in this • complexity of cells
organelle • complexity of the whole organism
Nucleus Chromosomes are found in • size
this structure in eukaryotic • total biomass on the planet
cells
• range of habitats they can live in
Ribosomes These are found on rough
• ability to survive adverse conditions • ability to
endoplasmic reticulum (RER)
regulate their internal environments
Cell wall This structure contains • number and variety of species.
cellulose as a strengthening Is this a meaningless question?
material This question probably is a meaningless question,
Nucleolus Makes ribosomes although it doesn’t stop people discussing it. All
living things have evolved to be adapted to their
particular environments (ecological niches) and to
3
, CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
this extent are equally successful. Judgements can be (smooth) endoplasmic
made in various ways. Eukaryotes are far more reticulum ; rough endoplasmic
complex than prokaryotes. Humans (probably the reticulum ; 25 nm / larger / 80S
most intelligent eukaryotes) have the greatest control ribosomes ; linear / non-circular
over their environments. Bacteria have the greatest DNA ; chromatin ; lysosome(s)
biomass and are the most versatile of cells. They are ; Golgi apparatus ;
probably the most likely cells to survive a mitochondrion / mitochondria ;
catastrophic change in the Earth’s climate/ microtubule(s) ; centriole(s) ;
environment. centrosome ; vacuole(s) ;
microvillus / microvilli ;
cilium / cilia ; nucleolus /
Exam-style questions nucleoli ; nuclear envelope ;
The mark schemes, suggested answers and comments nuclear pore(s) ;
that appear here were written by the author(s). In AVP ; [max. 10] [Total: 10]
examinations, the way marks would be awarded to
answers like these may be different. 5 a magnification is the number of times
larger an image is compared with the real
Notes about mark schemes size of the object ; AW
A or accept indicates an alternative acceptable resolution is the ability to
answer. distinguish between two separate points / the
R = reject. This indicates a possible answer that greater the resolution, the greater the detail
should be rejected. that can be seen ; AW
; The bold semicolon indicates the award of 1 a statement linking the terms,
mark. such as both terms used with reference
to
/ This indicates an alternative answer for the same
microscopy ; [3] b light microscope
mark. The alternatives may be separated from the
rest of the answer by commas. ( ) Text in brackets uses light as a source of radiation ;
is not required for the mark. Underlining This is electron microscope uses electrons
used to indicate essential word(s) that must be used as a source of radiation ; [2] c b oth
to get the mark. organelles / both found in eukaryotic cells ;
AW means ‘alternative wording’. It is used to nucleolus is located inside
indicate that a different wording is acceptable nucleus ; nucleus controls cell
provided the essential meaning is the same, and it is
used where students’ responses are likely to vary activity ; nucleolus makes
more than usual. ribosomes ;
AVP means ‘additional valid point’. This means AVP ; ; e.g. nucleus surrounded
accept any additional points given by the student that by envelope, no membrane round
are not in the mark scheme, provided they are nucleolus [max. 4]
relevant. But accept only as many additional points d c hromatin and chromosomes both contain DNA
as indicated by the bold semicolons, e.g. AVP ; ; (and protein / histones /
means award a maximum of 2 extra marks. RNA) / both found in nucleus ;
ORA means ‘or reverse argument’ and is used when chromatin is the loosely coiled form of
the same idea could be expressed in the chromosomes ;
reverse way. For example: ‘activity increases chromatin is the form that exists
between pH2 and pH5 ORA’ means accept ‘activity between cell / nuclear divisions ;
decreases between pH5 and pH2’. max. This
chromosomes are formed just
indicates the maximum number of marks that can be
given. before / during, cell / nuclear division ;
[max. 3]
1 A; [1] 2 C; [1] 3 B;
e an envelope consists of two membranes
[1]
(one just inside / outside the other) ;
4 nucleus ; a membrane is a thin (partially
4
Coursebook answers
• the idea that increasing concentrations of
Chapter 1 carbon dioxide in the atmosphere are
causing global warming
Before you start • the controversy about whether the measles,
• Learners should be familiar with some of the mumps and rubella (MMR) vaccine can
structures seen in cells from work done at increase the risk of autism. Scientists are
IGCSE. Figures 1.4 and 1.5 will refresh their now agreed that there is no link between the
memory of cell structure. MMR vaccine and autism.
• The functions of the main structures are found
in the sections ‘Features that animal and plant Self-assessment questions
cells have in common’ and ‘Differences
between animal and plant cells’. 1 a Structures that animal and plant cells have
in common:
• As well as Figures 1.4 and 1.5, they will find
relevant information in the section ‘Differences • nucleus with nucleolus and chromatin
between animal and plant cells’. • cytoplasm containing mitochondria,
• Yes, there are organisms other than animals and Golgi apparatus and other small
plants. They are classified in different structures
kingdoms which learners will learn about later. • cell surface membrane.
Other types of organism include fungi, a group
b Structures found only in plant cells:
of mainly unicellular organisms called
protoctists, bacteria and viruses. • chloroplasts
• large, permanent central vacuole
Science in Context • cell wall with middle lamella and
• Two obvious examples are: plasmodesmata.
• Darwin and Wallace’s theory of evolution c Structure found only in animal cells:
by natural selection was highly controversial
• centriole.
because it appeared to be in conflict with the
religious belief that God created all species 2 • Use a sharp pencil.
of living things and that humans were a • Do not use shading / do not draw the
special creation. nucleus as a solid blob.
• Galileo was placed under house arrest for
• Do not cross label lines.
the rest of his life after putting forward the
idea that Earth and the other planets orbited • Do not use arrowheads on label lines.
the Sun rather than the Earth being at the • Use a ruler to draw label lines.
centre of the solar system. Again, this
• Make outline of cells less sketchy – lines
appeared to contradict the religious beliefs
should be continuous, not broken.
of the time. Other examples include:
• Write labels horizontally, not at the same
• Einstein’s theory of general relativity (which
angle as the label line.
challenged our ideas of the link between
space and time) • Interpret what you see. For example,
• Wegener’s theory of continental drift (which outlines are not very accurate (too rounded).
was controversial because scientists could 3 a a ctual diameter = 20 µm (see caption)
not explain how continents could move).
diameter on diagram = 58 mm =
• Modern scientific controversies include:
1
, CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
58 000 µm • The nucleus is seen to be surrounded by a
magnification, M = size of image, double membrane (envelope) with pores in
it.
= • Mitochondria have a surrounding double
actual size of specimen I A membrane (envelope), the inner layer
forming finger-like folds pointing inwards.
= • Endoplasmic reticulum is extensive
therefore magnification = ×2900 b throughout the cell, some with ribosomes
(rough) and some without (smooth).
magnification = ×16 000 (see caption)
• Small structures seen under the light
length on micrograph = 65 mm = microscope can be distinguished by the
65 000 µm electron microscope as lysosomes and
size of specimen, A = size of image, vesicles.
I • Free ribosomes are seen throughout the cell.
= • The centriole is seen to be two separate
magnification M
centrioles.
= • Finger-like extensions of the cell surface
therefore actual size of chloroplast = 4.1 µm membrane, known as microvilli, are seen.
4 The resolution of a microscope is limited by the • Microtubules are visible in the cytoplasm.
radiation used to view the specimen. Resolution
7 Detail in a plant cell seen with the electron
equals half the wavelength of the radiation used.
microscope but not apparent using a light
The shortest wavelength of light is 400 nm,
microscope:
therefore the resolution of a light microscope is
200 nm. The diameter of a ribosome is much • In the nucleus, chromatin can be
smaller than this, namely 25 nm. distinguished.
5 • The nuclear membrane can be seen as a
double structure (envelope), continuous with
Feature Light Electron
the rough endoplasmic reticulum, and with
microscope microscope pores in it.
source of light electrons • There is extensive rough and smooth
radiation endoplasmic reticulum throughout the cell.
wavelength 400–700 nm about 0.005 nm • Free ribosomes are visible in the cytoplasm.
of radiation
• Microtubules are visible in the cytoplasm.
used
• The mitochondria have a double membrane
maximum 200 nm 0.5 nm in (envelope), the inner layer having folds into
resolution practice the matrix.
lenses glass electromagnets • Chloroplasts have a double outer membrane
specimen living, nonliving non-living or (envelope).
or dead dead • Grana can be seen in the chloroplast, as
stains coloured dyes heavy metals stacks of sacs connected to other grana by
longer sacs (thylakoids).
image coloured black and white 8 a diameter of nucleus (I)
6 Detail in an animal cell seen with the electron = 75 mm = 75 000 µm
microscope but not apparent using a light magnification (M) = ×11 000 therefore
microscope:
• In the nucleus, chromatin can be actual diameter of nucleus (A) = =
distinguished. 6.8 µm
b The nucleus may not have been
sectioned at its widest part. If you try cutting
2
, CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
a model sausage, the cut surface of a Mitochondrion Site of ATP synthesis in
transverse section will look like a circle. The aerobic respiration
cut surface of a longitudinal section will
look like a sausage. Other sections will be Golgi apparatus Makes lysosomes
somewhere between circles and sausages.
Cilium Has a ‘9 + 2’ arrangement of
9 Cilia in the oviduct beat in order to move the microtubules
egg from the ovary to the uterus. (It may be
Lysosome Mainly contains digestive
fertilised along the way.)
enzymes
10 Cell surface membrane: essential because it
There are some obvious variations on this activity.
forms a partially permeable barrier between
Both the structures and the descriptions can be
the cell and its environment, regulating
varied. It is useful for students to choose their own
movement of materials into and out of the cell.
terms and descriptions because it reinforces learning.
This is necessary to maintain an environment
The number of cards can be varied to suit the number
inside the cell which is different from that
of students involved.
outside the cell.
Cytoplasm: site of metabolic activity. It contains
biochemicals in solution.
Reflection
Ribosomes: sites of protein synthesis, an
What is a cell?
essential activity of all cells. (DNA controls A cell is the basic unit of life. All living organisms
cells by controlling which proteins are made.) are made of cells. All cells have a cell surface
Protein synthesis is a complex process membrane, cytoplasm and genetic material in the
involving the interaction of many molecules; form of DNA. All cells have a protein synthesising
the ribosome provides a site where this can machinery which includes ribosomes.
happen in an organised way.
Why are all living things made of cells?
DNA: the genetic material. DNA contains the
The biochemistry of life must be separated from the
information which controls the activities of the
surrounding environment. This separation is achieved
cell. It can replicate itself, enabling new cells to
by the cell surface membrane. The cell surface
be formed.
membrane is partially permeable and controls what
Cell wall (absent in animal cells): prevents the enters and leaves the cell. Without it, the chemicals
cell from bursting as a result of osmosis if it is of life would mix with the chemicals of the
exposed to a solution of higher water potential. environment and a separate chemistry of life would
Flagellum: needed for locomotion by some cells. be impossible.
Practical activity Comparing the success of prokaryotic and
Here is the table with the terms and descriptions eukaryotic cells
correctly matched. Criteria could be:
Chloroplast Photosynthesis occurs in this • complexity of cells
organelle • complexity of the whole organism
Nucleus Chromosomes are found in • size
this structure in eukaryotic • total biomass on the planet
cells
• range of habitats they can live in
Ribosomes These are found on rough
• ability to survive adverse conditions • ability to
endoplasmic reticulum (RER)
regulate their internal environments
Cell wall This structure contains • number and variety of species.
cellulose as a strengthening Is this a meaningless question?
material This question probably is a meaningless question,
Nucleolus Makes ribosomes although it doesn’t stop people discussing it. All
living things have evolved to be adapted to their
particular environments (ecological niches) and to
3
, CAMBRIDGE INTERNATIONAL AS & A LEVEL BIOLOGY: COURSEBOOK
this extent are equally successful. Judgements can be (smooth) endoplasmic
made in various ways. Eukaryotes are far more reticulum ; rough endoplasmic
complex than prokaryotes. Humans (probably the reticulum ; 25 nm / larger / 80S
most intelligent eukaryotes) have the greatest control ribosomes ; linear / non-circular
over their environments. Bacteria have the greatest DNA ; chromatin ; lysosome(s)
biomass and are the most versatile of cells. They are ; Golgi apparatus ;
probably the most likely cells to survive a mitochondrion / mitochondria ;
catastrophic change in the Earth’s climate/ microtubule(s) ; centriole(s) ;
environment. centrosome ; vacuole(s) ;
microvillus / microvilli ;
cilium / cilia ; nucleolus /
Exam-style questions nucleoli ; nuclear envelope ;
The mark schemes, suggested answers and comments nuclear pore(s) ;
that appear here were written by the author(s). In AVP ; [max. 10] [Total: 10]
examinations, the way marks would be awarded to
answers like these may be different. 5 a magnification is the number of times
larger an image is compared with the real
Notes about mark schemes size of the object ; AW
A or accept indicates an alternative acceptable resolution is the ability to
answer. distinguish between two separate points / the
R = reject. This indicates a possible answer that greater the resolution, the greater the detail
should be rejected. that can be seen ; AW
; The bold semicolon indicates the award of 1 a statement linking the terms,
mark. such as both terms used with reference
to
/ This indicates an alternative answer for the same
microscopy ; [3] b light microscope
mark. The alternatives may be separated from the
rest of the answer by commas. ( ) Text in brackets uses light as a source of radiation ;
is not required for the mark. Underlining This is electron microscope uses electrons
used to indicate essential word(s) that must be used as a source of radiation ; [2] c b oth
to get the mark. organelles / both found in eukaryotic cells ;
AW means ‘alternative wording’. It is used to nucleolus is located inside
indicate that a different wording is acceptable nucleus ; nucleus controls cell
provided the essential meaning is the same, and it is
used where students’ responses are likely to vary activity ; nucleolus makes
more than usual. ribosomes ;
AVP means ‘additional valid point’. This means AVP ; ; e.g. nucleus surrounded
accept any additional points given by the student that by envelope, no membrane round
are not in the mark scheme, provided they are nucleolus [max. 4]
relevant. But accept only as many additional points d c hromatin and chromosomes both contain DNA
as indicated by the bold semicolons, e.g. AVP ; ; (and protein / histones /
means award a maximum of 2 extra marks. RNA) / both found in nucleus ;
ORA means ‘or reverse argument’ and is used when chromatin is the loosely coiled form of
the same idea could be expressed in the chromosomes ;
reverse way. For example: ‘activity increases chromatin is the form that exists
between pH2 and pH5 ORA’ means accept ‘activity between cell / nuclear divisions ;
decreases between pH5 and pH2’. max. This
chromosomes are formed just
indicates the maximum number of marks that can be
given. before / during, cell / nuclear division ;
[max. 3]
1 A; [1] 2 C; [1] 3 B;
e an envelope consists of two membranes
[1]
(one just inside / outside the other) ;
4 nucleus ; a membrane is a thin (partially
4
