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•
Water evaporates from this layer and diffuses out of a leaf through the stoma.
Many plants have modified stomata or leaf surfaces to reduce this water loss.
Handout 4.22
The leaf: an important
plant organ
•
A typical leaf is wide and flat to absorb as much sunlight as possible. The top
and bottom sides are different. One side is positioned facing the Sun. Leaves are
arranged on stems so that they shade each other as little as possible. They are
thin, to reduce the distance over which carbon dioxide has to diffuse into the
cells from the air (or water in the case of aquatic plants). They have a welldeveloped system of veins, which transports water to the cells and sucrose
away to be stored.
•
The palisade cells nearest the top side of the leaf have lots of chloroplasts.
•
Leaves have many pores (stomata, singular stoma) to let carbon dioxide in.
There are many air spaces (see diagram) in the leaf to allow the carbon dioxide
to diffuse easily to individual cells.
•
The mesophyll cells have a thin water layer on the outside, to allow carbon
dioxide to dissolve so that it can pass into the cells to the chloroplasts.
•
Water evaporates from this layer and diffuses out of a leaf through the stoma.
Many plants have modified stomata or leaf surfaces to reduce this water loss.
111 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Task L: Cross-sections of leaves
8 minutes
The last eight minutes of this session are available for participants to look at some
cross-sections of leaves under the microscope and see if they can identify and
name the various cells. This helps to consolidate the microscope work they did in
session 1 and gives them a feel for the range of specialist cells and the part they
play in photosynthesis.
Reproduction
18 minutes
Introduce this final part of the session by explaining that pupils in Year 9 need to
build on their earlier knowledge of reproduction and cells. They need to recognise
that an organism possesses features that are inherited from information contained
in the male and female reproductive cells, which join during fertilisation. Achieving
the Year 9 yearly teaching objectives is important to prepare pupils for the genetics
taught in more detail during Key Stage 4.
Handout 4.23
Read out the poem ‘Where do babies come from?’ (handout 4.23).
Where do babies come from?
Handout 4.23
I’m sure when you were very young,
you used to say to dad or mum,
Mummy? Daddy? Where do babies come from?
Well your mum and dad reply
and suddenly they go all shy.
Well my dear you’ll learn some day
how you have turned out this way.
A little creature called a sperm
which lives inside a man
will swim towards a tiny egg
as fast as it possibly can.
The egg just waits inside a woman
waiting for a sperm.
It really is quite complicated
there’s quite a lot to learn.
The sperm with his little wiggly tail
and its pointy head
finds its way into the egg
while your parents are in bed.
Slowly then the egg splits and grows
as large as it can go.
Then forms into a sort of baby
called an embryo.
The embryo then grows and grows
and gets all its bits and pieces.
Then it grows a little bit bigger
and this is called a foetus.
In the next nine months your mum gets fat,
starts buying the baby clothes and hats.
She then goes into hospital
to wait for the baby to be born.
She screams and shouts
while the baby comes out, and daddy holds her hand.
Then you give it the name you want
and then you can take it home!
The above poem is reproduced with kind permission of the ASE. ‘Where do babies
come from?’ is included in Science is like a tub of ice-cream – cool and fun, edited
by Rosemary Feasey and published by the ASE (2001). (ISBN 086357 3223)
112 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
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99 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
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Say that:
Slide 4.24
•
This poem was written by a 13-year-old schoolgirl and is one of the poems in
the ASE science poetry book, Science is like a tub of ice cream – cool and fun.
•
This session focuses on basic knowledge of how the cell nucleus transfers
characteristics from parent to offspring. It does not deal with the mechanics of
reproduction.
Show slide 4.24.
Slide 4.24
Inherited characteristics – a common misconception
• By Year 9, pupils are confident that organisms inherit characteristics from both
male and female parents.
• However, when they consider the characteristics they have inherited, many believe
that boys, because they are male, inherit most of their dominant characteristics
from their father. Girls, because they are female, inherit most of their dominant
characteristics from their mother.
Say that this misconception is often not well articulated by the pupils but is just
something they believe to be true.
Slide 4.25
Show slide 4.25, which indicates the inheritance themes embodied in the Year 9
yearly teaching objectives.
Slide 4.25
Inheritance themes in Year 9
• Offspring generally have two parents.
• This results in variation of combinations of inherited characteristics.
• Offspring will therefore vary.
• Selective breeding increases the chances of desirable characteristics being
inherited.
• It is the chromosomes in the cell nucleus which carry the information to determine
the characteristics of the organism.
Handout 4.26
More information is given in handout 4.26. Refer participants to the handout and
go through the points made.
Handout 4.26
The inheritance themes in
Year 9
•
Offspring generally have two parents unless they are hermaphrodites.
•
This means that offspring inherit characteristics from both parents but in
combinations which are difficult to predict.
•
This means that offspring will vary both from their parents and from their
siblings.
•
By choosing the parents carefully, it is possible to increase the chances that
certain desirable characteristics are inherited by the offspring.
•
Nature exerts some influence over which parents mate successfully, because
only those which are best adapted to their environment survive long enough to
reproduce.
•
In some species of animal it is the strongest male that mates with the females,
and this increases the chance that the offspring will also be strong.
•
Humans, by choosing which animals are allowed to mate (such as in dogs or
horses), can increase the chances of desirable characteristics being passed on
to the offspring.
113 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
100 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Slide 4.27
Show slide 4.27, which shows a cross-section of a typical animal cell.
Slide 4.27
Typical animal cell
nucleus
Handout 4.28
Ask participants to retrieve handout 4.28 and go through the points made on it,
referring as appropriate to slide 4.27.
Handout 4.28
The cell nucleus and its
chromosomes
•
The nucleus of a cell contains rod-shaped structures called chromosomes.
•
Chromosomes are only visible when a cell is about to divide.
•
There are two full sets of chromosomes, one set which comes from the female
parent and the other from the male parent.
•
Chromosomes comprise a very long chain molecule of a complex chemical
known as DNA (deoxyribonucleic acid).
•
Short lengths of DNA control different characteristics of an organism. Each
such section of a chromosome is called a gene.
•
Any chromosome is made up of many genes, often with non-functioning
lengths of DNA between them.
•
When a normal cell divides in two for growth of an organism, the process
ensures that each new cell has exactly the same chromosomes as the original
cell.
•
In order to produce male sex cells (such as sperm) or female sex cells (such as
eggs), the process of cell division ensures that the sex cells have only half the
chromosomes of the normal cells in the organism.
•
This means that when a male sex cell fertilises a female sex cell, the offspring
regains the full number of chromosomes.
•
It is vitally important that all the cells of any organism have the same, and
correct, number of chromosomes for the species. Even one more or less can
have disastrous effects on the individual, often leading to death.
114 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
Slide 4.29
© Crown copyright 2003
Use slide 4.29 when explaining the final few points on the handout.
101 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Sex cell division
Slide 4.29
We can look at plant cells in a similar way. Make the point that all plants can
reproduce sexually.
Plants can also reproduce asexually, e.g. from cuttings or runners, as those
participants who are keen gardeners will know. The new plants produced in this
way have exactly the same chromosomes as the plant they were taken from, which
means that they will have the same characteristics as the parent plant. This is an
important feature which gardeners exploit to increase the number of plants with
one or more desirable characteristics. Most animals, and certainly all ‘higher’
animals, cannot reproduce asexually.
Variation between offspring and parents
Explain that the idea of variation between offspring and parents can be quite easily
illustrated by considering maleness and femaleness in humans. A human cell
contains 46 chromosomes in 23 pairs. One pair is special. Its chromosomes are
called the X and Y chromosomes, because they look a bit like an X or Y. A female
human always has two X chromosomes in this pair. A male human always has one
X and one Y chromosome. The Y chromosome is quite small and carries fewer
genes. The X chromosome is normal size.
Slide 4.30
Show slide 4.30, which is about the first part of the reproductive process – the
production of sperm or eggs. Remember that sperm cells and egg cells contain
half the chromosomes of normal cells.
102 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
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Chromosome allocation in sperm and eggs
Slide 4.30
• The male parent’s cells contain an X and a Y chromosome.
• Because a sperm cell has half of the chromosomes of normal cells, it will have
either an X or a Y chromosome.
• The female parent’s cells contain two X chromosomes.
• Because an egg cell has half of the chromosomes of normal cells, it will have only
one X chromosome.
Slide 4.31
Slide 4.31 gives a mathematical representation of possible combinations of X and
Y chromosomes (called a Punnett square). It is used to predict the range of
combinations of characteristics in offspring and the chance of any particular
combination occurring. Explain that this format is commonly used when
considering genetics and inheritance, but other ways of working and displaying the
information are also possible.
Determining sex in humans
Slide 4.31
Say that:
•
Each human has a 50% chance of being a boy.
Further detail on genetics is not part of the Key Stage 3 programme of study.
However the following task, which uses some Key Stage 4 understanding, will help
participants to follow how variation in offspring is ensured by sexual reproduction.
This activity can be used with Key Stage 3 pupils in school.
Task M: The chromosome game
5 minutes
Remind participants that each chromosome contains many genes and that genes
control the characteristics of an organism. Explain that many characteristics are
controlled by two or more genes working together, but some characteristics are
controlled by a single gene only. In task M we will assume that a single gene
controls each characteristic.
103 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Slide 4.32
Show slide 4.32, which has the instructions for the task. Handout 4.33 includes
these instructions, some additional notes and the materials for the task.
Handout 4.33
Task M: The chromosome game
Slide 4.32
• Work in pairs.
• Cut out the four chromosomes on handout 4.33 and sort them into a male pair
and a female pair.
• The male pair divide to become part of two different sperm cells. The female pair
divide to become part of two different egg cells.
• Move the chromosomes to identify what combinations of characteristics are
possible in any baby.
• If, over time, two babies are produced, what is the chance that they will have the
same hair colour?
You have 5 minutes for this task.
Handout 4.33
Task M: The chromosome
game
Sex
Male
Male
Female
Female
1 Hair colour
blonde
blonde
black
black
2 Hair type
straight
curly
curly
3 Eye colour
curly
blue
grey
grey
grey
4 Nose shape
round
pointed
round
pointed
5 Nose size
small
large
large
small
•
To make the game manageable, each ‘chromosome’ contains five genes, but in
reality a chromosome will contain many more than this. The five genes and the
variations which we are considering (this is also a considerable
oversimplification) are:
1
2
3
4
5
hair colour (black or blonde)
hair type (straight or curly)
eye colour (blue or grey)
nose shape (pointed or rounded)
nose size (large or small)
The embolded terms are dominant.
•
Work in pairs.
•
Cut out the four chromosomes and put them into a male pair and a female pair.
•
The male pair divides to become part of two different sperm cells. The female
pair divides into two different egg cells.
•
Sort the chromosomes to identify what combinations of characteristics are
possible in any baby.
•
If, over time, two babies are produced, what is the chance that they will have
the same hair colour?
115 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
T
© Crown copyright 2003
Additional guidance
This exercise makes use of hypothetical genes. It is almost certain that the reality of
the inheritance of hair, eye colour and nose shape is much more complex than
depicted. However the exercise will suit the purposes of the session.
When trying to resolve the characteristics of each potential offspring, discussion
may turn to what happens in the case where there are two different genes for a
characteristic. You may have to explain that some genes are dominant and others
recessive, and also that in some cases the genes are co-dominant or incompletely
dominant. For the purposes of this task, let us consider that the following are
dominant and will show in the offspring whenever they are present: black hair,
straight hair, blue eyes, pointed nose, large nose.
Take feedback and make the following points if they do not emerge explicitly:
•
Sexual reproduction inevitably leads to variation in the offspring in terms of the
combinations of characteristics which are possible.
•
The offspring will vary among themselves and will also have different
combinations of characteristics to either parent.
104 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
•
Evolution is dependent on variation; the combination of characteristics which
enable the organism to live most successfully are the ones most likely to be
passed on to the next generation of offspring. However, because variation
occurs at every generation there is never a guarantee of certain characteristics
being passed on.
•
Selective breeding means that by using only parents with desirable
characteristics (such as milk yield in cows, amount of grain produced by wheat,
disease resistance in food crops), humans can increase the chances that the
offspring will also have these characteristics. But note that it is inevitable that
(with enough offspring) some will not show the desired characteristics.
•
There are some unwanted consequences of inadvertent selective breeding. One
example is antibiotic resistance in harmful bacteria such as MRSI (multiple
resistant staphylococcal infection). The extensive use of antibiotics has killed off
many harmful bacteria, so that mainly those variants which are (for some
reason) naturally resistant to antibiotics survive to reproduce. This has led to an
increase in the proportion of resistant bacteria and hence to an increased
chance that humans will be infected by a resistant variant.
Make the following final points:
•
Inheritance of genes is much more complex than the simple example implies.
The principle of half the genetic material being supplied by each parent is real.
Pupils’ questions about characteristics need handling with sensitivity. The way
this is taught in individual schools is determined by each school’s relevant
policies, e.g. sex education, inclusion, PSHE and citizenship. Participants who
are not familiar with these policies should make sure that they read them before
teaching this element of cells.
•
The mechanisms of sexual reproduction have not been covered in this training.
Plenary
Slide 4.34
5 minutes
Show slide 4.34. Say that this session has included a great deal of theory about
the Year 9 yearly teaching objectives, but that you have tried to emphasise the
themes and threads which are needed by pupils for full understanding rather than
simple recall of knowledge.
Objective and outcomes for session 4
Slide 4.34
• To clarify the importance of some cellular processes, described in the Year 9
yearly teaching objectives, to the success of an organism
By the end of this session, participants should:
• be clear about the importance of respiration and photosynthesis, including some
misconceptions about them held by pupils;
• be able to articulate the similarities and differences between respiration and
photosynthesis;
• have clarified some aspects of how cells influence and control the characteristics
of an organism.
105 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Handout 4.26
The inheritance themes in
Year 9
•
Offspring generally have two parents unless they are hermaphrodites.
•
This means that offspring inherit characteristics from both parents but in
combinations which are difficult to predict.
•
This means that offspring will vary both from their parents and from their
siblings.
•
By choosing the parents carefully, it is possible to increase the chances that
certain desirable characteristics are inherited by the offspring.
•
Nature exerts some influence over which parents mate successfully, because
only those which are best adapted to their environment survive long enough to
reproduce.
•
In some species of animal it is the strongest male that mates with the females,
and this increases the chance that the offspring will also be strong.
•
Humans, by choosing which animals are allowed to mate (such as in dogs or
horses), can increase the chances of desirable characteristics being passed on
to the offspring.
113 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Handout 4.28
The cell nucleus and its
chromosomes
•
The nucleus of a cell contains rod-shaped structures called chromosomes.
•
Chromosomes are only visible when a cell is about to divide.
•
There are two full sets of chromosomes, one set which comes from the female
parent and the other from the male parent.
•
Chromosomes comprise a very long chain molecule of a complex chemical
known as DNA (deoxyribonucleic acid).
•
Short lengths of DNA control different characteristics of an organism. Each
such section of a chromosome is called a gene.
•
Any chromosome is made up of many genes, often with non-functioning
lengths of DNA between them.
•
When a normal cell divides in two for growth of an organism, the process
ensures that each new cell has exactly the same chromosomes as the original
cell.
•
In order to produce male sex cells (such as sperm) or female sex cells (such as
eggs), the process of cell division ensures that the sex cells have only half the
chromosomes of the normal cells in the organism.
•
This means that when a male sex cell fertilises a female sex cell, the offspring
regains the full number of chromosomes.
•
It is vitally important that all the cells of any organism have the same, and
correct, number of chromosomes for the species. Even one more or less can
have disastrous effects on the individual, often leading to death.
114 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003
Handout 4.33
Task M: The chromosome
game
Sex
Male
Male
Female
Female
1 Hair colour
blonde
blonde
black
black
2 Hair type
straight
curly
curly
curly
blue
grey
grey
grey
4 Nose shape
round
pointed
round
pointed
5 Nose size
small
large
large
small
3 Eye colour
•
To make the game manageable, each ‘chromosome’ contains five genes, but in
reality a chromosome will contain many more than this. The five genes and the
variations which we are considering (this is also a considerable
oversimplification) are:
1
2
3
4
5
hair colour (black or blonde)
hair type (straight or curly)
eye colour (blue or grey)
nose shape (pointed or rounded)
nose size (large or small)
The embolded terms are dominant.
•
Work in pairs.
•
Cut out the four chromosomes and put them into a male pair and a female pair.
•
The male pair divides to become part of two different sperm cells. The female
pair divides into two different egg cells.
•
Sort the chromosomes to identify what combinations of characteristics are
possible in any baby.
•
If, over time, two babies are produced, what is the chance that they will have
the same hair colour?
115 | Strengthening teaching and learning of cells | Notes for tutors | Session 4
© Crown copyright 2003