Reproduction
Most cells in a multicellular organism can be described as diploid (2n).
A diploid cell contains two copies of each of the chromosomes for that species.
For example, a healthy human cell consists of 46 different chromosomes.
A diploid human cell therefore has 46 chromosomes, or 23 pairs of chromosomes.
We can see these chromosomes pairings in the karyotype diagram below.
These images are used to detect genetic mutations such as Downs Syndrome which appears
as an extra chromosome on pair 21.
Image from Wikimedia Commons
In order for sexual reproduction to occur, two cells must fuse to form the new individual.
If two diploid cells were to fuse the number of chromosomes in the offspring would be
double that of the parents.
Multicellular organisms therefore need to reduce the chromosome complement of the cells
involved in sexual reproduction by half.
Cells which have only one set of the species' chromosomes are described as being haploid
(n). In humans these haploid cells are egg and sperm cells, but they are pollen and ovule in
plants. These types of cells are known as gametes.
During sexual reproduction, two haploid gametes fuse to form a new individual.
This process is known as fertilisation and the new diploid cell is called a zygote. This process
is summarised in the diagram below.
To simplify the diagram, the diploid number in the diagram is 4 and the haploid number is 2.
In humans the diploid number is 46 and the haploid 23.
Gamete Production in Plants
In order to reproduce sexually plants will need to produce haploid gametes which can fuse
to form a diploid zygote.
The organ of sexual reproduction for many plants is the flower.
Gamete production and fertilisation occurs in flowers and we therefore need to explore the
structure of flowers in some detail.
The diagram below shows the generalised structures found in most flowers. If you cut a
flower in half -length ways- you should be able to identify most of these structures.
Each structure in a flower carries out a particular function which are shown in the table
below.
Structure
Petals
Sepals
Stamen
Carpel
Function
In flowers which are pollinated by insects petals are
prominent and brightly coloured to attract insects to
the flower.
Wind pollinated flowers have petals which are much
smaller and are not colourful.
Sepals are modified leaves which protect the flower
when it is developing. These form the green outer layer
of a 'bud'.
When the flower opens the sepals can be found at the
base.
The stamen consist of the anther and the filament.
The anther produces and releases pollen grains which
contain the male gamete.
In wind pollinated plants the filaments are often
extremely long to ensure the anthers are outside the
flower. .
The carpel consists of the stigma, style and ovary.
The ovary contains the female gamete, ovules.
Pollen grains which land on and attach to the stigma
grow a pollen tube down the style to the ovary to allow
the male gametes to travel down to and fertilise the
ovules.
The ovary and the anthers are the sites of gamete production.
The 'female' gamete in plants are called ovules. These are produced and stored in the ovary.
The ovary is also the site of fertilisation, so the 'male' gamete must travel here.
The 'male' gamete in plants is found in the pollen grains. These are produced in the anther
and are dispersed in various methods depending on the species of plant, such as by wind or
insect, in a process called pollination.
Once a pollen grain lands on the stigma of a flower the 'male' gamete travels down the style
in a pollen tube to the ovary where it fuses with an ovule to form a diploid zygote.
This diploid zygote then develops into a seed which is then dispersed from the plant by
various methods depending on the species of plant, such as wind or animals, to avoid the
offspring competing with the parent plant for resources.
Gamete Production in Animals
The haploid gametes in animals are egg cells and sperm cells.
When these cells fuse they form a diploid zygote, but where do they come from in the first
place?
In mammals, sperm is produced in testes and egg cells are produced in ovaries.
Male reproductive system
Sperm tube
The penis inserts sperm into the female to allow the sperm cells to come into contact with an
egg cell and for fertilisation to occur. Sperm cells are produced in the testes.
Female reproductive system
Oviduct
The diagram above shows the main structures of the human female reproductive system.
Eggs are produced in the ovaries.
Unlike in males, mammalian females are born with all their egg cells already produced.
Human females develop their haploid egg cells when they themselves are developing in their
own mother's uterus.
When human females reach puberty they begin the process of releasing their own eggs,
known as ovulation, normally at the rate of one per month.
Not all of the egg cells which a female is born with are released, many of them die over time.
If sperm cells are present during or soon after ovulation then fertilisation can occur to form a
diploid zygote. This diploid zygote can then develop into an embryo.