Reproduction
Living organisms all reproduce in order to make a new generation of organisms . There are two types of reproduction that living organisms do:-
Sexual Reproduction - A 'male' and 'female' organism each contribute 1/2 the DNA required to produce a new offspring.
Asexual Reproduction - a single organism contributes all the DNA required to produce a new offspring. The new offspring is a clone of the parent.
Note - This course will focus only on Sexual Reproduction.
Recap of Cells
There are two types of cells that take part in reproduction:-
Egg Cell
The image above shows an egg cell. Egg cells are part of the female reproductive system. A egg cell is specialised:-
Nucleus contains only 1/2 the DNA required to create a new organism.
Has a lot of cytoplasm containing nutrients to keep the cell alive until implantation.
Sperm Cell
The image above shows a sperm cell. Sperm cells are part of the male reproductive system. A sperm cell is specialised:-
Nucleus contains only 1/2 the DNA required to create a new organism.
Has a tail, to help it 'swim' to the egg cell.
Fertilisation
Fertilisation occurs when a sperm cell fuses with an egg cell. The sperm cell loses its tail and the two cell nuclei join together to form a new cell called a zygote. A membrane then forms around the egg cell, stopping any more sperm cells entering.
Types of Fertilisation
Fertilisation can take place in two different ways:-
Internal fertilisation.
External fertilisation.
Internal Fertilisation
The process of fertilisation takes place within the female organism. To do this, the male sperm cells must be placed within the female through sexual intercourse.
As the sex cells are close together, they have a good chance of meeting. This means only a small number of egg cells have to be produced. Also, the developing cells can be protected within the organism, keeping them safe from predators.
External Fertilisation - (Only Aquatic Species)
The process of fertilisation takes place outside the female organism. The female lays eggs and the male releases sperm into the water around them.
As there is only a small chance of the two sex cells meeting, lots of egg cells are produced. Also, the developing cells are not protected within the organism, so many will be eaten by predators. This means lots of fertilised cells are needed, to ensure some survive to become adults.
