Topic 3 Sexual Reproduction Flashcards
(10 cards)
Describe the structure of a sperm cell.
They have a flagellum which is used to propel the cell.
This is done using microtubules found within the flagellum.
Next to the flagellum is the mid piece.
This contains many mitochondria to provide the energy needed for movement.
The head contains a haploid nucleus which stores the genetic information.
The tip of the head contains an acrosome which contains hydrolytic digestive enzymes.
What is the nature of an acrosome?
It is a specialised Lysosome.
Describe the structure of ova
They are surrounded by a “jelly-like”zona pellucida.
The cytoplasm contains many food stores to support the zygote at first.
The genetic material is in the last stages of meiosis and only finishes when a sperm enter.
Until this division is completed, the cell is called an oocyte and not an ovum.
What are the two types of fertilisation?
Fertilisation can either be internal or external.
External fertilisation is more common in aquatic animals.
Internal fertilisation is often less wasteful.
Describe the acrosome reaction.
The front of a sperm touches the zona pellucida of the oocyte and the acrosome reaction is triggered.
Digestive enzymes pour out of the open acrosome and begin to digest the follicle cells and the zona pellucida.
Projections of the sperm surface shoot forward as the result of actin-like proteins.
Describe the cortical reaction.
Cortical granules in the cytoplasm of the ovum release enzymes into the zona pellucida.
These enzymes destroy the sperm-binding sites and also thicken and harden the jelly of the zona pellucida.
This then forms a tough fertilisation membrane around the fertilised ovum preventing polyspermy.
Describe fertilisation in mammals.
The sperm approach the oocyte in the oviduct.
When a sperm head reaches the zona pellucida, the acrosome breaks open triggering the acrosome reaction.
When a sperm reaches the cell membrane, its head enters the cell and the tail does not.
This immediately causes all ion channels to switch from being electrically negative to positive.
This prevents all other sperm from entering the cell temporarily until the cortical reaction takes place.
The oocyte finishes meiosis and is now called and ovum.
The head absorbs water and swells, releasing its chromosomes to fuse with the ovum’s chromosomes.
A diploid nucleus is now formed and the cell is called a zygote.
Describe the structure of a pollen grain
It has two haploid cells, a generative cell and a tube cell.
Once the pollen grain reaches the stigma of a flower, the generative nucleus divides by mitosis forming two male gametes.
Describe the process of fertilisation in flowering plants.
Once a pollen grain lands on stigma of a flower, a pollen tube begins to grow out from the tube cell of the pollen grain.
The tip of the pollen tube produces hydrolytic enzymes to digest the tissue of the style, so that the pollen tube can make its way towards the ovule.
The digestive material acts as a nutrient source for the pollen tube as it grows.
Eventually the tip of the pollen tube will pass through the micropyle of the ovule.
Then, one of the male nucleus fuses with the egg cell to form a diploid zygote.
the other male nucleus fuses with the two polar bodies to form a triploid endosperm.
This is called a double fertilisation.
Describe the function of the tripolid endosperm.
It is envolved in supplying the embryo plant with food when it begins to germinate.
