B10: Reproduction

B10.1: Asexual and sexual reproduction

Asexual Reproduction

Asexual reproduction is a process resulting in the production of genetically identical offspring from one parent
Bacteria divide themselves asexually by binary fission, which happens through the following steps

Replicating and doubling the contents within the cell
Diviing into two identical daughter cells

Some plants reproduce asexually by the process of budding, which happens by the following steps

A small part of the parent plant breaks off
The new small part becomes a new individual plant and becomes independent and a clone of the parent

Some disadvantages of asexual reproduction include...

The population of the offspring do not vary much, meaning that a large amount of the population would perish under a change in conditions
For example, a whole population would be likely to be affected by a disease

Some advantages of asexual reproduction include...

In the right condition, population can increase rapidly
Asexual reproduction is more efficient and faster

Sexual Reproduction

Sexual Reproduction is a process involving the fusion of the nuclei of two gametes (sex cells) to form a zygote and the production of offspring that are genetically different from each other
Gamete is a sex cell. They have a haploid nucleus, meaning that they carry half the chromosomes of other cells
Cells normally have 46 chromosomes, while gametees have 23 chromosomes
Haploid cells have one set of chromosomes (23 chromosomes), whie diploid cells have two copies (46 chromosomes)
Gametes contain a copy of each chromosome, while other cells in the body have two copies
Male and female gametes fuse to become a zygote, containing 46 chromosomes and 2 pairs of each chromosome (half from the father and half from the mother). This means that the nucleus of the zygote is diploid
Some advantages of sexual reproduction include...

The population will have more variation
Species are therefore more likely to adapt because of the variation they have, allowing them to have a better chance of survival as a population
This is because a certain genetic variation may fare better under different circumstances
Disease may not affect the entire population because of variation

Some disadvantages of sexual reproduction include...

It is less efficient to find a partner, as there are two parents involved in sexual Reproduction
Therefore, if a species cannot find a partner, they cannot reproduce

B10.2: Sexual reproduction in plants

Reproduction in the Flower

Flowers are the organs in plants that reproduce, often containing male and female parts
The male part of a flower is called the stamen, and the female part is called the pistil
Plants produce pollen, that produce the male gamete (sex cell)
Pollen cannot move on its own, and instead reaches the female part of the plant through other factors like insects or wind
Insects will move from plants to plants, with pollen grains stuck on their body. This means that when it travels, some of the pollen grains from one plant may be transferred to the female part o another
The wind could also carry the pollen grains from the male part to a female part of a flower
Fertilisation occurs when a pollen nucleus fuses with a nucleus in an ovule

The pollen grain lands on a stigma of a flower of the same species
The nucleus of the pollen grain goes down the pollen tube and grows to the ovary
The ovary contains ovules that have the female sex cell (ovum). The ovum (female) nucleus fuses with the (male) pollen nucleus
After the nuclei fuse, they form a zygote
The zygote then divides and forms a seed in the ovule

Seeds are inactive until the ideal conditions are meant
They require water, oxygen, 68°F to 86°F to germinate

Parts of the flower

Sepal: Protects the inside unopened parts of the flower
Petals: Brifhgtly coloured to attract insects
Anther: Produces the pollen grain, which is the male sex cell
Filament: a thin cylindrical stalk that suppotrs the anther
Stigma: The top of the female part of the flower that collects pollen grains
Ovary: Produces female gametes/sex cells (ovum)
Ovule: Carries female gametes/sex cells (ovum)

Wind-pollinated and insect-pollinated flowers

Feature	Wind-pollinated	Insect-pollinated
Petals	Small and dull in colour	Large and bright in colour (to attract insects)
Scent	A scent is present to attract insects to the plant (in order for it to move its pollen grains to the other plants)	There is no scent (no energy needs to be wasted to create a scent as there is no need to attract insects)
Pollen grains	There are a medium amount of pollen grains which are large and sticky to attach to insects	There are large amounts of small pollen grains (for more chances of pollination). They are small so they can be carried easily by the wind
Anthers	The anther is attached strongly to the inside of the flower to make sure that it can withstand insects	The anther is loosely attached to the outside of the flower to allow pollen grains to be carried by the wind more easily
Stigma	The stigma is sticky and within the flower, so that the pollen grains will stick to the stigma when an insect carries the grains to the flower	Found outside the flower to catch the pollen grains in the wind

Diagram of a flower

Types of Pollination

There are two types of pollination: Self-Pollination and Cross-Pollination
Cross-pollination is the more popular way in which the pollen of one plant goes to another plant (this allows more variation in the population)
Self-pollination is when the male and females sex cells are from the same plant, causing the offspring to be identical
The lack of variation causes the offspring to not be able to adapt to new conditions as well as offspring with genetic variation
However, cross-pollination has the disadvantage of relying on pollinators such as insects. If these pollinators are affected, then the plants dependent on them will be affected as well

B10.3: Sexual reproduction in humans

Humans sexually reproduce

Fertilisation is defined the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell/ovum)

The fertilised cell is called a zygote

The zygote forms an embryo (a ball of cells) which implant themselves into the wall of the uterus

The male reproductive system (Definitions from IGCSE Syllabus)

Testes: Produces male gametes (sperm)
Scrotum: sac that holds the testes outside the body
Sperm Ducts: Transfers sperm to the urethra
Prostate Gland: secrete fluids for sperm to swim in forming semen
Urethra: carries urine and semen out of the body
Penis: transfers semen to vagina during sexual intercourse

The female reproductive system

Ovaries: Release female gametes (eggs)
Oviducts: Transfers egg to uterus and the site of fertilisation
Uterus: Where the fetus develops
Cervix: Ring of muscle at the opening of the uterus
Vagina: receives penis during sexual intercourse

Male and female gametes (sex cells)

Gametes are specialised for fertilisation and developing an embryo
For this reason, the sperm and egg cells have different features
Sperm cells have a tail called the flagellum which helps it move to the egg cell
Sperm cells are very small in size
Sperm cells have the ability of movement (locomotion)
Around 100 million sperm cells are produced per day
Sperm cells have extra mitochondria to supply energy, and has enzymes at its head to penetrate the egg cell
The egg cell contains a cytoplasm with nutrients to allow embryo to develop
The cell membrane changes after fertilisation to make sure that only one sperm enters
The size of an egg cell is large
Egg cells are shaped as round cells covered in a jelly membrane
Thousands of immature eggs are in each ovary, and one is released per month

Development of the Fetus

After fertilisation, the zygote develops into an embryo, which implants itself in the walls of the uterus and grows and develops
The embryo receives nutrients that diffuse through the uterus lining
This allows the embryo to develop its organs greatly during the first 12 weeks of the pregnancy
When the organs have been developed, a placenta forms and the embryo is now a fetus
The following time is for the general development and growth of the fetus
The fetus is surrounded by the amniotic sac containing amniotic fluid, which protects a baby from bumps in the abdomen
An umbilical cord connects the fetus to the placenta
The placenta provides nutrients and filters waste products through a connection of the blood supply of the fetusand the mother

These nutrients include gluocse, amino acids, fats, water, and oxygen
Waste (eg. urea, CO₂)is absorbed through diffusion by the mother's blood to make sure that the baby can stay safe
The placenta has thin walls and a large surface area, allowing diffusion to be efficient. It also prevents dangerous molecules from entering the blood (but not all)
The umbilical cord is cut after birth, which is painless because the umbilical cord does not have nerves that sense pain. It then falls off later
The placenta detaches from the walls of the uterus and is pushed out

Sexually Transmitted infections (STIs) and HIV/AIDS

Sexual intercourse without protection can transmit disease through the transmission of microorganisms through body fluids
STIs are controlled through controlling the amount of sexual partners, the use of protection, tests, and educational campaigns
An example of an sexually transmitted disease is HIV), which can also be transmitted through other exchanges of body fluids
HIV infects a lympocyte (a type of white blood cell that fights against infection) in the immune system and multiplies
However, HIV is not destroyed by lymphocytes as it is not recognised
Therefore, the number of lymphocytes and amount of antibodies that can be made to fight infection are reduced
This causes impaired ability of the body to fight infection, which leads to AIDS

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