ما هي معادلة البناء الضوئي الكيميائية الموزونة؟ What is the balanced equation for photosynthesis?

المعادلة الموزونة هي: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂ (ستة جزيئات ثاني أكسيد الكربون + ستة جزيئات ماء في وجود طاقة الضوء وصبغة الكلوروفيل تعطي جزيء جلوكوز وستة جزيئات أكسجين). وتتطلب العملية تفاعلات ضوئية لإنتاج ATP و NADPH يعقبها تفاعلات لاضوئية لتثبيت الكربون. [الصيغة الرمزية الموزونة: 6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂]

2.1.1 Sexual and Asexual Reproduction: Differences · 2. Reproduction and Inheritance | Edexcel International GCSE Science Double Award (Modular): Biology Unit 2

Section 1 of 9Two kinds of reproduction

Transport systems in a flowering plant and the human body

Bacterial cell division (binary fission)

Reproduction produces new individuals of a species. There are two main types.

Sexual reproduction involves the joining (fusion) of the nuclei of two specialised sex cells called gametes from two parents. The fusion is called fertilisation, and it forms a single cell called a zygote, which divides to form an embryo. The offspring have a mixture of genetic information from both parents, so they show variation.

Asexual reproduction involves only one parent and no gametes. The offspring are genetically identical to the parent and to each other, and are called clones. The cells divide by mitosis.

Most animals and flowering plants reproduce sexually, but many plants and microorganisms can do both, switching according to conditions. Sexual reproduction in humans involves internal fertilisation, while many fish and amphibians shed gametes into water, where fertilisation is external and far more gametes are needed to give a few survivors.

Section 2 of 9Gametes

The gametes in animals are the sperm cells in males and the egg cells (ova) in females.

In flowering plants, the male gamete is in pollen and the female gamete is the egg cell in the ovule.

Haploid

Gametes are haploid: they contain a single set of chromosomes, which in humans is 23 chromosomes.

Meiosis

They are made by a special type of cell division called meiosis (6.1.2).

Diploid

When two gametes fuse, the zygote has a full set of 46 chromosomes, a diploid number, with one set from each parent.

Body cells, in contrast, are diploid and divide by mitosis for growth, repair and asexual reproduction.

Section 3 of 9Why sexual reproduction gives variation

Fertilisation: how a sperm fertilises an egg cell

Genetic Variation in a Species — Interactive 2D Simulation

Variation arises in sexual reproduction for several reasons.

1

Each gamete carries a different mixture of the parent's chromosomes, because meiosis shuffles the genes.

2

Fertilisation is random, so any sperm can fuse with any egg.

3

Offspring therefore get a unique combination of alleles.

4

This variation can help a species to survive if the environment changes, as some individuals may be better adapted than others.

5

In contrast, in asexual reproduction, any differences between offspring result only from the environment or rare mutations.

Section 4 of 9Examples of asexual reproduction

Asexual reproduction in plants: buds and tubers

Strawberry runner producing a genetically identical plantlet

Runners — Asexual Reproduction in Strawberry

Asexual reproduction by bulbs and tubers

Vegetative propagation by runners in strawberry plants

Organism	Method
Bacteria	Binary fission: the cell copies its DNA and divides in two
Fungi (e.g. mushrooms, yeast)	Release of spores; yeast can bud
Strawberry plants	Runners growing from the parent
Potatoes, daffodils	Tubers and bulbs that grow into new plants
Malarial parasite	Asexual reproduction in the human host (mitosis)

Some organisms use both. Plants such as strawberries reproduce sexually through seeds and asexually through runners, and the malarial parasite uses sexual reproduction in mosquitoes and asexual reproduction in humans. Fungi also may switch depending on the conditions.

Section 5 of 9Comparing the two

THE PROCESS OF CELL DIVISION BY MITOSIS — INTERACTIVE SIMULATION

Feature	Sexual	Asexual
Number of parents	Two	One
Gametes	Yes	No
Cell division	Meiosis then fertilisation	Mitosis
Offspring	Genetically different	Genetically identical clones

This topic links to cell division by mitosis in cell biology, to meiosis (6.1.2), and to the advantages of each type (6.1.3). In humans, the fusion of sperm and egg and the growth of the embryo link to the reproductive system and hormones (5.3).

Section 6 of 9Worked comparison: strawberry plant

Pollen tube growth to the ovule (stages)

A strawberry plant shows both kinds of reproduction side by side, which makes it a good model for exam answers.

1

In summer it grows stems called runners that creep along the ground.

2

At intervals a bud on the runner puts down roots and grows into a new plant.

3

All of these plants are genetically identical to the parent, because the runner cells arise by mitosis.

4

They spread quickly across a bed and, if the parent is well suited to the soil, so are they.

The same plant also makes flowers.

1

Pollen from the anther of one flower lands on the stigma of another, a pollen tube grows, and the male gamete fuses with the egg cell in the ovule.

2

The ovule develops into a seed.

3

Seedlings grown from these seeds differ from the parent and from each other,

so there is a chance that some will resist a disease or cope with a dry summer.

4

Writing a short comparison like this, naming the cell division, the gametes and the outcome for variation, scores well in extended answers.

For humans, identical twins are a natural example of clones, produced when an embryo splits early in development, but they arise from sexual reproduction and so are different from either parent.

Section 7 of 9Exam-style questions

Human fertilisation: sperm and egg nuclei fuse

Egg cell and sperm: fertilisation (nuclei fuse)

Fertilisation: haploid gametes to diploid zygote

How the diploid number is maintained through meiosis and fertilisation

Describe the differences between sexual and asexual reproduction. [4]4 marks
State the number of chromosomes in a human gamete and in a zygote. Explain the difference. [3]3 marks
Explain why offspring from sexual reproduction are not identical to their parents. [3]3 marks

Section 8 of 9Key terms

Term	Meaning
Gamete	A haploid sex cell such as a sperm or egg.
Fertilisation	The fusion of the nuclei of two gametes.
Zygote	The cell formed when two gametes fuse.
Clone	A genetically identical organism produced asexually.
Haploid	Having one set of chromosomes.
Diploid	Having two sets of chromosomes.
Mitosis	Cell division producing identical cells.
Meiosis	Cell division producing genetically different gametes.

Section 9 of 9Exam technique

1

Saying asexual reproduction involves gametes.

2

Saying clones are produced by meiosis. Asexual reproduction uses mitosis.

3

Confusing fertilisation (fusion of gametes) with pollination.

4

Writing that gametes have 46 chromosomes in humans.

5

Saying variation is only due to mutation; sexual reproduction mixes genes.