Science - Combined (0653) | Reproduction

🔬 Biology B14: Reproduction Study Notes

Welcome to the chapter on Reproduction!
Reproduction is one of the seven characteristics of living organisms (Remember MRS GREN?). It is the fundamental process by which life continues—making more of the same kind of organism. Understanding this process is key to understanding how both plants and humans survive and evolve.

Don't worry if some of the terms seem new; we will break down the complex processes into easy, manageable steps!

Part 1: Sexual Reproduction in Plants (B14.1)

Sexual reproduction in plants usually involves two parents and results in offspring that are genetically different from the parents. The main structure responsible for this process is the flower.

1.1 Flower Structure and Function (Core)

Think of a flower like a specialised machine designed to help the plant reproduce.

Key Structures and Their Functions:

• Sepals: Often green, leaf-like structures that enclose and protect the flower bud before it opens. (Think of them as the bud's security guard.)
• Petals: Often large, colourful, and scented to attract insects for pollination. In wind-pollinated flowers, petals are usually small and dull.
• Stamen (Male Part): The entire male structure.
  
  • Anther: Produces the pollen grains (which contain the male gametes).
  • Filament: A stalk that holds the anther up.
• Carpel or Pistil (Female Part): The entire female structure.
  
  • Stigma: The sticky tip that receives the pollen during pollination.
  • Style: The stalk connecting the stigma to the ovary.
  • Ovary: Contains one or more ovules.
  • Ovule: Contains the female gamete (egg cell). After fertilisation, the ovule develops into a seed, and the ovary develops into a fruit.

Quick Review: The male parts are the Stamen (Anther + Filament); the female parts are the Carpel (Stigma + Style + Ovary/Ovules).

1.2 Pollination: The Pollen Journey (Core & Supplement)

Pollination is defined as the transfer of pollen grains from the anther (male part) to the stigma (female part) of a flower.

Types of Pollination:

1. Self-Pollination: Pollen is transferred to a stigma on the same plant.
2. Cross-Pollination: Pollen is transferred to a stigma on a different plant of the same species. Cross-pollination increases genetic variation, which is beneficial for survival.

Comparing Insect-Pollinated and Wind-Pollinated Flowers (Supplement)

The structure of a flower is adapted specifically for how it receives pollen.

Feature	Insect-Pollinated Flower	Wind-Pollinated Flower
Petals	Large, brightly coloured, often scented.	Small, dull, often green or absent.
Nectar/Scent	Present, used to attract insects.	Absent.
Pollen	Small quantity, sticky or spiky, large grains.	Large quantities, smooth, very light grains.
Anthers	Stiff, contained within the flower petals.	Large, loosely attached, often hanging outside the flower (B14.1.6).
Stigma	Small, sticky, contained within the petals.	Large, feathery, often hanging outside the flower to catch airborne pollen (B14.1.6).

Memory Aid: For wind-pollinated plants, think of the wind spreading dust—you need loads of light pollen and big, feathery targets (stigma) to catch it!

1.3 Fertilisation (Core)

Pollination is just the transfer; fertilisation is the actual fusion.

Fertilisation occurs when a pollen nucleus (male gamete) fuses with a nucleus in an ovule (female gamete).

Step-by-Step Process:

1. Pollen lands on the stigma (Pollination).
2. A tube, called the pollen tube, grows down the style towards the ovule in the ovary.
3. The male nucleus travels down the pollen tube.
4. The male nucleus fuses with the female nucleus inside the ovule (Fertilisation).
5. The fertilised ovule develops into a seed and the ovary develops into a fruit.

1.4 Seed Germination (Core)

After the seed has been dispersed (moved away from the parent plant), germination is the process where the seed starts to grow into a new plant.

The seed contains an embryo and a food store, but it needs the right conditions to start growing.

The environmental conditions that affect germination are:

1. Water: The seed needs water to activate the enzymes that break down the stored food (like starch) so the embryo can use it for growth.
2. Oxygen: Oxygen is needed for aerobic respiration to release energy for growth.
3. Suitable Temperature: A specific range of warmth is needed for the enzymes to work efficiently. If it's too cold or too hot, germination won't happen.

Did You Know? Light is generally NOT required for germination. Many seeds actually germinate better in the dark, as they are usually buried underground!

Part 2: Sexual Reproduction in Humans (B14.2)

Human sexual reproduction also involves the fusion of male and female gametes, produced by specialised organs.

2.1 The Male Reproductive System (Core)

The male system is designed to produce sperm and testosterone, and to transfer the sperm into the female reproductive system.

Key Structures and Functions:

• Testes (Singular: Testis): Produce sperm (male gametes) and the hormone testosterone.
• Scrotum: A sac that holds the testes outside the body. This keeps the temperature slightly lower than body temperature, which is essential for healthy sperm production.
• Sperm Ducts (Vas Deferens): Tubes that carry sperm from the testes/epididymis to the urethra.
• Prostate Gland: Produces fluid that nourishes the sperm and makes up a large proportion of the seminal fluid (semen).
• Urethra: Tube running through the penis that carries urine or semen out of the body.
• Penis: Structure used to transfer semen into the vagina during sexual intercourse.

2.2 The Female Reproductive System (Core)

The female system is designed to produce eggs, receive sperm, provide a site for fertilisation and development of the fetus, and give birth.

Key Structures and Functions:

• Ovaries: Produce egg cells (ova/female gametes) and female sex hormones.
• Oviducts (Fallopian Tubes): Tubes that connect the ovaries to the uterus. This is where fertilisation usually takes place.
• Uterus (Womb): A muscular organ where the fertilised egg implants and develops into a fetus. It has a thick lining (the endometrium).
• Cervix: A ring of muscle at the base of the uterus, opening into the vagina.
• Vagina: A muscular tube that leads from the cervix to the outside of the body. It receives the penis during sexual intercourse.

2.3 Human Fertilisation (Core)

As in plants, fertilisation is the fusion of the gamete nuclei.

Fertilisation is the fusion of the nucleus from a male gamete (sperm) and the nucleus from a female gamete (egg cell).
The resulting single cell is called a zygote.

If sexual intercourse occurs, millions of sperm travel up the vagina, through the cervix and uterus, and into the oviducts. If an egg is present, one sperm will successfully fertilise it in the oviduct.

2.4 The Menstrual Cycle (Core)

The menstrual cycle is a repeating cycle (usually lasting around 28 days) involving changes in the ovaries and the lining of the uterus, preparing the female body for potential pregnancy.

Remember: In Combined Science, you only need to know the physical changes, NOT the role of specific sex hormones!

The cycle can be simplified into four main stages:

Stage 1: Menstruation (Day 1 – 5)

• The uterus lining, which had thickened in the previous cycle, is shed along with blood. This is known as a period.

Stage 2: Repair and Thickening (Day 5 – 14)

• The lining of the uterus starts to thicken again, becoming rich in blood vessels, ready to receive a fertilised egg.
• Meanwhile, in the ovary, an egg cell starts to mature.

Stage 3: Ovulation (Around Day 14)

• A mature egg is released from the ovary into the oviduct. This is called ovulation.

Stage 4: Maintenance (Day 14 – 28)

• The uterus lining remains thick and ready for implantation.
• If the egg is fertilised, it implants into this thick, spongy lining, and pregnancy begins.
• If the egg is not fertilised, the cycle restarts (back to Stage 1).

You have successfully completed the core concepts of Reproduction! Keep practising those key term definitions!