Biology (0610) | Sexual reproduction

Welcome to the Chapter on Sexual Reproduction!

Hello future Biologists! This chapter is all about how living things create the next generation, focusing specifically on **sexual reproduction**. It's the process that drives life's diversity, from the flowers blooming in your garden to the development of human beings.
Don't worry if some terms look intimidating; we'll break down the mechanisms in plants and humans step-by-step. Let's explore how organisms mix their genes to create unique offspring!

1. Introduction to Sexual Reproduction (16.2)

What is Sexual Reproduction?

Sexual reproduction is a biological process involving two parents, resulting in offspring that are **genetically different** from each other and from their parents.

Key Processes and Terms:

• Gamete: A sex cell (e.g., sperm or egg/ovule). Gametes contain half the number of chromosomes. Their nuclei are therefore haploid (single set of chromosomes).
• Fertilisation: This is the moment when the nuclei of the two gametes (male and female) fuse together.
• Zygote: The single cell formed immediately after fertilisation. Its nucleus contains a full set of chromosomes, making it diploid (two sets of chromosomes).
• Offspring: The new organisms produced are genetically unique due to the mixing of genetic material from the two parents.

Analogy: Think of gametes as half-decks of cards. When they fuse (fertilisation), they form a full, shuffled deck (the zygote), which is unique from the original two decks.

Quick Review:
Gamete (n) + Gamete (n) → Zygote (2n)

Advantages and Disadvantages of Sexual Reproduction (Supplement)

Sexual reproduction is powerful because it creates variation. This helps populations survive when the environment changes.

To a Population (in the Wild):

• Advantage: High genetic variation. If a disease strikes or the climate changes, some individuals will likely have the right genes to survive and reproduce.
• Disadvantage: Takes time and energy to find a mate; fewer offspring are typically produced than in asexual reproduction.

To Crop Production:

• Advantage: Allows for the development of new, improved strains (e.g., disease-resistant crops) through selective breeding.
• Disadvantage: The traits you want (e.g., high yield) might be mixed with undesirable traits in the offspring, leading to unpredictable results. Growing crops from seed (sexual) can also be slower than using cuttings (asexual).

2. Sexual Reproduction in Flowering Plants (16.3)

2.1 Flower Structure (Core)

The flower is the reproductive organ of a plant. We focus on two types: insect-pollinated and wind-pollinated flowers.

Parts of an Insect-Pollinated Flower (You need to be able to identify and draw these!):

• Sepals: Small, green leaves that enclose and protect the flower bud before it opens.
• Petals: Often large, brightly coloured, scented, and may contain nectar guides to attract insects.
• Stamen (Male Part): Consists of the filament and the anther.
  • Filament: Stalk that holds the anther up.
  • Anther: Produces and contains the pollen grains (which contain the male gametes).
• Carpel/Pistil (Female Part): Consists of the stigma, style, and ovary.
  • Stigma: The receptive tip where pollen grains land. Often sticky.
  • Style: Stalk connecting the stigma to the ovary.
  • Ovary: Contains the ovules (which contain the female gametes).

2.2 Pollination: Getting the Pollen There (Core)

Definition: Pollination is the transfer of pollen grains from the anther to the stigma.

Comparing Pollination Methods:

The structure of a flower is adapted to its method of pollination.

Feature	Insect-Pollinated Flower	Wind-Pollinated Flower
Petals	Large, bright, scented, often have nectar (to attract insects).	Small, dull, or absent (no need to attract anything).
Pollen Grain	Relatively large, sticky or spiky (to stick to insects).	Very small, light, smooth (easily carried by wind). Produced in massive quantities.
Anther	Firmly attached, positioned inside the flower.	Loosely attached, often hanging outside the flower (to shed pollen easily).
Stigma	Small, sticky, often enclosed within the petals.	Large, feathery, often hanging outside the flower (to catch pollen in the air).

Types of Pollination (Supplement)

• Self-Pollination: Pollen is transferred from the anther to the stigma of the same flower OR a different flower on the same plant.
• Cross-Pollination: Pollen is transferred from the anther of a flower on one plant to the stigma of a flower on a different plant of the same species.

Did you know? Self-pollination leads to less variation, which is risky for the population, but it guarantees reproduction even if pollinators are scarce! Cross-pollination increases variation, which is essential for adapting to changes in the environment.

2.3 Fertilisation in Plants and Seed Germination (Core & Supplement)

The Journey to Fertilisation (Core & Supplement)

Fertilisation is the key event: the fusion of the pollen nucleus with a nucleus in the ovule.

1. Pollen lands on the stigma.
2. The pollen grain absorbs nutrients and grows a tube called the pollen tube (Supplement).
3. The pollen tube grows down the style, guided by chemicals, towards the ovary.
4. The pollen tube enters the ovule (Supplement).
5. The nucleus from the pollen grain moves down the tube and fuses with the nucleus in the ovule. This is fertilisation.
6. The fertilised ovule develops into a seed, and the ovary develops into a fruit.

Requirements for Seed Germination (Core)

The seed contains an embryo waiting for the right conditions to start growing (germination). It needs three things:

1. Water: To rehydrate the seed and activate enzymes (like amylase) to break down stored food for energy.
2. Oxygen: For aerobic respiration, which provides the energy needed for growth.
3. Suitable Temperature: For the enzymes involved in growth and respiration to work efficiently (their optimum temperature).

Memory Aid: Think W.O.T. - Water, Oxygen, Temperature.

3. Sexual Reproduction in Humans (16.4)

Let's now look at the anatomy and physiology required for reproduction in mammals, using humans as the example.

3.1 The Human Reproductive Systems (Core)

Male Reproductive System: Identification and Functions

• Testes (Singular: Testis): Produce male gametes (sperm) and the hormone testosterone. Located in the scrotum outside the body to keep the temperature slightly lower than body temperature (optimal for sperm production).
• Scrotum: Sac that holds the testes outside the body.
• Sperm Ducts: Tubes that carry sperm from the testes/epididymis to the urethra.
• Prostate Gland: Produces fluid that mixes with sperm to form semen, providing nutrients and a fluid medium for transport.
• Urethra: Tube that passes through the penis, carrying semen (or urine) out of the body.
• Penis: Structure adapted for copulation (transfer of sperm into the female vagina).

Female Reproductive System: Identification and Functions

• Ovaries: Produce female gametes (egg cells or ova) and the hormones oestrogen and progesterone.
• Oviducts (Fallopian Tubes): Tubes connecting the ovaries to the uterus. The site where fertilisation usually occurs.
• Uterus (Womb): Muscular organ where the embryo implants and develops during pregnancy. Its lining (endometrium) thickens each month.
• Cervix: The neck of the uterus, forming a narrow opening into the vagina.
• Vagina: Muscular tube leading from the cervix to the outside; receives the penis during sexual intercourse.

3.2 The Gametes and Fertilisation (Core)

Comparison of Male and Female Gametes (Core)

Sperm and egg cells are vastly different because they have different roles.

Feature	Male Gamete (Sperm Cell)	Female Gamete (Egg Cell/Ovum)
Size	Very small.	Very large (largest cell in the body).
Structure	Tail (flagellum), head (acrosome), middle section (mitochondria).	Spherical, large cytoplasm, protected by a jelly coat.
Motility	Motile (can swim).	Non-motile (cannot swim).
Numbers	Produced in huge numbers (millions).	Produced in small numbers (usually one per cycle).

Adaptations of Gametes (Core)

• Sperm Adaptations:
  • Flagellum (Tail): Provides motility (allows it to swim towards the egg).
  • Mitochondria: Concentrated in the middle section to provide the large amount of energy (ATP) needed for the tail's movement.
  • Acrosome: A cap containing powerful enzymes that digest the egg's outer jelly coat, allowing the sperm nucleus to enter.
• Egg Cell Adaptations:
  • Energy Stores: Large volume of cytoplasm containing nutrient stores to support the early development of the zygote before implantation.
  • Jelly Coat: This protective layer changes structure immediately after fertilisation to prevent other sperm from entering (polyspermy).

3.3 Pregnancy and Fetal Development (Core & Supplement)

Fertilisation to Implantation

Fertilisation (fusion of nuclei) occurs in the oviduct. The resulting **zygote** immediately begins dividing by mitosis, forming a ball of cells called the embryo. This embryo travels down to the uterus and implants into the thick, spongy lining of the uterus (endometrium).

Key Structures of Fetal Development (Core & Supplement)

Once implanted, the embryo needs support systems to grow into a fetus.

1. Placenta (Supplement Focus): This is the temporary organ responsible for the essential exchange between the mother's blood and the fetus's blood.
2. Umbilical Cord: Connects the fetus to the placenta. Carries the fetus's blood supply.
3. Amniotic Sac: A membrane surrounding the fetus.
4. Amniotic Fluid: Fluid inside the amniotic sac. It acts as a shock absorber, protecting the fetus from bumps and sudden movements, and maintains a constant temperature.

Functions of the Placenta and Umbilical Cord (Supplement)

The placenta is vital for survival, acting as a communication hub between two blood systems (mother's and fetus's), though their blood rarely mixes directly.

• Exchange of Dissolved Nutrients: Glucose, amino acids, fats, water, and mineral ions pass from the mother's blood into the fetus's blood by diffusion or active transport.
• Exchange of Gases: Oxygen diffuses from the mother's blood to the fetus's blood; Carbon dioxide diffuses from the fetus's blood to the mother's blood.
• Excretory Products: Urea (waste product) diffuses from the fetus's blood to the mother's blood for the mother's kidneys to excrete.
• Protection and Risk: The placenta also allows the transfer of antibodies (providing passive immunity) but can also allow harmful substances like some pathogens (e.g., viruses causing Rubella, HIV) and toxins (e.g., nicotine or alcohol) to pass across and affect the fetus.

4. Human Sex Hormones and Cycles (16.5)

4.1 Roles of Sex Hormones (Core)

Hormones control the development of secondary sexual characteristics during puberty (the time when the body becomes sexually mature).

• Testosterone (Male): Produced by the testes. Role in the development of male secondary characteristics (e.g., deep voice, hair growth, muscle development).
• Oestrogen (Female): Produced by the ovaries. Role in the development of female secondary characteristics (e.g., breast development, fat deposition).

4.2 The Menstrual Cycle (Core & Supplement)

The menstrual cycle is a monthly sequence of changes in the ovaries and the lining of the uterus, typically lasting 28 days. Its primary purpose is to prepare the uterus for pregnancy.

Changes in the Uterus Lining (Core)

The most visible change is in the lining of the uterus (endometrium):

1. Day 1 - 5 (Menstruation): The existing lining breaks down and is shed (period).
2. Day 6 - 14 (Repair & Thickening): The lining repairs itself and starts thickening, becoming rich in blood vessels, ready for implantation.
3. Day 14 - 28 (Maintenance): The lining is kept thick and ready. If fertilisation does not occur, the cycle restarts.

Hormonal Control (Supplement)

Four key hormones (secreted by the pituitary gland and the ovaries) regulate this cycle:

1. Follicle-Stimulating Hormone (FSH): Produced by the pituitary gland. Stimulates the development of a follicle (containing an egg) in the ovary. Also stimulates the ovary to produce oestrogen.
2. Oestrogen: Produced by the ovaries. Repairs and thickens the uterus lining. Also inhibits FSH production and stimulates LH production.
3. Luteinizing Hormone (LH): Produced by the pituitary gland. Causes ovulation (release of the egg) around Day 14. Stimulates the corpus luteum to form and secrete progesterone.
4. Progesterone: Produced by the ovary (specifically the corpus luteum). Maintains the thick uterus lining. If pregnancy occurs, high levels of progesterone continue to inhibit FSH and LH. If no pregnancy, progesterone levels drop, triggering menstruation.

Analogy: FSH starts the engine (egg development). Oestrogen builds the nest (uterus lining). LH launches the egg (ovulation). Progesterone keeps the nest stable (maintains lining).

5. Sexually Transmitted Infections (STIs) (16.6)

What are STIs? (Core)

A Sexually Transmitted Infection (STI) is an infection that is transmitted through sexual contact.

Human Immunodeficiency Virus (HIV)

HIV is a major pathogen (disease-causing organism) that causes an STI.

• HIV attacks the body's **immune system** (specifically the lymphocytes/white blood cells).
• Infection with HIV may eventually lead to Acquired Immune Deficiency Syndrome (AIDS), where the immune system is so weak that the individual cannot fight off simple infections.

Transmission of HIV (Core)

HIV is passed from an infected person to an uninfected person, primarily through the exchange of **body fluids** (blood, semen, vaginal fluids, and breast milk).

Methods of transmission include:

• Unprotected sexual intercourse (the main route).
• Sharing contaminated needles (e.g., during drug use or tattooing).
• Transfusion of infected blood products (less common in modern healthcare).
• From mother to fetus across the placenta, or via breast milk after birth.

Controlling the Spread of STIs (Core)

The spread of STIs, including HIV, can be controlled by:

• Using barrier methods of contraception, such as condoms, during sexual contact.
• Limiting the number of sexual partners.
• Screening blood donations and blood products before transfusions.
• Using sterilized needles and syringes.
• Education programmes promoting safe sexual practices and awareness.