Life Sciences Grade 12: Biodiversity of Plants and Reproduction

Sexual Reproduction

• Involves the presence of gametes (sex cells) produced during meiosis. Meiosis ensures that each gamete receives only half of the chromosomes of the parent cell, promoting genetic diversity upon fertilization.

• Two types of gametes:

  • Egg cells: The female gametes, typically large and non-motile.

  • Sperm cells: The male gametes, typically small and motile.

• A sperm cell fertilizes an egg cell to form a zygote. This fusion restores the full complement of chromosomes, creating a genetically unique individual.

• The zygote develops into a multicellular organism through mitosis and cellular differentiation. Mitosis ensures that each new cell receives an identical set of chromosomes, while differentiation allows cells to specialize in function.

• Diagram illustrating the life cycle with mitosis and meiosis:

  • Adult male and adult female undergo meiosis to produce sperm and egg, respectively. Meiosis involves two rounds of cell division, resulting in four haploid daughter cells.

  • Fertilization occurs, forming a zygote. Fertilization activates the zygote, initiating development.

  • The zygote undergoes mitosis to develop into a multicellular organism. Repeated mitotic divisions lead to the formation of tissues, organs, and organ systems.

Advantages:

• Results in genetic variation, which enhances the survival of a species in changing environments. Genetic variation allows populations to adapt to new conditions and resist diseases.

• Can lead to the formation of new species (speciation). Speciation occurs when populations diverge genetically and can no longer interbreed.

• Decreases the likelihood of inheriting diseases or weak characteristics from parents. Genetic recombination during meiosis shuffles genes, reducing the chances of offspring inheriting harmful traits.

Disadvantages:

• The process is lengthy due to the time required for gamete formation. Meiosis is a complex process that takes considerable time to complete.

• Fewer offspring are produced, reducing the chances of survival. Sexual reproduction typically results in fewer offspring compared to asexual reproduction.

• Requires more energy. Finding a mate and undergoing meiosis and fertilization require significant energy expenditure.

• Relies on pollinating agents for pollination to occur. Pollination can be unreliable, especially in the absence of pollinators.

Asexual Reproduction

• Occurs in the absence of gametes. Therefore, meiosis and fertilization do not occur.

• Involves only one parent. The offspring is derived from a single individual.

• Offspring have the same genetic composition as the parent (no variation). This results in a clone of the parent.

Advantages:

• Large numbers of offspring are produced. Asexual reproduction allows for rapid population growth.

• Occurs rapidly. Asexual reproduction is generally faster than sexual reproduction.

• Requires less energy. No need to find a mate or undergo meiosis.

• Offspring are likely to be successful in the same environment as the parent, since they are genetically identical. If the parent is well-adapted to its environment, the offspring will also be well-adapted.

Disadvantages:

• Offspring are genetically identical to the parent. This lack of genetic variation can be detrimental in changing environments.

• If environmental changes occur, all offspring may be affected, decreasing the chances of survival for the species. A disease that affects one individual can affect the entire population.

• Can lead to overpopulation. Rapid reproduction can lead to competition for resources.

Reproductive Organ in a Plant

• The flower is the reproductive organ. Flowers are specifically adapted for sexual reproduction in plants.

Parts of a Flower and Their Functions

• Petal (Kroonblare):

  • Often large and colored to attract pollinators (Groot en kleurvol om bestuiwers aan te lok). The color and shape of the petals can attract specific pollinators.

• Sepal (Kelkblare):

  • Protects the flower while in bud (Beskerm die toe blom knop). Sepals enclose and protect the developing flower.

• Anther (Helmknop):

  • Contains pollen sacs and produces the male gamete (Bevat stuifmeel en produseer manlike gamete). Pollen grains contain the sperm cells.

• Filament:

  • Supports the anther (Beweeglike steel wat helmknop ondersteun). The filament elevates the anther, making it accessible to pollinators.

• Stigma (Stempel):

  • Sticky surface that receives the pollen (Taai oppervlak wat die stuifmeel ontvang). The sticky surface helps pollen grains adhere.

• Style (Styl):

  • Connects the stigma to the ovary (Verbind die stempel aan die ovarium). The style provides a pathway for the pollen tube to reach the ovary.

• Ovary (Ovarium):

  • Contains the ovules and turns into the fruit when fertilized (Bevat ovulums en raak die vrug wanneer bevrugting plaasvind). The ovary protects the ovules and develops into the fruit after fertilization.

• Ovule (Ovulum):

  • Contains the female gametes and turns into the seeds after fertilisation (Bevat vroulike gamete en raak die sade na bevrugting). The ovule contains the egg cell and develops into the seed after fertilization.

Pollination and Fertilization

• Pollination: Transfer of ripe pollen grains from the anther to the stigma of a flower (Bestuiwing is die oordrag van stuifmeel vanaf die helmknop na die stempel).

Types of Pollination:

• Cross-pollination: Pollen grains from the anther of one flower are transferred to the stigma of another flower (Kruisbestuiwing is wanneer die stuifmeel van ‘n helmknop van een blom oorgedra word na die stempel van ‘n ander blom).

  • Pollen is transferred between different flowers on separate plants of the same species. This promotes genetic diversity.

  • Beneficial for promoting genetic diversity, leading to stronger, more adaptable offspring. Genetic diversity increases the chances of survival in changing environments.

  • Requires pollinators like insects, wind, or other animals. Pollinators play a crucial role in transferring pollen between flowers.

• Self-pollination: Pollen grains from the anther of one flower are transferred to the stigma of the same flower (Selfbestuiwing is wanneer die stuifmeel van ‘n helmknop van een blom oorgedra word na die stempel van dieselfde blom).

  • Pollen transfer occurs within the same flower or on the same plant. This ensures pollination even in the absence of pollinators.

  • Advantageous in stable environments, ensuring pollination even with limited pollinators. Self-pollination can be a reliable reproductive strategy.

  • May result in reduced genetic diversity. Reduced genetic diversity can make populations more vulnerable to diseases and environmental changes.

• Pollen grains are released from the anther when it bursts. The release of pollen grains is essential for pollination to occur.

• Pollinating agents include wind, water, insects, and birds (Die wind, water, insekte en voëls word beskou as bestuiwers). Different pollinators have different adaptations for pollination.

Fertilization

• Occurs after pollination. Fertilization is the fusion of gametes to form a zygote.

• Fusion of male and female gametes to form a zygote (Bevrugting is wanneer die manlike en vroulike gamete saamsmelt om ‘n sigoot te vorm). The zygote is the first cell of the new organism.

• Pollen lands on the stigma and grows a pollen tube down the style to the ovary. The pollen tube carries the sperm cells to the ovule.

• When the pollen tube enters the ovule, an embryo and seed start to form. The embryo develops into the new plant, and the seed provides nourishment and protection.

• The embryo is located within the seed and consists of three parts:

  • Radicle: the first root, develops into the root system. The radicle anchors the plant and absorbs water and nutrients.

  • Plumule: the first shoot. The plumule develops into the stem and leaves.

  • Axis: The embryonic stem connecting the radicle and plumule.

• The mature ovule develops into the seed. The seed contains the embryo and a food supply.

• The ovary becomes the fruit. The fruit protects the seed and aids in dispersal.

How Asexual Reproduction Improves Crops

• A mutation could occur, producing plants with superior traits. Mutations can lead to desirable characteristics.

• Grafting: Inserting tissue from one plant into another. Grafting combines the desirable traits of two plants.

  • Rootstock: Plant chosen for its roots. The rootstock provides the root system for the graft.

  • Scion: Plant chosen for its superior fruits. The scion provides the desired fruit characteristics.

How Sexual Reproduction Improves Crops

• Cross-pollination to create cultivars: Plants selected for desired characteristics. Cross-pollination allows for the combination of desirable traits from different plants.

• Self-pollination to bring out the desired characteristics. Self-pollination can stabilize desirable traits in a cultivar.

Terminology

• Multicellular organism: Made up of many cells. Complex organisms with specialized cells and tissues.

• Genetic variation: Offspring are genetically different from each other. This is essential for adaptation and evolution.

• Cloning: A process during which identical offspring are produced from a parent. Cloning results in genetically uniform individuals.

• Clone: The offspring produced during cloning. Clones share the same genetic makeup as the parent.

• Complete flower: Has all four whorls. The four whorls are the sepals, petals, stamens, and carpels.

• Regular flower: Parts of each whorl are identical. This refers to the symmetry of the flower.

• Style: A long tube-like structure that holds the stigma and provides a passage for the pollen tube. The style ensures that the pollen tube can reach the ovary.

• Stigma: A sticky structure on top of the style that receives pollen grains. The stigma facilitates the capture of pollen.

• Ovary: Contains ovules with ova or eggs. The ovary protects the developing ovules.

• Receptacle: The part of the flower from which all the whorls arise. The receptacle is the base of the flower.

• Pedicel: A stalk that attaches the flower to the stem. The pedicel supports the flower.

• Locules: Chambers or cavities within the ovary. Locules contain the ovules.

• Bilocular: The ovary has 2 chambers (locules). This describes the structure of the ovary.

• Tri-locular: The ovary has 3 chambers or locules. This describes the structure of the ovary.

• Carpels: Floral whorls of a flower. Carpels are the female reproductive structures.

• Pistil: Formed from the carpels and made up of stigma, style, and ovary. The pistil is the female reproductive organ of the flower.

• Superior ovary: The ovary arises from above the receptacle. This describes the position of the ovary relative to the other floral parts.

• Incomplete flower: Made up of 3 whorls instead of 4. Missing one or more of the four typical floral whorls.

• Perianth: The fused calyx and corolla. When the sepals and petals are indistinguishable.

• Tepals: Individual petal-like structures that make up the perianth. Found in flowers with a perianth.

• Perianth tube: When the tepals are fused at the bottom to form a tube. A characteristic of some flowers with a perianth.

• Nectaries: Produce nectar. Nectaries attract pollinators.

Questions and Answers

• Question 1: A wind-pollinated flower has…

  • Answer: C. Small green flowers with no nectar.

• Question 2: Insect-pollinated flowers have…

  • Answer: A. Large showy petals and a pleasant smell.

• Question 3: Some flowers have markings on the petals to…

  • Answer: A. Direct the insect to the nectar.

• Question 4: The nectaries produce the…

  • Answer: A. Nectar

• Question 5: Light dry pollen grains…

  • Answer: A. Are wind pollinated

• Question 6: Sticky or rough pollen grains are…

  • Answer: B. insect pollinated

• Question 7: Some small flowers are grouped together…

  • Answer: A. To make them more visible to the insects.

• Question 8: A large sticky stigma occurs in…

  • Answer: A. All flowers

• Question 9: Self-pollination is when pollen grains of the anther…

  • Answer: B. From one flower lands on the stigma of the same flower.

• Question 10: Cross pollination is when pollen grains of the anther…

  • Answer: A. From one flower lands on the stigma of another flower.

• Question 11: Seed bearing plants with flowers are called…

  • Answer: D. Both A and C

• Question 12: Seed bearing plants without flowers are called…

  • Answer: B. Gymnosperm

• Question 13: The part of the flower from which all other whorls arise is…

  • Answer: D. Receptacle

• Question 14: Arrangement of ovules within the ovary is called…

  • Answer: C. Axile Placentation

• Question 15: The chambers within the ovary are called…

  • Answer: C. Locules

• Question 16: The mature ovary develops into the…

  • Answer: A. Fruit

• Question 17: The ovule develops into the…

  • Answer: B. Seed

• Question 18: The highly modified floral leaves make up the pistil is called…

  • Answer: A. The gynaecium