Plant Reproduction IGCSE PDF

Unit 8: Plant Reproduction (IGCSE Pages 197-209) Reproduction is the production of an offspring (a new individual). In this process, genetic information is passed on from the parent to the offspring. Reproduction can be sexual or asexual. Characteristic Asexual S...

Unit 8: Plant Reproduction (IGCSE Pages 197-209) Reproduction is the production of an offspring (a new individual). In this process, genetic information is passed on from the parent to the offspring. Reproduction can be sexual or asexual. Characteristic Asexual Sexual Definition The offspring is produced from The offspring is produced from one parent. two parents. Process Several methods can be 1. The parents produce used, including: gametes (sex cells). binary fission 2. The male gamete is spore formation transferred to the female reproductive organ. budding 3. Fertilization takes place. During this process, the gametes fuse to produce a zygote (the first cell of the offspring). Genetic make- The offspring is genetically The offspring inherits genetic up of the identical to the parent. information from both offspring parents. Therefore, the offspring is not genetically identical to neither one parent nor the other. Advantages The process is fast. The offspring produced are A large number of offspring genetically different from is produced. each other and from their parents. This is known as All advantageous genes genetic diversity. are passed on to the offspring. Genetic diversity, allows species to adapt to changes in the environment. Ms Marika Pisani 1  Characteristic Asexual Sexual Disadvantages All disadvantageous genes Two parents are required. are passed on to the Not all advantageous offspring. genes are passed on to the Due to a lack of genetic offspring. diversity, the species is less able to adapt to changes in the environment. Types of Asexual Reproduction Binary Fission (Cell Division) Binary fission is a process in which a parent cell divides into two identical daughter cells. 1. An animal cell that 2. The nucleus divides 3. The cytoplasm pinches off is about to divide has into two daughter between the nuclei. The other a large nucleus. nuclei. Other organelles organelles are shared more or also divide. less equally between the two. 4. Two daughter cells are 5. One of the cells may become specialized. The formed. unspecialized cell may continue to divide. Figure 1: Binary fission in animal cells Ms Marika Pisani 2  middle lamella 1. A plant cell that is 2. The nucleus divides to 3. The cytoplasm adds about to divide has a form two daughter layers of cellulose to large nucleus, dense nuclei. Other organelles each side of the new cell cytoplasm and no also divide. A new cell wall. Vacuoles start to vacuole. wall begins to form. This is from in the cytoplasm of known as the middle one of the daughter lamella. cells. 4. The vacuoles absorb water. This 5. The vacuoles join to form one results in an increase in pressure which vacuole. This daughter cell has makes the cell larger. become specialized. The other cell will divide again. Figure 2: Binary fission in plant cells Before the parent cell divides, it must make copies of its DNA molecules. This process is known as DNA replication. In this way, each daughter cell ends up with its own complete set of DNA molecules. Figure 3: The importance of DNA replication prior to cell division Ms Marika Pisani 3 Binary fission has two important roles: It can take place in any part of a multicellular organism for growth and repair. It allows unicellular organisms to produce offspring. Spore Formation in Fungi Some fungi are made up of thread like structures called hyphae (singular: hypha). As these fungi grow, some hyphae start to grow vertically. The tip of each vertical hypha swells to form a sporangium (plural: sporangia). The sporangia contain hundreds of spores. Spores are microscopic reproductive structures that generally consist of one cell. When the sporangium matures, it bursts. The spores are released and spread. If a spore lands in favourable conditions, it develops into a new fungus. Figure 4: Asexual reproduction in fungi using spore formation Ms Marika Pisani 4 Budding in Yeast Cells Yeast is a unicellular fungus. It reproduces by means of budding. 1. A bud develops on the surface of 2. The nucleus of the cell divides into two. a yeast cell. One of the nuclei moves into the bud. 3. The bud separates and continues When budding occurs rapidly, the to grow. Eventually it becomes individuals do not separate at once. As a completely independent. result, small groups of attached cells may sometimes be seen. Figure 5: Asexual reproduction in yeast using budding Ms Marika Pisani 5 Sexual Reproduction in Flowering Plants Gamete Formation Pollination Fertilization The flower produces Pollen is The male and pollen grains (each transfered to the female containing a male female part of gametes fuze to gamete) and ova the flower. from a zygote. (female gametes). Seed Dispersal Seed Formation Germination The seed is transported The zygote A plant away from the parent develops into an grows from plants. The seed may be embryo. A seed the seed. transported to an area (which contains that has ideal conditions the embryo) is for germination. formed. Figure 6: The process of sexual reproduction in flowering plants The Flower The flower is the reproductive organ of flowering plants. Some species of plants have separate male flowers and female flowers. Other species have hermaphrodite flowers which contain both female and male reproductive parts. Figure 7: Cross-section through a hermaphrodite flower Ms Marika Pisani 6  Structure Description Function Receptacle The expanded end of the It supports the flower. flower stalk Sepals Modified leaves which form a They protect the flower bud (the ring at the base of the flower structure that develops into a flower). Nectary Small glands found at the Produce nectar (a sugary solution base of insect pollinated which attracts animals) flowers Petals Modified Leaves They enclose the reproductive organs. In insect pollinated flowers, they attract insects. Structure Description Function Stamen Anther: It produces pollen grains. (The male Each pollen grain contains a reproductive male gamete. The pollen grain organ) protects the male gamete from drying out. Filament: A stalk that supports the anther. Carpel Stigma: The section of the carpel that (The female receives pollen. reproductive Style The section of the carpel that organ) supports the stigma. Ovary The lower part of the carpel that contains one or more ovules. Each ovule contains an ovum (the female gamete). Ms Marika Pisani 7 Pollination When the anthers are ripe, the pollen sacs split open and release the pollen. Pollination is the transfer of pollen from an anther to a stigma. Pollination may be either: Self-pollination: Pollen is transferred from an anther to the stigma of the same plant. Some plant species have adaptations that prevent self- pollination from taking place (since self-pollination does not allow for genetic diversity). Cross-pollination: Pollen from a flower reaches the stigma of a flower of another plant. The two plants must be of the same species for fertilization to occur. Figure 8: Self-pollination versus cross-pollination Ms Marika Pisani 8 There are two main types of pollination. In insect pollination, the pollen is transferred by means of insects (such as bees, butterflies and ants). In wind pollination, the pollen is transferred by wind. Species of flowering plants are adapted for either insect pollination or wind pollination. Insect Pollinated Flower Wind Pollinated Flower Figure 9: Structural differences between insect pollinated flowers and wind pollinated flowers. Feature Insect Pollinated Flowers Wind Pollinated Flowers Flower Large Small and inconspicuous Petals Large and colourful Small and dull Nectar Nectar is produced. Nectar is not produced. As the insect reaches into the flower for the nectar, the pollen attaches itself to the insect. Anthers The stigma is hooked. The stigma is sticky and and The stigma is located inside the feathery (it acts as a net which Stigma flower, surrounded by the anther traps pollen grains). and petals. The stigma and stamen hang outside the flower, where they are exposed to wind. Pollen The pollen is relatively heavy. Pollen is produced in large numbers (to increase the possibility that some of the Ms Marika Pisani 9  It has a hooked surface (so pollen reaches the stigma of a that it can easily become flower of the same species) attached to the insect). It is very light (so that it can be easily carried by wind currents). It has a smooth surface. Fertilization Fertilization takes place as follows: 1. A pollen grain reaches a stigma. The pollen grain absorbs liquid form the stigma. 2. A pollen tube grows out of the pollen grain and moves down the style. (The pollen grain forms a pollen tube only if it reaches the stigma of a flower of the same species.) 3. The pollen tube reaches an ovule. The male gamete moves down the pollen tube. 4. The male gamete reaches the ovule. The male nucleus fuses with the female nucleus. A zygote is formed. 5. The zygote divides many times to produce an embryo. Figure 10: The pollen tube When all the female gametes present in the ovary are fertilised, the development of pollen tubes by other pollen grains is stopped. Ms Marika Pisani 10 Seed and Fruit Formation Once the ovules present in an ovary are fertilized: The sepals, petals, stamens, stigma and style wither away and fall off. The ovules develop into seeds. The ovary develops into a fruit, which may or may not be edible. In some species of flowering plants, the ovary and the receptacle swell together to form the fruit. The ovaries are fertilized. The petals, stamens and The fruit carpel fall. The ovary continues to starts to grow. grow. Figure 11: Changes in floral structures during fruit formation The seed is made up of: An embryo which consists of a young shoot (or plumule) and a young root (or radicle) One or two cotyledons, that is, food stores formed when sugar is carried into the ovule through the phloem vessels. A seed coat (testa), that is, the thickened outer wall of the ovule which prevents dehydration of the embryo. Seeds also have a micropyle. This is a small hole which marks the point where the pollen tube entered the ovule. For germination to start, water must be absorbed through the micropyle. Ms Marika Pisani 11 Figure 12: Cross-section through a seed Seed Dispersal The main function of fruit is seed dispersal. Seed dispersal is the process by which seeds are carried to a distance away from the parent plant. Seed dispersal has the following advantages: It allows plant populations to establish themselves in new habitats. Since the offspring are not growing in the same area as the parents, there is less competition between parent plants and their offspring. The main methods of seed dispersal are summarised in the following table. ‘Parachute’ Fruits and ‘Winged’ Fruits ‘Explosive’ Fruits Seeds The ovary wall has The pod of the flower The fruit or seed has wing-like structures dries in the sun and feathery hairs which which slow down the shrivels. The fruit splits allow it to float over sinking of the fruit to in half down two lines Wind long distances before the ground. The fruit, of weakness. The two Dispersal sinking to the ground. therefore, is likely to halves curl back and As the fruit or seed, be carried away by flick out the seeds. sinks to the ground it is air currents. carried away by air currents. Hooked Fruits Succulent Fruits Animal The fruit has hooks which become Succulent fruits are eaten by birds Dispersal attached to the fur of passing and mammals. The seeds are not digested and are thus egested a Ms Marika Pisani 12  mammals. The seeds fall out as distance away from the parent the mammal moves about. plant. Many marine, beach and aquatic plants have seeds which can float. Water This Is because the fruits of these seeds are corky and/or contain air. Dispersal Water carries the seeds from one shore to another. Figure 13: Adaptations of seeds for seed dispersal Ms Marika Pisani 13 Germination Germination is the development of a seed into a young plant (or seedling). For a seed to germinate, it must land in a location which provides favourable conditions. These conditions include: Warm temperatures: In warm temperatures, the chemical reactions needed for germination to take place occur more quickly. − Germination is more rapid between temperatures of 30°C and 40°C. − Germination does not take place above 45°C. At this temperature the enzymes are denatured and the seedlings are killed. − Germination may not take place in cold temperatures such as below 5°C. Light: Most seeds are planted below the ground therefore, they do not need light to germinate. However: − The seeds of certain species of plants have to be exposed to light to germinate. − Once the seedling develops, it needs light for photosynthesis. Oxygen: The testa of most seeds is not permeable to oxygen. Therefore, the first stages of germination are anaerobic. Once the seed testa splits, oxygen is used in aerobic respiration. Respiration provides the energy needed for the seedling to grow. Water: For germination to start, water must be absorbed through the micropyle of the testa. Once the radicle emerges from the testa, it absorbs water from the soil. The water is used to: − activate the enzymes that convert the stored starch to sugar (which is then used for respiration) − transport the sugar from the cotyledons to the growing parts of the seed − expand the vacuoles of the new cells (thus making the root, shoot and leaves expand) − provide the water needed for photosynthesis If a seed falls in an area where the conditions are not favourable, it remains dormant (that is, it does not germinate) until favourable conditions are present. Dormancy allows the seed to resist low or high temperatures and water loss. Ms Marika Pisani 14 Challenges in Flowering and Non-Flowering Plants There are five main plant groups: algae, mosses, ferns, conifers and flowering plants. The last four of these groups have developed adaptations for a terrestrial mode of life. The following table summarises the challenges faced by terrestrial plants with regards to reproduction. Plant Group Mosses and Ferns Conifers Flowering Plants Production 1. The male 1. Male cones 1. The stamen of Offspring gametes swim in produce pollen. produce pollen. water to reach 2. Pollination (mostly 2. Wind or insect the female by wind) allows pollination allows reproductive the pollen to the pollen to organ. reach the female reach the carpel. 2. Fertilization takes cones. 3. Fertilization takes place. 3. Fertilization takes place. The ovules 3. Sporangia place. The develop into containing spores female cones seeds. The ovary develop. produce ‘naked develops into a seeds’ that are fruit. not enclosed in fruit. Challenges Fertilization To facilitate wind Insect pollination and cannot take pollination, the cannot occur in Adaptations place in the pollen is light and the absence of absence of water. powdery. In some pollinators. Gametes can be species, the Pollen can be lost lost in the pollen is winged. during wind process. Pollen can be lost pollination. It must Therefore, they during wind be produced in must be pollination. It must large amounts. produced in large be produced in Pollen from wind amounts. large amounts. and insect pollinated flowers have different characteristics. Ms Marika Pisani 15  Plant Group Mosses and Ferns Conifers Flowering Plants Dispersal of 1. The spores mature. 1. When the seeds are Fruits are Offspring When water is mature, the female adapted for present, the cone opens to wind, animal, sporangia open to release the seeds. explosive or release the spores. 2. The seeds may have water dispersal. 2. Spores are wings, which dispersed by wind promote wind or water. dispersal or bright colours and/or extensive food supplies to encourage birds and animals to help with dispersal. Challenges Water is required for Cotyledons store nutrients for growth of and the sporangia to the embryo. Adaptations open. Seeds require less water for germination. Spores are light and The testa allows the seed to tolerate harsh easily dispersed. environmental conditions. Spores do not However, if seed dispersal is not effective contain food stores. and the offspring germinates close to the Therefore, they parent, the offspring and parent will require more compete for light, water and minerals. favourable conditions to germinate. Spores require more water for germination. Although they have a protective coating, they are less prone to survive in harsh environmental conditions. Ms Marika Pisani 16 Practice Questions Sexual and Asexual Reproduction Germination AQA Biology page 197 questions 1-3 SEC Sept 2012 Paper 1 question 5 SEC Sept 2016 Paper 2B question 4 The Flower SEC May 2017 Paper 1 question 5 SEC May 2013 Paper 1 question 1b Challenges in Flowering and Non- Pollination flowering Plants Sample Paper Section A question 3 AQA Biology page 275 question 2 SEC Sept 2015 Paper 1 question 8a-b, d Seed Dispersal Sample Paper Section B SEC Sept 2012 Paper 1 question 4a SEC Sept 2018 Paper 1 question 3 SEC Sept 2020 Paper 1 question 10 Ms Marika Pisani 17