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Biology Form 6
Topic 1: Transportation
Transportation is the movement of substances from one part of an organism to another. In advanced biology, it explains how water, mineral salts, manufactured food, respiratory gases, hormones and wastes move efficiently in large multicellular organisms.
1.1Meaning and Importance of Transport
- Transport is necessary because large organisms have small surface area to volume ratio. Diffusion alone cannot supply every cell quickly enough, especially when cells are far from the external environment.
- Transport systems connect exchange surfaces with body cells. They help organisms obtain nutrients and oxygen, distribute hormones, remove metabolic wastes, maintain temperature, and keep internal conditions stable.
- Small organisms may depend mainly on diffusion, but large plants and animals require specialized conducting tissues or circulatory systems.
1.2Biophysical Principles of Transport
- Diffusion is movement of molecules from a region of high concentration to a region of low concentration until equilibrium is reached. It is important in gaseous exchange and movement of small solutes.
- Osmosis is the movement of water molecules through a selectively permeable membrane from a region of higher water potential to a region of lower water potential.
- Active transport is movement of substances against a concentration gradient using energy from ATP. It is important in mineral uptake by root hairs and reabsorption in animal kidneys.
- Mass flow is movement of a fluid in one direction due to pressure difference. In animals it occurs in blood circulation, while in plants it occurs in xylem and phloem.
1.3Transport in Plants
- Plants transport water and mineral salts mainly through xylem. Xylem vessels are dead, hollow, lignified tubes adapted for rapid upward movement of water and support.
- Water enters root hairs by osmosis because the cell sap has lower water potential than the surrounding soil solution. Mineral ions are often absorbed by active transport.
- Water moves across the root through the apoplast pathway, symplast pathway, and vacuolar pathway before entering the xylem.
- The ascent of sap is mainly explained by cohesion-tension theory. Water evaporates from leaves during transpiration, creating tension that pulls a continuous column of water upward. Cohesion keeps water molecules together and adhesion helps water attach to xylem walls.
- Transpiration is the loss of water vapour from aerial parts of the plant, mainly through stomata. It helps mineral transport, cooling, and maintaining a transpiration stream, but excessive transpiration may cause wilting.
- Phloem transports organic food such as sucrose from sources to sinks. A source is a region where food is produced or released, such as leaves. A sink is a region where food is used or stored, such as roots, fruits, seeds and growing buds.
- Translocation in phloem is explained by the pressure-flow hypothesis. Sucrose is actively loaded into sieve tubes, water enters by osmosis, pressure rises, and sap moves toward regions of lower pressure.
1.4Transport in Mammals
- Mammals have a closed, double circulatory system. Blood remains inside vessels and passes through the heart twice during one complete circulation: pulmonary circulation and systemic circulation.
- The heart is a muscular pump with four chambers: right atrium, right ventricle, left atrium and left ventricle. Valves prevent backflow and ensure one-way movement of blood.
- Arteries carry blood away from the heart under high pressure. They have thick elastic walls and narrow lumen. Veins carry blood toward the heart under low pressure and have valves. Capillaries are thin-walled vessels where exchange occurs between blood and tissues.
- Blood is made of plasma, red blood cells, white blood cells and platelets. Plasma transports dissolved substances. Red blood cells transport oxygen using haemoglobin. White blood cells defend the body. Platelets help in blood clotting.
- Tissue fluid forms when plasma is forced out of capillaries by hydrostatic pressure. It surrounds cells and allows exchange of materials. Excess tissue fluid returns to blood through lymphatic vessels.
- The lymphatic system returns excess tissue fluid to the blood, transports fats from the ileum, and supports body defence through lymph nodes and lymphocytes.
1.5Common Examination Points
- Explain why diffusion alone is not enough in large organisms.
- Compare xylem and phloem in structure and function.
- Explain cohesion-tension theory and pressure-flow hypothesis.
- Compare arteries, veins and capillaries.
- Describe the formation and importance of tissue fluid and lymph.
Biology Form 6
Topic 2: Growth and Development
Growth is a permanent and irreversible increase in size, dry mass and number of cells. Development is the progressive change in structure and function leading to maturity. Growth and development are controlled by cell division, cell enlargement, differentiation, hormones and environmental conditions.
2.1Meaning of Growth and Development
- Growth involves an increase in cell number by mitosis, increase in cell size by enlargement, and increase in dry mass due to synthesis of new cellular materials.
- Development includes differentiation, specialization and morphogenesis. Differentiation is the process by which cells become specialized to perform particular functions.
- Growth can be measured using length, height, fresh mass, dry mass, surface area, number of cells, or volume. Dry mass is more reliable than fresh mass because fresh mass is affected by water content.
2.2Growth Curves
- A sigmoid growth curve is common in many organisms. It has lag phase, log or exponential phase, decelerating phase and stationary phase.
- During the lag phase, growth is slow because cells are adapting and preparing for division. During the log phase, growth is rapid because cell division and cell enlargement are active.
- During the decelerating phase, growth rate slows due to limiting factors such as nutrients, space or accumulation of wastes. During the stationary phase, growth becomes stable as maturity is reached or resources become limited.
2.3Growth in Plants
- Plant growth occurs mainly in meristematic regions. Apical meristems at tips of roots and shoots cause primary growth, which increases length. Lateral meristems such as vascular cambium and cork cambium cause secondary growth, which increases girth.
- In roots and shoots, three main zones occur: zone of cell division, zone of elongation and zone of differentiation. Cells divide actively in the meristem, elongate behind it, and later specialize into tissues such as xylem, phloem, epidermis and cortex.
- Plant hormones regulate growth. Auxins promote cell elongation and apical dominance. Gibberellins promote stem elongation and seed germination. Cytokinins promote cell division. Abscisic acid promotes dormancy and stress responses. Ethene promotes fruit ripening and leaf abscission.
2.4Seed Germination
- Germination is the process by which a seed resumes growth and develops into a seedling. It requires water, oxygen and suitable temperature. Some seeds also require light or special treatment to break dormancy.
- Water activates enzymes, softens the seed coat and causes swelling. Oxygen is needed for aerobic respiration to release energy. Suitable temperature allows enzymes to work efficiently.
- In epigeal germination, cotyledons are brought above the soil surface, as in beans. In hypogeal germination, cotyledons remain below the soil surface, as in maize.
2.5Growth in Animals
- Animal growth occurs throughout the body in early stages, but many tissues later stop growing or grow slowly after maturity. Growth is controlled by hormones, nutrition, genes and environmental conditions.
- In insects, growth is discontinuous because the exoskeleton restricts expansion. The insect must moult by shedding the old cuticle. Growth occurs rapidly after moulting before the new cuticle hardens.
- Metamorphosis is a change in body form during development. Complete metamorphosis has egg, larva, pupa and adult stages. Incomplete metamorphosis has egg, nymph and adult stages.
2.6Factors Affecting Growth
- Internal factors include genes and hormones. External factors include nutrition, temperature, light, oxygen, water, mineral salts, disease and competition.
- Balanced nutrition provides energy and raw materials for synthesis of new tissues. Lack of proteins, vitamins or mineral salts may retard growth.
- Temperature affects enzyme activity. Very low temperature slows metabolism, while very high temperature may denature enzymes.
Biology Form 6
Topic 3: Reproduction
Reproduction is the biological process by which organisms produce new individuals of their own kind. It ensures continuity of species and transfer of genetic information from one generation to another.
3.1Types of Reproduction
- Asexual reproduction involves one parent and does not involve fusion of gametes. It produces offspring that are genetically identical to the parent, unless mutation occurs.
- Sexual reproduction involves formation and fusion of male and female gametes. It produces genetically variable offspring, which increases chances of adaptation and survival in changing environments.
- Asexual reproduction is rapid and useful in stable environments, while sexual reproduction is slower but produces variation.
3.2Asexual Reproduction
- Binary fission occurs when one organism divides into two equal daughter cells, for example in Amoeba and bacteria.
- Budding occurs when a new individual develops as an outgrowth from the parent, for example in Hydra and yeast.
- Fragmentation occurs when a body breaks into pieces and each piece grows into a new organism, for example in some algae.
- Spore formation occurs when spores are produced and later germinate under favourable conditions, for example in fungi and ferns.
- Vegetative propagation in plants involves production of new plants from vegetative parts such as stems, roots and leaves. Examples include runners, rhizomes, tubers, bulbs and suckers.
3.3Sexual Reproduction in Flowering Plants
- The flower is the reproductive structure of angiosperms. Stamens produce pollen grains, while carpels contain ovules inside the ovary.
- Microsporogenesis produces pollen grains in the anther. Megasporogenesis produces the embryo sac in the ovule.
- Pollination is the transfer of pollen grains from anther to stigma. It may be self-pollination or cross-pollination. Cross-pollination increases genetic variation.
- After pollination, pollen germinates on the stigma and forms a pollen tube. The male nuclei move down the pollen tube toward the embryo sac.
- Double fertilization is unique to flowering plants. One male nucleus fuses with the egg cell to form a diploid zygote, while the other male nucleus fuses with polar nuclei to form the triploid endosperm nucleus.
- After fertilization, the ovule develops into a seed, the ovary develops into a fruit, and the zygote develops into an embryo.
3.4Sexual Reproduction in Mammals
- The male reproductive system produces sperm in the testes. Sperm production is called spermatogenesis. Testosterone supports sperm production and development of male secondary sexual characteristics.
- The female reproductive system produces ova in the ovaries. Ovulation is the release of a mature ovum from the ovary. Oestrogen and progesterone regulate the menstrual cycle and prepare the uterus for pregnancy.
- Fertilization usually occurs in the oviduct. The zygote divides by mitosis to form an embryo, which implants in the uterine lining.
- The placenta allows exchange of materials between mother and foetus. It supplies oxygen and nutrients, removes carbon dioxide and wastes, and secretes hormones that maintain pregnancy.
3.5Menstrual Cycle
- The menstrual cycle is controlled by hormones from the hypothalamus, pituitary gland and ovaries. The main hormones are FSH, LH, oestrogen and progesterone.
- FSH stimulates development of follicles in the ovary. Oestrogen repairs and thickens the uterine lining. A surge of LH causes ovulation. Progesterone maintains the uterine lining after ovulation.
- If fertilization does not occur, progesterone and oestrogen levels fall, causing menstruation. If fertilization occurs, the embryo produces hormones that maintain the corpus luteum and pregnancy.
3.6Examination Points
- Compare asexual and sexual reproduction.
- Explain double fertilization in flowering plants.
- Describe the roles of FSH, LH, oestrogen and progesterone.
- Explain adaptations of placenta for exchange.
- Describe advantages and disadvantages of vegetative propagation.
Biology Form 6
Topic 4: Genetics
Genetics is the study of heredity and variation. It explains how characteristics are passed from parents to offspring through genes, how variation arises, and how genetic information is expressed in organisms.
4.1Basic Genetic Terms
- A gene is a unit of heredity found on a chromosome. It controls a particular characteristic by coding for a polypeptide or functional RNA.
- An allele is an alternative form of a gene. For example, a gene for seed shape may have a dominant allele and a recessive allele.
- Genotype is the genetic constitution of an organism. Phenotype is the observable expression of the genotype as influenced by the environment.
- Homozygous means having two identical alleles for a gene. Heterozygous means having two different alleles for a gene.
- Dominant alleles express themselves in heterozygous condition, while recessive alleles express themselves only when homozygous.
4.2Mendelian Inheritance
- Mendel used pea plants to study inheritance. His work led to the law of segregation and the law of independent assortment.
- The law of segregation states that the two alleles of a gene separate during gamete formation so that each gamete receives only one allele.
- The law of independent assortment states that alleles of different genes assort independently during gamete formation, provided the genes are on different chromosomes or far apart on the same chromosome.
- Monohybrid inheritance involves one pair of contrasting characteristics. Dihybrid inheritance involves two pairs of contrasting characteristics.
4.3Non-Mendelian Inheritance
- Incomplete dominance occurs when the heterozygous phenotype is intermediate between two homozygous phenotypes, such as red and white flowers producing pink flowers.
- Codominance occurs when both alleles are fully expressed in the heterozygote, such as AB blood group in humans.
- Multiple alleles occur when more than two alleles exist for one gene in a population. The ABO blood group system is a common example.
- Sex-linked inheritance involves genes found on sex chromosomes. X-linked recessive traits are more common in males because males have only one X chromosome.
- Epistasis occurs when one gene masks or modifies the expression of another gene at a different locus.
4.4Linkage and Crossing Over
- Linkage is the tendency of genes on the same chromosome to be inherited together. Linked genes do not assort independently unless crossing over separates them.
- Crossing over occurs during prophase I of meiosis. It involves exchange of genetic material between non-sister chromatids of homologous chromosomes.
- Crossing over produces new combinations of alleles and increases genetic variation. The frequency of crossing over can be used to estimate the distance between genes on a chromosome.
4.5Mutation
- Mutation is a sudden change in genetic material. It may occur at gene level or chromosome level.
- Gene mutations include substitution, insertion and deletion of bases. Chromosomal mutations include deletion, duplication, inversion, translocation and changes in chromosome number.
- Mutations may be harmful, beneficial or neutral. They are important because they introduce new genetic variation, which is raw material for evolution.
- Mutagens are agents that increase mutation rate. Examples include radiation, some chemicals and certain viruses.
4.6Application of Genetics
- Genetics is applied in plant and animal breeding, diagnosis of inherited diseases, genetic counselling, forensic science, biotechnology and conservation biology.
- Selective breeding uses desired traits to improve crops and animals. Genetic engineering allows transfer of specific genes into organisms to produce useful products such as insulin or pest-resistant crops.
- Knowledge of inheritance helps predict genetic disorders and understand biological variation among individuals and populations.
Biology Form 6
Topic 5: Evolution
Evolution is the gradual change in genetic composition of populations over successive generations. It explains unity and diversity of living organisms, adaptation, speciation and the origin of new forms from pre-existing forms.
5.1Meaning of Evolution
- Evolution may be defined as a change in allele frequencies in a population over generations. It may also be understood as gradual development from simple to more complex forms.
- Organic evolution refers to changes in living organisms that may eventually lead to formation of new species. A species is a group of organisms that can interbreed naturally and produce fertile offspring.
- Evolution does not occur in individuals during their lifetime. It occurs in populations across generations.
5.2Forces of Evolution
- Mutation is the original source of new alleles. Without mutation, no completely new genetic variation would arise.
- Gene recombination creates new combinations of existing alleles through crossing over, independent assortment and random fertilization.
- Natural selection acts on variation. Organisms with favourable traits survive and reproduce more successfully than those with less favourable traits.
- Gene flow is movement of alleles between populations through migration. It can increase variation in a population or reduce differences between populations.
- Genetic drift is random change in allele frequency, especially in small populations. It may occur through founder effect or bottleneck effect.
- Isolation prevents interbreeding between populations and allows differences to accumulate, leading to speciation.
5.3Theories of Origin of Life
- Special creation proposes that life was created by a supernatural power. It is based on faith and is not tested by scientific experiment.
- Spontaneous generation proposed that life arose from non-living matter. It was rejected after controlled experiments showed that living organisms arise from pre-existing living organisms.
- Biochemical evolution proposes that life arose gradually from simple chemicals under early Earth conditions. Organic molecules formed, combined into polymers, and eventually formed self-replicating systems.
- Panspermia proposes that life or life-forming materials came from space. It suggests a possible source of life but does not fully explain the original origin of life.
5.4Lamarckism and Darwinism
- Lamarck proposed that organisms change because of use and disuse of organs and that acquired characteristics are inherited. The theory is historically important but inheritance of acquired characteristics is not accepted as a general mechanism.
- Darwin proposed evolution by natural selection. Individuals in a population vary, organisms produce more offspring than can survive, resources are limited, and individuals with favourable variations survive and reproduce.
- Natural selection leads to adaptation because favourable traits become more common in a population over generations.
5.5Natural and Artificial Selection
- Natural selection occurs when the environment acts as the selecting agent. Examples include resistance to antibiotics in bacteria and industrial melanism in peppered moths.
- Artificial selection occurs when humans choose organisms with desired traits and breed them. It is used to improve crop yield, milk production, disease resistance and other useful characteristics.
- Artificial selection can produce rapid change but may reduce genetic diversity if only a few individuals are repeatedly used for breeding.
5.6Speciation and Isolation
- Speciation is the formation of new species from pre-existing species. It occurs when populations become isolated and accumulate genetic differences until they can no longer interbreed successfully.
- Geographical isolation occurs when physical barriers such as mountains, rivers, oceans or distance separate populations.
- Reproductive isolation prevents mating or production of fertile offspring. It may be mechanical, behavioural, seasonal, ecological, gametic or post-zygotic.
- Adaptive radiation occurs when one ancestral species gives rise to many species adapted to different ecological niches.
5.7Evidence for Evolution
- Fossil records show that organisms in older rock layers are generally simpler, while younger layers contain more complex and more recent forms.
- Comparative anatomy provides evidence through homologous structures, analogous structures and vestigial organs. Homologous structures suggest common ancestry.
- Embryology shows similarities in early development of related organisms. Biochemistry shows that organisms share DNA, RNA, ATP and many similar proteins.
- Biogeography shows how distribution of organisms is related to continental drift, isolation and adaptive radiation.
- Direct observation includes rapid evolution such as antibiotic resistance in bacteria and pesticide resistance in insects.