- Have large brightly coloured petals; which attract insects.
- Presence of nectaries that secrete sweet-scented nectar which serve as bait for the insects.
- Have small, sticky stigmas which enable the pollen grains from the insects' body to stick onto it.
- Anthers are small and held firmly onto the filaments to ensure that they don't break when the insects rub against them as they crawl into the flower.
- Have large, heavy, and sticky pollen grains to stick on the insects' bodies.
- The anthers produce few but large grains to increase the chances of their transfer to a stigma for pollination.
- Flowers normally have shapes that are convenient for the insects' bodies thus the insects fit comfortably e.g. flower petals forming a corolla tube
Distinguish Between Cross-pollination and Self-pollination
Self-pollination is the transfer of pollen grains from the anther to the stigma of the same flower; while cross-pollination is the transfer of pollen grains to the stigma of another flower either on the same plant or another but of the same species.
Describe Pollination, Fertilization and Seed and Fruit Formation in Plants
Pollination
- The pollen grains land and stick onto the stigma.- The stigma secretes/produces a sticky chemical fluid that causes pollen grains to adhere to it.
- The sticky secretions also stimulate the pollen grain to germinate into a pollen tube.
- The pollen tube grows into the stigma and protrudes down the style tissues.
- The pollen tube derives its nutrition from the style tissues.
- As the tube grows downwards, the tube nucleus occupies the position just behind the tip.
- The generative nucleus then enters the tube; and divides mitotically to give a pair of male gamete nuclei, which follow the tube nucleus, as the pollen tube grows further down the style.
- On reaching the ovary the pollen tube enters the ovary via the micropyle.
- It then penetrates the embryo sac wall and bursts open.
- The tube (vegetative) nucleus then disintegrates, leaving a clear passage for the entry of the two male gamete nuclei.
Fertilization
- Fertilization takes place when one male gamete fuses with both polar nuclei to form a triploid nucleus called the primary endosperm nucleus.- The other male nucleus fuses with the functional egg cell to form a Diploid zygote.
- This is Double fertilization, a unique feature of flowering plants.
- After fertilization, a number of changes (post-fertilization changes) occur in the flower; Main changes include the following: -
- The zygote divides mitotically, growing and developing into an embryo.
- The embryo becomes differentiated into a radicle (embryonic root), plumule (embryonic shoot), and one or 2 cotyledons.
- The triploid primary endosperm nucleus divides mitotically to form endosperm, which surrounds the developing embryo and provides it with nourishment.
Seed and Fruit Formation
- The embryo sac expands to accommodate the developing and growing embryo.- The nucellus becomes crushed so that the embryo and endosperm come to fill the whole area inside the integuments.
- The ovules develop into the seed. The integuments become the seed coat (The outer integument becomes the testa and the inner integuments become the tegmen).
- The final transformation of the ovule into seed involves the removal of water from an initial 90% to the final 15% thus forming a dormant resistant structure that can withstand adverse conditions.
- The ovary develops into a fruit; this protects the seed and aids in dispersal.
- The ovary wall becomes the pericarp (wall of the fruit).
- Petals, stamen, style, and stigma wither, die, and fall off.
Question
Distinguish between diffusion and active transport
Answers
- In diffusion molecules move/along concentration gradient from a highly concentrated region to a lowly concentrated region while in active transport molecules move/against concentration gradient from a lowly concentrated region to a highly concentrated region
- No energy is required in diffusion while energy is required in active transport
- In diffusion no carrier molecules are required while in active transport, carrier molecules are required
- No energy is required in diffusion while energy is required in active transport
- In diffusion no carrier molecules are required while in active transport, carrier molecules are required
Structure and Functions of the Cell Organelles
Cell membrane
- The cell membrane, also known as plasma membrane or plasmalemma consists of three layers when viewed under the electron microscope.
- The three layers are composed of one layer of phospholipid sandwiched between two protein layers.
- It is flexible and has pores. The cell membrane is important in that:
- The three layers are composed of one layer of phospholipid sandwiched between two protein layers.
- It is flexible and has pores. The cell membrane is important in that:
- It encloses the cell contents.
- It allows for selective movement of materials in and out of the cells. The pores allow materials particularly of small molecular size to move in and out of the cells.
Cytoplasm
- Cytoplasm consists of a fluid medium in which chemical reactions take place.
- It contains organelles and other inclusions such as starch, glycogen, fat droplets and many other dissolved substances.
- Cytoplasm is not static; it undergoes a movement known as cytoplasmic streaming.
- It provides a suitable medium for cellular reactions to take place.
- It contains organelles and other inclusions such as starch, glycogen, fat droplets and many other dissolved substances.
- Cytoplasm is not static; it undergoes a movement known as cytoplasmic streaming.
- It provides a suitable medium for cellular reactions to take place.
Mitochondrion
- Mitochondrion is a sausage shaped organelle that provides sites for respiratory reactions that yield energy for the cell. Mitochondria is thus, referred to as the powerhouse of the cell.
- It is bound by two membranes. The inner membrane is greatly folded into cristae to increase surface area for respiration.
- Such cells include muscle cell, sperm cell, apical meristem cells, and kidney cell.
- Mitochondria are self replicative that is they can divide to form new ones.
Endoplasmic Reticulum
- Endoplasmic reticulum appears as a series of interconnected channels, running throughout the cytoplasm.
- Their membranes are continuous with the outer membrane of the nuclear membrane.
- Some endoplasmic reticula have granules called ribosomes on their surfaces and are referred to as rough or granular endoplasmic reticula.
- Endoplasmic reticula that are not associated with ribosomes are called smooth endoplasmic reticula.
- Generally, endoplasmic reticula also act as storage areas for synthesized molecules such as enzymes. They also contribute to mechanical support.
Ribosomes
- These are spherical in shape. While some are bound to the endoplasmic reticula, some ribosomes are scattered within the cytoplasm (free ribosomes). Their largest dimension is 25 nanometres.
- They are synthesised in the nucleolus.
- They form sites for protein synthesis.
Lysosomes
- These are spherical sac-like organelles bound by a single membrane. They contain lytic enzymes which break down large molecules, destroy worn out organelles or even the entire cells.
- Lysosomes also play crucial role in digestion in unicellular organisms.
- The lysosomes are also vital in breakdown of bacteria and other harmful microbes that might have been ingested in food. This explains their high relative abundance in injured or infected cells.
- The membrane of the lysosomes are intact. This is important because if the enzymes leak out, they may destroy the whole cell.
Golgi bodies/Golgi apparatus
- These are stacks of membrane bound tube like sacs. They are found close to the cell membrane.
- Golgi bodies perform the following functions;
- Golgi bodies perform the following functions;
- They package and transport glycoproteins.
- They are involved in secretion of synthesized proteins and carbohydrates.
- They manufacture lysosomes.
Centrioles
- These are rod shaped structures located just outside the nuclear membrane.
- They take part in cell division and also in the formation of cilia and flagella in lower organisms.
- Plant cells lack centrioles.
- They take part in cell division and also in the formation of cilia and flagella in lower organisms.
- Plant cells lack centrioles.
Chloroplasts
- Chloroplasts are egg-shaped structures surrounded by two membranes and contain a gel-like stroma through which runs a system of membranes that are stacked together to form grana.
- The granum contains chlorophyll which traps light energy that is used during photosynthesis.
- It is in the chloroplasts that photosynthesis takes place.
- The granum contains chlorophyll which traps light energy that is used during photosynthesis.
- It is in the chloroplasts that photosynthesis takes place.
Vacuoles- These are sacs that are filled with fluid called cell sap. Vacuoles vary in size.
- Animal cells contain small vacuoles which may be numerous in the cells while plant cells contain one large centrally placed vacuole.
- Sap vacuoles store sugars and salts thereby contributing to the osmotic properties of the cell. This influences how materials move in and out of the cell.
- In some unicellular organisms, food vacuole stores and digests food substances while the contractile vacuole excretes unwanted materials from the cell.
Cell wall
- This is the rigid outer cover of plant cells and some lower organisms.
- In plants it is composed of cellulose fibres.
- Cell wall is important in that:
- In plants it is composed of cellulose fibres.
- Cell wall is important in that:
- It gives plant cells their definite shape
- It provides mechanical support and protection against mechanical injury.
- The cell wall allows gases, water and other substances to pass through it.
Nucleus
Nucleus is a double membrane bound structure made up of a viscous fluid known as nucleoplasm in which nucleolus and chromatin materials are suspended. The nuclear membrane has minute pores, nuclear pores which allow materials to move in and out of the nucleus.
- Nucleus controls all the activities of the cell.- Nucleolus is responsible for manufacture of ribosomes while chromatin contains hereditary materials.
- Nucleus generally takes a spherical or oval shape.
Comparison between Plant Cells and Animal Cells
| Plants | Animals |
|---|---|
| Usually Large | Smaller in Size |
| Regular in shape | Irregular in shape |
| Has a cell wall | Has no cell wall |
| Usually has a large vacuole | Usually has no vacuole but when present, they are often temporary and small structures within the cytoplasm |
| Cytoplasm and Nucleus are usually located towards the periphery of the cell | Cytoplasm occupies most space in the cell with the nucleus usually centrally placed |
| Some have chloroplasts | Has no chloroplasts |
| Usually most store oils, starch and proteins | Store glycogen and fats |
| Has no centriole | Has centrioles |
Summary: Introduction to Biology (Cell, Physiology and Nutrition)
1. Introduction to Biology
- Biology is the study of life (Greek bios = life, logos = study).
- Main branches: botany (plants), zoology (animals), microbiology (microorganisms), anatomy and morphology (internal and external structure), physiology (function), biochemistry, cytology (cells), genetics, ecology, taxonomy, histology (tissues), virology, bacteriology, entomology (insects), ichthyology (fish).
- Value: it explains how the body works and supports careers in medicine, nutrition, public health, dentistry, agriculture, environmental studies and teaching.
2. Characteristics of Living Things
Living organisms show: nutrition, respiration, gaseous exchange, excretion, growth and development, reproduction, irritability (response to the environment) and movement.
3. Collecting and Observing Organisms
- Equipment: knives, polythene bags, collecting and killing jars, gloves, sweep nets, pooters, traps, forceps, hand lens.
- Observation: note the habitat, feeding, interactions and numbers. Record colour, texture and size.
- Biological drawings: show the magnification, calculated as length of drawing ÷ length of specimen (no units).
- Plants vs animals: plants are fixed, respond slowly, have cellulose cell walls and make their own food. Animals move, respond quickly, have no cell walls and feed on ready-made food.
4. Classification
- Organisms are grouped by shared external features and structure. This makes study easier, aids identification and shows evolutionary relationships.
- Hierarchy (smallest to largest): species → genus → family → order → class → phylum (division in plants) → kingdom.
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Five kingdoms:
- Monera: prokaryotes such as bacteria.
- Protoctista: unicellular eukaryotes such as algae and protozoa.
- Fungi: no chlorophyll, mostly saprophytic.
- Plantae: multicellular, autotrophic (mosses, ferns, seed plants).
- Animalia: multicellular, heterotrophic (e.g. annelids, molluscs, arthropods, chordates).
- Binomial nomenclature: a two-part scientific name, with the genus capitalised and the species in lower case. Both are italicised or underlined (Homo sapiens, Zea mays).
5. The Cell
- Light microscope: total magnification = eyepiece × objective (e.g. ×10 × ×4 = ×40). Key parts include the eyepiece, objectives, nosepiece, coarse and fine focus, stage, condenser, diaphragm and mirror. Handle it carefully with both hands, clean lenses with lens paper, and store it dust-free.
- Light microscope view: cell membrane, cytoplasm, nucleus, and in plants a cellulose cell wall, a large vacuole and chloroplasts.
- Electron microscope: magnifies up to 500,000× and has higher resolution, but cannot be used on living specimens.
-
Organelles:
- Plasma membrane: controls what enters and leaves the cell.
- Endoplasmic reticulum: transport (rough carries proteins, smooth carries lipids).
- Ribosomes: protein synthesis.
- Golgi bodies: package materials and produce lysosomes.
- Mitochondria: respiration and energy release.
- Lysosomes: digest foreign or worn-out material.
- Nucleus: holds DNA and chromosomes.
- Chloroplasts: photosynthesis.
- Plant vs animal cells: plant cells have a cell wall, chloroplasts and a large central vacuole, and are larger and more regular. Animal cells have centrioles, store glycogen and fats, and are smaller and irregular.
- Organisation: specialised cells → tissues → organs → organ systems → organism.
- Animal tissues: epithelial, muscular (skeletal, smooth, cardiac), supporting (cartilage, bone), blood and nerve.
- Plant tissues: meristematic, parenchyma, collenchyma, sclerenchyma, and vascular (xylem, phloem).
6. Cell Physiology (Movement Across Membranes)
- The membrane is a phospholipid bilayer with embedded proteins. It is semi-permeable, charged (polar) and damaged by extreme heat or pH.
- Diffusion: particles move from high to low concentration with no energy needed. It is faster with a steeper gradient, higher temperature, smaller molecules, thinner surfaces and a larger surface-area-to-volume ratio. It is used in gaseous exchange, absorption and excretion.
-
Osmosis: diffusion of water through a semi-permeable membrane.
- Solution types: hypertonic (cell loses water), hypotonic (cell gains water) and isotonic (no net movement).
- Plant cells: they become turgid when full of water and undergo plasmolysis in a hypertonic medium. Wilting is loss of turgor through excess transpiration.
- Animal cells: red blood cells burst (haemolysis) in hypotonic solutions and shrivel (crenation) in hypertonic ones.
- Active transport: solutes move from low to high concentration, using respiratory energy and carrier proteins. It depends on oxygen, temperature and carbohydrate supply, and is stopped by metabolic poisons such as cyanide. It is used in root mineral uptake, absorption of digested food, kidney reabsorption and nerve sodium pumps.
- Practicals: potassium permanganate diffusion, visking tubing and potato osmosis, and turgor and plasmolysis in onion epidermis.
7. Nutrition in Plants
- Leaf structure: epidermis and cuticle (protection), palisade mesophyll (main photosynthetic layer), spongy mesophyll (gas exchange), vascular bundles (transport) and stomata.
- Chloroplasts: the grana hold chlorophyll and carry out the light stage. The stroma carries out the dark stage.
-
Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, using light and chlorophyll.
- Light stage: water is split (photolysis), oxygen is released, and ATP and hydrogen carriers are produced.
- Dark stage (Calvin cycle): CO₂ is fixed into sugars. The first product is a 3-carbon sugar, which becomes glucose, sucrose (transport form) and starch (storage form).
- Limiting factors: CO₂ concentration, light intensity, water, temperature (optimum about 30°C) and chlorophyll.
8. Chemical Compounds of Life
-
Carbohydrates: contain C, H and O.
- Monosaccharides (glucose, fructose, galactose) are reducing sugars used for energy.
- Disaccharides (sucrose, maltose, lactose) are formed by condensation and broken down by hydrolysis. Sucrose is non-reducing.
- Polysaccharides: starch and glycogen (storage), cellulose (cell walls), chitin (arthropod exoskeleton), inulin.
- Lipids: a triglyceride is made of glycerol and three fatty acids. They provide energy, insulation, protection and metabolic water, and form membranes.
-
Proteins: chains of amino acids joined by peptide bonds.
- They provide structure (keratin, elastin), protection (antibodies, fibrinogen), regulation (enzymes, hormones), transport (haemoglobin) and movement (actin, myosin), and are a last-resort energy source.
- Amino acids are either essential (must be in the diet) or non-essential.
- Enzymes: protein catalysts that are specific, reusable, effective in small amounts and fast. Their activity depends on temperature (denatured above about 40°C), pH, and enzyme and substrate concentration. Inhibitors slow them, and cofactors (ions, prosthetic groups, coenzymes) help them.
9. Nutrition in Animals
- Feeding types: holozoic, saprophytic and parasitic. Animals can be carnivores, herbivores or omnivores.
-
Mammalian teeth (heterodont):
- Incisors cut, canines tear, premolars and molars grind.
- Herbivores: chisel-like incisors, a diastema, ridged grinding teeth and open roots.
- Carnivores: pointed incisors, long canines, carnassial teeth and closed roots.
- Omnivores: a mix of both.
- Tooth structure: enamel, dentine, pulp cavity, cement.
-
Dental problems and care:
- Caries: bacteria form acid that corrodes the enamel.
- Periodontal disease (gum disease): caused by plaque and poor hygiene.
- Care: brushing, flossing, a balanced diet with fluoride in small amounts, less sugar and regular dental check-ups.
10. Human Digestion
- Pathway: mouth → oesophagus → stomach → small intestine (duodenum, ileum) → large intestine → anus. The liver and pancreas assist.
- Processes: ingestion, digestion, absorption, assimilation and egestion.
-
Where digestion happens:
- Mouth: amylase turns starch into maltose.
- Stomach: pepsin (protein), rennin (milk) and hydrochloric acid, at about pH 1.5–2.
- Duodenum: bile emulsifies fats, and pancreatic trypsin, amylase and lipase act.
- Ileum: intestinal enzymes complete digestion.
-
Ileum adaptations for absorption: long and coiled, with villi and microvilli, thin epithelium, a rich blood supply and lacteals.
- Glucose and amino acids go into the blood, then to the liver via the hepatic portal vein.
- Fatty acids and glycerol go into the lacteals.
- Water is absorbed mainly in the colon.
- Assimilation: glucose is used for energy or stored as glycogen or fat. Amino acids build proteins, and excess is deaminated to urea in the liver. Fats are stored, used for energy or used in membranes.
11. Diet and Deficiency
-
Vitamins: needed in small amounts and not stored. B and C are water-soluble and the rest are fat-soluble.
- Deficiencies: A (night blindness), B₁ (beriberi), B₃ (pellagra), B₁₂ (pernicious anaemia), C (scurvy).
- Minerals: iron (anaemia), iodine (goitre, cretinism), fluorine (strong enamel), and others such as potassium, chloride, magnesium, sulphur, copper, zinc, cobalt, manganese, molybdenum and chromium.
- Water: makes up 60–70% of body weight. It acts as a solvent, a reaction medium and a coolant.
- Roughage: cellulose fibre adds bulk and aids peristalsis but is not digested by humans.
- Energy needs vary with age, sex, health and occupation.
- Balanced diet: carbohydrates, proteins, lipids, vitamins, minerals, water and fibre in the right proportions.
- Malnutrition: marasmus (lack of food overall), kwashiorkor (lack of protein), rickets, goitre and anaemia.