INTRODUCTION TO BIOLOGY
What is Biology?
Biology is the branch of science that deals with the study of living things. In Greek, Bios means life while Logos means knowledge.
Branches of biology
There are two main branches:
- Botany: Study of plants
- Zoology: Study of animals
The others include:
- Ecology: Study of living things in their surroundings.
- Genetics: The study of inheritance and variation.
- Entomology: Study of insects
- Parasitology: Study of parasites
- Taxonomy: Study of classification of organisms
- Microbiology: Study of microscopic organisms
- Anatomy: Study of structure of cells
- Cytology: Study of cells
- Biochemistry: Study of chemical changes inside living organisms
Name at least six other smaller branches of biology (6 marks).
Importance of Biology
- Solving environmental problemsg. Food shortage, poor health services, pollution, misuse of environmental resources etc.
- Choice of careers e.g. Medicine, Agriculture, public health, Veterinary, Animal husbandry, Horticulture, Dentistry etc.
- Acquiring scientific skills g. observing, identifying, recording, classification, measuring, analyzing, evaluating etc.
- International co-operation g. Development of HIV\AIDS vaccine, fight against severe Acute respiratory Syndrome (SARS), fight to save ozone layer from depletion, management of resources through international depletion.
- Help on study of other subjects
- Learn what living things are made up of and their bodies work
- Acquire knowledge about plant and animal diseases and their treatment.
- Know the effects of our bodies on drug and substance abuse and can kill.
- Learn about HIV\AIDS diseases and other viral diseases e.g. its treatment—balanced diets, proper hygiene, spreading, sexual behavior, cultural practices etc.
List five professional occupations that require the study of biology. (5 marks)
Characteristics of living things;
- Nutrition: Process by which living things acquire and utilize nutrients: plants photosynthesize; animals feed on already manufactured foods.
- Respiration: energy-producing process occurring in all the cells of living things.
- Gaseous Exchange: where living things take in air (oxygen) and give out air(carbon iv oxide) across respiratory surfaces.
- Excretion: Process by which waste or harmful materials resulting from chemical reactions within cells of living things are eliminated. Excess of such materials poison living things.
- Growth and Development: Growth –is the irreversible increase in size and Mass.—Essential for body function. Development –Irreversible change in complexity of the structure of living things.
- Reproduction: Process by which living things give rise to new individuals of the same kind.
- Irritability: Is the ability of living things to perceive changes in their surroundings and respond to them appropriately. E.g. reaction to changes in temperature, humidity, light, pressure and to the presence of certain chemicals.
- Movement: Change in position by either a part or the whole living thing. Locomotion – Progressive change in position by the whole living thing. In animals, movement include; swimming, walking, running, flying. In plants, closing of leaves, folding of leaves, closing of flowers, growing of shoots towards light etc.
- List four uses of energy obtained from the process of respiration. (4 marks).
- List six characteristics of living things (6 marks).
Collection of specimens
- Sweep net: for catching flying insects.
- Fish net: For trapping small fish and other small water animals.
- Pooter:For sucking small animals from rock surfaces and tree barks.
- Bait trap: For attracting and trapping small animals e.g. rats.
- Pit fall trap: For catching crawling animals.
- Pair of forceps: picking up small crawling animals e.g. stinging insects.
- Specimen bottles: keeping collected specimen. Larger specimens require large bottles.
- The magnifying lens: Instrument used to enlarge objects. Lenses are found in microscope and the hand lens (magnifier). Its frame is marked e.g. x8 or x10—indicating how much larger will be the image compared to object.
Precautions during Collection and Observation of specimens
- Collect only the number of specimen you need.
- Do not harm the specimens during the capture or collection exercise.
- Handle dangerous or injurious specimens with care e.g. stinging plants or insects i.e. use forceps or hand gloves.
- The teacher will immobilize highly mobile animals. (diethyl ether, formalin, chloroform)
- Do not destroy the natural habitat of the specimens.
Practical activity 2
Practical activity 3
Comparison between plants and animals
|1. Green in colour( have chlorophyll)||1. Lack chlorophyll thus feed on readymade food.|
|2. Their cells have cellulose cell walls.||2. Cells lack cellulose cell walls.|
|3. Respond slowly to changes in the environment.||3. Respond quickly.|
|4. Lack specialized excretory organs.||4. Have complex excretory organs.|
|5. Do not move about.||5. Move about in search of food and water.|
|6. Growth occurs in shoot and root tips.(apical growth)||6.Growth occurs in all body parts9intercalary growth).|
Living things are also known as living organisms.
Organisms (forms of life) have distinguishing characteristics and therefore are grouped.
The Magnifying lens
-Is used for enlarging small objects.
Procedure of its use
- Place the object on the bench.
- Move the hand lens from the object to the eye.
- An enlarged image is seen.
Drawing magnification = Length of the drawing/ drawing Length
Length of the object/Actual Length
External features of plants and animals
External features of plants
- Rhizoids as in moss plant.
- Fronds in ferns.
- Roots, stems, leave, flowers, seeds, fruits, and cones in higher plants.
External features of animals
- Tentacles in hydra
- Feathers in birds
- Shells in snails
- Wings in birds
- Fur and hair in mammals
- Scales and fins in fish
- Proglotids in tapeworms
- Mammary glands in mammals
- Locomotory Structures e.g. limbs in insects
- Body pigmentation
Practical activity 1
To collect and observe animal specimens
To collect and observe plant specimens
What is classification?
-Is an area of biology that deals with the grouping of living organisms according to their structure. Organisms with similar structures are put under one group referred to as a taxon—taxa (plural).
The groupings also consider evolutionary relationships (phylogeny)—since all living organisms had a common origin at one time.
Taxonomy—Science of classification.
Taxonomist—Biologist who studies taxonomy.
Need for classification.
- To identify living organisms into their correct groups for reference and study
- To bring together living organisms with similar characteristics but separate those with different features.
- To arrange information of living organisms in an orderly manner. This avoids chaos and confusion.
- To understand the evolutionary relationship between different organisms
Are groups (taxa) into which organisms are placed as a matter of convenience.
Groups are based on observable characteristics common in the group.
In a classification scheme (taxonomic units or groups, a hierarchy of groups are recognized starting with the first largest and highest group; the Kingdom to the smallest and lowest unit; the species.
There are 7 major taxonomic units.
There are five Kingdoms of living organisms, namely:
- Kingdom Monera: bacteria
- Kingdom protoctista: algae, protozoa, amoeba, paramecium
- Kingdom Fungi: Moulds, Yeast, Mushrooms
- Kingdom Plantae: Moss plants, ferns, maize, garden pea, pine, meru oak, bean etc.
- Kingdom Animalia: hydra, tapeworms, bees, human beings etc.
A kingdom is divided into Phyla in animals or divisions in plants and sorts out organisms based on body plan and form.
Plan is the adaptation to a special way of life.
The Class is further divided into small groups; Orders using structural features.
Orders are divided into families using structural features, then Families into Genera (singular genus) –based on recent common ancestral features that are less adaptive.
Genus is divided into species i.e. kind of plant, or animal.
Down the hierarchy, the number of organisms in each group decreases but their similarities increases.
The Species group members naturally interbreed to produce fertile off springs.
Minor differences are exhibited in the species groups e.g. on colour of the skin in human beings and varieties of plants.
The groups of the species are termed to as varieties, races or strains.
Classification of A human being and a maize plant
|Taxonomic unit||Human being||maize||bean|
|Phylum or division||Chordata||Angiospermaphyta||Angiospermae|
Scientific name Homo sapiens Zea mays phaseolus vulgaris
Scientific Naming Of Living Organisms
Present naming was developed by carolus Linnaeus 18th c, where organisms were given 2 names in Latin language.
Living organisms have their scientific names and common names i.e. local or vernacular names.
Scientific naming uses the double naming system—Binomial system.
In binomial system, an organism is given both the genus and species name.
Binomial nomenclature (Double –naming system)-Is the assigning of scientific names to living organisms governed by a definite set of rules recognized internationally.
Principles of binomial nomenclature
- The first, genus name, should begin with a capital letter and the second name, species, should begin or written in small letters e.g.
Lion---- Panthera leo
Leopard----- Panthera pardus
Domestic dog----- Canis farmiliaris
Human being--- Homo sapiens
Maize plant---Zea mays
Lion and Leopard are closely related ---Same genus but distantly related—different species.
- The scientific names must be printed in italics in textbooks and where hand written to be underlined e.g. Panthera leo.
- The specific name (species) is frequently written with the name of the scientist who first adequately described and named the organism e.g.Phaseolus vulgaris i.e. Vulgaris is the scientist who described and named the bean plant.
- Biologists should give a Latinized name for a newly described animal or plant species where Latin name is missing e.g.
Meladogyne kikuyuensis – Is a scientific name of a nematode from kikuyu.
Aloe kilifiensis --- A member of Aloeceae family from Kilifi discovery.
Garinsoga parviflora waweruensis --- a member of Macdonald eye family discovered by Waweru.
Study Question 1
Complete the table below
|Taxon||Lion||Domestic dog||Garden pea||Napier grass|
Scientific name --------------------- ------------------------ ----------------------- ------------------------
- Review of the magnification lens
- Calculating Magnification
- External characteristics of plants and animals
Diversity of Living Organisms
- Organisms with similar characteristics are placed under one group called taxon (taxa).
- The science of classification is known as taxonomy.
- Biologists who study taxonomy are called taxonomists.
Need For Classification
- Help in identifying living organisms into their correct groups for reference.
- It brings together organisms with similar characteristics and separates those with different features.
- Help to organize information about living organisms in an orderly manner avoiding any confusion.
- Help to understand the evolutionary relationship between different living organisms.
Historical Background of Classification
- Long time ago classification was artificial where living things were classified as either plants or animals.
- Plants were classified as herbs, shrubs and trees.
- Animals were further divided into carnivores, herbivores and omnivores.
- Today modern classification uses evolutionary relationships between living organisms.
Taxonomic Units of Classification
- This refers to the groups into which living organisms are placed in classification.
- These units start from the first largest and highest group (kingdom) to the smallest and lowest unit (species).
- There are seven taxonomic units as shown below:
Carolus Linnaeus (1707-1778) initially introduced the two kingdom system of classification. However many new life forms have been discovered which are neither animals nor plants. This has led to a more accepted classification system that adopts five kingdoms. These are;
- ) Monera .eg bacteria
- Protoctista e.g algae and protozoa
- Fungi e.g. mushrooms, moulds and yeast.
- Plantae e.g. maize, ferns and all types of trees.
- Animalia e.g. man, cow tapeworm, flies etc.
Kingdom is further divided into several phyla in animals or divisions in plants.
- Phylum (phyla) or Division in plants.
It is the second largest and further divided into classes.
Each class is divided into several orders.
Orders are divided into smaller groups called families.
Family is divided into several Genera.
Here members are closely related. It is further divided into the species.
This is the smallest unit of classification.
Species is defined as a group of organisms whose members naturally interbreed to produce fertile offspring’s.
Members of a given species have small differences such as skin colour, height etc.
Classification of Man and Maize plant. ( Table 2.1 Page 15 KLB Bk 1)
Scientific Naming of Living Organisms.
- Today organisms are given two names in Latin language. This was developed by Carolus Linnaeus.
- Latin language was used because it was widely spoken during his time.
- In scientific naming, an organism is given the genus and the species name.
- This double naming system is known as Binomial system (two name System)
This is the double naming system of organisms where organisms are assigned two names i.e. the generic name and the specific name.
In binomial nomenclature the following rules are observed.
- Generic name is written first followed by the specific name. First letter in the generic name is in capital and the rest are in small letters. Specific name is written in small letters.
- The two names are underlined separately when handwritten or italicised when printed.
- Newly discovered species must be given Latinized names.
- Specific name is frequently written with the name of the scientist who first adequately described and named the organism.
- This is the study of the functions of cell structures.
Membrane Structure and Properties
- A membrane is a surface structure which encloses the cell and organelles. Membranes regulate the flow of materials into out of the cell or organelle.
- Examples of membranes: cell membrane, tonoplast (membrane surrounding the vacuole), nuclear membrane, mitochondrial membrane, chloroplast membrane etc.
The Cell Membrane
- It has three layers, two protein layers and a phos-pholipid layer sandwiched in between the two.
Properties of Cell Membrane
- Semi-permeability. – It has small pores allowing for the passage of molecules of small size into and out of the cell. Cell Wall however allows all materials to pass through it hence it is referred to as being Permeable.
- Sensitivity to Changes in Temperature and pH – Extreme temperature and pH affects the cell membrane since it has some protein layers. Such changes alter the structure of the membrane affecting its normal functioning.
- Possession of Electric Charges – it has both the negative and positive charges helping the cell to detect changes in the environment. These charges also affect the manner in which substances move in and out of the cell
- The ability of the cell to control the movement of substances in and out of the cell is achieved through physiological processes such as Diffusion, Osmosis and Active Transport.
- This is a process by which particles move from a region of high concentration to a region of low concentration.
Practical Activity 1
To demonstrate diffusion using potassium permanganate (VII)
The difference in concentration of particles between the region of high concentration and the region of low concentration is known as the diffusion gradient.
Role of Diffusion in Living Organisms
- Absorption of Materials
- Mineral salts in the soil enter the root by diffusion since their concentration in the soil is greater than in the root hair cells.
- Digested food (glucose and amino acids) diffuse across the wall of the ileum into the blood for transport to rest of the body.
- Gaseous Exchange in Plants and Animals
- In both plants and animals, respiratory gases (oxygen and Carbon (IV) oxide) are exchanged through simple diffusion depending on their concentration gradient.
- Excretion of Nitrogenous Wastes
- Transport of Manufactured Food form Leaves to other Plant Parts.
Factors Affecting Diffusion
- Diffusion Gradient
- A greater diffusion gradient between two points increases the rate of diffusion.
- Surface Area to Volume Ratio
- The higher the ratio the greater the rate of diffusion and the lower the ratio the lower the rate.
- This means that small organisms expose a large surface area to the surrounding compared to large organisms.
- Small organisms therefore depend on diffusion as a means of transport of foods, respiratory gases and waste products.
- Thickness of Membranes and Tissues
- The thicker the membrane the lower the rate of diffusion because the distance covered by the diffusing molecules is greater. The thinner the membrane, the faster the rate.
- Size of the Molecules
- Small and light molecules diffuse faster than large and heavy molecules.
- Increase in temperature increases the energy content in molecules causing them to move faster.
- This is the process where solvent molecules (water) move from a lowly concentrated solution (dilute) to a highly concentrated solution across a semi-permeable membrane.
Diagram fig 4.6
- The highly concentrated solution is known as Hypertonic Solution.
- The lowly concentrated solution is called Hypotonic solution.
- Solution of the same concentration are said to be Isotonic.
- Osmosis is a special type of diffusion because it involves the movement of solvent (water) molecules from their region of high concentration to region of low concentration across a semi permeable membrane.
Practical activity 2
Practical activity 3
- This is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semi permeable membrane. This is the pressure needed to nullify osmosis.
- Osmotic pressure is measured using the
- This is the measure of the pressure a solution would develop to withdraw water molecules from pure water when separated by a semi permeable membrane.
Water Relations in Animals
- Cell membrane of the animal cell is semi permeable just like the dialysis/visking tubing.
- Cytoplasm contains dissolved sugars and salts in solution form.
- If an animal cell e.g. a red blood cell is placed in distilled water (hypotonic solution), water flows in by osmosis.
- The cell would swell up and eventually burst because the cell membrane is weak. The bursting of the red blood cell when placed in hypotonic solution is called Haemolysis.
- If a similar red blood cell is placed in a hypertonic solution, water is drawn out of the cell by osmosis. The cell will shrink by a process called Crenation.
- Body fluids surrounding the cells must therefore have same concentration as to that which is found inside the cell.
Water Relations in Plants
- When a plant cell is placed in a hypotonic solution it gains water by osmosis and distends outwards.
- As the cell gains more water, its vacuole enlarges and exerts an outward pressure called turgor pressure. As more water is drawn in, the cell becomes firm and rigid and is said to be turgid.
- The cell wall in plant cell is rigid and prevents the cell from bursting unlike the case in animal cells.
- The cell wall develops a resistant pressure that pushes towards the inside. This pressure is equal and opposite the turgor pressure and is called wall pressure.
- When a plant cell is placed in hypertonic solution, water molecules move out of the cell into the solution by osmosis. The cell shrinks and becomes flaccid.
- If the cell continues to lose more water, plasma membrane pulls away from the cell wall towards the center.
- The process through which plant cells lose water, shrink and become flaccid is called
- Plasmolysis can be reversed by placing a flaccid cell in distilled water and this process is called
Study Question 5
Practical Activity 4
- When plants lose water through evaporation and transpiration, cells lose turgidity, shrink and the plant droops. This is called
- If water supply from the soil is inadequate, plants do not recover hence permanent wilting.
Study Question 6
Role of Osmosis in Organisms
- Absorption of water from the soil
- Root hair cells of plants absorb water from the soil by osmosis.
- Cells of herbaceous plants, which are less woody, absorb water, become turgid hence support.
- Opening and closing of the stomata
- During the day, guard cells synthesize glucose, draw in water, become turgid hence open the stomata.
- During the night, they lose turgidity since there is no photosynthesis. As a result, they shrink thus closing the stomata.
- Feeding in insectivorous plants
- These plants are able to change their turgor pressure on the leaves which close trapping insects which are digested to provide the plant with nitrogen.
- In the kidney tubules, water is reabsorbed back to the body by osmosis
Factors Affecting Osmosis
- Concentration of Solutions and Concentration Gradient. The greater the concentration gradient between two points, the faster the rate of osmosis.
- Optimum Temperature as long as it does not destroy the semi-permeability of the membrane.
- This is the process that moves substances across cell membranes against a concentration
- This process requires energy to move these substances across cell membranes and involves
- Substances such as amino acids, sugar and many ions are taken in by living organisms through active transport.
Role of Active Transport
- Re-absorption of sugars and useful substances by the kidney
- Absorption of some mineral salts by plant roots
- Absorption of digested food from the alimentary canal into the blood stream
- Accumulation of substances in the body to offset osmotic imbalance in arid and saline environment
- Excretion of waste products from body cells
Factors Affecting Active Transport.
- Oxygen concentration.
- Change in pH.
- Glucose concentration.
- Enzyme inhibitors.
NB/ Any factor affecting energy production affect the rate of active transport.
Cell Specialization, Tissues, Organs and Organ Systems
- Cell specialization
- This is where cells are modified to perform specific functions. Such cells are said to be specialized.
- Examples include the sperm cell which has tail for swimming and the root hair cell which is extended creating large surface area for water absorption.
- These are cells of a particular type that are grouped together to perform the same function.
Animal tissues include;
- Epithelial tissue – which is a thin continuous layer of cells for lining and protection of internal and external surfaces.
- Skeletal – it is a bundle of elongated cells with fibres that can contract. Its contraction and relaxation brings about movement.
- Blood tissue – this is a fluid containing red blood cells, white blood cells and platelets. It transports many substances and protects the body against infections.
- Connective tissue – made up of strong fibres that connect other tissues and organs holding them together.
Plant tissues include:
- Epidermal tissue of a plant – this is a single layer of cells protecting the inner tissues of the plant.
- Palisade tissue – this is a group of cells rich in chloroplasts containing chlorophyll. They absorb light energy during photosynthesis.
- Parenchyma tissue – it is made thin walled irregularly shaped cells. They store water and food.
- Vascular bundle – consists of the xylem and phloem. Xylem conducts water and mineral salts while phloem conducts food substances.
- Many tissues become specialized and grouped together to perform a functional unit called the organ.
- Examples of organs in plants include; roots, leaves, flowers and stem.
- In animals they include heart, lungs, kidney, brain, stomach and the liver.
- Organ systems.
- This is made of several organs whose functions are coordinated and synchronized to realize an effective action is called an organ system. Examples include; digestive, circulatory, excretory, respiratory, reproductive and nervous system.
Microscope Parts & Function
Parts of the Microscope
Handling and Care of the Microscope
The following rule should be observed:
- Use both hand when carrying the microscope. One hand should hold the base and the other holds the limb.
- Never place the microscope too close to the edge of the bench.
- Do not touch the mirror and the lenses with the fingers.
- Clean dirty lenses using soft tissue.
- Clean other parts using a soft cloth.
- Do not wet any part of the microscope.
- Make sure the low power clicks into position in line with the eye piece before and after use.
- Always store the microscope in a safe place free from dust and moisture.
Using the Microscope
- Place microscope on the bench with the stage facing away from you.
- Turn the low power objective lens until it clicks into position.
- Ensure the diaphragm is fully open.
- Look through the eyepiece with one eye. Adjust the mirror to ensure maximum light can pass through.
- Place the slide containing the specimen on the stage and clip it into position. Make sure the slide is at the centre of the field of view.
- Again look through the eyepiece while adjusting the mirror to ensure maximum light reach the specimen.
- Use the coarse adjustment knob to bring the low power objective lens to the lowest point. While viewing through the eyepiece, turn the coarse adjustment knob gently until the specimen comes into focus.
- Use the fine adjustment knob to bring the image into sharp focus.
- Make a drawing of what you see.
- For higher magnification, turn the medium power into position and adjust the focus using the coarse knob. Use the fine adjustment knob for sharper focus.
- For even large magnifications, turn the high power objective lens into position. In this case use only the fine adjustment knob to bring details into sharper focus.
- Magnification of the object viewed under the microscope is calculated by;
Magnification = Eye Piece Lens Magnification X Objective Lens Magnification.
- If the eyepiece lens has the magnification of x5 and the low power objective lens has a magnification of x10, the total magnification is 5x10=50.
Study Question 1
Fill the table below.
|Eye piece lens
Practical Activity 1
Cell Structures as Seen Under the Light Microscope
- The following cell organelles can be seen under the light microscope.
- Cell wall.
- Cell membrane
Diagrams- plant and animal cells
The Electron Microscope.
- It is more powerful than the light microscope.
- It can magnify up to 500,000 times and has high resolving power.
- The high resolving power of the electron microscope enables it to separate objects which lie close to one another.
- Electron microscope uses a beam of electrons instead of light to illuminate the object.
Study Question 2
Practical Activity 2
Cell Structures as Seen Under the Electron Microscope
Diagrams – Plant and Animal Cells
The Cell Organelles
- Cell membrane (Plasma Membrane).
- It has three layers i.e. one layer of phospho-lipid layer sandwiched between two protein layers.
- It is flexible with pores and ahs the following main functions.
- Encloses all the cell contents.
- It allows selective movement of substances into and out of the cell since it is semi-permeable.
- It is s fluid medium in which chemical reactions take place.
- It has some movement called cytoplasmic streaming.
- It contains organelles, starch, glycogen, fat droplets and other dissolved substances.
- It has double membrane called the nuclear membrane.
- The membrane has pores allowing passage of materials into and out of the cell.
- Nucleus has a fluid called nucleoplasm in which the nucleolus and chromatin are suspended.
- Nucleolus manufactures ribosomes while chromatin contains the hereditary material.
- They are sausage shaped and are the respiratory sites.
- Mitochondrion has two membranes. Inner membrane is greatly folded into cristae to increase the surface area for respiration.
- Cells that require a lot of energy have large number of mitochondria e.g. muscle cell, sperm cell, kidney cell etc.
- Endoplasmic Reticulum (ER)
- Some endoplasmic reticulums have granules called Ribosomes on their surfaces hence referred to as rough endoplasmic reticulum.
- Others do not contain ribosomes hence the name smooth endoplasmic reticulum.
- Rough endoplasmic reticulum transport proteins while the smooth endoplasmic reticulum transports lipids.
- They are spherical in shape and form the site for protein synthesis.
- They contain lytic enzymes which break down large molecules, destroy worn out organelles or even the entire cell.
- Golgi Bodies (Golgi apparatus)
- Their function is to package and transport glyco-proteins.
- They are also associated with secretion of synthesized proteins and carbohydrates.
- They are rod shaped structures that are used in cell division and in the formation of cilia and flagella.
- Plant cells lack the Centrioles.
- They are egg shaped and contain two membranes.
- Chloroplast has chlorophyll which traps light energy to be used during photosynthesis.
- This are sacs filled with a fluid called cell sap.
- Animal cells contain small vacuoles while plant cells have large vacuoles.
- Sap vacuoles store sugars and salts.
- Food vacuole store and digest food while contractile vacuoles excrete unwanted materials from the cell.
- Cell wall
- It is a rigid outer cover of the plant cells made of cellulose.
- It gives the plant cell a definite shape while providing mechanical support and protection.
- Cell wall also allows water, gases and other materials to pass through it.
Study Question 3
Differences between Plant and Animal Cells
Preparation of Temporary Slides
Practical Activity 3
Estimation of Cell Sizes.
NUTRITION IN PLANTS AND ANIMALS
- This is the process by which organisms obtain and Assimilate
- There are two modes of nutrition; Autotrophism and Heterotrophism.
- This is where living organism manufacture its own complex food substances from simple substances such as carbon (iv) oxide, water, light or chemical energy.
- Where sunlight is used as a source of energy, the process is referred to as photosynthesis.
- Photo means light while synthesis means to make.
- Some none green plants make their own food using energy obtained from certain chemicals through a process called
- Organisms that make their own food are referred to as autotrophs.
- This is where organisms take in complex food materials such as carbohydrates, proteins and fats obtained from bodies of plants and animals.
- Organisms that feed on already manufactured foods are called Heterotrophs.
External Structure of a Leaf
A leaf is a flattened organ which is attached to the stem or a branch of a plant.
Parts of a leaf
Lamina: This is the flat surface. It is green in colour and contain the photosynthetic tissue.
Midrib: This is a thick structure running through the middle of the leaf
Veins: They arise from the midrib to forming an extensive network of veins.
Leaf Apex: This is the tip of the leaf and usually it is pointed.
Petiole: It attaches the leaf to the stem or branch.
In some monocotyledonous plants the leaves are attached to the stem by the leaf sheath.
Practical Activity 1: To examine the External Features of a Dicotyledonous and Monocotyledonous leaf
Study Question 1
Internal Structure of a Leaf
- Internal structure of the leaf is composed of the following parts.
- It is a thin waterproof and transparent layer that coats the upper and lower surfaces of the leaf.
- It reduces excess water loss and protects the inner tissue of the plant against mechanical injury.
- It also prevents entry of disease causing micro organisms.
- Since it is transparent, it allows penetration of light for photosynthesis.
- It is a one cell thick tissue on both the upper and lower leaf surfaces.
- It secretes the cuticle and also protects the inner tissues from mechanical damage and prevents entry of pathogens.
- Epidermal cells have no chloroplast except the guard cells.
- Guard cells are special bean shaped cells. They have chloroplast and are able to carry out photosynthesis hence controlling the opening and closing of the stomata.
- Air moves into and out of the leaf through the stomata.
- Palisade layer.
- This is layer of cells located beneath the upper epidermis.
- It is made of cylindrical shaped cells closely packed They have numerous chloroplasts containing chlorophyll.
- Their position and arrangement enables them to receive maximum light.
- Spongy Mesophyll Layer.
- This is below the palisade layer. The cells are irregularly shaped and loosely packed creating large air spaces in between them.
- The air spaces allow gases to diffuse in between the cells. They contain fewer chloroplasts as compared to the palisade cells.
- Leaf Veins.
- Each vein is a vascular bundle consisting of xylem and phloem.
- Xylem conducts water and mineral salts from the roots to the leaves while the phloem translocates manufactured food from the leaves to the rest of the plant.
Study Question 2
Adaptations of Leaves to Photosynthesis.
- Broad and flat lamina to increase surface area of Carbon (IV) oxide and sunlight absorption.
- Thin transparent cuticle and upper epidermis; to allow easier penetration of light to photosynthetic cells;
- Thin; for faster diffusion of gases;
- Palisade cells placed next to the upper surface; to trap maximum light for photosynthesis;
- Palisade cells with numerous chloroplasts; to trap maximum amount of light for photosynthesis;
- Large/ intercellular air spaces in the spongy mesophyll layer; for storage of Carbon (IV) oxide for easier gaseous exchange;
- Waxy water proof cuticle; to reduce water loss sand reflect excess light;
- Leaf mosaic/ non-overlapping leaves; for maximum exposure to light;
- Guard cells, modified cells to open and close stomata; to control amount of water loss from the leaf and allows gaseous exchange;
- Leaves have leaf veins; xylem to conduct water to photosynthetic cells, Phloem to translocate products of photosynthesis to other parts of plant;
- They are disc shaped organelles found in the cytoplasm of plant cells.
- Each chloroplast has a double membrane; the inner and outer membrane.
- Chloroplasts are made of layers of membranes called lamellae contained in a fluid matrix called stroma.
- Several lamellae come together to form the granum (grana).
- Granum contains chlorophyll molecules and other photosynthetic pigments.
- The stroma contains enzymes that speed up the rate of photosynthesis.