January 2023

Movement is a characteristic of all living organisms. Most animals move from place to place but some are sessile (i.e. fixed to the substratum). However, though not easily observed all living protoplasm shows movement of one type or another. Necessity for support and movement in plants They enable plants to be held upright to trap maximum light for photosynthesis and gaseous exchange. It enables animals and plants to adjust to their environment. To hold flowers and fruits in appropriate position for pollination and dispersal respectively. To enable plants to grow to great heights and withstand forces of environment e.g. strong winds. Movement of male gametes to effect fertilization and ensure perpetuation of a species. Plant parts move in response to certain stimuli in the environment of tropisms. Tissue distribution in Monocotyledonous and Dicotyledonous plants Vascular bundles are the main support tissues in plants. In monocotyledonous stem they are scattered all over the stem. while in dicotyledonous stem they are found in a ring or rings. In monocots the xylem and phloem alternate around with pith in the centre. In dicots of the xylem forms a star in the centre – there is no pith. Phloem is found in between the arms of xylem. Dicotyledonous plants have cambium which brings about secondary growth resulting in thickening of the stem and root hence providing support. Secondary xylem becomes wood, providing more support to the plant. Role of support tissues in young and old plant Plants are held upright by strengthening tissues ; parenchyma, collenchyma, sclerenchyma xylem tissue. Parenchyma and collenchyma are the main support tissues in young plants. Parenchyma – They are found below the epidermis. They form the bulk of packing tissue within the plant between other tissues . They are tightly packed and turgid they provide support. Collenchyma – Their cell walls have additional cellulose deposited in the corners. This provides them with extra mechanical strength. Sclerenchyma – Their cells are dead due to large deposits of lignin on the primary cell wall. The lignified wall is thick and inner lumen is small, hence provide support. Sclerenchyma fibres are arranged in elongated and in longitudinal sheets giving extra support. They are found in mature plants. Xylem – Has two types of specialised cells. Vessels and tracheids. Vessels are thick-walled tubes with lignin deposited in them. They give support and strength to the plant. Tracheids are spindle-shaped cells arranged with ends overlapping. Their walls are lignified. They help to support and strengthen the plant.  Plants with weak stems obtain their support in the following ways. Some use thorn or spines to adhere to other plants or objects. Some have twinning stems which grow around objects which they come into contact with. Others use tendrils for support. Tendrils are parts of a stem or leaf that have become modified for twinning around objects when they gain support.  In passion fruit and pumpkin, parts of lateral branches are modified to form tendrils. In the morning glory, the leaf is modified into a tendril. Support and Movement in Animals Necessity for support and movement in animals. Animals move from place to place: In search of food. To escape from predators. To escape from hostile environment. To look for mates and breeding grounds. The skeleton, which is a support structure helps to maintain the shape of the body. Movement is effected by action of muscles that are attached to the skeleton.   Types and Functions of Skeletons Two main types will be considered. These are exoskeleton and endoskeleton. Exoskeleton Exoskeleton is hard outer covering of arthropods made up of mainly chitin.  Which is secreted by epidermal cells and hardens on secretion. It is strengthened by addition of other substances e.g. tannins and proteins to become hard and rigid. On the joints such as those in the legs the exoskeleton is thin and flexible to allow for movement. Functions of Exoskeleton Provide support. Attachment of muscles for movement. Protection of delicate organs and tissues. Prevention of water loss. Endoskeleton: It forms an internal body framework. This is a type of skeleton characteristic of all vertebrates. The endoskeleton is made of cartilage, bone or both. It is made up of living tissues and grows steadily as animal grows. Muscles are attached on the skeleton. The muscles are connected to bones by ligaments. Functions The functions of endoskeleton include support, protection and movement. Locomotion in a finned fish e.g. tilapia. Most of the fishes are streamlined and have backward directed fins to reduce resistance due to water.   External features-of Tilapia Scales tapers towards the back and overlap forwards to provide a smooth surface for a streamlined body. The head is not flexible. This helps the fish to maintain forward thrust. Slimy mucous enables the fish to escape predators and protects the scales from getting wet. The pectoral and pelvic fins are used mainly for steering, ensuring that the fish is balanced. They assist the fish to change direction. The dorsal and anal fins keep the fish upright preventing it from rolling sideways. The caudal or tail fin has a large surface area, and displaces a lot of water when moved sideways creating forward movement of the fish. In order to change position in water the fish uses the swim bladder. When filled with air the relative density of the body is lowered and the fish moves up in the water. When air is expelled, the relative density rises and the fish sinks to a lower level. Swimming action in fish is brought about by contraction of muscle blocks (myotomes). These muscles are antagonistic when those on the left contract, those on the right relax. The muscles are attached to the transverse processes on the vertebra. The vertebra are flexible to allow sideways movement.   Mammalian skeleton The mammalian skeleton is divided into two: Axial and appendicular. Axial skeleton is made up of the skull and the vertebral column. Appendicular skeleton is made up of the pelvic and pectoral girdles and limbs (hind limb and forelimbs).  The

Introduction The structures involved in detecting the changes may be located far away from the ones that respond. There is need for a communication system within the body. The nervous system and the endocrine system perform this function, i.e. linking the parts of the body that detect changes to those that respond to them. Irritability Living organisms are capable of detecting changes in their internal and external environments and responding to these changes in appropriate ways. This characteristic is called irritability, and is of great survival value to the organism. Stimuli A stimulus is a change in the internal or external environment to which an organism responds. Examples of stimuli include light, heat, sound, chemicals, pH, water, food, oxygen and other organisms. Response A response is any change shown by an organism in reaction to a stimulus. The response involves movements of the whole or part of the body either towards the stimulus or away from it.  It also results in secretion of substances e.g. hormones or enzymes by glands.   Co-ordination Co-ordination is the working together of all the parts of the body to bring about appropriate responses to change in the environment. Reception Reception is the detection of changes in the environment through receptors.   Irritability in Plants Response in plants is not as pronounced as in animals. This does not in anyway diminish the importance of irritability in plants. It is as important to their survival as it is in animals. Plants respond to a variety of stimuli in their environment. These stimuli include light, moisture, gravity and chemicals. Some plants also show response to touch. Tropisms Plants often respond by growing in a particular direction. Such growth movements are called tropisms. They are the result of unequal growth in the part of the plant that responds. The stimulus cause unequal distribution of growth hormones (auxins) produced in the plant. One side grows more than the other resulting in a bend either towards the stimulus (positive tropism) or away from the stimulus (negative tropism).   Phototropism If seedlings are exposed to light from one direction, their shoots grow towards the light. This response is called phototropism. Shoots are said to be positively phototropic because they grow towards the light. The tip of the shoot receives the light stimulus from one direction (unilateral stimulus) but the response occurs below the tip. The response of the shoot is due to a hormone called auxin produced at the tip. It diffuses down the shoot to this zone of cell elongation where it causes the cells to elongate. Light causes auxin to migrate to the darker side. The auxin is more concentrated in the dark side than on the light side. The cells on the dark side grow faster than the ones on the light side. A growth curvature is therefore produced.      Survival value: Positive phototropism by shoots ensure that sufficient light is absorbed by leaves for photosynthesis. Geotropism Geotropism is a growth response to gravity. Roots are positively geotropic because they grow down towards the direction of the force of gravity; shoots are negatively geotropic because they grow away from direction of force of gravity. If a seedling is kept in the dark with its plumule and radicle in a horizontal position, the plumule will eventually grow vertically upwards while the radicle will grow vertically downwards. The effect of gravity on roots and shoots can be explained as follows: When the seedling is placed in a horizontal position, more auxin settles on the lower side of the root and shoot due to the effect of gravity. Shoots respond to a higher concentration of auxin than roots. The lower side of the shoot grows faster than the upper side. Resulting in a growth curvature that makes the shoot grow vertically upwards. Root growth is inhibited by high concentrations of auxin. Therefore, the lower side of the root grows at a slower rate than the upper side where there is less auxin concentration. This results in a growth curvature that makes the root grow vertically downwards. Survival Value: Roots in response to gravity grow downwards where they absorb water and get anchored in the soil. This results in absorption of nutrients needed for growth. Hydrotropism Hydrotropism is the growth of roots towards water (moisture) .       Survival Value It ensures that plant roots grow towards moisture to obtain water needed for photosynthesis and transport of mineral salts. Chemotropism Chemotropism is the response of parts of a plant towards chemical substances, e.g. the growth of the pollen tube towards the ovule in flowering plants is a chemotropic response.  Survival Value This ensures that fertilisation take place and the perpetuation of the species continues. Thigmotropism Thigmotropism is a growth response to touch. e.g. tendrils of climbing plant bend around objects that they come in contact with.       Survival Value This provides support and the leaves stay in a position suitable for absorption of light and gaseous exchange for photosynthesis. Tactic Movements in Plants and other Organisms A tactic movement is one made by a whole organism or a motile part of an organisms (e.g. a gamete) in response to a stimulus. Tactic movements are named according to the nature of the stimulus that brings about the response. Phototaxis is movement in response to direction and intensity of light. Free-swimming algae such as Chlamydomonas usually tend to concentrate where light intensity is optimum and will respond to light by swimming towards it. This is an example of phototactic response. Osmotaxis is movement in response to changes in osmotic conditions e.g. freshwater amoeba. Survival Value Ensures favourable conditions for existence. Chemotaxis is movement in response to concentration of chemical substances.  Survival  Value  In bryophytes, antherozoids move towards archegonia to effect fertilisation Survival Value of taxis: These ensure conditions favourable for life bring maximum benefit to the organism. Nastic Movements A nastic movement is one made by part of a plant in response to stimulus which is not coming from any particular direction. Nastic movements

Meaning of Evolution and Current Concepts Evolution is the development of organisms from pre-existing simple organisms over a long period of time. It is based on the similarities in structure and function that is observed in all organisms. All are made up of cells, and similar chemical compounds are present. This indicates that all organism may have had a common origin. Evolution seeks to explain the diversity of life and also to answer the question as to the origin of life, as well as its present state. The Origin of Life Human beings have tried to explain how life began.   Currently held views are listed below: Special creation -life was created by a supernatural being within a particular time. Spontaneous generation life originated from non-living matter all at once. e.g. maggots arise from decaying meat. Steady state – life has no origin. Cosmozoan – life on earth originate from elsewhere, outer space. Bio-chemical evolution-life originated according to chemical and physical laws. Only special creation and chemical evolution will be discussed. Special Creation The earliest idea is that of special creation which is recorded in the old testament (Genesis 1: 1-26). It states that God created the world and all living things in six days. Some hold the six days literally, while others say it may represent thousands of years. According to his theory, the earth and all organisms were created mature. Similarities in structure and function denote the stamp of a “common Designer” Evidence for this view arises from observations of life itself. Faith explains it all. By faith we understand that the universe was created by the command of God. Several scientists hold this view and their research confirms accounts in the old testament of a universal flood explains the disappearance of dinosaurs as vegetation decreased.   Chemical Evolution The following is the line of thought held in this view to explain origin of life: The composition of atmospheric gases was different from what it is today: There was less oxygen, more carbon (IV) oxide, hence no ozone layers to filter the ultra-violet light. The high solar energy reached the earth and brought together hydrogen, carbon (IV) oxide and nitrogen to make organic compounds. These were: hydrocarbons, amino acids, nucleic acids, sugars, amino acids and proteins. The proteins coalesced and formed colloids. Proteins and lipids formed a “cell membrane” that enclosed the organic compounds, to form a primitive cell. The cell was surrounded by organic molecules that it fed on heterotrophically. This took place in water. From this cell progressively autotrophs evolved. That were similar to blue-green algae. They produced oxygen and as more oxygen was evolved ozone layer formed an blocked ultra violet radiation. This allowed formation of present day photo-autotrophs. Evidence for Organic Evolution Most of the evidence for evolution is indirect . e. it is based on studies carried out on present-day animals and plants. Direct evidence is obtained from studying the remains of animals and plants of the past. Fossil Records The study of fossils is called paleontology. Fossils are remains of organisms that lived in ancient times. Most fossils are remains of hard parts of the body such as bones, teeth, shells and exoskeletons. Some fossils are just impressions of the body parts, e.g. footprints, leaf-vennation patterns, etc. Fossils are usually found in sedimentary rocks which have been formed by deposition of sediments over millions of years. The deeper the layer of sediments, the older the fossils found in that layer. Modem man, Homo sapiens, evolved from ape-like creatures 25 million years ago. These evolved to upright, tool using creature called Australopithecus afarensis which had a cranial capacity of 400-500 cc. This evolved through several intermediates; Homo habilis and Homo erectus to modem day human. Homo sapiens has a cranial capacity of 1350 – 1450 cc. Homo sapiens is more intelligent. Main features in human evolution include bipedal posture, is an omnivore and has an opposable thumb. Limitations of the Fossil Evidence Only partial preservation was usually possible because softer parts decayed. The fossil records are therefore incomplete. Distortion – parts of organisms might have become flattened during sedimentation. Subsequent geological activities e.g. erosion, earthquakes, faulting and uplifting may have destroyed some fossils. Geographical Distribution Until about 250 million years ago, all the land masses on earth formed a single land mass (Pangaea). This is thought to have undergone continental drift, splitting into different continents. Consequently, organisms in certain regions became geographically isolated and did not have a chance to interbreed with other organisms in other regions. Such organisms underwent evolution in isolation and have become characteristically different from organisms in other regions. For example, pouched mammals (e.g. kangaroo, wallaby, koala bear) are found almost exclusively in Australia. The opossum is the only surviving representative of the pouched mammals in North America. Comparative Embryology During the early stages of development, the embryos of different vertebrates are almost indistinguishable.  Fish, amphibian, bird and mammalian embryos have similar, features, indicating that they arose from a common ancestor. Similarities include: Visceral clefts, segmental muscle blocks (myotomes) and a single circulation. Comparative Anatomy Comparative anatomy is the study of organs in different species with the aim of establishing whether the organism are related. Organisms which have the same basic features are thought to have arisen from a common ancestor. The vertebrate pentadactyl limb evolved in different ways as an adaptation to different modes of life. e.g. as a flipper in whales, as a wing in bats and as a digging hand in moles. Such organs are said to be homologous, i.e. they have arisen from a common ancestor but they have assumed different functions. This is an example of divergent evolution . The wing of a butterfly and that of a bird are said to be analogous.  i.e. they have originated from different ancestors but they perform the same function. This is an example of convergent evolution.   Cell Biology All eucaryotic cells have organelles such as mitochondria, membrane-bound nuclei, ribosomes, golgi bodies. Thus indicating that

Introduction Genetics is the study of inheritance. The fact that the offspring of any species resemble the parents indicates that the characters in the parents are passed on to the offspring. Factors that determine characters (genes) are passed on from parent to offspring through gametes or sex cells. In fertilisation the nucleus of the male gamete fuses with the nucleus of the female gamete. The offspring show the characteristics of both the male and the female. Genetics is the study of how this heritable material operates in individuals and their offspring.   Variations within Plant and Animal Species Variation The term variation means to differ from a standard. Genetics also deals with the study of differences between organisms belonging to one species. Organisms belonging to higher taxonomic groups e.g. phyla or classes are clearly different. Although organisms belonging to the same species are similar, they show a number of differences or variations such that no two organisms are exactly the same in every respect. Even identical twins, though similar in many aspects, are seen to differ if they grow in different environments. Their differences are as a result of the environment which modifies the expression of their genetic make-up or genotype. The two causes of variations are the genes and the environment. Genes determine the character while the environment modifies the expression of that character. Continuous and Discontinuous Variation Continuous Variations The differences between the individual are not clear-cut. There are intermediates or gradations between any two extremes. Continuous variations are due to action of many genes e.g. skin complexion in humans. In continuous variation, the environment has a modifying effect in that it may enhance or suppress the expressions of the genes. Continuous variation can be represented in form of a histogram. Example of continuous variation in humans is weight, height and skin complexion. Linear measurements: In humans, height shows gradation from tall, to tallest. So does the length of mature leaves of a plant. In most cases, continuous variation is as a result of the environment. Discontinuous Variations These are distinct and clear cut differences within a species. Examples include: Ability to roll the tongue. An individual can either roll the tongue or not. Ability to taste phenylthiourea (PTC); some individuals can taste this chemical others cannot. Blood groups – and individual has one of the four blood groups A, B AB or O. There are no intermediates. Albinism – one is either an albino or not. Discontinuous variations is determined by the action of a single gene present in an individual. Structure and Properties of Chromosomes These are threadlike structures found in the nucleus. They are normally very thin and coiled and are not easily visible unless the cell is dividing. When a cell is about to divide, the chromosomes uncoil and thicken. Their structure, number and behaviour is clearly observed during the process of cell division. The number of chromosomes is the same in all the body cells of an organism. In the body cells, the chromosomes are found in pairs. Each pair is made up of two identical chromosomes that make up a homologous pair. However sex chromosomes in human male are an exception in that the Y-chromosome is smaller. Number of Chromosomes  Diploid Number (2n) This is the number of chromosomes found in somatic cells. For example, in human 2n = 46 or 22 pairs (44 chromosomes) are known as autosomes (body chromosomes”) while 1 pair is known as the sex chromosomes. In Drosophila melanogaster, 2n =   Chromosome Structure All chromosomes are not of the same size or shape. In human beings; each of the twenty­ three pairs have unique size and structure . On this basis they have been numbered 1 to 23. The sex chromosomes formthe 23rd pair. Properties of Chromosomes Chromosomes are very long and thin. They are greatly and loosely coiled and fit within the nucleus. During cell division they shorten, become thicker and are easily observable. Each consists of two chromatids. The two chromatids are held at same position along the length, at the centromere. Chromatids separate during cell division in mitosis and in the second stage of meiosis. Chromosomes take most dyes and stain darker than any other part of the cell. This property has earned them the name “chromatin material” Each chromosome is made up of the following components: Deoxyribonucleic acid (DNA) – this carries the genes. It is the major component of the genetic material. Protein e.g. histones. Ribonucleic acid (RNA) is present in very small amounts. Enzymes concerned with DNA and RNA replication – these are DNA and RNA polymerases and ligases. Structure of DNA The structure of DNA was first explained in 1953 by Watson and Crick. DNA was shown to be a double helix that coils around itself. The two strands are parallel and the distance between the two is constant. Components of DNA DNA is made up of repeating units called nucleotides. Each nucleotide is composed of: A five-carbon sugar (deoxyribose). Phosphate molecule. Nitrogenous base, four types are available i.e, Adenine – (A) Guanine – (G) Cytosine – (C) Thymine – (T) The bases are represented by their initials as A, G, C and T respectively. The sugar alternates with the phosphate, and the two form the backbone of the strands. The bases combine in a specific manner, such that Adenine pairs with Thymine and Guanine pairs with Cytosine. The bases are held together by hydrogen bonds. A gene is the basic unit of inheritance consisting of a number of bases in linear sequence on the DNA. Genes exert their effect through protein synthesis. The sequence of bases that make up a gene determine the arrangement of amino acids to make a particular protein. The proteins manufactured are used to make cellular structures as well as hormones and enzymes. The types of proteins an organism manufactures determines its characteristics. For example, albinism is due to failure of the cells of an organism to synthesise the enzyme tyrosine required for the

TOPIC 1: GENETICS – Click to view TOPIC 2: EVOLUTION – Click to view TOPIC 3: RECEPTION, RESPONSE AND CO-ORDINATION IN PLANTS AND ANIMALS – Click to view TOPIC 4: SUPPORT AND MOVEMENT IN PLANTS AND ANIMALS – Click to view

Concept of Growth and Development Growth is a characteristic feature of all living organisms. Most multicellular organisms start life as a single cell and gradually grow into complex organisms with many cells.  This involves multiplication of cells through the process of cell division. This quantitative permanent increase in size of an organism is referred to as growth. For growth to take place the following aspects occur Cells of organisms assimilate nutrients hence increase in mass. Cell division (mitosis) that lead to increase in the number of cells. Cell expansion that   leads   to enlargement an increase in the volume and size of the organism. It is therefore possible to measure growth using such parameters as mass, volume, length, height, surface area. On the other hand development is the qualitative aspect of growth which involves differentiation of cells and formation of various tissues in the body of the organism in order for tissues to be able to perform special functions. It is not possible to measure ac aspects of development quantitative. Therefore development can be assessed terms of increase in complexity of organism e.g. development of leaves, flowers and roots. A mature human being has millions of cells in the body yet he or she started from; single cell, that is, a fertilised egg. During sexual reproduction mammals an ovum fuses with a sperm form a zygote. The zygote divides rapidly without increasing in size, first into 2, 4, 8, 16,32, 64 and so on, till it forms a mass cells called morula. These first cell division is called cleavages. The morula develops a hollow part, resulting into a structure known as a blastula (blastocyst). Later, blastocyst cells differentiate into an inner layer (endoderm) and the outer layer (ectoderm). The two-layered embryo implants into the uterine wall and, by obtaining nutrients from the maternal blood, starts to grow and develop. As the embryo grows and develops, changes occur in cell sizes and cell -types. Such changes are referred to as growth and development respectively. These processes lead to morphological and physiological changes in the developing young organism resulting into an adult that is more complex and efficient. In the early stages, all the cells of the embryo look alike, but as the development process continues the cells begin to differentiate and become specialised into different tissues to perform different functions. Growth involves the synthesis of new material and protoplasm. This requires a continuous supply of food, oxygen, water, warmth and means of removing waste products. In animals, growth takes place all over the body but the rates differ in the various parts of the body and at different times. In plants however, growth and cell division mostly take place at the root tip just behind the root cap and stem apex. This is referred to as apical growth which leads to the lengthening of the plant.  However, plants do not only grow upwards and downwards but sideways as well. This growth leads to an increase in width (girth) by the activity of cambium cells. The increase in girth is termed as secondary growth. Study Question 1-State two major differences between growth and development Measurement of growth Growth can be estimated by measuring some aspect of the organism such as height, weight, volume and length over a specified period of time. The measurements so obtained if plotted against time result into a growth curve. Study Question 2 The following results were obtained from a study of germination and early growth of maize. The grains were sown in soil in a greenhouse and.at two-day intervals. Samples were taken, oven dried and weighed. See table Plot a graph of dry mass of embryo against time after sowing. Describe the shape of the graph. For most organisms when the measurements are plotted they give an S-shaped graph called a sigmoid curve such as in figure . This pattern is due to the fact that growth tends to be slow at first and then speeds up and finally slows down as adult size is reached. A sigmoid curve may therefore be divided into four parts. Lag phase (slow growth) This is the initial phase during which little growth occurs. The growth rate is slow due to various factors namely: The number of cells dividing are few. The cells have not yet adjusted to the surrounding environmental factors. Exponential phase (log phase) This is the second phase during which growth is rapid or proceeds exponentially. During this phase the rate of growth is at its maximum and at any point, the rate of growth is proportional to the amount of material or numbers of cells of the organism already present. This rapid growth is due to: 1. An increase in number of cells dividing,2-4-8-16-32-64 following a geometric progression, 2. Cells having adjusted to the new environment, 3. Food and other factors are not limiting hence cells are not competing for resources, 4. The rate of cell increase being higher than the rate of cell death. Decelerating Phase This is the third phase during which time growth becomes limited as a result of the effect of some internal or external factors, or the interaction of both. The slow growth is due to: 1. The fact that most cells are fully 2. Fewer ceils still dividing, 3. Environmental factors (external and internal) such as: shortage of oxygen and nutrients due to high demand by the increased number of cells. space is limited due to high number of cells. accumulation of metabolic waste products inhibits growth. limited acquisition of carbon (IV) oxide as in the case of plants. Plateau (stationary) phase This is the phase which marks the period where overall growth has ceased and the parameters under consideration remain constant. This is due to the fact that: The rate of cell division equals the rate of cell death. Nearly all cells and tissues are fully differentiated, therefore there is no further increase in the number of cells. The nature of the curve during this phase may vary depending on

Introduction The process by which mature individuals produce offspring is called reproduction. Reproduction is a characteristic of all living organisms and prevents extinction of a species. There are two types of reproduction: sexual and asexual reproduction. Sexual reproduction involves the fusion of male and female gametes to form a zygote. Asexual reproduction does not involve gametes. Cell Division Cell division starts with division of nucleus. In the nucleus are a number of thread-like structures called chromosomes, which occur in pairs known as homologous chromosomes. Each chromosome contains-genes that determine the characteristics of an organism. The cells in each organism contains a specific number of chromosomes. There are two types of cell division: Mitosis – This takes place in all body cells of an organism to bring about increase in number of cells, resulting in growth and repair. The number of chromosomes in daughter cells remain the same as that in the mother cell. Meiosis – This type of cell division takes place in reproductive organs (gonads) to produce gametes. The number of chromosomes in the gamete is half that in the mother cell. Mitosis Mitosis is divided into four main stages. Prophase, Metaphase, Anaphase and Telophase. These stages of cell division occur in a smooth and continuous pattern.   Interphase The term interphase is used to describe the state of the nucleus when the cell is just about to divide. During this time the following take place: Replication of genetic material so that daughter cells will have the same number of chromosomes as the parent cell. Division of cell organelles such as mitochondria, ribosomes and centrioles. Energy for cell division is synthesised and stored in form of Adenosine Triphosphate (ATP) to drive the cell through the entire process. During. interphase, the following observations can be made: Chromosomes are seen as long, thin, coiled thread-like structures. Nuclear membrane and nucleolus are intact. Prophase The chromosomes shorten and thicken. Each chromosome is seen to consist of a pair of chromatids joined at a point called centromere. Centrioles (in animal cells) separate and move to opposite poles of the cell. The centre of the nucleus is referred to as the equator. Spindle fibres begin to form, and connect the centriole pairs to the opposite poles. The nucleolus and nuclear membrane disintegrate and disappear. Metaphase Spindle fibres lengthen. In animal cells they attach to the centrioles at both poles. Each chromosome moves to the equatorial plane and is attached to the spindle fibres by the centromeres. Chromatids begin to separate at the centromere. Anaphase Chromatids separate and migrate to the opposite poles due to the shortening of spindle fibres . Chromatids becomes a chromosome. In animal cell, the cell membrane starts to constrict.  Telophase The cell divides into two. In animal cells it occurs through cleavage of cell membrane. In plants cells, it is due to deposition of cellulose along the equator of the cell.(Cell plate formation). A nuclear membrane forms around each set of chromosome. Chromosomes later become less distinct. Significance of Mitosis It brings about the growth of an organism: It brings about asexual reproduction. Ensures that the chromosome number is retained. Ensures that the chromosomal constitution of the offspring is the same as the parents. Meiosis Meiosis involves two divisions of the parental cell resulting into four daughter cells. The mother cell has the diploid number of chromosomes. The four cells (gametes) have half the number of chromosomes (haploid) that the mother cell had. In the first meiotic division there is a reduction in the chromosome number because homologous chromosomes and not chromatids separate. Each division has four stages Prophase, Metaphase, Anaphase and Telophase.  Interphase As in mitosis the cell prepares for division. This involves replication of chromosomes, organelles and build up of energy to be used during the meiotic division. First Meiotic division Prophase I Homologous chromosomes lie side by side in the process of synapsis forming pairs called bivalents. Chromosomes shorten and thicken hence become more visible. Chromosomes may become coiled around each other and the chromatids may remain in contact at points called chiasmata (singular chiasma). Chromatids cross-over at the chiasmata exchanging chromatid portions. Important genetic changes usually result. Metaphase I Spindle fibres are fully formed and attached to the centromeres. The bivalents move to the equator of the spindles. Anaphase I Homologous chromosomes separate and migrate to opposite poles. This is brought about by shortening of spindle fibres hence pulling the chromosomes. The number of chromosomes at each pole is half the number in the mother cell. Telophase I Cytoplasm divides to separate the two daughter cells. Second Meiotic Division Usually the two daughter cells go into a short resting stage (interphase) but sometimes the chromosomes remain condensed and the daughter cells go straight into metaphase of second meiotic division. The second meiotic division takes place just like mitosis. Prophase II Each chromosome is seen as a pair of chromatids. Metaphase II Spindle forms and are attached to the chromatids at the centromeres. Chromatids move to the equator. Anaphase II Sister chromatids separate from each other Then move to opposite poles, pulled by the shortening of the spindle fibres. Telophase II The spindle apparatus disappears. The nucleolus reappears and nuclear membrane is formed around each set of chromatids. The chromatids become chromosomes. Cytoplasm divides and four daughter cells are formed. Each has a haploid number of chromosomes. Significance of Meiosis Meiosis brings about formation of gametes that contain half the number of chromosomes as the parent cells. It helps to restore the diploid chromosomal constitution in a species at fertilisation. It brings about new gene combinations that lead to genetic variation in the offsprings. Asexual Reproduction Asexual reproduction is the formation of offspring from a single parent. The offspring are identical to the parent.    Types of asexual reproduction. Binary fission in amoeba. Spore formation in Rhizopus. Budding in yeast. Binary fission This involves the division of the parent organism into two daughter cells. The nucleus first divides into two and then the cytoplasm separates into

Introduction Ecology is the study of organisms and their environment. All organisms show interdependence on one another. Organisms are affected by their environment, and they in turn affect the environment. Green plants manufacture food by photosynthesis which other organisms obtain directly or indirectly. Growth of plants is mainly affected by environmental factors such as soil and climatic factors. On the other hand, organisms modify the environment through various activities. This interrelationship comprises the study of ecology. The study of ecology is important in several fields of study such as agriculture and environmental studies. Concepts and Terms Used in Ecology Habitat: This is the place or “home” that an organism lives or is found, g., forest or grassland. Niche: A niche is the functional unit in the habitat. It includes not only the specific place in which an organism lives but also how the organism functions. To avoid or reduce competition, organisms are separated or segregated by their niches, for example, different species of birds make their nest on one tree, some at tips of terminal branches, and others feed on leaves, some on flowers and yet others on fruits of the same tree, i.e., food niche. Yet others feed on same food, e.g., worms in the same place but at different times – time niche. Population: The term population refers to the total number of individuals of a species living in a given area at a particular time. Density is the number of individuals of a population found in a unit area, i.e., Dispersion: This is the distribution of individuals in the available space. Dispersion may be uniform as in maize plants in a plantation; random as in cactus plants in the savannah ecosystem or clumped together as in human population in cities. Community: This is the term used to describe all the organisms living together in an area. During the development of an ecosystem, the species composition of a community changes progressively through stages. Finally a steady state is reached and this is described as the climax community. This development of an ecosystem is termed succession. Each stage in development of an ecosystem is a sere. Succession is primary when it starts with bare ground, and secondary when it starts in a previously inhabited area e.g. after clearing a forest. The Ecosystem: The community and the abiotic or non-living environment together make up an ecosystem or ecological system. In this system energy flow is clearly defined from producers to consumers and nutrient cycling takes place in paths that links all the organisms and the non-living environment. Biomass: This is the mass of all the organisms in a given area. Ideally, it is the dry mass that should be compared. Carrying capacity: This is the maximum sustainable density in a given area e.g. the number of herbivores a given area can support without overgrazing.  Factors in an Ecosystem Abiotic factors (environmental factors) Temperature Is the hotness or coldness of an area or habitat. It directly affects the distribution and productivity (yield) of populations and communities. Most organisms are found in areas where temperature is moderate. However, certain plants and animals have adaptations that enable them to live in areas where temperatures are in the extremes such as the hot deserts and the cold polar regions. Temperatures not only influence distribution of organisms but also determine the activities of animals. High temperature usually accelerates the rates of photosynthesis, transpiration, evaporation and the decomposition and recycling of organic matter in the ecosystem.  Light – Light is required by green plants for photosynthesis. Light intensity, duration and quality affect organisms in one way or another. Atmospheric Pressure The force per unit area of atmospheric air that is exerted on organisms at different altitudes. Growth of plants and activity of animals is affected by atmospheric pressure g., rate of transpiration in plants and breathing in animals. Salinity This is the salt content of soil or water. Animals and plants living in saline conditions have special adaptations. Humidity This describes the amount of moisture (water vapour) in the air. Humidity affects the rate of transpiration in plants and evaporation in animals.  pH Is the measure of acidity or alkalinity of soil solution or water. pH is very important to organisms living in water and soil. Most prefer a neutral pH. Wind: Is moving air currents and it influences the dispersion of certain plants by effecting the dispersal of spores, seeds and fruits. Air currents also modify the temperature and humidity of the surroundings. Topography: These are surface features of a place. The topographical factors considered include altitudes, gradient (slope), depressions and hills. All these characteristics affect the distribution of organisms in an area g., the leeward and windward sides of a hill.   Biotic factors: These are the living components in an ecosystem, competition predation, symbiosis, parasitism, human activities. Inter-relationships Between Organisms The relationships between organisms in a given ecosystem is primarily a feeding one. Organisms in a particular habitat have different feeding levels referred to as trophic levels. There are two main trophic levels: Producers: These organisms that occupy the first trophic level. They manufacture their own food hence are autotrophic. Consumers: These are the organisms that feed on organic substances manufactured by green plants.   They occupy different trophic levels as follows: Primary consumers: These are herbivores and feed on green plants. Secondary consumers: These are carnivores and feed on flesh. First order carnivores feed on herbivores while second order carnivores feed on other carnivores, i.e., tertiary consumers. Omnivores: These are animals that feed on both plant and animal material. They can be primary, secondary or tertiary consumers.   Competition: This describes the situation where two or more organisms in the same habitat require or depend on the same resources. Organisms in an ecosystem compete for resources like food, space, light, water and mineral nutrients. Competition takes place when the environmental resource is not adequate for all. Intraspecific competition. This is competition between organisms of the same species. For example, maize plants