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Ecology - Biology Form 3

Background information

All living organisms live, grow and develop in a certain area of the environment. For them to survive, they need food, water and even shelter. These resources are provided by the surrounding environment where a particular organism is found. For example, a lion in maasai mara will feed on the animals such as gazelles and even drink water found in the same reserve. There is therefore a constant interaction between living and non-living things in a certain area.In this topic we shall look at such relationships and even their impacts to each other and the environment.

Click on the play button below to see living and non-living things interacting in an ecosystem


By the end of this topic, you should be able to:-

  • Define the term ecology, habitat, biomass, ecosystem and carrying capacity.
  • Identify the physical (abiotic) and biological (biotic) factors in a given ecosystem.
  • Describe the interrelationship of organisms in an ecosystem
  • Differentiate between saprophytism and symbiosis
  • Explain the importance of fungi and bacteria as decomposers
  • Relate the mode of transmission to prevention / control of a named parasite
  • Describe the adaptive characteristics of a named parasite to the host.
  • Describe the nitrogen cycle
  • Explain the flow of energy in an ecosystem.
  • Identify and construct food chains, food webs, pyramid of numbers and pyramid of biomass.
  • Explain the various methods of estimating population.
  • Relate the adaptations of plants to various habitats.
  • Describe the effects of pollutants in air, water and soil on humans and other living organisms.
  • Identify the symptoms of different types of human diseases and methods of transmission and control.



Ecology is derived from two words ECO or oikos which means house or place to live in and logos which means study of. Therefore ecology is the study of the interrelationships of organisms to each other and to their environment. To bring out this let us look at this illustration. For a cabbage to grow it requires light, rainfall among other factors. Worms rely on cabbages for survival. Therefore there exist a relationship between a cabbage, worm and the weather.

Ecology is subdivided in to two main branches, that is, synecology and autecology.

Synecology is the study of different species living and interacting in the same ecosystem e.g. wild beasts, warthogs, zebras and termites living in the same ecosystem.

Play the video below to see different species of organisms living and interacting in the same ecosystem for example wild beasts, warthogs, zebras and termites living in the same ecosystem.

Autecology involves the study of a single species or individual group of organisms, for example the wildebeest in Maasai Mara.

Play the video clip to view wild beasts migrating; This is a study of a single species

Terms used in ecology

Some of the terms that are used in ecology include: - Biosphere, habitat, niche, population, community, ecosystem and carrying capacity.

Biosphere (Ecosphere) - It is the part of the earth and atmosphere inhabited by living organisms.

Click on the play button to watch the video.


This is the physical location that an organism lives in with a particular set of conditions. Examples are a fish living in a pond, a grasshopper in a field, and monkeys in a forest. The pond, the field and forest are habitats. Some habitats are formed of land therefore referred to as terrestrial or can be formed of water therefore referred to as aquatic.

The photographs below show organisms and their habitats.

Fish in a pond

Grasshopper in a field


Describes where an organism lives and how it lives. This includes the position that an organism occupies in a habitat such as physical space and the role the organism plays in the habitat in terms of feeding relationships and interactions with other species.

For example, buffaloes, zebras and egrets can live in the same habitat but each organism is playing a different role. Giraffes are browsers, zebras, buffaloes and gazelles are grazers, while the egrets are commensals. 








-Refers to the total number of a particular species of organisms living in a particular habitat at a particular time.

For example: The number of flamingos at Lake Nakuru is approximately 250,000. The Photograph shows flamingoes in lake Nakuru.

Example two: The population of people in Kibera as per 2009 census is approximately 170,070. Play the video to view kibera slum.


This refers to all the organisms of different species that live within the same habitat. For example, in an old log of wood you can find termites, mushrooms, caterpillars, garden worms and many other organisms living together though they belong to different species.  A community changes gradually from few organisms to many organisms. The photograph shows an example of a community.


-An ecosystem is a unit comprised of several habitats and their communities of organisms living together in a self sustaining environment. Play the video clip below to see an example of an ecosystem.

Carrying capacity

This is the maximum population of a species that a particular habitat can support indefinitely under a given set of environmental conditions without depletion of resources in that habitat.

Photograph one shows an area inhabited by few organisms that the resources can sustain while in photograph two, there are very many organisms that the available resources cannot sustain and therefore the resources have been depleted.

Photograph one








photograph two

Factors in an ecosystem.

These factors are divided into two groups namely:

  • Abiotic factors
  • Biotic factors

Abiotic factors

This refers to all the non-living environmental factors which affect the distribution of organisms in an ecosystem.


The sun is the main source of energy for all life on earth. Green plants and photosynthetic bacteria need light energy to manufacture food. Animals depend on plants for food either directly or indirectly. Light affects growth and distribution of plants e.g. a forest that has a thick canopy there is limited undergrowth, but in a well light forest i.e. without a thick canopy there is thick undergrowth. Light intensity is measured by a photographic light meter while light penetration in water is measured using a seechi disc.

Photograph one show a photographic light meter while photograph two shows a seechi disc.










Biochemical processes in most organisms function efficiently within a narrow range of temperature. This therefore affects the distribution of organisms in a habitat. Organisms must develop physiological and behavioral adaptations to cope with extreme temperatures.

For example, the bear has thick fur to cope with extremes of low temperatures in the polar regions. Air temperature is measured by a thermometer while air temperature range is measured by the maximum and minimum thermometer.

Atmospheric pressure.

The atmosphere exerts pressure on the earth. This varies with altitude and determines the relative concentration of oxygen needed for respiration and carbon (IV) oxide for photosynthesis. This affects the distribution of organisms in a habitat. Decrease in atmospheric pressure increases the rate of transpiration and therefore the need to conserve water by plants. The illustration shows the variation of oxygen and carbon IV oxide concentration with attitude and the distribution of organisms. Atmospheric pressure is measured using a barometer.


Humidity refers to the amount of water vapour in the atmosphere. When humidity is high there is much water vapour in the atmosphere creating fog and mist e.g. in the Limuru area, but when it is low there is less water vapour in the atmosphere like in the desert where the sky is clear. This in turn affects the distribution of organisms on earth. Humidity is measured using a wet and dry bulb hygrometer.


Wind is moving air. It increases the rate of water loss from organisms therefore affecting their body. It carries water vapour to the upper parts of the atmosphere where it condenses and precipitates as rain. In desert areas wind aids in formation of sand dunes. Stable sand dunes may become habitats for growth of desert plants. Play the video to see a sand dune being formed.

Wind causes wave formation in lakes and oceans, which enhances aeration of water in these water bodies.

In the video clip, sea storms are formed by strong waves of water blown by wind. Click on the play button to view the video clip.

Trees growing in areas experiencing strong winds may have stunted and distorted growth. Wind also disperses spores and seeds and influence dispersal.

Play the video to see strong wind blowing through the forest trees producing whistling sound.

Biotic factors:

These are factors in an organism's environment that arise from other living organisms.These factors may affect an organism in many ways. For example,



This is the food relationship in which one organism kills the other for food and feeds on it. The organism which kills another for food is referred to as the predator while the one killed is the prey. An example is the antelope and the cheetar. Play the video to see predation.


This is an association in which one organism (i.e. the parasite) lives on or in the body of another organism (i.e. the host) from which it obtains nutrients causing harm to the host. E.g. Ticks on the body of a cow. Parasites that live in the host are known as endoparasites while those that live on the host are reffered to as ectoparasites.  

The photograph shows a cow infested with ticks.


This is an interaction between individuals of different species (symbionts) in which both individuals benefit e.g. the Rhizobium bacteria in the root nodules of the leguminous plants.

The photograph shows symbiosis between a leguminous plant and the rhizobium bacteria forming the root nodules.

Nitrogen cycle

Nitrogen is one of the elements that make up proteins. Proteins are needed for growth in both plants and animals. Animals depend on plants for their protein needs since they cannot synthesis their own food. Plants on their part can synthesize proteins (using nitrogen) but cannot utilize free nitrogen in the atmosphere. Plants obtain nitrogen in two forms; either in the ammonium form or as nitrates.

The illustration below shows the nitrogen cycle

Nitrogen fixation

This is the conversion of atmospheric nitrogen into nitrates. It occurs in the following three ways:

  • Oxidation of nitrogen by lightening during thunderstorms.
  • Free-living blue-green algae and other bacteria such as Azotobacter and Clostridium.
  • Symbiotic bacteria and other bacteria such as Rhizobium in root nodules of legumes.

The nitrates are absorbed by plants which use them to synthesize proteins. Plants are eaten by animals which convert plant protein to animal protein through assimilation. When both plants and animal die they are fed on by saprophytic bacteria and fungi in the process releasing ammonia (NH3). Animals excrete, releasing ammonia and urea. Ammonia gets into the soil, and combines with water and other elements to form ammonium compounds (salts). The ammonium salts are then converted to nitrates by nitrifying bacteria of the genera Nitrosomonas and Nitrococcus. The nitrites are then converted to nitrates by nitrifying bacteria of the genus Nitrobacter. The nitrates are absorbed by plants. Some nitrates are converted to either free gaseous nitrogen or nitrites by denitrifying bacteria called Pseudomonas denitrificans. This process takes nitrogen back to the atmosphere from where it was obtained, and in the process reducing the fertility of the soil.

The illustration below shows the process of nitrogen fixation from free nitrogen in the atmosphere to nitrates in the soil.

Energy flow in an ecosystem

The main source of energy in any ecosystem is the sun. This energy is trapped by green plants during photosynthesis to form food which is a potential energy in chemical form stored in the plant. The plants are therefore called Producers in an ecosystem. Some organisms such as herbivores feed directly on plants and are known as primary consumers. The primary consumers are in turn fed on by carnivores which are called secondary consumers. The secondary consumers are fed on by tertiary consumers. On death of a tertiary consumer, they are eaten by a vulture which is called a quaternary consumer. These feeding levels are called trophic levels. When all living organisms die, they are decomposed by saprophytic bacteria and fungi, which are together referred to as decomposers.

This flow of energy from one trophic level to the next results in loss of some energy in the form of heat. Energy is also lost in the processes of excretion, respiration or defaecation. When organic materials are fully decomposed, all the energy is lost from the ecosystem. As such energy flows through an ecosystem and is never recovered.

The illustration shows energy flow in an ecosystem. The energy flows from the sun to the producers, primary consumers, secondary consumers, tertiary consumers and finally decomposers.

Food Chains

A food chain is a linear representation of the flow of energy from a producer through a sequence of living organisms in which each eats the one below it in the chain and is eaten by the one above.The video clip shows a feeding relationship whereby the gazelle feeds on the grass and the cheetar feeds on the gazelle. Click the play button to view the video.

Arrows are used to point to the eater. If decomposers are included in the chain, they are always placed at the end of the chain.

-Examples of food chains are given below.

Food Webs

A single organism in a food chain can be a source of energy or nutrients for more than one organism. For example: green grass may be fed on by a gazelle, zebra, water buck, dik dik and many other herbivores. Each of these herbivores may in turn be fed on by different carnivorous animals and so on.As such feeding relationships may be more complex than is revealed by food chains. Therefore feeding relationships will always consist of a network of different food chains forming a food web. Food web may be regarded as a system of food chains that are linked to one another in a given community.

The following is an example of a food web.

The photographs show different organisms that are represented in the food web.

Pyramid of numbers

This is a diagrammatic representation of the number of organisms at each trophic level of a food chain in an ascending manner. The number of organisms at each trophic level of a food chain can be counted or estimated. The numbers of organisms can be obtained by totaling the population of all the species at that trophic level e.g. total number of plants (producers), Total number of herbivores (primary consumers), and Total number of carnivores (secondary consumers). The numbers can then be used to draw a diagram (pyramid of numbers) which shows the relationship between the numbers of organisms occupying each trophic level.

The figure below shows a pyramid of numbers.

From the pyramid, it is realized that the number of organisms transferring energy to the next energy level decrease as one ascends the pyramid.

Sometimes the number of organisms feeding on a primary producer are more than the number of producer(s). For instance if we were to draw a pyramid of numbers for caterpillars feeding on a leaf of cabbage and themselves are being fed on by a few birds, the pyramid of numbers would look different as shown. This is called an inverted pyramid of numbers.

The figure shows an inverted pyramid of numbers.

Pyramid of Biomass

Biomass refers to the total dry weight of organisms at each given trophic level of a food chain. The total dry mass is the mass of tissue only without any water. A pyramid drawn using total dry mass of organisms at each trophic level is called a pyramid of biomass. When drawing a pyramid of biomass, a scale must be used in order to draw boxes whose lengths are proportional to the dry mass of organisms while the heights remain the same.

An example of a pyramid of biomass is given below.

Population estimation methods

The number of individual of a given species in an ecosystem should be determined regularly to enable proper planning of the utilization of the resources available to avoid exceeding the carrying capacities. To determine the population of an organism in a given habitat, the following methods are used.

a) Quadrat method

b) Line and belt transect methods

c) Capture recapture method

Below are illustrations of a Quadrat, line transect and belt transect population estimation methods.


A quadrat is a square of a wooden or metallic frame of a known area. It can be 1square metre, 0.5square metre or any area.It is made by joining four pieces of wood/metal together.

The following are the steps followed when using a quadrat.

  • Identify a selected area
  • Toss the quadrat over the shoulder in the selected area
  • Make a table with rows indicating the number of throws while the columns have the titles to show results after every throw.
  • Count the number of the organism of interest and record.Sometimes it is impossible to say where one plant stops and another begins. In this case use a percentage area of the quadrat covered.
  • Repeat the above procedure for at least four times for a good sampling
  • Add the total number of all the values of organism obtained and divide by the number of throws done to get an estimate of the population.

Play the video below to see secondary school students estimating population using the quadrat method.

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Transect method

This is a tape or string running along the ground in a straight manner between two poles. There are two types of transects:line and belt transects.

Line transect

A single line of either a string or a tape is tied to two pieces of wood as pegs and marked at intervals of one metre. The photograph shows the line transect method.

Belt transect

Two single lines running parallel to each other separated by known width through a selected area in a habitat. The photograph shows the belt transect method.

Steps to follow when using transect method to estimate populations

  • Select the area of study.
  • Make a sketch of the whole area selected above.
  • Fix a one peg firmly on the ground and tie one end of string/ tape or any other material that be used to the peg.
  • Fix the other peg firmly on the ground at a different part of the area but to cover the total length of the string to be used.
  • Mark the string starting from one end to the other at 1m intervals.
  • Count and record the number of the organism of interest that falls along the line in intervals on a table.
  • Find the average number of organisms and use it to estimate the population of the organism of interest in the whole area of study.

The video clips below show students estimating the population using line and belt transect methods. Click on the play button to view the video clip.

a) Line transect method

b) Belt transect method. Click on the play button to view the video clip.

Capture recapture method

This method involves the following steps:
  • Capture of organisms using an appropriate method e.g. fish net for fish and a pitfall trap for crawling insects.
  • Mark the captured organisms using an ink or varnish which should not have any effect on the organism. Count them and release them.
  • After a fixed interval of time e.g. 24 hours, capture the organisms for the second time and count them.
  • Among this group some will be marked while others will be unmarked. Count them separately and record.
The population is then estimated by: Number of organisms initially captured multiplied by the Number of organism recaptured divided by the number of marked- recaptured organisms.


Play the video clip below to see students estimating the populations of mosquitoes using capture recapture method.

This method is based on the following assumptions

  1. Organisms mix freely in the population
  2. Enough time is allowed for mixing between capture and recapture
  3. Organisms movement is restricted to the geographic area under study
  4. That immigration, emigration, birth and death rates are negligible
  5. That the marking does not interfere with the free movement of animals or make them less camouflaged to their predators or poison them to death.

Adaptations of plants to life in various habitats

In this section we shall learn about the distinguishing characteristics that plants posses to enable them thrive in desert, terrestrial, water and salty marshes.

Different habitats are inhabited by different species of plants with varying abundance. These plants thrive in different habitats because they have characteristics that suit them to these habitats. These characteristics are called adaptations. The adaptations enable the plants to grow in areas with different levels/quantities of the abiotic and biotic factors.

There are four main groups of plants namely: -

  • Xerophytes
  • Mesophytes
  • Halophytes
  • Hydrophytes.


These are plants that are adapted to life in a dry habitat to endure prolonged conditions of drought as in arid and semi-arid areas. These habitats are characterized by low humidity, winds, very high day temperatures and low night temperatures; and unpredictable and poorly distributed rainfall.

Adaptations of Xerophytes

-Leaves are modified to spines or thorns to reduce the surface area over which transpiration can occur. E.g. Acacia.

-Leaves have thick waxy cuticle to minimize the rate of cuticular transpiration

-Some plants roll or fold their leaves to reduce the rate of transpiration by not exposing stomata.

Play the animation see rolling of leaves by plants on hot days to reduce the surface area exposed to sunlight

-They shed their leaves during the dry season to reduce the surface area exposed to transpiration

The animation shows a tree shedding off its leaves during dry weather conditions. Click on the play button to view the animation

-Have reversed stomata rhythm to reduce the rate of water loss by transpiration


-Have reduced number of stomata to reduce the rate of transpiration


-Swollen stems and leaves for storage of water e.g. Cactus and Baobab.


-Leaves have resin coatings to increase reflection of solar radiation hence lower transpiration rate.


-Others posses very deep roots to absorb water from deep in the soil e.g. Acacia


-Some have superficial roots that grow horizontally e.g. acacia


-Roots are close to the soil surface to absorb water after a short/light rain.


Mesophytes are plants living under normal conditions of water supply or in well watered soils. Their habitats have adequate rainfall, high humidity, less winds, shallow water table with moderate to high temperature. Such ecosystems as savannah, rainforests and reserve forests have mesophytes growing in them.

Adaptations of Mesophytes to their habitats

  • Those found in forests (trees) grow tall due to competition for light.
  • Some have slender, delicate stems to climb larger trees in order to reach light
  • Epiphytic mesophytes grow and support themselves on the branches of tall trees.

The photograph shows a thick forest with tall trees, climbers and epiphytes.

  • Mesophytes in areas with adequate water supply have broad leaves and thin cuticles to encourage water loss by transpiration.
  • Shallow rooted mesophytes have buttress/prop roots for extra support, and also to absorb water after light rains e.g. Baobab.
  • Those in drier areas have deep roots to absorb water from relatively deeper ground waters e.g. Eucalyptus
  • Some have waxy and shiny surfaces to reflect the strong light rays and drip off rain water.
  • The leaves are arranged in a regular mosaic pattern to minimize overlapping and overshadowing such that each leaf receives adequate light for photosynthesis.

In this photograph, leaf mosaic arrangement is shown.


These are plants that live in fresh water or in very wet places. Usually part or whole of the plant is in the water and hence their adaptations are those that minimize the possibility and effects of absorption of excess water, obtaining enough light and gases from air.

Adaptations of hydrophytes to their habitats

-Floating hydrophytes have fibrous roots for absorption of mineral salts e.g. papyrus and sedges.

-They have an aerenchyma tissue (air-filled tissue) which reduce density for buoyancy and gaseous exchange e.g. water hyacinth

-Have photosensitive chloroplasts hence can photosynthesize at low light intensities

-Leaves are covered by a waxy substance to reduce excessive water absorption e.g. water hyacinth.

-Most emergent and floating types have broad leaves with maximum number of stomata on the upper surface for efficient gaseous exchange and rapid loss of excess water through transpiration.

-Flowers are raised above the water to allow for pollination


The photograph below shows a water hyacinth broad leaves floating on water and its flowers raised above the water to facilitate pollination.

-Most of them lack xylem and support tissues and hence pliable. This avoids breakage by water currents e.g. sedges.

-Most of them lack cuticles, but for those which have cuticles, it is very thin to allow for absorption of water, mineral salts and carbon (IV) oxide gas.


It may be defined as the release of substances or forms of energy into the environment by human activities in such quantities whose effects are either harmful or unpleasant to humans and other living organisms.

There are various types of pollution. These include the following: -

  • Air pollution.
  • Water pollution
  • Soil pollution

Air Pollution

It is the release of substances or forms of energy into the air by human activities in excess amounts which are harmful to living organisms dependent on that part of the air. Air pollution is caused by the following:

Sulphur based chemicals e.g. sulphur (iv) oxide and hydrogen sulphide; produced by industries will lead to bronchitis, pneumonia and heart failure when inhaled.

Oxides of Nitrogen
e.g. Nitrogen II oxide (NO) and nitrogen (iv) oxide (NO2). Produced from burning petroleum fuel in motor vehicles. They are also released during industrial manufacture of Nitric acid. These are poisonous to animals affecting respiratory systems when inhaled.

Smoke and fumes
containing carbon (II) oxide (CO), carbon (IV) oxide (CO2) and carbon particles. These are produced from industries, burning of coal, burning of petroleum fuels, motor vehicles exhaust, burning of natural gases and charcoal. Their effects include affecting visibility due to smoke on roads, block stomata hence no photosynthesis co is a respiratory poison. Carbon (IV) oxide causes green house effect

Play the video clip to see factory chimneys releasing thick smoke into the air while a lorry is releasing thick smoke from the exhaust.

Dust from quarries and cement manufacture which affects the respiratory surfaces of organisms. Dust also clogs the stomata and blocks the stomata affecting photosynthesis.

Play the video to see a lot of dust being produced from a road construction site while a tanker is spraying water on the road under construction to reduce dust.

Aerosols which contain CFCs, whose main effect causes the depletion of the ozone layer.

Noise is produced by machines in factories, heavy vehicles, aeroplanes, music players and jua kali workshops. This affects hearing in animals e.g. Human beings become insensitive to low pitched sound. On the video clips below, jua kali artisans are hammering metals in a workshop producing a lot of noise.

There are many measures that are put in place by the government to enforce legislative cuts to ban use of leaded and sulphur based petroleum products.

The photograph shows a billboard of a petrol station advertising unleaded petrol and low sulphur diesel.

In controlling dust e.g. in road construction the government is enforcing the use of wet methods where water is sprayed to reduce the amount of dust raised. In industries that produce a lot of noise it is a government policy now that everybody wears ear muffs.

Play the video to see factory workers wearing ear muffs to minimize noise.

Water pollution

Water pollution refers to the release of substances or energy forms in water bodies in quantities that are harmful to living organisms that depend on that water. The causes include release of domestic and industrial wastes in the ocean and oil spillage from tankers whose effects are harmful to the living organisms. Play the video clips to see the following:-

Oil spillage at the sea showing water birds trapped in the oil.

In controlling water pollution apart from enforcing and providing for heavy penalties, there is also proper treatment and disposal of domestic and industrial effluents before discharge.

Play the video to view industrial effluent treatment ponds and the raw sewage treatment ponds.

Soil pollution

The other type of pollution is soil pollution which is the release of chemicals or solids that accumulate to levels that cause harm to soil organisms.The causes include petroleum products spilled on land when oil tankers involved in an accidents, then household wastes and industrial wastes which are non-biodegradable e.g. rubber, plastic containers crap metals and glass bottles.

The control of soil pollution involves recycling of plastic, glass and metal.

Human diseases

A disease is a condition in which the body cells, tissues, organs or the entire organism's body is affected and therefore normal function is impaired or not carried out normally.

In human beings diseases can be caused by the following:

a) Bacteria

b) Virus

c) Protozoa

d) Fungi

e) Nutritional deficiencies

f) Genetic disorders

A pathogen is an organism that causes impairment of the normal body function.

In this sub-topic, we shall consider Bacterial diseases, protozoan diseases and parasitic worms.

Bacterial diseases

These are diseases caused by bacteria. Examples include cholera and typhoid. The bacteria are spread by houseflies and through eating contaminated food. The diseases spread easily after the initial infection to cause an epidemic.The video shows individuals with cholera and typhoid symptoms such as abdominal pains, diarrhearing and vomiting. Click on the play button to view the video.



Cholera is caused by a bacterium called Vibrio cholerae which thrives in contaminated water.

The diagram shows Vibrio cholerae bacterium.

The bacterium is transmitted through coming into contact with contaminated food. The disease is spread easily after the initial infection to cause an epidemic.


They appear between one to six days after infection depending on the health of a person.

  • The bacteria secrets enzyme mucinase which digests the inner lining of the intestines
  • Exposed wall of the intestine becomes irritated and damaged by the toxins produced by the bacteria
  • Abdominal pains
  • Violent diarrhea and vomiting
  • Dehydration

The animation shows the process of infection and symptoms of cholera.

Prevention and treatment

  1. Proper disposal of faeces to prevent contamination of water
  2. Pit latrine should be dip and kept clean
  3. Boiling drinking water or chlorinating it.
  4. Wash hands after visiting the latrine
  5. Oral rehydration should be administered
  6. Isolate infected persons.



Caused by bacterium called Salmonella typhi.

The diagram below shows a bacterium Salmonella typhi

Poor disposal of urine and faeces may cause contamination of the water supply from rivers, dams and lakes. Healthy individuals can be infected by taking contaminated food or water.


Its incubation lasts for about two weeks, after which a fever and rush develop, followed by severe diarrhoea, leading to dehydration.

The bacteria attacks walls of the intestine and cause patches of sores.

In severe attacks the sores may burst and cause perforation in the intestine.

Prevention and Treatment

There should be proper sewage disposal

There should also be proper sewage treatment

Boiling drinking water or chlorinating it.

Proper handling of food.

Regular medical checking for all food handlers

Vaccination of healthy people with weakened typhoid bacteria

Treatment involves administering antibiotics.

Protozoan diseases

Examples of diseases are malaria and amoebic dysentery.


Caused by a protozoan parasite of the genus Plasmodium, which are of four different species: Plasmodium vivax, Plasmodium ovale, Plasmodium falciparum and Plasmodium malariae.

The photograph shows a plasmodium

The parasite is transmitted from one infected person to a healthy person by a female anopheles mosquito.

The photograph shows a anopheles mosquito

Steps of infection

  • Mosquito bites an infected person & sucks blood
  • Plasmodium develops into an infective stage
  • Same mosquito bites a health person and pass the parasite from its salivary glands into blood stream.
  • Plasmodium migrates to liver cells and multiplies.
  • Plasmodium leaves liver cells and back into blood stream when they start destroying the red blood cells.

The animation shows the process of infection of a health person by malaria. Click on the play button to view the animation.


  • High regular fevers, Lack of appetite, Profuse sweating, vomiting, headaches, muscle and joint pain, enlargement of liver & Spleen and convulsions.

Prevention and treatment

  • Vaccination
  • Use of treated bed nets
  • Use of mosquito repellants
  • Spraying of their dwelling places e.g. stagnant water using insecticides
  • Draining stagnant water and clearance of vegetation near homes
  • Empty containers e.g. tins, polythene papers should be destroyed to avoid stagnant
  • water which is a breeding ground for mosquitoes.
  • Use fish in ponds to feed on mosquito larvae
  • Use of sterilized male mosquitoes to mate the females reducing the mosquitoes in number
  • Taking of anti malarial drugs when traveling to malaria prone areas
  • Proper diagnosis through a laboratory test, administering a full dosage of the malaria drugs.
  • Use natural pyrethroids in mosquito control

Parasitic worms

Ascaris lumbriocoides

It is a parasitic round worm. It is widespread and infects small intestines of pigs and human being though other organs may be infected. The female is slightly larger than the male though both appear in a brown yellow color.

The photographs show ascaris lumbricoides worms and a packet of dewormers.

Mode of transmission

- Swallowing contaminated food & drinking infected water.

- Consumption of contaminated fruits & vegetables

- Direct infection from faeces to mouth especially in children.

Effects of the parasite on host

Constipation, intestinal blockage, obstruction of bile duct, pancreatic duct and appendix, anaemia, irritation of the trachea leading to damage of lungs as the larvae migrate and coughing.

Play the animation to see blocked small intestine, bile duct, and appendix leading to large rounded stomach.

Prevention and treatment

  • Boiling drinking water or chlorinate
  • Proper sanitation disposal
  • Personal hygiene
  • Regular deworming
  • Using drugs to kill the worms or inactivate the eggs

Schistosomiasis (bilharzia)

It is caused by the flat worms of genus schistosoma. Examples of the species that cause schistosomiasis are Schistosoma mansonii and Schistosoma haematobium. The parasite inhabits fresh water canals, dams and rivers.

The photograph shows a bilharzia worm.

Modes of transmission

The worm is transmitted between humans and tiny snails found in fresh water streams, rivers and irrigation canals.

The adult worms deposit their eggs into the hepatic portal vein of the humans.


Play the animation to see bilharzia worms depositing eggs in the hepatic portal vein of human.


The eggs are then released to water where they hatch into free swimming ciliated larva which then finds its way into a snail and develops to infective stage.

Effects of parasite on the host

  • The host skin is destroyed when the larvae is penetrating
  • Severe abdominal pains
  • Blood in the urine due to destruction of tissues by the parasite.
  • It can cause death if untreated due to lowered body immunity.

Prevention and treatment

  • Treating or chlorinating drinking and bathing water.
  • Proper sanitation disposal.
  • Use of chemicals to destroy snails which are the intermadiate hosts.
  • Using drugs to kill the adult worms or inactivate the eggs.

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For Best results INSTALL Adobe Flash Player Version 16 to play the interactive content in your computer. Test the Sample e-Content link below to find out if you have Adobe Flash in your computer.

Sample Coursework e-Content CD

Other Goodies for KCSE ONLINE Members!

Coursework e-Content CD covers all the topics for a particular class per year and costs 1200/- ( Per Subject per Class ).

Purchase Online and have the CD sent to your nearest Parcel Service. Pay the amount to Patrick 0721806317 by M-PESA then provide your address for delivery of the Parcel. Alternatively, you can use BUY GOODS TILL NUMBER 827208 Ask for clarification if you get stuck.

Install ADOBE Flash Player for Best Results

For Best results INSTALL Adobe Flash Player Version 16 to play the interactive content in your computer. Test the link below to find out if you have Adobe Flash in your computer.


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