Animal Nutrition Revision Notes

Nutrition is the process of taking in food and converting it into living matter. Animal feed by taking in complex organic matter (holozoic nutrition).

In animals, nutrition consists of the following processes:

  • Feeding/ingestion – Food taken into the body
  • Digestion – Large food molecules broken down into smaller soluble molecules that can be absorbed into the body cells
  • Absorption – Digested food substances absorbed into the body cells
  • Assimilation – Some of the absorbed food substances are converted into new protoplasm or used to provide energy



  • Mouth and buccal cavity – Food enters mouth which leads to the buccal cavity,
    • teeth breaks down food into smaller pieces
    • salivary glands produce saliva which flows into oral/buccal cavity via tubes known as ducts
    • tongue helps to mix food with salivabuccal cavity.jpg
  • The pharynx – Passageway for food and air into the body
    • connects to the oesophagus and larynx (contains a slit-like opening known as the glottis)
    • larynx leads to the trachea (windpipe; leading to the lungs)
    • flap-like tissue epiglottis above the larynx, behind the root of the tongue, prevents food from going down the wrong waylarynx.jpg
  • The oesophagus/gullet – Narrow muscular tube which pass through the thorax (chest) and diaphragm to join the stomach
    • wall of oesophagus made up of two layers of muscles (longitudinal muscles on the outside, circular muscles on the inside of gut) –> peristalsisoesophagus.jpgperistalsis.jpg
  • The stomach – Distensible (can be stretched/expanded) muscular bag
    • when fully distended sends signals to the brain that it is full (or sated)
    • pits present on the stomach wall lead to gastric glands that secrete gastric juice
    • pyloric sphincter (ring of muscle) located at the junction between stomach and small intestine (contract/relax to close/open the entrance to small intestine)

pyloric sphi.jpg

gastric glands.jpg

  • The small intestine – Consists of U-shaped duodenum, jejunum and coiled ileum
    • lining of the walls of small intestine contains glands which secrete digestive enzymes, walls also adapted to absorb digested food products and water

small inetstine.jpg

  • The large intestine – Shorter and broader
    • colon absorb water and mineral salts from undigested food)
    • rectum is a temporary storage of faeces
    • caecum is a sac-like structure between colon and intestine, attached to the appendix (no specific function)large intestine and cecum.jpeg
  • Accessory organs involved in digestion –
    • Liver – largest gland in the body, secrete bile which is a greenish-yellow liquid that contains bile salts & bile pigments which help to speed up digestion of fats by emulsifying fats into small fat molecules
    • gall bladder is the temporary storage area for bile, connected and release bile to duodenum via bile duct
    • pancreas is connected to the duodenum by pancreatic duct, produces pancreatic juice containing digestive enzymes along with the hormones insulin and glucagonaccessory.jpg

Enzyme Revision Notes

Enzymes are biological catalysts which speed up a biological reaction through lowering the activation energy required for a reaction to start. They are classified based on the reactions they catalyse (e.g. hydrolases catalyse hydrolytic reactions).

Enzyme-catalysed reactions can be classified into two types – synthesis of substances (anabolic reactions) and breaking down of substances (catabolic reactions).

The energy required to start a chemical reaction is known as the activation energy of the reaction.

activation energy.png

In the presence of enzymes, a greater number of reactant particles possess more energy than the activation energy and the reaction occurs at a faster rate. The reactant molecules on which enzyme act on are called substrates.

Characteristics of enzymes:

  • Highly efficient – Remained unchanged in the reactions they catalyse, this means that only a small amount of the enzyme is required and these enzymes can be reused over and over again.enzyme-reuse.PNG
  • Highly specific – Each enzyme only interact with one particular type of substrate (enzyme specificity) to form a unique enzyme-substrate complex. This is dependent on the 3D surface configuration  of the enzyme. The depressions or ‘pockets’ on the surface of an enzyme which the substrate fits into is known as the active site of the enzyme. There are two main forms of models which explains how the shape of an enzyme affects the way it functions – Lock and key hypothesis and induced fit model.

e-s specificity.PNG

==> Lock and key hypothesis – Enzyme is the lock and substrate is the key, substrate fits perfectly into the active sites of the enzymes as the shape of the substrate is complementary to the shape of the active site of the enzyme

==> Induced fit model – Original active site is not exactly complementary to the substrate, enzyme molecules undergoes slight adjustments in its shape to fit more tightly around the substrate moleculeenzyme.png

  • Catalyse reversible reaction – Enzyme catalyse both the forward and reverse reaction until a state of equilibrium is reachedreversible reactions.png
  • Affected by temperature changes – Temperature at which an enzyme is most active (catalyses the greatest number of reactions per second) is called the optimum temperature and most human enzymes perform best at temperatures between 37 to 40 degree Celcius

enz temp.gif
==> When temperature rises: Increase in kinetic energy of reacting particles, particles are more likely to collide with each other and substrates fitting into active sites, enzyme-substrate complexes formed at a faster rate and products are formed at a faster rate
==> When temperature exceeds optimum: High temperature cause hydrogen bonds in the enzymes to break, resulting in the loss of its unique three-dimensional shape and active site (denaturation), irreversible and the performance and rate of enzyme catalysed reactions drop rapidly

  • Affected by pH changes – Optimum pH at which an enzyme works the best is not the same for each enzyme, slight changes in pH may change the electrostatic charges on the surface of enzyme active site and substrate resulting in electrostatic repulsion and affecting interaction between substrate and active site, extreme changes in pH of the solution will denature an enzymeenz ph.PNG
  • Affected by substrate & enzyme concentration – When the substrate concentration is kept at a high level, increasing the enzyme concentration will increase the rate of reaction proportionally; For a fixed enzyme concentration, as the substrate concentration increases, the rate of reaction increases till it reaches the point of saturation (all active sites of the enzyme is taken up and the current reactions must be completed before further reactions can start)

enz con.jpg

sub con.jpg

Temperature, pH and enzyme and substrate concentrations become limiting factors when they are above or below the optimum required for the enzyme to be the most active.


Nutrients Revision Notes

All living organisms require energy to stay alive. Green plants are able make use of light energy from the sun to convert raw materials from their surroundings into stores of chemical energy such as carbohydrates (in the form of glucose or starch), in a process known as photosynthesis. Animals must consume green plants, or organisms that feed on green plants, in the form of food in order to get stored energy.


Nutrients are chemical substances found in the food we consume which provides energy and the raw materials needed by the body for growth and repair of worn out body parts. They can be classified based on the presence (organic) or absence (inorganic) of carbon. There are five main types of nutrients that we consume – carbohydrates, fats, proteins, water and vitamins and minerals. In this topic, we are only looking at carbohydrates, fats and proteins.

Carbohydrates – inorganic compounds made up of carbon, hydrogen and oxygen with the general formula of CnH2mOm, one gram of carbohydrate provide an average of 16 kJ of energy

There are three main groups of carbohydrates – monosaccharides, disaccharides and polysaccharides.


types of carbs.jpg

Obtained from Perfect Guide: ‘O’ Level Biology 2nd Edition, Marshall Cavendish

Functions of carbohydrates:

  • substrate for formation of supporting sturctures
  • substrate for respiration, provides energy for cellular activities
  • substrate for the synthesis of other organic compounds such as amino acids and fats
  • required for the synthesis of nucleic acids
  • main component of nectar in flowers
  • used to synthesise natural lubricants

Food tests involving carbohydrates:

  • Benedict’s test – Blue Copper (II) sulfate solution (Benedict’s solution) when boiled with reducing sugar (any sugar that is capable of acting as a reducing agent; glucose, maltose, fructose, galactose or lactose), produce brick-red precipitate of copper (I) oxideResult-Interpretation-of-Benedicts-Test.jpg
  • Iodine test – Triiodide anion (I3) present in the iodine solution form a complex with amylose of starch, resulting in the formation of a blue-black complex330px-IodineStarch_en.svg.pnggcse-igcse-biology-by-syllabus-points-54-638.jpg

Fats – organic compounds made up of carbon, hydrogen and oxygen, contains less oxygen in proportion to hydrogen, have no general formula (proportion of elements not fixed)

There are two main kinds of fats – saturated or unsaturated fats. Unsaturated fats can be further characterised into monounsaturated or polyunsaturated fats.

saturated and unsat fats.JPGfats comparison.PNG

Functions of fats:

  • Insulating layer that reduces heat loss
  • Essential component of cell membranes
  • Reduce water loss from skin surface by forming a hydrophobic layer which prevents evaporation of water
  • Solvent for fat-soluble vitamins and vital substances, such as hormones

Food test involving fats:

  • Emulsion test – Lipids are dissolved in alcohol before it gets precipitated out,  in the form of a cloudy emulsion, in the presence of water

emulsion test.png

Proteins – complex organic compounds made up amino acid subunits with carbon, hydrogen, oxygen and nitrogen, sulfur may sometimes be present

An amino acid has an amino group (-NH3), and acidic (or carboxyl) group (-COOH) and a variable side chain (-R). Hence the general formula for an amino acid is:


Amino acids can be linked together by peptide bonds, which is formed through a condensation reaction between a amino group and a carboxyl group, to give a polypeptide or peptone.

peptide bond.jpg

Functions of proteins:

  • Synthesis of antibodies required for adaptive immunity
  • Growth and repair of body cells and tissues
  • Key components of some enzymes and hormones
  • Synthesis of haemoglobin, a protein in red blood cell that binds and carries oxygen to all parts of the body

Food test involving proteins:

  • Biuret test – Biuret solution (blue solution of sodium hydroxide and copper (II) sulfate) turns deep purple in the presence of protein and pink in the presence of short chain polypeptidesbiuret test.PNG

In order for us to obtain the chemical energy stored in the nutrients of the food we consume, our body must break the nutrients down into simpler substances.

Hydrolysis (or hydrolytic reaction) is a chemical reaction in which water molecules is used to break up a complex molecule into simpler molecules.

Reversibly, simpler molecules can also be chemically combined to form a more complex substance, through a process known as condensation (or dehydration reaction).


Hydrolysis of carbohydrates:

  • Glycosidic bonds between monosaccharides broken downhydro dissacaride.jpg
  • For complex carbohydrates such as starch, a two step reaction is adopted. Amylase first digests starch into smaller maltose molecules before maltase hydrolyses maltose to give glucose molecules.starch.gif

Hydrolysis of fats:

  • Ester bonds between fatty acids and glycerol in the lipid molecule are broken downfats hydrolysis.PNG

Hydrolysis of proteins:

  • Peptide bond between amino acid residues in the protein are broken down by enzyme-catalysed hydrolytic reactionsAmino-acid-300x220.png

Movement of Substances Revision Notes

Every cell is surrounded by a liquid or semi-liquid environment. The cell transport system produces a nearly continuous movement of substances, both into and out of the cell, to ensure that the cell receive the correct and enough raw ingredients and discard toxic wastes fast enough for survival.

The concepts of diffusion, osmosis and active transport are required to understand the movement of water and substances between the cell and its surroundings.

Diffusion – net movement of substances (atoms, ions, molecules) from a region of higher concentration to a region of lower concentration, down a concentration gradient.

passive diffusion.gif

Movement of substance across a fully permeable membrane

The concentration gradient refers to the difference in the concentration of solutes present in a solution between two regions. The steeper the concentration gradient, the faster the rate of diffusion. As the substances diffuse, the concentration gradient becomes less steep.

When the substances have evenly spread out, we say that a state of dynamic equilibrium has been reached. The substances are still in constant random movement in the space it is confined with but there is no net change in concentration any further. Not applicable to water molecules.

Osmosis – movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane, down the water potential gradient.

Water potential is simply the measure of relative tendency of water molecules to move from one area to another. A dilute solution for instance contains more water molecules per unit volume than a concentrated solution, and hence have a higher tendency of water to move to the other areas of the solution and a higher potential.

A water potential gradient is the difference in water potential between two solutions of different water potential, separated by a partially permeable membrane.

A partially permeable membrane is a barrier which allows the passage of small molecules but not the large solute molecules.

When a solution have a higher/lower/same water potential as another solution we are comparing with, it is hypotonic/hypertonic/isotonic with respect to the second solution it is compared with.

The cell wall of a plant cell is permeable and allows most dissolved substances to pass through, which cause the plant cell to behave differently from an animal cell, when placed in solutions with different water potentials.


When the surrounding environment is hypertonic:

  • Plant cell – water leave cell by osmosis, resulting in the shrinkage of cytoplasm and cell surface membrane from the cell wall (plasmolysis; can be restored by placing in water or solution with higher water potential)
  • Animal cell – water leave cell by osmosis, resulting in the shrinkage of cells and the formation of spikes on surface of cell (crenation; cannot be restored, dehydration and eventual death)

When surrounding environment is hypotonic:

  • Plant cell – water enters cell by osmosis and cause the enlargement of vacuole which pushes the cell contents against the cell wall, resulting in swollen/turgid state (turgor important in maintaining shape of soft tissues in cells; pressure exerted by water on cell is turgor pressure), cell does not burst as the cell wall is strong and relatively inelastic which prevents overexpansion by exerting opposing pressure as water enters the cell
  • Animal cell – water enters cell by osmosis, cell burst when too much water enters the cell

When surrounding environment is isotonic:

  • No change in water movement between the surrounding and the plant/animal cell since there is no difference in water potential

Active transport – is the energy-dependent process of moving substances from a region of low concentration to a region of high concentration, against a concentration gradient.

active transport.png


The rate at which a substance enters a cell across the cell surface membrane is dependent on the permeability of the membrane and the surface area of a cell.

For a given concentration gradient and volume, the greater the area of the cell surface membrane, the faster the rate of diffusion of a substance.

However, as the cell grows in size, even though its surface area also increase, the surface area to volume ratio (SA:V) decreases. A decrease in SA:V results in a decrease in rate of movement of substances across the cell membrane, per unit volume. Take the following figure as an example.

SA to V ratio.png

Both combination of cells, on the left and right side of the figure, results in the same volume occupied in the body even though there are more cells on the right hand side of the figure.

Let’s take the volume of the surrounding environment to be 200 cm square. The SA:V on the represented by the left hand side of the figure is 5400:200 (or 27:1) while the SA:V for the right hand side of the figure is 16,200:200 (or 81:1). There will be a faster rate of substance diffusion in the scenario on the right hand side than the scenario on the left hand side.

This is also the reason to which why as small, actively growing cells increase in size, their level of activity and metabolism decreases and they stop growing once a maximum size is reached.

Cells can compensate for their small SA:V by adopting some of these strategies:

  • Self-restriction – staying small maximises SA:V for greatest efficiencies in diffusion, bacteria and yeast cells adopt this strategy
  • Increasing surface area through physical modifications – elongating and flattening out of the body of the cell (e.g. flat discs of red blood cells) or formation or microfolds, fingers or indentations within the surface of the cell (e.g. human intestinal microvilli)
  • Bulk acquisition – take in nutrients through water found in vacuoles in which they are dissolved, allows intake of large quantities of nutrients in bulk (e.g. intestinal epithelium cells)
  • Movement of cytoplasmic organelles – streaming motion of parts of cytoplasm transfers nutrients away from the plasma membrane, help maintain a steady concentration gradient for continuous diffusion (e.g. leaf and root hair cells)
  • Active relocation – mobile unicellular organisms can search out areas of high concentrations of nutrients (e.g. protists)

Ecology Revision MCQ Questions

1. ln the diagram, arrows represent the movements of carbon compounds in the carbon cycle. The circles represents carbon compound in animals, decomposers, plants and the atmosphere.seng kang q38.PNG

2. A bird feeds on a population of insects. The insects feed on the leaves of a tree. Which of the following shows the pyramid of numbers for the food chain?kuo chuan q40.PNG

3. The diagram shows a food web.

st joseph Q37.PNG
At which trophic levels is the leopard feeding?

A.    first and second
B.    second and third
C.    second and fourth
D.    third and fifth

4.  The graph shows the quantities of pesticide that accumulate in four populations, A, B, C and D, each at different trophic levels in a food chain. Which population is most likely to be herbivores?chij katong Q40.PNG

5. A single plant provides food for many herbivores and the herbivores are eaten by the carnivores.

seng kang q39.PNG

6. The graph shows changes in biomass of different organisms in an ecosystem over a 12 month period. Each line represents a different trophic level. Which line represents the herbivores?st joseph q39.PNG

7. The diagram represents the flow of substances within a balanced ecosystem. The boxes are various trophic levels. Which box represents the decomposers?serangoon q40.PNG








Ecology Revision Structured Questions

1. The humpback whale, Megaptera novaeangliae, is one of the world’s largest aquatic mammals. It can grow to a length of up to 15 metres and a mass of up to 36 000 kg. A large proportion of the mass of a humpback whale is a very thick layer of fat-filled cells stored under the skin, called blubber.
The humpback whales are seasonal feeders. They feed in polar regions during the summer
and then migrate to warmer temperate and tropical waters to mate and have their young during the polar winter.
(a) The figure below shows the organism in the food web. The humpback whale is a carnivore, feeding on krill and herring. The herring feeds on krill. Both the krill and herring feed on phytoplankton. Add arrows on the figure to show the direction of energy flow in the food web.

chij katong q7.PNG

(b) Construct a food chain in which the humpback whale is a tertiary consumer.

(c) Draw and label a pyramid of numbers that comprises three trophic levels.

(d)  One reason that the humpback whale has managed to reach its enormous size is
because it is a member of a simple food web.
In terms of energy transfer, explain how the humpback whale is able to reach such a large size.

2. The figure below shows the feeding relationships of organisms in a food web found in the South Pole region.

northland q6.PNG

(a) With reference to the figure above, draw a pyramid of biomass with 4 trophic levels.

(b)  Explain your answer to part (a).

(c)  Explain why large amounts of pesticides are found in the bodies of penguins despite no farming activity in the South Pole region.

3.  The figure below shows how the energy flows through part of the food chain.

serangoon q3.PNG

(a) (i)  What is the source of energy input in the above system?

(ii)  Write an equation to show how the animal receives energy from the food when oxygen is present.

(b) State and explain how energy is lost from the cow at B and C.

(c) What percentage of the energy is available to consumers and decomposers? Show your working.

(d)  The figure below shows a hom fly, a parasite which feeds on the blood of the cattle. lt leads to lower weight and milk production in the cattle. There can be up to 50,000 horn flies feeding on one cattle.serangoon q3 b.PNG

(i)  Draw a food chain of the above situation when the cow is infested with the hom flies. Also indicate where in the food chain does enter.

(ii)  Draw a Pyramid of numbers and a Pyramid of biomass of the above food chain.

(e) What is a carbon sink? Give an example of a carbon sink.



1. (a) phytoplankton to herring / krill, krill to herring, herring and krill to whale

(b) phytoplankton –> krill –> herring –> humpback whale

(c)  correct labels (phytoplankton/ krill/ humpback whale); correct shape


abundant food available

– feeding on more than one trophic level;
– phytoplankton efficient at converting light energy –> can support large population of primary consumers;
– no competition from other predators for food
– efficient feeding mechanism
– food consumed has few indigestible / inedible parts/ contains high calories;

efficient energy transfer
– less energy lost in short food chains 9 more energy reaches the top consumer
– example of how energy is lost – energy used to support the organisms in the different trophic levels, energy lost to surrounding as heat

2. (a)

northland q6 ans.PNG

(b) Trophic level 1 has a smaller biomass than trophic level 2 because microscopic algae are extremely small and hence the total biomass is small;

Lesser energy is transferred from one trophic level to the next because energy is lost through heat by respiration;

As lesser energy is transferred, it can only support a smaller biomass, hence biomass decreases from trophic level 2 to 4

(c) The microscopic algae may have absorbed large amounts of insecticides at an area outside the south pole region;

The shrimp feeds on the algae, squid feeds on the algae and the insecticides accumulates in the body of the squid and is not excreted;

Penguin may feed many squids which contains insecticides and insecticides accumulate and amplifies in concentration

3. (a) (i) The Sun

(ii) Glucose + oxygen –> carbon dioxide + water + energy


  • Energy is lost as heat energy in respiration at B
  • And as waste e.g urea and faeces in excretion / egestion at C

(c)  4+62= 66 J ; 66/100 x100= 66%

(d) (i) Light energy from the sun –> grass –> cow –> horn flies

(ii) serangoon q3 ans.PNG

(e) A carbon sink is an area that stores carbon compounds for an infinite period. lt stores more carbon than it releases. e.g Forest , ocean


Cell Structure and Organisation Revision Notes

Cells are the simplest basic building blocks of life. Many chemical reactions occur inside our cells to keep us alive.

Examples of cells:


Brain cell (Neuron)

metal mining bacteria.jpg

Bacteria Cell

plant cell.jpg

Plant cell

All cells takes in raw materials, process these raw materials to make new molecules and make use of these new molecules in the cell or transported to the other parts of the body for other body functions.

Most cells are too small to be seen with our eyes and we use a light microscope to help see these cells and their parts. A light microscope is able to magnify objects to more than 1000x its original size. For anything much smaller than a cell, it can be observed using a electron microscope, which is able to magnify objects up to 200,000x its original size. The diagram below shows the size of an animal cell with respect to a normal adult human (female).

size of cells.png

The different types of cells in our body and in plants may not look the same, but they all have similar parts. Each living cell consist of 2 major components, namely the cell surface (or plasma) membrane and the protoplasm.

cell membrane and protoplasm.png

  • The cell surface membrane is partially permeable membrane that encloses the protoplasm of a cell. A partially permeable membrane is a barrier of the cell which allows only some substances to cross and enter or exit the cell.
  • The protoplasm is a living material in a cell which consists of the cytoplasm and the cell nucleus. Its composition is unique to each type of cell and suited for the cell function.

animal cell.GIF

The cytoplasm is the part of the protoplasm found between the cell surface membrane and the nucleus, where most of the cell activities occurs.

  • Consists of specialised structures known as the organelles such as the ribosome, along with many enzymes which carry out specific functions in the cell. These organelles are often much smaller in size compared to the cell as a whole, and require an electron microscope to see them. Listed below are the organelle functions.

organelles and fn.PNG

  • Can exist can exist either in the sol (liquid) or gel (semi-solid) state, dependent on the composition of substances and organelles in the cells.


The nucleus is an organelle within the cell which is essential for cell division and controls cell activities such as repairing of worn out parts in the body. It consists of 4 major components – nuclear envelope, nucleoplasm, nucleolus and chromatin.

  • Nuclear envelope is the two-layered membrane of the nucleus which separates the contents of the nucleus from the rest of the cytoplasm
  • Nucleoplasm is the dense material within the nucleus which acts similarly like the “protoplasm” of a nucleus
  • Nucleolus (plural: -li) plays a part in making of proteins in the cell
  • Chromatin is a network of long thread like structures, made up of deoxyribonucleic acid (DNA) which stores hereditary information and instructions a cell needs to carry out the chemical reactions within itself.

plant cell.GIF

In addition to the above mentioned parts of a living cell, a plant cell have two distinct components absent in the animal cell – cell wall and chloroplast (Marked by a red rectangular box in the diagram above.

  • Cell wall is a non-living part which surrounds the cell surface membrane, protecting the plant cell from injury and give the plant cell its fixed shape. It is made up of cellulose and is fully permeable.
  • Chloroplast organelle with green pigment, chlorophyll, essential for photosynthesis (process by which plants make food)


Each animal cell carries a pair of centriole (also known as basal bodies) which is absent in the plant cell. Centrioles are small structure found outside of the body which helps in cell division (in mitosis) and the formation of flagella and cilia, through making microtubules. In addition, vacuoles may or may not be present in animal cells. If present, it is likely to be small and numerous (Plant: large, central vacuole).

Depending on the location in our body, our cell have different cell structures which allows them to perform specific cellular function. However, our cells do not start off this way.


All our cells originated from a single parent known as the embryonic stem cell, which have the capability to differentiate into the different types of cells we see in our body. Differentiation is a process which occur during development, when cells with general function develops to become one with specific function. Cells may acquire special structured or lose certain structures during differentiation.

specilaised cell fn.png

When the same type of cells combine to serve a particular set of functions, they are collectively known as a simple tissue (e.g. muscle tissue). Complex tissues on the other hand are made up of different types of cells (e.g. Blood, xylem tissue). Different tissues may combine to form organs (e.g. stomach, leaf). When the different organs work together for a common purpose, they form a organ system. The organism is made up of numerous organ systems working together to support the basic survival needs.






Environment & Humans Revision Structured Questions

1. (a) Before 1977, many of the rivers in Singapore were polluted with waste materials discharged from factories. The rivers were so polluted that the number of fishes decreased drastically. Since then, efforts have been ramped up to reduce pollution in the rivers. Sewage treatment plants were built so that raw sewage will not be discharged into the river.

Explain how sewage treatment uses biotechnology to improve water quality.

Explain how this treatment would affect the number of fishes in the river.

(b) Human beings are continually using and depleting the Earth’s natural resources. This
upsets the balance of nature and causes lasting damage to the environment. Human activities have led to deforestation and over-fishing.
Suggest ways that humans can conserve the forests and fishing grounds.

2.  The figure below shows the results of an investigation on the effect of sewage discharged in a

kuo chuan.PNG

(a) (i) From which point X, Y or Z, is the sewage being discharged into the river?

(ii) Explain your answer.

(b) what process could have taken place after 30m downstream?

(c) Based on the evidence shown in Fig. 10.1, state and explain what could have happened to the number of algae at the following distances

(i)  15 m to 35 m from downstream

(ii) after 35 m downstream

(d) State and explain changes in the oxygen level in the river.

(i) downstream from point Y to 15 m.

(ii) downstream from 20 m to 30 m.

(iii) downstream from 30 m to 60 m.



1. (a) Sewage is channelled into an aeration tank where aerobic microorganisms like bacteria and fungi digest organic pollutants into harmless soluble substances;

The digested produced are absorbed and used by bacteria for their growth;

Sewage is aerated by bubbling air through it to ensure sufficient oxygen for aerobic respiration;

Sludge removed from the settling tanks is treated by anaerobic digesters. Methane and carbon dioxide gas that are produced in
this process are used to generate electricity;

The number of fishes in the river would increase;

The decrease in concentration of organic pollutants and bacteria will cause oxygen levels to increase and hence providing sufficient oxygen for the fishes to take in to perform cellular respiration

(b) Prevent tree-felling by enacting laws to protect remaining forests;

These trees are cut down selectively and at a regulated rate;

Young trees are not felled and new seedlings are planted to replace trees that were cut down for timber;

Perform reforestation;

Check trees regularly and control the number of insects and diseases that harm them;

Research are carried out to improve the quality of forest trees and make the forest more productive;

Banning the use of drift nets which indiscriminately trap all forms of marine life;

Using nets with certain mesh size so that young or immature fish are not caught;

Regulating the entry of ships into fishing grounds;

Limiting period of fishing in fishing grounds;

Banning the harvesting of fishing of endangered species;

Raising endangered species of fish in hatcheries and releasing them into the fishing grounds where the fish populations are decreasing;

2. (a) (i) point Y

(ii) Sewage contains organic matter. When it is discharged at point Y, there is a sudden increase in organic matter in the river.

(b) eutrophication

(c) (i) The number of algae increases tremendously. Nitrates and phosphates are good sources of nutrients for the growth of algae. As a result, they grow and multiply rapidly.

(ii) The number of algae decreases rapidly. This could be due to insufficient sunlight reaching the submerged algae. The sunlight is blocked by the thick layer of algae growing near the water surface.

(d) (i)  Oxygen level starts to decrease when sewage is discharged from point Y. The decrease is caused by the rapid growth of bacteria that uses up the oxygen in the river.

(ii) Oxygen level increases may be due to the rapid growth of algae that releases large amounts of oxygen when they carry out photosynthesis

(iii)  Large concentration of algae have built up and deplete oxygen through respiration. Oxygen is also used up when the death of algal cells and other
marine life becomes increasingly significant.




Environment & Human Revision MCQ Questions

1. Which processes result in organisms gaining carbon compounds, and in the addition of carbon dioxide to the atmosphere?

st joseph Q40.PNG

2. Which statement(s) state(s) the purpose of a carbon sink?
I      Release carbon into the air
II     Absorb carbon dioxide from the atmosphere
lll    Create energy for fossil fuels
lV    Keep carbon dioxide from accumulating at a more rapid rate in the atmosphere
A.    ll only
B.    l and lI
C.    ll and lV
D.    l, ll, lll and lV

3. Which graph shows the most likely effect of pollution by sewage on the amount of oxygen dissolved in a river?

macpherson q40.PNG

4. Which property of modern insecticides helps to keep environmental pollution at the lowest level?

A.    They accumulate in the bodies of predators
B.    They are broken down by soil bacteria
C.    They are easily washed into lakes and rivers
D.    They are taken up by plant roots

5. Which of the following practices does NOT help in the maintenance of biodiversity on Earth?

A.    Designating land as nature reserves
B.    Use drift nets for catching large numbers of fish
C.    Planting of forests to replace trees that have been felled for timber
D.    Limit the period of fishing in fishing grounds


Answer: DCABB


Environment and Humans Revision Structured Question

1. Marine conservationists are concerned that fish stocks in the sea are decreasing. Drastic measures will have to be taken to stop the extinction of many fish species.

The figure below shows a marine food web. Tuna are large carnivorous fishes that are an important human food. Dolphins may be caught in fishemien’s nets and die.

yuyin Q8.PNG

(a) State the names given to organisms occupying trophic levels 1 and 3.

(b) (i)  Draw a food chain with four links from the food web shown, involving herbivorous fish.

(ii)  Draw and label a pyramid of energy for this food web.

(iii) Explain why it is more energy efficient for humans to eat herbivorous fish rather than tuna.

(c) Explain why it is necessary to conserve animals, such as tuna and dolphins, which are at trophic level 4.

(d) A toxin was introduced to the ecosystem through an accident involving a cargo ship. The toxin is directly assimilated into plankton and photosynthetic algae and remains in the organisms throughout their lifetime.
Describe its effect on the tuna population.

2. (a) The figure below shows a food chain. The figures show the relative numbers of organisms at each level.

pasir ris Q8.PNG

(i) Draw a pyramid of numbers for this food chain.

(ii)  How would the shape of a pyramid of biomass for this food chain be different from this pyramid? Explain this difference.

(iii) What is the importance of sunlight in this food chain?

(iv) Suggest why this food chain could not have another trophic level.

(b) A farmer sprays insecticide on his crops. The insecticide washed off into a lake where it is absorbed by the producer to enter the food chain shown below.

pasir ris Q8 b.PNG

The numbers give the levels of insecticide in the organisms in parts per million (ppm).

(i) Describe the difference in the levels of insecticide in the organisms in the food chain.

(ii) Suggest a cause for this difference.

(iii) The fish-eating birds are the only organism in this food chain that dies from insecticide poisoning. Suggest a reason why.



1. (a) Producer and secondary consumers

(b)(i) Any food chain with 4 trophic levels

(ii)  Lowest trophic level producers, highest trophic level are tertiary consumers. Decreasing amount of energy down trophic level

(iii) As you go up the trophic level, energy is lost. As herbivorous fish is of a lower trophic level, more energy will be available to the humans.

(c)  If tuna and dolphins are removed, the ecosystem will be imbalance / upset.
Any reference to the increase in secondary consumer/ producers
This causes the ecosystem to be untenable l unsustainable / humans will not be able to carry out fishing

(d)  The tuna population will decrease
As secondary and primary consumers consume plankton with toxins, the toxins will accumulate in these trophic levels with increasing concentration.
The tuna will receive toxins at a greater concentration / amount/ called bioaccumulation.

2. (a)(i) 4 levels shown and labelled clearly; width shows relative numbers at each level, narrow at bottom

(ii) pyramid of biomass would be widest at the bottom (rain tree) and narrowest at the top (tree sparrows); the single rain tree is the largest organism and thus has the most
carbon compounds. Organisms that are smaller in size have a smaller amount of carbon compounds in total. The total amount of carbon compound decreases as we move down the food chain

(iii) Sunlight is the source of energy for the food chain; green plant photosynthesizes to convert light energy to chemical energy for the living organisms / all living organisms are dependent on the this energy to survive

(iv) As energy is lost / used up at each trophic level, the last trophic level does not have enough energy to support another population of organisms

(b)(i) The unicellular green plants had the lowest level of insecticide, 0.05 ppm. The levels increase down the food chain and is the highest in the fish eating birds at 25. ppm

(ii) The insecticide is not broken down or excreted and thus
accumulates in the bodies of the organism/ At each trophic level, the organisms feed on many organisms from the previous trophic level. Thus a higher amount of insecticides
enter the bodies of these organisms

(iii) The concentration of the insecticide is the highest in the fish eating birds. The concentration of insecticides in the other organisms are lower and does not cause death.