iGCSE Biology

1 - STRUCTURES AND FUNCTIONS IN LIVING ORGANISMS 

Characteristics of Living Organisms - 

1. Movement - eg: towards food, away from predators
2. Reproduction - for species to survive
3. Sensitivity - reacting to changes in their surroundings
4. Nutrition - energy, growth and repair, eg: proteins, fats, carbohydrates, vitamins, minerals
5. Excretion - waste products removed, eg: carbon dioxide, urine
6. Respiration - releases energy from food
7. Growth - developing

Cells - 

Animal cells - 
- Nucleus - contains genetic material and chromosomes
                 control's cells activities
                 surrounded by its own membrane
- Cell membrane - outer surface of the cell
                           controls substances that enter/exit
- Cytoplasm - gel-like substance
                     where most of the chemical reactions happen
                     enzymes to control reactions
- Mitochondria - carry out some of the respiration reactions
                        most energy from respiration is released here

Plant cells - 
- Nucleus - contains genetic material and chromosomes
                 control's cells activities
                 surrounded by its own membrane
- Cell membrane - outer surface of the cell
                           controls substances that enter/exit
- Cytoplasm - gel-like substance
                     where most of the chemical reactions happen
                     enzymes to control reactions
- Mitochondria - carry out some of the respiration reactions
                        most energy from respiration is released here
- Chloroplasts - where photosynthesis takes place
                       contain chlorophyll
- Cell wall - rigid structure made of cellulose
                 surrounds cell membrane
                 supports and strengthens cell
                 freely permeable
- Vacuole - contains cell sap (weak solution of sugars and salts)
                 supports cell
                 permanent

Specialised cells - specialised to carry out particular functions, eg: red blood cells (carrying oxygen), white blood cells (defending against disease) 


Levels of organisation - 

1. Similar cells -> tissues 
2. Tissues -> organs 
3. Organs -> organ systems 

1. Similar cells are organised into tissues 

• Tissue - group of similar cells that work together to carry out a particular function 
• eg: plants - xylem tissue (transporting water and mineral salts) and phloem tissue (transporting sucrose and amino acids) 
• Can contain more than one cell type 

2. Tissues are organised into organs 

• Organ - group of different tissues that work together to perform a function 
• eg: lungs in mammals, leaves (both made up of different tissue types) 

3. Organs make up organ systems 

• Each system does different job 
• eg: digestive system - stomach, intestines, pancreas, liver 

The Kingdoms - 


Plants - 

• Multicellular 
• Have chloroplasts - they photosynthesise 
• Cells have cell walls (made of cellulose) 
• Store carbohydrates as sucrose or starch 
• eg: cereals (maize etc.), herbaceous legumes (peas, beans etc.)

Animals - 

• Multicellular 
• No chloroplasts 
• No cell walls 
• Most have a form of nervous coordination (respond quickly to changes in environment) 
• Usually can move around 
• Often store carbohydrates as glycogen 
• eg: mammals (humans etc.), insects (houseflies, mosquitos etc.)

Fungi - 

• Some are single-celled, other have body called a mycelium (made up of hyphae - thread-like structures, contain lots of nuclei) 
• Can't photosynthesise 
• Cells have cell walls (made of chitin) 
• Most feed by saprotrophic nutrition (secrete extracellular enzymes - dissolve the food - absorb nutrients) 
• Store carbohydrates as glycogen 
• eg: yeast (single-celled fungus), nucor (multicellular, mycelium and hyphae) 

Protoctists - 

• Single-celled 
• Microscopic 
• Some have chloroplasts (
• Some are similar to plant cells, others more like animal cells 
• eg: chlorella (plant cell-like), amoeba (animal cell-like, live in pond water) 

Bacteria (prokaryotes) - 

• Single-celled 
• Microscopic 
• No nucleus 
• Have a circular chromosome of DNA 
• Some photosynthesise
• Most feed off other organisms 
• eg: lactobacillus bulgaricus (used to make milk go sour to make yoghurt, rod shaped), pneumococcus (spherical shape) 

Viruses - 

• Particles, not cells 
• Smaller than bacteria
• Only reproduce inside living cells (parasitic) 
• Infect all types of living organisms 
• Different shapes and sizes 
• No cellular structure - protein coat around some genetic material (DNA or RNA) 
• eg: influenza virus, tobacco mosaic virus (makes the leaves of tobacco plants stop producing chloroplasts - discoloured), HIV 

Pathogens - cause disease 
eg:

• Protoctist - Plasmodium, causes malaria 
• Bacterium - Pneumococcus, cause pneumonia 
• Viruses - Influenze virus, causes flu 
• Viruses - HIV, causes AIDS

Enzymes 

• Catalysts produced by living things - biological catalysts
• Catalyst - a substance which increases the speed of a reaction without being changed or used up in the process 
• Allows individual reactions to be sped up (without raising temperature and speeding up unwanted reactions too/damaging cells) - reduce the need for high body temperatures
• Only have enzymes to speed up useful chemical reactions (metabolic reactions) 
• All proteins (chains of amino acids) 

Specificity -

• Substrate - molecule changed in a reaction 
• An enzyme molecule has an active site (where a substrate then joins on) 
• Suited to one particular reaction - needs correct substrate to fit into active site 
• Lock and key model: 

Temperature -

• Changing temperature changes rate of an enzyme-catalysed reaction 
• Higher temperature increases reaction rate at first (more heat - particles have more energy, higher collision rate) 
• Lower temperatures slow reaction down - lower collision rate 
• Denaturing - If the enzyme gets too hot, some of the bonds break and the shape of the active site changes (substrate doesn't fit anymore). Reaction eventually stops once all enzymes are denatured. It's irreversible. 
• Optimum temperature - reaction is at its fastest just before it gets too hot and denatured. (eg: most important enzymes in humans have an optimum temperature of 37 degrees) 

Effect of temperature on enzyme activity -

How fast a product appears:
-Catalase catalyses the breakdown of hydrogen peroxide into water and oxygen

• Collect oxygen given off - how much in a set time? 
• Use a water bath - how does temperature affect amount of product produced/activity level of catalase? 
• Control any variables (eg: enzyme concentration, pH, volume of solution etc.) 

How fast a substrate disappears:
-Amylase catalyses breakdown of starch to maltose 

• Use iodine to test for starch (iodine solution turns from brown/orange to blue/black) 
• Time how long it takes for starch to disappear (sample starch solution regularly) - compare 
• Use water bath - how does temperature affect activity of amylase? 
• Control all variables

pH - 

• Affects enzymes 
• Too high/low - interferes with bonds in enzyme, can cause it to become denatured 
• Optimum pH - (generally pH7), pH at which the rate of reaction is the fastest 
• eg: pepsin (breaks down proteins in stomach), optimum pH = pH 2 

 

Diffusion

• The gradual movement of particles from a high concentration to a lower concentration 
• Happens in liquids and gases - particles free to move about 
• eg: perfume in air

Cell membranes -

• Hold cell together and control substances entering/exiting 
• Substances move in and out of cells by osmosis, diffusion and active transport 
• Only small molecules can dissolve through (eg: glucose, amino acids, water, oxygen) 
• Large molecules (eg: starch, proteins etc.) can't diffuse through cell membranes  

Diffusion experiment (non-living system) -
- Phenolphthalein - pH indicator, pink in alkaline, colourless in acidic solution

1. Make agar jelly with phenolphthalein and dilute sodium hydroxide (jelly will be pink) 
2. Fill beaker with dilute hydrochloric acid 
3. Use a scalpel to cut out cubes of jelly - put them in the beaker (use different size cubes and a timer to investigate surface area and diffusion)
4. leave cubes - turn colourless (acid diffuses into agar jelly, neutralises sodium hydroxide)

Osmosis 

• The net movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration
• Partially permeable membrane - small holes in it (only small molecules - eg: water can get through), eg: cell membrane 
• Water molecules pass both ways during osmosis (move randomly) 
• Steady net flow of water into region with fewer water molecules 
• Other solution gets more dilute 

Osmosis in cells -

• Water moves in and out of cells by osmosis 
• Tissue fluid (water, with oxygen, glucose etc. dissolved in it) surrounds cells in body. Squeezed out of blood capillaries to supply cells 
• Tissue fluid has different concentration to fluid inside cell - water will move into/out of cell by osmosis (to/from tissue fluid) 

Turgid cells - 

• Turgid - when the cells in a plant are all hydrated (plump and swollen) 
• Hydrated plant - all cells draw in water by osmosis = plump and swollen (turgid)  
• Contents of cell push against cell wall - turgor pressure - helps support plant tissues 
• Dehydrated plant - cells lose water, lose turgor pressure - cells become flaccid (plant wilts) 
• Plant doesn't completely lose shape - inelastic cell well keeps it supported

Osmosis experiments -

1. Living system - potato cylinders

• Cut potato into equal cylinders, measure and record lengths
• Put cylinders in beakers with different sugar solutions (eg: pure water, different levels of concentrated sugar solution, very concentrated sugar solution) 
• Leave in beaker for at least half an hour 
• Take out cylinders - measure and record 
• Drawn in water (osmosis) = will be longer 
• Water drawn out = slightly smaller 

2. Non-living system - visking tubing

• Tie wire around 1 end of visking tubing, put glass tube in other end, tie visking tubing around it 
• Pour sugar solution down glass tube into visking tubing 
• Measure where sugar solution comes up to in glass tube 
• Put in beaker of pure water 
• Leave overnight 
• Measure liquid in glass tube again 
• Water drawn in by osmosis - level in glass tube higher 

Active transport 

• The movement of particles against the concentration gradient (from an area of lower concentration to an area of higher concentration) using energy released during respiration

eg:

1. Higher concentration of nutrients in blood, lower concentration in gut 
2. Active transport - against diffusion/osmosis - against concentration gradient
3. Allows nutrients to be taken into blood 
4. Needs energy from respiration 

Things that affect movement of substances -

1. Surface to area volume ratio -(larger surface area to smaller volume ratio = substances move in/out faster)
2. Temperature -(warmer = more energy, move faster substances move in/out faster)
3. Concentration gradient -(big difference in concentration = substance move faster) - doesn't affect active transport

2 - HUMAN NUTRITION 

Biological molecules 

Carbohydrates - 

• Made up of simple sugars 
• Contain carbon, hydrogen and oxygen 
• Starch and glycogen are large, complex carbohydrates - made up of many smaller units (eg: glucose or maltose molecules) joined in a long chain 

Proteins - 

• Made up of long chains of amino acids 
• Contain carbon, nitrogen, hydrogen and oxygen 

Lipids - 

• Fats and oils 
• Made up of fatty acids and glycerol 
• Contain carbon, hydrogen and oxygen 

Testing for glucose (Benedict's reagent) - 

• Add Benedict's reagent (blue) to a sample (an excess) 
• Heat - do not boil 
• Positive = coloured precipitate (blue-green-yellow-orange-brick red) 
• The higher the concentration, the further the colour change goes - can compare 

Testing for starch (iodine test) - 

• Add iodine solution (iodine dissolved in potassium iodide solution - brown/orange) 
• Present = blue/black 
• No starch = stays brown/orange 

Balanced diet

• Gives all essential nutrients in right proportions 
• Carbohydrates, proteins, lipids, vitamins, minerals, water 
• Fibre

Carbohydrates - 

• Pasta, rice, sugar 
• Provide energy 

Lipids (fats and oils) - 

• Butter, oily fish 
• Provide energy 
• Act as energy store 
• Provide insulation 

Proteins - 

• Meat, fish 
• Growth and repair of tissue 
• Provides energy in emergencies 

Vitamins -

1 - Vitamin A 

• Liver 
• Improves vision 
• Keeps skin and hair healthy 

2 - Vitamin C  

• Oranges 
• Prevents scurvy 

3 - Vitamin D 

• Eggs 
• Calcium absorption 

Mineral ions - 

1 - Calcium 

• Milk, cheese 
• Makes bones and teeth 

2 - Iron 

• Red meat 
• Makes haemoglobin 
• Healthy blood 

Water - 

• Food, drink 
• Most bodily functions 
• Homeostasis 
• Replaces water lost through urinating, breathing and sweating etc 

Dietary fibre - 

• Wholemeal bread 
• Movement of food through gut 

Energy requirements - 

• Activity level - active people need more energy 
• Age - children/teenagers need more energy than older people - growth, generally more active 
• Pregnancy - pregnant women need more energy - provide energy baby needs for development 

Digestive enzymes 

• Break down big molecules into smaller ones 
• Starch, proteins and fats are too big to pass through walls of digestive system and insoluble 
• Sugars, amino acids, glycerol and fatty acids are smaller, soluble and can pass through walls of digestive system 

Amylase - starch to maltose 

Maltase - maltose to glucose 

Protease(s) - proteins to amino acids 

Lipase(s) - lipids to glycerol and fatty acids 

Bile - 

• Produced in liver, stored in gall bladder, released into small intestine 
• Neutralises stomach acid 
• Emulsifies fats 
• HCl in stomach makes pH too acidic for enzymes in small intestine - bile neutralises/makes conditions alkaline 
• Emulsifies fats - breaks fats into tiny droplets (bigger surface area for lipase to break down - speeds up digestion) 

Alimentary canal 

Mouth - 

- Salivary glands in the mouth produce amylase in saliva 

- Teeth break down food mechanically 

Oesophagus - 

- Muscular tube 

- Connects mouth and stomach 

Liver - 

- Produces bile 

Gall bladder - 

- Stores bile 

Stomach - 

- Pummels food with muscular walls 

- Produces pepsin (protease enzyme) 

- Produces HCl - kills bacteria, right pH for proteas enzyme (pH 2) 

Pancreas - 

- Produces protease, amylase, lipase 

- Releases enzymes into small intestine 

Small intestine -

- Produces protease, amylase, lipase 

- Nutrients absorbed into body 

- Contains villi 

Large intestine - 

- Excess water absorbed by blood 

Peristalsis - 

• Muscular tissue all the way down alimentary canal 
• Squeeze boluses through gut 
• Squeezing action - waves of circular muscle contractions 

Digestive process - 

1. Ingestion 

• Putting food in mouth 

2. Digestion 

• Break-down of large, insoluble molecules to small, soluble molecules 
• Mechanical - teeth and stomach muscles 
• Chemical - enzymes and bile 

3. Absorption 

• Process of moving molecules through the walls of the intestines into the blood 
• Digested food molecules absorbed in small intestine 
• Water mainly absorbed in large intestine 

4. Assimilation 

• Digested molecules have been absorbed, moved into body cells - become part of cells (assimilation) 
• eg: amino acids used by cells to make cellular proteins 

5. Egestion 

• Undigested materials form faeces 
• Egested 

Villi (small intestines) - 

• Small intestine - adapted for absorption of food 
• Very long - time to break down and absorb all food 
• Large surface area for absorption - covered in millions of villi 
• Each cell on surface of villi has microvilli - increase surface area further 
• Villi - single permeable layer of surface cells, very good blood supple for quick absorption 

3 - PLANT NUTRITION AND TRANSPORT

Photosynthesis 

• Produces food for plant (glucose) 
• Happens in the leaves of all green plants 
• Happens inside chloroplasts - in leaf cells and other green parts of plant, contain green pigment (chlorophyll) 
• Chlorophyll - green pigment, absorb sunlight, uses its energy to convert carbon dioxide and water into glucose (oxygen also produced - waste product) 
• Converts light energy to chemical energy (stored in the glucose) 
• Chemical energy released when glucose is broken down during respiration 

Leaves - 

•  Broad - large surface area exposed to light 
• Most chloroplasts found in palisade layer - near top of leaf, more exposure to light 
• Upper epidermis - transparent, light can pass through it to palisade layer 
• Network of vascular bundles - xylem and phloem (transport vessels), deliver water and nutrients, take away glucose produced, support leaf structure 

• Waxy cuticle - reduces water loss by evaporation 
• Adaptions of leaves for efficient gas exchange also make photosynthesis more efficient (eg: lower surface full of small holes - stomata, let carbon dioxide diffuse directly into leaf) 

Rate of photosynthesis  

Limiting factor - 

• Depends on environmental conditions (eg: winter - low temp., night - light etc.) 
• Stop photosynthesis from happening any faster 
• Light intensity, CO2, concentration, temperature 

Limiting factor: light 

• Chlorophyll uses light energy to perform photosynthesis 
• Light intensity increased - rate of photosynthesis increases steadily (only up to a certain point - only limiting factors are then either CO2 levels or temp) 
• Not enough light = slows down rate 

Limiting factor: carbon dioxide 

• CO2 - raw material needed for photosynthesis (only about 0.04% of air, fairly scarce for plants) 
• Increases rate of photosynthesis up to a point 
• Graph flattens out - no longer the limiting facto 

Limiting factor: temperature 

• Affects enzymes involved 
• Temp increases - rate of photosynthesis increases up to a point 
• Too high - denatures enzymes 

Experiments 

Testing leaf for starch - 

• Put in boiling water with tweezers/forceps - stops any chemical reactions happening inside leaf 
• Put in boiling tube with ethanol 
• Heat in water bath - gets rid of any chlorophyll, leaf end up pale/white 
• Rinse leaf in cold water 
• Add a few drops of iodine solution
• Present = blue/black 

Showing whether photosynthesis is taking place (starch tests) - 

1 - Chlorophyll 

• Use variegated leaves - only green parts contain chlorophyll (eg: ivy leaves) 
• Take variegated leaf from plant that has been exposed to light 
• Record which bits are green etc. 
• Test leaf for starch (previous experiment) 
• Present = blue/black (will only occur in green parts of leaf) 
• Only parts of the leaf that contained chlorophyll are able to photosynthesise and produce starch 

2 - CO2 

• Set up plant and dish of soda lime in bell jar (sealed) 
• Light shining through bell jar 
• Soda lime will absorb CO2 out of the air in the jar 
• Test leaf for starch (with iodine solution) after set period of time = none present 
• No starch been made in the leaf, CO2 needed for photosynthesis 

3 - Light 

• Plant that has grown without any light 
• Cut leaf from plant - test for starch using iodine solution 
• Light needed for photosynthesis 

Showing rate of photosynthesis (oxygen production) - 

1 - Light 

• Canadian pondweed used to measure the effect of light intensity on the rate of photosynthesis - rate at which the pondweed produces oxygen corresponds to the rate at which photosynthesis is taking place 
• The faster the rate of oxygen production, the faster the rate of photosynthesis 

1. Source of white light placed at a specific distance from pondweed 

2. Leave to photosynthesise for set time 

3. Syringe draws up gas produced 

4. Record results 

5. Repeat with light at different distances 

• Can be altered to measure effect of temperature or CO2 on photosynthesis 
• eg: put in beaker of water, different concentrations of CO2 bubbled through 
• eg: changing temp of water 

Minerals for healthy growth 

Deficiencies - 

1 - Nitrates 

• Contain nitrogen for making amino acids and proteins 
• Needed for cell growth 
• Stunted 
• Yellow older leaves 

2 - Phosphates 

• Contain phosphorus for making DNA and cell membranes 
• Needed for respiration and growth 
• Poor root growth 
• Purple older leaves 

3 - Potassium 

• Helps enzymes needed for photosynthesis and respiration 
• Poor flower and fruit growth 
• Discoloured leaves 

4 - Magnesium (small amounts) 

• Needed for making chlorophyll 
• Yellow leaves