IGCSE Biology

MRS C GREN

• Movement - they move all or parts of themselves
• Respiration - they release energy from their food
• Sensitivity - they respond to their surroundings
• Control - they control their internal conditions
• Growth - they grow and develop
• Reproduction - they produce offspring
• Excretion - they remove waste products
• Nutrition - they need nutrients

Plants:

• These are multicellular organisms
• They contain chloroplasts (and chlorophyll) and are able to carry out photosynthesis
• They have cellulose cell walls
• They have permanent vacuoles
• They store carbohydrates as starch or sucrose
  • Examples include: maize, peas, beans

Animals:

• These are multicellular organisms
• They do not contain chloroplasts and are not able to carry out photosynthesis
• They have no cell walls or permanent vacuoles
• They usually have nervous system and are able to move from one place to another
• They often store carbohydrates as glycogen
  • Examples include: humans, houseflies, mosquitoes

Fungi:

• These are multicelluar and unicellular organisms that are not able to carry out photosynthesis
• Their body is organised into a mycelium made from thread-like structures called hyphae, which contain many nuclei
• They have chitin cell walls
• They feed by saprotrophic nutrition - extracellular secretion of digestive enzymes onto food material and absorption of the organic products
• They may store carbohydrate as glycogen
  • Examples include: multicellular - Mucor; unicellular - yeast

Bacteria:

• These are microscopic unicellular organisms
• They have a cell wall, cell membrane, cytoplasm and plasmids
• They lack a nucleus, but contain a circular chromosome of DNA
• Some bacteria can carry out photosynthesis, but most are parasitic and feed off others
  • Examples include: useful - Lactobacillus; pathogenic - Pneumococcus

Protoctists:

• These are microscopic unicellular organisms
• Some, like Amoeba, that live in pond water, have features like an animal cell
• While others, like Chlorella, have chloroplasts and are more like plants
• A pathogenic example is Plasmodium, responsible for causing malaria

Viruses:

• These are small particles, smaller than bacteria
• They are parasitic and can reproduce only inside living cells
• They infect every type of living organism
• They have a wide variety of shapes and sizes
• They have no cellular structure, but have a protein coat and contain one type of nucleic acid, either DNA or RNA
  • Examples include: tobacco mosaic virus - causes discolouring of the leaves of tobacco plants by preventing the formation of chloroplasts; influenza virus - causes ‘flu’; HIV - causes AIDS

Pathogens:

Pathogens are organisms that cause disease. They include some fungi, protoctists, bacteria and viruses.

PROTOCTIST: Plasmodium - causes malaria
BACTERIUM: Pneumococcus - causes pneumonia
VIRUSES: Influenza virus - causes 'flu'; HIV - causes AIDS

Organisms are made from organisations of smaller structures:

• Organelles - tiny structures that carry out different functions within cells
• Cells - specialised basic units from which all organisms are made
• Tissues - a group of similar cells that work together to carry out a particular function 
• Organs - a group of different tissues that work together to perform a function
• Organ Systems - organs work together to form organ systems and each system does a different job

e.g. cytoplasm --> cardiac muscle cell --> cardiac muscle --> heart --> circulatory system

Functions of the organelles:

• Nucleus - contains the genetic material that controls the cell's activities
• Cytoplasm - responsible for the cell's chemical reactions
• Cell Membrane - controls the substances that enter and exit the cell
• Mitochondrion - responsible for respiration
• Cell Wall - made of cellulose; supports the cell and strengthens it
• Chloroplast - responsible for photosynthesis; contains chlorophyll, the green pigment used in photosynthesis
• Vacuole - contains cell sap, a weak solution of sugars and salts; supports the cell

Carbohydrates, proteins and lipids (fats and oils) are large molecules made up from smaller basic units:

Carbohydrates:

Elements present - C, H, O
Carbohydrates are made up of simple sugars - glucose/maltose --> starch/glycogen

Proteins:

Elements present - C, H, O, N
Proteins are made up of amino acids - amino acids --> proteins

Lipids:

Elements present - C, H, O
Lipids are made up of fatty acids and glycerol - 3 fatty acids+glycerol --> lipid

Test for glucose:

Add Benedict's reagent in excess to the sample and heat it
If glucose is present, the sample turns from blue to form a coloured precipitate - green (low conc.) ~ brick red (high conc.)

Test for iodine:

Add iodine solution to the sample
If starch is present, the sample turns from brown to blue-black

Enzymes:

• Are proteins
• Are biological catalysts in metabolic reactions (speed up chemical reactions)
• Are thus chemically unchanged nor used up in the reaction
• Are specific to one particular substrate
• Are affected by temperature and pH

'Lock and key model':

The functioning/activity of enzymes are affected by changes in temperature:

1. Initially, increasing the temperature increases the rate of reaction
2. However, after the optimum temperature is reached, the enzyme's active site begins to change shape and it is no longer able to bind to its substrate
3. Therefore the enzyme becomes permanently denatured and stop working (the rate of reaction is zero at this point)

The functioning/activity of enzymes are affected by changes in pH: 

1. Initially, increasing the pH increases the rate of reaction
2. However, after the optimum pH is reached, the enzyme's active site begins to change shape and is no longer able to bind to its substrate
3. Therefore the enzyme becomes permanently denatured and stops working (the rate of reaction is zero at this point)
N.B. Different enzymes work best at different pH values e.g. intestinal enzymes ~ pH 7.5; gastric enzymes ~ pH 2

Experiments to investigate how enzyme activity can be affected by changes in temperature:

You can measure how fast a product appears...

• The enzyme catalase catalyses the breakdown of hydrogen peroxide --> water + oxygen
• The product oxygen can be collected by downward displacement of water
• Measure how much oxygen is given off in a minute
• Repeat the experiment, each with the water bath at different temperatures e.g. 10°C, 20°C, 40°C, etc. to see how temperature affects the activity of catalase
• Control all other variables e.g. enzyme concentration, pH, volume of solution, etc.

Experiments to investigate how enzyme activity can be affected by changes in temperature:

...Or how fast a substrate disappears

• The enzyme amylase catalyses the breakdown of starch --> maltose
• The substrate starch is detected using iodine solution
• Time how long it takes for the starch to disappear by sampling the starch and amylase mixture every minute, and use the times to compare rates between different tests
• Adjust the water bath temperature to see how temperature affects the activity of amylase
• Control all other variables e.g. enzyme concentration, pH volume of solution, etc.

Diffusion:
The net movement of particles from an area of higher concentration to an area of lower concentration

Osmosis:
The net movement of water molecules across a partially permeable membrane from a region of higher water potential to a region of lower water potential

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

Turgid cells are very important in plants as a means of support:

• When a plant is watered, all its cells draw water in by osmosis and become turgid
• The contents of the cell push against the cell wall - turgor pressure supports the plant tissues
• Cells are said to be flaccid when they lose their water and turgor pressure, as a result of insufficient moisture in the soil

Factors that affect the rate of movement of substances:

1) Surface area to volume ratio:

• The larger the SA:Vol ratio, the faster the rate of movement of substances
• The smallest cube has the largest SA:Vol ratio, which means substances would move into and out of this cube the fastest

Factors that affect the rate of movement of substances:

2) Temperature:

• When a substance is heated, the particles gain kinetic energy - so they move faster and collide with the cell membrane more frequently
• This means as temperature increases, the rate of movement of substances increases (until the temperature reaches too high, and the rate is zero as the enzymes have denatured)

3) Concentration gradient:

• If there are many more particles on one side than the other, there are more particles which move across more quickly in order to reach equilibrium
• This means that the greater the difference in concentration between the inside and outside of the cell, the faster the rate of movement of substances
  N.B. This factor only increases the rate of diffusion and osmosis - the rate of active transport is not afftected by concentration gradients

Experiment to investigate diffusion in a non-living system:
1) Place agar jelly in a beaker containing phenolphthalein and dilute NaOH (aq) - the jelly turns from colourless to pink
2) Fill another beaker will dilute HCl (aq)
3) Use a scalpel to cut out cubes from the jelly and put them in the beaker of acid

The pink cubes will gradually turn colourless, as HCl diffuses into the agar jelly and neutralises the NaOH

N.B. The rate of diffusion can be investigated by using different sized cubes of jelly and timing how long it takes for each cube to go colourless - the smallest cube with the largest SA:Vol ratio will lose its colour the quickest
IV=size of cubes , DV=time taken to lose colour

Experiment to investigate osmosis in a living system - potato cylinders:
IV=concentration of sugar solution , DV=length of cylinders
1) Cut up a potato into identical cylinders
2) Pour a sugar solution of different concentrations into each beaker (one must be pure water and another a very concentrated sugar solution)
3) Measure the initial length of the cylinders
4) Leave the cylinders in each beaker for half an hour, then measure their lengths again

In pure water, the cylinder will have drawn water in by osmosis, therefore being longer 
In concentrated sugar solution, water will have been drawn out by osmosis, therefore being shorter

Experiment to investigate osmosis in a non-living system - Visking tubing:

1) Tie a piece of wire around one end of the Visking tubing
2) Put a glass tube in the other end and fix the tubing around it with wire
3) Pour sugar solution down the glass tube into the Visking tubing
4) Place the Visking tubing in a beaker of pure water
5) Measure where the sugar solution comes up to in the glass tube
6) Leave the tubing overnight, then measure the liquid level in the glass tube again

Water will have been drawn into the Visking tubing by osmosis, and this forces liquid up the glass tube

Photosynthesis:

• The process that produces glucose using sunlight in plants
• Important because it converts light energy --> chemical energy (stored in glucose - this chemical energy is released when glucose is broken down during respiration)
• Takes place inside chloroplasts, which are found in leaf cells
• Chloroplasts contain the green pigment chlorophyll, which absorbs sunlight and uses its energy to convert carbon dioxide + water --> glucose (+ oxygen)

Factors that affect the rate of photosynthesis:

1) Carbon dioxide concentration:

• Increasing the concentration of CO₂ increases the rate of photosynthesis steadily, but only up to a certain point - this is called its maximum turning rate
• The graph then platteaus, showing that the rate cannot increase further as temperature or light intensity are now the limiting factor

2) Light intensity:

• Increasing the light intensity also increases the rate of photosynthesis steadily, but only up to a certain point
• The graph then platteaus, showing that the rate cannot increase further as temperature or CO₂ level are now the limiting factor

3) Temperature:

• Increasing the temperature increases the rate of photosynthesis up to the plant's optimal temperature
• If the temperature is too high (over 45°C), the plant's enzymes will be permanently denatured, so the rate rapidly decreases to zero
  N.B. Usually if temperature is the limiting factor, it is because it is too low

Leaves are adapted for efficient photosynthesis:

• Broad leaves - large surface area exposed to sunlight
• Chloroplasts in palisade layer - near the top of the leaf where light is most accessible
• Transparent upper epidermis - light passes through to the palisade layer
• Vascular bundles - carry water and nutrients to other parts of the leaf, take away the product glucose
• Waxy cuticle - reduces transpiration
• Stomata - pores on the lower surface which allows direct diffusion of CO₂ into and O₂ out of the leaf

In addition to CO₂ and water, plants also require mineral ions for growth:

• Magnesium ions are needed for making chlorophyll
• Nitrate ions are needed for making amino acids
• Phosphate ions are needed for making DNA and cell membranes
• Potassium ions are needed for photosynthesis and respiration
  

Experiment to investigate photosynthesis showing the evolution of oxygen from a water plant:
IV=distance from pondweed , DV=length of bubble

1) Place a source of white light 10 cm from the pondweed
2) Leave the pondweed to photosynthesise for 5 minutes
3) Then use the syringe to draw up the gas bubble and measure the length of the bubble -          the length is proportional to the volume of O₂ produced
4) Control all other variables e.g. temperature, time of photosynthesis
5) Repeat the experiment with the light source place at different distances from the pondweed
N.B. Rate of oxygen production = rate of photosynthesis

Experiment to investigate photosynthesis showing the production of starch:

1) Kill the leaf (held with forceps) by placing it in boiling water - this stops any chemical reactions happening inside the leaf
2) Put the leaf in a boiling tube containg ethanol, and heat the tube using a water bath (because ethanol is highly flammable) - this removes any chlorophyll inside the leaf
3) Rinse the pale leaf in cold water, and add a few drops of iodine solution

If starch is present, the leaf will turn blue-black; thus showing that photosynthesis has taken place.

Experiment to investigate photosynthesis showing the requirement of light:

To show that light is required for photosynthesis,
1) Grow a plant in a cupboard in the absence of light (destarching)
2) Cut a leaf from the plant, and test for starch using the experiment outlined above

The leaf will not turn blue-black as no starch has been made, thus showing that light is needed for photosynthesis.

Experiment to investigate photosynthesis showing the requirement of carbon dioxide:

To show that CO₂ is required for photosynthesis,
1) Set up apparatus as shown below

2) The soda lime absorbs CO₂ out of the air in the jar
3) Leave the plant in the jar overnight, and test for starch using the experiment outlined previously

The leaf will not turn blue-black as no starch has been made, thus showing that CO₂ is needed for photosynthesis.

Experiment to investigate photosynthesis showing the requirement of chlorophyll:

To show that chlorophyll is required for photosynthesis,
1) Take a variegated leaf from a plant which has been exposed to light for 20 minutes
2) Test for starch using the experiment outlined previously

Only the parts of the leaf that were green turned blue-black, thus showing that only the parts of the leaf which contained chlorophyll are able to photosynthesise.

A balanced diet includes appropriate proportions of all the essential nutrients:

• Carbohydrates provide energy
  • pasta, rice, sugar
• Proteins are needed for growth and repair of tissue
  • meat, fish
• Lipids provide energy, act as energy store and provide insulation
  • butter, oily fish
• Vitamins A improves vision and keeps skin and hair healthy; C prevents scurvy; D is needed for calcium absorption
  • A=liver, carrots; C=oranges, D=eggs, skin
• Minerals calcium ions make bones and teeth; iron ions make haemoglobin
  • Ca=milk, cheese; Fe=red meat
• Water supply is needed to replace water lost through urinating, breathing and sweating
  • watermelon, drinks
• Dietary fibre aids the movement of food through the gut
  • wholemeal bread

Energy requirements vary in different people with activity levels, age and pregnancy

Food is moved through the gut by peristalsis:
1) There is muscular tissue all the way down the alimentary canal
2) Peristalsis is carried out by waves of circular muscle contractions and squeezes boluses through the gut

Digestive enzymes:

Digestive enzymes break down big, insoluble molecules into smaller, soluble molecules to pass through the walls of the digestive system

• Amylase converts Starch into Maltose
  • Maltase converts Maltose into Glucose
• Proteases convert Proteins into Amino Acids
• Lipases convert Lipids into Glycerol + Fatty Acids

Bile:

Produced by the liver --> Stored in the gall bladder --> Released into the small intestine

• Alkaline bile neutralises the stomach acid for enzymes in the small intestine to function in optimal conditions (alkaline pH of 7.5)
• Bile also emulsifies fats - breaks down fat into tiny droplets, giving a bigger surface area of fat for lipase to work, thus making digestion faster

Villus:

• Thin wall - one-cell thick epithelium increases diffusion rate of molecules into blood
• Rich blood supply - efficiently carries absorbed molecules away from intestine, meaning there is always low conc. of product molecules in blood which maintains high conc. gradient
• Intestine length - roughly 7m long, provides plenty of time to complete digestion
• Large surface area - villi and microvilli increase SA

Experiment to investigate the energy content in a food sample - calorimetry:

1) Burn a dry sample of food e.g. pasta, biscuit (skewered on a mounted needle)
2) Fill a boiling tube with 25cm³ of water (held with a clamp), and measure the initial temperature
3) Hold the burning food under the boiling tube, until the food will not catch fire again
4) Measure the final temperature of the water

Calculate the amount of energy in Joules:
Energy in food (J) = mass of water (g) x temp. change of water (°C) x 4.2 (J/g•°С)

Calculate the amount of energy in Joules per Gram:
Energy per Gram of food (J/g) = Energy in food (J) ÷ Mass of food (g)
(This calculation ensures that the energy values of different foods can be compared fairly)

• The energy value calculated from the experiment is likely to be much lower than the indicated value on the packet due to significant energy lost to the surroundings in the experiment
• To improve the experiment's accuracy, insulate the boiling tube with foil in order to minimise heat loss and retain more energy in the water

Respiration:

• The process of releasing energy from glucose

1) AEROBIC RESPIRATION requires oxygen
 Glucose + Oxygen --> Carbon Dioxide + Water (+ Energy)
C₆H₁₂O₆ +    6O₂    -->         6CO₂         + 6H₂O (+ Energy)

• Releases much more energy than anaerobic resp.
• Occurs in the mitochondria

2) ANAEROBIC RESPIRATION takes place in the absence of oxygen
In animals: Glucose --> Lactic Acid (+ Energy)
In plants:    Glucose --> Ethanol + Carbon Dioxide (+ Energy)

• Releases much less energy than aerobic resp.
• Occurs in the cytoplasm

Experiment to investigate the evolution of carbon dioxide from respiration - "train" experiment:

• Hydrogen-carbonate indicator is orange with atmospheric CO₂ level, yellow with increased CO₂ levels and red/ purple with decreased CO₂ levels
  
• Atmospheric air first passes through NaOH (aq) to absorb all CO₂ present in it - this enables us to conclude any change in CO₂ levels as a direct product of the organisms' respiration
• The respiring organisms (maggots) can also be constituted with respiring seeds (beans)
• The control should be experimented using glass beads / dead seeds

Experiment to investigate the evolution of heat from respiration - vacuum flask:

1) Place each set of seeds in a vacuum flask with some air to allow aerobic respiration
2) Set up apparatus as shown above
3) Record the temperature of each flask daily for a week

• The boiled seeds flask acts as the control and will show no change in temp.
• The germinating seeds flask will show an increase in temp. as it is well-insulated to obtain any heat produced from respiration