BIOLOGY IGCSE NOTES Edexcel 2021

Maryamtyrionlay@gmail.com

Chapter 1 Life Processes

Characteristics

• Movement: an action by an organism or part of an organism causing a change of position or place.

• Respiration: the chemical reactions that break down nutrient molecules in living cells to release energy.

• Sensitivity: the ability to detect or sense changes (stimuli) in their surroundings and to make appropriate responses.

• Homeostasis: the ability to control their internal conditions.

• Growth and development: a permanent increase in size and dry mass (growth) and an increase in complexity (development).

• Reproduction: the processes that make more of the same kind of organism.

• Excretion: the removal from organisms of toxic materials and the waste products of metabolism.

• Nutrition: the taking in of materials for energy, growth and development.

Common cell structures

• Cytoplasm is found inside the cell and contains all the other cell structures

• The large nucleus is surrounded by a nuclear membrane to separate it from the cytoplasm

• The cell membrane surrounds the cell

• The cell wall is made of cellulose and surrounds the cell membrane in plant cells

• Mitochondria (singular: mitochondrion) are organelles found throughout the cytoplasm

• Chloroplasts are organelles found in the cytoplasm that are packed with the pigment chlorophyll and so are green in colour

• Ribosomes are tiny structures that can be free within the cytoplasm or attached to a system of membranes inside the cell

• Vacuoles are large vesicles that take up a large part of the interior of plant cells

Functions

Comparing animal and plant cells

Enzymes

Structure and function

Enzymes:

• Are proteins

• Are biological catalysts (biological because they are made in living cells, catalysts because they speed up the rate of chemical reactions)

• Are specific to one particular substrate as the active site of the enzyme, where the substrate attaches, is a complementary shape to the substrate – known as the lock and key hypothesis

• Are affected by changes in temperature and pH

• Are not used up in the reaction they catalyze

Effect of temperature • Enzymes are proteins and have a specific shape, held in place by bonds. This is

extremely important around the active site area as the specific shape is what ensures the substrate will fit into the active site and enable the reaction to proceed.

• Enzymes work fastest at their ‘optimum temperature’ – in the human body, the optimum temperature is 37⁰C.

• Heating to high temperatures (beyond the optimum) will break the bonds that hold the enzyme together and it will lose its shape. This is known as denaturation. Substrates cannot fit into denatured enzymes as the shape of their active site has been lost.

• Denaturation is irreversible – once enzymes are denatured they cannot regain their proper shape and activity will stop.

• Increasing the temperature from 0⁰C to the optimum increases the activity of enzymes as the more energy the molecules have the faster they move and the number of collisions with the substrate molecules increases, leading to a faster rate of reaction.

• This means that low temperatures do not denature enzymes, they just make them work more slowly.

Practical; effect of temperature • Starch solution is heated to a set temperature • Iodine is added to wells of a spotting tile • Amylase is added to the starch solution and mixed well • Every minute, droplets of solution are added to a new well of

iodine solution • This is continued until the iodine stops turning blue-black (this

means there is no more starch left in the solution as the amylase has broken it all down)

• Time taken for the reaction to be completed is recorded • Experiment is repeated at different temperatures • The quicker the reaction is completed, the faster the enzyme is

working

Effect of PH • The optimum pH for most enzymes is 7 but some that are

produced in acidic conditions, such as the stomach, have a lower optimum pH (pH 2) and some that are produced in alkaline conditions, such as the duodenum, have a higher optimum pH (pH 8 or 9)

• If the pH is too high or too low, the bonds that hold the amino acid chain together to make up the protein can be destroyed

• This will change the shape of the active site, so the substrate can no longer fit in

• The enzyme will denature and activity will stop

Practical effect of PH • Place single drops of Iodine solution in rows on the tile • Label a test tube with the pH to be tested • Use the syringe to place 2cm3 of amylase in the test tube • Add 1cm3 of buffer solution to the test tube using a syringe • Use another test tube to add 2cm3 of starch solution to the

amylase and buffer solution, start the stopwatch whilst mixing using a pipette

• After 10 seconds, use pipette to place one drop of mixture on the first drop of iodine, which should turn blue-black

• Wait another 10 seconds and place another drop of mixture on the second drop of iodine

• Repeat every 10 seconds until iodine solution remains orange-brown

• Repeat experiment at different pH values – the less time the iodine solution takes to remain orange-brown, the quicker all the starch has been digested and so the better the enzyme works at that pH

Types of respiration Aerobic and anaerobic respiration

• Aerobic respiration requires oxygen • It is the complete breakdown of glucose to release a relatively large

amount of energy for use in cell processes • It produces carbon dioxide and water as well as the useful energy

Anaerobic respiration

• Anaerobic respiration does not require oxygen

• It is the incomplete breakdown of glucose to release a relatively small amount of energy for use in cell processes

• It produces different breakdown products depending on the type of organism it is taking place in

Anaerobic Respiration in Animals

• Mainly takes place in muscle cells during vigorous exercise

• The lactic acid builds up in muscle cells and lowers the pH of the cells

• This could denature the enzymes in cells so it needs to be removed

• This happens by oxidising the lactic acid (combining it with oxygen)

• When this happens it can be broken down to carbon dioxide and water

• This is the reason we continue to breath heavily even after finishing exercise – we need to continue getting larger amounts of oxygen into the blood to combine with the lactic acid produced

• This is known as ‘repaying the oxygen debt’

Anaerobic respiration in plants and yeast cells

• We take advantage of the products of anaerobic respiration in yeast by using it in bread making (where the carbon dioxide produced helps dough to rise) and in brewing (where the ethanol produced makes beer)

ATP

• Respiration is the process that releases energy into every living cell of every organism

• The energy is essential for keeping the cell alive as it powers processes like protein synthesis, growth, repair, cell division etc

• Once the glucose molecule has been broken apart, the energy stored in the bonds is released and is then ‘locked up’ in the bonds of a molecule called adenosine triphosphate (ATP)

Functions of ATP

• ATP can release energy from its bonds very quickly which is why it is used as the energy carrying molecule in cells

• It does so by breaking the final phosphate off the molecule • The molecules can be recycled so there is always a ready supply of

energy rich ATP in the cell for all processes that need it

Investigating respiration Measuring carbon dioxide and heat Investigating the evolution (production) of carbon dioxide in respiring organisms

• A tube is connected from the air (inlet pipe) into a flask of sodium hydroxide

• This flask is connected by a tube to a flask of hydrogen carbonate (indicator A)

• Indicator A flask is connected by a tube to a flask of respiring organisms (in this diagram, worms)

• The flask of respiring organisms is connected by a tube to a flask of hydrogen carbonate (indicator B)

• Finally, this flask is connected by a tube to the air as an exit pipe

• As air enters the first flask, the carbon dioxide in the air will be absorbed by the sodium hydroxide, causing the hydrogen carbonate (indicator A flask) to turn red (low levels of carbon dioxide)

• The flask of respiring organisms will respire aerobically, producing carbon dioxide as a waste product, causing the hydrogen carbonate (indicator B flask) to turn yellow (high levels of carbon dioxide)

• This shows that living organisms produce carbon dioxide in respiration

Investigating the evolution (production) of heat in respiring seeds

• Two thermos flasks are set up, one with germinating seeds, another with boiled seeds (the control)

• Thermometers wrapped in cotton wool (prevents heat from escaping) are added

• The initial temperature is recorded

• The seeds are left for a few days

• The final temperature is recorded

•

As the germinating seeds are alive, they will be respiring aerobically, producing energy in the form of heat, causing the temperature of thermo flask to rise as a result

• As the boiled seeds have died, they will not be respiring aerobically, therefore no energy is produced, causing the temperature to remain the same

Movement across membranes Methods of transport

• All cells are surrounded by a cell membrane which is partially permeable • Molecules can move in and out of cells by diffusion or active transport • Water can move in and out of cells by osmosis

What effects rate of movement

Surface Area to Volume Ratio

• The bigger a cell or structure is, the smaller its surface area to volume ratio is, slowing down the rate at which substances can move across its surface

• Many cells which are adapted for diffusion have increased surface area in some way – eg root hair cells in plants (which absorb water and mineral ions) and cells lining the ileum in animals (which absorb the products of digestion)

Distance

• The smaller the distance molecules have to travel the faster transport will occur

• This is why blood capillaries and alveoli have walls which are only one cell thick, ensuring the rate of diffusion across them is as fast as possible

Temperature

• The higher the temperature, the faster molecules move as they have more energy

• This results in more collisions against the cell membrane and therefore a faster rate of movement across them

Concentration Gradient

• The greater the difference in concentration either side of the membrane, the faster movement across it will occur

• This is because on the side with the higher concentration, more random collisions against the membrane will occur

Diffusion and osmosis practical

Diffusion in a Non-Living System

• A crystal (Potassium permanganate) is added to water (solvent)

• The potassium permanganate particles will diffuse with solvent particles (move from areas of higher concentration to lower concentration), diffusing throughout the solvent

• The potassium permanganate particles will be less concentrated in one place in the end, so the colour will not be as deep

Diffusion in a Living System

• During inhalation, oxygen rich air enters alveoli

• There is a higher concentration of oxygen in the alveoli than in the blood running past the alveoli

• Oxygen will diffuse out of alveoli down the concentration gradient into blood

• At the same time, there is a higher concentration of carbon dioxide in the blood (from respiration) than in the alveoli

• Carbon dioxide will diffuse out of blood down the concentration gradient into the alveoli

• This is how gas exchange takes place in mammals

Osmosis in a Non-Living System

• A visking tubing bag (partially permeable) filled with sucrose solution is placed into a beaker of water

• A capillary tube (a very thin diameter tube) is placed in the bag and the level of liquid in the tube is observed

• The concentration of water particles is higher outside (in the beaker of water) than inside the visking tubing bag

• Water molecules will move through the partially permeable membrane of the visking tubing down the water concentration gradient

• Over time, the level of liquid in the capillary tube will rise as more water molecules move into the bag due to osmosis

Osmosis in a Living System

• The most common osmosis practical involves cutting cylinders of potato and placing them into distilled water and sucrose solutions of increasing concentration

• The potato cylinders are weighed before placing into the solutions

• They are left in the solutions for 20 – 30 minutes and then removed, dried to remove excess liquid and reweighed

• The potato cylinder in the distilled water will have increased its mass the most as there is a greater concentration gradient in this tube between the distilled water (high water concentration) and the potato cells (lower water concentration)

• This means more water molecules will move into the potato cells by osmosis, making them turgid and increasing the mass of the cylinder

• The potato cylinder in the strongest sucrose concentration will have decreased its mass the most as there is a greater concentration gradient in this tube between the potato cells (higher water concentration) and the sucrose solution (lower water concentration)

• This means more water molecules will move out of the potato cells by osmosis, making them flaccid and decreasing the mass of the cylinder

• If there is a potato cylinder that has not increased or decreased in mass, it means there was no overall movement of water into or out of the potato cells

• This is because the solution that cylinder was in was the same concentration as the solution found in the cytoplasm of the potato cells, so there was no concentration gradient

Cell differentiation Specialized cells

• Specialised cells are those which have developed certain characteristics in order to perform particular functions

• Cells specialise by undergoing differentiation: a process that involves the cell gaining new structures within the cell in order for it to be suited to its function

• Cells can either differentiate once early on or have the ability to differentiate their whole lives (these are called stem cells)

• In animals, most cells only differentiate once, but in plants many cells retain the ability

Stem cells

• Many tissues in the human body contain a small number of undifferentiated cells

• These are called stem cells and their function is to divide and produce new differentiated cells within the tissue for growth and repair

• Adult stem cells from bone marrow are quite useful in medicine as they can be directed to differentiate into several different cell types (unlike adult stem cells from other tissues which can only differentiate into related cell types)

• Stem cells are also found in great numbers in embryonic tissue as there is a large amount of development and differentiation going on during foetal development

• Scientists can clone these embryonic stem cells and direct them to differentiate into almost any cell type

Levels of organization

Chapter 2 Variety of living organisms

Eukaryotic organisms

• Eukaryotes are organisms that have a membrane-bound nucleus and organelles inside their cells.

• They can be unicellular (only one cell) or multicellular (more than one cell).

PLANTS • Eg: herbaceous legumes (peas), cereals (wheat) • Multicellular • Cells contain a nucleus, chloroplasts and cellulose cell walls • All feed by photosynthesis • Store carbohydrates as starch or sucrose • Contain a large, central cell vacuole

ANIMALS

• Eg: humans (mammals), butterflies (insects) • Multicellular • Cells contain a nucleus but no cell walls or chloroplasts • Feed on organic substances made by other living things • Store carbohydrates as glycogen

FUNGI • E.g. moulds, mushrooms, yeast • Most are multicellular with a mycelium of thread-like structures called hyphae which

have many nuclei but some are single-celled (eg yeast is single celled) • Cells have nuclei and cell walls made from chitin • Feed by saprotrophic (on dead or decaying material) or parasitic (on live material)

nutrition by secreting extracellular enzymes onto the food • May store carbohydrates as glycogen

PROTOCTISTS

• E.g. Amoeba (like an animal), Chlorella (like a plant) • Most are unicellular • All have a nucleus; some may have cell walls and chloroplasts, meaning some protoctists

photosynthesize like plants and some feed on organic substances made by other living things like animals

Prokaryotic organisms

• Prokaryotes do not have a membrane-bound nucleus or membrane-bound organelles inside their cells.

• E.g. bacteria (Lactobacillus d. bulgaricus (rod-shaped bacterium used in yoghurt production), Pneumococcus (spherical bacterium that causes pneumonia).

• Unicellular and microscopic. • Have a cell wall, cell membrane, cytoplasm and plasmids. • Some can carry out photosynthesis but they mainly feed off other organisms, either

dead or alive.

Pathogens

• An organism that causes disease • Fungi, protoctists, bacteria and viruses can all be pathogens

VIRUSES • E.g. Tobacco Mosaic Virus, Influenza virus • Much smaller than bacteria • They are not made from cells and are not considered living organisms as

they do not carry out all the life processes • Parasitic – reproduce inside host cells by hijacking the cell’s mechanisms to

make multiple copies and then bursts out of the cell to spread throughout the host

• Able to infect every type of living cell • The envelope is used to gain entry into host cells • The capsid is a protein coat used to protect the genetic information • The DNA or RNA contains the code for building new viruses

Chapter 3 breathing and gas exchange The cells get energy by oxidizing food such as glucose during the process for respiration.

Respiration is the oxidization reaction that releases energy from food such as glucose. Breathing is a mechanism that moves air into and out of the lungs allowing gas exchange to take place

gas exchange system is also known as respiratory system

the structure of the gas exchange system, the structure of the thorax

The lungs are enclosed in the chest or thorax by the rib cage and a muscular sheet of tissue called the diaphragm. Joining each rib to the next are two sets of muscles called intercostal muscles. The diaphragm separates the contents of the thorax from the abdomen.

the air passages of the lungs form a highly branching network. This is why it is sometimes called the bronchial tree

When we breathe in air enters our nose or mouth and passes down the windpipe or trachea. The trachea splits into two tubes called the bronchi (singular bronchus) one leading to each lung. Each bronchus divides into smaller and smaller tubes called bronchioles eventually ending at microscopic air sacs called alveoli. It is here that gas exchange with the blood takes place.

The walls of the trachea and bronchi contain rings of cartilage. These support the airways and keep them open when we breathe in.

The inside of the thorax is separated from the lungs buy 2 thin moist membranes call the pleural membranes. They make up a continuous envelope around the lungs forming and art tight seal. Between the two membranes is a space called the pleural cavity, filled with a thin layer of liquid called pleural fluid. This acts as a lubrication so that the surfacers of the lungs don't stick to the inside of the chest wall when we breathe.

Keeping the airways clean

The trachea and larger air ways are lined with a layer of cells.

Some cells in this lining secrete a sticky liquid called mucus, which traps particles of dirt or bacteria that are breathed in.

Other cells are covered with tiny hair-like structures called cilia. The cilia beat backwards and forwards, sweeping the mucus and trapped particles out towards the mouth.

In this way, dirt and bacteria are prevented from entering the lungs, where they might cause an infection.

One of the effects of smoking is that smoking destroys the cilia and stops this protection mechanism from working properly

Ventilation of the lungs

How we breathe

Ventilation means moving air in and out of the lungs. This requires a difference in air pressure- air moves from a place where the pressure is high to one where it is low. Ventilation depends on the fact that the thorax is an airtight cavity. When we breathe we change the volume of our thorax, which alters the pressure inside it. This causes air to move in or out of the lungs.

Gas exchange in the alveoli.

Effects of smoking