IGCSE Biology Section 1/A

These are notes for the first two chapters of the edexcel textbook.


The Eight Processes of Life.

Move- Living organisms can move with the use of muscles or can move because of slow growth

Respire- They get or make energy from their food, and can release it.

Sensitive- Living things respond to stimuli or changes in their surroundings.

Nutrition-  They need nutrition in order to provide them with energy and minerals.

Excrete- The removal of waste products such as carbon dioxide or urine. 

Reproduce- Organisms produce offspring, so that their species thrives.

Grow- They grow and develop, mostly increasing in size.

+ Living organisms can control their internal conditions (homeostasis) 

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The Levels of Organisation

Organelles < Cells < Tissues < Organs < Organ Systems < Organism

  • Organelles are tiny structures within a cell, and can only be seen by using a microscope.
  • These include structures like the nucleus and mitochondrion.
  • Cells are mostly specialised to carry out a particular function or activity.
  •  For example, red blood cells are specialised to carry oxygen, whereas white blood cells defend against disease.
  • Tissues occur when similar specialised cells form together, however this does not have to mean the same type of cell.
  • An example of a tissue is the xylem which carries water and mineral salts.
  • An organ is a group of various tissues working together, for example human lungs.
  • Organs work together in organ systems, so for example when the stomach, intestines and liver work together in the digestive system.
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Organelles and their functions

____ = only found in plant cells and not animal cells 

  • Nucleus- Most cells have a nucleus, if they do not, they are usually dead, The nucleus contains chromosomes, and controls the activities of the cell. Genes control which protein the cell can make.
  • Mitochondria- Carry out some of the reactions of respiration, which produces energy, so the cell can use it. They are found in the cytoplasm.
  • Cell Membrane- It selectively permeable, and controls what substances go in and out of the cell.
  • Cytoplasm- Is the living material that makes up a cell. Most chemical reactions happen here.
  • Cell wall- Is made of cellulose and surrounds the cell membrane. 
  • Vacuole- Contains cell sap, and supports the cell.
  • Chloroplasts- Contain chlorophyll so therefore photosynthesis can take place. 
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The Six Groups Of Living Organisms

  • Plants
  • Are multicellular, which means they are made up of different types of cells.
  • Have chloroplasts, so they can photosynthesise.
  • Their cell walls are made in cellulose, and they store carbohydrates as sucrose or starch.
  • Animals
  • Are multicellular.
  • They don't have cell walls or chloroplasts.
  • Most have a nervous coordination which means they can respond rapidly.
  • They can move around.
  • They store carbohydrates as glycogen.
  • Protoctists
  • They are single-celled and can only be seen under a microscope.
  • Some have chlorophyl like plant cells e.g. Algae, others are more like animal cells e.g Amoeba.
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The Six Groups Of Living Organisms


  • Most are multi-cellular organisms, however yeasts are single-celled,
  • The mushroom or toadstool is the reproductive structure of the organism (the fruiting body.)
  • Under the soil there are many fine thread-like filaments called hypae, the whole network is called Mycelium.
  • Moulds absorb nutrients from dead organisms, so they are found on decying fruit when the mould spores in the air have landed on the surface and have grown a mycelium of hypae.
  • The hypae branches on the food, so the mycelium covers the surface
  • The hypae secretes enzymes onto the food whih breaks it down into soluable structures such as sugar, which is then absorbed by the mould.
  • This is called saprotrophic nutrition, when extra-cellular enzymes are used.
  • Their cell walls are made of chitin.
  • They store carbohyrdates as glycogen. 
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The Six Groups Of Living Organisms


  • Are single celled and microscopic.
  • They can occur in spheres, rods or spirals.
  • They are surrounded by a cell wall, which helps it to mantain its structure.
  • This cell wall is made up of complex chemicals made out of polysaccharides and proteins.
  • There is no nucleus in a bacterium, instead there is a single chromosome loop containing the DNA, which is loose in the cytoplasm.
  • Some can swim and are propelled through the water by a corkscrew shaped structure called a flagellum.
  • Mostly three quarters of bacteriums have plasmids in the cytoplasm which carry genetic information. 
  • A few types can photosynthesise, but most are decomposers and feed off dead/living organisms.
  • Some species are pathogens.
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The Six Groups Of Living Organisms


  • They are particles rather than cells, so are not species. 
  • They are smaller than bacteria.
  • They can only reproduce inside living cells (the host cell.)
  • Their structure is simple: DNA or RNA, surrounded by a protein coat (capsid,) sometimes surrounded by a membrane (envelope.)
  • They do not carry out the regular caharcterstics of a living organism, and can only perform these parasitically.
  • Viruses reproduce by enterring the host cell and taking over the host's genetic machinery.
  •  When lots of these particles have been made, the host cell dies, and more virus particles are released to infect other cells.
  • However, usually the body's immune system responds and destroys the virus.


  • Organisms that cause disease, includng some fungi, bacteria, viruses, and protoctists.
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What Are Enzymes

  • The chemical reactions that happen inside a cell are controlled by proteins called enzymes.
  • The nucleus contains the genes which control the production of enzymes, which then catalyse the reactions in the cytoplasm.
  • We only have enzymes to speed up useful reactions in the body (metabolic reactions.)
  • There are so many types of enzymes because they are made up of proteins which occur in many different shapes and combinations.


  • Temperatures inside organisms are low, and without a catalyst, many reactions would not even take place. If you just raised the temperature to speed up reations, the rate of the useful but also UNWANTED reactions would speed up too. Also, past a limit, the temperture could damage cells and other organisms. 
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How Do Enzymes Work?

A substrate is a chemical that is changed in a reaction, and the enzyme acts upon these molecules.

Every enzyme molecule has an active site which is the part where the substrate joins onto the enzyme, this is a small area on the surface.

The substrate attaches itself to the active site of the enzyme, and this lowers the energy needed for the reation to start, allowing the products to be formed more easily.

Enzymes usually only speed up one reaction, because for an enzyme to work, a substrate has to be the correct shape to fit into the active site. 

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How Do Enzymes Work?


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Factors Affecting Enzymes


  • More heat energy means that the enzymes and substrate particles have more energy so therefore the reaction happens more quicly at first.
  • This is because they have a higher collision rate.
  • Lower temperatures mean a lower collision rate and a slower reaction.
  • However if the enzyme gets too hot, some of its bonds will break meaning that the active site will lose its shape and cannot function properly. This stops the reaction entirely since the enzyme is DENATURED and the change in shape is irreversible. 

Every enzyme has its optimum working temperature, most in the human body being at around 37 degrees celsius. 

pH - Paper Two

Similar to the temperature, if the pH is too acidic or alkaline, the bonds in the enzyme can break and this denatures the enzyme. All have their optimum pH.

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The Effects of Temperature on Enzyme Activity

1) You can measure how fast a product appears.

  • Catalase speeds up the decomposition of hydrogen peroxide into water and oxygen.
  • Using the water displacement method, you can record the volume of oxygen given off in a set time frame.
  • To witness the effect of temperature, you could change the temperature of the water bath in which the hydrogen peroxide test tube lies, (as your independant variable.)

2) You can measure how fast a substrate disappears.

  • Amylase catalyses the breakdown of starch to maltose.
  • You can see if starch is present by using iodine. If there is starch, the iodine solution will turn black.
  • You could time how long it takes for the starch to disappear.
  • You can therefore change your independant variable by altering the temperature of the water bath in which the amylase and starch solution lies. 
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Diffusion and Cell membranes

Diffusion is the movement of particles from an area of higher concentration to a lower concentration.

Can happen in liquids and gases, because the particles are free to move around.

Diffusion can occur, since the particles have kinetic energy to move.

Eventually there would be an even balance of particles and this is called an equilibrium. 


They are semi-permeable or selectively permeable which means that they only allow certain ions and soluable molecules through.

Only very small molecules are able to diffuse through cell membranes like glucose, amino acids, water and oxygen.

Starch and proteins cannot. 

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Is the movement of water molecules across a partially permeable membrane, from an area of high water concentration, to an area of lower water concentration. (http://www.bbc.co.uk/bang/images/446x251/osmosis.jpg)

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Turgid Flaccid and Plasmolysed cells

Paper Two

How free the water molecules are to move is called the water potential. Pure water has the highest water potential. The more concentrated it is, the lower its water potential.

The tough cellulose cell wall around the cell keeps the shape of the cell and is vital, as it can resist changes in pressure inside the cell.

There are many dissolved solutes in the cell such as sugars and ions, and if the cell is surrounded by pure water (with higher water potential,) the cell will allow water through the membrane by osmosis. The cytoplasm pushes against the cell wall and the cell is plump and turgid.

However, the opposite to this is when the water is lost from the cell by osmosis, and the cytoplasm is not pushed against the cell wall. The cell is known as flaccid. When this happens over time, the cytoplasm breaks away from the cell membrane and the cell is plasmolysed. 

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Turgor and Active Transport

Turgor is the state when many of the plant cells are turgid. When this happens, the cells push against eachother and the plant uses this pressure to support less wooden parts of the plant, such as the leaves. This means that the leaves can photosynthesise properly.

Also, turgor is vital for the functioning of the stomata. If a plant is left for too long without water, it will wilt and the cells will become flaccid. The leaves begin to droop and the stomata closes, so that less of the surface area of leaves is exposed, and it does not lose as much water. 


Is the opposite of diffusion. It is the movement of particles against a concentration gradient, using energy released from respiration. 

For example: When there is a higher concentration of nutrients in the gut, the diffuse into the bloodstream. Although, sometimes this is the oppsosite way round, and the concentration gradient is the wrong way. The cells use energy in active transport to pump these particles through, using proteins in the cell wall. 

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Diffusion and Osmosis Experiments.


In a non-living system:

Place agar jelly (PHENOLPHTHALEIN and sodium hydroxide) into a beaker in cubes. Pour in dilute HCl. If left, the cubes will turn colourless, as the acid diffuses into the agar jelly and neutralises the sodium hydroxide.


Living system:

Cut potato into identical cylinders (so that the same amount of surface area is exposed,) and place them in sugar solutions which are varying in concentration. Before you put them in, you measure the length of the potato, and measure afterwards too. If water has been gained by osmosis, then the potato will be longer.

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These are sooooooo good! Thank you so much. These will save me so much time and I love the really clear layout! Have you made any more revision sets like this one for biolgy? :)

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