B2 Revision 2012 specification

Revision notes for B2 using the 2012 specification. It is for additional science.


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Carbon dioxide + Water -> Glucose + Oxygen  Word Equation

CO2 + H2O -> C6H12O6 + O2 Symbol Equation

6CO2 + 6H2O -> C6H12O6 + 6O2 Balanced Symbol Equation

The glucose is stored as starch 

There are three limiting factors when talking about photosynthesis and these are:

  • Light
  • Temperature
  • CO2

Example: CO2

At first as CO2 increases so does photosynthesis but only up to a certain point. Here light or temperature is the limiting factor. Up until this point the graph is going up evenly but it plateaus and becomes even so that the more CO2 does not increase the rate of reaction

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The leaves have a waxy cuticle which keeps water in

Also have a palisade layer, where the cells contain lots of chloroplasts capture light

Stomata on lower surface to allow gases in and out

Spongy mesophyll layer has air spaces to allow gases to move between cells

Chlorophyll is found in the chloroplasts and this is where the photosynthesis actually takes place

Plants need:

  • Nitrate to make protein 
  • Magnesium to make chlorophyll

The plant gets these from the soil 

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Animal and Plant Cells

There are some features to animal and plant cells that they have in common but others that they do not.

One feature is that they both have a nucleus. This controls the whole cell and holds the genetic material.

They also both have a cell membrane which controls what goes in and out of the cell.

They both have cytoplasms which is where the chemical reactions take place.

Mitochondira is also present in both which is where aerobic respiration occurs.

Ribosomes is where protein is synthesised and is present in both cells.

However, features only found in plant cells is things like cell wall which provides strength and is made of celluslose

Also, chloroplasts are only present in plant cells and this is where photosynthesis occurs

A vacuole is only found in a plant cell and contains salts and sugars, and maintains rigidity

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Pyramid of Biomass and Numbers

A pyramid of  biomass shows the amount of material at each stage



The pyramid of biomass is always a pyramid shape as...

  • some is used for respiration to move and grow
  • some is lost as heat
  • some material is not digested
  • some is lost as faeces

A pyramid of numbers shows the number of organisms at each level so is not always a pyramid shape because the bottom layer could be something like a single tree or a couple of bushes

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Enzymes are biological catalysts that speed up reactions. An example is respiration in the mitochondria

There is an enzyme and a substrate and the substrate binds to the active site. Here it is broken down. 

Enzymes work well with a higher temperature until about 40 degrees where it drops suddenly. This is because the enzymes have become denatred and do not work.

There are many uses of enzymes are:

  • Biological washing powders
  • In baby foods
  • In slimming foods (fructose is made using isomerases and is sweeter than glucose so less is needed)
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Enzymes in Digestion

Enzymes are also used in digestion

Large molecules are broken down so they can be absorbed

Amylase breaks down starch into sugars in the mouth and small intestine

Proteases breakdown proteins into amino acids in the stomach and the small intestine 

Lipases breakdown fats into fatty acids and glycerol in the small intestine

If something has -ase at the end of it then it is most likely an enzyme

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Mitosis and Meiosis

Mitosis - used for normal cell growth

  • 1. Parent cell
  • 2. Chromosomes make identical copies of themselves
  • 3. They line up along the centre
  • 4. They move apart
  • 5. Two daughter cells form each with 46 identical chromosomes to the parent cell

Meiosis - used to make the sex cells (gametes)

  • The chromosomes are copied
  • Cell divides twice to give four cells

46 = chromosomes in normal cell

23 = chromosomes in sex cell

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Stem Cells

Stem cells are so useful as they can be used to tern into any other kind of cell 

There are many sources of stem cell. These are:

  • Early embryo - ball of cells
  • Adult cells (bone marrow) but there is a restircted supply of cells 
  • Umbilical cord

It is considered very unethical to acquire stem cells as the means to get them is not very good. Unborn babies are not very fair as they have no chance at life. With bone marrow, the process to get the bone marrow is very painful and very uncomfortable for those that is donating the bone marrow. 

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Fossils and Extinctions

Fossils are dead organisms that have somehow had their remains presevered usually down to rock that has formed over it before it rotted

Fossils show how organisms have changed over time or that some have become extinct

Horse fossils show how it evolved from small small dwelling animals to whay it is today


Extinctions can be caused by a number of factors such as...

  • new diseases
  • new predators
  • new competitors
  • environmental changes (e.g. climate)
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Aerobic and Anaerobic Respiration

Aerobic Respiration is what we normally do when our cells respire. The equation is:

Glucose + oxygen -> carbon dioxide + water (+lots of energy)

Aerobic Respiration is what our cells do when there is not enough oxygen. The equation is:

Glucose -> Lactic acid (+ a little energy)

Lactic acid is poisonous and hurts so our body has to get rid of it. There is also an oxygen debt after respiring anaerobically as the cells need the oxygen that they did not use to recover so we will breathe more heavily after having respired anaerobically. 

Aerobic respiration is much more efficient than anerobic respiration as a lot more energy is given back.

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