OCR Biology 21st Century (New) B4

Revision notes for the new syllabus for B4 section of the second module

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Cell Structure and Function

Plant and Animal cells have similarities and differences: 

Similarities: 

  • Nucleus- this contains the DNA 
  • Cyptoplasm- gel-like substance where proteins (e.g. enzymes) are made. 
  • Cell membrane- holds the cell together and controls what goes in and out. Lets gases and water pass through it.
  • Mitrochondria- these are where the enzymes needed for the reactions of aerobic respiration take place and are found.

Extra things that animal cells don't have:

  • Rigid cell wall- made of cellulose. it supports the cell and strengthens it.
  • Vacuole- contains cell sap, a weak solution of sugar and salts.
  • Chloropasts- this is where the reaction of photosynthesis takes place. They contain a green substance called chlorophyll and the enzymes needed for photosynthesis.
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Cell Structure and Function (cont.)

Yeast are a single-celled micro-organisms which is used to make bread.
They have;

  • cell membrane 
  • cell wall
  • cytoplasm
  •  nucleus 
  • mitrochondria

Bacteria have a simple cell structure;

  • cell wall
  • cell membrane
  • cytoplasm
  • circular DNA molecule-- They do not have a nucleus. Instead they have a circular molecule of DNA which floats around in the cytoplasm
  • They don't have mitrochondria either, but they can still respire areobically 
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Enzymes

  • Enzymes are proteins produced by living things
  • ENZYMES ARE PROTEINS THAT SPEED UP CHEMICAL REACTIONS
  • The instructions for making enzymes and other proteins are found in a cell's genes

Enzymes are very specific;

  • Chemical  reactions usually involve things either being split apart or together.
  • A substrate is a molecule that is changed in a reaction.
  • Every enzyme molecule has an active site -- the part where a substrate joins on to the enzyme.
  • Enzymes usually only speed up one reaction. This is beacuse the substrate has to correctly fit into the active site. This is called the 'lock and key' model

They also need to be the right temperature and pH in order to work. 
The higher point where the enzymes are most active is called the optimum.
If it gets too hot/the pH is weird, then the bonds holding the enzyme together break.  

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Respiration

RESPIRATION IS A SERIES OF CHEMICAL REACTIONS THAT RELEASE ENERGY BY BREAKING DOWN LARGE FOOD MOLECULES. IT HAPPENS IN EVERY LIVING CELL.

The energy released by respiration is used in order to power some chemical reactions in cells. Examples of this are;

  • Movement 
  • Active Transport
  • Synthesis of large molecules

There are two types of respiration--- aerobic and anaerobic

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Aerobic respiration

  • "aerobic" just means "with oxgen"
  • It releases more energy per glucose molecule that anaerobic respiration.
  • This is the type of respiration that you're using most of the time.
  • Aerobic respiration takes place in animal and plant cells, and in some microorgansims.

GLUCOSE + OXYGEN --> CARBON DIOXIDE + WATER (+energy released)

C6H12O6 + 6O2 --> 6CO2 + 6H2O (+energy released) 

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Anaerobic Respiration (1)

Anaerobic Respiration doesn't use oxygen

"Anaerobic" just means "without oxygen"
 Anaerobic respiration takes place in animal and plant cells and some microorganisms when there's little or no oxygen. For example;

  • HUMAN CELLS; when you do really vigorous excerise, your body can't supply you with enough oxygen to your muscles for aerobic respiration.
  • PLANT CELLS: If the soil becomes water logged than there is no oxygen available for the roots.
  • BACTERIAL CELLS; If the bacteria gets under you skin, there maybe no access to oxygen. 
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Anaerobic Respiration (2)

Anaerobic respiration can produce lactic acid:
In animal cells and some bacteria 

GLUCOSE --> Lactic acid (+energy released)

Anaerobic respiration can also produce ethanol and carbon dioxide:
In plant cells and some microorganisms (like yeast)

  GLUCOSE --> ETHANOL + CARBON DIOXIDE (+energy released)

Fermentation:

is when microorganisms break down sugars into other products as they respire anaerobically. Humans use fermentation to make lots of things, for example:

  • BIOGAS
  • BREAD
  • ALCOHOL
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Photosynthesis

Photosynthesis produces glucose:

"Photosynthesis is a series of chemical reactions that uses energy from sunlight to produce food"

Photosynthesis happens in:

  • the cells in green parts, e.g. leaf cells.
  • some microorganisms, e.g. phytoplankton.

Chlorophyll is needed for photosynthesis to happen-- it's a green substance which absorbs sunlight and allows the energy to be used to convert carbon dioxide and water into glucose.

CARBON DIOXIDE + WATER --> (light energy) --> GLUCOSE + OXYGEN

6Co2 + 6H2o --> (light energy) --> C6H12o6 +6o2 

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Photosynthesis (con.)

Plants use the glucose in 3 different ways:

1) RESPIRATION

2) USED TO MAKE CHEMICALS FOR GROWTH

3) IS STORED AS STARCH 

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Rate of Photosynthesis

Three factors affect the rate of photosynthesis:

  • amount of light
  • amount of carbon dioxide
  • temperature

Any of these 3 factors can become the limiting factor. This means that it stops photosynthesis fromm happening anhy faster.
Whether it is a limiting factor depends on the enviroment conditions. 

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Diffusion, Osmosis and Active transport

Diffusion

"Diffusion is the passive overall movement of particles from a region of their higher concentration to a region of their lower concentration"

Osmosis

"Osmosis is the overall movement of water from a dilute to a more concentrated  solution through a partially permeable membrane."

A partially permeable membrane is just one that only allows certain substances to diffuse through it.

Active transport

"Active transport is the overall movement of chemicals across a cell membrane from a region of lower concentration to a region of higher concentration using energy released by respiration."

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