Biology

B3

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Cells & DNA

Animal Cell

  • Nucleus
  • Cell Membrane
  • Cytoplasm
  • Mitochondria

Plant Cell

  • Nucleus
  • Cell Membrane
  • Mitochondria
  • Large Vacuole
  • Chloroplasts
  • Cytoplasm
  • Rigid Cell Wall

Nucleus contains DNA, its a double stranded helix which makes up lots of groups called nucleotides. Each nucleotide contains a base which has four different bases:-

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Cells & DNA

A - C

G - T

Each base forms cross links to a base on the other strand.

DNA can replicate itself.

  • Copies itself when a cell divides
  • DNA unzips itself to form two strands
  • As it unzips itself new nucleotides floating around attach to where the bases fit
  • The result is 2 molecules of DNA identical to the original
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DNA Fingerprinting

DNA is unique, therefore each person had there own unique fingerprint.

How it works

  • Isolate the DNA from cells
  • Special enzymes cut DNA into fragments where they recognise particular order of bases - this will be different for everyone.
  • If DNA contains the section of bases lots of times then its cut into little pieces - the same the other way round.
  • DNA bits are separated using chromatography. Suspended in gel and electric current passed through gel. DNA is negatively charged so it moves toward positive anode. Small bits get further through gel because their smaller.
  • DNA is tagged with radioactive marker, then placed onto photographic film. The film turns dark where radioactivity is, showing positions of DNA fragments.


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Protein Synthesis and Enzymes

Proteins are made by reading the code in DNA

  • DNA controls the production of proteins in a cell
  • A gene is a section of DNA that codes for a certain protein
  • Proteins are made up of chains of molecules called amino acids. Each different protein has its own number and order of amino acids
  • Each protein had a different shape, means proteins can have different functions
  • Its the order of the bases in the strand of DNA that decides order of amino acids in protein 
  • Each amino acid is coded for by a sequence of 3 bases in strand of DNA
  • Proteins are made from 20 different amino acids, found in cytoplasm of cell. They're stuck together to make proteins, following order of code on DNA  
  • We get amino acids from our diet. If we don't take in amino acids in right amounts our body changes them into others this is called transamination and happens in liver
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Protein Synthesis and Enzymes

Enzymes and Catalysts are produced by living things

  • Living things have lots of different chemical reactions going on inside them. E.g. Respiration, Photosynthesis and Protein synthesis.
  • These need to be carefully controlled to get right amount of substances and keep organism working.
  • You can make reaction work quicker by raising temp - this speeds up useful reactions but also the unwanted ones.
  • There is a limit to how far you can raise the temp inside a living creature before cells get damaged.
  • Living things produce enzymes  which act as biological catalysts.
  • Enzymes reduce the need for high temps.
  • An enzyme is a biological catalyst which increases the speed of a reaction
  • Enzymes are all proteins - which is why proteins are important
  • Every different biological reaction has a different enzyme designed for it.
  • Each enzyme is coded for a different gene and has a unique shape which it need for its job.
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Enzymes

  • Chemical reactions involve things being split or joined
  • The substrate is the molecule changed in the reaction
  • Enzymes have an active site, this is where it joins on to substrate to catalyse the reaction
  • Enzymes usually only work with one substrate. "Enzymes have a high specificity for their substrate."
  • For an Enzyme to work the substrate has to fit in the active site, if it doesnt match, then the reaction wont be catalysed

THIS IS CALLED THE LOCK AND KEY MECHANISM - Because they fit into each other like a lock and key.

  • Changing the temperature changing rate of reaction
  • More heat means enzyme and substrate particles have more energy - they move more so are more likely to meet up and have a reaction.
  • If it gets too hot, some bonds holding enzymes together break


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Enzymes 2

  • This makes enzyme loose its shape which makes active site not fit substrate - Cannot catalyse the reaction and reaction stops
  • Enzyme is now denatured - shape change is permanent (it wont go back to normal when it cools down)
  • Each enzyme has its own temp where reaction goes fastest. This will be the temp just before it gets too hot and starts to denature
  • Optimum temp in most humans is 37 - same as body temp

Enzymes like it the right PH

  • If its too high or low, it interferes with bonds holding enzyme together This changes shape of active site and denatures
  • Most enzymes work best at PH7, but not always.
  • Pepsin is an enzyme that breaks down proteins in stomach - it works best at PH2 which means its well suited to acid conditions in stomach.
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Diffusion

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