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Biology · What Happens in Cells

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Biology · OCR Paper 1

What Happens in Cells

The Structure of DNA

DNA is a polymer made of two strands wound into a double helix, and a gene is a section of DNA that codes for a protein.

  • DNA The molecule that stores the genetic information of a cell.
  • Polymer A large molecule made of many smaller units joined together.
  • Double helix The twisted ladder shape of DNA: two strands wound round each other.
A twisted ladder shape made of two strands joined by rungs, with a section marked as a gene.
DNA is a double helix: two polymer strands joined by pairs of bases.

DNA in two kinds of cell

  • Eukaryotic cell (plant or animal) DNA is in chromosomes; Chromosomes are inside the nucleus; Usually many chromosomes in pairs
  • Prokaryotic cell (bacterium) DNA is a single loop; It floats in the cytoplasm, with no nucleus; Often has extra rings of DNA called plasmids

Extracting DNA from a living organism

  • Break up the tissue Mash the fruit in a bag to separate the cells.
  • Break open the cells Add a mixture of detergent and salt.
  • Filter Filter the mixture to remove the pieces of cell, leaving a…
  • Add cold ethanol Carefully pour ice-cold ethanol down the side of the tube so…

Nucleotides and Protein Synthesis (Separate only)

DNA is a polymer of four nucleotides, whose bases pair A with T and G with C.

  • Nucleotide A unit of DNA made of a phosphate group, a sugar and one base.
  • Base One of the four chemicals A, T, G and C in DNA.
  • Complementary base pairing The rule that A pairs with T and G pairs with C.
A nucleotide with a phosphate group, a sugar and a base, and a section of DNA with pairs of bases A with T and G with C.
A and T always pair, and G and C always pair.

DNA and mRNA (Higher tier)

  • DNA Two strands in a double helix; Stays in the nucleus; Contains the whole gene and many genes
  • mRNA A single strand; Leaves the nucleus; A copy of one gene
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Biology · OCR Paper 1

What Happens in Cells continued

Enzymes

An enzyme is a biological catalyst. Its active site fits only its own substrate, and a denatured enzyme cannot catalyse the reaction.

  • Enzyme A biological catalyst, made of protein, that speeds up a reaction in a living thing.
  • Catalyst A substance that speeds up a chemical reaction and is not used up.
  • Substrate The substance that an enzyme acts on.
Four stages of the lock and key model: the substrate approaches, fits the active site, is split into two products, and the products leave while the enzyme is unchanged.
The substrate fits the active site like a key in a lock. After the reaction the enzyme is unchanged and can be used again.

Below and above the optimum temperature

  • Below the optimum The rate rises as the temperature rises; The particles move faster and collide more often; The enzyme keeps its shape
  • Above the optimum The rate falls as the temperature rises; Bonds in the enzyme break; The active site changes shape: the enzyme is denatured

Investigating the Effect of pH on Amylase

The rate of an enzyme-controlled reaction is fastest at the optimum pH.

  • Continuous sampling Taking a small sample at regular intervals during a reaction and testing each one.
  • Buffer solution A solution that keeps the pH constant at a particular value.
  • Spotting tile A tile with small wells, used to test many small drops one after the other.
A water bath with a test tube and thermometer, a dropping pipette above a spotting tile with eight iodine drops that change from blue-black to orange-brown, and a stopwatch.
A drop of the mixture is tested every 30 seconds. The time at which the iodine stays orange-brown is the time taken for the starch to be digested.

The variables in this investigation

  • Independent variable What you change; The pH of the buffer solution; Five values: pH 5, 6, 7, 8 and 9
  • Dependent variable What you measure; The time for the starch to be digested; Used to work out the rate of reaction

The method

  • Prepare the tile Put a drop of iodine solution into each well of a spotting tile.
  • Set up the tubes Put 2 cm³ of amylase solution and 2 cm³ of the pH 5 buffer into…
  • Start the reaction Pour the starch into the amylase and buffer, mix.
  • Sample every 30 seconds Use a dropping pipette to take a drop of the mixture and add it…