B1

• Magnification


  The degree to which the size of an image is larger than the real object
• Standard form examples


  • 1.5 x 10^-5 = 0.000015
  • 3.4 x 10^3 = 3400
• Disadvantages of electron microscopes


  • Samples need to be placed in a special vacuum so living specimens cannot be viewed
  • Extremely expensive
  • Very large and not easily movable
  • Requires specialist training and skill
• Subcellular structures in animal and plant cells

  • Nucleus
  • Cytoplasm
  • Cell membrane
  • Mitochondria
• Cell structures

  • Microscopes
  • Light (optical) microscopes
• Standard form can be useful when working with very large or small numbers in microscopy
• Advantages of electron microscopes

  • Can see very small organelles
  • Can produce 3D images (SEM)
• Cell membrane


  Contains receptor molecules to identify and selectively control what enters and leaves the cell
• Mitochondria


  Where aerobic respiration reactions occur
• Nucleus contains the genetic material


  Which codes for a particular protein and is enclosed in a nuclear membrane
• Advantages of light microscopes


  • Relatively cheap
  • Can be used in the field
  • Does not require specialist training
  • Can look at living specimens
• Using an electron microscope


  1. It uses electrons instead of light to form images

  2. Electrons have a much smaller wavelength than light waves, allowing the viewing of smaller subcellular structures

  3. There are two types: SEM and TEM
• Resolution


  The ability to distinguish between two points that are close to each other
• Disadvantages of light microscopes

  • Low resolution (200nm)
  • Low magnification strength (x1500)
  • Staining is required for some organelles to be visible
• Cytoplasm


  Is a liquid substance in which chemical reactions occur, contains enzymes, and organelles are found in it
• Using a light microscope

  1. The specimen is placed onto a slide, which is a thin piece of glass

  2. If the specimen is colourless, a stain is needed to identify the organelles

  3. A cover slip is placed on top to protect the lens

  4. The microscope has a lamp at the bottom that shines constant light on the slide

  5. The objective lens magnifies the image, which is further magnified by the eyepiece lens
• Only in plant cells


  • Chloroplasts
  • Permanent vacuole
• Chloroplasts


  • Where photosynthesis takes place, providing food for the plant
  • Contains chlorophyll pigment (which makes it green) which harvests the light needed for photosynthesis
• Cell wall (also present in algal cells)


  • Made from cellulose
  • Provides strength to the cell
• In prokaryotic cells (e.g. bacteria)


  • Cytoplasm
  • Cell membrane
  • Cell wall
  • Single circular strand of DNA
  • Plasmids
• Cell membrane


  • Contain receptor molecules to identify and selectively control what enters and leaves the cell
• Mitochondria


  • Where aerobic respiration reactions occur, providing energy for the cell
• Enzymes

  Biological catalysts (a substance that increases the rate of reaction without being u
• Permanent vacuole


  • Contains cell sap
  • Found within the cytoplasm
  • Improves cell’s rigidity
• DNA

  • It is a double helix made from 2 strands that have twisted around each other
  • It is a polymer, meaning that it is made of many different molecules that join up to make a long strand: in the case of DNA these molecules are called nucleotides
  • Each nucleotide is made from one sugar molecule, one phosphate group (which forms the backbone) and one of the four different organic bases
  • The 4 bases are A, C, G, T
  • These nucleotides pair by complementary base pairing, meaning that only certain bases can join together: C joins to G and A joins to T
  • Each group of three bases codes for an amino acid and these then join together to make a protein
  • Chromosomes are structures made up of long molecules of DNA
• Cell


  • Contains enzymes (biological catalysts, i.e. proteins that speed up the rate of reaction)
  • Organelles are found in it
• Protein synthesis


  1. DNA helix is untwisted and unzipped

  2. mRNA nucleotides (messenger RNA: a different type of nucleotide) match to their complementary base on the strand

  3. The mRNA nucleotides themselves are then joined together, creating a new strand called a template strand of the original DNA. This process is called transcription

  4. The template strand of mRNA then moves out of the nucleus to the cytoplasm and onto structures called ribosomes

  5. At the ribosomes, the bases on the mRNA are read in threes to code for an amino acid (the first three bases code for one amino acid, the second three bases code for another etc). This is called translation

  6. The corresponding amino acids are brought to the ribosomes by carrier molecules

  7. These amino acids connect together to form a protein. It is therefore the triplet code of bases that determines which protein is produced and therefore expressed

  8. When the chain is complete the protein folds to form a unique 3D structure
• Ribosomes


  • Where protein synthesis occurs
  • Found on a structure called the rough endoplasmic reticulum
• DNA stands for deoxyribonucleic acid
• Temperature affecting enzyme reactions

  • Optimum around 37 degrees Celsius
  • Rate increases with temperature up to the optimum, then decreases rapidly
  • Denaturation occurs when temperature is too high, changing the active site shape
• Anaerobic respiration


  • Occurs when there is not enough oxygen
  • Yields less energy than aerobic respiration
  • Used as a last resort, e.g., during sprints
• Enzyme-substrate interaction


  1. Enzyme specificity is based on the complementary shape of the substrate to the active site

  2. Formation of an enzyme-substrate complex

  3. Reaction takes place and products are released from the enzyme surface
• Respiration

  1. Occurs in every cell to supply ATP

  2. Exothermic reaction

  3. Two types: aerobic and anaerobic
• In animals

  Glucose is converted to Lactic acid
• Substrate concentration affecting enzyme reactions

  • Higher concentration generally increases reaction rate
  • After a point, increasing substrate concentration has no effect as active sites are full
• Triplet code of bases

  Determines which protein is produced and expressed
• In plant and yeast cells

  Glucose is converted to Ethanol + Carbon dioxide
• pH affecting enzyme reactions

  • Optimum pH around 7
  • Affects forces holding amino acid chains in proteins
  • Changes in pH can denature enzymes by altering the active site shape
• Protein synthesis

  When the chain is complete, the protein folds to form a unique 3D structure
• Enzyme concentration affecting enzyme reactions

  • Initially increases reaction rate
  • After a while, reaction stops due to insufficient substrate molecules