Topic 2

• How are channel proteins involved in passive transport

  Channel protein allows the movement of large charged molecules by diffusion from high concentration to low concentration
• How are mammalian lungs adapted for efficient gas exchange

  There is a large surface area due to many alveoli and many capillaries. There is a short diffusion distance due to alveoli/capillaries being one cell thick. There is a good blood supply due to many capillaries and this ensures a steep concentration gradient.
• How is transcription involved in enzyme synthesis?

  DNA unzips and hydrogen bonds between complementary strands broken. the template strand is used for mRNA synthesis. RNA polymerase is used to join RNA nucleotides. Complementary base pairing of A with U, not T
• Describe the effect of ethanol on the permeability of beetroot membrane cells

  Ethanol causes membrane to be disrupted due to phospholipids dissolving in ethanol. Membrane proteins are denatured by ethanol. Betalin can escape from the cell when the membrane is disrupted
• Why do results differ in beetroot investigation?

  Beetroot cells may have been damaged when cutting. Beetroot pieces not rinsed before being placed in ethanol solution. Colorimeter not calibrated properly
• Why the Darson Danielle model does not support the fluid mosaic model?

  molecules cannot diffuse through 2 protein layers. No carrier or channel proteins for facilitate diffusion
• What does the rate of diffusion depend on? State Fick's law

  surface area, difference in concentration, thickness of gas exchange surface
• What is the structure of the cell membranes?

  Phospholipid molecules form a continuous bilayer. It contains proteins, cholesterol, glycoproteins and glycolipids. Some proteins are in the inner or outer layer only, some are in both.
• What is a glycoprotein?

  Proteins with a polysaccharide chain attached
• Why is the membrane partially permeable?

  Small molecules can move through the gaps between the phospholipids but large molecules and ions can only pass through channel and carrier proteins
• What is a glycolipid?

  Lipids with a polysaccharide chain attached
• Why is this bilayer fluid?

  The phospholipids are constantly moving
• Describe the phospholipid bilayer and its interaction with water.

  Phosphate head is hydrophilic and the two fatty acid tails are hydrophobic. In the bilayer the hydrophilic heads face out towards the water and the hydrophobic tails are on the inside making the center hydrophobic.
• Why doesn't the membrane allow water soluble substances through it?

  Because the centre of the membrane is hydrophobic due to the tails
• What role does cholesterol have in a cell membrane?

  It fits between the phospholipids forming bonds with them. This increases the rigidity of the membrane, making it less fluid
• Evidence for the fluid mosaic model

  EM images showed a bilayer in cell membranes and new methods for analysing proteins showed that they were randomly distributed in cell membranes. Also did experiments to find out cell is fluid
• Define diffusion

  Diffusion - Net movement of particles from an area of higher concentration to an area of lower concentration. Passive process
• Define concentration gradient

  Path from an area of higher concentration to an area of lower concentration. Particles diffuse down it
• Define osmosis

  Diffusion of water molecules from a dilute solution (an area of higher concentration of water molecules) to a concentrated solution (an area of lower concentration of water molecules) across a partially permeable membrane
• Define facilitated diffusion

  Larger molecules and charged particles cannot diffuse through the phospholipid bilayer as they are insoluble. They diffuse through carrier or channel proteins in the cell membrane instead. Passive process
• What are carrier proteins?

  Move large molecules into or out of the cell down their concentration gradient. Different carrier proteins facilitate the diffusion of different molecules
• How do carrier proteins work?

  A large molecule binds to a specific site in the carrier protein. The protein changes shape and releases the molecule on the opposite side of the membrane
• What is a channel protein?


  Channel proteins - Form pores in the membrane for charged particles to diffuse through. Different channel proteins facilitate the diffusion of different charged particles. All have a specific shape and can be opened or closed
• Define active transport

  Active transport - Uses energy to move molecules and ions across the plasma membranes against a concentration gradient. Use carrier proteins
• Explain how active transport works

  A molecule attaches to the carrier protein. ATP is hydrolysed in the cell to release energy. This energy is used to make the carrier protein change shape. This moves the molecule across the membrane, releasing it on the other side.
• Describe the process of endocytosis

  A cell surrounds a substance with a section of its cell membrane. The cell membrane pinches off to form a vesicle inside the cell containing the ingested substance. Process uses ATP for energy
• Why are substances taken in to the cell by endocytosis?

  They are too large to be taken in by carrier proteins
• Describe the process of exocytosis

  Some substances produced by the cell need to be released from the cell. Vesicles containing the substances fuse with the cell membrane and release their contents outside the cell. Uses ATP for energy
• Describe the process of translation

  Ribosome attaches to mRNA. tRNA carries amino acid to ribosome. Complementary base pairing between anticodon on tRNA and codon on mRNA. Hydrogen bonds form between the tRNA and mRNA. A peptide bond forms between amino acids by condensation reaction. The tRNA is released from the ribosome. The ribosome detaches from the polypeptide chain on mRNA when it reaches a stop codon
• How can a gene mutation result in a protein not being synthesised?

  Mutation changes sequence of bases, A premature stop codon or frame shift is changed by the mutation. Transcription/translation does not occur
• Describe the structure of tRNA

  tRNA is folded, has hydrogen bonds holding the structure together, is a fixed size, has an anticodon, has an amino acid binding site
• How are amino acids joined together to form the 3D structure of a protein?

  Peptide bonds between amine group and carboxyl group of another. Primary structure is sequence of amino acids in a protein. The primary structure is folded and held together by bonds. These bonds are disulphide bonds, ionic bonds, hydrogen bonds and are between the R groups
• Why does transcription occur?

  Because DNA is too large to move out of the nucleus so a section is copied onto mRNA
• What is mRNA?

  Made in the nucleus during transcription, has a codon, carries the genetic code from the DNA in the nucleus to the cytoplasm. Single stranded. Shorter than DNA
• What is a codon?

  Three adjacent bases
• What is tRNA?

  Found in cytoplasm, has an amino binding site at one end and an anticodon on the other end, carries amino acids that are used to make proteins to the ribosomes
• Define the genetic code

  The sequence of base triplets in DNA or mRNA which codes for specific amino acids
• What does it mean by 'non overlapping'?

  Each base triplet is read in sequence, separate from the triplet before and after it. base triplets do not share their bases
• Explain the term 'degenerate'?

  There are more possible combinations of triplets than there are amino acids. Some amino acids are coded for by more than one base triplet
• What are the stop and start codons?

  Triplets that tell the cell when to start and stop production of the protein. Found at the beginning and end of the gene