Week 7

• What is Vmax?

  Reaction rate when enzyme is fully saturated by substrate (all active sites occupied).
• What are the functions of cells?

  • Uptake materials from environment
  • Excrete waste products
  • Respond to changes in environment
  • Excitable (membrane potential)
  • Selectively permeable (cell membrane)
• What is simple diffusion used to transport?


  Small, non-polar solutes down their concentration gradient

  (e.g, oxygen)
• What is active transport used to transport?


  Solutes against their concentration gradient (e.g, K+ into cells)
• What is facilitated diffusion used to transport?


  Solutes with low permeability coefficient being transported down their concentration gradient (e.g, glucose)
• How do carrier (transporter) proteins work?

  Specifically bind the solute and undergo conformational change to release the solute on the other side of the membrane
• How do channel proteins work?

  Channels weakly interact with solutes, forming aqueous pores in the membrane which allow the solutes (usually small ions) to diffuse through them

  • channels may be right size for solutes to fit through
  • specific ion channels (e.g, Na+ channels)
  • Can be gated
• How do cells have polarised membranes?


  More negative inside cell than outside (higher concentration of Na+ outside cell than K+ inside cell)
• Influx of Ca2+ into neurones can be...


  Dangerous & damaging to cells & cause excitotoxicity
• What can pass across the membrane easily?


  • Hydrophobic molecules most permeable (e.g, steroid hormones (use intracellular receptors))
  • Small, uncharged polar molecules (2nd most permeable)

  e.g, urea, water & ethanol
• What can't pass across the membrane easily?


  • Large molecules (3rd most permeable) e.g, glucose
  • Ions least permeable (move through ion channels)
• Examples of passive transport:


  • Simple diffusion
  • Osmosis
  • Carrier/channel proteins
• Where are carrier & channel proteins found?


  Plasma membranes
• What do primary active transporters use for energy?


  ATP hydrolysis

  (e.g, sodium potassium ATPase pump- 2K+ in & 3NA+ out (electrogenic))

  1 ATP hydrolysed as this happens (net positive charge lost from cell)
• What is secondary active transport:


  Use ion gradient set up by primary active transport processes

  (e.g, sodium-dependent glucose transporter in gut villi)
• Examples of gated channel proteins:


  • Ligand gated- acetylcholine binding site to open sodium channel
  • Voltage gated (opens depending on membrane voltage (ion environment))
  • Ligand gated channels are fast
• What are aquaporins?


  Water channels in kidneys
• What is exocytosis?


  Export from cell
• How does exocytosis work?


  Membrane of vesicle fuses with plasma membrane & its contents are released outside cell (Ca2+ influx dependent process)

  (Insulin released from beta cells in this way)
• What are endosomes?


  • Transport vesicles
  • Substances too large to enter through carriers/channels can be endocytosed
• What is phagocytosis used for?

  Solid import

  Cell binds to item it wants to engulf on cell surface & draws item inwards whilst engulfing

  e.g, WBC
• What is pinocytosis used for?


  Liquid import

  (e.g, in thyroid follicle- hormones stored in colloid & they are engulfed)
• Types of gland secretion:

  • Endocrine
  • Exocrine
• What is endocrine secretion?


  Internal secretion of hormones into blood
• What is exocrine secretion?


  Secretion via a duct

  (e.g, secretions of glands for sweat, sebum, milk (mammary glands), salivary secretions, digestive enzymes to epithelial surface)
• What is Merocrine?


  Exocytosis (most common type of exocrine secretion)

  
• What is apocrine?


  Products still membrane bound & bit of cytoplasm comes out

  (e.g, milk released from mammary glands)

  
• What is holocrine?


  Entire cell secreted & breaks up

  (e.g, sebum from sebaceous glands)

  
• What are the 3 types of exocrine secretion?


  Merocrine, apocrine, holocrine
• What do carriers display?


  Saturation kinetics determined by the number of carriers available & rate of transport
• What 3 things can carriers be?

  • uniports
  • symports
  • antiports (exchangers)
• What do uniports do?

  Transport 1 solute

  e.g, glucose transporters

  Ca2+ uniporter electrogenic (can alter cell's electrical membrane potential)
• What do symports do?


  Co-transport

  e.g, sodium iodide symport in thyroid gland (electroneutral)
• What do antiports do?


  Carry 2 substances in opposite directions
• What are ABC proteins?

  ATP-binding cassettes proteins

  • Family of primary active transporters only using ATP to facilitate active transport
  • Move nucleotides, drugs, peptides & bile salts