biology chapter 5

• functions of membranes in cells:

  • separating cell contents from outside environment
  • separating the contents and activities of organelles from each other
  • cell recognition and signalling
  • site of chemical reactions
  • regulating the transport of materials into or out of cells
• membranes are mainly made of phospholipids

  • phosphate group
  • glycerol
  • fatty acid tails
  • hydrophilic polar head
  • hydrophobic non-polar tail
• plasma membranes have a double layer of phospholipids
• the fluid mosaic model:

  • the phospholipid molecules can move freely laterally, so the membrane is fluid
  • the proteins are distributed throughout the membrane unevenly in a mosaic pattern
• structure and function of the glycocalyx:

  • consists of glycoproteins (proteins with carbohydrate chains)
  • used for cell recognition/receptors
• types of carrier proteins:

  • carrier protein (passive)
  • carrier protein (active)
  • gated-channel protein
  • channel protein
• factors affecting membrane fluidity:

  • temperature
  • cholesterol
  • unsaturated or saturated fatty acids
• at low temperatures, the phospholipids in the cell membrane group together, causing low fluidity
• at high temperatures, the phospholipids in the cell membrane spread out, causing high fluidity as there is more space to move around
• at low temperatures, cholesterol increases fluidity because it inserts itself into different parts of the membrane, spreading out the phospholipids
• at high temperatures, cholesterol decreases fluidity because it pulls the phospholipids together
• pentane = saturated

  pentene = unsaturated
• saturated fats decrease the fluidity

  unsaturate fats increase the fluidity
• conformational change: carrier protein changes shape
• gated channel proteins:

  • passive process
  • facilitated diffusion
  • the channel only opens when a specific ligand binds
• ligand gated ion channels have a binding site where a ligand binds to it, causing it to open. this is because the ligand binding to the binding site changes the shape.
• 3 active processes that allow passage of substances across membranes:

  • active transport
  • endocytosis
  • exocytosis
• active transport: the movement of molecules across a membrane from low to high concentration using ATP and carrier proteins
• the energy needed for active transport is supplied by the hydrolysis of ATP
• active transport requires a conformational change in the protein
• bulk transport:

  • large molecules, e.g. enzymes, hormones, bacteria
  • need to move in and out of the cell by bulk transport as they are too large for carrier proteins
  • exocytosis and endocytosis
• endocytosis: bulk transport of materials into the cell, there are 2 types:

  1. phagocytosis for solids
  2. pinocytosis for liquids
• exocytosis: bulk transport of materials out of the cell. vesicles move towards and fuse with the membrane.
• in bulk transport, ATP is required for:

  • movement of vesicles along cytoskeleton
  • changing the shape of the membrane to engulf materials
  • fusion of cell membranes as vesicles form
  • fusion of cell membrane when it meets vesicles or materials outside the cell
• water potential - a measure of the energy available in an aqueous solution to cause the movement of water molecules across selectively permeable membranes during osmosis
• water potential values fall as the solute concentration increases
• when red blood cells are placed in:

  • isotonic solution: nothing happens, the solute concentrations are balanced
  • hypertonic solution: water moves out of the red blood cell, it becomes crenulated
  • hypotonic solution: water moves into the red blood cell, the pressure will eventually cause it to burst
• cytolysis:

  1. water moves into the cell by osmosis
  2. this increases the hydrostatic pressure inside the cell
  3. the cell surface membrane cannot stretch much and cannot withstand pressure
  4. the cell membrane will burst, called cytolysis
• • animal cells don't have a cell wall to protect them from excess hydrostatic pressure
  • so, multicellular animals must have osmo-regulatory mechanisms which keep the water potential of bodily fluids within narrow ranges
  • this maintains the cell membrane integrity
• effects of osmosis on plant cells:

  • cells in a dilute solution become turgid
  • cells in the same solution stay the same
  • cells in concentrated solutions become flaccid
  • plasmolysed cell: cytoplasm is pulled away from the cell wall
• the voltage-gated sodium channel:

  • the voltage changes, causing the channel to open
  • the ions can then diffuse down the open channel from high to low concentration
  • the inactivation gate then blocks the channel
• role of the membrane on the RER:

  • maintain different conditions from the cytoplasm
  • hold ribosomes in place
• plant cells in solutions

  A) plasmolysed

  B) flaccid

  C) turgid