Biology Chapter 5

• plasma membrane (cell membrane)

  Regulates what enters and leaves the cell and separates the internal environment of the cell from the external environment;
• What do plasma membranes surround?

  the cell

  internal organelles
• what is the size of a plasma membrane

  7nm
• fluid mosaic model of membrane structure


  states that a membrane is a fluid structure with proteins and lipids that can move around

  "mosaic" of various proteins and lipids embedded in membrane.

  
• What does the cytoskeleton do in the cell membrane?

  aid cell to cell attachment;

  involved in cell movement
• what are filaments of the cytoskeleton made of?

  actin
• Structure of a phospholipid

  made of 1 glycerol, 2 fatty acids and 1 phosphate

  phosphate forms a hydrophilic head, fatty acids form hydrophobic tails

  forms a phospholipid bilayer, basic structure of membranes
• Structure of phospholipid bilayer


  a two-layered arrangement of phospholipid molecules that form a cell membrane

  their hydrophilic heads are in contact with the watery exterior or interior (cytoplasm). Their hydrophobic tail is in the centre away from water.

  
• function of phospholipids

  create phospholipid bilayer to create a partially permeable barrier between the cell and its environment;

  keep the membrane fluid but stable as phospholipids move around each-other
• What are the types of proteins in membranes

  intrinsic (channel or carrier)

  extrinsic (glycoproteins)
• what are transmembrane proteins

  Integral proteins that span the whole width of the membrane;

  transport water soluble particles that cant get through the bilayer
• What are intrinsic proteins

  transmembrane proteins that are embedded in the phospholipid bilayer
• Function of intrinsic proteins

  help transport substances that cant diffuse across membrane e.g ions sugars amino acids;

  can be receptors or enzymes
• Channel proteins

  have a hydrophilic channel that certain polar molecules or ions can use as a passageway across membrane by diffusion down the conc gradient
• carrier proteins


  a protein that changes shape to move substances from each side of the membrane;

  • in active transport, it transports substances from a low concentration to a high concentration across a cell membrane.
  • in facilitated diffusion, they transport large polar particles down the concentration gradient

  
• What are extrinsic proteins


  proteins bound to the inner or outer surface of the membrane

  acts as receptors for hormones or transport molecules
• Glycoproteins

  Protein with a chain of carbohydrates attached;

  form H bonds in water to give membrane a stable structure;

  important in cell signalling as they act as receptors for certain molecules
• transmembrane channels

  spans the entire of the membrane creating a watery channel that is partially permeable and specific to the molecule it lets through.

  
• Glycolipids

  phospholipid with a chain of carbohydrate molecules attached
• Function of glycoproteins and glycolipids

  carbohydrate chain allows them to act as receptors

  bind with certain substances at cell surface

  form antigens on surface - via immune system
• what can pass through the phospholipid bilayer?

  small, non-polar molecules can pass (O2, CO2)
• what cannot pass through the phospholipid bilayer?

  large, polar molecules cannot pass (glucose,amino acids and water)
• Cholesterol

  spread about in the phospholipid bilayer.

  controls membrane fluidity - more cholesterol = less fluid/permeable;

  binds to phospholipid to prevent membrane fluidity;

  increase mechanical strength and stability of membrane
• state two roles of membranes in cells

  reactions are separated (compartmentalize) and protects the cell from the contents of lysosomes.
• What is membrane fluidity needed for?

  diffusion of substances across membrane;

  membranes to fuse e.g vesicles fuse with membrane;

  cell to move and change shape
• Factors affecting membrane fluidity/permeability

  temperature - (kinetic energy) (proteins)

  lipids

  cholesterol
• When temperature increases in the cell membrane

  phospholipids gain more kinetic energy and move around more in a random way, increasing membrane fluidity.

  increased fluidity reduces effect as barrier.

  permeability increases, increasing diffusion.

  
  

  (cholesterol in the membrane buffers temperature change)
• When temperature drops in the cell membrane

  At lower temperatures lipids have less kinetic energy and they are packed closer together decreasing fluidity

  
  

  (cholesterol in the membrane buffers temperature change)
• What effect does long saturated fats have on fluidity?

  has a high melting point which decreases fluidity

  straight chains mean lipid molecules are packed close together
• What effects do short unsaturated fats have on fluidity?

  has low melting point which increases fluidity;

  double bonds create kinks in lipid molecules so they are packed loosely together
• proteins and temperature

  high temperatures cause proteins to permanently change shape (denature). just under the membrane is a cytoskeleton made of protein. If the proteins denature then the plasma membrane will begin to fall apart.
• Effects of cholesterol on the membrane fluidity

  1. reduces membrane fluidity at moderate temperatures by reducing phospholipid movement

  2. stops solidification by disrupting the regular packing of phospholipids at low temperatures

  3. low levels of cholesterol at normal temperatures makes the membrane too fluid, increasing risk of the cell bursting
• cell signaling

  The process of cell-to-cell communication mediated by signalling molecules and membrane receptors
• What do cells communicate through

  chemical messenger molecules transported to target cell
• What do cells need to detect messenger molecules

  correct cell membrane receptors
• Where can cells communicate with each other?

  inside cells - between components

  outside cell - between cells
• What are cell receptors?

  glycoproteins
• Why are only some cells effected by messenger molecules?

  receptors are specific to certain cell signalling messengers;

  may not have specific receptor
• complementary (cell signalling)


  shape of receptors are specfifc to messengers
• How many receptors can cells have

  100s