Transport Across Cell Membranes Flashcards
what is the importance of the cell membrane
-prevent organelles from escaping
-controls what can enter and leave
-cell recognition and communication
-compartmentalise
-site of biochemical reactions
what is the model of the cell surface membrane called
fluid mosaic model
what does the membrane consist of
-phospholipid bilayer being free to move about by lateral diffusion
-tails point inwards to the interior of the membrane (hydrophobic) and heads point outwards (hydrophilic)
-the tails can be saturated or unsaturated
-cholesterol- plays an important role in the mechanical stability of the membrane and prevents uncontrolled diffusion
-protein molecules that are free too move about
-intrinsic membrane protein - completely span the bilayer
-extrinsic ‘’’- dont go fully through
what is the difference between sat and unsat
unsat makes the membrane more fluid as they fit together more loosely- higher percentage
what other type of lipid does the bilayer contain
cholesterol
how does cholesterol help the membrane
provides mechanical stability/support
the higher the cholesterol the more fluid the membrane is and vice-versa
Cholesterol molecules sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing.
Interaction between cholesterol and phospholipid tails also stabilises the cell membrane at higher temperatures by stopping the membrane from becoming too fluid
Cholesterol molecules bind to the hydrophobic tails of phospholipids, stabilising them and causing phospholipids to pack more closely together
what other substance does the bilayer contain
proteins
-intrinsic - completely span the bilayer
-extrinsic - only on one side of the bilayer
what are the roles of intrinsic proteins
-transportation
-channel proteins
-gated channel proteins
-receptor proteins
what are the roles of extrinsic proteins
-informational
-enzymes
-receptors
-cell surface antigens
-components in energy transfer
what do some lipids and proteins have in the bilayer
short, branchy carbs
gylcoproteins and gylcolipids
why is the membrane described as fluid and mosaic
The fluid mosaic model describes cell membranes as ‘fluid’ because:
The phospholipids and proteins can move around via diffusion
The phospholipids mainly move sideways, within their own layers
The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
The fluid mosaic model describes cell membranes as ‘mosaics’ because:
The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
what is the membrane also described as
partially/selectively permeable barrier
what is diffusion
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration. - across partially permeable membrane
what factors affect simple diffusion
-size and nature of the particles - small-lipid soluble and uncharged can pass through whereas large polar water molecules struggle
-conc gradient- greater concentration gradient/difference the faster the overall net movement
-length of diffusion path-shorter = quicker eg capillary walls and villi and alveoli walls
-surface area bwt two regions -larger SA = faster rate
eg villi and microvilli in small intestine, alveoli , fish gills and tracheal system in insects
-temp - higher temp = more kinetic energy and diffuse quicker
what is fricks law
calcs rate of diffusion
surface area x conc difference/ thickness of membrane
what is facilitated diffusion
refers to the transport of large polar molecules and ions into and out of a cell with aid of channel/carrier protein
what helps these ions / polar molecules diffuse
These substances can only cross the phospholipid bilayer with the help of certain protein
what is a carrier protein
carrier proteins can switch between two shapes
This causes the binding site of the carrier protein to be open to one side of the membrane first, and then open to the other side of the membrane when the carrier protein switches shape
The direction of movement of molecules diffusing across the membrane depends on their relative concentration on each side of the membrane
Net diffusion of molecules or ions into or out of a cell will occur down a concentration gradient (from an area containing many of that specific molecule to an area containing less of that molecule)
substance deposited on the other side of the membrane