Plasma Membranes Flashcards
What are the roles of the membrane?
-Separates internal and external environments
-intercellular membranes form compartments(nucleus, mitochondria and RER) and vacuoles.
-Material exchange control(Partially permeable)
done by diffusion, facilitated diffusion, osmosis and active transport
-Partially permeable barriers
-Cell signalling/ communication
When was the fluid mosaic model outlined?
1972
Why is the model fluid?
-Phospholipids move around via diffusion
-phospholipids move sideways in their layers
-Different types of proteins inside the bilayer that move around
Why is the model mosaic?
Scattered patterns due to proteins within the bilayer
What are the main components of the bilayer
-Phospholipids
-Cholesterol
-Glycolipids, glycoproteins
-Transport proteins
Function and structure of phospholipids
Tails form hydrophobic core, act as a barrier to most water soluble substances, non polar fatty acid tails stop polar molecules from passing across the membrane.
- can be chemically modified to act as signalling molecules
(move in the bilayer to activate other molecules)
Function and structure of Cholesterol
Increases the fluidity of the membrane by stopping it from becoming too fluid when the temperature is too low, stops phospholipids from
packing too closely together
At higher temperatures interactions between cholesterol and phospholipid tails
Cholesterol increases the mechanical strength and stability of membranes
what are the two types of proteins in membranes
intrinsic and extrinsic membranes
what are intrinsic proteins
transmembrane proteins that are embedded through both layers of the membrane. There amino acids have hydrophobic R groups on their external surface which interacts with the hydrophobic core of the membrane and keeps it in place
Channel proteins
provides hydrophilic(polar channel) for passive movement of polar molecules/ions down the concentration gradient through membranes
held in place by hydrophobic interactions
carrier proteins
passive transport and active transport into cells, protein changes shape
glycoproteins
embedded in the cell-surface membrane attached to a carbohydrate chain varying lengths/ shapes. Used in cell adhesion(joining of cells to make tissues) and cell signalling(receptors)
how does cell signalling work
chemical binds to receptor and elicits response from the cell. can cause a direct response or many events in the cell
glycolipids
lipids with attached carbohydrate chain, cell markers/ antigens recognised by the cells immune system as self or non self
extrinsic proteins
present in one side of the bilayer hydrophilic interactions hold it up, can be present in either layer or move between them
cholesterol
lipid with hydrophilic end and hydrophobic end maintains fluidity of the membrane
how does cholesterol act
between phospholipids with hydrophilic end interacting with the head and hydrophobic acting on the tail pulling them closer together
adds stability without making them too rigid
stops them being solid by stopping them from grouping too much together
how does temperature affect the bilayer
more Ke so phospholipids move faster, makes bilayer more fluid and loses its structure, the higher the temperature the more it breaks down
increases permeability of the membrane proteins in the membrane denature and permeability also increases
how do solvents affect the membrane
organic solvents dissolves the membrane disrupting the cells
strong alcohols are toxic and destroy bodily cells less concentrated solutions dissolve the membranes and non polar molecules enter the membrane and their presence disrupts the membrane
what is diffusion
net movement of particles from an area of high concentration from a high concentration down the concentration gradient
occurs passively until there is an equilibrium in concentration between the two areas
diffusion speeds reason
particles are constantly colliding slowing down overall next movement so movement of particles over a long distance is slower that’s why cells are similar so they can have faster rates of diffusion
what are the factors that affect diffusion
temperature- more Ke so molecules will move faster(faster diffusion)
concentration difference- higher the difference faster the rate of diffusion because overall movement from high to low con is higher
diffusion across membranes
only molecules that are non polar and
charged molecules/ions cant pass through easily as they are repelled by hydrophobic interior
polar molecules can pass through (H20 partially positive but at slow rate)
smaller polar molecules can pass through more easily
factors that affect the rate of diffusion across membranes
surface area- larger Surface area higher the rate of diffusion
thickness of membrane- thinner exchange surface, higher thee rate of diffusion
Facilitated diffusion
way for polar molecules/ ions to pass
most protein molecules are specific to one molecule/ ion
facilitated diffusion involves carrier proteins which change shape when a specific molecule binds onto it
active transport definition
active movement of moleculels/ ions in or out of a cell from a area of low concentration to an high concentration which requires energy(and carrier proteins)
supplied by ATP
Na+ K+ pump
-molecule that needs to be transporrted binds to receptors in channel protein on the outside of the cell
-on inside of cell ATP binds onto the carrier protein and is hydrolysed into ADP and Phosphate
-binding of phosphate molecule causes the protein to change shape opening up to inside the cell
-molecule released into the cell
-phosphate molecule released from carrier protein and combines with ADP to form ADP
-carrier protein returns to normal
Bulk transport
for molecules that are too big to fit through channel/carrier proteins
endocytosis
phago/pinocytosis cell surface membrane bends inwards when comes in contact with the material, membrane enfolds the material and eventually membrane fuses and forms a vesicle which pinches off into the cytoplasm for further processing
exocytosis
reverse of exocytosis vesicle from the Golgi apparatus fuses with the cell membrane which bends and released the contents
ATP required for both processes
what is water potential
pressure exerted by water molecules with a membrane/ container measured in Pa or Kpa
Osmosis
net movement of water molecules from an area oh high water potential to an area of low water potential through a partially permeable membrane
increase of pressure in water
if solution is in a closed system and water diffuses in there is an increase in hydrostatic pressure
cytolysis (hypotonic)
higher water potential outside the cell than inside so water moves in the cell cant handle the increase in water so it bursts
crenation(hypertonic)
Lower water potential outside than inside so water in cytoplasm moves out and the cell shrinks
how do animals combat this
they are able to regulate the water potential of liquids in the blood so there is equal water potential
how do plant cells differ
they cannot control the water potential of things in their immediate environment (roots surrounded by almost pure water)
they have a strong cell wall that so hydrostatic pressure only causes turgor
plasmolysis
placed in a lower Water potential solution, cytoplasm reduces in volume which pulls the membrane away from the cell wall (plasmolysed)
