cell membrane and transport Flashcards
cell membrane contents
made up of entirely proteins and phosolipids
Membrane contains glycoproteins, glycolipids and sterols
Phospholipids in membrane
Forms by layers, one sheet of phospholipid molecules opposite another
Inside layer - hydrophilic, heads pointing in towards cell, intercat w water in cytoplasm
Outer layer - hydrophyllic head pointing outwards interacting with water surrounding cell
Hydrophobic tails of both layers point towards each other to centre of membrane
Phospholipid component of membrane allows lipid soluble molecules across but non water soluble molecules are not
Proteins in membrane
Scattered throughout bilayer of membrane
Extrinsic proteins
On either surface of Bilayer, provide structural support and form recognition sites by identifying cells and receptor sites for hormone attachment
Intrinsic proteins
Extend across both layers of Bilayer. The transport proteins, which use active or passive transport to move molecules and ions across cell membrane
fluid mosaic model of membrane structure
Introduced by singer and Nicholson in 1972
Why is it called fluid mosaic
Individual phospholipid can move within a layer relative to one another (fluid)
Proteins embedded in bilayer vary in shape and size among phospholipid (mosiac)
Cholesterol in animal cell membrane
occurs between
Occurs between phospholipid molecules, making membrane more stable at high temperature and more fluid at low temperatures
Glycocalyx
Carbohydrate layer around cell
Some molecules in this have roles as hormone receptors, in cell to cell recognition and in cell to cell cohesion
Permeability of membranE
Eg O and CO2
Small molecules move between phospholipid molecules and diffuse across the membrane
Eg. Oxygen and carbon dioxide
Lipid soluble substances
Dissolve in phospholipid and diffuse across membrane
Phospholipid layer is hydrophobic so liquid soluble molecules move through the cell membrane more easily than water soluble substances
Eg. Vitamin A
DIFFUSION
water soluble substances
Cannot readily diffuse across membrane through phospholipids and must pass through intrinsic protein molecules which form water filled channels across membrane
Therefore cell surface membrane is selectively permeable to water and some solutes
FACILITATED DIFFUSION
Diffusion
Simple diffusion - passive transport
Movement of molecules from a region of high concentration to a region of low concentration eg. Down a concentration gradient until equally distributed
Rates of diffusion is affected by
Concentration gradient
The greater difference in concentration of molecules in two areas, the more molecules diffuse in given time
Rates of diffusion is affected by
Thickness of exchange surface/ Distance of travel over which diffusion takes place
The thin of the membrane or the shorter the distance, the more molecules diffuse in a given time
Rate of diffusion is affected by
Surface area of the membrane
The larger the area, the more molecules have room to diffuse across in a given time
Rate of diffusion equation
SA x diff in conc/ length of diffusion pass
Other factors affecting rate
- Size of the diffusing molecule: smaller molecules diffuse faster than larger, higher KE o pass thru PP molecules
- Nature of diffusing molecule: fat soluble molecules diffuse faster than water soluble and non polar molecules diffuse faster than polar
Temperature: increase temperature increases rate, as molecules have more KE
Facilitated Diffusion
‘made easier’
- Allows movement of insoluble molecules across membrane
- Passive transfer of molecules or ions down a concentration gradient, across a membrane, by channel or carrier protein molecules in the membrane
Channel proteins
Molecules with pores lined with polar groups
As the channel are hydrophilic, ions are water soluble so can pass through
Channels open and close according to needs of the cell
Carrier proteins
Allows the fusion of larger polar molecules across membrane
Eg. Sugars and amino acids
Molecule attaches to binding site on carrier protein
Carrier protein changes shape and releases molecule on the other side of membrane before changing back to its original shape
Active transport
Movement of molecules across a membrane against the concentration gradient using energy from the hydrolysis of ATP
Features of active transport
- Ions and molecules are moved from a lower to higher concentration eh. Against concentration gradient
- Process requires energy from ATP. Anything that affects respiration will affect active transport
- Process occurs through intrinsic carrier proteins spanning membrane
- Rate is limited by number and availability of carrier proteins
Active uptake of single molecule
1
- molecule/ion combines with specific carrier protein on outside of membrane
Active uptake of single molecule
2
- ATP transfers phosphate group to carry a protein on outside of membrane
Active uptake of single molecule
3
- Carrier protein changes shape and carries molecule across the membrane to the inside of cells
- Molecule released into cytoplasm
Active uptake of single molecule
4
Phosphate ion released from carrier molecule back to cytoplasm and recombines with ADP to form ATP
Carrier protein returns to original shape
Active transport and respiration
At a higher concentration difference across a membrane, the rates of uptake increases and reaches a plateau at which the carrier proteins are saturated and all solute binding sites are occupied
Rate of uptake is reduced with an addition of respiratory inhibitor implying process requires ATP and so active transport must be taking place
Cyanide
A respiratory inhibitor which prevents aerobic respiration and production of atp in the mitochondria
Without ATP, active transport cannot occur so cyanide reduces active transport
Active transport and respiration - experiments in oxygen
Experiments shown an increase in active transport if more oxygen is available to the cells, When it was previously limiting
Indicates active transport as the oxygen would have increased the production of ATP by aerobic respiration
Co transport
Transport mechanism in which facilitated diffusion brings molecules and ions across the cell membrane together into the cell
Difference between active transport and facilated diffusion
AT:
Uses membrane proteins
Pumps against a concentration gradient.
Requires energy
Occurs in living cells
FD:
Use membrane proteins
Passively moves particles down a concentration gradient.
No need of energy
cell doesn’t need to be alive
Osmosis
Net passive diffusion of water molecules across a selectively permeable membrane from a region of higher water potential to a region of lower water potential
Water potential
Tendency for water to move into a system, water moons from a solution with high water potential (less neg) to one with a lower water potential (more neg)
what the potential is decreased by addition of solute
Pure water has a water potential of zero as there’s no tendency for water molecules to move into pure water
Solute potential
Measure of osmotic strength of a solution
Reduction in water potential due to presence of solute molecules
The more solid presence, the more tightly water molecules are held, the lower the tendency of water to move out. So a higher concentration solution has a lower, more negative solid potential
turgid
A plant cell that holds as much water as possible. feather entry of waters prevented as the cell wall cannot expand further
Pressure potential explain
Water entering a plant cell by osmosis expands the vacuum and pushes cytoplasm against the cell wall. Pressure outwards builds up, resisting entry of more water- turgid
Prussia is a pressure potential, and as it is a push outwards= positive sign
Water potential equation
water potential of cell= pressure P + solute P
Pressure potential definition
The hydrostatic pressure exerted by the cell contents on the cell wall. it is equal and opposite to the pressure exerted by the cell Wall on the cell
plasmosysis
Retraction of the cytoplasm and cell membrane from the cell wall as a cell loses water by osmosis
incipient plasmosysis
Cell membrane and cytoplasm are partially detached from the cell wall due to insufficient water to make cell turgid
hypotonic
if the water potential of external solution is LESS negative ( higher) than solution in cell, external solution is HYPOTONIC to the cell and water flows into the cell
hypertonic
if the water potential of external solution is MORE negative ( lower) than solution inside cell, external solution is HYPERTONIC to the cell and water flows out of the cell
isotonic
if the cell has the same water potential as the surrounding solution, the external solution and cell are isotonic and there is no net movement
plasmosyis process
Plant cells in a hypertonic solution lose water by osmosis. Process is plasmolysis and when complete the cell is flaccid - floppy
Plant wilts- cells can’t provide support
Incipient plasmolysis process
If the external concentration is high enough that the South has lost just enough water that its membrane begins to be pulled away from the cell wall, the cell is at incipient plasmolysis
Salworld does not exert any pressure on the cytoplasm so there is no pressure potential= 0
A plant cell in a hypotonic solution
Takes in water until prevented by the opposing pressure from cell wall
As water enters the cells, The contents expand and push out more on the cell wall, increasing the pressure potential.
The pressure potential rises until it is equal and opposite to the pull inwards of solute potential. No more water can enter and as there is no tendency for the cell to absorb water- water potential is 0
Pp= -Sp
significant of turgor to plants
Provide support, maintains their shape and holds them upright
Osmosis and animal cells
An animal cell has no cell wall and so pressure potential isnt considered
Water potential= solute potential
Haemolysis
if red blood cells are in distilled water, water enters by osmosis and without a cell wall they burst
Red blood cells in concentrated salt solution
Water leaves the cells and they shrink becoming crenated
Endocytosis
Active process of the cell membrane engulfing material, bringing it into the cell in a vesicle
Two types: phagocytosis and pinocytosis
Exocytosis
Active process of a vesicle fusing with the cell membrane, releasing the molecules it contains
Phagocytosis
Active process of the cell membrane engulf in large particles, bringing them into the cell in a vesicle
pinocytosis
Active process of the sound membrane engulfing droplets of fluid, bringing them into the cell of a vesicle
When endocytosis and exocytosis occur
Cell membrane has to change shape this requires energy. Processes are therefore active, using ATP, generated by the cells respiration.
In both cases the cell membrane flows. The property of fluidity of the cell membrane is essential for these processes to occur
