3. Cell Membranes and Transport Flashcards
phospholipid
describe phospholipid head
charged
hydrophillic
micelle
how are phospholipids held together in bilayers
weak hydrophobic interactions between the hydrocarbon tails allowing some membrane fluidity
Fluid Mosaic Model of Membrane Structure
Singer and Nicholson - 1972
-fluid, components of the membrane, particularly phospholipids can move between the two layers of membrane
-mosaic, looked at freeze fracture electron microscopy
-cell membrane is fractured along the line of least resistance – the centre of the bilayer. The images produced have a speckled effect caused by the proteins exposed, looked like a mosaic
why is it called the fluid mosaic model 2 marks
mosaic of protein molecules, randomly arranged
lipid layer is fluid - can move
fluid mosaic model membrane diagram
two types of membrane proteins
extrinsic
intrinsic
extrinsic proteins
on or in one side of the bilayer
eg receptors
intrinsic proteins
runs through both sides of the bilayer
eg transport proteins
eg of intrinsic protein and where it is found
ATP synthase
embedded on cristae of mitochondria
diagram of intrinsic and extrinsic proteins
characteristics of intrinsic proteins
hydrophilic and hydrophobic parts
span the membrane
some are channel proteins
channel protein
have hydrophilic R groups lining the space down the middle of the protein. These channels are specific to particular charged or polar particles and allow them to diffuse across the membrane in facilitated diffusion.
have hydrophilic groups lining the pore
protein carriers
involved in active transport and facilitated diffusion across the membrane; these are specific to the polar or charged molecules carrier
channel and carrier
cell membranes
selectively permeable
hydrophobic layer in middle of bilayer is impermeable to polar and charged particles
function of cell membrane
Taking up nutrients/other requirements/reference to selective permeability;
phagocytosis/secreting chemicals; cell recognition;
adhesion; receptor sites.
How do non polar molecules get through cell membrane
Non-polar molecules like oxygen, carbon dioxide and fat-soluble vitamins (A, D, E and K) dissolve in the hydrophobic layer and can cross the membrane by simple diffusion.
glycoprotein
These are proteins with carbohydrate chains attached
These carbohydrate chains also project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)
glycolipid
These are lipids with carbohydrate chains attached
These carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)
glycocalyx
All of the carbohydrates projecting outside of the cell
glycocalyx is mainly involved in cell to cell recognition
only found on the side of the bilayer that faces out of the cell
act as antigens in cell recognition
cell membrane - carbohydrates
act as specific antigens.
-An individual’s specific antigens are unique to themselves
- are involved in cell recognition and signaling, playing a critical role in immune responses
-If the carbohydrate structures on a cell are recognized as foreign, they trigger an immune response.
providing energy needed by the cell to survive
cell membrane - hydrophobic region, molecules of cholesterol
regulate the fluidity of the membrane and are found between fatty acid tails.
If there is not enough cholesterol, the membrane becomes too fluid
if there is too much cholesterol, the membrane becomes too rigid.
label
A - Hydrophilic heads
B - Hydrophobic tail
C - Intrinsic protein
D - Oligosaccharide/ glycoprotein
explain at temp below 40 degrees celcius effect on pigment in cell
the increased kinetic energy of the phospholipids and proteins in the membrane causes them to move more. This creates gaps between the phospholipids. The pigment will also have more kinetic energy so will move more and diffuse out of the cell into the water through these gaps.
explain at temp above 40 degrees celcius effect on pigment in cell
protein components of the membrane begin to denature, forming pores through which the pigment can diffuse out more easily. The graph will level off as all proteins denature and the cell membrane becomes fully permeable to the pigment.
is diffusion active or passive
passive
what does passive mean?
ATP/energy from respiration is not required
Respiratory inhibitors that inhibit ATP production do not affect passive transport methods.
In passive transport, movement is from high concentration to low concentration.
diffusion
passive
the random movement of particles from a region of their high concentration to a region of their low concentration
vis phospholipid bilayer
transports -
small, non-polar or lipid-soluble molecules
factors affecting rate of diffusion
- surface area of the membrane
- length of diffusion pathway
- steepness of concentration gradient
- temperature
- membrane permeability
diffusion -
surface area of the membrane
A higher surface area gives more places over which diffusion can happen and increases the rate of diffusion. Folds in the cell membrane increase the surface area.
diffusion -
length of diffusion pathway
the shorter the diffusion pathway, the faster diffusion can happen. Flattened cells, thinner membranes and less layers of cells decrease diffusion pathways.
diffusion -
steepness of concentration gradient
the larger the difference between the high and low concentrations, the faster diffusion happens. Circulation and ventilation increase concentration gradients.
diffusion -
temperature
the higher the temperature, the more kinetic energy particles have and the faster they move. This increases the rate of diffusion.
diffusion -
membrane permeability
is affected by salt concentration, presence of detergents and organic solvents.
diffusion -
lipid solubility
more lipid soluble a molecule is
faster it will diffuse across a membrane
facilitated diffusion
passive process
uses specific protein carriers or channels
transports - Charged particles and polar molecules, large molecules (too big to cross via simple) accross the membrane
high conce to low conc
eg glucose and protons
co-transport
two different particles are transported through the same protein at the same time
via a carrier protein
eg of co transport
glucose and Na+, which are transported into cells together.
protein carriers and facilitated diffusion
rate of facilitated diffusion depends on number of protein carriers
factors affecting facilitated diffusion
temperature
concentration gradient
number of protein carriers
osmosis
net movement of water from an area of high water potential to an area of low water potential across a selectively permeable membrane
passive process (requires no energy from ATP is unaffected by ATP inhibitors)
define water potential
Water potential is defined as the tendency of water to leave a system by osmosis.
The difference between the free energy of water molecules in a system and the free energy of molecules in pure water/the tendency for water molecules to leave/move out of a system.
what does water potential depend on
solute concentration (solute potential)
and
the pressure exerted on the solution (pressure potential).
highest water potential
and unit of water potential
0
kPa
hypotonic solution
lower solute concentration and therefore a higher water potential
red blood cell
lysis
lack cell wall, haemolysis occurs, red pigment released (haemoglobin), remove debris, measure with colorimeter
isotonic solution
same concentration of solute and the same water potential
hypertonic solution
higher concentration of solute and a lower water potential
crenation - water moves out of the cells
hypertonic solution
effect on
pressure potential
The vacuole shrinks,
The cytoplasm pulls away from the cell wall (a process called plasmolysis),
The cell becomes flaccid, and the pressure potential becomes zero, as there is no internal pressure exerted on the cell wall.
The relationship between the solute, water and pressure potentials can be expressed by the equation:
Ψw = Ψs + Ψp
(pressure potential is a positive number, solute and water potentials are negative)
Incipient Plasmolysis
Water potential, solute potential and bathing soultion relationship
Explain how this pressure potential is built up in cell X. [3]
water passes into cell by osmosis; cytoplasm expands;
cell becomes turgid;
as cytoplasm/contents push against wall;
wall inelastic/resists further expansion. (not: rigid) [3 max]
turgid
water potential
is 0
solute potential and pressure potential cancel out
plasmolysis
The vacuole and cytoplasm shrink because of the loss of water and the cytoplasm pulls away from the cell wall.
When cells are plasmolysed the pressure potential is 0 kPa; the water potential and solute potential of the cells are equal.
incipient plasmolysis
The point where 50% of the cells are plasmolysed is taken to be the point where the water potentials of solution and tissue are equal.
Wp = 0
crenation vs plasmolysis
Crenation occurs in animal cells, whereas plasmolysis occurs in plant cells.
lysis
only occurs in animal cells - burst
plant cells - surrounded by a rigid cell wall that can withstand the turgor pressure of the turgid cell contents.
Turgid means cells or tissues that are swollen due to water uptake.
Diagram of turgid and plasmolysed cells
Active transport
moves substances from low to high concentrations (against a concentration gradient) using specific protein carriers and energy from ATP.
factors affecting active transport
Respiration rate - Aerobic respiration uses oxygen. The more oxygen that is available, the faster the rate of respiration and active transport.
Temperature can also affect respiration rate - increased temperatures can result in higher rates of active transport.
Cyanide is a respiratory inhibitor - that binds to cytochrome in the electron transport chain, preventing ATP production. As a result, cyanide prevents active transport. Passive transport is unaffected by cyanide.
bulk transport
movement of solids or liquids
endocytosis
type of bulk transport
Two Types:
phagocytosis is the movement of solids – like cells or large proteins;
pinocytosis is the movement of liquids.
cell membrane extends around the particles being taken into the cell.
Eventually the particles are surrounded and the membrane of the cell fuses around the particle,
forming a vesicle in the cytoplasm containing the particle.
exocytosis
mechanism by which large particles such as enzymes and hormones are secreted from cells.
The vesicle containing the particles migrates to the cell membrane and fuses with it,
releasing the particles to the outside of the cell.
