Transport Across Membranes Flashcards
Explain the orientation of phospholipids in the membrane
The phosphate group has a negative charge, so hydrophilic
So orientate outwards towards other polar molecules such as aqueous cytoplasm or extracellular fluid
Hydrophobic fatty acid tail orientated toward the middle away from water
Forming a self sealing double membrane (bilayer)
Why is it called the fluid mosaic model
Model = representation of a structure Fluid = phospholipid bilayer is constantly moving Mosaic = proteins unevenly distributed throughout so look like a mosaic
What does selectively permeable mean
Only allows the passage of certain substances
What determines selective permeability
The type, number and distribution of specific proteins in the membrane
What is the function of the phospholipids in membrane
Allows lipid soluble substances to enter/leave cell
Prevents water soluble substances entering/leaving cell
Makes the membrane flexible and self sealing
What is the function of the phospholipid bilayer
Keeps organelles in cell
Protects organelles
Controls what enters/leaves cell
Transports toxic substances out of cell
What is found in the cell membrane
Proteins (carrier/channel) Phospholipids Glycoproteins Glycolipids Cholesterol Receptor molecules Aquaporins Enzymes
Cholesterol function in membrane
Decreases permeability and increases stability
More = less fluid
~25% (20-30 in most cells)
Different cells have different amounts because different functions
Channel proteins in cell membrane
Pore like structures
Only allow specific charged ions or small molecules to move across membrane by facilitated diffusion
Open or closed
Intrinsic
Proteins so have specific tertiary structure so complementary to one type of molecule so only transport one type of molecule
Channel proteins transport…
Small molecules
Charged ions
By facilitated diffusion
Carrier protein function in cell membrane
Aid transport of ions/polar molecules and large molecules like glucose and amino acids
By active transport of facilitated diffusion
Proteins so have specific tertiary structure so complementary to one type of molecule so only transport one type of molecule
Carrier proteins transport…
Ions/polar molecules
Large molecules
By active transport or facilitated diffusion
Receptor protein function in cell membrane
Proteins so have specific tertiary structure so complementary to one type of molecule so only one type of molecule can bind
Specific cells have specific receptors based on function
Hormones can bind to receptors
Allows the cell to respond (eg by increasing permeability)
Enzyme function in cell membrane
Active site complementary to substrate so can only form an ESC with one that type of molecule
Embedded within membrane
Glycoprotein function in cell membrane
Carbohydrates and proteins on the surface of the cell membrane important for cell recognition
Often act as antigens
Immune cells detect specific shapes of glycoproteins to identify whether self/non-self
Produced in the golgi body by the cell displaying them
All cells have them
Glycolipid function in cell membrane
Carbohydrates and lipids on the surface of the cell membrane important for cell recognition
Often act as antigens
Immune cells detect specific shapes of glycolipids to identify whether self/non-self
Produced in the golgi body by the cell displaying them
All cells have them
Types of transport across membranes
Diffusion Facilitated diffusion Active transport Osmosis Bulk transport (exocytosis/endocytosis)
How can proteins be integrated into phospholipids
Proteins are charged in parts and so attracted to hydrophilic phosphate head and aqueous solutions in/out of cell
Non-polar parts associate with hydrophobic fatty acid tails
What is diffusion
A passive process
Not requiring ATP
The net movement of a substance from a higher concentration to a lower concentration across a partially permeable membrane
Diffusion transports…
Small, lipid soluble molecules
Oxygen, Carbon Dioxide, Oestrogen
When will net diffusion stop
When number of molecules either side of a partially permeable membrane reaches equilibrium
Equal number on either side
Net stays 0 but diffusion can still occur
Ficks Law
Rate of diffusion = (concentration gradient x surface area)/diffusion distance
Factors that affect the rate of diffusion
Temperature
Surface area
Concentration gradient
Diffusion distance
How does temperature affect rate of diffusion
Increased temperature means increased kinetic energy of molecules
Faster rate of diffusion
How does surface area affect the rate of diffusion
Larger surface area means more space for molecules to pass through
So more can pass through at once
More space for proteins for active transport or facilitated diffusion
Microvilli
How does concentration gradient affect the rate of diffusion
As difference increases so does the rate of diffusion
How does diffusion distance affect the rate of diffusion
Shorter means faster because shorter distance to diffuse through
Time to cross membrane decreases
Determined by length of fatty acid tails
What is facilitated diffusion
Passive process
Does not require ATP
Uses channel or carrier proteins to transport water soluble, charged or large molecules that are unable to pass between the hydrophobic fatty acid tails across the membrane
Transports molecules from an area of higher concentration to an area of lower concentration across a partially permeable membrane
Facilitated diffusion transports…
Water soluble molecules
Charged molecules
Large molecules
Why can’t hydrophilic substances pass through membrane
They can’t pass through the hydrophobic tails of phospholipid bilayer
Because the fatty acid tails repel them due to being non-polar
When does facilitated diffusion stop
When all carrier proteins are saturated (binding sites all occupied)
Substance concentration has reached equilibrium (isotonic)
What is osmosis
The net movement of water from an area of higher water potential to an area of lower water potential
Across a selectively permeable membrane
Via aquaporins
What is water potential
Free water molecules are able to move around and collide with membrane
Exerting a pressure on it
This is known as water potential in kPa
More free molecules means more collisions and so greater potential and higher water potential
Explain a lower water potential
More solute dissolved in water, more negative the water potential
Because water is a dipole molecule its attracted to the solutes molecules charges and so the number of free moving water molecules decreases
Explain higher water potential
Less solute dissolved in water, less negative water potential
More free moving water molecules so more collisions and more pressure means higher water potential
Explain pure water
0kPa
Nothing dissolved in it and so every water molecule is free moving
Highest water potential
What happens when water potential is higher than in a cell
Water moves in to cell via osmosis down a water potential gradient
Causing swelling and osmotic lysis
Destroying cells contents
Cellulose cell wall prevents lysis in plants
What happens when water potential is lower than in a cell
Water moves out of cell via osmosis down a water potential gradient
Animals = crenation = shrivelling
Plant = plasmolysed = cell membrane pulls away from cell wall
Describe isotonic
No net movement of water in or out of cell
No water potential gradient so no gain or loss in mass
What is active transport
Transport of molecules across a membrane against a concentration gradient from an area of lower concentration to an area of higher concentration
By carrier proteins with specific complementary shapes
Active process so requires ATP
How is ATP used in active transport
Carrier proteins require energy source, supplied by ATP which is produced during aerobic respiration via condensation reaction
The hydrolysis of ATP into ADP and Pi provides a small amount of energy/phosphorylates the carrier protein
Causing a conformational change in shape so it can transport the molecule into an area of higher concentration against a gradient
Explain exocytosis
Method of bulk transport that uses golgi vesicle to move very large quantities of molecule from inside to outside of cell
Used to move enzymes and glycoproteins from the golgi apparatus to the cell surface membrane to secrete proteins
ATP is required to move the vesicles to the cell surface
Vesicle fuses with cell membrane
Why is ATP required in exocytosis
Requires the energy provided by breakdown of ATP into ADP and Pi to move the vesicles to the cell surface membrane so it can fuse with the membrane
Explain endocytosis
Method of bulk transport in which the cell surface membrane is pulled inwards to create a vesicle
Any molecules next to that part of cell membrane are enclosed in the vesicle
Requires breakdown of ATP into ADP and Pi
Part of reason why membrane must be flexible
Why do cells using active transport have lots of mitochondria
Mitochondria are the site of aerobic respiration and therefore ATP production
Active transport needs the small amount of energy and inorganic phosphate group (Pi) produced from the breakdown of ATP during aerobic respiration
So there is enough energy and Pi for the carrier protein
What happens if the tertiary structure of a channel/carrier protein changes
The binding site will change shape No longer complementary to its molecule Won't bind No ESC Can't be transported and so facilitated diffusion or active transport can't occur
How does a respiratory inhibitor affect active transport
A respiratory inhibitor prevents mitochondria from making ATP
When the inhibitor is added, the rate of uptake will decrease rapidly, indicating energy is needed
Graph will also plateau when all carrier proteins saturated
How does a respiratory inhibitor affect diffusion
As concentration gradient across membrane increases so does the rate of uptake
Uptake not affected by respiratory inhibitor because does not require energy provided by ATP since it is a passive process
Curve won’t plateau because no proteins to be saturated and concentration keeps increasing
How does a respiratory inhibitor affect facilitated diffusion
Prevents mitochondria making ATP
When inhibitor added no change on rate of uptake since its passive
Curve plateaus because all carrier proteins saturated
Define intrinsic and extrinsic
Intrinsic = spans both sides of membrane Extrinsic = spans only one side of the membrane
