2.3 Transport across cell membrane Flashcards
What model do all membranes have?
Fluid-mosaic
What are the major components of the cell membrane?
- Phospholipid
- Bilayer
- Channel Protein
- Carrier Protein
- Glycoprotein
- Glycolipid
- Cholesterol
Describe the arrangement of phospholipid molecules in the phospholipid bilayer.
- Two layers of phospholipids
- Hydrophilic heads face outside
- Hydrophobic tails face inside
Describe and explain the advantages of the phospholipid bilayer in the cell membrane.
- Selectively-permeable barrier
- Only non-polar molecules are able to pass through via simple diffusion
- Bilayer is fluid
- Can bend to take up different shapes, forming vesicles
Describe and explain the advantages of channel proteins in the cell membrane.
- Protein through bilayers
- Allows water soluble substances through via facilitated diffusion
Describe and explain the advantages of carrier proteins in the cell membrane.
- Protein through bilayer
- Allows substances to move through via facilitated diffusion and active transport
Explain the advantages of glycoproteins in the cell membrane.
- Cell recognition and signalling
- Cell attachment
Explain the advantages of glycolipids in the cell membrane.
- Cell recognition and signalling
- Cell attachment
What is a glycolipid?
Phospholipid with short carbohydrate chain
What is a glycoprotein?
Protein with short carbohydrate chain
Describe and explain the advantages of cholesterol in the cell membrane.
Regulates membrane fluidity
What is simple diffusion?
Movement of molecules along concentration gradient from higher concentration to lower concentration
Is simple diffusion active or passive?
- passive
- no ATP hydrolysed
What does simple diffusion occur through?
Diffuse through phospholipid bilayer
What type of molecule can move through the membrane by simple diffusion?
non-polar
What is facilitated diffusion?
Movement of molecules along concentration gradient from higher concentration to lower concentration
Is facilitated diffusion active or passive?
- passive
- no ATP hydrolysed
What do the molecules move through in facilitated diffusion?
Channel AND carrier proteins
What type of molecule can move through the membrane in facilitated diffusion?
polar/charged
What is osmosis?
Movement of water along a water potential gradient from an area of high water potential to an area of low water potential
What is osmosis regarding solute concentration?
Low to high solute concentration
Is osmosis passive or active?
- passive
- no ATP hydrolysed
What do the molecules move through in osmosis?
Water channel proteins (aquaporins)
What does the cell do when water moves IN by osmosis?
turgid
What happens to the cytoplasm when water moves in by osmosis?
DOESN’T shrink from cell wall
What solution causes water to move IN by osmosis?
hypotonic
Water IN by osmosis - Water potential of cell and solution
Solution has higher water potential than cell
Water OUT by osmosis - Cell appearance
Plasmolysed
Water OUT by osmosis : Cytoplasm
Shrunk from cell wall
Water OUT by osmosis : Cell in what solution
Hypertonic
Water OUT by osmosis : Water potential of cell and solution
Solution has a lower water potential than the cell
Water potential of pure water:
0kPa
What is the water potential of a solution MORE concentrated than pure water?
LESS than 0kPa
What is the relationship between water potential and solute concentration?
More concentrated solute = lower water potential
What is active transport?
Movement of molecules against concentration gradient from lower concentration to higher concentration
Is active transport active or passive?
- Active
- Requires ATP hydrolysis
How does active transport occur?
- Through carrier protein
- Protein changes shape, as substrate binds
What occurs when a substrate binds to a carrier protein?
the protein changes shape
Describe exocytosis.
Fusion of a vesicle with the cell-surface membrane
Describe endocytosis.
Engulfment of molecules by cell-surface membrane to form vesicles
State the ways in which cells adapt to increase the rate of transport across a membrane.
- Increased number of channel and carrier proteins
- Folded membrane
- Increased number of mitochondria
Describe how increasing the number of channel/carrier proteins increases the rate of transport across a membrane.
More facilitated diffusion / active transport
Describe how a folded membrane increases the rate of transport across a membrane.
Increases surface area
Describe how an increased number of mitochondria increases the rate of transport across a membrane.
Supply more ATP for active transport
Describe two features you would expect to find in a cell specialised for absorption.
- Folded membrane so large surface area for absorption
- Large number of co-transport/carrier/channel proteins so fast rate of absorption
Why is the structure of a plasma membrane described as a fluid mosaic?
- Fluid = molecules move around
- Mosaic = proteins floating among phospholipids
Give two functions of proteins in plasma membranes.
- receptors
- carriers
- channels
Explain how three features of a plasma membrane adapt it for its functions.
- phospholipid bilayer forms a barrier to water soluble / charged substances
- bilayer is fluid, so can bend to take up different shapes for phagocytosis / form vesicles
- channel proteins let water soluble / charged substances through - facilitated diffusion
Explain how sodium ions are transported through the membranes. (2)
active transport;
by specific carrier proteins/pumps;
Explain how three features of a plasma membrane adapt it for its functions. (6)
phospholipid bilayer (as a barrier);
forms a barrier to water soluble
allows non-polar substances to pass
bilayer is fluid;
can bend to take up different shapes for phagocytosis / form vesicles / self repair;
channel proteins (through the bilayer)/intrinsic protein;
let water soluble/charged substances through / facilitated diffusion;
carrier proteins (through the bilayer);
allow facilitated diffusion / active transport;
surface proteins / extrinsic proteins, glycoproteins / glycolipids;
cell recognition / act as antigens / receptors;
cholesterol;
regulates fluidity / increases stability;
Cholesterol function
regulates membrane fluidity/ stability
less fluid at higher temps
regulates lateral movement of phospholipids
Many different substances enter and leave a cell by crossing its cell surface membrane. Describe how substances can cross a cell surface membrane.
- (Simple / facilitated) diffusion from high to low concentration / down concentration gradient;
Q Do not allow across / along / with concentration gradient - Small / non-polar / lipid-soluble molecules pass via phospholipids / bilayer;
Reject: named molecule passing through membrane by an incorrect route
Accept: diagrams if annotated
OR
Large / polar / water-soluble molecules go through proteins; - Water moves by osmosis / from high water potential to low water potential / from less to more negative water potential;
- Active transport is movement from low to high concentration / against concentration gradient;
Only penalise once if active transport is not named
e.g. ‘movement against the concentration gradient involves proteins and requires ATP’ = 2 marks - Active transport / facilitated diffusion involves proteins / carriers;
Accept: facilitated diffusion involves channels
Reject: active transport involves channels - Active transport requires energy / ATP;
- Ref. to Na+ / glucose co-transport;
against concentration
Active transport
Contrast the processes of facilitated diffusion and active transport. (3 marks)
Facilitated diffusion occurs down a concentration gradient but active transport occurs against a concentration gradient.
Active transport is active so requires ATP but facilitated diffusion is passive so does not require ATP.
Facilitated diffusion used both carrier proteins and protein channels but active transport only uses carrier proteins.
contrasport brief
1- cotransport protein
2- carrier protein used
2- NA + conc kept low
3- Sodium potassium pump
Atp energy
Active transport of NA + into blood
glucose facilitated diffusion into blood
Describe the process of the co-transport of glucose and sodium ions.
Sodium ions are actively transported out of epithelial cells into the blood via a Na+/K+ pump.
This maintains a lower concentration of sodium ions in the cell than in the ileum.
Sodium ions then diffuse into cells down a concentration gradient from the ileum through the co-transport protein.
Glucose is carried into the cells with the sodium ions via the co-transport protein.
Sodium ions move down a concentration gradient but glucose molecule move against a concentration gradient.
Glucose molecules move into the blood via facilitated diffusion (high concentration in epithelial cell to low concentration in blood).
The addition of a respiratory inhibitor stops the absorption of amino acids. Use the diagram about cotransport to explain why?
less ATP
NO active transport
Sodium not moved into cell
NO diffusion gradient for sodium
NO concentration gradient for sodium
Glucose is absorbed from the lumen of the small intestine into epithelial cells.
Explain how the transport of sodium ions is involved in the absorption of glucose by epithelial cells.
- Na+ ions leave epithelial cell and enter blood;
Penalise for Na without ions once.
- (Transport out is by) active transport / pump / via carrier protein using ATP;
Reject channel protein
- So, Na+ conc. in cell is lower than in lumen (of gut);
Maintains diffusion gradient for Na+ from lumen/into cells;
- Sodium/Na+ ions enter by facilitated diffusion;
Accept diffusion/from high to low concentration through a symport/cotransport protein
- Glucose absorbed with Na+ ions against their concentration/diffusion gradient / glucose absorbed down an electrochemical gradient;
