transport across cell membranes Flashcards
Describe the fluid mosaic model of membranes
Fluid: phospholipid bilayer in which individual phospholipids can move so is flexible
Mosaic: extrinsic and intrinsic proteins of different sizes and shapes are embedded
Explain the role of cholesterol and glycolipids in membranes
- Cholesterol: connects phospholipids and reduces fluidity to make bilayer more stable
- Glycolipids: cell signalling and cell recognition
Explain the function of extrinsic proteins in membranes
- Binding sites/receptors
- Antigens
- Bind cells together
- Involved in cell signalling
Explain the function of intrinsic proteins in membranes
- Electron carriers
- Channel proteins
- Carrier proteins
Explain the functions of membranes within cells
- Provide an internal transport system
- Selectively permeable to regulate the passage of molecules into/out of organelles
- Provide reaction surface
- Isolate organelles from cytoplasm for specific metabolic reactions
Explain the functions of the cell surface membrane
- Isolates cytoplasm from the extracellular environment
- Selectively permeable to regulate transport of substances
- Involved in cell signalling/recognition
Name and explain 3 factors that affect membrane permeability
- Temperature: high temperature denatures membrane proteins, phospholipid molecules have more kinetic energy and move further apart
- pH changes tertiary structure of membrane proteins
- Solvent may dissolve membrane
Describe when membranes become less fluid
- An increased proportion of saturated fatty acid chains as the chains pack together tightly and therefore there is a high number of intermolecular forces between the chains
- A lower temperature as the molecules have less energy and therefore are not moving as freely which causes the structure to be more closely packed
Describe when membranes become more fluid
- An increased proportion of unsaturated fatty acid chains as these chains are bent, which means the chains are less tightly packed together and there are fewer intermolecular forces
- At higher temperatures, the molecules have more energy and therefore move more freely, which increases membrane fluidity
Define osmosis
Water diffuses across semi-permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established
Explain water potential in a dilute solution
- Low concentration of solute
- High concentration of free water
- High water potential
Describe water potential in a concentrated solution
- High concentration of solute
- Low concentration of free water
- Low water potential
What is the water potential of pure water?
0 kPa
Explain how osmosis can negatively affect plant and animal cells
Osmosis into the cell:
- Plant = swells, cell turns turgid
- Animal = cell lysis (bursts)
Osmosis out of the cell:
- Plant = shrinks, flaccid
- Animal = shrinks, dies
Define simple diffusion
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration.
What type of process is simple diffusion and why?
Diffusion is a passive process because it doesn’t require ATP from energy.
Define facilitated diffusion
The net movement, as a result of the random motion of its molecules or ions, of a substance from a region of its higher concentration to a region of its lower concentration via specific proteins in the phospholipid bilayer
Explain how to channel protein work
- Hydrophilic channels bind to specific ions
- One side of the protein closes and the other opens
Explain how carrier protein work
- Binds to complementary molecule
- Conformational change releases molecules on other side of membrane
Name 5 factors that affect the rate of diffusion
- Temperature
- Diffusion distance
- Surface area
- Size of molecule
- Concentration gradient
State Fick’s Law
Rate of diffusion = (surface area x concentration difference) / diffusion distance
How are cells adapted to maximise the rate of transport across their membranes?
- Many carrier/channel protein
- Folded membrane increases the surface area
Explain how the concentration gradient affects the rate of diffusion
A greater difference in concentration means a greater difference in the number of molecules passing in the two directions and therefore a faster rate of diffusion.
Explain how temperature affects the rate of diffusion
Molecules and ions have more kinetic energy at higher temperatures therefore they will move faster, resulting in a higher rate of diffusion.
Explain how surface area affects the rate of diffusion
The greater the surface area across which diffusion is taking place, the greater the number of molecules or ions that can cross it at any moment and therefore the faster diffusion occurs.
Describe how the properties of molecules or ions affect the rate of diffusion
- Large molecules diffuse more slowly than smaller ones as they require more energy to move
- Uncharged and non-polar molecules diffuse directly through the phospholipid bilayer
- Non-polar molecules diffuse more quickly than polar ones as they are soluble in the non-polar phospholipid bilayer
Define active transport
The movement of molecules/ions into or out of a cell from a region of lower concentration to higher concentration using ATP and carrier proteins
Compare and contrast active transport and facilitated diffusion
- Both involve carrier protein
- Active transport requires energy from ATP hydrolysis, facilitated diffusion is a passive process
- Facilitated diffusion may also involve channel protein
Define co-transport
The movement of a substance against its concentration gradient is coupled with the movement of another substance down its concentration gradient
Explain how co-transport is involved in the absorption of glucose AND/OR amino acids in the small intestine
(glucose diffuses from the lumen of the gut into the ileum, then into a capillary)
- Na+ is actively transported out of epithelial cells and into the bloodstream
- Na+ concentration is lower in epithelial cells than in the lumen of the gut, creating diffusion gradient of Na+
- Transport of glucose/amino acids from the lumen to epithelial cells is coupled to the facilitated diffusion of Na+ down the concentration gradient
How does the cell get energy for active transport?
The energy required is provided by ATP (adenosine triphosphate) produced during respiration. The ATP is hydrolysed to release energy
Why is energy required in active transport?
The energy is required to make the carrier proteinchange shape, allowing it to transfer the molecules or ions across the cell membrane
Define endocytosis
Type of active transport that moves particles into a cell by enclosing them in a vesicle made out of plasma membrane
Define exocytosis
Cells transporting signalling chemicals or waste products from the inside of the cell to the outside of the cell in vesicles that fuse with the plasma membrane
Why is active transport a selective process
It’s a selective process because molecules have the right shape and size to fit into the tertiary structure of a carrier protein
