4.1 - Cell Transport Mechanism Flashcards
(22 cards)
Explain the structure of the cell surface membrane with reference to the fluid mosaic model.
it’s a phospholipid bilayer, a partially permeable membrane composed of phospholipids with protein molecules between them. The components of the membrane are:
- Phospholipids,
have a hydrophobic head and a hydrophillic tail. - Cholesterol,
located between the phospholipids and phospholipid bilayer helps the plasma membrane to retain fluidity. - Proteins:
- Integral Proteins: main transport system of the membrane forming either permanent pores or other transport mechanisms like carrier proteins or active pumps
- Peripheral Proteins: may be enzymes, can be involved in regulating transport like by cell signalling
- Glycoproteins: are proteins with a carbohydrate part added to the molecule, they stabilize the membrane and are responsible for intercellular communication.
This model of the floating proteins in a lipid sea is known as the ‘Fluid Mosaic Model’
What is passive transport?
Transport of substances as a result of concentration, pressure or electrochemical gradients, and involving no energy from the cell
Explain diffusion (passive transport)
- movement of the particles in a liquid or gas down a concentration gradient from an area of high concentration to an area of low concentration, through a lipid bilayer.
- once they reach a uniform distribution, they do not stop moving but the movement no longer causes a net change in concentration as equal numbers are moving in all directions
What are the factors that affect the rate of diffusion?
- CONCENTRATION GRADIENT:
The rate of diffusion is higher where there is a more significant difference between the concentrations of the two regions - DISTANCE MOVED
Diffusion over a shorter distance occurs at a greater rate than over a larger distance - Surface area involved:
The larger the area across which diffusion occurs, the greater the diffusion rate. - BARRIERS TO DIFFUSION:
Thick barriers have a slower rate of diffusion than thin barriers - TEMPERATURE:
Particles at a higher temperature diffuse at a greater rate than at low temperatures as they have more kinetic energy
What molecules does diffusion transport?
lipid-soluble molecules like O2
Explain facilitated diffusion (passive transport)
- diffusion takes place through carrier proteins or protein channels.
- they transport polar, charged and water-soluble molecules across the membrane
- carrier proteins transport larger molecules like glucose or amino acids
- the protein channels transport ions
Explain the process of facilitated diffusion.
- Intrinsic globular proteins within the membrane function as carriers for transporting larger molecules like glucose
- These carriers are specific to the molecules they transport and ‘ping-pong’ them across the membrane
- These large molecules are taken up by the carrier proteins which causes them to change shape.
- For this shape change to happen they must have lots of kinetic energy to move the molecule they carry around
- After the larger molecule has passed through, the carrier protein returns to its original shape allowing for more molecules enter
Explain osmosis (passive process)
- movement of water down a concentration gradient through bilayer or protein pores, it is a specialised form of diffusion of water through the partially permeable membrane
Osmotic concentrations
Isotonic solution - the osmotic concentration of the solutes in the solution is the same as that in the cells
Hypotonic solution - osmotic concentration of solutes in solution is lower than that in the cytoplasm
Hypertonic solution - osmotic concentration of solutes in the solution is higher than that in the cytoplasm
What is osmotic concentration?
the measure of the conc of solutes in a solution that have an osmotic effect
Osmosis in animals
Osmosis needs to be controlled within animals, where the net movements of water in and out of the cell need to be kept at a minimum.
Too much water = cell to burst
Too little water = the cell to shrivel up = lose its internal structure and the chemical reactions that normally take place will stop moving
Osmosis in plants when placed in a hypotonic solution
- has a cellulose cell wall that prevent the cells from bursting
- when placed in a hypotonic solution to the cytoplasm of the cell water will enter the cell by osmosis
- causes the cell to swell and press on the cell wall which generates hydrostatic pressure
- then an inward pressure of the cell wall on the cytoplasm called the ‘pressure potential’ increases until it cancels out the tendency for more water molecules to move into the cell
- when the osmotic force moving water in the cell is balanced to the pressure potential forcing it out the plant is rigid, this state is ‘turgor’ which makes the plant more efficient at photosynthesis as leaves are horizontal and the stem vertical so facing towards the sunlight.
Osmosis in plants when placed in an isotonic solution
- since the solution has the same concentration as the protoplast of the plant cell it will also have the same water potential
- the membrane starts to pull away from the cell wall as the protoplasm shrinks, this is called ‘incipient plasmolysis’
- there is no net movement of water so no pressure potential as the protoplast ceases to press against the cell wall
Osmosis in plants when placed in a hypertonic solution
- A hypertonic solution is more concentrated than the protoplast of the plant cell, so has a lower water potential than the cell
- this means the net movement of the water by osmosis is out of the cell and the protoplast shrinks
the protoplast is completely pulled away from the cell wall and the cell is fully plasmolysed - this is irreversible as it completely damages the cell membrane
Why does the shape and size not change when fully turgid or fully plasmolysed?
This is due to cell wall keeps the structure consistent under any conditions therefore the shape and size do not change only the contents of the cell change
What is the equation to find water potential of cell?
water potential (tendency of water to leave the cell, usually 0 or negative) = turgor pressure (pressure potential, usually - or 0, the tendency for water to be forced out of the cell) + osmotic potential (solute potential, the effect of solute concentration on water potential, always - )
What molecules go through the lipid bilayer?
Hydrophobic molecules like O2 and small uncharged polar molecules like H2O go through
Large uncharged polar molecules like glucose and ions do not go through
Explain what active transport is.
Is the movement of molecules (or ions) from regions of low to high concentrations across a cellular membrane by the transport protein. This transportation requires energy and that energy comes from ATP
Explain the process of active transport.
- ATP binds to a transport protein
- A molecule or ion to be transported binds to the transport protein
- ATP is hydrolysed and the energy releases are used to transport the molecule or ion across the membrane, this is done by the protein changing shape which requires energy.
- The molecule or ion is released and the transport protein reverts to its previous state
What is the difference between primary and secondary active transport?
Primary directly uses ATP for the energy to transport molecules, whereas in secondary, energy is stored in a concentration gradient. The transport of one molecule is coupled to the movement of another down its concentration gradient so ATP is not directly involved.
What is endocytosis?
particles are enclosed in phagocytic vesicles formed by pinching off the cell surface membrane.
Lysosome-containing lytic enzymes fuse with the vesicle.
Enzymes are released into vesicles to break down bacteria.
Soluble products are absorbed into the cytoplasm. requires ATP
What is exocytosis?
Secretory vesicles containing secretory products fuse with the cell surface membrane to secrete secretory products out of the cell.
Requires ATP
