4.1 - Cell transport and mechanisms Flashcards
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:
1. Phospholipids, have a hydrophobic head and a hydrophobic tail.
2. Cholesterol, located between the phospholipids and phospholipid bilayer helps the plasma membrane to retain fluidity.
3. 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?
It is the transport of substances as a result of concentration, pressure or electrochemical gradients and involves no energy from the cell
Diffusion (passive transport)
It is the 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 have reached a uniform distribution, they do not stop moving but the movement no longer causes a net change in concentration because equal numbers are moving in all directions.
The factors that affect the rate of diffusion
- Concentration gradient: the rate of diffusion is higher when there is a greater difference between the concentrations of two regions
- The 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 rate of diffusion
- Barriers to diffusion: thick barriers have a slower rate of diffusion than thick barriers
- Temperature: Particles at a high 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
Facilitated diffusion
Diffusion that 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 channel transports ions.
Explain the process of facilitated diffusion. (carrier proteins)
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 carrier proteins which results in them changing shape, for this to happen they must have a lot of kinetic energy to move the molecule they carry around. After the large molecule has passed through, the carrier protein returns to its original shape allowing more molecules to enter.
What is osmosis?
It is the movement of water down a concentration gradient through bilayer or protein pores, it is a specialised form of diffusion of water through a partially permeable membrane
Do the water particles stop moving once they reach equilibrium?
Once equilibrium is reached diffusion stops but the movement continues at an equal rate
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 = cell to shrivel up = loses its internal structure and the chemical reactions that normally take place will stop moving
Osmosis in plants when placed in an 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 cell is balanced to the pressure potential forcing it our the plant is rigid, this state is ‘turgor’ which makes the plant more efficient at photosynthesis as leaves are horizontal and 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 places in a hypertonic solution
- hypertonic solution is one that 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.
