Membrane Transport Flashcards
Define the term diffusion.
The net movement of a substance from an area of high concentration to an area of low concentration. It is passive, does NOT require ATP.
Define the term facilitated diffusion.
Movement of molecules from a high concentration to a low concentration, across a partially permeable membrane, via specific channel or carrier proteins. It is passive, does NOT require ATP.
What is a channel protein?
A protein which creates a fluid filled pore in the cell membrane through which ions and small polar molecules can pass.
What is a carrier protein?
A protein which changes shape to allow larger molecules to pass through the membrane. In facilitated diffusion this requires no energy, in active transport it requires ATP.
What types of molecules can diffuse directly across the phospholipid bilayer?
Small molecules such as oxygen and carbon dioxide; lipid-soluble molecules such as steroid hormones and alcohol.
Describe the movement of water across the phospholipid bilayer.
Water is polar and insoluble in lipid, BUT because it is present in such high quantities, significant direct, simple diffusion does occur. In membranes (e.g. collecting duct) where a very high rate of water movement is required, special channel proteins known as aquaporins are inserted into the membrane.
State 5 factors that affect the rate of simple diffusion.
Temperature, surface area, diffusion distance, size of molecule, concentration gradient.
Define the term active transport.
Movement of molecules, against their concentration gradient (using energy liberated from ATP hydrolysis) using specific protein channels or carriers.
Describe how carrier proteins are used in active transport.
Molecule binds to specific site in carrier protein; ATP binds to separate binding site; carrier protein changes shape (conformational change) and transports molecule across membrane.
Define bulk transport and give two examples.
The movement of large molecules that are too big to pass across the plasma membrane. Endocytosis (phagocytosis or pinocytosis) brings large molecules INTO the cell, enclosed in a vesicle. Exocytosis transports large molecules OUT of cells.
Define and describe phagocytosis.
The intake of solid particles into the cell by engulfing. Pseudopodia surround the particles, the membrane fuses together, to form a vesicle.
Define and describe pinocytosis.
The intake of liquids into the cell by engulfing. The plasma membrane invaginates, and the membrane fuses around the substance, forming a vesicle.
Define and describe exocytosis.
The bulk transport of particles too large to pass through the membrane, out of the cell. It works like a reversal of pinocytosis. A vesicle containing the substance fuses with the plasma membrane. The fused site opens, releasing the contents of the secretory vesicle.
Describe the role of ATP in bulk transport.
ATP is required to provide energy for the movement of vesicles along microtubules of cytoskeleton (via motor proteins); ATP is needed to fuse vesicle membrane and plasma membrane together (PM is changing shape).
What is the main difference between active transport and facilitated diffusion?
Active transport requires ATP; facilitated diffusion is passive.
Define the term osmosis.
The movement of water molecules from a region of higher water potential to a region of lower water potential, across a partially permeable membrane.
What is water potential?
The tendency of water molecules to move from one region to another.
What substance has the highest possible water potential of 0 kPa?
Pure water.
State the equation that links water potential, solute potential and pressure potential.
WP = SP + PP.
As more solute is added to a solution, what happens to the solute potential and hence the water potential?
Solute potential decreases, hence water potential decreases.
Describe what would happen to a red blood cell placed in a solution with a more negative water potential than that of its cytoplasm.
Water would move OUT, down a WP gradient, shrinking to crenation.
Describe what would happen to a liver cell placed in a solution with a more positive WP than its own cytoplasm.
Water would move IN, down a WP gradient, swelling the cell and causing it to burst (CYTOLYSIS) due to the presence of no cell wall.
Describe what would happen to a root hair cell placed in a solution with lower WP than its own cytoplasm/vacuole.
Water would move OUT, down a WP gradient, causing the cell to become flaccid and then plasmolysed.
Describe what would happen to a guard cell placed in a solution of less negative WP than its own cytoplasm.
Water would move IN, down a WP gradient, causing the cell to swell and become TURGID.
