Membrane transport
When you drink a sports drink after a camogie match with your school team, glucose and ions have to get from your gut into your cells. Every substance that enters or leaves a cell does so by a specific mechanism, and that mechanism depends on three factors: the size of the substance, its electrical charge and whether it moves down or against its concentration gradient. By the end of this page you will be able to explain how that glucose gets in.
Simple diffusion: down the gradient
If a drop of ink is added to a glass of still water, the dye spreads spontaneously until it colours all the liquid evenly, without any need to stir it: why do dissolved particles always tend to spread out through all the space available without using any energy?
Distinguish between passive transport and active transport across the membrane, indicating which of them requires an input of ATP and in which direction substances move…
Distinguish between passive transport and active transport across the membrane, indicating which of them requires an input of ATP and in which direction substances move in each case.
What is simple diffusion? Give two examples of substances that enter or leave cells by this mechanism and explain what common characteristics they share.
What is simple diffusion? Give two examples of substances that enter or leave cells by this mechanism and explain what common characteristics they share.
Facilitated diffusion
Glucose enters most cells by facilitated diffusion. Explain what this mechanism consists of, which proteins are involved and how it differs from simple diffusion.
Glucose enters most cells by facilitated diffusion. Explain what this mechanism consists of, which proteins are involved and how it differs from simple diffusion.
Active transport: the sodium-potassium pump
In a resting neuron, the concentration of $\text{K}^+$ in the cytosol is about 20-30 times higher than outside the cell, while the concentration of $\text{Na}^+$ is about 10 times higher outside the cell: how do cells manage to maintain this imbalance constantly, against the natural physical tendency for concentrations to even out?
Without ATP the pumps stop. The accumulated ion (for example K⁺ in a root) then leaks out by passive diffusion through channels, down its gradient, until its concentrations inside and outside the cell are equal and the gradient is dissipated. This is why a poison such as DNP, which stops ATP being made, makes K⁺ uptake stop and the ion drain out of the root: the pumps were holding the gradient, and nothing holds it once they stop.
Observe the diagram of the sodium-potassium pump. State which ion corresponds to particles A and which to particles B, explain why this mechanism is considered active…
Observe the diagram of the sodium-potassium pump. State which ion corresponds to particles A and which to particles B, explain why this mechanism is considered active transport and what consequence the unequal exchange of ions has for the electrical charge of the membrane.
The uptake of $\text{K}^+$ in barley seedling roots in a hydroponic medium is analyzed. Upon adding dinitrophenol (DNP) to the culture—a substance that uncouples…
The uptake of $\text{K}^+$ in barley seedling roots in a hydroponic medium is analyzed. Upon adding dinitrophenol (DNP) to the culture—a substance that uncouples mitochondrial ATP synthesis without halting $\text{O}_2$ consumption—$\text{K}^+$ uptake stops immediately and the ion begins to slowly leak out of the root, so that the accumulated $\text{K}^+$ is progressively lost. Interpret these data by deducing the transport mechanism involved in $\text{K}^+$ uptake, justify why blocking ATP causes the dissipation of the accumulated electrochemical gradient, and predict what would happen to uptake if, in the absence of DNP, the temperature were lowered to $0\ \text{°C}$.
Ouabain is a drug that inhibits the $\text{Na}^+/\text{K}^+$ pump. Predict what will happen, in the medium term, to the sodium gradient of the cell and to the coupled…
Ouabain is a drug that inhibits the $\text{Na}^+/\text{K}^+$ pump. Predict what will happen, in the medium term, to the sodium gradient of the cell and to the coupled transport of glucose that uses this gradient in the intestine, and reason your answer.
Bulk transport: endocytosis and exocytosis
Receptor-mediated endocytosis in detail
In receptor-mediated endocytosis, receptor proteins in the cell membrane bind a specific ligand (for example LDL, which carries cholesterol). The cytosolic tail of each receptor binds adaptin, which recruits clathrin. The clathrin forms a coated pit that buds inwards as a coated vesicle, so the ligand is taken in. If the receptor lacks this tail, it still binds the ligand but cannot be internalised.
Explain what endocytosis and exocytosis are and give a specific biological example of each of these processes.
Explain what endocytosis and exocytosis are and give a specific biological example of each of these processes.
A white blood cell engulfs a bacterium to destroy it. What type of endocytosis does it carry out and what is the vesicle formed called? Briefly describe the process.
A white blood cell engulfs a bacterium to destroy it. What type of endocytosis does it carry out and what is the vesicle formed called? Briefly describe the process.
A patient suffers from a form of familial hypercholesterolemia due to a mutation in the LDL receptor gene. The mutated receptor is synthesized and inserted into the…
A patient suffers from a form of familial hypercholesterolemia due to a mutation in the LDL receptor gene. The mutated receptor is synthesized and inserted into the plasma membrane, where it is capable of binding circulating LDL normally, but lacks the cytosolic tail that interacts with adaptin and clathrin. As a consequence, cholesterol does not enter the cells and accumulates in the bloodstream, despite pinocytosis of extracellular solutes remaining intact. Justify what type of vesicular transport is blocked and why the cell cannot internalize the ligand, and reason whether a compensatory increase in pinocytosis would effectively compensate for this deficiency.
Electrochemical gradient and secondary active transport
For an ion, what matters is its electrochemical gradient: its concentration and the difference in electrical potential. Facilitated diffusion uses channels or transporters down that gradient. Active transport requires a source of energy, either direct or indirect.