After a soccer game in the sun, your cells keep absorbing glucose, getting rid of CO₂, and keeping ions where they belong. 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 electric charge, and whether it moves with or against its concentration gradient.
Simple diffusion: moving down the gradient
If you put a drop of ink into a glass of still water, the dye spreads until it colors all the liquid, without you stirring it. Why do dissolved particles tend to spread throughout all the available space without using energy? Because they move randomly, and from where there are many of them toward where there are few, more leave than come back.
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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 is specific (each transporter recognizes its molecule) and saturable: if all the proteins are occupied, the rate no longer increases even if there is more glucose outside. Simple diffusion has no such limits.
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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.
In a resting neuron, the concentration of $\mathrm{K^+}$ inside is about 30 times higher than outside, and the concentration of $\mathrm{Na^+}$ is about 10 times higher outside. How do cells maintain this imbalance despite the natural tendency toward equalizing?
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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.
How the vesicle forms in receptor-mediated endocytosis
Receptors (such as the LDL receptor) span the membrane. On the outside they bind to their molecule; on the inside they have a cytosolic tail. This tail binds to adaptor proteins, the adaptins, and these recruit clathrin, which assembles like a basket on the membrane. The basket deforms the membrane until it closes into a clathrin-coated vesicle, which pinches off and carries its cargo inside.
If the tail is missing, the receptor still binds its molecule, but it cannot recruit adaptin or clathrin: no vesicle forms and the substance does not enter. General pinocytosis does not need that receptor, so it can keep working.
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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: the sum of the difference in concentration and the difference in electric charge across the membrane. Facilitated diffusion uses channels or transporters down that gradient. Active transport needs a source of energy, direct or indirect.
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