When you eat mangú with avocado, glucose and other nutrients have to pass from your intestine into your blood and then enter every cell in your body. But the membrane doesn't let just anything through. How does each substance get in? It depends on its size, its electric charge, and whether it moves with or against its concentration gradient.
Simple diffusion: down the gradient
If you drop a bit of dye into a glass of still water, the color spreads throughout the whole glass on its own without you stirring it. Why do particles tend to spread out through all the available space without using energy?
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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.
Notice the differences from simple diffusion: facilitated diffusion needs a specific protein (so it can become saturated when all the proteins are occupied), while simple diffusion depends only on the bilayer. In both, movement goes down the gradient and no ATP is used.
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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 $\text{K}^+$ inside the cell is about $30$ times higher than outside, while the concentration of $\text{Na}^+$ is about $10$ times higher outside. How do cells maintain this imbalance against the natural tendency to even out?
In the diagram, the purple particles (A) are $\text{Na}^+$ ions, which leave the cell; the green ones (B) are $\text{K}^+$ ions, which enter. The pump hydrolyzes ATP to change shape and move them against their gradient.
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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.
A white blood cell in your blood meets a bacterium, too big to cross through a protein. How does it capture it?
Go back to the beginning: the white blood cell captures the bacterium by phagocytosis, and the vesicle that forms is called a phagosome. Then it fuses with a lysosome, which digests the bacterium. An example of exocytosis: pancreas cells release insulin packaged in vesicles.
How receptor-mediated endocytosis works
In the membrane there are specific receptors that recognize only one molecule (the ligand). When the ligand binds to the receptor, the receptors cluster in an area called a coated pit. On the inner face of the membrane, the tail of the receptor that faces the cytoplasm binds to an adaptor protein (adaptin), and this attracts clathrin, which forms a coat around the pit. The pit sinks inward and closes: a coated vesicle forms with the receptor and its ligand inside. Then the vesicle loses its coat and releases its contents into the cell.
One example is the entry of cholesterol. It is carried in the blood attached to particles called LDL, which bind to LDL receptors on cells. If the receptor cannot bind to adaptin and clathrin (because of a mutation in its cytosolic tail), the LDL binds but the vesicle does not form: the cholesterol does not enter the cell and builds up in the blood. This causes familial hypercholesterolemia. Pinocytosis keeps working, because it does not depend on that receptor.
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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 between the two sides of 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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