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Which of the following is a function of membranes?
The question asks about membranes in general, not just the cell plasma membrane; therefore, all of the answers are true. The plasma membrane's most important functions are protecting the internal environment of the cell and selectively allowing nutrients into the cytoplasm (semi-permeability). The final answer describes a function of the inner-membrane of the mitochondria, which houses the electron transport chain.
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Which of the following substances would be most capable of passing through a plasma membrane?
Membranes are semi-permeable, meaning that they only allow certain compounds to cross without protein assistance. The two factors that determine permeability across a membrane are size and polarity. Polar and charged compounds very rarely cross the membrane by simple diffusion, and often require a carrier protein or pump in order to cross. Glucose and amino acids are polar, and a sodium ion carries a positive charge. These compounds will be blocked by the hydrophobic interior of the membrane, formed by the fatty acids tails of the phospholipids bilayer.
Nonpolar compounds, on the other hand, can very easily cross the cell membrane. Even larger nonpolar molecules, like steroid hormones, can easily cross the membrane via simple diffusion.
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Name the type of transport that requires an expenditure of energy in order to move a compound across the membrane.
Energy is not necessary when a compound is being moved down its electrochemical gradient. Diffusion, facilitated diffusion, and osmosis all involve a compound moving from a higher to a lower concentration. Since this is the spontaneous direction of flow, no energy input is required.
In order to move a compound against its electrochemical gradient, energy is needed. This type of transport is called active transport.
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