You are researching sexual reproduction in primates. while you are examining some cells under the microscope you see what appear to be four sister chromatids bound together. you also noticed a remnant of the nuclear envelope. what stage of meiosis are you looking at?

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

the solute can move "downhill," from regions of higher to lower concentration, relying on the specificity of the protein carrier to pass through the membrane. this process is called passive transport or facilitated diffusion, and does not require energy.

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