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How various mechanisms by which substances cross the cell me

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How various mechanisms by which substances cross the cell me

Homeostasis is essential to the cell's survival. The cell membrane is

responsible for homeostasis. The membrane has a selective permeability

which means what moves in and out of the cell is regulated. Amino

acids, sugars, oxygen, sodium, and potassium are examples of substances

that enter the cell. Waste products and carbon dioxide are removed from

the cell. All of these substances cross the membrane in a variety of

ways. From diffusion and osmosis, to active transport the traffic

through the cell membrane is regulated.

Diffusion is the movement of molecules form one area of higher

concentration to an area of lower concentration. Concentration gradient

causes the molecules to move from higher concentration to a lower

concentration. The side of the membrane that has the higher

concentration is said to have the concentration gradient. It drives

diffusion because substances always move down their concentration

gradient. The pressure gradient also plays a role in diffusion. Where

this is a pressure gradient there is motion of molecules. The pressure

gradient is a difference in pressure between two different points. If

the concentration of one side of the membrane is greater than the

molecules will travel from the higher to lower concentration.

Eventually there will be a dynamic equilibrium and there will be no net

movement of molecules from one side to the other.

Osmosis is the diffusion of water. Like diffusion, the water moves

from a region of higher water potential to a region of lower water

potential. Solutions have three different stages that the solutes can

be classified in: isotonic, hypertonic, and hypotonic. Isotonic is when

the solutions have equal amounts of solutes. Like equilibrium, there is

no net change in the amount of water in either solution. When the

solutions have different concentration of solutes then the one with less

solute is hypotonic and the one with more solute is hypertonic.

Hypotonic takes in the solute from the hypertonic side that gives away

the solute. There will be a net movement in these types of solution.

The molecules will move from the hypotonic solution into the hypertonic


The third way a substance can cross the cell membrane is through

facilitated diffusion. This occurs when special carrier proteins carry

solutes dissolved in the water across the membrane by using active

transport. When the concentration gradient can not allow travel from


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Keywords: what are the various mechanisms by which substances cross membranes

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