Biology Basics: Homeostasis

Essay by mjj328High School, 10th gradeA+, April 2004

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1. What is diffusion? What is osmosis?

Diffusion is the process by which substances spread from regions of high concentration to regions of low concentration. It is a method of passive transport, and thus costs the cell no energy. Electrically charged ions and large molecules will not diffuse freely across the membrane.

Facilitated diffusion is used for molecules that can't diffuse through the cell membrane because they are too large or not soluble in the lipid bilayer. Proteins in the membrane acts as channels and enable them to pass through more quickly. First, the carrier protein binds to molecule on outside of cell membrane. Then the carrier protein changes shape, shielding the molecule from the interior of the membrane, then the molecule is released inside the cell. Facilitated diffusion only works in the direction of the concentration gradient, and does not expand any energy.

Ion Channels are another form of passive transport. They allow passage of important ions, some are always open, some have gates. Also require no energy, but are with gradient.

Osmosis is the diffusion of water across a semipermeable membrane.

~Direction of osmosis:

Environment is : Cell is :Water Will Move:

If solute concentration in the env is lower than in cellhypotonichypertonic into cell

If solute concentration in the env is higher than in cellhypertonichypotonicout of cell

If solute concentration in the env is = to that of the cellisotonic isotonicWater will not move- no net +-

-When water diffuses into a plant cell, turgor pressure increases.

-When animal cells take in too much water, cytolysis occurs= cells take in so much water that they burst.

-Plasmolysis= when water diffuses out of a plant cell, the plant wilts

-Cell wall, solute pumps, and contractile vacuoles help cells deal with osmotic pressure, animal cells= cells...