The Plasma Membrane - Southington High School



The Plasma Membrane

Some materials need to be brought into cells and wastes need to be let out. Most of the materials outside of cells need to be kept outside if a cell is to survive. How does a cell accomplish this?

Homeostasis—the process of maintaining a cell’s internal environment.

Selective permeability—the process in which the plasma membrane

allows some molecules to enter cells and keeps many others out.

Structure of the Plasma Membrane

Plasma membrane consists of two layers back to back (called a bilayer). Each layer is made of phospholipids.

Phospholipid—an important molecule that has a phosphate group as a

“head” and two lipid chains as “tails”.

Fluid mosaic model—the plasma membrane has been described as a

fluid mosaic. It is fluid because it is flexible and the molecules can move around. It is a mosaic because it consists of several different molecules besides the phospholipids.

Parts of the “mosaic”

Cholesterol—prevents the lipid chains from sticking together.

(Keeps the membrane “fluid”)

Transport proteins—complex proteins that form a “tunnel” that can

open or close to allow large molecules into or out of the cells.

Review

1. How is the plasma membrane a bilayer?

2. How does selective permeability maintain homeostasis?

3. Why is the plasma membrane referred to as a fluid mosaic?

4. What might happen if cells grew in an environment w/o cholesterol?

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