Cells Maintaining Homeostasis Cell Membrane Cell membranes control

Cells Maintaining Homeostasis

Cell Membrane • Cell membranes control movement of materials in and out of cells. • They are composed of a double layer of lipid molecules with proteins embedded in the lipid bilayer.

Some proteins go part way in, others completely penetrate. The lipids are fluid and the proteins can move about. - fluid mosaic model

Brownian Movement

Passive Transport • no energy used • Molecules move from areas of high concentration to areas of lower concentration • With the concentration gradient Examples: 1. Diffusion 2. Osmosis 3. Facilitated Diffusion

Diffusion • Substances move from higher to lower areas of concentrations • Example: O 2 and CO 2

• Diffusion stops when molecules are evenly spread out - equilibrium • Diffusion is one of the major ways materials move in and out of cells.

Diffusion of Oxygen into a Cell The concentration of oxygen molecules is greater outside the cell than inside So the oxygen molecules diffuse into the cell 9

Why do we use the term selectively permeable when describing the cell membrane?

• Selectively permeable = selecting what penetrates

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• Water and other small molecules that do not dissolve in lipids diffuse through openings formed by proteins that go through the entire membrane. • Diffusion of water through a membrane is called osmosis

Osmosis • Since water makes up 70 - 95% of a living cell, its movement in and out is of vital importance. • The cell has no control over osmosis. Osmosis will end when equilibrium is reached. • The water concentration is determined by the concentration of solutes in the water.

• isotonic solution concentration of solutes equal in and out of cell iso = same

• hypertonic solution concentration of solutes outside is higher • less water, more solutes • ex: saltwater hyper = above

What is the effect of hypotonic solutions on cells?

• Cells in hypotonic solutions take in water. • When cells are put in distilled water and burst due to cytolysis the membranes and contents can be separated by a centrifuge for detailed study.

What is the effect of hypertonic solutions on cells?

• Cells in hypertonic solutions lose water. This causes cells to shrink - plasmolysis • If humans drink seawater, their cells lose more water than they take in. • crenate = plasmolysis

Normal Plasmolysis Cytolysis

Since osmosis and diffusion take place without any use of energy by the cell, they are called forms of passive transport