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Return to Biology:  Osmosis & Diffusion                    Return to A&P I      (updated 9/24/05)

In biology, Osmosis is defined as the diffusion of water across a cell membrane. This process accounts for many functions that maintain life. In relation to blood and plant cells, blood cells placed in pure distilled water will swell and burst, while plant cells remain turgid because the cell wall prevents bursting. If these cells are placed in a Hyperosmotic (hypertonic) solution, i.e., the solution has more dissolved particles, salts, sugar, etc., than is in the cells, they will shrivel up (a process called crenation in the case of blood cells, and plasmolysis in plant cells. The animal may die from blocked blood flow, while the plant may wilt and die. Crenation also occurs during sickle-cell anemia during a crisis.

Osmosis, diffusion, filtration (when fluid is under pressure), facilitated diffusion (diffusion through a membrane with help from a transport protein), and active transport (use of energy to move materials through a membrane), all occur in the kidneys, as they function to maintain the proper pH, blood volume, blood composition, and eliminate wastes.

The fluid mosaic model of the cell membrane.

How cell membranes are studied (preserving & sectioning).

Staining & examination of membranes.

Functions of proteins: transport of ions and molecules; serve as enzymes; receptors for hormones & other substances; self-identity markers; cell to cell adhesion; attachment to internal cytoskeleton.

The polar areas are closer to the phospholipid 'heads' (which bear electrical charges, while the nonpolar areas are attracted to the nonpolar fatty acid portions of the phospholipids.

Details of anchoring proteins within cells.

Hydrophilic and hydrophobic regions of proteins in relation to ion transport and chemioasmosis.

Pore proteins allow certain molecules to enter and leave the cell.

Passive transport: Diffusion - the movement of a substance from an area of high concentration to an area of lower concentration (a concentration gradient).

Facilitated diffusion - diffusion through carrier proteins within cell membranes.

Osmosis - diffusion of water through a selectively permeable membrane.

Osmotic pressure always moves water toward the hypertonic side (the side containing the least amount of water) of a membrane.

Endocytosis - An active process of taking in something through a cell membrane, which uses energy (ATP). Phagocytosis - cell eating.

Pinocytosis - Cell drinking.

Receptors help to attach molecules to the membrane before taking them in.

Exocytosis - the opposite of endocytosis, is also an active process.

Active transport - transport proteins within the membrane must use energy (ATP) to move substances either to the inside or outside of the membrane.

The sodium-potassium pump must break ATP down into ADP in order to pump sodium three ions outside the cell (continued below),

while it pumps two potassium ions into the cell. The ATP phosphorylates (adds a phosphate to) the membrane protein as it binds to the sodium and breaks down, and it dephosphorylates the protein as it binds with the potassium. Cellular respiration must occur to add the phosphate back to ADP, thus restoring the ATP.

In cotransport, one process (the Na-K pump), is coupled with movement of a molecule of sugar (glucose) out of the cell, while allowing sodium to enter through the protein.

Proton pumps operate both within the inner mitochondrial membranes in the electron transport system of cellular respiration, and in chloroplast membranes during photosynthesis.

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