Lecture Notes-13 Biology 1004

North Arkansas College

Topic: Membranes

1. All living cells are surrounded by a membrane that separates the internal and external environments.
2. The membrane helps to maintain cellular integrity (living material cannot be dissipated or it becomes non-functional).
3. Helps to regulate the flow of nutrients into the cell and waste materials out of the cells. This is a called a semi permeable membrane - partially regulates the size of materials that can pass through it.
4. Helps to maintain homeostasis.
5. Allows communication between cells.

1. Phospholipids - glycerol, phosphate and fatty acids
2. Proteins

Phospholipids - A suspension of phospholipids in water produces a bilayer that separates the environment on each side of the bilayer. Structurally, phospholipids have a hydrophilic head region and a hydrophobic tail region. The two layers of phospholipids form the lipid bilayer in which the hydrophobic tails assemble.

Click below for a slide show- see slide #4 and #6

http://www-class.unl.edu/bios201/chapter4WEB/sld001.htm

Proteins are imbedded in phospholipid bilayer to form a fluid mosaic model in that membrane. There are two types of proteins in the membrane:

1. Peripheral proteins are out near surface, and are easily removed.
2. Integral proteins extend all the way through the lipid bilayer, and are hard to remove (extract with detergents or surfactants). Membrane proteins function in several ways:

1. Channel proteins are hollow connectors between interior and exterior of cell.
2. Carrier proteins "pick up" and help move materials through the membrane.
3. Receptors on the surface of the cells, chemicals, viruses, etc. seek out and attach themselves to specific receptors in/on membrane surface.
4. Enzymes - Many membrane proteins have enzymatic activity.

1. Glycolipids (sugars & lipids) and glycoproteins (sugars & proteins) found on membrane act as fingerprint of cells. These vary from species-to-species and individual-to-individual (15-100’s).
2. They help tissue and organs during differentiation. Changes in isomer forms of sugars, and changes in the type of sugar and chain lengths are responsible.
3. They are involved in tissue/organ transplantation (acceptance/rejection).
4. They are part of the "self recognition" process. They makeup the major histocompatibility complex (MHC).
5. They make up blood groups (A, B, AB & O).
6. They keep blood in developing fetuses from mixing with mother’s blood (which would be rejected as foreign tissue).

1. Passive Transport (No Energy Required) - There are two types of passive transport:

1. Diffusion - solute moves from higher to lower concentration. Osmosis is a type of diffusion in which the solvent (normally H₂0) moves from higher to lower concentration. Cells are usually filled with H₂0 which provide cell shape - turgidity.
2. Facilitated Transport (diffusion) - which requires carrier proteins to move the solute from area of higher concentration to lower concentration.

2. Active Transport (Requires Energy (ATP) & Carrier Proteins) - can move the solute against the concentration gradient from area of lower concentration to an area of higher concentration. A particular type of active transport occurs with the sodium-potassium pump in the membrane of higher organisms. There keeps the sodium concentration high outside of the cell and the potassium (K) concentration high inside of the cell. Some leakage occurs, and sodium is pumped into the cell, while potassium is pumped out of the cell.

Response of cells to environmental changes in tonicity:

1. Isotonic - solute concentration is the same inside/outside of cell. No change occurs. 0.975% NaCl is isotonic to blood (5% glucose is isotonic).
2. Hypotonic - solute concentration is greater inside cell. Water flows into cell, cell swells and may burst: osmotic lysis.
3. Hypertonic - solute concentration is greater outside cell. Cell loses water and collapses/shrinks: crenation or plasmolysis.

3. Membrane Assisted Transport - There are two types of membrane-assisted transport.

1. Exocytosis - Internal cellular components are expelled from the cell as the membrane buds outward. The material is trapped in budded vesicle. New membrane is produced expelling material in the vesicle/vacuole to the outside.
2. Endocytosis - Cell membrane invaginates inward trapping external material in the vesicle or vacuole. New membrane is produced - the vacuole winds up inside the cell.

1. Phagocytosis - a type of endocytosis that usually involves WBC’s (leukocytes). Phagocytosis is the engulfment and destruction of foreign particles, and is part of the body’s natural defense mechanism.
2. Pinocytosis - the engulfment of liquids or very small particles.
3. Receptor Mediated Endocytosis (RME) - a foreign particle attached to a receptor site in cell membrane. This causes the membrane to invaginate. Several other receptor sites reach the area causing the foreign particle(s) to bind to several sites. These sites are enclosed in the invaginated area enhancing the process of endocytosis.

Membrane Junctions - There are 3 types in animal cells and an additional type in plant cells. These allow cells to communication and help make cells stronger.

1. Adhesion Junctions - helps hold cells together (desosomes). Found in tissue that needs to stretch, e.g. stomach, bladder, etc.
2. Tight Junctions - membrane proteins attach between adjacent cells. Found in tissue that serve as barriers (e.g. in intestine, kidney, to prevent leakage).
3. Gap Junctions - allows communication. Narrow channels for communication in heart, etc. for electrolyes to move smooth muscle.
4. Plant Junctions - Plasmodesmata are channels involved in cytoplasmic streaming (bound by membrane); connect one cell to another cell.

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