Unit I Molecules and Cells Learning Goal One Describe how the cell membrane and other cell structures function in a cell.

Cell Membranes: Transport and Communication Cell Membrane Structure Fluid Mosaic Model Passive Transport Diffusion Facilitated Diffusion Osmosis Cell Environments Cell Membrane Proteins Transport Proteins Channel Carrier Recognition Proteins Receptor Proteins Cell Adhesion Proteins Active Transport Endocytosis Exocytosis Overview of Cell Communication Reception Transduction Response

Cell Membrane Structure Lipids Cell membranes contain a double layer of phospholipid molecules. They have nonpolar (water repellant) fatty acid chains at one end. The other end is polar (water loving). They form a bilayer because the polar ends face the aqueous areas of the cell’s exterior and interior.

The Fluid Mosaic Model This model of cell membrane structure was proposed in It states that the membrane consists of a fluid phospholipid bilayer in which proteins are embedded and float freely.

Passive Transport Transport of substances through cell membranes that does not require energy on the part of the cell. It depends on concentration differences of the substances on either side of the membrane.

Diffusion Some passive transport of molecules is based on simple diffusion. This is net movement of ions or molecules from a region of higher concentration to a region of lower concentration. This concentration difference is called the concentration gradient.

Simple vs Facilitated Diffusion In facilitated diffusion molecules move through the membrane with the help of membrane proteins. This is necessary when charged molecules need to be transported across the membrane that would normally be repelled by the polar regions of the phospholipid bilayer.

Osmosis This term refers to the passive transport of water through cell membranes. Water diffuses directly through the lipid bilayer and through aquaporins. Much of the energy budget of many cells particularly animal cells is spent controlling water content.

Cell Environments Hypotonic The solution that surrounds a cell contains dissolved substances at lower concentrations than the cell. This would cause a cell to swell and possibly burst.

Hypertonic The solution surrounding the cell contain salts or other molecules at a higher concentration than in the cell. This will cause water to leave the cell and the cell will shrink.

Isotonic If extracellular fluid has a concentration of ions, proteins, and other molecules that is the same as the concentration in the cell.

Cell Membrane Proteins Types of Proteins Transport Proteins Form channels that allow selected polar molecules and ions (charged atoms) to pass across.

Transport Proteins and Diffusion Channel Proteins Form hydrophilic (water loving) channels in the membrane through which water and ions can pass. Facilitated diffusion of water through membranes occurs through specialized water channels called aquaporins.

Carrier Proteins Bind a specific molecule and transport it across the lipid bilayer. An example is glucose.

Recognition Proteins Identify a cell as part of the same individual or as foreign. These proteins are responsible for rejecting foreign tissue in the case of transplants.

Receptor Proteins Recognize and bind molecules from other cells that act as chemical signals such as insulin for example.

Cell Adhesion Proteins Bind cells together by recognizing and binding receptors or chemical groups on other cells or in the extracellular matrix.

Active Transport If substances moved across the membrane are moved against a concentration gradient it requires energy. This requires the use of a high energy molecule called ATP.

Exocytosis and Endocytosis Exocytosis Molecules that are too large to move though the plasma membrane or through membrane proteins leave the cell in this process. Secretory vesicles move through the cytoplasm and contact the plasma membrane. The vesicle membrane fuses with the plasma membrane, releasing the contents of the vesicle into the cell exterior.

Endocytosis Proteins and other substances are trapped in pit-like depressions that bulge inward from the plasma membrane. The depression then pinches off as an endocytotic vesicle.

An Overview of Cell Communication Reception A signal molecule binds with a specific receptor of a target cell. Example: Secretion of the hormone epinephrine from adrenal glands in response to stress.

Transduction Process of changing the signal into the form necessary to cause a cellular response. Surface receptor of epinephrine sends a signal through cytoplasm setting off a series of reactions.

Response Transduced signal causes a specific cellular response. With the epinephrine pathway the response is the activation of an enzyme that causes cells to convert stored glycogen to glucose.

Vocab Quiz Cell Membrane Transport 1. Fluid Mosaic Model 2. Passive/Active Transport 3. Diffusion/Facilitated Diffusion 4. Hypotonic/Hypertonic/Isotonic 5. Transport Proteins 6. Recognition Proteins 7. Receptor Proteins 8. Adhesion Proteins 9. Endocytosis/Exocytosis 10. Reception/Transduction/Response