Structure and Function of Cell Membranes Structure of Lipid Bilayer • Made of lipid (fat) and protein molecules. • The lipid bilayer forms the framework of the cell membrane. • Protein molecules are embedded in the bilayer to allow substances to enter and exit the cell.
4. The phospholipid molecules that make up the bilayer are polar. The hydrophilic (water loving) phosphate heads face the water and the hydrophobic (water hating) tails face away from the water.
Cell Membrane Function • Maintenance • Identification- protein molecules are unique on the membrane surface. • Communication- hormones and neurotransmitters bind to receptor proteins that start changes in the cell’s activities. • Transportation- Most substances move through the membranes. • Defense- antibodies are located on the membrane surface and bind to harmful substances.
Chapter 7.3“Cells & Their Environment” What is Passive Transport? Movement of substances across the cell membrane that does not require energy from the cell. Concentration gradient- A difference in the concentration of a substance. Equilibrium- A condition in which the concentration of a substance is equal.
Movement of Substances Which Requires No Energy From the Cell • Diffusion- movement of a substance from an area of higher concentration to an area of lower concentration. diffusion
Which Direction Will Water Flow? • Hypertonic Solution- water leaves the cell which causes the cell to shrink • Hypotonic Solution- water will enter the cell and cause the cell to swell. • Isotonic Solution- water moves in both directions, both into and out of the cell at equal rates. The cell stays the same size.
Facilitated Diffusion • Some substances need a little “help” to enter or exit. They use the aid of transport proteins. Two Types 1. Channel proteins- tube-like and provide an opening through which small, particles can diffuse.
2. Carrier proteins- has a shape that fits a specific substance. When the proper molecule binds with the protein, the protein changes its shape and moves it across the cell membrane.
Active Transport • When a cell moves a substance from an area of lower concentration to a higher concentration. This requires ENERGY produced inside the cell.
Some carrier proteins are used as “pumps” to move a substance against its concentration gradient.
Movement of Larger Substances • Larger substances are transported into and out of the cell by vesicles. The movement of a substance into a cell is called endocytosis.
Two types of endocytosis: • phagocytosis- “cell eating” • pinocytosis- “cell drinking”
The movement of a substance by a vesicle to the outside of a cell is called exocytosis.