Cells and Their Environment Ch. 4 Biology

Membrane Structure Phospholipid Bilayer 2 layers of phospholipids Proteins Transport Receptors Cholesterol Maintains fluidity

Fluid-Mosaic Model Plasma membrane contains a variety of molecules Molecules move laterally

Selectively Permeable Because of polar and nonpolar regions of the phospholipid bilayer, the membrane allows certain materials in and certain materials out of the cell Small molecules Small ions Nonpolar molecules

Passive Transport Diffusion caused by the random movement of particles across a membrane Movement due to concentration gradient Moving from a higher concentration to a lower concentration No energy used Movement continues until equilibrium reached Concentration is the same on both sides of the membrane

Osmosis Diffusion of water across a selectively permeable membrane Does not use energy

Direction of Water Movement Hypertonic solution Solution contains more solute than the solution it is compared with Hypotonic solution Solution contains less solute than the solution it is compared with Isotonic solution Solution contains same amount of solute as the solution it is compared with

Facilitated Diffusion Passive transport—no energy used Uses transport proteins embedded in the plasma membrane (ion channels) Potassium (K+) Sodium (Na+) Calcium (Ca+) Chloride (Cl-) Pores—always open Gated channels—open/close in response to stimuli

Active Transport Substances are transported across a membrane, against the concentration gradient Use carrier proteins embedded in the membrane Use energy (ATP)

Sodium-Potassium Pump

1. 3 Na+ ions inside the cell bind to the pump. ATP donates a phosphate to the pump 2. The pump changes shape, transporting 3 Na+ across the membrane, and are released 3. 2 K+ ions outside the cell bind to the pump and are transported across the membrane 4. The phosphate group is released and the 2 K+ are released

Sodium-Potassium Pump

Vesicles Move Substances Substances that are too large for carrier proteins (proteins and polysaccharides) Endocytosis—movement of substances into the cell (a.k.a. phagocytosis) Exocytosis—movement of substance out of the cell

Receptor Proteins Protein that binds a specific signal molecule, allowing the cell to respond to the signal molecule Some receptor proteins are attached to ion channels Changes permeability to a specific ion Some may cause the formation of a second messenger, which acts as a signal molecule in the cytoplasm Some may act as enzymes to speed up chemical reactions by lowering the activation energy