Energy and Metabolism Chapter 4 Part 2

4.5 Movement of Ions and Molecules  For metabolism to work, a cell must keep its internal composition stable – even when conditions outside are greatly different  Selective permeability Membrane property that allows some substances, but not others, to cross

Selective Permeability of Cell Membranes

Diffusion  Molecules or ions tend to follow their own concentration gradient and diffuse into an adjoining region of fluid in which they are less concentrated  Diffusion Net movement of molecules or ions from a region of higher concentration to a region of lower concentration

Osmosis and Tonicity  Water molecules tend to diffuse in response to their own concentration gradient  Osmosis Net diffusion of water molecules across a selectively permeable membrane between two fluids with different water concentrations

Osmosis

Osmosis and Tonicity  Tonicity describes relative concentrations of solutes in fluids separated by a selectively permeable membrane Hypotonic: Low solute concentration relative to another fluid Hypertonic: High solute concentration relative to another fluid Isotonic: Same solute concentration relative to another fluid

Fig (a), p. 71

Fig (b-d), p. 71 B Red blood cells immersed in an isotonic solution do not change in volume. The fluid portion of blood is typically isotonic with cytoplasm. C Red blood cells immersed in a hypertonic solution shrivel up because more water diffuses out of the cells than into them. D Red blood cells immersed in a hypotonic solution swell up because more water diffuses into the cells than out of them.

4.6 Membrane Crossing Mechanisms  Gases, water, and small nonpolar molecules can diffuse across a lipid bilayer  Most other molecules and ions cross only with the help of transport proteins, which gives a cell or membrane-enclosed organelle control over which substances enter and exit

Transport Proteins  Each type of transport protein moves a specific ion or molecule across a membrane  The types of transport proteins in a membrane determine which substances cross it Examples: glucose transporters, calcium pumps

Passive and Active Transport  Passive transport Concentration gradient drives a solute across a cell membrane through a transport protein Requires no energy input  Active transport A transport protein use energy, usually from ATP, to pump a solute across a cell membrane against its concentration gradient

Fig. 4-13a, p. 72 Extracellular Fluid glucose 1 Cytoplasmlipid bilayer Passive Transport

Fig. 4-13b, p. 72 2

Fig. 4-13c, p. 72 3

Fig. 4-14, p. 73 Sarcoplasmic Reticulum Cytoplasm calcium ABC Active Transport

Cotransport  Cotransporter Active transport protein that moves two substances across a membrane in opposite directions at the same time  Example: sodium-potassium pump ATP powers an active transport protein that pumps Na + out of and K + into a cell

Cotransport: Sodium-Potassium Pump

Endocytosis and Exocytosis  Endocytosis Process by which a cell takes in a small amount of extracellular fluid by a ballooning inward of its cellular membrane  Exocytosis Process by which a cell expels a vesicle’s contents to extracellular fluid by merging the vesicle with the plasma membrane

Phagocytosis  Phagocytosis (“cell eating”) Endocytic pathway by which cells such as macrophages and other white blood cells engulf particles such as microbes or cellular debris Amoebas also are phagocytic cells

Phagocytosis