Cell Physiology: Membrane Transport

Cell Physiology: Membrane Transport Membrane transportmovement of substances into and out of the cell Two basic methods of transport Passive transport No energy is required Active transport Cell must provide metabolic energy (ATP) Solutions and Transport Solutionhomogeneous mixture of two or more components

Solventdissolving medium; typically water in the body Solutescomponents in smaller quantities within a solution Intracellular fluidnucleoplasm and cytosol Interstitial fluidfluid on the exterior of the cell Selective Permeability The plasma membrane allows some materials to pass while excluding others This permeability influences movement both

into and out of the cell Passive Transport Processes Diffusion Particles tend to distribute themselves evenly within a solution Movement is from high concentration to low concentration, or down a concentration

gradient Figure 3.9 Passive Transport Processes Types of diffusion Simple diffusion An unassisted process Solutes are lipid-soluble materials or small enough to pass through membrane pores Passive Transport Processes Figure 3.10a

Passive Transport Processes Types of diffusion (continued) Osmosissimple diffusion of water Highly polar water molecules easily cross the plasma membrane through aquaporins Passive Transport Processes Figure 3.10d Passive Transport Processes Facilitated diffusion

Substances require a protein carrier for passive transport Transports lipid-insoluble and large substances Passive Transport Processes Figure 3.10bc Passive Transport Processes Filtration Water and solutes are forced through a membrane by fluid, or hydrostatic pressure A pressure gradient must exist

Solute-containing fluid is pushed from a high-pressure area to a lower pressure area Active Transport Processes Substances are transported that are unable to pass by diffusion Substances may be too large Substances may not be able to dissolve in the fat core of the membrane Substances may have to move against a concentration gradient ATP is used for transport

Active Transport Processes Two common forms of active transport Active transport (solute pumping) Vesicular transport Exocytosis Endocytosis Phagocytosis Pinocytosis Active Transport Processes Active transport (solute pumping) Amino acids, some sugars, and ions are

transported by protein carriers called solute pumps ATP energizes protein carriers In most cases, substances are moved against concentration gradients Extracellular fluid Na+ K+ Na

+ Na+ P P Na+ Na+ K+

Na + K+ P K+ ATP ADP Binding of cytoplasmic Na+ to the pump protein stimulates phosphorylation

by ATP, which causes the pump protein to change its shape. The shape change expels Na+ to the outside. Extracellular K+ binds, causing release of the phosphate group. Loss of phosphate restores the original conformation of the pump protein. K+ is

released to the cytoplasm and Na+ sites are ready to bind Na+ again; the cycle repeats. Cytoplasm Figure 3.11 Active Transport Processes Vesicular transport Exocytosis

Moves materials out of the cell Material is carried in a membranous vesicle Vesicle migrates to plasma membrane Vesicle combines with plasma membrane Material is emptied to the outside Active Transport Processes: Exocytosis Figure 3.12a

Active Transport Processes: Exocytosis Figure 3.12b Active Transport Processes Vesicular transport (continued) Endocytosis Extracellular substances are engulfed by being enclosed in a membranous vescicle Types of endocytosis

Phagocytosiscell eating Pinocytosiscell drinking Active Transport Processes: Endocytosis Extracellular fluid Cytoplasm Pit Extracellular

fluid Plasma membrane Recycling of membrane and receptors (if present) to plasma membrane Ingested substance Transport to plasma

membrane and exocytosis of vesicle contents Vesicle Lysosome Detachment of vesicle Plasma membrane

Vesicle containing ingested material Vesicle fusing with lysosome for digestion Release of contents to cytoplasm (a)

Figure 3.13a Active Transport Processes: Endocytosis Figure 3.13bc