2.5 Membrane Permeability

Cells have membranes that allow them to establish and maintain internal environments that are different from their external environments. Explain how the structure of biological membranes influences selective permeability. The structure of cell membranes results in selective permeability. Cell membranes separate the internal environment of the cell from the external environment. Selective permeability is a direct consequence of membrane structure, as described by the fluid mosaic model. Small, nonpolar molecules, including N2, O2, and CO2, freely pass across the membrane. Hydrophilic substances, such as large polar molecules nad ions, move across the membrane through embedded channel and transport proteins. Polar uncharged molecules, including H2O, pass through the membrane in small amounts. Describe the role of the cell wall in maintaining cell structure and function. Cell walls provide a structural boundary, as well as a permeability barrier for some substances to the internal environments. Cell walls of plants, prokaryotes, and fungi are composed of complex carbohydrates.

semipermeable/selectively permeable plasma membrane due to bilayer

• essential for existence - separates internal and external environment

• small, nonpolar molecules (N2, O2, CO) can dissolve in and cross the bilayer

• hydrophilic substances (large polar molecules, ions) are less likely to cross the bilayer without assistance from embedded channel and transport proteins

• polar uncharged molecules (H2O) can pass through membrane in small amounts

cell walls

extracellular structure of plant cells, fungi, and prokaryotes, provide structural boundary and permeability barrier for some substances to internal environments

plasmodesmata channels

microscopic channels traversing cell walls of plant cells and some algal cells, allowing transport and communication in and out of the cell

complex carbohydrates in cell walls of plants, prokaryotes, and fungi

• plant cell walls made of cellulose

• fungi cell walls made of chitin

• prokaryotic cell walls made of peptidoglycan