A Closer Look at Membranes Chapter 6

Cell Membrane Main Functions Controls Transport in & out of the Cell Cell membrane is selectively- permeable, meaning it is a barrier to some, but not all molecules

Plasma Membrane Composition Phospholipid bilayers with the hydrophobic regions facing each other and the hydrophilic regions facing out.

Fluid Mosaic Model The theory by which the properties of the plasma membrane are explained Mosaic: mixture of phospholipids, sterols (cholesterol), proteins, glycoproteins Fluid: the components are not fixed in place and may move or shift but are kept ordered due to hydrophobic forces

Fluid Mosaic Model

Membrane proteins Transport: move molecules or ions from one side of the membrane to the other. Receptors: bind substances secreted by other cells which trigger changes in the receiving cell. Anchors proteins: help cells stick to each other and stay positioned; found in cell junctions

Membrane Proteins Many of the proteins of the membrane are glycoproteins, which contain sugar groups

Movement Across the Membrane Passive Transport:  Diffusion  Osmosis  Facilitated Diffusion Active transport Vesicular Transport

Passive transport Movement of substances from high concentration to low concentration (diffusion) Movement down concentration gradient Requires no energy

Diffusion The net movement of a substance from an area of high concentration to an area of low concentration. Substances move down their concentration gradient H 2 O, CO 2, & O 2 are among the few molecules that can pass across a cell membrane by diffusion

Osmosis Diffusion of H 2 O across a semi-permeable membrane Type of Passive transport – Requires No Energy As the concentration of solute increases the concentration of solvent (H 2 O) decreases. Compared to an equal volume of water, the solute/H 2 O mixture has less H 2 O molecules as that space is taken up by the solute.

Osmosis

Tonicity Refers to the relative solute concentrations of two fluids separated by a semi- permeable membrane. The one with fewer solutes is hypotonic The one with more solute is hypertonic Isotonic refers to equal solute concentrations.

Tonicity in Animal vs. Plant Cells

Facilitated Diffusion Diffusion “with help” – from high conc. to low conc. Requires no energy – form of passive transport Molecules that are too big or too charged to cross membrane, can cross membrane via protein channels (transport proteins)

Active transport Movement of substances from low concetration to high concentration Requires energy to pump substances against concentration gradient Example: the Sodium/Potassium Pump:

Vesicle Transport Vesicles allow for transportation of large molecules or large numbers of molecules Exocytosis: vesicle moves to the cell surface and fuses with the plasma membrane spilling its contents out of the cell Endocytosis: a vesicle forms as an indentation on the plasma membrane and engulfs a substance

Vesicle Transport Exocytosis Endocytosis

Phagocytosis  cell engulfs a large piece of food or even another cell (“cell eating”) Pinocytosis  cell takes up small amounts of the surrounding extracellular fluid (“cell drinking”) Receptor Mediated endocytosis  Cells forms a coated vesicle only when specific molecules bind to cell surface receptors

Endocytosis

Phagocytosis (White Blood Cell)