Plasma Membrane

Plasma Membrane• Separates intracellular (within the cell) fluids from

extracellular (outside the cell) fluids

• Made up of lipids, proteins and carbs (on outer surface)

• Plays a dynamic role in cellular activity & homeostasis– Controls and regulates what enters and leaves the cell

Lipids in the Membrane• Glycolipids are found only in the outer membrane

surface

• 20% of all membrane lipid is cholesterol– Maintain mobility of phospholipids in membrane

• Lipid Rafts:– Make up 20% of the outer membrane surface

– Composed of sphingolipids and cholesterol

– Are concentrating platforms for cell-signaling molecules

Glycocalyx• a fuzzy, sticky carb.-rich area (“sugar-coating”)

• Made up of glycoprotein on the surface of the cell

• Acts as highly specific biological markers by which cells recognize one another

– Examples: • sperm recognize ovum by its glycocalyx

• immune cells recognize bacteria by their glycocalyx.

Cell Membrane Proteins

• Proteins play a major role in membrane functions

– 2 main types:• Peripheral-on outside or inside of membrane

• Integral- are imbedded into membrane– Some go through entire membrane, others do not

Functions of Membrane Proteins

• Transport-spans the membrane and is selective for a particular solute– Some use ATP to actively pump across

membrane

• Enzymatic activity- some membrane proteins may be enzymes

• Receptors for signal transduction-have a specific binding site on outside of cell, once stimulated may initiate a chain reaction in the cell.

Figure 3.4.1

Functions of Membrane Proteins

• Intercellular adhesion-adjacent cells can hook together

• Cell-cell recognition- glycoproteins (proteins bonded to short sugar chains) act as an ID tag to other cells

• Attachment to cytoskeleton and extracellular matrix- anchor cell, help maintain cell shape and/or maintain location.– Some play role in cell movement or binding to

adjacent cells

Figure 3.4.2

Microvilli

• “little shaggy hairs”

• Minute, fingerlike extensions that project from an exposed cell surface

• Increase plasma membrane surface area greatly– Found typically on surface of absorption cells

Membrane Junctions• Most cells are bound to other cells

• 3 factors typically act to bind cells together:1. Glycproteins act as an adhesive

2. Wavy contours of membranes on adjacent cells fit together in a zipper fashion

3. Special membrane junctions are formed- 3 types:

1. Tight Junction

Figure 3.5a

Examples: between epithelial cells in digestive tract keep unwanted material from seeping into blood stream

• impermeable junction that encircles the cell

2. Desmosome

Figure 3.5b

Examples: Skin and heart muscle

• anchoring junction scattered along the sides of some cells

• Keeps cells from being pulled apart during mechanical stress

3. Gap Junction

Figure 3.5c

Example: electrically excitable tissues like heart and smooth muscle

• a nexus that allows chemical substances to pass between cells