Biology Chapter on Plasma Membrane Function

Plasma Membrane: Structure and Function

• Separates the living cell from its surroundings.
• Exhibits selective permeability, allowing some substances to cross more easily than others.

Fluid Mosaic Model

• Membranes are fluid structures with a mosaic of proteins embedded in a phospholipid bilayer.
• Phospholipids are amphipathic (hydrophilic and hydrophobic regions).
• Protein distribution within the membrane is not random.

Membrane Proteins

• Determine most membrane functions.
• Peripheral proteins are bound to the membrane surface.
• Integral proteins penetrate the hydrophobic core; transmembrane proteins span the entire membrane.

Six Major Functions of Membrane Proteins

• Transport

• Enzymatic activity

• Signal transduction

• Cell-cell recognition

• Intercellular joining

• Attachment to the cytoskeleton and extracellular matrix (ECM)

• HIV infection requires binding to CD4 and CCR5 proteins on immune cells; individuals lacking CCR5 are resistant.

Membrane Carbohydrates

• Involved in cell-cell recognition.
• Covalently bonded to lipids (glycolipids) or proteins (glycoproteins).

Membrane Permeability

• Selective permeability regulates molecular traffic.
• Hydrophobic molecules cross easily; hydrophilic molecules do not.

Transport Proteins

• Facilitate passage of hydrophilic substances.
• Channel proteins provide hydrophilic channels.
• Aquaporins are channel proteins for water.
• Carrier proteins bind molecules and change shape to shuttle them across.
• Transport proteins exhibit specificity.

Passive Transport: Diffusion

• Movement of substances across a membrane without energy investment.
• Substances diffuse down their concentration gradient (high to low concentration).
• Dynamic equilibrium: equal movement in both directions.
• No work is required; it’s passive.

Osmosis

• Diffusion of water across a selectively permeable membrane.
• Water moves from lower to higher solute concentration.

Tonicity

• Isotonic solution: no net water movement.
• Hypertonic solution: cell loses water.
• Hypotonic solution: cell gains water.
• Osmoregulation is crucial for maintaining water balance in different environments.

Water Balance in Cells

• Plant cells with cell walls: turgid (hypotonic), flaccid (isotonic), plasmolysis (hypertonic).
• Animal cells: cell lysis (hypotonic), crenation (hypertonic).

Facilitated Diffusion

• Passive transport aided by proteins.
• Channel proteins create corridors.
• Gated channels open/close in response to stimuli.
• Carrier proteins change shape to translocate solutes.

Active Transport

• Moves substances against their concentration gradient.
• Requires energy (usually ATP).
• Maintains concentration gradients different from surroundings.
• Sodium-potassium pump is an example.

Cotransport

• Active transport of one solute indirectly drives transport of others.
• Plants use proton pumps to drive nutrient uptake.

Bulk Transport

• Exocytosis: vesicles fuse with membrane, releasing contents outside.
• Endocytosis: substances are taken into the cell via vesicles (types: phagocytosis, pinocytosis, receptor-mediated endocytosis).

This quiz covers the structure and function of the plasma membrane, highlighting its role in cell separation and selective permeability. It also delves into the fluid mosaic model and the various functions and types of membrane proteins, emphasizing their importance in cellular processes. Test your knowledge on this fundamental biological concept!