Plasma Membranes

Structure and Function of the Plasma Membrane

The plasma membrane is a critical boundary that separates the living cell from its non-living surroundings. It is typically about 7-8 nm thick and plays a vital role in regulating the movement of substances into and out of the cell.

• Selectively permeable: The plasma membrane allows some substances to cross more easily than others, maintaining homeostasis.

• Composed mainly of lipids and proteins, with carbohydrates attached to the surface.

Early models of membrane structure were developed from indirect evidence:

• Lipid solubility: Lipid and lipid-soluble materials enter cells more rapidly than substances that are insoluble in lipids.

• Phospholipid content: Membranes isolated from red blood cells contain phospholipids arranged in two layers (bilayer).

• Protein content: Membranes also contain proteins as well as lipids.

These findings led to the fluid mosaic model of membrane structure, proposed by J.F. Danielli and H. Davson (1935) and refined by S.J. Singer and G.L. Nicolson (1972):

• Membrane is a phospholipid bilayer with proteins embedded or attached.

• Proteins may be peripheral (attached to the surface) or integral (embedded in the bilayer).

• Membrane structure and function are influenced by lipid composition and protein orientation.

Functions of the Plasma Membrane

The plasma membrane performs several essential functions:

• Regulates passage of nutrients, waste molecules, respiratory gases, and ions.

• Allows cell recognition and communication.

• Provides attachment sites for the cytoskeleton and extracellular matrix (ECM).

• Contains specific transport proteins for movement of large or charged molecules.

Transport Across Membranes

Modes of Transport

There are two main modes for the transport of molecules across membranes:

1. Passive transport (diffusion, osmosis, facilitated diffusion)

2. Active transport

Passive Transport

Passive transport is the movement of molecules across a biological membrane without the need for cellular energy (ATP).

• Diffusion: Movement of a substance down its concentration gradient (from higher to lower concentration).

• Diffusion is a spontaneous process ().

• Continues until dynamic equilibrium is reached.

Osmosis is the diffusion of water across a selectively permeable membrane.

• Water moves down its concentration gradient.

• Direction is determined by solute concentration differences.

Types of Solutions

• Hypotonic solution: Lower solute concentration compared to inside the cell.

• Isotonic solution: Equal solute concentration to that inside the cell.

• Hypertonic solution: Higher solute concentration compared to inside the cell.

Water Balance in Cells

• In a hypotonic environment, animal cells swell and may burst (lysis); plant cells become turgid due to the cell wall.

• In a hypertonic environment, animal cells shrink (crenation); plant cells undergo plasmolysis.

• In an isotonic environment, there is no net movement of water; animal cells are normal, plant cells may become flaccid.

Facilitated Diffusion

Facilitated diffusion is the passive movement of molecules across membranes with the help of transport proteins.

• Helps diffusion of polar molecules and ions.

• Transport proteins are specific for their solutes.

Summary Table: Effects of Solutions on Animal and Plant Cells

Active Transport

Active transport is the movement of molecules across a membrane against their concentration gradient, requiring energy input (usually ATP).

• Transport proteins (e.g., sodium-potassium pump) harness energy from ATP to move molecules.

• Active transport is necessary for maintaining concentration gradients of ions and other substances.

• Requires energy:

Exocytosis and Endocytosis

These are processes for moving large macromolecules into and out of the cell via vesicles derived from the plasma membrane.

• Exocytosis: Exporting macromolecules from the cell.

• Endocytosis: Importing macromolecules into the cell.

Types of endocytosis:

1. Phagocytosis: Engulfing solid particles or bacteria with pseudopodia, forming a food vacuole.

2. Pinocytosis: Engulfing fluid droplets into small vesicles. (When you see pino, think of pinot noir which is a wine. Wine is a liquid so Pino means you drink liquids)

3. Receptor-mediated endocytosis: Importing specific macromolecules in response to binding of specific ligands to cell surface receptors.

Key Terms and Definitions

• Phospholipid bilayer: Double layer of phospholipids forming the basic structure of the plasma membrane.

• Integral proteins: Proteins embedded within the lipid bilayer.

• Peripheral proteins: Proteins attached to the surface of the membrane.

• Transport proteins: Proteins that facilitate movement of substances across the membrane.

• Dynamic equilibrium: State where molecules continue to move but there is no net change in concentration.

• Osmoregulation: Regulation of water balance by cells or organisms.

Examples and Applications

• Sodium-potassium pump: Maintains ion gradients in animal cells, essential for nerve impulse transmission.

• Plant cell turgor: Maintained by osmosis, crucial for structural support.

• Receptor-mediated endocytosis: Uptake of cholesterol via LDL receptors in animal cells.