Plasmalemma

BIOLOGY:

Membrane Structure and Function

History of the Membrane Idea • 1925— Gorter & Grendel—hydrophobic tails inward. • 1940s— Daniel and Davson—sandwich model: (protein, phospholipid, and protein.) • 1972— Singer and Nicholson—fluid mosaic model.

Membrane Models • Fluid-Mosaic Model – Membrane is a fluid phospholipid bilayer in which protein molecules are either partially or wholly embedded.

Functions of the Plasma Membrane 1. Regulates the passage of materials into and out of the cell. 2. Receives chemical messages from other cells, e.g. hormones, growth factors, neurotransmitters. • Maintains structural and chemical relationships with other cells. • Protects the cell, helps in cell movement, secretion, and in transmitting impulses.

Plasma Membrane as a Fluid • At body temperature, consistency of olive oil. • Each phospholipid molecule can move sideways at ~ 2 mm/s • Most proteins are free to drift along it. • Cholesterol stiffens and strengthens the membrane, helping to regulate fluidity.

Plasma membrane is differentially permeable. – Passive Transport - No ATP requirement. • Diffusion - Movement of molecules from a high to low concentration until equilibrium is reached. • Osmosis - Diffusion of water across a selectively permeable membrane due to concentration differences. – Osmotic pressure, the greater the pressure, the more water will diffuse in that direction.

– Active Transport - Requires carrier protein and ATP.

Figure 8.12 The water balance of living cells

Membrane-Assisted Transport • Large marcomolecules are transported into or out of the cell by vesicle formation. – Endocytosis - Cells take in substances with vesicles • Phagocytosis – solid material. • Pinocytosis - Liquid particles.

– Exocytosis - Vesicles secrete substances out of cell

Plasma Membrane Structure • Plasma membrane is a phospholipid bilayer. • Hydrophilic polar heads face outside, and hydrophobic nonpolar tails face each other. • Proteins may be peripheral or integral. – Peripheral proteins are found on the inner membrane surface. – Integral proteins are embedded in the membrane.

Carbohydrate Chains

• In animal cells, the carbohydrate chains give the cell a “sugar coat,” called the glycocalyx which helps – protect the cell – adhesion between cells – in the reception of signal molecules – cell-to-cell recognition. – give a “fingerprint” (tissue rejection) – give rise to A, B, and O blood groups

Protein Functions

• Channel Proteins - pass molecules through • Carrier Proteins - bond with substance to help it through • Cell Recognition Proteins - Help body recognize foreign substances and itself. • Receptor Proteins Protein changes shape to bring about cellular change. • Enzymatic Proteins Carry out metabolic reactions directly.

Transport by Carrier Proteins • Carrier proteins combine with molecules which are then transported through the membrane. – Facilitated Transport • (facilitated diffusion) • molecules move with the gradient by combining with carrier proteins.

– Active Transport • molecules move against the gradient by combining with carrier proteins. (requires ATP)

Figure 8.15 The sodium-potassium pump: a specific case of active transport

Cell Surface Modifications • Cell Surfaces in Animals—junctions between cells Adhesion Junctions • Intercellular filaments between cells. • bladder & stomach Tight Junctions • Impermeable barriers. • intestines and kidneys. Gap Junctions • Membrane channels join • Allows small ions & molecules to pass between cells.

Cell Surface Modifications – Extracellular Matrix • Meshwork of polysaccharides and proteins • Can be flexible or hard, as in bone.