Biological Membranes

Biological membranes Complex, dynamic structures made of lipid and protein molecules Perform many functions Define cell as a compartment Regulate passage of materials Participate in chemical reactions Transmit signals between cell interior and the environment Act as part of energy transfer and storage

Biological membranes Physically separate cell interior from extracellular environment Form compartments within eukaryotic cells Plasma membrane Regulates passage of materials Participates in biochemical reactions Receives information about environment Communicates with other cells

Fluid mosaic model Membranes consist of fluid phospholipid bilayer with a mosaic pattern of associated proteins Phospholipid molecules are amphipathic and contain Hydrophobic regions which are repelled by water Hydrophilic regions which are attracted to water

Phospholipid molecules

Since the cell is a very watery environment, the heads of the phospholipid molecules face toward the cell. The fatty acid “tails” of the molecules face inward toward the center of the bilayer.

Phospholipids in water Detergent in water Phospholipids form bilayers in water Phospholipids in water Detergent in water

Plasma membrane of mammalian red blood cell

Membrane properties Orderly arrangement of phospholipid molecules make the cell membrane a liquid crystal Allow molecules to move rapidly Proteins move within membrane Lipid bilayers are Flexible Self-sealing Can fuse with other membranes

Detailed structure of the plasma membrane

Proteins are embedded in the bilayer Integral membrane proteins Embedded in the bilayer Transmembrane proteins Integral proteins that extend completely through the membrane Peripheral member proteins Associated with the surface of the bilayer

Asymmetrically positioned to bilayer Membrane proteins, lipids, and carbohydrates Asymmetrically positioned to bilayer Sides have different composition and structure Function of member proteins Transport of materials Acting as enzymes or receptors Cell recognition Structurally linking cells

Asymmetry of the plasma membrane

Functions of membrane proteins

Membranes are selectively permeable (only some materials are allowed in and out) Physical processes Osmosis Diffusion Carrier-mediated processes Channel proteins Carrier proteins

Diffusion: the movement of molecules from a region of higher concentration to one of lower concentration.

Rate of diffusion depends on: Temperature (higher temperature, more movement of molecules) Size of molecules (smaller molecules tend to diffuse faster) Electrical charge (like charges repel and opposites attract) Difference in concentration (concentration gradient)

Some molecules easily diffuse through the membrane Water Gases Small polar molecules Large, lipid soluble molecules

Osmosis The diffusion of water across a selectively permeable membrane from a region where there is a greater concentration of water to a region where there is less water. Osmotic pressure: the tendency of water to move into a solution.

Osmotic pressure Concentration of dissolved substances in a solution Isotonic: equal solute concentration Hypertonic: solution has a greater concentration of solutes than the cell, loses water in plasmolysis Plasmolysis (collapse of a cell due to loss of water. Hypotonic: solution has a lesser concentration of solutes than the cell, gains water and swells

Turgor pressure The internal hydrostatic pressure usually present in walled cells. Turgor pressure provides structural support in plants which do not have wood.

Movement of particles through proteins If substances cannot pass through the lipid bilayer, they may still move through the membrane via protein channels. This may or may not require energy from the cell. Passive transport: no energy expenditure Active transport: energy expended by the cell.

Facilitated diffusion does not require energy Occurs down a concentration gradient Active transport requires energy Moves ions or molecules against a concentration gradient Cotransport requires energy ATP-powered pump maintains a concentration gradient

Sodium-Potassium Pump

Sucrose proton cotransport

Cells expend metabolic energy to carry on physiological processes Exocytosis: large molecules leave the cell through fusion with the membrane Endocytosis: large molecules enter the cell through fusion with the membrane Phagocytosis Pinocytosis Receptor-mediated endocytosis

Exocytosis

Phagocytosis

Phagocytosis is used by macrophages to help defend the body from bacteria

Amoebae feed using phagocytosis

Pinocytosis

Receptor-mediated endocytosis

Cells communicate by cell signaling Signaling molecules include Neurotransmitters Hormones Regulatory molecules

Cell signaling involves Synthesis and release of signaling molecule Transport to target cells Reception by target cells involving special surface receptors where the signaling protein docks. Signal transduction cells convert an extracellular into an intracellular one. Response by the cell: interior of the membrane protein undergoes a change in conformation, which activates proteins in the cytoplasm—often in a chain reaction. Termination of signal

Signal transduction

Cells in close contact often develop intercellular junctions Anchoring junctions connect epithelial cells Desmosomes composed of filaments which hold cells subject to mechanical stresses together (in animal cells) Adhering junctions cement cells together with proteins Tight junctions seal off intercellular spaces in some animal cells. Seals are protein links.

Gap junctions: permit transfer of small molecules and ions Gap junctions: permit transfer of small molecules and ions. These contain pores that connect cells. Allow rapid chemical and electrical communication between cells. Plasmodesmata: allow movement of certain molecules and ions between plant cells. Plasma membranes are continuous though the gaps.

Desmosomes

Tight junctions

Gap junctions

Plasmodesmata