Membrane Structure and Function The Fluid Mosaic Model

Introduction plasma membrane separates the living cell from its nonliving surroundings. membrane is selectively permeable: allowing some substances to cross more easily than others.

Basic Structure Made of lipids, proteins and some carbohydrates. The most abundant lipids are phospholipids. Phospholipids are amphipathic molecules. Have both hydrophobic regions and hydrophilic regions. The phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model.

How is it like a fluid? Membrane molecules are held in place by relatively weak hydrophobic interactions. Most of the lipids and some proteins can drift laterally in the plane of the membrane, but rarely flip-flop from one layer to the other.

Temperature and Saturation As temperatures cool, membranes switch from a fluid state to a solid state as the phospholipids are more closely packed. Membranes rich in unsaturated fatty acids are more fluid that those dominated by saturated fatty acids (kinks prevent tight packing).

Cholesterol The steroid cholesterol is wedged between phospholipid molecules in the plasma membrane of animal cells. warm temperatures: restrains the movement of phospholipids and reduces fluidity. cool temperatures: maintains fluidity by preventing tight packing

How is it like a mosaic? collage of different proteins determine most of the membrane’s functions membranes each have unique collections of proteins There are two populations of membrane proteins: Peripheral: bound to the surface Integral: embedded in the lipid bilayer

Protein functions

Cell-cell recognition Membrane plays a key role in cell-cell recognition (ability of a cell to distinguish one type of neighboring cell from another) important in cell sorting and organization as tissues and organs in development. basis for rejection of foreign cells by the immune system. Cells recognize other cells by keying on surface molecules, often carbohydrates, on the plasma membrane.

Selective Permeability A steady traffic of small molecules and ions moves across the plasma membrane in both directions. Examples: sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave. cell absorbs oxygen and expels carbon dioxide. cell regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by shuttling them across the membrane. Substances do not move across the barrier indiscriminately, membrane regulates movement

Hydrophobic core Permeability depends on the interaction with the hydrophobic core of the membrane. Hydrophobic molecules: can dissolve in the lipid bilayer and cross easily Examples: hydrocarbons, CO2, O2 Ions and polar molecules: difficulty passing Examples: small molecules, like water, and larger critical molecules, like glucose and other sugars. Proteins can assist and regulate the transport of ions and polar molecules.

Transport Proteins Specific ions and polar molecules can cross the lipid bilayer by passing through transport proteins that span the membrane. Some have a hydrophilic channel (tunnel) Others bind to these molecules and carry their passengers across the membrane physically Each transport protein is specific as to the substances that it will translocate (move).

Passive Transport: Diffusion! Diffusion is the tendency of molecules of any substance to spread out in the available space Diffusion is driven by the intrinsic kinetic energy (thermal motion or heat) of molecules. Passive transport: no energy required! Movements of individual molecules are random, but Movement of a population of molecules may be directional.

Concentration gradients a substance will diffuse down its own concentration gradient: more concentrated  less concentrated Example: dye will cross the membrane until both solutions have equal concentrations of the dye. At dynamic equilibrium molecules are moving both ways equally

Relative Concentration Differences in the relative concentration of dissolved materials in two solutions can lead to the movement of ions from one to the other. Hypertonic: The solution with the higher concentration of solutes. Hypotonic: The solution with the lower concentration of solutes. Isotonic: Solutions with equal solute concentrations These are comparative terms. Tap water is hypertonic compared to distilled water but hypotonic when compared to sea water

Osmosis Osmosis: the diffusion of water across a selectively permeable membrane water molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer In other words: More water  less water Osmosis continues until the solutions are isotonic

Cell Survival An animal cell immersed in an isotonic environment experiences no net movement of water across its plasma membrane. Water flows across the membrane, but at the same rate in both directions. The volume of the cell is stable. What happens when there environment is not isotonic?!

Animal Cells Hypotonic solution: cell will gain water, swell, and burst! Isotonic solution: normal! Hypertonic solution: cell will loose water, shrivel, and probably die!

What is happening and why?

Plant Cells Hypotonic solution: Turgid is a healthy state for most plant cells. contribute to the mechanical support of the plant. Isotonic solution: no movement of water into the cell - flaccid and the plant may wilt Hypertonic solution: plasmolysis - the plasma membrane pulls away from the wall – usually lethal!

Review Questions: Describe the structure and function of the cell membrane. Describe the different functions of the proteins in the cell membrane. How does relative concentration of solutes determine how substances diffuse? Explain what happens in each of the following: hypotonic solutions, isotonic solutions, hypertonic solutions.