Cell Membranes & Transport

Cell Membranes F5-1 Cell membrane distinguishes one cell from the next. Cell membranes do the following: a) Regulates exchange of salts, nutrients and waste with the environment. b) Mediate communication between the cytosol and environment. c) Maintain cell shape.

More metabolically active cells contain membranes with more protein.

Fluid Mosaic Model of Cell Membrane F5-2

F5-3 Membrane Phospholipids Allows lipid-soluable molecules to migrate across the membranes. Provides framework for membrane proteins.

Membrane Proteins and Cholesterol F5-2 Associated proteins include enzymes and, structural proteins which tie the cytoskeleton of the cell to the membrane. Cholestrol is hydrophobic. It makes membranes impermeable to water- soluable molecules and maintains the membrane over a wide range of temperatures.

Membrane-Spanning Proteins F5-4 Membrane-spanning proteins are tightly bound to the lipid bilayer and could only be removed from the membrane by disrupting it, eg. detergents. However, they can move around using the cytoskeletal framework. They also develop polarity. Carbohydrates bind to the outer loops (glycoproteins). They form a protective layer called the glycocalyx which plays a role in the body’s immune response. Phosphate groups attach to the inner loops. Cause conformational changes.

Map of Membrane Proteins F5-11

Enzymes F4-7 Enzymes act as biological catalysts to speed up reactions without themselves being changed.

Cell Membrane Receptors F5-6 Each receptor is specific to a particular molecule.

Membrane Transport If a substance could cross the cell membrane by any means, we say the membrane is permeable for that substance. Very small or lipid-soluable substances can cross the phospholipid bilayer readily. For example, water, oxygen, carbon dioxide and lipids. If the membrane does not allow a particular substance to cross its membrane, we say the membrane is impermeable for that substance. Larger polar molecules and some ions cannot cross the membrane and need to be transported across with the help of membrane proteins.

Transporters: Structure of Channel Proteins F5-7 Made up of amino acid chains which traverse the membrane back and forth to form a cluster of ‘protein cylinders’. Aqueous pore links the intracellular and extracellular compartments. See Fig.5-4. Channels are used for rapid transport. Most channels are selective to particular molecules based on their charge. Molecules of the same charge as the channel will be repelled, whereas of the opposite charge attracted. Eg. Cation channels pass +vely charged ions but repel - vely charged ions. Channels which are open to the flow of ions all the time are known as leak channels.

Transporters: Gated Channels F5-8 Gated channels are usually in the closed state. The opening and closing of channels are controlled by: A) Ligands (ligand-gated channels). B) Voltage (voltage-gated channels). C) Physical changes (mechanically-gated channels).

Transporters: Carrier Proteins F5-9 Carrier transport is slow, but show greater selectivity for ions or molecules. The molecules or ions they ferry are termed substrates. For example, 1000 to 10, 000 ions versus 10 7 ions through channels. Moves organic molecules such as glucose, Na + and K + ions, and other amino acids.

Differences Between Channel and Carrier Proteins Molecules being transported bind temporarily with carrier molecules but travel straight though the pore of the channel protein. Carrier proteins do not create a continuous passage from inside and outside of the cell whereas channel proteins do. Carriers have two gates; one on the extracellular side and the other on the intracellular side. One gate is always closed preventing free exchange between the compartments.

Movement of Molecules Across Cell Membranes F5-13 Passive Transport Active Transport

Fick’s Law of Diffusion F5-15 Diffusion F5-14

Active Transport Active transport moves molecules against their concentration gradient, that is, from low to high concentrations. The energy comes from breaking the high-energy phosphate bond of ATP (ATP hydrolysis). Usually carries more than one type of molecule at the one time. They are called cotransporters. Creates a state of disequilibrium by making concentration gradients more pronounced.

Primary Active Transport: Na + /K + -ATPase F5-20 The energy comes directly from ATP hydrolysis. Pumps 3 Na + ions out of the cell and 2 K + ions into the cell. Creates and maintains concentration gradients in order to transmit electrical signals in neurons. *

References 1.Tortora, G.J. & Grabowski, S.R (2003). Principles of Anatomy & Physiology.New Jersey: John Wiley & Sons. Ch.3, pp Silverthorn, D.U (1998). Human Physiology: An Integrated Approach. New Jersey: Prentice Hall. Ch.5, pp