Membrane Structure & Functions Novosibirsk State Agrarian University

Membrane Structure & Functions Novosibirsk State Agrarian University Professor Korotkevich O.S.

Membrane Structure A double layer of lipids molecules (type: phospholipids). Amphipathic molecules. Organized into a bimolecular layer with the nonpolar fatty acid chain in the middle. The polar regions of the phospholipids are oriented toward the surface of the membrane. No chemical bonds link the phospholipids to each other or to the membrane proteins. Lateral movements of both membrane lipids. The long fatty acid chains can bend and wiggle back and forth. Thus, the lipids bilayer has the characteristics of a fluid.

Membrane Structure Cholesterol: A steroids Slightly amphipathic because of a single polar hydroxyl group. Cholesterol associates with certain classes of plasma membrane phospholipids and proteins, forming organized clusters that functioning the pinching off of proteins of the plasma membrane to form vesicles that deliver their contents to various intracellular organelles.

Membrane Structure Proteins: There are two classes of proteins: Integral Peripheral

Membrane Structure Integral membrane protein: Are closely associated with the membrane lipids. Amphipathic: Having polar amino acid side chains in one region of the molecules. Nonpolar side chains clustered together in a separate region. Because of amphipathic, the polar regions are at the surfaces in association with polar water molecules. The nonpolar regions are in the interior associated with nonpolar fatty acid chains.

Membrane Structure Integral membrane protein: Many can move laterally in the plane of the membrane. But others are immobilized. Most integral proteins referred to as transmembrane proteins. Cross the lipids bilayer several times. These proteins have polar regions connected by nonpolar segments that associate with nonpolar regions of the lipids in the membrane interior, and the polar regions of transmembrane proteins extend far beyond the surface of the lipids bilayer. Some, form channels through which ions or water can cross the membrane. Others are associated with the transmission of chemical signals across the membrane.

Membrane Structure Peripheral Proteins: Not amphipathic. Thus, do not associate with the nonpolar regions of the lipids in the interior of the membrane. Located at the membrane surface where thy are bound to the polar regions of the integral membrane proteins. Most of the peripheral proteins are on the cytosolic surface of the plasma membrane where they are associated with cytoskeletal elements that influence cell shape and motility.

Membranes functions Membranes perform a variety of functions Membrane Functions: Regulate the passage of substances into and out of cells and between cell organelles and cytosol Detect chemical messengers arriving at the cell surface Link adjacent cells together by membrane junctions Anchor cells to the extracellular matrix

Movement of Molecules across Cell Membranes

Transports across plasma membrane either: Passive Transport: The diffusion of a substance across a biological membrane, without need for energy. Active Transport: The movement of a substance across a biological membrane against its concentration or electrochemical gradients, with the help of energy input and specific proteins.

Diffusion through Membranes Diffusion: The movements of substances down its concentration gradient, from a more concentrated to a less concentrated area. Flux: the amount of material crossing a surface in a unit of time.

Diffusion through the lipids bilayer: Nonpolar molecules diffuse more rapidly across plasma membranes. Because nonpolar molecules can dissolved in the nonpolar regions of the membrane regions occupied by the fatty acid chains of the membrane phospholipids.

e.g. Oxygen, carbon dioxide, fatty acids, and steroid hormones. Most of the organic molecules are ionized or polar molecule, thus have a low solubility in the lipid bilayer. Thus, Most polar molecules diffuse into cells very slowly or not at all.

Diffusion of ions through protein channels: Ions such as Na-, K+, Cl-, and Ca2+ diffuse a cross membranes at rates that are much faster than would be predicted from their very low solubility in membrane lipids. Because of that some of the integral membrane proteins form a channels through which ions can diffuse across the membrane.

Diffusion of ions through protein channels: Ion channels show selectivity for the type of ion that can pass through them. This selectivity is based on: Channel diameter. Charged and polar surfaces of the protein subunits that form the channel walls and electrically attract or repel the ion. The diameters of protein channels are very small. Thus, the small size of the channels prevents larger, polar, organic molecules from entering the channel.

Membrane potential: The exists a separation of electrical charges across plasma membrane.

Diffusion of ions through protein channels: Ion channels can exists in an open or closed state, and changes in a membrane׳s permeability to ions can occur rapidly as a result of opening or closing of these channels. Channel gating: the process of opening and closing ion channels.

Mediated Transport Systems: There are a number of other molecules including amino acids and glucose that are able to cross membranes yet are too polar to diffuse through the lipids bilayer and too large to diffuse through ion channels. The passage of these molecules and the non-diffusional movements of ions are mediated by integral membrane proteins known as transporter (or carriers). The movement of substances through a membrane by these mediated-transport systems depends on conformational changes in these transporters.

Mediated Transport Systems: Steps of Mediated Transport System The transported solute must first bind to a specific site on transporter. A portion of the transporter then undergoes a change in shape, exposing this same binding site to the solution on the opposite side of the membrane. The dissociation of the substance from the transporter binding site completes the process of moving the material through the membrane.

Mediated Transport Systems: Note: the binding of the solute is not necessary to trigger the conformational change. The oscillations in conformation are presumed to occur continuously whether or not solute is bound to the transport protein.

Mediated Transport Systems: There are many types of transporters in membranes, each type having binding sites that are specific for a particular substance or a specific class of related substances. e.g. amino acids and sugars undergo mediated transport, a protein that transports amino acids does not transport sugars, and vice versa. There are two types of mediated transport: Facilitated diffusion Active transport.

Facilitated diffusion: The net flux of a molecule across a membrane always proceeds from higher to lower concentration and continues until the concentrations on the two sides of the membrane become equal. Like diffusion, facilitated diffusion do not need ATP. e.g. Among the most important facilitated-diffusion systems in the body are those that move glucose across plasma membrane.

Active transport: Active transport differs from facilitated diffusion in that it uses energy to move a substances uphill across a membrane, in other words; against the substances electrochemical gradients. This energy can: Alter the affinity of the binding site on the transporter such that it has a higher affinity when facing one side of the membrane than when facing the other side. Alter the rates at which the binding site on the transporter is shifted from one surface to the other.

Active transport: Two means of coupling an energy flow to transporters are known: Primary Active Transport: the direct use of ATP. Secondary active transport: The use of ion concentration differences across membrane to drive this process.

Primary Active Transport: The hydrolysis of ATP by a transporter provides the energy. The transporter is actually an enzymes (called ATPase) that catalyzes the breakdown of ATP and, in the process, phosphorylates it self.

Primary Active Transport: The binding site for the transported solutes is exposed to the extracellular fluid and has a high affinity because the protein has been phosphorylated on its intracellular surface by ATP. The transported solute in the extracellular fluid binds to the high affinity binding site, causing a change in the conformation of the transporter.

The change in conformation of the transporter results in removal of the phosphate group from the transporter. This decreases the affinity of the binding site, leading to: The release of the transported solute into the intracellular fluid. When the-low affinity site is returned to the extracellular face of the membrane by the random oscillation of the transporter, it is in a conformation that again permits phosphorylation, and the cycle repeated.

e.g. The major primary active-transport proteins found in most cells are: Na+ / K+ -ATPase: present in all plasma membrane. H+ / K+ -ATPase: present in the plasma membranes of the acid-secreting cells in the stomach and kidneys, where it pumps one hydrogen ion out of the cell and moves one potassium in for each molecule of ATP hydrolyzed.

Secondary Active Transport: It is differs from primary active transport by its use of an ion concentration gradient across a membrane as the energy source. The flow of ions from a higher concentration (higher energy state) to a lower concentration (lower energy state) provides energy for the uphill movement of the actively transported solute.

It has a binding site for the actively transported solute and a binding site for an ion. This ion is usually sodium, but in some cases it can be another ion such as bicarbonate, chloride or potassium.

The binding of an ion can: Altering the affinity of the binding site for the transported solute. Altering the rate at which the binding site on the transport protein is shifted from one surface to the other. e.g. In most cells, amino acids are actively transported into the cell by this way.

Secondary Active Transport: Coupled transport. Energy needed for “uphill” movement obtained from “downhill” transport of Na+. Hydrolysis of ATP by Na+/K+ pump required indirectly to maintain [Na+] gradient.

Osmosis Osmosis is the passive transport of water Osmosis: The diffusion of water across a selectively permeable membrane. Water diffuses through cell membranes by a group of membrane proteins known as Aquaporins.

Bulk Transport Movement of fluids or gases or many large molecules that cannot be transported by carriers from region of higher pressure to one of lower pressure. Includes: Exocytosis Endocytosis

Endocytosis The cellular uptake of macromolecules and particulate substances by localized regions of the plasma membrane that surrounded the substance and pinch off to form an intracellular vesicle.

Exocytosis The cellular secretion of macromolecules by the fusion of vesicles with the plasma membrane. Functions: It provides a way to replace portion of the plasma membrane that have been removed by endocytosis (to add new membrane). It provides a route by which membrane-impermeable molecules (such as protein hormones) synthesized by cells can be secreted into the extracellular fluid.

Exocytosis

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