Membrane Function, Structure, and Transport PDF

MEMBRANE FUNCTION, STRUCTURE, AND TRANSPORT Membrane Function Membranes preform various functions in a cell. In this topic we study how membranes function as barriers which allow for the selective movement of substances Membrane proteins are what give different membranes their u...

MEMBRANE FUNCTION, STRUCTURE, AND TRANSPORT Membrane Function Membranes preform various functions in a cell. In this topic we study how membranes function as barriers which allow for the selective movement of substances Membrane proteins are what give different membranes their unique function Membrane Structure The Fluid Mosaic Model – is the model that describes the structure of membranes. The fluid part of the name derives from the fact that membranes have properties that are similar to a fluid because the molecules that make up the membrane are able to move around freely in certain ways. The mosaic part of the name refers to the membranes being a mosaic of lipid and protein. Phospholipids are a major component of membranes. A phospholipid has a hydrophilic region, which consists of the glycerol backbone and a phosphate group, and a hydrophobic region, which consists of two fatty acids. See the following illustration of a phospholipid: Phospholipids form a phospholipid bilayer. The phospholipid bilayer consists of two layers of phospholipids where the fatty acids of each layer are pointing towards each other in the center of the structure and the hydrophilic regions of each layer are pointing away from each other. See the following illustration of a phospholipid bilayer: 1  Phospholipids can move in different ways within the membrane: - Lateral Diffusion (this type of movement occurs frequently) - Rotation (spinning) (this type of movement occurs frequently) - Flip-Flop (this type of movement rarely occurs on its own) See the following illustration of the phospholipid movements: See the following table comparing the types of phospholipid movements: The membrane is a fluid structure and cells want the fluidity of their membrane to remain relatively constant. Membranes that have too high of a fluidity are leaky and low fluidity can inhibit the function of membrane proteins. Three things that affect fluidity are: Temperature. An increase in temperature will increase the fluidity, while a decrease will decrease fluidity. 2  Phospholipid molecules – the length of the fatty acids and the presence of C=C in the fatty acids affects membrane fluidity. Longer fatty acids will decrease the fluidity, while shorter fatty acids will increase fluidity. More C=C in the fatty acids will increase fluidity, fewer C=C will decrease fluidity. Cholesterol – is a “fluidity buffer” because its effects depend on temperature. At high temperatures, cholesterol decreases fluidity. At low temperatures, it increases fluidity. See the following table comparing these effects on fluidity: A second major component of membranes is proteins, and these are called membrane proteins There are two types of membrane proteins: Integral Membrane Proteins and Peripheral Membrane Proteins. See the following table comparing the these two types of membrane proteins: 3  Integral Membrane Proteins which are embedded in the phospholipid bilayer. Many Integral Membrane Proteins span the entire thickness of the phospholipid bilayer and are called Transmembrane Proteins. You should know some of the functions of transmembrane proteins, see the following illustration for the functions you should know: Peripheral Membrane Proteins are associated with the membrane but not embedded in the phospholipid bilayer Membrane proteins can be held in place within a membrane by anchoring them to something on the inside of the cell, like the cytoskeleton, or to something on the outside of the cell, like the extracellular matrix. Cholesterol is also a component of the plasma membrane of mammalian cells Membrane Transport One of the functions of membranes is to act as selectively permeable barriers. Some molecules (nonpolar molecules) can pass through a membrane without the aid of a protein because they can easily pass through the phospholipid bilayer. These types of molecules can pass through easily because they are highly lipid soluble and the hydrophobic interior of the bilayer is not a barrier to them. Some molecules (polar molecules and ions) can’t get across a membrane without the aid of a protein. These types of molecules or ions are poorly lipid soluble and the hydrophobic interior of the bilayer is a barrier to these types of substances. Polar molecules and ions will require membrane proteins to transport them from one side of the membrane to the other. See the following illustration of these concepts: 4  See the following table comparing the different types of substances shown above: 5  The following are different mechanisms of Membrane Transport: - Simple Diffusion - Facilitated Diffusion - Active Transport – Primary and Secondary - Endocytosis and Exocytosis - Osmosis When considering simple diffusion, facilitated diffusion, osmosis, primary active transport, and secondary active transport we can divide them into two categories: - Passive Transport - Active Transport Passive Transport is transport that does not require the input of energy from the cell and the substance transported moves down its concentration gradient. The passive forms of transport are simple diffusion, facilitated diffusion, and osmosis. Active Transport is transport that requires the input of energy from the cell and the substance transported is moved against its concentration gradient. See the following table comparing passive and active transport: Useful things to point out now: A concentration gradient is simply a difference in the concentration of a substance between two areas. For example: the inside and outside of the cell. See the following illustration for concentration gradient: 6  If something moves down a concentration gradient then it is moving from an area of high concentration to an area of low concentration If something moves up or against a concentration gradient then it is moving from an area of low concentration to an area of high concentration - See the following table comparing down and up: An electrochemical gradient is the combination of two gradients. The chemical gradient is the concentration gradient and the electrical gradient is the membrane potential. A membrane potential is a separation of charge across the membrane (Example: most plasma membranes have a slight excess of negative charges on the inside of the cell and a slight excess of positive charges on the outside. This type of membrane potential favors the movement of positive ions into the cell and the movement of negative ions out of the cell.) See the following illustration which describes a membrane potential: 7  See the following illustration which describes an electrochemical gradient: 8 Diffusion Diffusion is the net movement of a molecule or ion from and area of high concentration to an area of low concentration Diffusion rate measures the number of molecules or ions that move across a membrane in a given amount of time. Diffusion rate is affected by: Temperature: the higher the temperature, the faster the diffusion rate. So, an increase in temperature will increase the diffusion rate and a decrease will decrease the rate. Concentration Gradient: the larger the concentration gradient, the faster the diffusion rate. So, an increase in the concentration gradient will increase the diffusion rate, and a decrease will decrease the rate. Molecular Weight: the larger the substance, the slower the diffusion rate. See the following table that compares the effects of these things: 9 Simple Diffusion See the following illustration of simple diffusion: 10  Simple diffusion does not require a membrane protein and does not require the input of energy In simple diffusion a molecule simply passes through the membrane down its concentration gradient The direction of movement (for example: into or out the cell) is determined by the concentration gradient The molecules that move by simple diffusion are nonpolar and are able to pass through the membrane without the aid of a protein. Examples include: oxygen, carbon dioxide, and fatty acids. Transporting Polar Molecules and Ions The transport of polar molecules and ions requires the aid of membrane proteins because these types of substances can’t pass through the membrane on their own. See the illustration below: Transmembrane proteins that aid in the movement of substances across a membrane are called transport proteins. There are two types: - Channels - Carriers (Transporters) See the illustration below: 11  Channels and carriers both display specificity for what they transport. This simply means that they are specific for what they transport. Channels, when they are open, form a selective pore (channel) in the membrane which allows the transported substance to flow through it. Channels only allow for passive transport. See the following illustration of channels: Carriers (Transporters) will bind the substance to be transported on one side of the membrane, change their shape, and then drop the substance off on the other side of the membrane. Some carriers perform passive transport while others perform active transport. Carriers have a binding site that is specific for the substance(s) that are transported. See the following illustration of carriers: 12  There are different kinds of channels, see the following table for the different types: See the following illustration of how a ligand-gated channel works: 13  There are different types of carriers, see the following table for the different types: See the following illustration of how the different types work: 14 Facilitated Diffusion See the following illustration of simple diffusion: Facilitated diffusion is diffusion that is mediated (facilitated) by a membrane protein. This form of transport does not require energy but does require a membrane protein Channels and Carriers (Transporters) can both be used in facilitated diffusion 15  In facilitated diffusion a molecule or ion moves down its concentration gradient across a membrane using either a channel or carrier The concentration gradient determines the direction of movement (example: into or out of the cell) Ions and polar molecules can be moved by facilitated diffusion Active Transport See the following illustration of active transport: Active transport requires energy from the cell and a membrane protein The type of membrane proteins used in active transport are carriers In active transport, a molecule or ion will be moved up (against) its concentration gradient across the membrane using a carrier 16  Carriers that perform active transport are sometimes called pumps There are two types of active transport: primary active transport and secondary active transport The difference between the two types is the energy source for the carrier, in primary active transport the energy source is ATP and in secondary active transport the energy source is an electrochemical gradient Primary Active Transport: is active transport in which the carrier uses the hydrolysis of ATP as its power source to move substances against their concentration gradient. The Na+/K+-ATPase (also known as the Na+/K+-Pump) is a carrier that performs primary active transport. The Na+/K+-ATPase is a carrier that is present in plasma membranes. This carrier moves Na+ ions out of the cell against their concentration gradient and K+ into the cell against their concentration gradient. This carrier hydrolyzes ATP to power this transport. See the following illustration of primary active transport: Secondary Active Transport: is active transport in which the carrier uses an electrochemical gradient as its power source. In this type of active transport one substance passes through the carrier down its electrochemical gradient while a second type of substance is transported against its concentration gradient. It is the movement of the substance down its concentration gradient that powers the carrier so that it can transport something else against its concentration gradient. See the following illustration of secondary active transport: 17  See the table below for a comparison of the types of transport we have discussed so far: 18 Endocytosis and Exocytosis Endocytosis and exocytosis are forms of transport that involve vesicles In Endocytosis, a portion of the plasma membrane surrounds something on the outside of the cell and then pitches off into the inside of the cell to form a vesicle. The contents of the vesicle now contain something that was on the outside of the cell. So, endocytosis is used to take substances into the cell. There are different kinds of endocytosis: Pinocytosis: is a type of endocytosis in which the cell takes in extracellular drops of fluid. Sometimes referred to as “cell drinking”. Phagocytosis: is a type of endocytosis in which the cell takes in a particulate and in some cases an entire cell. Sometimes referred to as “cell eating” Receptor-Mediated Endocytosis: this is endocytosis that involves a receptor the binds a specific molecule on the outside of the cell. In this form the substance that is taken in binds to receptors in the plasma membrane. In exocytosis, a vesicle on the inside of the cell moves to the plasma membrane where its membrane fuses with the plasma membrane and releases the contents of the vesicle to the outside of the cell Osmosis Osmosis is the net diffusion of water across a membrane In osmosis water is the substance diffusing and it is going to diffuse towards the area with a higher solute concentration So, if there is a higher solute concentration on the inside of the cell, then water will diffuse into the cell. However, if the solute concentration is higher on the outside of the cell, then water will diffuse out of the cell We are now going to look at different types of solutions that have distinct effects on the movement of water: Hypertonic solution, hypotonic solution, and isotonic solution. 19  A hypertonic solution causes water to move out of the cell and the cell to shrink. A hypertonic solution has a solute concentration that is greater than (hyper) the solute concentration on the inside of the cell. When a cell is placed in a hypertonic solution water diffuses out of the cell toward the higher solute concentration. A hypotonic solution causes water to move into the cell and the cell to expand. A hypotonic solution has a solute concentration that is less than (hypo) the solute concentration on the inside of the cell. When a cell is placed in a hypotonic solution water diffuses into the cell towards the higher solute concentration. An isotonic solution causes no movement of water either into the cell or out of cell. The cell will remain the same size in this type of solution. An isotonic solution has a concentration of solute that is equal to (iso) the concentration of solute on the inside of the cell. Because the concentration of solute is equal on both sides, there will no movement of water. See the following table for a comparison of the different solutions: 20