Lesson_5_Membranes - Movement of Molecules Across Membranes...

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Unformatted text preview: Movement of Molecules Across Membranes Lesson 5 Dr. Pamela Vandevord BME 5005 Objectives Following this lesson, you should be able to describe: 0 Membrane Structure ° Acme TranSPOFt . Diffusion — Primary active . . . transport — S1mple d1ffus1on . _ _ _ _ — Secondary act1ve — Fac111tated d1ffus1on transport - Channels - Carrier Proteins . EndocytOSIS . Osmosis - Exocytos1s Introduction As previously discussed, the membrane is the outside wall of the cell, and serves as a very important barrier between the intracellular contents and the extracellular fluid. The membrane selectively allows molecules to cross into and out of the cell. Phospholipids The membrane is primarily composed of phospholipids. Phospholipids are made of a ? 2 :3 phosphate head (gray), and 2:: two fatty acid chain tails 2:: $3: (yellow). The phosphate head :3: Symbm is charged and polar, so it is hydrophilic (likes water). The 2:; fatty acid tails are Tails‘\\‘c§§3 hydrophobic (dislikes water). ‘“ So when you put a bunch of these phospholipids together, H dm hmc Water they prefer to line up in a hé’ads" bilayer, with all the Hydrophobic a hydrophilic heads on the tails outside, and the hydrophobic tails on the inside. Other Membrane Components Besides phospholipids, there are proteins, cholesterol and carbohydrates associated with the membrane as well. Proteins (purple) can be embedded into the membrane, and act as receptors or tranporters, as we will discuss later in this lecture. Cholesterol (yellow) molecules are also embedded in the phospholipid bilayer, and work to improve the fluidity of the membrane. Small carbohydrate chains (green) can be attached to the proteins or phospholipids, and serve as recognition sites. Glycoprotein Carbohydrate h“: (ofglycoprotein) Ax -. 4* Fibers of the J ‘ extracellular Phosph'o- lipid - Cholesterol [ _. Microfilaments of moi-(gins the cytosketeton CYTOPLASM Iiix".Ll=_|I5u:,-n'I‘J'mzlu',‘ L:I";ll\ii‘l lI'L' Membrane Proteins Proteins in the membrane are used for many different functions. They can act as enzymes to facilitate reactions on the membrane. They can also be receptors for extracellular messengers (like hormones), and transfer the signal inside the cell. Or they can be channels, or pores, for certain molecules that could not otherwise cross the membrane. We’ll talk more about these later in this lesson... Messenger Channel molecule E Receptor _ Activated - molecule Diffusion Diffusion is a natural process of molecules wanting to spread out. Just as you do not enjoy being in a crowded room, molecules also do not like to be crowded. Therefore, if given extra space, molecules will spread out as evenly as possible. Notice in the first figure, how all the dye molecules are on the left side of the membrane. In the middle picture, some will move to the other side of the membrane, until there are approximately equal numbers on both sides. This point is called equilibrium. At equilibrium, the rate of particles moving to the left side is the same as the particles moving to the right. So the numbers on each side remain constant. Molgcules of dye Membrane EQUILIBRIUM f1: i .l'a I I l ' «I a i I I —7-~ ' a ' I + II i l. I i ‘l' i a ' a i i a I. I I g I i I I 9 I n Diffusion When molecules are not at equilibrium, there is said to be a concentration gradient, and molecules Will move along the gradient until they reach their equilibrium point. Each type of molecule has its own concentration gradient. In the figure below, notice how initially all the purple dots are on the left and the green dots are on the right. The purple dots’ concentration gradient favors movement to the right side (click to see). The green dots’ gradient favors movement to the left side (click). At equilibrium, there are equal purple dots on both sides, and green dots on both sides. Therefore, both molecules have reached their respective equilibrium points. EQUILIBRIUM . U I I I Concentration . . i ' I . gradient O . I ' I I . I .. g i i I I I O . Hypotonic Hyperionic solution solution H20 —" _—,-— _h —* __.i_ . a o. .. I. \_ a .' Solute 1.x o 0 It. .0 molecule ' . . . . Selectively permeable I J “'L”"' membrane J HYPOTONIC SOLUTION HYPERTONIC SOLUTION - Water ,jp molecule ' _-- -- I ,3; I- .i ,4. _ Concentration gradient of 3.-. I, if water ' . -- . Solute molecule with membrane cluster of water molecules —p NET FLOW OF WATER lit-\inlnon'p'i'l:::-.I', .Cll-éll'fA'l I'L' Osmosis Sometimes there is a semi- permeable membrane that separates two compartments that is not permeable to the solute, but is permeable to water. Water also prefers to move down its concentration gradient (click), and this process is called osmosis. A solution with fewer solute particles is called hypotonic. A solution with many solute particles is called h ypertonic. Water will move from the hypotonic solution, towards the hypertonic solution, until the concentration is equal on both sides. This is now equilibrium for the solution, and the solutions are called isotonic. Semi-permeable Cell Membranes Cell membranes are considered semi-permeable. Water can flow in and out pretty easily, but many of the solute particles cannot. Therefore, it is important for the cell to be in an isotonic environment, where water concentration is at equilibrium between the inside and outside of the cell. If the surrounding solution was hypotonic (less solute), water would flow into the cell, causing it to burst. If the cell was in a hypertonic solution (more solute), water would leave the cell, and shrivel it up. I‘SOTONlC HYPOTONIC HYPERTONIC SOLUTION SOLUTlDN SOLUTION H20 , H20 “20 H20 ANIMAL CELL __ , (1) Normal {2) Lysing (3) Shriveled - 1 Some solute molecules need to cross Slmp 6 VS- the cell membrane. If the molecule is small and nonpolar (like 02 or FaCIhtated C02), it can cross the hydrophobic . . barrier on its own, like the pink lefUSIOD molecules in this diagram. This is called simple diffusion (click). However, charged or polar molecules cannot easily pass “WE” Wei“ through the hydrophobic membrane. ' They need some sort of special protein that provides a favorable ‘ environment to pass through. This ' I I; ', is called facilitated diffusion (click). In this diagram, the yellow lipid_i,,3,|ub,, molecules pass through a channel W9: protein, which acts as the facilitator. / Note that the molecules still travel with their natural concentration gradient! Extracellular fluid Lipid l-_i|ayer Lipid—soluble solutes CY'tflplflfim Facilitated Diffusion Another kind of protein that can be used for facilitated diffusion is a carrier protein. Carrier proteins allow hydrophilic molecules to travel through the hydrophobic membrane, like the channel proteins, but instead of acting like a pore, they act more like a revolving door. They trap a molecule, then change conformations to let it out on the other side. Again, since the molecules are traveling with their concentration gradient, this process is still a type of diffusion. If the cell wants to move molecules against their concentration gradient, then energy needs to be used, just w like swimming upstream. “5 war W binding Energy in the cell is in the form of a molecule called ATP. If ATP is used to go against the gradient, the process is called active transport. In this diagram, ATP is being used to shuttle sodium ions out of the Ma, cell, and potassium ions into the Na“ 53:32 ' + cell. This is called a sodium- + potassium ATPase pump, and is very common on most cell membranes, especially nerve cells! Active Transport + Na Na+ Na_+ + + Na Extracellular fluid Na Endocytosis innit (-‘t-‘UU ,3: .,. L' . tF[JF§’{££-.;{gf'ifi'§ in HIE-5:55“ I: . Membrane Sometimes, the membrane can actually be used to bring molecules into the cell. Endocytosis is the process of the membrane creating a pocket around molecules, and pinching off into the cell. Often, this process will bring in several molecules at once. Inside the cell, the membranous pocket can be broken down and the contents can be let out, into the cytoplasm. Exocytosis Exocytosis is the opposite of endocytosis. When molecules inside of the cell need to leave, they can be packaged inside membranous vesicles and pushed towards the cell membrane. When the vesicle arrives, its wall fuses with the cell’s membrane, and lets the contents of the vesicle spill out into the extracellular fluid. In the next lesson, we will see that nerve cells use this method to emit neurotransmitters! FLUID OUTSIDE CELL 0 o CYTOPLASM Conclusions The cell membrane is a dynamic barrier between the intracellular contents and the extracellular fluid. It selectively allows molecules to pass in and out of the cell. This barrier is extremely important for normal functioning of the cell, and the proteins embedded in the membrane serve just as much an important role as the phospholipids that make up most of the membrane. The proteins can act as channels or carrier molecules. Sometimes, they can even use ATP for active transport. The membrane is an important cell part to understand, since it governs the properties of the cell contents. Things to Remember! The membrane is made up of phospholipids, proteins, cholesterol, and carbohydrates Diffusion is the natural tendency of molecules to spread out as evenly as possible Active transport uses energy to work against the normal diffusion pattern Endocytosis and exocytosis use pieces of the membrane to transport molecules into and out of the cell respectively ...
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Lesson_5_Membranes - Movement of Molecules Across Membranes...

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