Plasma Membrane Structure and Function PDF

Structure and function of the plasma membranes Topic 5 BIOL 130 SLOs Understand the cell membrane fluid mosaic model Describe phospholipid, protein, and carbohydrate functions in membranes Discuss membrane fluidity Explain why and how passive transport occurs Understand the osmosis and di...

Structure and function of the plasma membranes Topic 5 BIOL 130 SLOs Understand the cell membrane fluid mosaic model Describe phospholipid, protein, and carbohydrate functions in membranes Discuss membrane fluidity Explain why and how passive transport occurs Understand the osmosis and diffusion processes Define tonicity and its relevance to passive transport Understand how electrochemical gradients affect ions Distinguish between primary active transport and secondary active transport Describe endocytosis, including phagocytosis, pinocytosis, and receptor- mediated endocytosis Understand the process of exocytosis Eukaryotic cells: the plasma membrane Plasma membrane: phospholipid bilayer with embedded (integral) or attached (peripheral) proteins and separates the cell's internal content [cytoplasm] from its surrounding environment. Cytoplasm: entire region between the plasma membrane and the nuclear envelope, consisting of organelles suspended in the gel-like cytosol, the cytoskeleton, and various chemicals The plasma membrane is described with the fluid mosaic model. Integral membrane proteins span the plasma membrane Proteins can span the plasma membrane with one or multiple alpha- helices or beta- sheets. What types of amino acids could we expect to find in the regions spanning the membrane? Plasma membrane components and functions Component Location Phospholipid Main membrane fabric Attached between phospholipids Cholesterol and between the two phospholipid layers Embedded within the Integral proteins (for example, phospholipid layer(s); may or integrins) may not penetrate through both layers On the phospholipid bilayer's inner or outer surface; not Peripheral proteins embedded within the phospholipids Selective permeability of the plasma membrane results in two distinct classes of molecular transport. Passive transport requires no input of cellular energy—no ATP involved. Free diffusion Facilitated diffusion Osmosis Active transport requires the input of cellular energy—ATP is involved. Primary active transport Diffusion is primary force at play in passive transport. Diffusion is defined as the movement of molecules from an area of high concentration to an area of low concentration. The selectively permeable nature of the plasma Larger and electrically charged molecules require facilitated diffusion to freely cross the plasma membrane. Facilitated diffusion requires the presence of proteins. Proteins can form channels that allow molecules to passively diffuse into or out of the cell. Proteins can also form more elaborate structures that pair the diffusion of one molecule to another, like carrier proteins. The rate of diffusion is governed by various physical characteristics of the molecules and the environment. The steepness of the concentration gradient Molecular size Temperature of the environment Environmental media Solubility Surface area of plasma membrane Distance Selectively permeable membranes can stop the movement of solutes resulting in osmosis. Osmosis is the diffusion of water molecules across a semi- permeable membrane. When solute movement is blocked water will diffuse down its own concentration gradient. Biology is concerned with the cell therefore tonicity is comparing the outside to the inside of the cell Hypotonic = more solute inside cell than outside Isotonic = equal solutes inside and out Hypertonic = less solutes inside cell than outside The selective permeability of the plasma membrane results in osmotic pressure having great effects on cells Animal cells in hypertonic solution will experience crenation (shriveling) Animal cells in an isotonic solution are in the “Goldie-locks” zone (just right) Animal cells in a hypotonic solution will experience lysis (cellular explosion) Cells with cell walls are protected from cell lysis Organism with cell walls utilize osmotic pressure in order to maintain structural integrity. In plant cells we refer to this as turgor pressure, the pressure exerted by water inside the cell pushing out against the cell wall. Hypotonic solution are the “Goldie-locks” zone for organisms with cell walls Active transport is defined by the use of energy in order to move something in or out of a cell Active transport requires ATP Active transport is the process of moving something against its concentration gradient This becomes a means for cells to establish concentration gradients that can be used to do work Active transport is used by cells to develop and maintain electrochemical gradients Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients. This represents the process the nervous system utilizes to send electrical signals This is also used in our muscle cells to drive muscular contractions There are two different classes of active transport Primary active transport is a cell using energy (ATP) to move ions across the membrane—this leads to an electric gradient Primary active pumps frequently pair the movement of different ions Secondary active transport harnesses the electrochemical gradient created by primary active transport This is in effect a “piggy-back effect” It is relatively easier for cells to move small molecules compared to larger molecules Secondary active transport increases the efficiency of the cellular ATP output Endocytosis is perhaps the most expensive means of actively transporting something into a cell 3 classes of endocytosis Phagocytosis: classified as the larger package ingulfed—a vacuole Pinocytosis: classified as the smaller package ingulfed—a vesicle Receptor mediated: driven by the activation of surface receptors—highly selective Exocytosis is perhaps the most expensive means of actively transporting something out of a cell In exocytosis, a vesicle is directed to the plasma membrane, attaches to the membrane and opens releases its contents outside of the cell. This is a means for nerve cells to communicate with one another