Plasma Membrane The membrane at the boundary of every cell Functions as a selective barrier for the passage of materials in and out of cells Membrane Composition Phospholipids Proteins Carbohydrates Cholesterol (Animal cells only) Phospholipid Bilayer Hydrophilic head Hydrophobic tail Molecules with hydrophilic and hydrophobic parts are called Ampipathic molecules Fluid Mosaic Model Refers to the way the phospholipids and proteins behave in a membrane, it accounted for the selective permeability of cell membranes Fluid Mosaic Model Fluid Phospholipid bilayer Molecules are not bonded together(just held together by hydrophobic interactions so are free to shift) Cholesterol helps animal cells remain fluid for membranes to function
Fluid Mosaic Model Mosaic Proteins: Float in a sea of phospholipids but stick in a certain place because their functional groups make part of the protein hydrophilic and part hydrophobic (Amphipathic) Proteins form a collage or mosaic pattern that shifts over time Protein Function in Membranes - Transport Passive: Protein has a hydrophilic core for hydrophilic ions and large polar molecules to move across membrane without using energy Active: Protein can use ATP to change shape and allow transport Protein Function in Membranes Enzymatic Activity Enzymatic proteins are built into membranes to carry out rxns Protein Function in Membranes Receptor Sites for Signals (Signal transduction)-membrane proteins have binding sites for specific molecules to relay messages inside a cell
Protein Function in Membranes Cell Division (Intercellular joining)- membrane proteins that hook together to form gap junctions and tight junction between cells Protein Function in Membranes Cell to Cell Recognition Membrane glycoprtoeins serve as ID tags on the outside of a cell so that cells can recognize each other Protein Function in Membranes Attachment to the Cytoskeleton Membrane proteins attach to the cytoskeleton on the inside of a cell and the ECM on the outside to stabilize the location of proteins and maintain the cells shape Types of Membrane Protein Intergral-inserted into the phospholipid bilayer Peripheral-not embedded in the phospholipid bilayer but are attached to the membrane surface
Membrane Carbohydrates Branched short chains of carbohydrates on external surface of the cell (only)(where the ID tag is needed) Function- Recognition of self vs other How do Materials get Across a Cell s Membrane? Problems Inside of the phospholipid bilayer is hydrophobic. Hydrophilic materials DON T cross easily Large molecules DON T cross easily. Transport in Passive transport (no energy (ATP)neededused to move materials from a [high]to[low] Active transport (needs energy (ATP)- used to move materials from a [low]to[high] Passive Transport Movement across membranes that does NOT require cellular energy (ATP) Small uncharged nonpolar molecules like N2, O2, and CO2 move freely across the membrane from [high->low] Hydrophillic ions and large polar molecules need specific transport proteins to move them from [high->low] Two Types of Passive Transport Channel proteins: have a hydrophilic core so polar substances can move down their concentration gradient Carrier proteins: bind specific molecules and change shape to move large molecules across membranes **even though proteins are being used to move materials across the membrane, it is still passive b/c the materials are moving from [high->low] so no ATP is used to move them!**
Diffusion The net movement of atoms, ions, or molecules down a concentration gradient Movement if from [high-low] concentration Equilibrium When the concentration is equal on both sides There is no NET movement of materials -Individual molecules still move but concentration of each is equal on both sides Factors that Effect Diffusion Rates Concentration (higher=faster) Temperature (higher=faster) Pressure (higher=faster) Particle size (smaller=faster) Mixing (stirring=faster) Osmosis Diffusion of water Water moving from an area of high to an area of low concentration No cell energy is used Tonicity The concentration of water inside a cell relative to that outside a cell 1. Isotonic 2. Hypotonic 3. Hypertonic
Isotonic Cell and water are equal in solute concentration No net movement of water in or out of the cell No change in cell size Animal cells love this Hypotonic Cell s [water] is LOWER than the outside water (cell has more solutes) Water moves into the cell from the solution Cell swells, may burst (animal- lyse) or the cell is turgid (plants- cell wall) Plant cells love this Hypertonic Cell s [water] is HIGHER than the outside water (cell has less solutes) Water moves out of the cell into the solution Cell shrinks or plasmolysis occurs (plants) Facilitated Diffusion Transport protein that helps hydrophilic and large polar molecules through the cell membrane Doesn t require energy (ATP) Works on a downhill concentration gradient [high-low] Aquaporins Newly found channels for water transport (Osmosis)
Active Transport Movement across membranes that DOES require cellular energy (ATP) to move materials against their concentration gradient [low->high] Membrane proteins are needed for active transport Used to establish concentration gradients for cellular processes Carrier-Mediated Transport General term for the active transport of materials into cells AGAINST the concentration gradient Movement is from [low->high] Examples 1. Na+-K+ pump 2. Electrogenic or H+ pumps 3. Contransport Na+-K+ Pump Moves Na+ ions out of cells while moving K+ ions in against their concentration gradient Electrogenic or H+ Pumps A.k.a proton pumps Create voltages across the membranes for other cell processes Used by plants, fungi, and bacteria
Contransport Uses a proton pump for movement of H+ against concentration gradient across a membrane, which creates an unstable membrane and allows other materials to be transported into the cell as the H+ diffuses back across the cell membrane to stabilize it Example Sucrose transport in plants Exocytosis Internal vesicles fuse with plasma membrane to secrete large macromolecule out of the cell Example Secretion of enzymes made by RER Endocytosis Cell takes in marcomolecules by forming new vesicles derived from plasma membrane around it Types Pinocytosis-moving liquids in Phagocytosis-moving solids in Receptor Mediated Endocytosis-uses receptors to catch specific kinds of molecules and then bring them in, like cholesterol