Chapter 5 The Working Cell Membrane Structure and Function 5 1 Membranes are fluid mosaics of lipids and proteins with many functions Fluid Mosaic Model describe membrane structure diverse protein molecules suspended in a fluid phospholipid bilayer Plasma membrane is selectively permeable allows some substances to cross more easily than others Pg 74 Diverse Functions 5 2 The spontaneous formation of membranes was a critical step in the origin of life Phospholipids key ingredients of biological membrane one of 1st organic molecules that formed from chem reactions on early earth o Spontaneously self assemble into simple membranes Plasma membrane that allows cells to regulate their chemical exchanges w environment is a basic need for life evolutionary unity of life 5 3 Passive transport is diffusion across a membrane with no energy investment Molecules have thermal energy from their constant motion Diffusion is a result particles of any substance spread out into available space Diffuses down concentration gradient until equilibrium is reached molecules continue to move around but there is no net change in conc Passive transport cell doesn t do work when molecules diffuse across membrane o Ex Oxygen into cells CO2 waste out O2 and CO2 are small nonpolar molecules that diffuse easily across phospholipid bilayer of membrane ions and polar molecules can do so if they are moving down conc gradient w a transport protein 5 4 Osmosis is diffusion of water across a membrane If one side of membrane has higher concentration of solute water will pass through to even it out Polar water molecules cluster around hydrophilic solute molecules there are fewer water molecules free to cross membrane 5 5 Water balance between cells and their surroundings is crucial to organisms Tonicity ability of a surrounding solution to cause a cell to gain or lose water depends on the concentration of a solution relative to the concentration of solutes inside the cell 3 levels of tonicity 1 Isotonic equal solute levels Animal Normal Plant Flaccid 2 Hypotonic lower solute levels Animal Lysed burst Plant Turgid normal 3 Hypertonic higher solute levels Animal shriveled Plant shriveled plasmolysis Osmoregulation control of water balance NOT DONE 5 6 Transport proteins can facilitate diffusion across membranes Facilitated diffusion assisted transport of hydrophilic molecules and ions type of passive transport Transport proteins help specific substance diffuse across a membrane down its concentration gradient no input of energy Substances that use facilitated diffusion for crossing cell membrane include a number of sugars aa ions water Aquaporin protein channels make very rapid diffusion of water in and out of cells possible 5 7 Research on another membrane protein led to the discovery of aquaporins NOT DONE 5 8 Cells expend energy in the active transport of a solute Active transport cells must expend energy to move a solute against its concentration gradient Allows cell to maintain internal concentration of small molecules and ions that are dif from concentrations in the surroundings Ex animal cell higher K conc inside the cell and lower Na that the outside sodium potassium pump maintains concentration differences 5 9 Exocytosis and endocytosis transport large molecules across membranes Exocytosis export bulky materials such as proteins and polysaccharides o Transport vesicle filled w macromolecules buds from the Golgi and moves to plasma membrane fuses and its spilled out of cell o Ex cells in pancreas manufacture insulin and secrete it into extracellular matrix where its picked up by bloodstream Endocytosis cells take in large molecules o 2 kinds Phagocytosis cellular eating cell engulfs particle by wrapping pseudopodia around it and packaging it within a membrane enclosed sac vacuole Ex amoeba take in food WBCs engulf bacteria Receptor mediated endocytosis enables a cell to acquire specific solutes o Receptors for specific molecules are embedded in regions of membrane that are lined by a layer of coat proteins o Plasma membrane indents to form a coated pit receptors pick up particular molecules from extracellular fluid Ex take in cholesterol from blood synthesis of membranes and as a precursor for steroids cholesterol circulates in blood in particles called low density lipoproteins LDLs bind to receptor proteins enter cell by endocytosis Energy and the Cell structure of matter 5 10 Cells transform energy as they perform work Energy capacity to cause change or perform work Kinetic energy energy of motion Thermal energy type of kinetic energy associated w the random movement of atoms or molecules transfer of this energy is heat Potential energy energy that matter possesses as a result of its location and Chemical energy potential energy available for release in a chemical reaction most important type of energy for living organisms transformed to power the work of the cell Thermodynamics study of energy transformations that occur in a collection 1st law of thermodynamics law of energy conservation energy in the universe is constant energy can be transferred and transformed but it can be created or destroyed When energy is transferred or transformed some energy becomes unavailable to do work it is converted to thermal energy and released as heat Entropy measure of disorder or randomness 2nd law of thermodynamics energy conversions increase the entropy of the Cellular respiration chemical energy stored in organic molecules is used to universe produce ATP 5 11 Chemical reactions either release or store energy Exergonic reaction releases energy o Begins w reactants whose covalent bonds contain more potential energy than those in the products o Amount of energy released is the dif in potential energy between the reactants and the products Endergonic reaction require a net input of energy and yield products that are rich in potential energy o Starts w reactants w little potential energy o Ex photosynthesis Metabolism total of an organisms chemical reactions Metabolic pathway series of chemical reactions that either builds a complex molecule or breaks down a complex molecule into simpler compounds o slow burn of cell resp Energy coupling use of energy released from exergonic reactions to drive endergonic reactions crucial in all cell 5 12 ATP drives cellular work by coupling exergonic and endergonic reactions Adenosine triphosphate powers nearly all forms of cellular work Bonds connecting the 3 phosphate groups are unstable