Metabolism Ch 5

Cellular Transport Cellular Transport: movement of substances across a membrane [in or out] and within the cell Includes substances needed for reactions in the cell, and substances created by reactions in the cell Passive Transport: does not require energy input Diffusion, facilitated diffusion, osmosis Active Transport: requires the input of energy [ATP!!] Protein pumps, endocytosis, exocytosis

Diffusion and Membranes Diffusion: spontaneous spreading of molecules or atoms from an area of high concentration to low concentration Concentration: the amount of particles in a given amount of space Facilitated Diffusion: solute binds to a transport protein and moves across membrane [transport proteins are specific to certain solutes]

Diffusion and Membranes The following factors all effect the rate of diffusion Bigger concentration difference will increase rate of diffusion Higher temps increases rate of diffusion Small ions/molecules diffuse more quickly Ions will flow towards the opposite charge Higher pressure increases rate of diffusion Dynamic Equilibrium: equal concentrations are reached, the particles will still move, but no overall change

Osmosis Because lipid bilayers [cell membrane] are selectively permeable, only some substances can diffuse across them water, hydrophobic molecules, and gases CAN cross, but ions and most large polar molecules [sugar] cannot

Osmosis Osmosis: diffusion of water across a semi-permeable membrane Occurs in response to differences in solute concentration The direction and rate of this diffusion is described in terms of tonicity Hypotonic, isotonic, hypertonic

Tonicity Isotonic: equal solute concentrations, no osmosis HypOtonic: fluid with low solute concentration compared to another Hypertonic: fluid with high solute concentration compared to another

Osmosis If a cell is hypotOnic, water will diffuse into the cell [swell] If it is hypertonic, water will diffuse out [shrink] Diffusion will continue until the fluids are isotonic **Water is moving in both directions, but there is a net movement in one direction**

Turgor An inflow of water causes pressure to build, in plant cells the cell wall counteracts the increase to control the amount of expansion Turgor: pressure that a fluid exerts against a structure Osmotic Pressure: amount of turgor that prevents osmosis into cytoplasm or other hypertonic fluid

Membrane Trafficking Some substances are too large to move through the membrane by diffusion or transport proteins Exocytosis: cell expels a vesicle’s contents to extracellular fluid Endocytosis: cell takes in a small amount of extracellular fluid (and its contents) by the ballooning inward of the plasma membrane Both require energy, both continually replace and withdraw patches of the membrane

Membrane Trafficking Phagocytosis: “cell eating,” a cell engulfs a large solid particle like another cell The object is digested by lysosomal enzymes Some white blood cells engulf viruses and bacteria this way

Cellular Energy Remember chapter 3? ATP [adenosine triphosphate] is a nucleic acid that serves an important role as an energy carrier in cells ATP is composed of adenine, ribose [sugar], and three phosphate groups Bonds between phosphate groups hold A LOT of energy which can be transferred when the bonds are broken

Cellular Energy When a phosphate group is transferred from ATP to another molecule, ADP is formed [adenosine diphosphate] Phosphate groups are then added back onto the ADP to create ATP and so on  ATP/ADP cycle These reactions release energy that allows the cell to do work and perform necessary functions

Energy Energy: the capacity to do work The energy that fuels most life on Earth comes from the sun, however, the dispersal of energy is resisted by chemical bonds During a chemical reaction, one or more reactants become one or more products Some reactions require energy, some release it The energy in chemical bonds of a reaction contributes to a molecule’s free energy [energy available for work]

Energy Work occurs as a result of energy transfers Energy lost from a transfer is usually in the form of heat, which is not useful for doing work Due to this loss, energy in the universe must be replenished for life to continue ENERGY ONLY FLOWS IN ONE DIRECTION

Energy Metabolism: chemical reactions by which cells acquire and use energy All living things are “chemical factories”  driven by chemical reactions! Cells store energy by running reactions that build organic molecules [endergonic] Cells use energy by running reactions that break down organic compounds [exergonic]

Energy However, chemical bonds do not break without at least a small input of energy Activation Energy: minimum amount of energy required to start a chemical reaction At this state, the reaction can run without any additional energy input Sometimes the amount of energy needed for a reaction to occur is so high, that the reactions would occur incredibly slow if allowed to occur on their own

Enzymes Metabolism requires the use of enzymes Enzyme: organic molecule [PROTEIN] that speeds up a reaction without being changed by it In a process called catalysis, an enzyme makes a reaction run much faster than it would on its own by lowering the activation energy An enzyme recognizes only a specific substrate (reactants), and they fit together like a LOCK AND KEY

Enzymes Active Site: pocket in an enzyme where substrates bind and a reaction occurs An enzyme is does not change the amount of product that is made, it only SPEEDS UP THE REACTION

Enzymes When the enzyme and substrate bind, subtle changes occur in the active site occur, known as an induced fit. Induced fit enhances catalysis as the enzyme converts substrate to product. When the products are released, the enzyme is restored to its original form and can repeat the reaction over and over

Enzymes Environmental factors (pH, temperature, salt) influence an enzyme’s shape and function Enzymes functions best in a particular range of conditions that reflect the environment in which it evolved

Metabolic Pathways Building, rearranging, or breaking down organic molecules often occurs in steps Metabolic Pathway: series of enzyme- controlled reactions by which cells build, remodel, or break down an organic molecule Linear pathway: reactions run straight from reactant to product Cyclic pathway: the last step regenerates a reactant for the first step

Electron Transfers Metabolic pathways that capture or release energy can be dangerous for cells to run because the bonds of organic molecules hold enough energy to be harmful if released all at once [explosive] Instead, cells break the bonds one by one so energy is released in small, manageable amounts Most are oxidation–reduction reactions [REDOX] In a typical redox reaction an electron is transferred from one molecule (oxidized) to another molecule (reduced)

Electron Transfers Electron Transfer Chain: array of enzymes and other molecules that accept and give up electrons in sequence The energy of the electrons is released with each step of the sequence Important for photosynthesis and aerobic respiration