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Metabolism
Cell Energetics
Metabolism = total of all the chemical reactions taking place in
an organism
Metabolism
• Anabolism = ‘build up’ processes; ‘consume’ (store) energy by assembling macromolecules (photosynthesis)
• Catabolism = ‘break down’ reactions; release energy by breaking down (lyse) molecules (digestion)
Concept 8.1: An organism’s metabolism transforms matter and energy, subject to the laws of thermodynamics
Metabolism
• Energy - the ability to do work
• Closed Systems - system without energy input
• Open systems – system with energy input
Types
• Potential = capacity to do work
– Caused by POSITION
– Stored chemical energy; (glucose, glycogen)
• Kinetic – energy of motion
• Equilibrium = energy runs outEquilibrium = energy runs out
Thermodynamics
• Thermodynamics - the study of energy transformations
• 1st Law of Thermodynamics - energy cannot be created nor destroyed transformed
• 2nd Law of Thermodynamics - in a closed system, when energy is transformed, some is ‘lost’ as heat
• Entropy – decreasing available energy of the universe is increasing (disorder)
• Energy systems become more disordered/random;
• Total entropy increases; ‘stuff’ runs down
Free Energy
• The energy in a system available for work
• A spontaneous change can cause free energy to ‘flow’– System becomes more stable– Less work capacity– Free energy decreases (entropy)
Biological Order and Disorder
• Organisms live at the expense of free energy
• Organisms are open systems with low entropy – Use free energy to maintain order and
organization – Convert complex molecules into simpler ones;
digestion – Trade organization for heat (increases
randomness and entropy)
Types of Reactions
• Exergonic reaction - net release of free energy
– Less stable, more work– Fire, respiration
• Endergonic reaction - energy-requiring reaction; energy is absorbed/stored
– Photosynthesis
Concept 8.3: ATP Powers Cellular Work by Coupling Exergonic
Reactions to Endergonic Reactions
• Cells do three main kinds of work
– Mechanical - movement
– Transport – of stuff
– Chemical – polymerization, bioluminescence
Coupled Reactions• Coupled Reactions - endergonic reactions
are coupled with exergonic reactions
• Energy from an exergonic reaction (respiration) is stored in phosphate bonds
• Phosphate group is added to a molecule– Phosphorylation
– Molecule ‘works’
ATP
• Adenine + ribose + phosphate group
• Phosphate bond is easily broken/formed
• Controlled by enzymes
Uses of Energy
• Mechanical - beating of cilia/flagella, muscle, cytoplasmic flow, movement of chromosomes (mitosis)
• Transport - H+ ‘pump’, receptors
• Chemical – polymerization, bioluminescence
Figure 8.1
Concept 8.4: Enzymes speed up metabolic reactions by lowering
energy barriers
• Enzymes - biological catalysts– Accelerate reactions without being changed – Proteins (700)
• Catabolic or anabolic• All chemical reactions require activation
energy– Activation energy, EA - the initial amount of
energy needed to start a chemical reaction
• Often supplied as heat from the environment (spontaneous)
EnzymesEnzymes
Enzymes • Cellular T needs to remain
low, but metabolism is too slow at low T
• Enzymes reduce activation energy
• Transition state - reactants have absorbed energy
Enzymes • Substrate = substance
enzyme acts upon• Active site = area on the
enzyme which the substrate binds to (attaches) – Verryyy specific– Groove, pocket = 3d shape
• 2 mechanisms describe how enzymes function:– ‘Lock and Key’– Induced Fit - enzyme
may change shape to allow better reaction on substrate
Enzymes
Active site
Induced fit
Chemical bonds broken
• Enzymatic action is reversible – E + S P + E – Enzyme unaffected by reaction– Dependent upon concentration of reactants vs
products
• Reaction rate of 1000’s per second
• Speeding up enzyme reactions: – Add more substrate; until saturated with substrate…– Add more enzyme
• DNA controls cell’s activities by storing the code for protein synthesis (enzymes)
Enzymes
Factors That Affect Enzyme Activity • Temperature and pH
• Inorganic salts – disrupt H, ionic bonds, hydrophobic interactions
• Cofactors
• Inhibitors
Cofactors• Cofactors - nonprotein enzyme helpers
– Metals – Fe, Zn, Cu
• Coenzymes - organic – Vitamins
• Inhibitors = substances that inhibit the actions of enzymes (2 kinds:)
– Competitive inhibitors
– Noncompetitive inhibitors
Competitive inhibitors - resemble substrate, block active site Neurotoxin, Disulfiram
• Noncompetitive inhibitors - causes enzyme to change shape– Destroys conformation
(active site)– DDT, nerve gas (DSF)– May be allosteric
regulation
Enzymes
Concept 8.5: Regulation of enzyme activity helps control
metabolism
• A cell’s metabolic pathways must be tightly regulated
Allosteric Regulation of Enzymes
• Allosteric regulation - a protein’s function at one site is affected by binding of a regulatory molecule at another site
• Receptor site located away from the active site (quaternary structure)
• Allosteric site has to be activated, (may be inhibited)
Allosteric activator
Allosteric inhibitor
• Cooperativity - one substrate molecule can activate all other subunits of an enzyme
• Only requires a small concentration of substrate to activate enzyme– Hemoglobin
Enzymes
Feedback Inhibition• Metabolic pathways –
series of enzymes creates small steps to a final product
• Controlling the enzymes (activity or production) controls the pathway and product(s)
• Feedback Inhibition - end product of the pathway inhibits the pathway
Isoleucine – allosteric inhibitor
• Feedback inhibition prevents cells from wasting resources– “don’t need gas if you don’t have a car.”
Enzymes
Structure and Metabolism
• Cells are organized • Enzymes are grouped into
complexes or incorporated into membranes
• Multi-enzyme complex =
enzymes are assembled in correct physical position for a sequence of events to happen– Mitochondria, chloroplasts
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