Chapter 6
6.1 The Flow of Energy in Living Systems
Thermodynamics
Branch of chemistry concerned with energy changes
Cells are governed by the laws of physics and chemistry
Energy
Capacity to do work
2 states
Kinetic - energy of motion
Potential - store energy
Many forms - mechanical, heat, sound, electric current, light, or radioactivity
Measuring Energy
Heat is most confident way of measuring energy
1 cal = heat required to raise 1 gram of water 1 degree C
Cal on food labels (with a capital C) is actually a kilocalorie (1000 calories)
Energy Flow
Energy flows into the biological world from the SUN
Photosynthetic organisms capturing this energy
Stored as potential energy in chemical bonds
Breaking bonds between atoms requires energy; energy stored in chemical bonds may be used to make new bonds
Redox Reactions
Oxidation
Atom or molecule loses an electron
Reduction
Atom or molecule gains an electron
Higher level of energy than oxidized form
Oxidation-reduction reaction (redox)
Reactions always paired
6.2 The Laws of Thermodynamics and Free Energy
First Law of Thermoodynamics
Energy can no be created or destroyed
Energy can only change from one from to another
Total amount of energy in the universe remains constant
During each conversion, some energy is lost heat
Second Law of Thermodynamics
Entropy (disorder) is continuously increasing
Energy transformations proceed spontaneously to convert matter from a more ordered/less stable form to a less ordered/more stable form
Free energy OR Available energy
G = energy available to do work
G = H-TS
H = enthalpy, energy in a molecule’s chemical bonds
T = absolute temperature
S = entropy, unavailable energy
ΔG = ΔH − TS
Positive ΔG
Products have more free energy than reactants
H is higher or S is lower
Not spontaneous, requires input of energy
ENDERGONIC (Ex: Photosynthesis)
Negative ΔG
Products has less free energy than reactants
H is lower or S is higher or both
Spontaneous (may not be instantaneous
EXERGONIC (Ex: Cellular respiration)
Activation Energy
Extra energy required to destabilize existing bonds and initiate a chemical reaction
Rate of exergonic reaction depends on the activation energy
Larger activation energy processes more slowly
Rate can be increased 2 ways:
Increasing energy of reacting molecule (heating)
Lowering activation energy
Catalysts
Subtances that influence chemical bonds in a way that lowers activation energy
Cannot…
violate laws of thermodynamics
Make an endergonic reaction spontaneous
Alter the proportion of reactant turned into product
6.3 ATP: The Energy Currency of Cells
ATP or Adenosine triphosphate
Primary energy “currency” used by cells
Composed of:
Ribose - five carbon sugar
Adenine
Chains of 3 phosphate
Key to energy storage
Bonds are unstable, release energy when broken
ADP - adenosine diphosphate = 2 phosphates
AMP - adenosine monophosphate = one phosphate
Lowest energy form
ATP Cycle
ATP hydrolysis drives endergonic reactions
Couple reaction results in net -G (exergonic and spontaneous)
ATP not suitable for long-term energy storage
Phosphate bonds are too unstable
Fats and carbohydrates better
Cells store only a few seconds worth of ATP
6.4 Enzymes: Biological Catalysts
Most enzyme are proteins
Some RNA
Shape of enzyme stabilizes a temporary association between substrates
Enzymes do not change or consumed in reaction
EX: carbonic anhydride
200 molecules of carbonic acid per hour WITHOUT enzyme
600,000 molecules formed per second WITH enzyme
Enzyme Binding
substrates binds in the active site
Active site
pockets or clefts for substrate binding
Forms enzyme - substrate complex
Precise fit of substrate into activity site
Applies stress to distort particular bond to lower activation energy
Induced fit
It describes the dynamic interaction between an enzyme and its substrate.
The enzyme undergoes conformational changes upon substrate binding.
This conformational change enhances the enzyme's catalytic activity.
Induced fit allows for a tighter binding between the enzyme and substrate.
It increases the efficiency and specificity of enzymatic reactions.
Forms of Enzymes
enzymes may be suspended in the cytoplasm or attached to cell membrane and organelles
Multienzyme complexes
subunits that work together to form molecular machine
Product can be delivered easily to next enzyme
Unwanted side reactions prevented
All reactions can be controlled as a unit
Enzyme Function
Rate of enzyme-catalyzed reaction depends on concentrations of substrate and enzyme
Chemical or physical condition that affects the enzyme’s three-dimensional shape can change rate:
Optimum temperature
Optimum pH
Inhibitors- substances that binds to enzyme and decreases its activity
Competitive inhibitor - competes with substrate for active site
Noncompetitive inhibitor
binds to enzyme at a site other than active site
causes shape change that makes enzyme unable to bind substrate
Allosteric Enzymes
Allosteric enzymes - enzymes exist in active and inactive forms
Most noncompetitive inhibitors bind to allosteric site - chemical on/off switch
Allosteric inhibitor - binds to allosteric site and reduces enzyme activity
Allosteric activator - binds to allosteric site and increases enzyme activity
Cofactors and Coenzymes
Cofactors
Assist enzymes
Can be metal ions
EX: Zinc, molybdenum, manganese
Often found in the active site
Coenzymes
Cofactors that are no protein organic molecules
Vitamins
6.5 Metabolism: The Chemical Description of Cell Function
Metabolism
Total of all chemical reactions carried out by an organism
Anabolic reactions/anabolism
Expend energy to build up molecules
Catabolic reaction/catabolism
Harvest energy by breaking down molecules
Biochemical Pathways
Reaction occurs in a sequence
Product of one reaction is the substrate for the next
Many steps take place in specific organelles
Feedback Inhibition
End-product of pathways increases in concentration as synthesized
More product increases probability that it binds to an allosteric site on an enzyme in the pathways and cause change so it cannot bind normal substrates
Shut down pathway so raw materials and energy are wasted