Cell Biology Metabolism
Metabolism
- All the chemical reactions that occur within a cell
- 1000s of chemical reactions
- network of metabolic pathways that alter molecules by a series of
steps
- enzymes control by selectively accelerating each step
Energy
- capacity to do work
- kinetic – energy of motion
o electrical, heat, and electromagnetic (light) energy
- potential
o chemical bond energy (stored in organic molecules like
sugars), concentration gradients
- forms are interconvertible
- all come from sun; plants convert light to chemical bond energy. We
eat plants or animals that have eaten plants.
Cell Performs 3 main Kinds of Work
- Mechanical work
o beating cilia, movement of chromosomes
- Transport work
o Pumping of substances across membranes
- Chemical work
o Driving endergonic reactions (monomers  polymers)
Bioenergetics
- study of how organisms manage their energy resources
Catabolic reactions
- degradation of larger molecules to small molecules with the
production of useful energy
Biosynthetic or Anabolic reactions
- consume energy to build complex molecules from simpler ones.
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Energy Coupling
- As cells use energy (e.g. food molecules), heat energy is produced.
Since the cell cannot use heat energy to drive energy-requiring
processes, chemical bond energy that is released in the exergonic
reactions must be directly transferred to chemical bond energy in
the products of the endergonic reactions.
- Energy Coupling
o Linking of heat-generating reactions (exergonic) to endergonic
reactions (e.g. synthesis of macromolecules)
o Requires enzymes
o Energy is stored temporarily in intermediate molecules called
“activated carriers”
Activated Carriers
- small molecules that allow coupled reactions to occur
- captures and transfers energy where needed
- energy stored in high-energy bonds or as high-energy electrons
- ATP, NADH, NADPH
Adenosine Triphosphate (ATP)
- energy “currency” of the cell; most widely used carrier
- Structure
o 3 phosphate groups
o 5C ribose sugar
o adenine base
- phosphate groups linked by phosphoanhydride bonds
o bonds are unstable partly due to charge repulsion
o hydrolysis (adding H2O) of terminal P group is highly
exergonic
(G = -7.3 kcal/mol)
o reverse reaction is endergonic (ADP + Pi  ATP + H2O)
ATP Function
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- enzymes transfer P group from ATP to other compounds
(phosphorylated)
- forms high-energy intermediate that is more unstable than original
compound
- compound undergoes some kind of change that performs work
- examples
o changing a proteins conformation
o linking 2 molecules together via a condensation reaction
(removes H2O)
ATP
B-OH

A-H
B-O~P

ADP
A-B
Pi
Oxidation-Reduction (Redox) Reactions
- all cells obtain energy by oxidizing organic molecules (e.g.
nutrients)
o carbohydrates, fats, proteins
o their oxidation is highly exergonic
o requires enzymes
o oxidized molecule has less energy
- Oxidation
o Loss of electrons to another substance
- Reduction
o Addition of electrons to another substance
- Rarely involves direct addition of oxygen
- Occurs simultaneously
- Reducing agent
o The electron donor
- Oxidizing agent
o The electron acceptor
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Organic molecules and Redox reactions
- Can have partial shift of electrons between atoms (polar covalent
bonds)
- protons (H+) are frequently transferred with electrons
- If C-H bonds decrease, carbon is being oxidized
o Methanol to formaldehyde
Cellular Respiration
- process whereby all plant and animal obtain energy by the gradual
oxidation of organic molecules
o carbohydrates, proteins, fats
o if burn these, all energy released as heat
o in cells, energy released gradually so that it can be coupled to
ATP generation
- oxidation of glucose (very exogenic)
o glucose + O2  CO2 + H2O + energy
 many intermediate steps involved
 O2 is final electron acceptor
NADH and NADPH
- electrons are rarely passed from an oxidizable substrate to oxygen
- NAD+ and NADP+ are intermediate electron acceptors
o Accept 2 electrons and 1 proton to become their reduced
forms, NADH and NADPH
- Derivatives of B vitamins
- NADH and NADPH really like to give up their electrons to other
molecules. G of transfer is negative
- NADH functions in cellular respiration; NADPH in photosynthesis
Photosynthesis
- conversion of light energy into chemical energy by plants
o light energy + CO2 + H2O  sugars + O2 + heat energy
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o many intermediate steps involved
Cellular Respiration and Photosynthesis are Complementary
Processes (see fig. 3-10)
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