Ground Rules of Metabolism
- Energy and the Underlying Organization of Life
- Potential energy measured in kcals.
- Kinetic energy - includes heat energy.
- How Much Energy Is Available?
- First law of thermodynamics.
- A cell cannot produce energy; it can only be borrowed.
- Energy can be well channeled or lost as heat
- The One-Way Flow of Energy
- Second law of thermodynamics (Entropy) (H ® L quality
- Each conversion releases heat energy that is unavailable for work.
- The world of life (plant and animal) maintains a high degree of organization only
because it is being resupplied with energy from the sun. Nobel Prize -
- The Directional Nature of Metabolism
- Which Way Will a Reaction Run?
- AB + CD ® AC + BD
- depends on [ ] and the collision of molecules (heat).
- When a reaction approaches chemical equilibrium, the
forward and reverse reactions proceed at equal rates
- There is no net change in concentrations.
- Every reaction has its own ratio of products to reactants at equilibrium.
- Conservation of Mass
- The law of conservation of mass states that the total mass
of all substances entering a reaction equals the total mass of all the products. (Not
- Balance chemical equations.
- Energy Transfers and Cellular Work
- The Structure of ATP
- Before cells can use the energy of sunlight or that stored in carbohydrates, they must
transfer the energy to molecules of ATP.
- ATP is composed of adenine, ribose, and three phosphate groups.
- Phosphate-Group Transfers
- Energy input links phosphate to ADP to produce ATP (phosphorylation).
- ATP can in turn donate a phosphate group to another molecule, which then becomes primed
and energized for specific reactions. (glucose-6 phosphate, AT
- ADP « ATP very rapidly in the ATP/ADP cycle.
- ATP Output and Metabolic Pathways
- Metabolic pathways form series of reactions that regulate the concentration of
substances within cells by enzyme-mediated linear and circular sequences.
- In biosynthetic pathways, small molecules are assembled
into large molecules; for example, simple sugars are assembled into complex carbohydrates.
- In degradative pathways, large molecules such as
carbohydrates, lipids, and proteins are broken down to form products of lower energy.
Released energy can be used for cellular work.
- Participants in metabolic pathways are defined as follows:
- Substrates are substances that enter reactions (reactants).
- Intermediates are the compounds formed between the start and the end of a
- End products are the substances
present at the conclusion of a pathway.
- Energy carriers are mainly ATP.
- Enzymes are proteins that catalyze (speed up) reactions.
- Cofactors are small molecules and metal ions that help enzymes by carrying atoms
- Transport proteins are membrane-bound
proteins that participate in adjusting concentration gradients that will influence the
direction of metabolic reactions.
- Enzyme Structure and Function
- Specificity - molecular recognition (directionality)
- Enzyme-Substrate Interactions
- Enzymes increase V by lowering activation energy.
- The active site is a crevice where the substrate; in Koshlands induced-fit model,
structural changes during binding allow a more precise fit.
- Factors Influencing Enzyme Activity
- Enzymes and Environmental Conditions
- T° disrupts the bonds that maintain 3D shape (denaturation)
- pH optimal near 7 (pepsin in the stomach is an exception);
- Control of Enzyme Function
- Genetic control regulates the number of enzyme molecules available by speeding
up/slowing down their synthesis.
- Allosteric enzymes have (in addition to active sites) regulatory sites where
control substances can bind to alter enzyme activity; if this control substance is the end
product in the enzymes metabolic pathway, feedback inhibition occurs.
- Enzyme Helpers
- Cofactors are nonprotein groups that bind to many enzymes and make them more
- Coenzymes are large organic molecules such as NAD+, FAD, and NADP+
that transfer protons and electrons from one substrate to another.
- Heavy metals (ions) such as Fe++ also serve as cofactors when assisting
membrane cytochrome proteins in their electron transfers in chloroplasts and mitochondria.
- Electron Transfers Through Transport Systems
- Energy is released from storage molecules (such as glucose) in controlled steps via a
series of intermediate molecules.
- Electrons released during bond breaking are transferred stepwise through the components
of electron transport systems located on various cell membranes.
- Each time a donor gives up an electron it is oxidized; if
it gains, it is reduced.
- Electron transport systems are similar to staircases where electrons flow down the steps
from the top (most energy) to the bottom (least energy), releasing a small amount at each
- The energy is harnessed to move hydrogen ions, which in turn establish pH and electric
gradients necessary for ATP production.