Major Concepts to Review for AP Biology

Cell Energetics

Energy -

the forms of energy are: kinetic, potential, light (radiation), mechanical, chemical, etc....

how energy is distributed -- radiation, convection, conduction

Thermodynamics -- importance of the first and second laws to life

First Law of Thermodynamics - Conservation of Energy
Energy can be stored (potential energy) in chemical bonds to be used later or to be supplied in small equal doses to do work

Second law of Thermodynamics

The energy available to do work (free energy) can never equal the total energy (stored within chemical bonds) because entropy is involved in all energy conversions. Entropy is an increase in the randomness of a system - generally involving the conversion of some useful energy to heat (friction for example)

Free energy G; enthalpy H and entropy S.

G = H - TS

Exergonic and Endergonic reactions. Some reactions produce excess energy (exergonic) while some require an input of energy (endogonic).

Exergonic

Endergonic

Energy of activation.

Energy's Implications for life on earth

  1. Life requires a constant supply of new energy (sun light)
  2. All closed systems must eventually run out of energy (cave analogy - ie No perpetual motion machines)
  3. Earth is an open system where sunlight constantly supplies the energy needed to overcome the degradation caused by entropy

 

Enzymes are:

  • made of protein
  • catalysts, which speed up both forward and reverse reactions until equilibrium is reached
  • able to lower the energy of activation
  • not used up in the reaction
  • needed in tiny amounts
  • form weak, temperary bond with the substrate
  • affected by environmental conditions such as pH, temperature, salt, and concentration
 
Enzymes often require:
cofactors --metal Ions or
coenzymes --vitamins NAD etc.
 

Enzyme activity is descriped by the induced fit hypothesis where the enzyme and its substrate form a complex

Influences - factors affecting enzyme action

 
1. negative feed back possible with allosteric site which when occupied by a product reduces the "fit" between enzyme and substrate

2. Effects of pH allows ionization of amino acid R groups. Excess pH change results in denaturing of enzyme (loss of shape)

3. Effects of temperature. 10 degree higher temp doubles reaction rate, but too high a temperature will denature enzyme and stop activity.
 

A typical graph of enzyme activity where product grows exponentially until reactant is used up.

Metabolic Pathways - Teams of Enzymes, Coupled, assembly line in which the product of enzyme A becomes substrate for enzyme B and so on.

Enzyme Control Mechanisms

Activating Mechanizms - (positive feedback)
Cell membrane recognition sites activate enzyme in membrane which begins "cascade" of events which activate other enzymes. This is called signal transduction (pg 155 Campbell)

Inhibiting Mechanisms

competitive inhibition -- similar substrate occupies the active site of the allosteric enzyme but does not react and remains stuck

non competitive inhilation -- agent bonds to a locale other than active site but by so doing it makes the active site inoperable.

Examples include

Penicillin - competitive inhibitor

Allosteric control - Allosteric Enzymes -

Large enzymes ususally in metabolic pathways react to two kinds of modulators --activators and inhibitors which bind weakly to a secondary site (allosteric) usually where polypeptides mesh together in their quartenary structure.

The presence of the modulator affects the shape of active sites - opening or closing them. In metabolic pathways the modulators are frequently products or reactants in the pathway.

Example: The inactivation of allosteric enzymes by an end product of the pathway is called Negative feedback inhibiton.

Equilibrium

In chemical terms, equilibrium is reached when forward and reverse reactions occur at the same rate and system is no longer changing

ATP

Energy Currency of all living things is - ATP a nucleotide (see below)

All metabolic activity and energy requirements are mediated by a single universal molecule of energy transfer ATP structure.

Terminal phosphate bond has 7.3 kcal/mole ĘG. Rather than high energy those bonds have readily available energy.

Phosphorylation as a way to activate enzymes

Cycling of ATP. Cells don't need a large supply of ATP since it is efficiently recycled.

 

Coenzymes and other electron carriers generally Take on 2 e- . Examples:
NAD -- NADH- mitochondria

NADP -- NADPH - chloroplasts (All 3 are involved in redox reactions)

FAD --

 

Oxidation - removal of electrons or hydrogen. Usually exothermic (reducing agents)

Reduction - Gain of electrons (and proton H+). Usually endothermic (oxidizing agents accept electron)

 

The ability to reduce other substances is called reducing power

Stationary Electron carriers are proteins with active prosthetic groups. Most common are cytochromes containing heme groups -- a ring with Nitrogen which traps Fe at its center.

Electron Transport is usually coupled with proton "pumping"

 

Formation of ATP

  1. Substrate level phosphorylation - directly from redox coupling. (glycolysis for example)
  2. Chemiosmotic phosphorylation

    Mitchell 1961 - neither chloroplasts nor mitochondria can make ATP unless intact.

    ATP function depends upon the creation of steep proton concentration gradiants - which requires sac within a sac membrane arrangement.

    electron pumps also concentrate H+ thus increasing the free energy of the system



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