7 Cellular Pathways that Harvest Chemical Energy

7Cellular Pathways that Harvest Chemical Energy

7 Energy and Electrons from Glucose

• The sugar glucose (C6H12O6) is the most common form of energy molecule.

• Cells obtain energy from glucose by the chemical process of oxidation in a series of metabolic pathways.


• The equation for the metabolic use of glucose:

C6H12O6 + 6 O2 6 CO2 + 6 H2O + energy

• About half of the energy from glucose is collected in ATP.

G for the complete conversion of glucose is negative.

• The reaction is therefore highly exergonic, and it drives the endergonic formation of ATP.


• Three metabolic processes are used in the breakdown of glucose for energy:

Glycolysis

Cellular respiration

Fermentation

Figure 7.1 Energy for Life

=Glucose


• Glycolysis produces some usable energy and two molecules of a three-carbon sugar called pyruvate.

• Glycolysis begins glucose metabolism in all cells.

• Glycolysis does not require O2; it is an anaerobic metabolic process.


• Cellular respiration uses O2 and occurs in aerobic (oxygen-containing) environments.

• Pyruvate is converted to CO2 and H2O.

• The energy stored in covalent bonds of pyruvate is used to make ATP molecules.


• Fermentation does not involve O2. It is an anaerobic process.

• Pyruvate is converted into lactic acid or ethanol.

• Breakdown of glucose is incomplete; less energy is released than by cellular respiration.


• Redox reactions transfer the energy of electrons.

• A gain of one or more electrons or hydrogen atoms is called reduction.

• The loss of one or more electrons or hydrogen atoms is called oxidation.

• Whenever one material is reduced, another is oxidized.

Figure 7.2 Oxidation and Reduction Are Coupled


• An oxidizing agent accepts an electron or a hydrogen atom (it itself is reduced).

• A reducing agent donates an electron or a hydrogen atom (it itself is oxidized).

• During the metabolism of glucose, glucose is the reducing agent (and is oxidized), while oxygen is the oxidizing agent (and is reduced).


• The coenzyme NAD is an essential electron carrier in cellular redox reactions.

• NAD exists in an oxidized form, NAD+, and a reduced form, NADH + H+.

• The reduction reaction requires an input of energy:

NAD+ + 2H NADH + H+

• The oxidation reaction is exergonic: NADH + H+ + ½ O2 NAD+ + H2O

Figure 7.3 NAD Is an Energy Carrier


• The energy-harvesting processes in cells use different combinations of metabolic pathways.

• With O2 present, four major pathways operate:

Glycolysis Pyruvate oxidation The citric acid cycle The respiratory chain (electron transport chain)

• When no O2 is available, glycolysis is followed by fermentation.

Table 7.1 Cellular Locations for Energy Pathways in Eukaryotes and Prokaryotes

7 Glycolysis: From Glucose to Pyruvate

• Glycolysis can be divided into two stages:

Energy-investing reactions that use ATP

Energy-harvesting reactions that produce ATP


• The energy-investing reactions of glycolysis:

In separate reactions, two ATP molecules are used to make modifications to glucose.

Phosphates from each ATP are added to the glucose molecule.

The molecule is split into two 3-C molecules that become glyceraldehyde 3-phosphate (G3P).

Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part1)


Dihydroxyacetone phosphate (DAP)

Glycerladehyde 3-phosphate (G3P) –

2 molecules


• The energy-harvesting reactions of glycolysis: The first reaction (an oxidation) releases free

energy that is used to make two molecules of NADH + H+, one for each of the two G3P molecules.

Two other reactions each yield one ATP per G3P molecule.

The final product is two 3-carbon molecules of pyruvate.


Figure 7.6 Glycolysis Converts Glucose to Pyruvate (Part 4)

7 Pyruvate Oxidation

• Pyruvate is oxidized to acetate which is converted to acetyl CoA.

• Pyruvate oxidation is a multistep reaction catalyzed by an enzyme complex attached to the inner mitochondrial membrane.

• One NADH is generated during this reaction.

Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 1)

7 The Citric Acid Cycle

• The citric acid cycle begins when the two carbons from the acetate are added to oxaloacetate, a 4-C molecule, to generate citrate, a 6-C molecule.

• A series of reactions oxidize two carbons from the citrate. With molecular rearrangements, oxaloacetate is reformed, which can be used for the next cycle.

• For each turn of the cycle, three molecules of NADH + H+, one molecule of ATP, one molecule of FADH2, and two molecules of CO2 are generated.

Figure 7.8 Pyruvate Oxidation and the Citric Acid Cycle (Part 2)

7 The Respiratory Chain:Electrons, Protons, and ATP Production

• The respiratory chain uses the reducing agents generated by pyruvate oxidation and the citric acid cycle (i.e. NADH and FADH2).

• The electrons flow through a series of redox reactions.

• ATP synthesis by electron transport is called oxidative phosphorylation.

Figure 7.10 The Oxidation of NADH + H+


• As electrons pass through the respiratory chain, protons are pumped by active transport into the intermembrane space against their concentration gradient.

• This transport results in a difference in electric charge across the membrane.

• The potential energy generated is called the proton-motive force.


• Chemiosmosis is the coupling of the proton-motive force and ATP synthesis.

• NADH or FADH2 yield energy upon oxidation.

• The energy is used to pump protons into the intermembrane space, contributing to the proton-motive force.

• The potential energy from the proton-motive force is harnessed by ATP synthase to synthesize ATP from ADP.

Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 1)

Figure 7.12 A Chemiosmotic Mechanism Produces ATP (Part 2)

7 Fermentation: ATP from Glucose, without O2

• Some cells under anaerobic conditions continue glycolysis and produce a limited amount of ATP if fermentation regenerates the NAD+ to keep glycolysis going.

• Fermentation uses NADH + H+ to reduce pyruvate, and consequently NAD+ is regenerated.

• Lactic acid fermentation occurs in some microorganisms and in muscle cells when they are starved for oxygen.

Figure 7.14 Lactic Acid Fermentation

7 Fermentation: ATP from Glucose, without O2

• Alcoholic fermentation involves the use of enzymes to metabolize pyruvate, producing acetaldehyde.

• Then acetaldehyde is reduced by NADH + H+, producing NAD+ and ethanol (a waste product).

Figure 7.15 Alcoholic Fermentation

Figure 7.16 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 1)

Figure 7.16 Cellular Respiration Yields More Energy Than Glycolysis Does (Part 2)

Figure 7.17 Relationships Among the Major Metabolic Pathways of the Cell

Glucose utilization pathways can yield more than just energy. They are interchanges for diverse biochemical traffic.

Intermediate chemicals are generated that are substrates for the synthesis of lipids, amino acids, nucleic acids, and other biological molecules.

7 Regulating Energy Pathways

• Metabolic pathways work together to provide cell homeostasis.

• Control points regulated by enzymes use both positive and negative feedback mechanisms.

• For example, some enzymes are inhibited by ATP and activated by ADP and AMP.

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7 Cellular Pathways that Harvest Chemical Energy