Pathways that Harvest and

Store Chemical Energy

6

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Energy is stored in chemical bonds and can be released and transformed by metabolic pathways.

Chemical energy available to do work is termed free energy (G).

Why are exergonic and endergonic reactions always coupled?

The energy released from an exergonic reaction is used to power an endergonic reaction!

What is the major energy source for endergonic reactions?

ATP!!!!

What are the two major ways that ATP is generated?

ATP can be formed by substrate-level phosphorylation or oxidative phosphorylation.

Characteristics of Substrate Level Phosphorylation:

Phosphate is transferred directly from a substrate to ADP

Characteristics of Oxidative Phosphorylation:

Involves Reduction/Oxidation reactions

Electron Transport via electron carriers

Chemiosmosis (proton gradient)

How do redox reactions transfer energy?

•Reduction is the gain of one or more electrons.

• Oxidation is the loss of one or more electrons.

The transfer of electrons is a transfer of energy!

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Oxidation and reduction always occur together.

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e–).

When a molecule loses a hydrogen atom, it becomes oxidized.

LEO says GER!

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

The more reduced a molecule is, the more energy is stored in its bonds.

Energy is transferred in a redox reaction.

Which coenzyme is a key electron carrier in redox reactions?

Coenzyme NAD+ is a key electron carrier in redox reactions.

NAD+

NADH

Which is the oxidized form? Which is reduced? Which form has more energy?

Figure 6.4 A NAD+/NADH Is an Electron Carrier in Redox Reactions

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Reduction of NAD+ is highly endergonic:

Oxidation of NADH is highly exergonic:

Figure 6.4 B NAD+/NADH Is an Electron Carrier in Redox Reactions

How does the oxidation of electron carriers enable the formation of ATP?

• Oxidative Catabolic reactions release energy that is trapped by the reduction of coenzymes such as NADH

• Oxidative phosphorylation transfers energy from NADH to ATP. (via chemiosmosis)

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Oxidative phosphorylation couples oxidation of NADH:

with production of ATP:

Which reaction is endergonic? Which is exergonic?

Figure 6.5 A Chemiosmosis

Describe the process of chemiosmosis?

The transfer of electrons provides energy to actively pump protons across a membrane creating a large proton gradient

Resulting flow (diffusion) of protons back across the membrane (through ATP Synthase), drives the synthesis of ATP.

Electron Transport Chain and Chemiosmosis

Chemiosmosis and ATP Synthase

What is ATP synthase?

ATP synthase—membrane protein with two subunits that is required for ATP synthesis

F0 is the H+ channel; potential energy of the proton gradient drives the H+ through.

F1 has active sites for ATP synthesis.

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Cellular respiration is a major catabolic pathway. Glucose is oxidized:

Photosynthesis is a major anabolic pathway. Light energy is converted to chemical energy (CO2 is reduced):

Figure 6.7 ATP, Reduced Coenzymes, and Metabolism

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

Five principles governing metabolic pathways:

1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway.

2. Each reaction is catalyzed by a specific enzyme.

3. Most metabolic pathways are similar in all organisms.

Concept 6.1 ATP, Reduced Coenzymes, and Chemiosmosis Play Important Roles in Biological Energy Metabolism

4. In eukaryotes, many metabolic pathways occur inside specific organelles.

5. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.