La nuova biologia.blu Le cellule e i viventi David Sadava, David M. Hillis, H. Craig Heller, May R. Berenbaum La nuova biologia.blu Le cellule e i viventi

Energy and Metabolism

What Physical Principles Underlie Biological Energy Transformations? Metabolism: the sum total of all chemical reactions occurring in a biological system at a given time. Metabolic reactions involve energy changes.

How Does Glucose Oxidation Release Chemical Energy? Fuels: molecules whose stored energy can be released for use. In cells, energy from fuel molecules is used to make ATP. Glucose is the most common fuel in cells. C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + free energy

How Does Glucose Oxidation Release Chemical Energy? Oxidation–Reduction (Redox) reactions: one substance transfers electrons to another substance. Reduction: gain of one or more electrons by an atom, ion, or molecule. Oxidation: loss of one or more electrons.

How Does Glucose Oxidation Release Chemical Energy? Coenzyme NAD+ is a key electron carrier in redox reactions.

How Does Glucose Oxidation Release Chemical Energy? Three catabolic processes harvest the energy from glucose.

Where do Energy Pathways Work?

What Are the Aerobic Pathways of Glucose Metabolism? Glycolysis Takes place in the cytosol Converts glucose into 2 molecules of pyruvate Produces 2 ATP and 2 NADH Occurs in 10 steps. Glucose + 2 ATP + 4 ADP + 2 Pi + 2 NAD+ 2 pyruvate + 4 ATP + 2 ADP + 2 NADH + 2 H+ + 2 H2O

Glycolysis Converts Glucose into Pyruvate Steps 1–5 require ATP (energy-investing reactions). Figure 9.5 Glycolysis Converts Glucose into Pyruvate Glucose is converted to pyruvate in ten enzyme-catalyzed steps. Along the way, two ATP are used (Steps 1 and 3), two NAD+ are reduced to two NADH (Step 6), and four ATP are produced (Steps 7 and 10).

Glycolysis Converts Glucose into Pyruvate Steps 6–10 yield NADH and ATP (energy-harvesting reactions). Figure 9.5 Glycolysis Converts Glucose into Pyruvate Glucose is converted to pyruvate in ten enzyme-catalyzed steps. Along the way, two ATP are used (Steps 1 and 3), two NAD+ are reduced to two NADH (Step 6), and four ATP are produced (Steps 7 and 10).

How Does Oxidative Phosphorylation Form ATP? Many bacteria and archaea have evolved pathways that allow them to exist where O2 is scarce or absent, by using other electron acceptors— anaerobic respiration.

How Is Energy Harvested from Glucose in the Absence of Oxygen? Without O2, ATP can be produced by glycolysis and NADH can be oxidased by fermentation.

The Respiratory Chain and ATP Synthase Produce ATP Figure 9.8 The Respiratory Chain and ATP SynthaseProduce ATP by a Chemiosmotic Mechanism As electrons pass through the transmembrane protein complexes in the respiratory chain, protons are transferred from the mitochondrial matrix into the intermembrane space. As the protons return to the matrix through ATP synthase, ATP is formed.

What Are the Aerobic Pathways of Glucose Metabolism? Pyruvate Oxidation: occurs in the mitochondrial matrix produces acetate and CO2 acetate binds to coenzyme A to form acetyl-CoA is a multistep reaction catalyzed by the pyruvate dehydrogenase complex. One NAD+ is reduced to NADH.

What Are the Aerobic Pathways of Glucose Metabolism? Citric acid cycle: Acetyl-CoA is the starting point. The acetyl group is completely oxidized to 2 molecules of CO2. Energy released is captured by ADP, NAD+, FAD, and GDP.

What Are the Aerobic Pathways of Glucose Metabolism? The complete oxidation of one glucose molecule yields: 6 CO2 10 NADH 2 FADH2 4 ATP

How Does Oxidative Phosphorylation Form ATP? Oxidative phosphorylation: ATP is synthesized by reoxidation of electron carriers in the presence of O2.

Relationships among the Major Metabolic Pathways of the Cell Figure 9.13 Relationships among the Major Metabolic Pathways of the Cell Note the central positions of glycolysis and the citric acid cycle in this network of metabolic pathways. Also note that many of the pathways can operate essentially in reverse.

What Is Photosynthesis? Photosynthesis is the “synthesis from light”. Energy from sunlight is captured and used to convert CO2 to more complex carbon compounds.

What Is Photosynthesis? Two pathways occur in different parts of the chloroplast. Light reactions: Convert light energy to chemical energy as ATP and NADPH. Light-independent reactions: Use ATP and NADPH (from the light reactions) plus CO2 to produce carbohydrates.

Photosynthetic Organisms Changed Earth’s Atmosphere About 2.5 billion years ago, photosynthesis changed the nature of life on Earth. Early photosynthetic cells were probably similar to cyanobacteria (prokaryotes). The atmosphere of early Earth had no O2, but it began to increase as photosynthetic prokaryotes increased. Organisms that could tolerate O2 proliferated. Abundant O2 opened up new avenues of evolution. Figure 1.4 Photosynthetic Organisms Changed Earth’s Atmosphere (A) Colonies of photosynthetic cyanobacteria and other microorganisms produced structures called stromatolites that were preserved in the ancient fossil record. This section of fossilized stromatolite reveals layers representing centuries of growth. (B) Living stromatolites can still be found in appropriate environments.

Inc. All rights reserved Adapted from Life: The Science of Biology, Tenth Edition, Sinauer Associates, Sunderland, MA, 2014 Inc. All rights reserved