Chapter 4. Chapter 4: Energy

Fossil fuels are made of plant and animal remains. They require millions of years to form, which essentially makes them a non-renewable resource: any fossil used today will not be replaced for a great deal of time—possibly long after humans have become extinct.

The ball has the greatest amount of potential energy at the top of the ramp. As it rolls down the ramp, the amount of potential energy decreases. This energy is converted to the energy of motion, or kinetic energy, and heat from friction between the ramp and the ball and between the air and the ball. Once it reaches the bottom of the ramp and stops moving, the ball has the least amount of potential energy and no longer has kinetic energy.

Some energy is always converted to heat, which is the least useful form of energy. Energy conversions in which a high percentage of energy forms heat are considered less efficient than energy conversions in which a smaller percentage of energy is converted to heat.

Each of the three phosphate groups of ATP carries a negative charge, causing repulsion between them. Bonds between these three phosphate groups readily break, releasing energy that can be utilized by the cell to power energy-consuming processes. Cells can easily regenerate ATP by attaching a phosphate group to an adenosine diphosphate (ADP), with the input of energy. These characteristics make ATP an ideal molecule to carry and store energy.

Oxygen is formed during photosynthesis as water is split in the water-splitting photosystem to supply electrons to the electron transport chain and to supply hydrogen ions. Animals inhale oxygen and use it in cellular respiration (as an electron acceptor). If oxygen were not available, there would be no final electron acceptor for electrons that have passed down the mitochondrial electron transport chain. In this case, only glycolysis would take place and relatively little ATP would be produced.

Most chloroplasts are located in the leaves, particularly in cells close to the leaf surface. Leaves are designed to capture light; many leaves have flat blades that face the sun, and they are often located in the highest parts of the plant.

Chlorophyll, found in chloroplasts, is a green pigment, meaning that it reflects light in the green part of the electromagnetic spectrum. The observed color of any object is a result of the light it reflects. Pigments are chemicals with the ability to absorb specific portions of the electromagnetic spectrum.

Energy moves through the cell as electrons, which carry energy, are passed from one substance to another. The substance with a stronger “pull” receives the electron as the substance with less of a pull loses the electron. In chloroplasts, electrons originate in the water-splitting photosystem from the splitting of water molecules: oxygen is a by-product, and hydrogen is used by the chloroplasts. Electrons are energized by light and pass down an electron transport chain, where ATP is produced, and enter the NADPH-producing photosystem, where they are again energized by light—but this time they are picked up by the electron carrier NADPH. The NADPH shuttles the electrons to the Calvin cycle, where they are used in the synthesis of glucose. In mitochondria, glucose enters the cellular respiration pathway, where electrons are removed and ultimately pass down the electron transport chain in the inner membrane of the mitochondria. The energy is used to concentrate hydrogen ions (protons) in the intermembrane space, and this proton gradient is used to make ATP. The electrons that pass down the electron transport chain are accepted by oxygen, which combines with hydrogen to form water.

An electron in a photosynthetic pigment that is excited to a higher energy state generally has one of two fates. The electron returns to its resting unexcited state, releasing energy in the process, some of which may bump electrons in a nearby molecule to a higher energy state, or the excited electron itself is passed to another molecule.

Rubisco helps plants pluck off (remove) the carbons from carbon dioxide for use in the building of carbohydrates and other organic molecules. Carbons are used in the Calvin cycle to synthesize glucose.

When stomata are closed to conserve water, the exchange of gases—specifically, carbon dioxide and oxygen—is disrupted. If carbon dioxide is prevented from entering the leaf, it will not be available for fixation in the Calvin cycle. As a result, photosynthesis is halted.

Energy is derived from food by the breaking of chemical bonds. When the chemical bonds of food molecules, such as glucose, are broken, energy is released and captured in ATP. The chemical bond between the second and third phosphate groups of ATP stores energy for use by the cell.

Glycolysis takes place in the cytoplasm, outside the organelles.

Mitochondria are the site of aerobic cellular respiration, where a majority of the cell’s ATP is produced. Glycolysis occurs in the cytoplasm, outside the mitochondria. During glycolysis, only two ATP molecules are produced per glucose molecule. Cellular respiration in the mitochondria, requiring oxygen, includes the Krebs cycle (citric acid cycle) and the electron transport chain and produces an additional 30–32 ATP molecules per glucose molecule. Therefore, the vast majority of the cell’s ATP is produced in the mitochondria, making them the “ATP factories.”

In cellular respiration, electrons removed from glucose ultimately pass down the electron transport chain in the inner membrane of the mitochondria. The energy derived from electrons as they pass down the electron transport chain is used to concentrate hydrogen ions (protons) in the mitochondrion’s intermembrane space. This proton gradient in then used to make ATP, and the electrons that pass down the electron transport chain are accepted by oxygen, which combines with hydrogen to form water. It is possible to concentrate hydrogen ions in the intermembrane space because the mitochondrial membranes are impermeable to ions; the concentration gradient provides the potential energy that can be used to make ATP.

Oxygen acts as an electron acceptor at the end of the electron transport chain. Imagine that the electron transport chain is like a set of stairs. Every time an electron falls down one step, some energy is lost from the electron and is used for ATP production. Once each electron makes its way to the end of the electron transport chain (the bottom of the stairs), it is accepted by oxygen, combining with hydrogen to form water. If no oxygen were available, electrons would have no place to go and would collect at the bottom of the stairs. This would prevent other electrons from moving down the electron transport chain, halting aerobic cellular respiration and the majority of the cell’s ATP production. Glycolysis would continue, but would produce a net of only two ATP molecules per glucose molecule—not enough energy to support many cellular activities.

Dietary lipids are broken down into fatty acids and glycerol molecules. Glycerol molecules can enter glycolysis, and fatty acids can be converted to acetyl-CoA, eventually entering the cellular respiration pathway.