Energy Generation in Mitochondria and Chloroplasts

Essential Cell Biology, Fifth Edition Copyright © 2019 W. W. Norton & Company
Fundamentals of Cell and Developmental Biology – Module 3

Catabolic and anabolic pathways together constitute the cell’s metabolism.
- During catabolism:
- A major portion of the energy stored in the chemical bonds of food molecules is dissipated as heat.
- Some of this energy is converted into useful forms needed to drive the synthesis of new molecules in anabolic pathways.
A series of enzyme-catalyzed reactions forms a linked pathway.
- The set of enzymes acts in a series to convert molecule A to molecule F.
- Each chemical reaction is catalyzed by a distinct enzyme.
Biological order is made possible by the release of heat energy from cells.
Cells can convert energy from one form to another.
Photosynthetic organisms use sunlight to synthesize organic molecules.
Cells obtain energy by the oxidation of organic molecules.

Biological structures are highly ordered:
- Examples of biological orders:
- Protein molecules in the coat of a virus.
- Regular array of microtubules in a sperm tail.
- Surface contours of a pollen grain.
- Cross-section of a fern stem showing patterned arrangements of cells.
- Spiral array of leaves made of millions of cells.
The spontaneous tendency toward disorder:
- Reversing this natural tendency requires an intentional effort and an input of energy.
- According to the second law of thermodynamics, human intervention releases enough heat to compensate for the reestablishment of order.
Living cells do not defy the second law of thermodynamics:
- Both the cell and the rest of the universe exist in a relatively disordered state.
- Cells take in energy from food molecules, carry out reactions to give order, and release heat into the environment, increasing disorder in surroundings.

Different forms of energy are interconvertible, but total energy must be conserved:
- Example:
- The large amount of chemical-bond energy released when forming water (H2O) from hydrogen (H2) and oxygen (O2) is first converted into rapid thermal motion in H2O molecules.
- This energy spreads to surroundings (heat transfer) and makes the H2O molecules indistinguishable from others.
Examples:
- Chemical-bond energy in batteries is converted to electrical energy for appliances.
- Cells convert chemical-bond energy into kinetic energy using molecular motor proteins.
- Some cells harvest energy from sunlight to form chemical bonds via photosynthesis.

The radiant energy of sunlight sustains all life:
- Trapped by plants and microorganisms through photosynthesis; sunlight is the ultimate energy source for humans and animals.
Photosynthesis occurs in two stages:
- Stage 1: Activated carriers ATP and NADPH are generated.
- Stage 2: Chemical-bond energy is formed and stored.
Photosynthesis and cellular respiration are complementary processes:
- Photosynthesis uses sunlight to produce sugars and organic molecules from atmospheric CO2.
- Cellular respiration oxidizes food molecules using O2, releasing CO2 back into the atmosphere.
- Useful energy is obtained for survival from this process.

Carbon atoms cycle continuously through the biosphere:
- Uptake of CO2 occurs via photosynthesis, and CO2 is released during respiration and fossil fuel combustion.
- Individual carbon atoms are incorporated into organic molecules by photosynthetic organisms and passed through the food chain.

Oxidation and reduction involve electron transfers:
- A simple reduced carbon compound like methane can be oxidized stepwise by replacing hydrogen with oxygen atoms.
- Each step shifts electrons away from carbon, making it more oxidized.
- Carbon dioxide, conversely, becomes more reduced when hydrogen atoms replace oxygen in the synthesis of methane.

Cells obtain most of their energy through membrane-based mechanisms:
- Oxidative phosphorylation in mitochondria uses energy from food oxidation to generate a proton (H+) gradient across a membrane.
- Photosynthesis in chloroplasts uses sunlight energy to create a proton gradient across a membrane.
- This proton gradient drives ATP synthesis in both processes

Membrane-based systems utilize stored energy in an electrochemical proton gradient for ATP synthesis:
- Step 1: A proton pump harnesses electron transfer energy to pump protons (H+) across a membrane, creating a proton gradient.
- High-energy electrons can come from organic/inorganic sources or be generated by light acting on chlorophyll.
- Protons are derived from water.
- Step 2: The proton gradient acts as an energy store driving ATP synthesis via ATP synthase—a process known as chemiosmotic coupling.

Batteries can harness the energy of electron transfer:
- Directly connecting a battery's terminals converts energy to heat.
- Connecting to a pump allows energy to do work, such as pumping water.
- Cells can similarly harness electron transfer energy to perform work, like pumping H+ across membranes.

Mitochondria exhibit dynamic structural and functional characteristics:
- They can divide similarly to bacteria and undergo fission processes.
- Mitochondria and chloroplasts have features resembling their bacterial ancestors, possessing their own DNA and machinery for RNA and protein synthesis.
Mitochondrial structures consist of:
- An outer membrane.
- An inner membrane.
- Two internal compartments (mitochondrial matrix and chloroplast stroma).
Mitochondria are often located near ATP-utilizing sites, such as:
- Cardiac muscle cells.
- Cilia and flagella of sperm cells.
Mitochondrial networks may form elongated and tubular structures extending throughout the cytoplasm:
- In cultured cells and yeast, mitochondria can create continuous networks adjacent to the plasma membrane.

Motile mitochondria segment to nodes, supporting Na+/K+ ATPases crucial for high-velocity nerve impulse conduction.
Abnormalities in mitochondrial transport may relate to neurodegenerative diseases.
Complex dynamics of axonal mitochondrial transport exist, with one-third of axonal mitochondria being motile, influenced by various physiological states.

Mitochondria are compartmentalized for specialized functions, enabling effective ATP production and energy management.
Key Processes Explained:
- The citric acid cycle generates high-energy electrons.
- Electrons move through three large enzyme complexes in the inner mitochondrial membrane.
- Electron movement is coupled to proton pumping creating a steep electrochemical gradient, which elevates ATP synthesis efficiency.
The high ATP/ADP ratio in cells is maintained by rapid conversion of ADP to ATP within mitochondria.
The process of cellular respiration is depicted as highly efficient in providing energy to living organisms.