One of the essential features of life is the ability to capture and harness energy from the environment and use this energy to build, move, grow, and replicate. What energy is used and where does it come from?
Organisms eat carbohydrates and fats that contain chemical energy,
digesting these molecules to trap their chemical energy in a molecule called adenosine triphosphate (ATP). Cells use ATP to do most activities that require energy input to occur. Processes requiring energy input will not occur on their own, catalyzed or not. In fact, without energy input, most of the molecules fundamental to life tend to move in the other direction, toward oxidation and a loss of structure. By capturing food energy and converting it into ATP, life uses energy to drive forward all of the reactions it needs to perform. This process is known as cellular respiration.
Where does ATP come from? Cells in humans and other organisms use a common set of biochemical reactions to make ATP, including pathways such as glycolysis, the Krebs cycle, and electron transport. The
process of generating energy in the form of ATP begins with the glucose molecule. In humans, glucose is present in the blood as a fuel for all cells. Cells take in glucose, leading to the glycolytic pathway that is the first step in the path to ATP.
Glycolysis
A metabolic pathway is a linked series of biochemical reactions that have a common purpose. Glycolysis is a very ancient pathway in the evolution of life, present in all of the kingdoms of life, from bacteria to humans. Glycolysis is important because it is the first biochemical pathway in the capture of energy from glucose, which makes ATP. The glycolytic pathway consists of ten steps, each catalyzed by an enzyme uniquely evolved to catalyze that reaction. You will not need to know all of the individual reactions or the individual enzymes, but being familiar with the idea of metabolic pathways and the function of glycolysis is a good idea. Glycolysis takes glucose, a sugar molecule with six carbon atoms, and breaks it into two pyruvate molecules, each with three carbons, that capture energy in different ways. Energy is captured to make NADH, an energy carrier the cell uses to make ATP through electron transport.
Fermentation
In glycolysis, NAD+ is required, and it is converted to NADH. Obviously,
NAD+ must be regenerated or glycolysis would run out of it and stop, halting ATP production as well (and probably the life of the cell or
organism involved). NAD+ is regenerated in one of two ways. In the first, in the presence of oxygen, NADH goes on to the electron transport chain and is used to produce more ATP, as described in the sections that
follow; during this process it is converted back to NAD+. The second way to regenerate NAD+ occurs in the absence of oxygen or in anaerobic organisms that do not use oxidative metabolism. This alternate pathway is called fermentation.
Fermentation allows glycolysis to continue even in the absence of oxygen. In fermentation, NADH is regenerated back to NAD+ in the absence of oxygen to allow glycolysis to continue to produce ATP, producing either ethanol or lactic acid as by-products.
Aerobic Respiration
Although glycolysis produces two ATP and two NADH for every molecule of glucose, this is not where the eukaryotic cell extracts most of its
energy from glucose. Glycolysis is only the beginning; aerobic
respiration is the rest of the story. During aerobic respiration, glucose is fully combusted by the cell as an energy source, going through the Krebs cycle and electron transport to trap energy ultimately used to make ATP.
To accomplish this more efficient form of energy production, pyruvate from glycolysis is oxidized all the way to carbon dioxide in a pathway called the Krebs cycle. The Krebs cycle and the other steps of oxidative metabolism occur in mitochondria. It is not important to know all the details about the Krebs cycle, but you should understand that the Krebs cycle is a series of reactions linked in a circle that extracts energy from the products of glycolysis to make the high-energy electron carriers.
Finally, electron transport is the mechanism used to convert the energy held by these carriers into a more useful form that ultimately results in ATP production.
Photosynthesis
Photosynthesis is the foundation of all ecosystems because it is the primary source of energy. Plants are autotrophs, or self-feeders, that use photosynthesis to generate their own chemical energy from the energy of the sun. There are also many prokaryotic and eukaryotic photosynthetic organisms, such as algae, that contribute significantly to biological production. The chemical energy that plants get from the sun is used to produce the glucose that can be burned in mitochondria to make ATP, which is then used to drive all of the energy-requiring processes in a plant, including the production of proteins, lipids, carbohydrates, and
nucleic acids. Animals eat plants to extract this energy for their own metabolic needs. In this way, photosynthesis supports almost all living systems.
In plants, photosynthesis occurs in the chloroplast, an organelle that is specific to plants. In prokaryotes, there are no chloroplasts, and
photosynthesis occurs throughout the cytoplasm. Chloroplasts are found mainly in the cells of the mesophyl, green tissue in the interior of leaves.
A leaf contains pores in its surface called stomata that allow carbon dioxide in and oxygen out, facilitating photosynthesis in the leaf.
Chloroplasts have an inner and outer membrane; within the inner
membrane there is a fluid called the stroma. Photosynthesis involves the reduction of carbon dioxide (CO2) to a carbohydrate. It can be
characterized as the reverse of respiration, in that the reduction of CO2 produces glucose instead of the oxidation of glucose making CO2. Oxygen, one of the by-products of photosynthesis, is of keen interest to all of us air-breathers since we need it to survive.