How Cells Make ATP

ATP is the universal energy carrier, involved in a wide range of energy transformations, including muscle contraction, building proteins, cognitive functions, etc. The principle ways that cells have of making ATP are:

I. Photosynthesis

Photosynthesis is the process by which plant cells make ATP and glucose from sunlight, carbon dioxide and water. As a process, it is almost the reverse of aerobic respiration. Photosynthesis has two stages:

The Light Reactions requires sunlight and water to produce ATP, NADPH, and oxygen.

The Dark Reactions use the ATP and NADPH to "fix" carbon dioxide in glucose.

The light reactions involve a photosynthetic pigment which absorbs the sunlight and elevates electrons to a high energy state. Chlorophyll is the most common pigment, but accessory pigments like the carotenoids and xanthophils also absorb solar radiation. These secondary pigments are responsible for the brilliant colors found in leaves of deciduous trees in autumn.

The process of photosynthesis involves a process in which high energy electrons originating from chlorophyll are "handed off" to a special electron carrier, NADPH. The electrons must be replace in the photosynthetic pigments, and this occurs by the process of photolysis (literally, "splitting with light"). Water molecules are split, with electrons going to the photosystem, protons being left behind, and oxygen being evolved. It was the evolution of this system some 2 billion years ago that began the process by which oxygen is put in our atmosphere, making possible the organic world as we know it.

Although both ATP and NADPH are high energy compounds, they need to be converted to a simple sugar, glucose. Glucose is a more stable and convenient way to store the energy trapped from sunlight. The dark reactions involve processing carbon dioxide through a series of reactions, collectively called the Calvin Benson Cycle to produce glucose. The energy for this conversion is supplied in the form of ATP and NADPH.
Because animal cells cannot make ATP by photosynthesis, they must rely on the following processes:

II. Glycolysis
Glycolysis involves breaking glucose (sugar) into two pyruvate molecules, with the corresponding release of ATP. Also as a result of the reaction, high energy electrons are produced that must be picked up by a special compound (NAD) that carries electrons around in the cell. What happens next depends on whether oxygen is available or not. If no oxygen is available (anaerobic), the NAD eventually becomes saturated with electrons and the cell must find a way to get them off the NAD. It can do this either by converting pyruvate to ethyl alcohol (yeast cells) or to lactic acid (many animal cells, including human muscle cells). The result is that NAD is regenerated, more glucose is split in half, and more ATP is produced. Anaerobic glycolysis occurs in muscle cells when oxygen can't be delivered fast enough. But this is only a temporary solution to the energy needs of the cell. Both lactic acid and ethyl alcohol can interfere with cell function when they reach high concentrations--leading to fatigue of muscles and death of yeast cells. If oxygen is available, then the pyruvate is transported to the mitochondria where it is burned to produce ATP during aerobic respiration.
III. Aerobic Respiration

Aerobic respiration involves stripping high energy electrons (associated with carbon-hydrogen bonds) from organic compounds (including carbohydrates, proteins, and lipids) and using the energy to make ATP. Aerobic respiration takes place inside the mitochondria.

Pyruvate is converted, through a complex cycle of reactions (called the Kreb's cycle) into carbon dioxide (CO2) and NADH (a compound carrying the high energy electrons). These reactions take place in the middle of the mitochondria (the matrix) and results in the production of lots of high energy NADH.

The electron transport system is a system of molecules found in the inner membrane of the mitochondria that convert the energy of NADH into ATP..

So where does the oxygen come in? Oxygen is the final electron acceptor that picks up the "energetically spent" electrons as they come tumbling out of the electron transport system. The compound produced when this happens is water. For many desert animals, this metabolic water is the primary source of water in their water balance.

IV. Variations
The electron transport system is vulnerable to certain types of poisons, such as cyanide. Cyanide causes cell death by preventing aerobic ATP formation.

Some mitochondria are designed to burn substrates (glucose, fat, etc.) without producing ATP. These mitochondria are found in a special tissue called brown fat. Brown fat functions to produce heat, and is found in small mammals, and also in the young of larger mammals. Human babies, for example, have brown fat around the shoulders and along the spinal column, which keeps them warm when they are cold-stressed. It has been suggested that some people retain brown fat into adulthood. These are the people who can eat all day long and never gain weight. Brown fat works by decoupling the electron transport system from phosphorylation. The pyruvate is burned (oxidized), but no ATP is produced.

Bacteria and Archeae do not have mitochondria, does that mean that they are incapable of aerobic respiration? No--in the case of prokaryotes, the electron transport system is in the cell membrane, and the ATP is produced there. The entire cell functions as a mitochondrion, consistent with the theory of endosymbiosis.

Anaerobic respiration is a mechanism for producing ATP in which some compound other than oxygen is the final electron acceptor. This is found in a number of different types of bacteria, and results in the productions of compounds like methane (swamp gas), hydrogen sulfide ("rotten egg" smell), and ammonia. In the case of methane producing bacteria, carbon is the final electron acceptor and for the bacteria producing hydrogen sulfide, sulfur is the final acceptor.

A final look--to build a single glucose molecule, over 90 ATP are required. When that glucose is burned in a mitochondria (either plant or animal), only 36 ATPare released. This is a manifestation of the second law of thermodynamics.