Cellular respiration produces ATP (adenosine triphosphate), the energy currency in biochemical processes. Oxygen-aided respiration – aerobic respiration – produces much more ATP than anaerobic respiration. The details of these processes are covered in later topics.
For now, we will look at ATP in isolation, as the product of a series of special reactions, themselves part of respiration. ATP is made through ADP by adding back its third phosphate, and it reacts reversibly by losing that phosphate and releasing energy.
We are talking about adenosine triphosphate breaking down into adenosine diphosphate, inorganic phosphate and energy; and the latter joining back together to make adenosine triphosphate again.
When the hydrolysis of ATP (via the enzyme ATP hydrolase) is coupled to other reactions requiring energy, it enables these processes to take place. The inorganic phosphate released can itself take part in a further phosphorylation reaction with another chemical, often increasing its reactivity.
The condensation of ADP and inorganic phosphate takes place during photosynthesis and respiration, and is catalysed by the enzyme ATP synthase. Because it synthesises ATP. Get it get it.
ATP synthase (also known as ATP synthetase) produces ATP when hydrogen ions, H+, pass through its transmembrane channel. The reason they pass through is that there are more of them on one side of the semipermeable membrane than the other, thus being driven by an electrochemical gradient. This movement is called chemiosmosis.
As protons (H+) are driven to cross the membrane via ATP synthase, this rotates and catalyses one ADP and one P (inorganic phosphate) into one molecule of ATP.
The question, of course, is how is this higher concentration of protons maintained on that side of the membrane all the time? The answer is that proton pumps across the membrane pump protons (genius I know). Their activity is driven by a chain of electrons being passed down between the pumps from a higher excitation level of electrons to a lower level (the lowest level is where the oxygen in aerobic respiration comes in; it accepts the final electron and hence forms one of the respiration byproducts: water).
The chain of electrons driving the proton pumps is called the electron transport chain.
This membrane is inside the mitochondrion, making this organelle the cell “powerplant”. Funnily enough, these ATP-making processes aren’t isolated to mitochondria. In fact, much of the same thing happens in plant chloroplasts (say what?).
During photosynthesis, ATP must be made in order to synthesise organic compounds using carbon dioxide and water. Obviously the purpose of these compounds, such as glucose, is in itself energy through ATP made during cellular respiration. Let’s assume that the ATP that will be extracted from these compounds subsequently via respiration far outweighs the ATP needed in their making during photosynthesis (remember plants, too, undergo respiration to make ATP as their key mechanism of energy release).
Chloroplasts contain the photosynthetic pigments used to capture specific light wavelengths. These provide the energy that kickstarts the light-dependent reactions in the chloroplast.
Just like mitochondria, chloroplasts have inner membranes. In chloroplasts, these form the thylakoids which are flattened stacked discs. These membranes are the site of ATP synthesis during photosynthesis. The principles of chemiosmosis and the electron transport chain apply all the same.
As light photons interact with the chlorophyll (green circle) on the thylakoid membrane, the electron transport chain is established starting with the highest level energy electrons making their way across photosystems. As they decrease their energy level, proteins pump protons across the membrane. This powers up ATP synthase as seen before, and results in the synthesis of ATP. The ATP in this scenario goes into making the organic products of photosynthesis, rather than straight as cellular energy for diverse processes as seen with mitochondrial ATP production during aerobic respiration.