Metabolism, cellular respiration and photosynthesis

Metabolism gathers all chemical reactions in a cell.
Some reactions produce energy, others molecules or work needed by the cell.

Metabolism can be divided into catabolism, where molecules split into small pieces releasing some energy, and anabolism, where molecules are synthetized from smaller components which consume energy.

In living cells, ATP (adenosine triphosphate) is used as the energy toll. In fact, ATP hydrolysis in ADP and inorganic phosphate releases an amount of energy that can be used to carry out most of the other non-spontaneous reactions.

As ATP cannot be stored in the cells, different paths have been selected through evolution to keep regenerating ATP from ADP and inorganic phosphate.

Cells can use light or organic molecules as an energy source to synthetize ATP.
ATP can be used as well for different types of cellular functions.

Different forms of energy do coexist in the cell from light to chemical but also osmotic or kinetic.
There can be multiple energy conversions from one form to another.

Some reactions are simply coupling two chemical reactions and the energy released by one of them allows the second one to occur.

In other cases, energy is converted from one form to another in both ATP production or degradation. Light can be necessary to allow a non-spontaneous reaction to happen for example or an osmotic gradient can trigger ATP synthesis. ATP use can lead to an imbalance of molecules in two close compartments or the movement of proteins and contraction.

Most chemical reactions are sped up by enzymes.
Different regulations can affect their action and thus the efficiency of metabolism.

Enzymes are mainly proteins that increase specific reaction speed. Those reactions depend on enzyme production, availability and specificity. Production is controlled by gene expression. Availability can vary according to competitive inhibitors and specificity is affected by non-competitive inhibitors.

This leads to regulation in metabolism. For example, a cell producing lactate dehydrogenase can achieve lactic fermentation when a cell producing alcohol dehydrogenase will achieve alcoholic fermentation. Another important example is the down-regulation of different glycolysis reactions by their direct products.

If glycolysis occurs in the cytoplasm, respiration takes place in the mitochondrion.
Respiration produces ATP through chemiosmosis.

Glycolysis converts glucose into two pyruvate molecules plus ATP and reduced NAD cofactors. Those molecules may give alcohol or lactic acid in the cytoplasm but can also migrate into the mitochondria to produce more ATP molecules through chemiosmosis.

In that case, pyruvate is completely broken down into carbon dioxide through the Krebs cycle. This cycle produces more reduced NAD and FAD cofactors that will give their electrons to a transport chain located in the inner mitochondrial membrane. As the electrons pass along, protons are pumped actively from the mitochondrial matrix towards the intermembrane space. This osmotic gradient is dissipated with protons returning towards the matrix using the ATP synthase that couples their passing with ATP production.

Oxygen is necessary for cellular respiration as it is the final electron acceptor at the end of the chain.

Photosynthesis is the possibility of biosynthesis with light energy sourcing.
In eukaryotes it takes place in the chloroplast.

Photosynthesis can be divided in two sequential phases: light dependent and light independent reactions both occurring inside the chloroplast.

Light absorption is carried out by pigments such as chlorophyll. Those pigments are located in the thylakoid membrane and organized in photosystems. Light excites electrons that can be passed to a transport chain leading to reduced NADP and proton relocation into the thylakoid lumen. This gradient will give ATP production as protons return passively to the stroma through the ATP synthase. In this phase, water is split into protons and oxygen.

Reduced NADP and ATP are used in the light-independent reactions. They give electrons and energy allowing carbon dioxide to be attached to a 5-carbon compound in the Cavin cycle. This cycle ends up producing a 3-carbon compounds (glycerate 3-phosphate, the first molecule of sugar synthesis) and regenerating its 5-carbon compound. RubisCO (ribulose bisphosphate Carboxylase Oxygenase) is the key enzyme of this cycle.