Photosynthesis and Cell Respiration

Cellular Respiration

Cellular respiration is the set of metabolic reactions and processes that take place in the cells of organisms to convert biochemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products.
  1. Glycolysis. In glycolysis, glucose (a six-carbon sugar) undergoes a series of chemical transformations and is ultimately converted into two molecules of pyruvate, a three-carbon organic molecule. In these reactions, ATP is made, and NAD^+​+​​start superscript, plus, end superscript is reduced to NADH.

  2. Pyruvate oxidation. The pyruvate from glycolysis goes into the mitochondrial matrix (the innermost compartment of mitochondria). There, it’s converted into a two-carbon molecule bound to Coenzyme A (acetyl CoA). Carbon dioxide is released and NADH is generated.

  3. Citric acid cycle. The acetyl CoA made in the last step combines with a four-carbon molecule and goes through a cycle of reactions, ultimately regenerating the four-carbon starting molecule. ATP, NADH, and FADH_2​2​​start subscript, 2, end subscript are produced and carbon dioxide is released.

  4. Oxidative phosphorylation. The NADH and FADH_2​2​​start subscript, 2, end subscript made in other steps deposit their electrons in the electron transport chain. As electrons move down the chain, energy is released and used to pump protons out of the matrix, forming a gradient. Protons flow back into the matrix through an enzyme called ATP synthase, making ATP. At the end of the electron transport chain, oxygen accepts electrons and takes up protons to form water.

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Photosynthesis

Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities.


In plants, photosynthesis usually occurs in the leaves. This is where plants can get the raw materials for photosynthesis all in one convenient location. Carbon dioxide and oxygen enter/exit the leaves through pores called stomata. Water is delivered to the leaves from the roots through a vascular system. The chlorophyll in the chloroplasts inside leaf cells absorbs sunlight.

The process of photosynthesis is divided into two main parts: light dependent reactions and light independent or dark reactions. The light dependent reaction happens when solar energy is captured to make a molecule called ATP (adenosine triphosphate). The dark reaction happens when the ATP is used to make glucose (the Calvin Cycle).


Chlorophyll and other carotenoids form what are called antenna complexes. Antenna complexes transfer light energy to one of two types of photochemical reaction centers: P700, which is part of Photosystem I, or P680, which is part of Photosystem II. The photochemical reaction centers are located on the thylakoid membrane of the chloroplast. Excited electrons are transferred to electron acceptors, leaving the reaction center in an oxidized state.

The light-independent reactions produce carbohydrates by using ATP and NADPH that was formed from the light-dependent reactions.