Photosynthesis

Mr. Andersen introduces photosynthesis as a vital process that provides oxygen and food. He mentions that it's found in plants, bacteria, and algae, and explains its long evolutionary history and importance. The primary site for photosynthesis in eukaryotic cells is the chloroplast.

The chloroplast contains thylakoid membranes, which are organized into stacks called grana, where light reactions occur. The liquid filling the chloroplast is called the stroma, the site of the Calvin cycle. Plants use various pigments, not just chlorophyll A, to absorb light. Chlorophyll A and B absorb blue and red light, reflecting green light, which is why plants appear green. The general equation for photosynthesis involves water, carbon dioxide, and light producing glucose and oxygen. Plants create sugar for their own cellular respiration and for structural components like cellulose.

Photosynthesis is divided into two stages: the light reaction and the Calvin cycle. The light reaction takes place in the thylakoid membrane, using light and water to produce oxygen, NADPH, and ATP. The Calvin cycle, formerly known as the dark reactions, occurs in the stroma and uses carbon dioxide, ATP, and NADPH to produce glucose. The Calvin cycle was named after Melvin Calvin, who elucidated the process.

Delving into the light reaction, which occurs in the thylakoid membrane, light and water are key inputs. Light energy powers the movement of electrons through an electron transport chain, specifically through photosystem II (discovered second) and photosystem I (discovered first). Water is split, releasing oxygen (a waste product), electrons, and protons (hydrogen ions). As electrons move, they pump protons into the thylakoid lumen, creating a proton gradient. These protons then flow through ATP synthase, generating ATP. The products of the light-dependent reactions are NADPH and ATP, which are used in the Calvin cycle, and oxygen as a byproduct.

The Calvin cycle utilizes the ATP and NADPH from the light reactions as energy. Carbon dioxide enters the cycle and combines with a five-carbon molecule called RuBP, catalyzed by the enzyme rubisco, forming a short-lived six-carbon molecule that immediately breaks into two three-carbon molecules. With energy from ATP and NADPH, these three-carbon molecules are converted into G3P. Some G3P is used to synthesize glucose (or other sugars), while the rest is recycled to regenerate RuBP, continuing the cycle. The Calvin cycle will shut down if ATP, NADPH, or carbon dioxide are absent.

A problem for photosynthesis is photorespiration, which occurs when carbon dioxide levels are low. In such cases, rubisco can bind with oxygen instead of carbon dioxide, forming a useless chemical that the cell must break down, thus wasting energy. This is particularly problematic for C3 plants (named because G3P is a three-carbon molecule). Photorespiration wasn't originally an issue as oxygen levels were low when photosynthesis first evolved. The problem arises when plants close their stomata in hot conditions to conserve water, limiting CO2 intake. To combat this, CAM plants (e.g., jade, pineapple) open stomata at night to absorb CO2 and convert it into malic acid, which is stored; during the day, with stomata closed, they release CO2 for the Calvin cycle. C4 plants (e.g., corn) take in CO2 and use enzymes to form a four-carbon molecule, which is then moved to bundle sheath cells deeper within the leaf, where CO2 is released for the Calvin cycle, minimizing photorespiration. These adaptations, while requiring more energy, are beneficial in hot environments.