本期是光合作用的第一期。如果有不太明白的或者有錯(cuò)誤的地方隨時(shí)來(lái)找UP主喔~ 文集本部分的參考文獻(xiàn) Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分內(nèi)容來(lái)自 khanacademy 與維基百科。本章的內(nèi)容很大程度上參考了 khanacademy.

14a Overview

Photosynthesis

Photosynthesis is the process in which light energy is converted to chemical energy in the form of sugars. In a process driven by light energy, glucose molecules (or other sugars) are constructed from water and carbon dioxide, and oxygen is released as a byproduct. The glucose molecules provide organisms with two crucial resources: energy and fixed-organic-carbon.

- Energy: The glucose molecules serve as fuel for cells: their chemical energy can be harvested through processes like cellular respiration and fermentation, which generate ATP for the cell’s immediate energy needs.

- Fixed Carbon: Carbon from carbon dioxide, the inorganic carbon, can be incorporated into organic molecules; this process is called carbon fixation, and the carbon in organic molecules is also known as?the?fixed carbon. The carbon that's fixed and incorporated into sugars during photosynthesis can be used to build other types of organic molecules needed by cells.

Ecological Importance

Photosynthetic organisms, including plants, algae, and some bacteria, play a key ecological role. They introduce chemical energy and fixed carbon into ecosystems by using light to synthesize sugars. Since these organisms produce their own food --- that is, fix their own carbon --- using light energy, they are called photoautotrophs (literally, self-feeders that use light).

Humans, and other organisms that can’t convert carbon dioxide to organic compounds themselves, are called heterotrophs, meaning different-feeders. Heterotrophs must get fixed carbon by eating other organisms or their by-products. Animals, fungi, and many prokaryotes and protists are heterotrophs.

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[Autotrophs]

The signature characteristic of autotrophs is that they can fix their own carbon, convert inorganic to organic carbon, given a suitable energy source.

- Photoautotrophs use light energy to convert carbon dioxide into organic compounds. This process is called photosynthesis.

- Chemoautotrophs extract energy from inorganic compounds by oxidizing them and use this chemical energy, rather than light energy, to convert carbon dioxide into organic compounds. This process is called chemosynthesis.

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[Heterotrophs]

Heterotrophs are unable to convert carbon dioxide to organic compounds themselves and must instead obtain fixed carbon from other organisms.

- Photoheterotrophs obtain energy from sunlight but must get fixed carbon in the form of organic compounds made by other organisms. Some types of prokaryotes are photoheterotrophs.

- Chemoheterotrophs obtain energy by oxidizing organic or inorganic compounds and, like all heterotrophs, get their fixed carbon from organic compounds made by other organisms. Animals, fungi, and many prokaryotes and protists are chemoheterotrophs.

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Besides introducing fixed carbon and energy into ecosystems, photosynthesis also affects the makeup of Earth’s atmosphere. Most photosynthetic organisms generate oxygen gas as a byproduct, and the advent of photosynthesis --- over 3 billion years ago, in bacteria resembling modern cyanobacteria --- forever changed life on Earth. These bacteria gradually released oxygen into Earth’s oxygen-poor atmosphere, and the increase in oxygen concentration is thought to have influenced the evolution of aerobic life forms --- organisms that use oxygen for cellular respiration.

Photosynthetic organisms also remove large quantities of carbon dioxide from the atmosphere and use the carbon atoms to build organic molecules. Without Earth’s abundance of plants and algae to continually suck up carbon dioxide, the gas would build up in the atmosphere. Although photosynthetic organisms remove some of the carbon dioxide produced by human activities, rising atmospheric levels are trapping heat and causing the climate to change. Many scientists believe that preserving forests and other expanses of vegetation is increasingly important to combat this rise in carbon dioxide levels.

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Photosynthesis vs. Aerobics Respiration

At the level of the overall reactions, photosynthesis and cellular respiration are near-opposite processes. They differ only in the form of energy absorbed or released, as shown in the diagram below.

At the level of individual steps,?photosynthesis?isn't just cellular respiration run in reverse. Instead, photosynthesis takes place in its own unique series of steps.

For instance, photosynthesis and cellular respiration both involve a series of redox reactions (reactions involving electron transfers). In cellular respiration, electrons flow from glucose to oxygen, forming water and releasing energy. In photosynthesis, they go in the opposite direction, starting in water and winding up in glucose?--- an energy-requiring process powered by light. Like cellular respiration, photosynthesis also uses an electron transport chain to make a H+ concentration gradient, which drives ATP synthesis by chemiosmosis.

See the upper left picture, photosynthesis has two parts:

[1] The “Photo”?part that uses light as energy source to make NADPH and ATP;

[2] The “Synthesis”?part that makes sugar.

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NADPH

NADPH is exactly the same molecule as NADH except for an extra phosphate group. See the upper right picture, the redox reaction happens at the “top”?of the NADPH molecule, so the extra phosphate group at the “bottom”?of the molecule makes no difference to the redox chemistry.

The extra phosphate group limits the enzymes that NADPH can interact with. NADPH is only used for anabolic synthesis reactions, while NADH interacts with different enzymes involved in catabolic breakdown reactions.

本次內(nèi)容到此結(jié)束，感謝閱讀！下一期內(nèi)容將是光合作用的第二節(jié)。

作者：離久-張所長(zhǎng)