本期是光合作用的第二期。如果有不太明白的或者有錯誤的地方隨時來找UP主喔~ 文集本部分的參考文獻 Essential Cell Biology, 5th ed. Alberts, et al. 2019. 部分內(nèi)容來自 khanacademy 與維基百科。本章的內(nèi)容很大程度上參考了 khanacademy.

14b Sites?and Steps of Photosynthesis

Sites?of Photosynthesis

Plants are the most common autotrophs in terrestrial-land-ecosystems. All green plant tissues can photosynthesize, but in most plants, the majority of photosynthesis usually takes place in the leaves. The cells in a middle layer of leaf tissue called the mesophyll are the primary site of photosynthesis.

small pores called stomata?(singular stoma), are found on the surface of leaves in most plants, and they let carbon dioxide diffuse into the mesophyll layer and oxygen diffuse out.

Each mesophyll cell contains organelles called chloroplasts, which are specialized to carry out the reactions of photosynthesis.

Within each chloroplast, disc-like structures called thylakoids are arranged in piles like stacks of pancakes that are known as grana?(singular granum).

The membrane of each thylakoid contains green-colored pigments called chlorophylls that absorb light. The fluid-filled space around the grana is called the stroma, and the space inside the thylakoid discs is known as the thylakoid space. Different chemical reactions occur in the different parts of the chloroplast.

Chloroplasts

Chloroplasts are found only in plants and photosynthetic algae. (Humans and other animals do not have chloroplasts.) The chloroplast's job is to carry out a process called photosynthesis.

In photosynthesis, light energy is collected and used to build sugars from carbon dioxide. The sugars produced in photosynthesis may be used by the plant cell, or may be consumed by animals that eat the plant, such as humans. The energy contained in these sugars is harvested through a process called cellular respiration, which happens in the mitochondria of both plant and animal cells.

Chloroplasts are disc-shaped organelles found in the cytosol of a cell. They have outer and inner membranes with an intermembrane space between them?(three layers). If you passed through the two layers of membrane and reached the space in the center, you’d find that it contained membrane discs known as thylakoids, arranged in interconnected stacks called grana (singular, granum).?So chloroplasts have three membranes: outer membranes,?inner membranes, and thylakoid?membranes.

The membrane of a thylakoid disc contains light-harvesting complexes that include chlorophyll, a pigment that gives plants their green color. Thylakoid discs are hollow, and the space inside a disc is called the thylakoid space or lumen, while the fluid surrounding the thylakoids is called the stroma.

Steps of Photosynthesis

Photosynthesis in the leaves of plants involves many steps, but it can be divided into two stages: the light-dependent reactions and the Calvin cycle.

- [Light Reaction, the “Photo”?part]: The light-dependent reactions take place in the thylakoid membrane and require a continuous supply of light energy. Chlorophylls absorb this light energy, which is converted into chemical energy through the formation of two compounds, ATP and NADPH (a reduced electron bearing electron carrier). In this process, water molecules are also converted to oxygen gas.

- [Dark Reaction, the “Synthesis”?part]: The Calvin cycle, also called the light-independent reactions, takes place in the stroma and does not directly require light. Instead, the Calvin cycle uses ATP and NADPH from the light-dependent reactions to fix carbon dioxide and produce three-carbon sugars?G3P molecules, glyceraldehyde-3-phosphate, which?can?join up to form glucose.

Overall, the light-dependent reactions capture light energy and store it temporarily in the chemical forms of ATP and NADPH. There, ATP is broken down to release energy, and NADPH donates its electrons to convert carbon dioxide molecules into sugars. In the end, the energy that started out as light winds up trapped in the bonds of the sugars.

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In the Light Reactions:

- Electrons from H2O ?→ stored in NADPH?;

- Energy from light ?→ stored in NADPH?& ATP?→?Used in chloroplast

In the Dark?Reactions:

- Electrons, Energy from NADPH?& Energy from ATP?→?gets to Sugars.

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Sugars are exported from the chloroplasts and are:

- Used to build cell components;

- Broken down by mitochondria to make ATP for cytoplasm.

Plants - Both Photosynthesis & Cellular Respiration

- Photosynthesis: only works with light;

- Cellular Respiration: makes ATP in the dark.

ATP made in chloroplasts never leaves chloroplasts, it is used to synthesize sugars; while ATP made in mitochondria is used for general cell needs.

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三節(jié)：光與光合色素。只想看生物內(nèi)容的朋友可以跳過下一期。

作者：離久-張所長