3.4 Photosynthesis

Keywords

English Term	中文翻译	Definition & Explanation
Photosynthesis	光合作用	The process that captures light energy and uses it to synthesize carbohydrates from $\ce{CO2}$ and $\ce{H2O}$, releasing $\ce{O2}$ as a byproduct.
Cyanobacteria	蓝细菌	Photosynthetic, oxygen-producing prokaryotes (formerly known as blue-green algae) that early in Earth's history oxygenated the atmosphere.
Chloroplast	叶绿体	The organelle in plant cells and photosynthetic eukaryotes where photosynthesis occurs.
Thylakoid	类囊体	Flattened, interconnected membranous sacs inside the chloroplast; the site of the light-dependent reactions.
Stroma	基质	The dense fluid within the chloroplast surrounding the thylakoid membrane; the site of the Calvin cycle.
Photosystem	光系统	A light-capturing unit located in the thylakoid membrane, consisting of a reaction-center complex surrounded by numerous light-harvesting complexes.
Chemiosmosis	化学渗透	An energy-coupling mechanism that uses energy stored in the form of a hydrogen ion gradient across a membrane to drive cellular work, such as $\ce{ATP}$ synthesis.
Calvin Cycle	卡尔文循环	The second of two major stages in photosynthesis, involving fixation of atmospheric $\ce{CO2}$ and reduction of the fixed carbon into carbohydrate.

1. Overview

Photosynthesis is the fundamental series of biological reactions that sustain life on Earth. It captures energy from the sun and uses it to build highly ordered molecules (sugars) that can be used in cellular processes or stored for later.

The overall conceptual reaction is:

\[\ce{CO2 + H2O + \text{Light Energy} \rightarrow \text{Carbohydrates} + O2}\]

Evolutionary Context: The Oxygenation of Earth

Photosynthesis did not initially evolve in complex plants; it first evolved in prokaryotic organisms. Scientific evidence strongly supports the claim that early prokaryotic photosynthesis (specifically by cyanobacteria) was directly responsible for the production of an oxygenated atmosphere on ancient Earth. Furthermore, these prokaryotic photosynthetic pathways laid the evolutionary foundation for eukaryotic photosynthesis (which aligns with the Endosymbiotic Theory).

2. Chloroplast

In eukaryotes, photosynthesis takes place in the chloroplast, which has a highly specialized double-membrane structure. The spatial separation of its internal compartments is essential for its function.

Thylakoids and Grana: Thylakoids are internal, interconnected membrane sacs. They are often stacked in columns called grana. The thylakoid membranes contain chlorophyll pigments, photosystems, and electron transport proteins. The light reactions occur here.
Stroma: The stroma is the fluid located inside the inner chloroplast membrane but outside the thylakoids. The Calvin cycle (carbon fixation) occurs in the stroma.

(Placeholder: A cross-section of a chloroplast, clearly labeling the stroma and the stacked thylakoids (grana).)

Light dependent and independent reactions.

3. The Light Reactions

The light reactions in eukaryotes involve a series of coordinated reaction pathways that capture light energy to yield $\ce{ATP}$ and $\ce{NADPH}$, which are necessary to power the Calvin cycle.

AP Exam Exclusion Statement: Light Reactions

The full names of the specific electron carriers in the electron transport chain (like plastoquinone or ferredoxin) are beyond the scope of the AP Exam. Focus on the flow of energy and electrons!

The Flow of Energy and Electrons

Capturing Light: Photosystem II (PS II) and Photosystem I (PS I) are embedded in the thylakoid membrane. Chlorophylls absorb light energy, which boosts electrons to a higher, unstable energy level.
Splitting of Water: To replace the electrons lost by PS II, $\ce{H2O}$ is split. This provides electrons, releases protons ($\ce{H+}$) into the thylakoid space, and releases Oxygen ($\ce{O2}$) as a byproduct.
Electron Transport Chain (ETC): The excited electrons are transferred between molecules in a series of oxidation/reduction reactions as they pass through the ETC connecting PS II and PS I. Ultimately, these electrons are transferred to $\ce{NADP+}$, reducing it to $\ce{NADPH}$.

Creating the Proton Gradient and $\ce{ATP}$

Electrochemical Gradient: As electrons move through the ETC, the energy released is used to pump protons ($\ce{H+}$) across the thylakoid membrane. This establishes an electrochemical gradient, separating a region of low proton concentration outside (in the stroma) from a region of high proton concentration inside the thylakoid lumen.
Photophosphorylation via Chemiosmosis: The formation of the proton gradient is linked to the synthesis of $\ce{ATP}$. Protons flow down their concentration gradient back into the stroma through the membrane-bound enzyme ATP synthase. This flow drives the formation of $\ce{ATP}$ from $\ce{ADP}$ and inorganic phosphate.

(Placeholder: A detailed view of the thylakoid membrane showing light exciting electrons, the splitting of water, the proton gradient building up inside the thylakoid space, and ATP Synthase producing $\ce{ATP}$.)

4. The Calvin Cycle

The energy captured in the light reactions (stored in $\ce{ATP}$ and $\ce{NADPH}$) now powers the production of carbohydrates from atmospheric carbon dioxide ($\ce{CO2}$). This process takes place in the stroma.

AP Exam Exclusion Statement: The Calvin Cycle

Memorization of the specific steps in the Calvin cycle, the structure of the intermediate molecules, and the names of the enzymes involved (with the exception of ATP synthase) is beyond the scope of the AP Exam. You do not need to memorize RuBP, G3P, or Rubisco!

Key Takeaway for the Calvin Cycle: It is an energy-consuming (endergonic) pathway. It relies completely on the products of the light reactions. $\ce{ATP}$ provides the chemical energy, and $\ce{NADPH}$ provides the reducing power (electrons) necessary to "fix" inorganic $\ce{CO2}$ into highly organized, energy-rich organic molecules (carbohydrates).

Quiz

Source: Campbell Biology Practice Test - Chapter 10 (Photosynthesis)