3.4 Photosynthesis
- Syllabus
- 2025
- Topic
- 3.4
- Level
- —
Learning objectives
3.4A—Describe the photosynthetic processes and structural features of the chloroplast that allow organisms to capture…Describe the photosynthetic processes and structural features of the chloroplast that allow organisms to capture and store energy.• Photosynthesis is the series of reactions that use carbon dioxide (CO2), water (H2O) and light energy to make carbohydrates and oxygen (O2).- i. Photosynthetic organisms capture energy fr om the sun and produce sugars that can be used in biological processes or stored.- ii. Photosynthesis first evolved in pr okaryotic organisms.- iii. Scientific evidence supports the claim that pr okaryotic (cyanobacterial) photosynthesis was responsible for the production of an oxygenated atmosphere.- iv. Prokaryotic photosynthetic pathways w ere the foundation of eukaryotic photosynthesis.• Stroma and thylakoids are found within the chloroplast.- i. The stroma is the fluid within the inner chloroplast membrane and outside the thylakoid. The carbon fixation (Calvin cycle) reactions of photosynthesis occur in the stroma.- ii. The thylakoid membranes contain chlorophyll pigments organized into two photosystems, as well as electron transport proteins.- iii. Thylakoids are organized in stacks called grana. The light reactions of photosynthesis occur in the grana.• The light reactions of photosynthesis in eukaryotes involve a series of coordinated reaction pathways that capture energy present in light to yield ATP and NADPH, which power the production of organic molecules in the Calvin cycle. This provides energy for metabolic processes.• Exclusion: Memorization of the steps in the Calvin cycle, the structure of the molecules, and the names of the enzymes involved, with the exception of ATP synthase, is beyond the scope of the AP Exam.3.4B—Explain how cells capture energy from light and transfer it to biological molecules for storage and useExplain how cells capture energy from light and transfer it to biological molecules for storage and use.• Electron transport chain (ETC) reactions occur in chloroplasts, in mitochondria, and across prokaryotic plasma membranes. In photosynthesis, electrons that pass through the thylakoid membrane are picked up and ultimately transferred to NADP+ reducing it to NADPH in photosystem I.- Exclusion: The full names of the specific electron carriers in the electron transport chain are beyond the scope of the AP Exam. Specific steps, names of enzymes, and intermediates of the pathways for these processes are beyond the scope of this course and the AP Exam.• During photosynthesis, chlorophylls absorb energy from light, boosting electrons to a higher energy level in photosystems I and II. Water then splits, supplying electrons to replace those lost from photosystem II.• Photosystems I and II are embedded in the thylakoid membranes of chloroplasts and are connected by the transfer of electrons through an ETC.• When electrons are transferred between molecules in a series of oxidation/reduction reactions as they pass through the ETC, an electrochemical gradient of protons (hydrogen ions) is established across the thylakoid membrane. The membrane separates a region of low proton concentration outside the thylakoid membrane from a region of high proton concentration inside the thylakoid membrane.• The formation of the proton gradient is linked to the synthesis of ATP from ADP and inorganic phosphate via ATP synthase. The flow of protons back through membrane-bound ATP synthase by chemiosmosis drives the formation of ATP from ADP and inorganic phosphate; this is known as photophosphorylation.• The energy captured in the light reactions and transferred to ATP and NADPH powers the production of carbohydrates from carbon dioxide in the Calvin cycle. This occurs in the stroma of the chloroplast.
Chloroplast Structure Organizes Photosynthesis
Photosynthesis uses carbon dioxide, water, and light energy to produce carbohydrates and oxygen. The carbohydrates store captured energy for later biological processes, while chloroplast compartments keep the light-capturing reactions and carbon-fixation reactions organized.
| Chloroplast region | Key structures | Photosynthetic role |
|---|---|---|
| Thylakoids, stacked as grana | Membranes containing chlorophyll, photosystems, and electron-transport proteins | Light reactions capture light energy and produce ATP and NADPH |
| Stroma | Fluid inside the inner chloroplast membrane and outside thylakoids | Calvin-cycle reactions use ATP and NADPH to fix carbon dioxide into organic molecules |
The two regions form one coupled system: products of the thylakoid light reactions supply energy and reducing power for carbohydrate production in the stroma. This spatial organization links membrane-based energy capture to chemical energy storage.
Photosynthesis first evolved in prokaryotes; cyanobacterial photosynthesis contributed to an oxygenated atmosphere and provided the foundation for eukaryotic photosynthesis. AP Biology requires the overall Calvin-cycle role, not memorization of its individual steps, molecular structures, or enzyme names other than ATP synthase.
From Light Energy to ATP, NADPH, and Carbohydrates
The light reactions transfer energy from light into ATP and NADPH by coordinating photosystems, electron transport, and chemiosmosis across the thylakoid membrane. ATP and NADPH then power carbohydrate production in the stroma.
- Chlorophyll in photosystems II and I absorbs light, raising electrons to higher energy levels.
- Water splits and replaces electrons lost from photosystem II; oxygen is produced.
- Electrons move between the photosystems through an electron transport chain in the thylakoid membrane.
- Redox transfers through the chain establish high proton concentration inside the thylakoid and lower concentration outside.
- Protons flow through ATP synthase, driving ADP + inorganic phosphate → ATP by chemiosmosis (photophosphorylation).
- Electrons ultimately reduce NADP+ to NADPH at photosystem I.
ATP supplies usable energy and NADPH supplies high-energy electrons for the Calvin cycle. In the stroma, these products of the light reactions power the conversion of carbon dioxide into carbohydrates, transferring captured light energy into stored chemical energy.
The proton gradient stores potential energy; ATP synthase uses proton flow rather than electron flow directly to make ATP. Specific ETC carrier names, enzyme names, and pathway intermediates are outside the required AP scope.