Photosynthesis
Photosynthesis is the biochemical process by which chlorophyll-containing organisms — plants, algae, and cyanobacteria — convert light energy into chemical energy stored as glucose, using carbon dioxide and water as raw materials and releasing oxygen as a by-product. The process occurs in two stages: the light-dependent reactions in the thylakoid membranes (which generate ATP and NADPH by splitting water) and the light-independent Calvin cycle in the stroma (which fixes CO₂ into three-carbon sugars). Photosynthesis underpins nearly all food chains on Earth and is responsible for maintaining atmospheric oxygen levels.
Key Formula
6CO2 + 6H2O → (light energy) → C6H12O6 + 6O2
LaTeX: 6CO_2 + 6H_2O \xrightarrow{\text{light}} C_6H_{12}O_6 + 6O_2
| Symbol | Meaning | Unit |
|---|---|---|
| CO₂ | Carbon dioxide (reactant) | molecules |
| H₂O | Water (reactant) | molecules |
| C₆H₁₂O₆ | Glucose (product) | molecules |
| O₂ | Oxygen (by-product) | molecules |
Worked Example
Problem
A leaf absorbs sufficient light to fix 12 molecules of CO₂ in one second via the Calvin cycle. How many molecules of glucose are produced per second, and how many molecules of O₂ are released?
Solution
Step 1: From the overall equation, 6 CO₂ produce 1 glucose and 6 O₂. Step 2: Moles of glucose = 12 CO₂ ÷ 6 = 2 glucose molecules per second. Step 3: Moles of O₂ = 12 CO₂ × (6 O₂ / 6 CO₂) = 12 O₂ molecules per second.
Answer
2 glucose molecules and 12 O₂ molecules are produced per second.
Light-Dependent vs Light-Independent (Calvin Cycle) Reactions
| Feature | Light-Dependent Reactions | Calvin Cycle |
|---|---|---|
| Location | Thylakoid membrane | Stroma of chloroplast |
| Inputs | Light, H₂O, ADP, NADP⁺ | CO₂, ATP, NADPH |
| Outputs | ATP, NADPH, O₂ | G3P (glucose precursor), ADP, NADP⁺ |
| Key process | Photolysis of water, photophosphorylation | Carbon fixation (RuBisCO enzyme) |
| Light required? | Yes | No (indirectly depends on ATP/NADPH) |
Interactive Tools
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Related Terms
Cellular Respiration
Cellular respiration is the set of metabolic reactions by which cells break down organic molecules — primarily glucose — in the presence or absence of oxygen to generate ATP, the universal energy currency of life. The complete aerobic pathway yields approximately 30–32 ATP molecules per glucose molecule through three sequential stages: glycolysis (cytoplasm), the citric acid (Krebs) cycle (mitochondrial matrix), and oxidative phosphorylation via the electron transport chain (inner mitochondrial membrane). Understanding cellular respiration is fundamental to nutrition science, exercise physiology, and the treatment of metabolic diseases.
Aerobic Respiration
Aerobic respiration is the form of cellular respiration that requires molecular oxygen (O₂) as the final electron acceptor in the electron transport chain, enabling the complete oxidation of glucose to carbon dioxide and water with maximum ATP yield (~30–32 ATP per glucose). It proceeds through glycolysis, the link reaction (pyruvate decarboxylation), the citric acid cycle, and oxidative phosphorylation, all of which are tightly coupled within the mitochondrion. Aerobic respiration is the predominant energy-yielding pathway in all eukaryotes and many prokaryotes under oxygen-sufficient conditions, underpinning sustained muscular activity, brain function, and virtually every energy-demanding cellular process.
ATP (Adenosine Triphosphate)
Adenosine triphosphate (ATP) is the primary energy currency of all living cells, consisting of an adenine base, a ribose sugar, and three phosphate groups linked by high-energy phosphoanhydride bonds. When the terminal phosphate group is hydrolysed by ATPase enzymes to yield ADP (adenosine diphosphate) and inorganic phosphate (Pᵢ), approximately 30.5 kJ/mol of free energy is released under standard conditions (and up to ~54 kJ/mol under physiological conditions), which drives endergonic cellular processes including muscle contraction, active transport, biosynthesis, and signal transduction. A typical human cell turns over its own body weight in ATP every day, with mitochondrial oxidative phosphorylation producing the vast majority of this ATP.
From Greek "photos" (φῶτος) meaning "light" and "synthesis" (σύνθεσις) meaning "putting together". The term was introduced by Charles Barnes in 1893, replacing earlier terms such as "assimilation" used by Julius von Sachs in the 1860s who established the chloroplast as the site of the process.