Module 02: Life's Origins
By the end you can
Photosynthesis captures light energy in glucose. Cellular respiration transfers food energy into ATP. Their overall reactants and products connect.
Work with one section at a time.
Describe what each label or arrow means.
Explain why the structure or process produces that result.
Answer a practice question and use its explanation to correct your reasoning.
Which substances are reactants in photosynthesis?
Carbon dioxide and water
Got it now? Scroll back up and re-read the section — it should land differently.
Plants, algae, and some bacteria carry out photosynthesis. Carbon dioxide and water are the reactants; light supplies energy. Glucose and oxygen are products. Light energy becomes chemical energy stored in glucose. The overall equation is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂.
Chloroplasts are the photosynthetic organelles in plant cells. Chlorophyll is the green pigment that captures light. Thylakoids are membrane structures inside chloroplasts. Stomata are leaf openings for gas exchange. Xylem transports water and minerals from roots; phloem transports food through the plant.
Chemical reactions rearrange atoms. The carbon, hydrogen, and oxygen present in the reactants remain in the products. Sunlight's energy is transformed into stored chemical energy. A reaction changes the arrangement and energy transfers, rather than making matter or energy disappear.
Aerobic cellular respiration uses glucose and oxygen, producing carbon dioxide, water, and energy transferred to ATP with heat released. The overall equation is C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy. Both plants and animals carry out respiration; plants need ATP even though they also make glucose.
Glycolysis begins the breakdown of glucose in the cytoplasm. In eukaryotic cells, later aerobic stages occur in mitochondria. The course names the Krebs cycle and shows later energy-transfer steps. The central goal is ATP production. Breathing moves air; cellular respiration is a set of chemical reactions.
Aerobic respiration requires oxygen and produces much more ATP per glucose than the fermentation pathway. When oxygen is unavailable, glycolysis can continue with fermentation supporting it. The course uses 'anaerobic' for this oxygen-free pathway. Its net yield is two ATP per glucose from glycolysis, compared with more than 30 for aerobic respiration in the course.
Lactic acid fermentation is associated with animal muscle cells when oxygen supply is limited and also occurs in some microbes. Alcoholic fermentation in yeast produces ethanol and carbon dioxide; the carbon dioxide makes dough rise. Both allow glycolysis to continue without oxygen.
Photosynthesis supplies glucose and oxygen used in respiration. Respiration supplies carbon dioxide and water used in photosynthesis. The overall equations have opposite arrangements of these substances, though their detailed reaction steps are different. Matter cycles while energy passes from sunlight to food to ATP and is eventually dispersed as heat.
A seed can use stored food in respiration before it grows leaves. In light, an aquatic plant photosynthesizes while also respiring. A snail uses oxygen and releases carbon dioxide through respiration. Explain an exchange by naming the process and its reactants/products; a plant still respires when it cannot photosynthesize.
