10.3 Photosynthesis, Cellular Respiration, and Energy Acquisition

Two Equations That Organize Life Science

The framework asks teachers to describe how organisms "obtain and use energy and matter." Almost every item in that space reduces to photosynthesis, cellular respiration, or the relationship between them, and those two equations also anchor the carbon cycle in Domain IV and the energy-pyramid material in Competency 015.

Photosynthesis

6 CO₂ + 6 H₂O + light energy → C₆H₁₂O₆ + 6 O₂

• Location: chloroplasts, mostly in the palisade mesophyll of leaves.
• Pigment: chlorophyll a and b absorb strongly in the blue-violet and red regions and reflect green, which is why leaves look green. Accessory pigments such as carotenoids absorb other wavelengths and become visible in autumn when chlorophyll degrades.
• Reactants: carbon dioxide from the air through stomata, water from the soil through xylem, light energy from the Sun.
• Products: glucose (chemical energy storage) and oxygen (released as a by-product).

Two stages are worth naming even at grades 4-8. The light-dependent reactions occur in the thylakoid membranes, split water, release O₂, and capture energy. The light-independent reactions (Calvin cycle) occur in the stroma and use that captured energy to build glucose from CO₂. Students do not need the biochemical detail, but they should know that oxygen comes from water, not from carbon dioxide.

Cellular Respiration

C₆H₁₂O₆ + 6 O₂ → 6 CO₂ + 6 H₂O + energy (ATP)

• Location: begins with glycolysis in the cytoplasm, completed in the mitochondria.
• Yield: roughly 36-38 ATP per glucose molecule under aerobic conditions.
• Performed by: plants, animals, fungi, and most protists and bacteria — essentially all organisms, not just animals.

Comparing the Two Processes

The row that matters most instructionally is the last two. A plant respires all the time and photosynthesizes only in light. During daylight photosynthesis usually outpaces respiration, so the plant is a net oxygen producer; in darkness only respiration continues, so the plant is a net oxygen consumer. Students who believe "plants give off oxygen and animals give off carbon dioxide" as a fixed rule cannot explain a sealed jar experiment run overnight.

The Mass Misconception

Ask students where a 500 kg tree's mass came from and most will answer "the soil." The classic evidence is Jan Baptist van Helmont's willow experiment: over five years the tree gained about 74 kg while the potted soil lost only about 60 g. Van Helmont wrongly concluded the mass came from water; the modern answer is that most of the added dry mass is carbon that entered as atmospheric CO₂ and was fixed into glucose and then into cellulose.

The classroom confront-and-resolve sequence is straightforward: mass a pot of dry soil and a seedling, grow the plant for several weeks, and re-mass both. The plant's gain vastly exceeds the soil's loss. The lesson generalizes to the whole carbon cycle, because it establishes that air is made of matter with measurable mass.

ATP: The Cell's Energy Currency

Adenosine triphosphate (ATP) consists of adenine, ribose, and three phosphate groups. Removing the terminal phosphate produces ADP and releases energy the cell can use for muscle contraction, active transport, and molecule synthesis. Adding a phosphate back to ADP stores energy again. The analogy worth teaching is a rechargeable battery: glucose is the fuel, ATP is the charged battery, and ADP is the discharged one.

Why the distinction matters: cells cannot use glucose energy directly. Respiration exists precisely to convert the energy in food into the ATP form that cellular machinery accepts.

Anaerobic Pathways

When oxygen is scarce, cells fall back on fermentation after glycolysis:

• Lactic acid fermentation — in human muscle during intense exercise; glucose → 2 lactic acid + 2 ATP. Lactate accumulation is associated with muscle fatigue.
• Alcoholic fermentation — in yeast; glucose → 2 ethanol + 2 CO₂ + 2 ATP. The CO₂ is what makes bread rise.

The yield contrast is the tested point: 2 ATP anaerobically versus 36-38 aerobically. Fermentation is a low-efficiency emergency pathway, not an equal alternative, which is why a sprinter cannot maintain top speed for long while a distance runner pacing aerobically can keep going.

From Organism to Ecosystem

Chemosynthesis is the framework's reminder that sunlight is not the only energy base for life: hydrothermal vent communities are built on bacteria that oxidize hydrogen sulfide. Decomposers close the loop by returning carbon, nitrogen, and phosphorus to inorganic forms that producers can reuse; without them, nutrients would remain locked in dead biomass and productivity would halt. That recycling role is what distinguishes the flow of energy — one-way, entering as sunlight and leaving as heat — from the cycling of matter, which is used repeatedly.