Photosynthesis & Cellular Respiration: Fifth Grade Science
The biosphere is powered by two complementary biochemical pathways: photosynthesis, which stores solar radiant energy in glucose molecules, and cellular respiration, which releases that energy as cellular ATP.
The Solar Engine of Life
Almost all biological life on Earth is energized directly or indirectly by sunlight. Through the biochemical process of photosynthesis, autotrophic organisms—plants, green algae, and cyanobacteria—capture radiant light energy and transform it into stored chemical potential energy inside the covalent bonds of glucose sugar. Every bite of food eaten by animals, every drop of oxygen breathed into lungs, and every piece of wood or coal burned for warmth owes its existence to this magnificent solar reaction.
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| THE CHEMICAL EQUATION OF PHOTOSYNTHESIS |
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| REACTANTS (Raw Ingredients In) PRODUCTS (Outputs Created) |
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| 6 CO₂ + 6 H₂O + Light Energy ---> C₆H₁₂O₆ + 6 O₂ |
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| Carbon Water Solar Radiant Glucose Oxygen |
| Dioxide from Energy from Sugar Gas |
| from Air Soil the Sun Food Released! |
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The Chloroplast: Solar Refinery
Photosynthesis takes place inside specialized green organelles called chloroplasts, concentrated primarily within the mesophyll cells of plant leaves. Chloroplasts contain flattened, membrane-bound discs called thylakoids, stacked neatly like piles of coins into structures called grana. Embedded inside the thylakoid membranes are millions of molecules of chlorophyll, a green photosynthetic pigment that absorbs blue and red wavelengths of visible sunlight while reflecting green wavelengths.
Two-Stage Reaction: Light and Dark Reactions
Photosynthesis occurs in two coordinated biochemical stages:
- The Light-Dependent Reactions: Occur inside the thylakoid membranes. Chlorophyll absorbs solar photons, energizing electrons and splitting water molecules (H₂O) into hydrogen ions and oxygen gas. The oxygen gas is released through leaf pores as a byproduct into the atmosphere, while energy-carrier molecules (ATP and NADPH) are synthesized.
- The Calvin Cycle (Light-Independent Reactions): Occurs in the fluid stroma surrounding the thylakoids. Using the ATP energy produced in the first stage, enzymes capture invisible carbon dioxide gas (CO₂) from the atmosphere and assemble the carbon and hydrogen atoms into glucose sugar (C₆H₁₂O₆).
Cellular Respiration: Releasing Stored Energy
While photosynthesis manufactures glucose, cellular respiration is the process by which all living cells—both plant and animal—break down glucose in the presence of oxygen to release usable chemical energy in the form of adenosine triphosphate (ATP).
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| THE CHEMICAL EQUATION OF CELLULAR RESPIRATION |
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| REACTANTS (Inputs Consumed) PRODUCTS (Outputs Released) |
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| C₆H₁₂O₆ + 6 O₂ ---> 6 CO₂ + 6 H₂O + Chemical ATP Energy |
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| Glucose Oxygen Carbon Water Usable Energy |
| Sugar Gas Dioxide Vapor (~36 to 38 ATP |
| from Food Inhaled Exhaled Byproduct molecules per glucose) |
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The Mitochondria: Cellular Power Plants
Cellular respiration occurs primarily inside the mitochondria. As glucose fragments enter the folded inner mitochondrial membranes (cristae), a cascade of biochemical reactions strips electrons from the food molecules, combining them with inhaled oxygen to generate water and carbon dioxide while synthesizing dozens of high-energy ATP molecules. Whenever your muscle cells contract, your nerve cells transmit electrical signals, or your skin cells divide, they consume ATP produced by mitochondria.
The Perfect Biochemical Cycle
Photosynthesis and cellular respiration form a closed, interdependent ecological loop. The products of one process serve as the exact raw reactants for the other process!
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| THE PHOTOSYNTHESIS-RESPIRATION CYCLE |
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| | PHOTOSYNTHESIS | |
| | (In Plant Chloroplasts) | |
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| O₂ (Oxygen) and | C₆H₁₂O₆ (Glucose) |
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| | CELLULAR RESPIRATION | |
| | (In Mitochondria of All) | |
| +--------------------+--------------------+ |
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| CO₂ (Carbon Diox) and H₂O (Water) |
| v |
| (Cycles back to power Photosynthesis!) |
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Plants perform photosynthesis during sunny daylight hours, producing oxygen and glucose. However, plants must also stay alive at night; therefore, plants carry out cellular respiration 24 hours a day, using oxygen and stored sugars in their mitochondria to fuel their nocturnal growth and root repair.
Exercises and Solutions
Exercise 1: Tracing Atoms in a Closed Biosphere Jar. A scientist creates a completely sealed glass terrarium containing a healthy green fern, rich soil with moisture, and sunlight streaming through the glass. The jar is airtight, meaning no gases can enter or escape. Explain how the fern can survive sealed inside this jar for years using the relationship between photosynthesis and cellular respiration.
Solution 1: The fern survives because photosynthesis and cellular respiration form a self-sustaining closed cycle of matter. During daylight hours, the fern's chloroplasts absorb light energy, taking in water from the soil and carbon dioxide released by cellular respiration to produce glucose and oxygen gas. At night, or whenever the plant requires metabolic energy, its mitochondria take in that oxygen gas and metabolize stored glucose, releasing carbon dioxide and water vapor back into the sealed jar. Because matter is conserved in a closed system, the carbon, hydrogen, and oxygen atoms continuously cycle between the plant's chloroplasts, mitochondria, and internal air without ever running out.
Exercise 2: Investigating Photosynthesis Rates Under Colored Light. A student sets up an experiment with three identical aquatic Elodea water plants submerged in beakers of water under three different light bulbs: Blue light, Red light, and Green light. The student counts the number of oxygen bubbles released per minute from the stems. Predict which plant will produce the fewest bubbles, and explain why using the properties of chlorophyll.
Solution 2: The plant illuminated by Green light will produce the fewest oxygen bubbles (the lowest rate of photosynthesis). Chlorophyll is a pigment that absorbs blue and red wavelengths of light with high efficiency, using that absorbed radiant energy to split water molecules and drive the light reactions that produce oxygen gas. In contrast, chlorophyll does not absorb green light; it reflects green light away (which is why leaves appear green to human eyes). Under pure green light, the chlorophyll cannot absorb sufficient photon energy to power the photosynthetic machinery, causing oxygen bubble production to slow to a near standstill.