grade 8 Photosynthesis & Respiration

Photosynthesis & Respiration

Plants use light energy to turn carbon dioxide and water into glucose and oxygen: photosynthesis. It happens in chloroplasts; the rate rises with light until it levels off above a saturation point. Cellular respiration releases living energy from glucose and oxygen, and its equation is the reverse of photosynthesis. Change the light intensity and the rate appears, side by side with respiration.

Plants Cook Their Own Food

🌿 Photosynthesis = a Plant Recipe

①Ingredients: CO₂ + H₂O

②Energy source: sunlight (solar energy)

③Kitchen: chloroplast (the green factory)

④Finished dish: glucose (food) + O₂

Photosynthesis Inside a Leaf

Light Intensity60%

💡 Take a Look

①Stronger light → faster photosynthesis

②Beyond a certain level, the rate plateaus (light saturation)

③With zero light, photosynthesis stops

Photosynthesis Equation

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

CO₂ + H₂O → (light) → glucose + O₂

Conditions Needed

Light + Chloroplast + CO₂ + H₂O

Missing any one of these slows down photosynthesis

Photosynthesis vs Cellular Respiration

Respiration Equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + Energy

Glucose + O₂ → CO₂ + H₂O + life energy (ATP)

💡 They are Opposite Reactions

①Photosynthesis: light energy → chemical energy (storage)

②Respiration: chemical → life energy (use)

③Photosynthesis only in light; respiration runs 24/7

④The two equations are exact reverses!

Summary

Core Equation

6CO₂ + 6H₂O + Light → C₆H₁₂O₆ + 6O₂

Site: chloroplast | When: daytime (with light)

🎯 Exam Points

①Photosynthesis site: chloroplast (chlorophyll absorbs light)

②Conditions: light, water, CO₂

③Respiration site: mitochondria (every living cell)

④Photo > Resp → releases O₂ (daytime)

⑤Resp > Photo → releases CO₂ (nighttime)

Examples and Unit Test

Example 1

What materials does photosynthesis need and what does it produce, and how is it written as an equation?

Photosynthesis uses light energy and takes carbon dioxide (CO₂) and water (H₂O) as its materials.

In the chloroplast it makes glucose (C₆H₁₂O₆) and oxygen (O₂): 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.

▸ Materials = CO₂ + H₂O, products = glucose + O₂ / 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Photosynthesis stores light energy as the chemical energy of glucose.

Example 2

In what way are photosynthesis and cellular respiration opposites, and how does a plant exchange gases by day and by night?

Photosynthesis is CO₂+H₂O→glucose+O₂ (chloroplast), and cellular respiration is glucose+O₂→CO₂+H₂O (mitochondria), so they are reverse reactions.

By day photosynthesis exceeds respiration, so the plant releases O₂; by night photosynthesis stops and only respiration occurs, so it releases CO₂.

▸ The two equations are reverse of each other / day = releases O₂ (photosynthesis > respiration), night = releases CO₂ (respiration only)

A plant respires 24 hours a day, while photosynthesis happens only in daylight.

Unit Test

Which statement about photosynthesis and cellular respiration is correct?

①Photosynthesis takes place in the mitochondria

②The materials of photosynthesis are glucose and oxygen

③Cellular respiration breaks down glucose to obtain usable energy

④A plant does not respire at night

⑤The equations of photosynthesis and respiration run in the same direction

▸ ③ Cellular respiration breaks down glucose to obtain usable energy

Photosynthesis takes place in the chloroplast (① wrong), and its materials are CO₂ and water while glucose and oxygen are products (② wrong).

A plant respires day and night (④ wrong), and the two equations run in reverse directions (⑤ wrong); cellular respiration breaks down glucose to get energy (③ correct).

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Exothermic & Endothermic Reactions

Digestion · Circulation · Excretion

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