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Biology: Life and Systems - Photosynthesis and Cellular Respiration

Grade 8IB

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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Photosynthesis is the process by which green plants and some other organisms use sunlight to synthesize nutrients from CO2CO_2 and H2OH_2O. This process occurs in the chloroplasts and involves the green pigment chlorophyll.

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The primary product of photosynthesis is glucose (C6H12O6C_6H_{12}O_6), which is stored as starch, and the byproduct is oxygen (O2O_2).

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Cellular Respiration is a set of metabolic reactions that take place in the cells of organisms to convert biochemical energy from nutrients into Adenosine Triphosphate (ATPATP).

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Aerobic respiration requires oxygen (O2O_2) and occurs primarily in the mitochondria, yielding a high amount of ATPATP per molecule of glucose.

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Anaerobic respiration (fermentation) occurs in the absence of oxygen. In muscles, it produces lactic acid (C3H6O3C_3H_6O_3), while in yeast, it produces ethanol (C2H5OHC_2H_5OH) and CO2CO_2.

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The relationship between photosynthesis and respiration is complementary; the products of photosynthesis (C6H12O6C_6H_{12}O_6 and O2O_2) are the reactants for aerobic respiration.

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Factors affecting the rate of photosynthesis include light intensity, CO2CO_2 concentration, and temperature (TT).

📐Formulae

6CO2+6H2O+light energy→C6H12O6+6O26CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2

C6H12O6+6O2→6CO2+6H2O+ATP energyC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{ATP energy}

Glucose→Lactic Acid+Energy (2 ATP)[Anaerobic in animals]\text{Glucose} \rightarrow \text{Lactic Acid} + \text{Energy (2 ATP)} \quad [\text{Anaerobic in animals}]

C6H12O6→2C2H5OH+2CO2+Energy[Anaerobic in yeast]C_6H_{12}O_6 \rightarrow 2C_2H_5OH + 2CO_2 + \text{Energy} \quad [\text{Anaerobic in yeast}]

💡Examples

Problem 1:

Explain why a plant kept in a dark room for 48 hours will show a negative result for a starch test using iodine solution.

Solution:

In the absence of light, the plant cannot perform photosynthesis to produce new glucose. Over 48 hours, the plant consumes its stored starch through cellular respiration to produce ATPATP for survival. Consequently, no starch remains to react with the iodine.

Explanation:

Starch is the storage form of C6H12O6C_6H_{12}O_6. Without light, the rate of photosynthesis is zero, while the rate of respiration remains constant, depleting the energy reserves.

Problem 2:

Calculate the number of molecules of O2O_2 produced if a plant consumes 1818 molecules of CO2CO_2 during photosynthesis.

Solution:

According to the balanced equation 6CO2+6H2O→C6H12O6+6O26CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2, the ratio of CO2CO_2 to O2O_2 is 1:11:1. Therefore, 1818 molecules of CO2CO_2 will produce 1818 molecules of O2O_2.

Explanation:

The stoichiometry of the photosynthesis reaction shows that for every molecule of carbon dioxide absorbed, one molecule of oxygen is released.

Problem 3:

During vigorous exercise, a student's muscles begin to ache. Identify the process occurring and the chemical responsible.

Solution:

The process is anaerobic respiration. Due to an oxygen debt, the muscles break down glucose into lactic acid (C3H6O3C_3H_6O_3) instead of CO2CO_2 and H2OH_2O.

Explanation:

When O2O_2 supply is insufficient for aerobic respiration, cells switch to anaerobic pathways, resulting in the accumulation of C3H6O3C_3H_6O_3, which can cause muscle fatigue.