Biology: AP Biology: Energy Flow and the Laws of Thermodynamics
Connecting free energy, entropy, and biological systems
Biology: AP Biology: Energy Flow and the Laws of Thermodynamics
Connecting free energy, entropy, and biological systems
Biology - Grade 9-12
- 1
State the first law of thermodynamics and explain how it applies to photosynthesis in plants.
Focus on how energy changes form rather than appearing from nothing.
The first law of thermodynamics states that energy cannot be created or destroyed, only transferred or transformed. In photosynthesis, plants transform light energy from the Sun into chemical energy stored in glucose and other organic molecules. - 2
State the second law of thermodynamics and explain why living organisms do not violate it when they build complex molecules such as proteins and DNA.
Remember that a cell is not an isolated system.
The second law of thermodynamics states that every energy transfer or transformation increases the entropy of the universe. Living organisms can build complex molecules because they use energy from their surroundings, and the total entropy of the organism plus its environment still increases, often through heat release. - 3
A cell uses ATP hydrolysis to drive the synthesis of a protein. Explain why ATP hydrolysis is often coupled to endergonic reactions.
Think about how one reaction can provide energy for another.
ATP hydrolysis is exergonic and releases free energy. When it is coupled to an endergonic process such as protein synthesis, the overall free energy change can become negative, allowing the combined reaction to proceed. - 4
The equation for Gibbs free energy is ΔG = ΔH - TΔS. Describe what it means when a biological reaction has a negative ΔG.
A negative ΔG means the reaction is exergonic and can proceed spontaneously under the given conditions. It also means the products have less free energy than the reactants. - 5
A reaction has ΔH = -40 kJ/mol, T = 300 K, and ΔS = -0.05 kJ/mol·K. Calculate ΔG and determine whether the reaction is spontaneous.
Use Kelvin for temperature and keep the units of entropy in kJ/mol·K.
Using ΔG = ΔH - TΔS, ΔG = -40 - 300(-0.05) = -40 + 15 = -25 kJ/mol. Because ΔG is negative, the reaction is spontaneous under these conditions. - 6
Explain why energy transfer between trophic levels in an ecosystem is inefficient.
Consider what happens to the energy an organism consumes before another organism eats it.
Energy transfer between trophic levels is inefficient because organisms use much of the energy they take in for cellular respiration, movement, growth, repair, and heat production. Only a fraction of the energy becomes biomass that is available to the next trophic level. - 7
In a simple food chain, producers store 20,000 kJ of energy. If approximately 10 percent of energy is transferred to each next trophic level, how much energy would be available to tertiary consumers after three transfers?
Multiply by 0.10 for each energy transfer.
After one transfer, primary consumers receive about 2,000 kJ. After two transfers, secondary consumers receive about 200 kJ. After three transfers, tertiary consumers receive about 20 kJ. - 8
Compare catabolic and anabolic pathways in terms of energy flow.
Catabolic pathways break down complex molecules into simpler molecules and usually release free energy. Anabolic pathways build complex molecules from simpler molecules and usually require an input of free energy, often supplied by ATP or reducing power. - 9
A graph shows free energy on the y-axis and reaction progress on the x-axis. The reactants are higher than the products, and there is a peak between them. Identify whether the reaction is exergonic or endergonic, and explain what the peak represents.
Compare the starting energy level with the ending energy level.
The reaction is exergonic because the products have lower free energy than the reactants. The peak represents the activation energy needed to reach the transition state before the reaction can proceed. - 10
Explain how enzymes affect activation energy and ΔG in a reaction.
Enzymes change the rate of a reaction, not whether the reaction is thermodynamically favorable.
Enzymes lower the activation energy of a reaction by stabilizing the transition state or providing an alternative reaction pathway. Enzymes do not change the overall ΔG of the reaction or the free energy of the reactants and products. - 11
During cellular respiration, glucose is broken down and oxygen is used to produce carbon dioxide, water, and ATP. Explain how this process illustrates energy transformation and entropy increase.
Cellular respiration transforms chemical energy in glucose into chemical energy in ATP, while some energy is released as heat. The process increases entropy because ordered glucose molecules are broken into smaller molecules and heat is dispersed into the surroundings. - 12
A student claims that cells recycle energy when they recycle matter such as carbon, nitrogen, and phosphorus. Evaluate this claim using the laws of thermodynamics.
Distinguish between cycling matter and flowing energy.
The claim is incorrect because matter can be recycled through biological and geochemical cycles, but energy is not recycled in the same way. Energy flows through organisms and ecosystems, is transformed, and is eventually dispersed as heat, consistent with the second law of thermodynamics.