2.2 Enzymes, Photosynthesis, and Respiration

Enzymes, Photosynthesis, and Respiration

This section sits at the meeting point of structure, function, matter, and energy. The New York State P-12 Science Learning Standards ask Life Science: Biology students to use models for photosynthesis and aerobic cellular respiration and to explain how carbon-based molecules and energy move through living systems. The Regents level is conceptual: know inputs, outputs, transformations, and evidence. Do not spend study time memorizing every biochemical intermediate in glycolysis, the Krebs cycle, or the Calvin cycle for this exam.

Enzymes: Shape Controls Rate

An enzyme is usually a protein that speeds up a chemical reaction by lowering activation energy. Its active site fits a substrate in a specific way. If the enzyme's shape changes, the reaction rate can drop because the substrate no longer fits well. This is why enzyme questions often show a graph of reaction rate versus temperature or pH.

A common Regents trap is thinking enzymes are used up. Enzymes can be reused, although they can be damaged by extreme conditions. Another trap is treating every enzyme as if it has the same optimum. Pepsin in the stomach works best under acidic conditions, while many enzymes in small-intestine fluid work at a more basic pH. The correct answer must fit the data provided.

Photosynthesis Stores Energy

Photosynthesis uses light energy to convert carbon dioxide and water into sugars and oxygen. In a Regents model, the most important idea is not the exact chemical pathway; it is the transformation of light energy into chemical energy stored in glucose and the rearrangement of atoms into new molecules. Chloroplasts contain chlorophyll and other molecules that help capture light.

A simple model is:

Plants do not get their food from soil. Soil can supply water and minerals, but the carbon atoms in glucose come mainly from carbon dioxide. If an answer says a plant gains most of its mass by absorbing food from soil, reject it. If a data table shows plants in brighter light produce more oxygen bubbles, the evidence supports increased photosynthesis, not increased absorption of food.

Respiration Releases Usable Energy

Cellular respiration breaks and rearranges bonds in food molecules and oxygen, forming carbon dioxide and water while transferring energy to ATP. ATP is useful because cells can use it for active transport, muscle contraction, synthesis, signaling, and repair. Respiration is not the same as breathing. Breathing exchanges gases at the organism level; cellular respiration is a chemical process inside cells.

Animal cells, plant cells, fungi, and many microorganisms carry out respiration. Plant cells respire in both light and dark, because they need ATP all the time. In light, photosynthesis may occur faster than respiration in green tissues, producing net oxygen release. In darkness, photosynthesis stops, but respiration continues.

Science-Data Examples

Example 1: An Elodea plant is placed under three light intensities. Average oxygen bubbles per minute are 4, 14, and 27 as light increases. The best claim is that higher light intensity increased the rate of photosynthesis under those conditions. A stronger constructed response would add that oxygen is an output of photosynthesis and the bubble count is a proxy for oxygen production.

Example 2: Yeast and sugar are placed in sealed test tubes at 5 degrees C, 30 degrees C, and 70 degrees C. Carbon dioxide production is highest at 30 degrees C, low at 5 degrees C, and nearly zero at 70 degrees C. The evidence supports an enzyme-temperature explanation: cold slows reactions, while high heat can disrupt enzyme shape. The answer should not say the yeast died immediately unless the data specifically show viability; stick to what the measured rate supports.

Matter and Energy Traps

• Trap: photosynthesis makes energy. It transforms light energy into chemical energy stored in sugar.
• Trap: respiration destroys matter. Atoms are rearranged; carbon dioxide and water still contain matter.
• Trap: oxygen is needed only by animals. Aerobic respiration in plants also uses oxygen.
• Trap: a graph peak means the enzyme is gone afterward. A peak usually marks optimum conditions, not enzyme disappearance.
• Trap: more substrate always means more product forever. Reaction rate can plateau when active sites are saturated.

When reading a Regents cluster, circle the measured variable before looking at choices. If the y-axis is oxygen production, the item is probably using oxygen as evidence for photosynthesis. If the y-axis is carbon dioxide production or temperature change in germinating seeds, the item may be using those data as evidence for respiration. Evidence first, vocabulary second.

A useful final check is to follow atoms and energy separately. Carbon atoms can move from carbon dioxide into glucose and later into carbon dioxide again, while energy changes form and some is released as heat. Answers that recycle energy the same way matter cycles usually miss the model.