Biology Practice Test: Photosynthesis and Cellular Respiration

The overall equation is 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. Glucose and oxygen are the products; they are the reactants of aerobic cellular respiration.

In the light-dependent reactions, water molecules are split to replace electrons lost by chlorophyll, and oxygen is released as a by-product. The oxygen atoms from carbon dioxide end up in glucose and water, not in the released O₂.

Chlorophyll and the electron transport chains of photosynthesis sit in the thylakoid membranes, so that is where light is absorbed and ATP and NADPH are made. The stroma, the fluid around the thylakoids, is where the Calvin cycle builds sugar.

The Calvin cycle takes in carbon dioxide and uses the energy in ATP and the electrons carried by NADPH, both made in the light-dependent reactions, to build sugar. Oxygen is a waste product of the light reactions, not an input to the Calvin cycle.

Glycolysis happens in the cytoplasm, does not need oxygen, and breaks one glucose into two pyruvate, with a net gain of 2 ATP. Most ATP is made later in the mitochondria, when oxygen is available.

Oxygen accepts the electrons at the end of the chain and joins with hydrogen ions to form water. Without oxygen the chain backs up, which is why cells switch to fermentation. Pyruvate turning into lactic acid is part of fermentation, not the electron transport chain.

The Krebs cycle runs in the matrix, the fluid inside the inner membrane, where it releases carbon dioxide and loads NADH and FADH₂ with electrons. Thylakoids are found in chloroplasts, not in mitochondria.

Glycolysis needs NAD⁺. Fermentation passes electrons from NADH to pyruvate, freeing NAD⁺ so glycolysis can continue to make its 2 ATP per glucose. Fermentation gives far less ATP than aerobic respiration, not more.

Alcoholic fermentation turns pyruvate into ethanol and carbon dioxide. The CO₂ gas gets trapped in the dough and makes it rise; the ethanol evaporates during baking. Lactic acid is made in muscle cells, not by yeast.

Moving the lamp farther away makes the light dimmer, and the bubble count falls from 40 to 6 per minute. So the rate of photosynthesis drops as light intensity drops. The plant still made bubbles at 30 cm and 40 cm, so it did not stop.

In bright light a plant photosynthesizes much faster than it respires, so it releases oxygen made by splitting water. Carbon dioxide is taken in by the plant, not given off, while it is photosynthesizing quickly.

In the dark the plant cannot photosynthesize but keeps respiring, so CO₂ builds up and the indicator turns yellow. In bright light the plant uses more CO₂ than it releases, so the solution stays blue. Tubes without organisms are controls and should not change.

Most of a tree's dry mass is carbon-based compounds such as cellulose, and that carbon comes from CO₂ fixed during photosynthesis. Sunlight supplies energy, not matter, and soil minerals make up only a small part of the mass.

Fermentation stops after glycolysis, so most of the energy stays locked in lactic acid or ethanol. In aerobic respiration the Krebs cycle and the electron transport chain fully oxidize the fuel to CO₂ and use oxygen to make many more ATP. Fermentation still makes a net 2 ATP from glycolysis.

Chlorophyll absorbs mostly red and blue-violet light and reflects green, which is why leaves look green. Because blue light is absorbed and used, it is not the color chlorophyll reflects.