GED Science Energy for Life: Photosynthesis and Respiration Summary

In photosynthesis, plants capture light energy and use it to turn carbon dioxide and water into glucose, a sugar that stores energy. Oxygen is released as a by-product.

Wrong choices

• Salt and heat: Photosynthesis does not make salt.
• Protein and nitrogen: Photosynthesis does not make protein or nitrogen.
• Carbon dioxide and water: Carbon dioxide and water are what photosynthesis uses up. They are also what cellular respiration gives off.

Chloroplasts contain chlorophyll, the green pigment that captures light energy. That is where photosynthesis happens.

Wrong choices

• Mitochondrion: The mitochondrion is where cellular respiration releases energy from food.
• Cell wall: The cell wall gives the plant cell support and shape.
• Nucleus: The nucleus holds the cell's DNA.

In aerobic cellular respiration, cells combine glucose with oxygen to release usable energy. Carbon dioxide and water are given off as wastes.

Wrong choices

• Nitrogen: Nitrogen makes up most of the air, but our cells do not use it in respiration.
• Carbon dioxide: Carbon dioxide is a waste product of respiration. It is breathed out, not used.
• Helium: Helium is not used by living cells.

The independent variable is the factor the scientist changes on purpose. Here the student changed the lamp distance. The dependent variable is what she measured in response: the number of bubbles.

Wrong choices

• The kind of plant used: The same plant was used each time, so the kind of plant was kept the same (a controlled variable).
• The time spent counting: The counting time was always one minute, so it was kept the same.
• The number of bubbles per minute: The number of bubbles is what was measured. That is the dependent variable.

As the lamp moved closer (10 cm), the bubble count went up (30). As it moved farther (40 cm), the count went down (4). More light gave faster photosynthesis, so more oxygen bubbles.

Wrong choices

• Lamp distance had no effect on the plant.: The count changed a lot as the distance changed, so distance did have an effect.
• The plant gives off the most oxygen in the dark.: The data show fewer bubbles with less light, so darkness would give the fewest bubbles, not the most.
• The bubble count doubled each time the lamp moved 10 cm closer.: From 20 cm to 10 cm the count went from 18 to 30, which is not double. The pattern is not exact doubling.

Photosynthesis uses carbon dioxide, water, and light to make glucose and oxygen. Cellular respiration uses glucose and oxygen to release energy, giving off carbon dioxide and water. Each process uses what the other makes.

Wrong choices

• Respiration stores energy in glucose, and photosynthesis releases it.: This is reversed. Photosynthesis stores energy in glucose, and respiration releases it.
• Photosynthesis uses oxygen and gives off carbon dioxide.: This is reversed. Photosynthesis uses carbon dioxide and gives off oxygen.
• Both processes take place only in animals.: Plants do both. Photosynthesis does not happen in animals at all.

When oxygen is short, cells can still break down glucose without oxygen. This is called fermentation (anaerobic). In human muscle, it makes lactic acid and gives much less energy than aerobic respiration.

Wrong choices

• Photosynthesis: Photosynthesis happens in plants and uses light. It does not happen in muscle.
• Aerobic respiration: Aerobic respiration needs oxygen. The cells here are low on oxygen.
• Digestion: Digestion breaks food down in the gut. It is not how muscle cells get energy from glucose.

1. One glucose molecule makes 6 CO₂ molecules.
2. Three glucose molecules make 3 × 6 = 18 CO₂ molecules.

Check: each glucose has 6 carbon atoms, so 3 glucose have 18 carbon atoms, and each CO₂ has 1 carbon atom.

Wrong choices

• 6: 6 is the number of CO₂ molecules from just one glucose molecule.
• 3: 3 is the number of glucose molecules, not the number of CO₂ molecules.
• 9: 9 comes from adding 3 and 6 instead of multiplying.