Life functions and energy intake

Evidence: the passage says plants "use light energy to turn carbon dioxide and water into glucose (a sugar) and oxygen."

Step by step:

1. Inputs (reactants): carbon dioxide and water. Outputs (products): glucose and oxygen.
2. Bubbles are a gas released by the plant, so they must be a product.
3. Glucose is a sugar that stays in the plant. The gas product is oxygen.
4. Carbon dioxide is taken in, not released, during photosynthesis. That is why the student added baking soda to supply it.

The answer is C. Oxygen.

Tip: many photosynthesis questions hinge on keeping reactants and products straight. Plants take in CO₂ and give off O₂ when they photosynthesize.

Evidence: the graph shows 42 bubbles per minute at 10 cm, falling to 30, 18, 10, and 6 at 20, 30, 40, and 50 cm.

Step by step:

1. Distance up → light on the plant dimmer.
2. Bubbles (oxygen) are a measure of the photosynthesis rate.
3. As distance increased, bubbles decreased at every step. So less light → slower photosynthesis.
4. Eliminate A (it reverses the trend), C (the count clearly depends on light), and B (the graph stops at 50 cm, and the count is still 6, not 0; predicting an exact stopping point goes beyond the data).

The answer is D.

Tip: translate the variable on the axis into what it means for the organism (distance → light intensity) before you judge a conclusion.

Evidence: the passage says the container was there "to absorb heat, so the water in the tube stayed at 22 °C for every trial."

Step by step:

1. The independent variable is light (lamp distance). The dependent variable is bubbles per minute.
2. A close lamp would also warm the water, and temperature affects photosynthesis too. Then you could not tell whether light or heat caused the change.
3. The water container keeps temperature the same, making it a controlled variable, so only light changes.
4. Eliminate A (the plant is already underwater), B (the container does not add light; it only blocks heat), and D (baking soda supplies CO₂).

The answer is C.

Tip: a good experiment changes only one thing at a time. When a setup step seems odd, ask which other variable it is keeping constant.

Evidence: the passage says BTB is blue with little CO₂, and "As more CO₂ dissolves, the water becomes more acidic, and the dye turns green and then yellow." Tube 1 (snail, light) went from green to yellow.

Step by step:

1. Yellow means the CO₂ in the water increased.
2. A snail is an animal. Animals carry out cellular respiration, which uses oxygen and glucose and releases carbon dioxide and water.
3. Snails do not photosynthesize, so nothing in Tube 1 removed the CO₂, even in the light.
4. Eliminate A (oxygen is taken in during respiration, not released), C (nitrogen is not part of respiration), and D (glucose is a sugar the snail uses up, not a gas it releases).

The answer is B. Carbon dioxide.

Tip: link the indicator color to a gas first (yellow = more CO₂), then ask which process in that tube adds that gas.

Evidence: paragraph 1: a cell "breaks one molecule of glucose all the way down to carbon dioxide and water. This releases enough energy to make about 36 molecules of ATP."

Step by step:

1. Aerobic respiration uses glucose and oxygen (reactants).
2. It produces carbon dioxide and water, and the released energy is stored as ATP (products).
3. Eliminate B (lactic acid comes from fermentation, which happens when oxygen runs short), C (oxygen is used up, not produced; glucose is the fuel), and D (that is photosynthesis, which muscle cells do not do).

The answer is A.

Tip: keep three processes straight. Aerobic respiration → CO₂ + water + lots of ATP; lactic acid fermentation → lactic acid + 2 ATP; photosynthesis → glucose + oxygen.

Evidence: the 25 °C line reads 0, 12, 20, 25, 26, 26 mg per hour at 0, 5, 10, 15, 20, and 25 mg/L of CO₂.

Step by step:

1. From 0 to 15 mg/L, oxygen output climbs quickly: 0 → 12 → 20 → 25.
2. From 15 to 25 mg/L, it barely changes: 25 → 26 → 26. The line levels off.
3. So adding CO₂ sped up photosynthesis at first, but above about 15 to 20 mg/L, more CO₂ made little difference. Something else, such as light or temperature, became the limiting factor.
4. Eliminate B (the rise did not continue at a steady rate), C (output went up, not down), and D (the line changed a lot at low CO₂).

The answer is A.

Tip: a curve that rises and then flattens means the factor on the x-axis stopped being the one that limits the rate.

Evidence: Jar 2 (plant, dark) rose from 400 ppm to 520 ppm. The passage says "Plants carry out cellular respiration all the time, day and night. Photosynthesis happens only when there is light."

Step by step:

1. In the dark, there is no photosynthesis, so the plant removes no CO₂.
2. Cellular respiration continues and releases CO₂ (see the respiration equation).
3. So CO₂ builds up in the sealed jar.
4. Eliminate A: plants respire all the time, not only in the dark. Eliminate C: photosynthesis uses CO₂ and cannot happen in the dark. Eliminate D: the jars were sealed, and Jar 3 (air only) stayed at 400 ppm, so no CO₂ leaked in.

The answer is B.

Tip: use the control jar. Jar 3 shows what happens with no organisms, which rules out leaks or measurement drift as explanations.

Evidence: Jar 1 (plant, bright light) fell from 400 to 250 ppm. The passage says plants respire "all the time" and photosynthesize "only when there is light." Jar 3 (air only, bright light) stayed at 400 ppm.

Step by step:

1. In the light, the plant does both processes: photosynthesis removes CO₂; respiration adds CO₂.
2. The net CO₂ level dropped by 400 - 250 = 150 ppm, so CO₂ was removed faster than it was added.
3. Therefore photosynthesis was faster than respiration.
4. Eliminate B (plants respire in light, too), C (Jar 3 had the same light and no change, so light alone does nothing), and D (the snail is in Jar 4, not Jar 1).

The answer is A.

Tip: a sealed-jar result shows the net effect of all processes. If the level falls, the process that removes the gas is winning, not the only one happening.

Evidence: the passage says: "When a person takes in more Calories than the body uses, the extra energy is stored, mostly as fat."

Step by step:

1. Energy balance = intake − use = 2{,}500 - 2{,}200 = +300 Calories per day.
2. A positive balance means a surplus. The passage says surplus energy is stored, mostly as fat.
3. Over months, the stored fat adds up, so Jamal will most likely gain weight.
4. Eliminate A (that would need intake below use), C (the body does not simply burn any amount of extra energy; the passage says extra is stored), and B (basal metabolism continues; it never stops while a person is alive).

The answer is D.

Tip: energy balance questions are subtraction plus one rule: surplus → stored (weight gain); deficit → stored energy used (weight loss).

Evidence: Tube 6 had "nothing added," was kept in the light, and stayed green, the starting color. All tubes began with "the same green BTB solution."

Step by step:

1. Tube 6 has everything the light tubes have (same solution, sealed, same light and temperature) except organisms. It is the control.
2. It stayed green, which shows that light and time alone do not change the color.
3. So the color changes in Tubes 1, 2, 4, and 5 must be caused by the organisms in them.
4. Eliminate B (the dye is in every tube; Tube 6 is not testing a new dye), C (light was not given only to Tube 6), and D (no organism was in Tube 6 to make CO₂).

The answer is A.

Tip: an "empty" or "nothing added" setup is almost always the control. Its job is to rule out the setup itself as the cause of a change.

Evidence: in the light, Tube 1 (snail) turned yellow (CO₂ added), and Tube 2 (plant) turned blue (CO₂ removed). Tube 3 (both) stayed green, the starting color. The passage says "When CO₂ is removed from the water, the color shifts back toward blue."

Step by step:

1. The snail carries out cellular respiration and adds CO₂.
2. In light, the plant carries out photosynthesis, which uses CO₂, faster than its own respiration adds CO₂, so overall it removes CO₂.
3. Together, the CO₂ added by the snail and the CO₂ removed by the plant roughly balanced, so the CO₂ level, and the color, stayed about the same.
4. Eliminate B (Tube 5 shows plants do respire; and the snail respires in any light), C (Tubes 1 and 2 show that each organism changes the CO₂ level on its own; in Tube 3 their effects balanced), and D (snails do not photosynthesize).

The answer is A.

Tip: when two organisms share a closed space, add up their effects. One that adds a gas and one that removes it can cancel out.

Evidence: lactate was 1.0, 1.3, and 2.0 mmol/L at 8, 10, and 12 km/h, then jumped to 4.1 and 8.0 at 14 and 16 km/h. The passage says "During very hard exercise, muscles may need oxygen faster than the blood can deliver it. Muscle cells then rely more on lactic acid fermentation."

Step by step:

1. Lactate in the blood comes from lactic acid made by fermentation.
2. At high speeds, the muscles needed oxygen faster than the blood could supply it, so they used more fermentation, which makes ATP without oxygen.
3. More fermentation → more lactic acid → the sharp rise in blood lactate.
4. Eliminate B (heart rate kept rising, so the heart did not stop pumping), C (fermentation does not use oxygen), and D (aerobic respiration makes carbon dioxide and water, not lactic acid).

The answer is A.

Tip: connect a product in the data (lactate) to the process that makes it (fermentation), then ask what condition turns that process up.

Evidence: the passage says "At every energy transfer, some energy is changed to heat. This is one reason the human body stays warm and why working muscles warm up even more."

Step by step:

1. Working muscles use a lot of ATP, so they carry out many energy transfers: glucose → ATP → movement.
2. At each transfer, some energy becomes heat. More work means more transfers and more heat.
3. That heat warms the muscles.
4. Eliminate B (true, but it says nothing about heat), C (chloroplasts are in plants and algae, not human muscles), and D (energy is not created; the passage says it cannot be).

The answer is A.

Tip: when a question asks which statement "best explains" something, pick the one that names the cause of that exact effect, not just any true statement.

Evidence: photosynthesis: 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2. Cellular respiration: \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{energy (ATP)}.

Step by step:

1. Products of photosynthesis: glucose and oxygen.
2. Reactants of cellular respiration: glucose and oxygen, the same substances.
3. Products of respiration (CO₂ and water) are the reactants of photosynthesis. The two processes are nearly reverses of each other.
4. Eliminate B (photosynthesis needs light energy; respiration releases energy as ATP), C (the passage says animals have mitochondria and carry out respiration), and D (they have opposite reactants and products, not the same ones).

The answer is A.

Tip: remember the cycle: what photosynthesis makes (glucose + O₂), respiration uses; what respiration makes (CO₂ + H₂O), photosynthesis uses.

Evidence: the passage says "Each step is controlled by enzymes" and "Enzymes can be permanently damaged, or denatured, by high heat." Bottle 3 collected 75 mL; the boiled-yeast bottle collected no gas.

Step by step:

1. Both bottles had 10 g of sugar, 7 g of yeast, and water at 35 °C. The only difference was boiling.
2. Boiling denatured the yeast's enzymes, and the damage is permanent, so cooling back to 35 °C did not fix them. Boiling also killed the yeast.
3. Without working enzymes, fermentation could not happen, so no CO₂ was made.
4. Eliminate B (sugar is not alive; the sugar was not boiled), C (the bottle was cooled to 35 °C, the same as the others), and D (the passage says yeast ferment without oxygen, so oxygen was not needed).

The answer is A.

Tip: "permanently" is the key word for denaturing. Cooling slows enzymes temporarily; high heat ruins them for good.

Evidence: CO₂ produced fell from 26 mL at 40 °C to 12 mL at 50 °C and 1 mL at 60 °C. The passage says enzymes "can be permanently damaged, or denatured, by high heat."

Step by step:

1. From 10 °C to 40 °C, CO₂ rose (2 → 26 mL): warmer temperatures speed up enzyme-controlled reactions.
2. Above 40 °C, production dropped sharply. The passage gives the reason: high heat denatures enzymes, so fermentation slows or stops.
3. Eliminate B: every flask had equal sugar, and higher temperatures produced less gas, not more, so sugar was not used up faster. Eliminate C: yeast at 10 °C made only 2 mL, so cold is not ideal. Eliminate D: warm water holds less dissolved gas, not more, and the passage points to enzymes.

The answer is A.

Tip: an up-then-down curve with temperature is the classic enzyme pattern: an optimum temperature, then a fast drop as heat damages the enzymes.

Evidence: the 15 °C line stays at 15 mg per hour from 20 to 25 mg/L, while the 25 °C line reaches 26 mg per hour under the same light and CO₂. The passage says enzymes "work more slowly when they are cool" and defines a limiting factor as one that holds back the rate.

Step by step:

1. Adding CO₂ from 20 to 25 mg/L did not raise the 15 °C rate, so CO₂ was no longer the limiting factor there.
2. The 25 °C algae got the same light and the same CO₂ but released much more oxygen. The main difference was temperature.
3. So at 15 °C, the cool temperature (slower enzymes) most likely limited the rate.
4. Eliminate B (more CO₂ did not help, so CO₂ was not limiting), C (the 15 °C algae released 15 mg per hour, so they were photosynthesizing), and D (both sets of flasks had the same bright light).

The answer is A.

Tip: to find a limiting factor, look for the condition that, when raised, would raise the rate. If adding more of something does nothing, it is not the limit.

Evidence: trail mix has 11 g fat, 15 g carbohydrate, 5 g protein. The passage gives 9 Calories per gram of fat and 4 per gram of carbohydrate or protein.

Step by step:

1. Fat: 11 \times 9 = 99 Calories.
2. Carbohydrate: 15 \times 4 = 60 Calories. Protein: 5 \times 4 = 20 Calories.
3. Total: 99 + 60 + 20 = 179 Calories.
4. Percent from fat: \frac{99}{179} \times 100 \approx 55\%
5. Trap: dividing grams instead of Calories, \dfrac{11}{31} \approx 35\%, ignores that fat has more Calories per gram.

The answer is C. About 55%.

Tip: "percent of Calories" means you must convert every nutrient to Calories before dividing.

Evidence: gas collected: 5, 40, 75, 110, 112 mL for 0, 5, 10, 20, and 40 g of sugar. Every bottle had "7 g of yeast" in water at 35 °C.

Step by step:

1. From 0 to 20 g of sugar, gas rose from 5 to 110 mL, so sugar was limiting the rate.
2. Doubling sugar from 20 to 40 g added only 2 mL. Sugar was no longer the limit.
3. The amount of yeast (and so the amount of enzymes) was fixed at 7 g. Once the yeast were working as fast as they could, extra sugar could not speed them up.
4. Eliminate B (40 g still produced the most gas, 112 mL, so it did not stop fermentation), C (all bottles were at 35 °C), and D (5 mL in Bottle 1 is small, and nothing suggests the yeast died in Bottle 5).

The answer is A.

Tip: when a rate stops rising even though you add more of one ingredient, look for another ingredient that was held constant. It has become the limiting factor.

Evidence: the glass beads used 0.0 mL of oxygen at both temperatures, while germinating peas used 0.4 and 1.0 mL. The passage says she used "equal volumes of three materials."

Step by step:

1. Glass beads take up the same space as the peas but cannot respire. That makes the beads tube a control.
2. If changes in temperature or pressure alone moved the drop, it would show up in the beads tube too.
3. The beads tube showed no change (0.0 mL), so the drop movement in the pea tubes can be credited to the peas using oxygen.
4. Eliminate B (beads do not supply oxygen), C (beads cannot respire, so they are not a living thing being compared), and D (beads do not absorb CO₂; the chemical in each tube does).

The answer is A.

Tip: a non-living object of the same size is a classic control in respiration experiments. It rules out the equipment or the conditions as the cause.

Evidence: germinating peas used 0.4 mL of oxygen at 10 °C and 1.0 mL at 25 °C. The passage says germinating seeds "carry out cellular respiration quickly to power their growth."

Step by step:

1. Cellular respiration is a series of chemical reactions, each sped up by enzymes.
2. Within a moderate range, enzymes work faster at warmer temperatures. 25 °C is warm but not hot enough to denature them.
3. Faster enzymes → faster respiration → more oxygen used: 1.0 \div 0.4 = 2.5 times as much at 25 °C.
4. Eliminate B (the cooler tube used less oxygen; temperature did not damage the enzymes), C (peas are seeds; they do not photosynthesize in a sealed tube, and no light was mentioned), and D (germinating peas used 0.4 mL of oxygen at 10 °C, so they respired below 20 °C too).

The answer is A.

Tip: for living things, warmer usually means faster up to a point, because enzymes speed up. Very high heat is different: it denatures enzymes.

Evidence: the passage defines denatured enzymes as "permanently damaged." The graph shows the best production at 40 °C (26 mL) and almost none at 60 °C (1 mL).

Step by step:

1. The claim is about permanence. To test it, you must find out whether yeast heated to 60 °C can recover.
2. Test C: heat yeast to 60 °C, then cool it to 40 °C (the best temperature) and measure CO₂. If production stays near zero, the damage is permanent; if it returns to about 26 mL, the yeast were only slowed.
3. B changes sugar, which does not address whether heat damage lasts. A extends the temperature range but still cannot tell slowed from permanently damaged. D checks for gas from sugar alone; useful as a control, but it says nothing about heated yeast recovering.

The answer is C.

Tip: to test a claim, design a test whose possible outcomes would clearly support or refute that specific claim. Ask: "What result would I expect if the claim were true? If it were false?"

Evidence: the passage says "chloroplasts capture light energy," "Only plants and algae have chloroplasts, but plants, algae, and animals all have mitochondria," and "In a plant, glucose made in the leaves is carried to other parts, such as the roots."

Step by step:

1. Root cells get no light, so they cannot carry out photosynthesis.
2. But root cells still need ATP to grow and build molecules. Plants have mitochondria, and mitochondria do not need light.
3. Glucose made in the leaves is carried to the roots, where mitochondria use it in cellular respiration to make ATP.
4. Eliminate B (root cells are alive and need ATP), C (there is no light underground, and cells get ATP from cellular respiration in mitochondria, not directly from light), and D (the passage says plants have mitochondria).

The answer is A.

Tip: every living cell, plant or animal, needs ATP from cellular respiration. Photosynthesis only makes the glucose; respiration turns it into usable energy.