Science Grade 7 Nutrition in Plants and Animals

Grade 7 Science Study Guide: Nutrition in Plants and Animals

1. The Driving Question

"If plants don’t eat food like we do, how do they get the energy to grow—and why can’t animals just do the same thing? What’s actually happening inside leaves and stomachs that turns sunlight or a burger into fuel for life?"

By the end of this guide, you’ll be able to trace the path of a single carbon atom from the air into a leaf, then into a cow’s muscle, and finally into your own cells—and explain why that journey is totally different for plants and animals.

2. The Core Idea — Built, Not Listed

Imagine a sunflower field in August. The plants stand tall, their leaves spread wide like solar panels, but they’re not plugged into anything. Instead, they’re pulling carbon dioxide (CO₂) from the air and water (H₂O) from the soil, then using sunlight to stitch those molecules together into sugar—a kind of edible battery. This process, photosynthesis, is how plants make their own food. Animals, on the other hand, are like a delivery truck that only unloads: we can’t build our own fuel, so we have to steal it from plants (or from animals that ate plants). Your stomach and intestines are basically a high-speed disassembly line, breaking down food into tiny pieces (like glucose) that your cells can burn for energy.

But here’s the twist: plants don’t just make food—they also eat it. At night, when there’s no sunlight, plants switch to cellular respiration, breaking down their own sugars to release energy, just like animals do. The difference? Plants start the food chain by making their own fuel, while animals depend on plants (or other animals) to survive.

Key Vocabulary:
- Photosynthesis
Definition: The process where plants (and some bacteria) use sunlight to convert CO₂ and water into glucose (sugar) and oxygen.
Example: A maple tree in Vermont uses photosynthesis to store energy in its sap, which humans later tap to make syrup.
Note: In college biology, you’ll learn about the light-dependent and light-independent reactions—the two stages of photosynthesis that happen in different parts of the chloroplast.

• Cellular Respiration
  Definition: The process where cells break down glucose to release energy (ATP), using oxygen and producing CO₂ and water as waste.
  Example: A cheetah sprinting after a gazelle relies on cellular respiration to power its muscles—its cells are burning glucose at top speed.
  Note: In advanced courses, you’ll study aerobic vs. anaerobic respiration and how mitochondria structure relates to efficiency.

• Autotroph
  Definition: An organism that makes its own food (usually through photosynthesis).
  Example: Kelp forests off the California coast are autotrophs—they produce food for entire ocean ecosystems.
  Note: Some autotrophs (like deep-sea bacteria) use chemosynthesis instead of sunlight, a concept you’ll explore in marine biology.

• Heterotroph
  Definition: An organism that must consume other organisms for energy.
  Example: A fungus growing on a fallen log is a heterotroph—it digests the wood externally before absorbing the nutrients.
  Note: In ecology, you’ll learn how heterotrophs are classified by their diet (herbivores, carnivores, omnivores, decomposers).

3. Assessment Translation

How This Appears on State Tests (Grade 7):
- Multiple Choice: Questions often ask you to identify inputs/outputs of photosynthesis or cellular respiration, or to compare autotrophs and heterotrophs.
Example: "Which of the following is a product of photosynthesis? A) Carbon dioxide B) Oxygen C) Glucose (correct) D) ATP" Distractor Pattern:* The wrong answers often mix up inputs/outputs of photosynthesis and respiration (e.g., CO₂ is an input for photosynthesis, not a product).

• Short Answer: You might be asked to explain the relationship between photosynthesis and cellular respiration, or to label a diagram of a plant/animal cell with organelles involved in these processes.
  Example: "Explain how the products of photosynthesis are used in cellular respiration. Include the role of mitochondria and chloroplasts in your answer."

• Evidence-Based Writing: Some states (like California) include a claim-evidence-reasoning (CER) prompt.
  Example: "A student claims that plants do not need oxygen. Use evidence from the processes of photosynthesis and cellular respiration to support or refute this claim."

Proficient vs. Developing Responses:
| Proficient | Developing | |----------------|----------------| | "Plants use photosynthesis to make glucose and oxygen. At night, they use cellular respiration to break down glucose for energy, which requires oxygen. So plants do need oxygen—they just make their own during the day." | "Plants make oxygen and don’t need it." (Missing the role of cellular respiration.) | | Labels both chloroplasts and mitochondria in a plant cell diagram. | Only labels chloroplasts. | | Explains that animals depend on plants for food and oxygen. | Says animals only depend on plants for food. |

Model Proficient Response (Short Answer):
"Photosynthesis and cellular respiration are opposite processes. During photosynthesis, chloroplasts in plant cells use sunlight to convert CO₂ and water into glucose and oxygen. The glucose is stored as food, and the oxygen is released. During cellular respiration, mitochondria in both plant and animal cells break down glucose using oxygen to release energy (ATP), CO₂, and water. This means plants produce the oxygen and food that animals need, while animals produce the CO₂ that plants need for photosynthesis."

4. Mistake Taxonomy

Mistake 1: Mixing Up Inputs and Outputs
- Question: "What are the products of photosynthesis?" - Common Wrong Answer: "Sunlight, water, and carbon dioxide." - Why It Loses Credit: The student confused inputs (what goes in) with outputs (what comes out). Sunlight, water, and CO₂ are used in photosynthesis, not produced.
- Correct Approach: 1. Remember the equation: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂.
2. Products are on the right side of the arrow.
3. Glucose (C₆H₁₂O₆) and oxygen (O₂) are the outputs.

Mistake 2: Forgetting Plants Do Cellular Respiration
- Question: "Do plants need oxygen? Explain." - Common Wrong Answer: "No, because they make oxygen during photosynthesis." - Why It Loses Credit: The student didn’t connect that plants also do cellular respiration (especially at night) and need oxygen for that process.
- Correct Approach: 1. Plants make oxygen and use it.
2. Photosynthesis happens in chloroplasts; respiration happens in mitochondria.
3. At night, plants rely on oxygen from the air (or their own stored oxygen) to break down glucose.

Mistake 3: Mislabeling Organelles in a Diagram
- Question: "Label the organelles involved in photosynthesis and cellular respiration in a plant cell." - Common Wrong Answer: Only labeling chloroplasts (for photosynthesis) and forgetting mitochondria (for respiration).
- Why It Loses Credit: The student didn’t recognize that plant cells have both organelles and that respiration happens in all cells, not just animal cells.
- Correct Approach: 1. Chloroplasts = photosynthesis (only in plant cells).
2. Mitochondria = cellular respiration (in all eukaryotic cells, including plants).
3. Label both in a plant cell diagram.

5. Connection Layer

1. Within Science: [Nutrition in plants/animals] → [Food webs and energy pyramids]
   Why it matters: Understanding how autotrophs and heterotrophs get energy explains why food chains always start with plants (or algae). Without autotrophs, there’d be no energy for herbivores, and thus no energy for carnivores.

2. Across Subjects: [Cellular respiration] → [Chemical reactions in chemistry]
   Why it matters: The equation for cellular respiration (C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy) is a combustion reaction—just like burning wood or gasoline. The difference? Your cells release energy slowly (in ATP) instead of all at once (as heat/light).

3. Outside School: [Photosynthesis] → [Why "eating local" reduces your carbon footprint]
   Why it matters: When you eat a tomato grown in your state instead of one shipped from Mexico, you’re reducing the CO₂ released from transportation. But here’s the twist: plants absorb CO₂ as they grow, so a local farm’s crops might actually offset some of the emissions from your food’s journey. (This is why some farms call themselves "carbon sinks.")

6. The Stretch Question

"If a plant is sealed in a glass jar with sunlight, will it grow forever? Why or why not?"

Pointer Toward the Answer:
- At first, the plant will grow because it has sunlight, CO₂, and water. But over time, the CO₂ levels will drop (used up in photosynthesis), and oxygen levels will rise (from photosynthesis). Eventually, the plant will run out of CO₂ to make more glucose, and cellular respiration will start to dominate—breaking down the plant’s own sugars and releasing CO₂. Without a way to replenish CO₂, the plant will slowly die. This is why closed ecosystems (like terrariums) need a balance of plants, animals, and decomposers to recycle nutrients.

Bonus twist: In space, NASA experiments with closed-loop life support (like the Biosphere 2 project) to see if plants can sustain humans long-term. The answer? Not yet—but understanding plant nutrition is key to making it work.