Unit 5 · Cells & Living Systems · Week 3
How does a living thing get and use energy?
Why do you get hungry and tired? Why does a plant left in a dark closet wither even though it was full of glucose? How do cells actually get the energy they need to do work?
Start here
How we work this lesson (our expectations)
- Wonder out loud — questions are the work
- Use evidence, not guesses
- Reasoning > being first or fastest
- Supports are for everyone — open any you want
30-second check-in self-management
Before we start, notice your energy level right now. On a scale of 1 (running on empty) to 5 (fully charged), where are you? Take one slow breath. If you're at a 1 or 2, that's okay — every step today is small on purpose, and you can listen to any part with the Listen button. Your cells are doing the work to re-fuel you even as you sit here.
Do-Now Quick recall
Pull from what you already know — no looking back. Retrieval makes ideas stick.
- W2 spiral Write the word equation for photosynthesis. Underline the part that provides most of the carbon for the plant's mass.
- W1 spiral The cell is the basic unit of life. Name two things a cell must do in order for the organism to stay alive.
- W1 spiral What is homeostasis? Give one example of how your body maintains it right now.
The phenomenon
You eat food, and within hours you are hungry again. A plant stores glucose in photosynthesis, but if you move it to a dark closet it wilts and eventually dies — even though it began with glucose stored in its cells. The glucose didn't disappear (matter is conserved) — but the usable energy did move somewhere. The answer is in the cell: every living cell, plant or animal, is constantly breaking down glucose in a process called cellular respiration, converting stored chemical energy into a form the cell can actually use to do work — to move, to grow, to maintain homeostasis. Understanding cellular respiration means understanding the inside of the cell.
You eat lunch, and two hours later you're hungry. A plant has glucose stored in its cells, but if it's moved to the dark it wilts and dies. Where does the energy go? Every living cell — yours, the plant's — breaks down glucose in a process called cellular respiration. It converts stored energy into usable energy for the cell. To understand this, we need to look inside the cell.
Notice & Wonder. Write one thing you notice and one thing you wonder. Keep your wonder — we are building toward it today.
Build it From cell structure to energy release, one step at a time
We will build the answer in steps. After each step there is a quick check before we move on.
Step 1 — Cell structure: organelles and their functions
A cell is not just a bag of liquid — it has specialized structures called organelles, each with a specific function. Structure determines function at every level of life. The cell membrane controls what enters and exits the cell, maintaining the internal environment. The nucleus contains DNA — the cell's instruction manual, directing all the cell's activities. The mitochondria are where cellular respiration happens: they convert glucose and oxygen into CO₂, water, and usable energy (ATP) — earning their nickname "the powerhouse of the cell." In plant cells, the chloroplast is where photosynthesis occurs, capturing sunlight to make glucose. The cell wall (plants only) is a rigid outer layer that provides structural support. The vacuole stores water and nutrients, helping the cell maintain turgor pressure.
A cell has parts called organelles, and each part has a job. The cell membrane is the gate — it controls what comes in and out. The nucleus holds DNA, the instruction manual. The mitochondria are where the cell breaks down glucose to get usable energy — the "powerhouse." In plant cells, the chloroplast makes glucose using sunlight. The cell wall (plants only) holds the plant upright. The vacuole stores water.
Step 2 — The cellular respiration equation
Every living cell performs cellular respiration continuously. The word equation is: glucose + oxygen → carbon dioxide + water + usable energy (ATP). This is the mirror-image of photosynthesis. In photosynthesis, the cell uses CO₂ and water plus light energy to build glucose — storing energy. In respiration, the cell breaks glucose apart — releasing that stored energy as ATP. The reactants of one reaction are the products of the other.
The equation for cellular respiration: glucose + oxygen → carbon dioxide + water + usable energy (ATP). Photosynthesis does the opposite — it builds glucose and stores energy. Respiration breaks glucose apart and releases that energy. They are mirrors of each other.
Worked example — tracing one glucose molecule (watch how an expert reasons)
Start: A plant in the sun makes glucose in the chloroplast during photosynthesis. The glucose is stored in the cell.
Move to mitochondria: The cell moves glucose to the mitochondria. There, glucose + oxygen → CO₂ + water + ATP.
ATP does work: The cell uses ATP energy to grow, divide, move materials across the membrane, and maintain homeostasis.
Matter is conserved: The CO₂ and water released in respiration contain the same number of atoms as the glucose + oxygen that went in — matter was rearranged, not destroyed.
Energy was transferred: The energy was never created or destroyed — it moved from stored chemical energy in glucose to usable chemical energy in ATP, then to the cell's work.
Step 3 — All living things do cellular respiration
One of the most important ideas in biology: all living organisms — plants, animals, fungi, bacteria — perform cellular respiration. Plants also photosynthesize, but they respire around the clock. The dark-closet plant is the clearest evidence: it has no sunlight, so it cannot photosynthesize new glucose. It runs on stored glucose, respiring continuously until the supply runs out. That is why it wilts and dies — not because it forgot to photosynthesize, but because every one of its cells kept doing the work of living.
All living things — plants, animals, fungi — do cellular respiration all the time. Plants also photosynthesize, but they never stop respiring. The closet plant dies because without sunlight it cannot make new glucose, but its cells keep using glucose through respiration until the supply is gone.
Show me the photosynthesis vs. respiration comparison table Tier 2 · organizer
Fill in the blank cells using this word bank: mitochondria · chloroplast · glucose · oxygen · CO₂ · water · ATP · stores energy · releases energy · all living things · plants only
| Feature | Photosynthesis | Cellular Respiration |
|---|---|---|
| Reactants (inputs) | CO₂ + H₂O + light | glucose + _______ |
| Products (outputs) | _______ + O₂ | CO₂ + _______ + ATP |
| Energy direction | _______ | _______ |
| Organelle | _______ | _______ |
| Who does it? | _______ | _______ |
Your turn Explain the phenomenon
Use a Claim–Evidence–Reasoning model to answer: Why does the plant in the dark closet wither, even though it started with glucose stored in its cells? Choose how you respond — all are equally valid.
The task: Make a claim about what is happening to the plant's glucose, give evidence from the lesson, and explain your reasoning connecting cell structure to energy transfer. Be sure to name the organelle responsible.
I want sentence starters Tier 2 · frames
Claim: The closet plant wilts because ________.
Evidence: I know this because all living cells, including plant cells, perform cellular respiration in the ________, which converts glucose + ________ into ________ + water + ________ (usable energy).
Reasoning: Without sunlight the plant cannot ________, so it cannot replace the glucose being used up by ________. When the glucose is gone, the cell has no source of ________ to do the work of staying alive.
Word bank: mitochondria · chloroplast · photosynthesis · cellular respiration · glucose · oxygen · CO₂ · ATP · usable energy · stored energy · organelle
Let's make it concrete first Tier 3 · re-teach
The battery analogy: Your phone needs a battery to do work. The charger (sunlight) fills the battery (glucose). When you use the phone (do life processes), the battery drains — that is respiration. A plant in the dark cannot recharge (no photosynthesis), so it runs on its battery until the glucose is gone. The battery is not creating electricity — it is releasing stored energy. Your cells do the same thing.
Fill in: The plant's cells keep ________ (the process) even in the dark, which uses up ________ (the molecule). Without sunlight, the plant cannot make new ________ (same molecule) through ________ (the other process). When the stored ________ runs out, the cells have no ________ to do work.
Now say your answer out loud before you write it. Two sentences is enough.
Push my thinking Extension
Why do athletes carb-load before a race? Research how the body stores glucose as glycogen and then converts it back to glucose for respiration. Explain the full chain: food → digestion → blood glucose → glycogen (liver/muscle) → glucose → cellular respiration → ATP → muscle contraction. Connect each step to the Unit 1 energy-transfer idea: energy is never created or destroyed, only transferred.
Check yourself Did it click?
- name the organelle where cellular respiration happens and explain its structural function;
- write the word equation for cellular respiration and explain how it mirrors photosynthesis;
- explain why all living cells — plant and animal — perform cellular respiration, using the closet-plant as evidence;
- connect cellular respiration to the Unit 1 idea that energy transfers, never appears from nothing.
Which statement correctly describes cellular respiration?
Re-teach: cellular respiration and energy conservation Tier 3
Misconception 1 — Only animals respire: The dark-closet plant is the direct evidence. It has no sunlight, so it cannot photosynthesize. Yet it keeps doing respiration — that is exactly why it dies when the glucose runs out. If it did not respire, the glucose would never be used and it would stay alive. Both plants and animals respire every second of every day.
Misconception 2 — Cells create energy: Return to Unit 1. Energy is never created or destroyed — only transferred. Cellular respiration does not make new energy. It converts stored chemical energy in glucose (which came from sunlight via photosynthesis) into ATP, the usable form of energy for the cell. The total amount of energy is the same before and after — just in a different form. Think of it as currency exchange, not printing money.
Complete this sentence: Cellular respiration happens in the ________, converts ________ + ________ into ________ + water + ________, and this process happens in ________ (all / only animal) living cells.
Reflect, then show what you know
Reflection metacognition + SEL
Which support did you use today (if any), and did it help? There is no wrong answer — noticing what helps you learn is a skill scientists use too. Also: the breath exercise just now was cellular respiration in action. Did that make the idea feel more real?
Exit ticket — answer on paper or in your notebook.
- New Write the word equation for cellular respiration. Then write one sentence explaining how it is the "mirror" of photosynthesis.
- New Name the organelle where cellular respiration happens and explain its structural function. Why do cells with high energy needs (like muscle cells) have more mitochondria than other cells?
- Spiral The glucose broken down in cellular respiration was made by photosynthesis. Trace one carbon atom from CO₂ in the air through photosynthesis and then through cellular respiration. What happens to it at the end?
- Spiral In Unit 1 you learned that energy is never created or destroyed — only transferred. How does cellular respiration follow this rule? Where was the energy before, and where does it go?
- Transfer Your muscle cells do a lot of respiration when you exercise. Your blood carries oxygen to those cells and carries CO₂ away. What do you think is happening at the body-system level to support all those cells doing respiration? Predict for next week.
Educator panel — facilitation, accommodations, answer key
At a glance
Standards: NGSS MS-LS1-2 (develop and use a model to describe the function of a cell as a whole and ways parts of cells contribute to the function) and MS-LS1-7 (develop a model to describe how food is rearranged through chemical reactions forming new molecules that support growth and/or release energy as this matter moves through an organism). 3D: SEP Developing & Using Models · DCI LS1.B & LS1.C · CCC Energy & Matter and Structure & Function. Throughline: connects the Unit 1 energy-transfer principle to living systems; bridges photosynthesis (W2) to body systems (W4).
Materials (fully digital-capable): student notebook; optional — live plant demo (two identical plants, one moved to a dark space two weeks prior). No lab required.
Facilitation arc (Rosenshine)
- Launch phenomenon — two-panel image, notice/wonder before vocabulary (3 min).
- Do-Now retrieval — photosynthesis equation from W2, cell basics from W1 (3 min).
- Build Steps 1–3 with worked example; stop and check after the comparison table (12 min).
- Differentiated CER — closet-plant explanation; circulate and name tier supports students reach for (15 min).
- Self-check + re-teach branch for A/B responders (5 min).
- Brain break — respiration as a somatic experience (2 min).
- Reflection + exit ticket (10 min).
IEP / 504 accommodation cards
Common accommodations mapped to this lesson
| Accommodation | How it is already built in / what to do |
|---|---|
| Text read aloud | Every passage has a Listen button (TTS); the simplified toggle shortens it. |
| Reduced reading load | "Reading: Simplified" swaps in shorter plain-language text site-wide. |
| Chunking / extended time | Lesson is in small steps with checks; progress saves automatically — students can stop and resume. |
| Sentence frames / word bank | Tier-2 panel under the practice task provides both; comparison table has a word bank. |
| Concrete / analogical support | Tier-3 battery analogy + cloze sentence; brain-break breathing exercise grounds respiration somatically. |
| Alternative response mode | Students may draw the energy-flow model, speak/record, or write the CER — all accepted. |
| Behavior / focus (504, ADHD) | Named expectations (PBIS), 30-sec check-in, and a breathing brain break that doubles as a focus reset. |
| Multilingual / EL | Simplified text, comparison table as visual organizer, word bank, home-language reflection allowed. |
Exit ticket answer key
- Glucose + oxygen → carbon dioxide + water + usable energy (ATP). It is the mirror of photosynthesis because the reactants of one (glucose, O₂) are the products of the other, and vice versa; photosynthesis stores energy, respiration releases it.
- Mitochondria; they convert glucose + oxygen into CO₂ + water + ATP, releasing stored chemical energy as usable energy. Muscle cells have more mitochondria because they need more ATP to power contractions — structure matches function demand.
- Carbon (as CO₂) is absorbed by the leaf in photosynthesis → fixed into glucose (C₆H₁₂O₆) → glucose moved to mitochondria → during respiration the C–C bonds break → carbon exits as CO₂ exhaled into the air. The atom returns to where it started.
- Energy conservation: before respiration, energy was stored as chemical bonds in glucose (which captured it from sunlight via photosynthesis). During respiration, those bonds break and the energy transfers to ATP; the cell then uses ATP energy to do work (move, grow, maintain homeostasis). Energy moved — it was never created or destroyed.
- Accept predictions that anticipate: the circulatory system delivers oxygen and glucose to muscle cells; the respiratory system brings in oxygen and removes CO₂; these systems work together to support all cells doing respiration. (Previews W4 levels of organization.)
Why it is designed this way
Phenomenon-first anchors the abstract biochemistry in a visible puzzle (NRC 2012). The two-misconception self-check (plants don't respire / energy is created) targets the two most common errors in this standard (AAAS Project 2061). The battery analogy in Tier-3 is a tested pedagogical bridge for students who lack the prior knowledge to decode the equation abstractly. The breathing brain break is a somatic anchor: students feel oxygen moving into blood and CO₂ leaving, making the unit-level process tangible before the body-systems lesson in W4. The exit ticket item 5 is a deliberate forward-transfer prompt — it activates prior knowledge schema in anticipation of W4's levels-of-organization build, reducing cognitive load next lesson (Ausubel, advance organizer).