What you'll learn
In plain words: plants catch sunlight and store it in sugar. Then plants and animals "burn" that sugar for energy.
Every living thing needs energy to grow, move, repair itself, and stay alive. This lesson follows that energy from sunlight to your muscles. You will learn:
- how photosynthesis captures light energy and stores it in glucose;
- how cellular respiration releases that stored energy as ATP, and how fermentation works without oxygen;
- why the two processes are nearly mirror images;
- what Calories and metabolism mean, and how energy balance affects body weight.
How it shows up on the GED: energy in living things is one of the two main focus areas of Life Science. Expect experiment passages: plants under lights, sealed jars, yeast making gas. Expect data tables about food energy, too. Questions ask you to:
- name what goes in (reactants) and what comes out (products);
- explain a result or choose a controlled variable;
- calculate Calories.
You do not need to memorize every chemical step. But you must know the two equations in words. You must know what goes in and what comes out.
Words to know
- photosynthesis — process in which plants, algae, and some bacteria use light energy to make glucose (sugar). They make it from carbon dioxide and water, and give off oxygen.
- chloroplast — the part of a plant cell where photosynthesis happens (leaf cells are full of them).
- chlorophyll — the green pigment in chloroplasts that absorbs light.
- glucose — a simple sugar, \text{C}_6\text{H}_{12}\text{O}_6, that stores chemical energy.
- cellular respiration — process in which cells break down glucose with oxygen to release usable energy.
- mitochondria — the cell parts where aerobic cellular respiration happens ("powerhouse of the cell").
- ATP — the molecule cells use as an energy "currency" to power their work.
- aerobic — uses oxygen (aerobic respiration).
- anaerobic — without oxygen (fermentation).
- fermentation — anaerobic breakdown of sugar; yeast make ethanol and CO₂; muscle cells make lactic acid.
- reactant — a substance that goes into a reaction (CO₂ in photosynthesis).
- product — a substance a reaction makes (O₂ in photosynthesis).
- enzyme — a protein that speeds up a reaction in cells; can be denatured by high heat.
- Calorie — a unit of food energy (one food Calorie equals one kilocalorie).
- metabolism — all the chemical reactions in the body; basal metabolic rate is the energy used at complete rest.
Photosynthesis: storing the Sun's energy
Think of a solar-powered kitchen. It takes in air and water, uses sunlight as power, and cooks up sugar. That is what a leaf does.
Plants, algae, and some bacteria are producers: they make their own food. In their chloroplasts, chlorophyll absorbs light energy and uses it to build glucose.
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}_2In words: carbon dioxide + water + light energy → glucose + oxygen.
- Carbon dioxide enters through tiny pores in leaves called stomata.
- Water comes up from the roots.
- Oxygen leaves through the stomata as a waste product, the oxygen we breathe.
- Glucose is used for energy, stored as starch, or built into cellulose for cell walls.
Energy transformation: light energy → chemical energy stored in glucose bonds.
Factors that limit the rate of photosynthesis:
| Factor | Effect |
|---|---|
| Light intensity | more light → faster, until another factor limits it |
| CO₂ concentration | more CO₂ → faster, up to a point |
| Temperature | faster as it warms to an optimum; too hot damages enzymes |
| Water | too little water → stomata close → less CO₂ enters |
Example: a lamp moved farther from a water plant gives less light, so fewer oxygen bubbles appear.
Cellular respiration: releasing the energy
Think of a car engine. It burns fuel with oxygen and gives off exhaust. Your cells do something similar, but slowly and without flames.
All living things, plants included, break down food to get usable energy. In most cells this happens in the mitochondria and needs oxygen:
\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)}In words: glucose + oxygen → carbon dioxide + water + energy.
- The energy is captured in ATP, which powers muscle contraction, nerve signals, building proteins, and active transport.
- Some energy is always lost as heat. That is why your body is warm and why you get hot when you exercise.
- You breathe in oxygen for this reaction and breathe out its carbon dioxide.
Fermentation (anaerobic): when oxygen is missing or scarce, some cells break down glucose without it. This releases much less energy per glucose molecule.
- Yeast: glucose → ethanol + carbon dioxide + a little energy. The CO₂ makes bread rise.
- Human muscle cells during hard exercise: glucose → lactic acid + a little energy.
Example: a balloon over a flask of yeast and sugar inflates because fermentation releases CO₂.
The two processes form a cycle: the products of one are the reactants of the other.
How the two processes connect
| Photosynthesis | Cellular respiration | |
|---|---|---|
| Who does it | plants, algae, some bacteria | almost all living things, including plants |
| Where in the cell | chloroplasts | mitochondria |
| Reactants | CO₂, water, light energy | glucose, oxygen |
| Products | glucose, oxygen | CO₂, water, ATP energy |
| Energy | stored | released |
| When | only in light | all the time |
The most tested idea: a plant in the light does both processes at once. Whether it adds or removes CO₂ from the air depends on which is faster. In bright light, photosynthesis usually wins and the plant removes CO₂. In the dark, only respiration happens, so the plant adds CO₂. An animal only respires, so it always adds CO₂.
Example: a sealed jar with a plant in the dark gains CO₂. The same jar in bright light loses CO₂.
Calories, metabolism, and energy balance
In plain words: Calories work like money. Food is income; activity is spending.
Food energy is counted in Calories. The three energy nutrients give:
- carbohydrate ≈ 4 Calories per gram
- protein ≈ 4 Calories per gram
- fat ≈ 9 Calories per gram
Your body spends energy in three ways:
- Basal metabolism: breathing, heartbeat, brain activity, and keeping warm at rest. For most adults this is the largest share, often more than half of daily use.
- Digestion: breaking down and absorbing food.
- Physical activity: the part you control most; more intense or longer activity uses more energy.
Metabolism varies with body size, age, sex, muscle mass, and activity. A larger body or one with more muscle generally uses more energy.
Energy balance: if intake > use, the extra is stored (as glycogen and mostly fat) and weight rises over time. If intake < use, stored energy is broken down and weight falls. If they are equal, weight stays about the same.
Example: someone eats 2,000 Calories a day but uses 2,300. That is a 300-Calorie daily shortfall (deficit). Over time they would lose weight.
Formulas you may use
\text{Calories} = 4 \times (\text{g carbohydrate}) + 4 \times (\text{g protein}) + 9 \times (\text{g fat}) \text{energy balance} = \text{Calories taken in} - \text{Calories used} \text{time needed} = \frac{\text{Calories to use}}{\text{Calories used per unit of time}}The Science test has no formula sheet. The test usually gives the Calories per gram or the activity rates in the passage, so read them from there.
Example 1: reactants and products
Passage: "A scientist gave a plant carbon dioxide containing a special tracer form of carbon. After several hours in light, the tracer carbon was found in the plant's sugars."
Question: Which process moved the carbon from the air into sugar?
Think-aloud:
- Carbon starts in CO₂ (a gas in the air) and ends in sugar (glucose).
- Which process takes in CO₂ and makes glucose? Photosynthesis.
- Cellular respiration does the opposite: it breaks glucose down into CO₂.
- The light condition also fits photosynthesis.
Answer: photosynthesis.
Example 2: identifying the controlled variable
Passage: "Students placed identical bean plants under red, blue, and green light of equal brightness for 2 weeks and measured growth."
Question: What is the independent variable, and name one controlled variable.
Think-aloud:
- What did the students change on purpose? The color of light. That is the independent variable.
- What was measured? Growth (dependent variable).
- What stayed the same? Brightness, plant type, and time (2 weeks) are controlled variables.
- On the GED, a wrong choice often lists the dependent variable as a controlled variable. Growth is not held constant; it is the result.
Answer: independent = light color; controlled = brightness (or plant type, or time).
Example 3: a Calorie calculation
Label: one cup of yogurt has 4 g fat, 20 g carbohydrate, 10 g protein.
Question: How many Calories are in one cup?
Think-aloud:
- Fat: 4 \times 9 = 36.
- Carbohydrate: 20 \times 4 = 80.
- Protein: 10 \times 4 = 40.
- Total: 36 + 80 + 40 = 156 Calories.
- Check: if every gram were 4 Calories, 34 \times 4 = 136; fat adds extra, so 156 is reasonable.
Answer: 156 Calories.
Example 4: interpreting a fermentation graph
Data: yeast in sugar water made 5 mL of CO₂ at 15 °C, 20 mL at 35 °C, and 2 mL at 55 °C.
Question: Which conclusion is best supported?
A. Yeast make the most CO₂ at the highest temperature. B. Yeast fermentation has an optimum temperature between the low and high values tested. C. Temperature does not affect yeast. D. Yeast will make 40 mL at 75 °C.
Think-aloud:
- The pattern rises then falls: 5 → 20 → 2.
- A is wrong: the highest temperature gave the least gas.
- C is wrong: the values change a lot.
- D extrapolates in the wrong direction; the trend is falling at high temperatures.
- B fits: the best performance is in the middle, around 35 °C, the enzyme optimum.
Answer: B.
Common traps
- "Plants photosynthesize; animals respire." Wrong: plants also carry out cellular respiration, all the time. Right: plants do both; animals only respire.
- Swapping reactants and products. Wrong: "Photosynthesis releases CO₂." Right: photosynthesis takes in CO₂ and releases O₂.
- "Respiration means breathing." Breathing moves air in and out of the lungs. Cellular respiration is the chemical reaction in cells that uses the oxygen.
- Fat = 4 Calories per gram. Wrong. Fat is about 9; carbohydrate and protein are about 4.
- Ignoring the control. A jar with no organisms tells you whether light or leaks change the gas level. Always check it before explaining a result.
- Extrapolating a curve. If the data stop at 50 cm or 60 °C, don't predict exact values beyond them.
GED tactics for energy in living things
- Write the two word equations on your erasable note board at the start: CO₂ + water + light → glucose + O₂, and the reverse.
- Ask "net or gross?" In sealed-container problems, the measured gas level shows the net result of everything happening.
- Find the control (no plant, no light, no yeast) and compare each result to it.
- Translate the setup: lamp distance = light intensity; bubbles = oxygen = photosynthesis rate; balloon size = CO₂ = fermentation rate.
- Check units in food data: grams vs. Calories, per hour vs. per minute.
- Estimate before calculating so you can eliminate answers that are far off.
Check yourself
Light and Photosynthesis in Elodea
During photosynthesis, plants use light energy to turn carbon dioxide and water into glucose (a sugar) and oxygen. Elodea is a plant that lives underwater. When it carries out photosynthesis, small bubbles of gas rise from the cut end of its stem.
A student placed a sprig of Elodea in a test tube of water with a pinch of baking soda, which adds dissolved carbon dioxide. She set a lamp at different distances from the tube. A clear container of water sat between the lamp and the tube to absorb heat, so the water in the tube stayed at 22 °C for every trial. At each distance she waited 5 minutes, then counted bubbles for 1 minute. She repeated each count three times and found the average.
The bubbles released by the Elodea are made mostly of which gas?
Hint
Photosynthesis takes in two things and gives off two things. Which product is a gas?
Step-by-step solution
Evidence: the passage says plants "use light energy to turn carbon dioxide and water into glucose (a sugar) and oxygen."
Step by step:
- Inputs (reactants): carbon dioxide and water. Outputs (products): glucose and oxygen.
- Bubbles are a gas released by the plant, so they must be a product.
- Glucose is a sugar that stays in the plant. The gas product is oxygen.
- 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.
Check yourself
Light and Photosynthesis in Elodea
During photosynthesis, plants use light energy to turn carbon dioxide and water into glucose (a sugar) and oxygen. Elodea is a plant that lives underwater. When it carries out photosynthesis, small bubbles of gas rise from the cut end of its stem.
A student placed a sprig of Elodea in a test tube of water with a pinch of baking soda, which adds dissolved carbon dioxide. She set a lamp at different distances from the tube. A clear container of water sat between the lamp and the tube to absorb heat, so the water in the tube stayed at 22 °C for every trial. At each distance she waited 5 minutes, then counted bubbles for 1 minute. She repeated each count three times and found the average.
Which conclusion is best supported by the graph?
Hint
Moving a lamp farther away makes the light on the plant dimmer. What happens to the bubble count as distance increases?
Step-by-step solution
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:
- Distance up → light on the plant dimmer.
- Bubbles (oxygen) are a measure of the photosynthesis rate.
- As distance increased, bubbles decreased at every step. So less light → slower photosynthesis.
- 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.
Summary
- Photosynthesis (chloroplasts): CO₂ + water + light energy → glucose + O₂; light energy becomes chemical energy.
- Cellular respiration (mitochondria): glucose + O₂ → CO₂ + water + ATP; happens in plants and animals all the time.
- Fermentation releases a little energy without oxygen: yeast make ethanol and CO₂; muscles make lactic acid.
- A plant in light does both processes; the faster one decides whether CO₂ goes up or down.
- Carbohydrate and protein ≈ 4 Calories per gram; fat ≈ 9. Intake above use is stored, mostly as fat.
- On the GED, identify variables and controls, read graphs carefully, and avoid extrapolating.