AP Biology Unit 3 covers the essential energy processes that... Show more
AP Biology82 views·Updated May 21, 2026·3 pagesAP Biology Unit 3: Cellular Energetics Review
Mariela Hilario@marielahilario_pkkw
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Photosynthesis: Capturing Energy from Light
Organisms are divided into autotrophs (producers that make their own food) and heterotrophs (consumers that get energy by eating other organisms). Photosynthesis is an endergonic process requiring energy input, transforming light energy into chemical energy stored in glucose.
Photosynthesis takes place in chloroplasts containing the green pigment chlorophyll, which absorbs light energy. These chloroplasts are found in the mesophyll tissue of leaves, with stomata (pores) allowing CO₂ to enter and O₂ to exit during the process.
The process has two main stages. Light reactions occur in the thylakoid membrane where light energy is captured by photosystems I and II. Water is split, releasing oxygen, and energy is transferred through electron transport chains to produce ATP and NADPH. The Calvin cycle uses these energy carriers in the stroma to incorporate CO₂ into sugar molecules called G3P, requiring 9 ATP and 6 NADPH for each complete cycle.
Try This! Draw a simple diagram connecting the light reactions to the Calvin cycle, showing how ATP and NADPH move between them. This visual connection will help you understand how these processes work together.
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Cellular Respiration: Releasing Energy from Food
Cellular respiration is the exergonic process that breaks down glucose to release stored energy for cellular work. The equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP shows how oxygen helps extract maximum energy from sugar.
The process begins with glycolysis in the cell's cytosol, which splits glucose into two pyruvate molecules while generating 2 ATP and 2 NADH. The pyruvate molecules then enter mitochondria where they're converted to acetyl-CoA through a process that releases CO₂ and creates 2 more NADH.
Next, the citric acid cycle (Krebs cycle) in the mitochondrial matrix completely breaks down the carbons from acetyl-CoA into CO₂, producing more electron carriers (NADH and FADH₂). Finally, oxidative phosphorylation uses these electron carriers in the inner mitochondrial membrane. The electron transport chain pumps H⁺ ions across the membrane, creating a gradient that drives ATP production through ATP synthase, generating 30-36 ATP molecules.
Remember this! Aerobic respiration (with oxygen) produces up to 38 ATP total, while anaerobic respiration (without oxygen) produces only 2 ATP through fermentation processes like those that create alcohol or lactic acid.
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Enzymes and Energy Concepts
All metabolic reactions can be categorized as either anabolic (building larger molecules from smaller ones using energy) or catabolic (breaking down larger molecules to release energy). Similarly, reactions are either exergonic (releasing energy) or endergonic (requiring energy input).
Enzymes are specialized proteins that dramatically speed up biochemical reactions by lowering the activation energy needed to start them. Each enzyme has a specific active site where the substrate fits like a lock and key. Enzymes can be affected by temperature, pH, and chemicals that alter their shape, potentially causing denaturation.
Enzyme activity can be regulated through allosteric regulation when molecules bind to locations other than the active site. Activators like cofactors (minerals) and coenzymes (vitamins) increase enzyme activity, while inhibitors decrease it. Feedback inhibition occurs when the product of a reaction stops the enzyme from making more product.
Make connections! Plants have evolved different photosynthetic adaptations: C3 plants perform normal photosynthesis, C4 plants separate processes spatially within the plant, and CAM plants separate processes temporally (at different times) to conserve resources in harsh environments.
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AP Biology82 views·Updated May 21, 2026·3 pagesAP Biology Unit 3: Cellular Energetics Review
Mariela Hilario@marielahilario_pkkw
AP Biology Unit 3 covers the essential energy processes that power life - photosynthesis and cellular respiration. These biochemical pathways explain how organisms capture, store, and use energy through complex molecular transformations that happen inside cells.
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Photosynthesis: Capturing Energy from Light
Organisms are divided into autotrophs (producers that make their own food) and heterotrophs (consumers that get energy by eating other organisms). Photosynthesis is an endergonic process requiring energy input, transforming light energy into chemical energy stored in glucose.
Photosynthesis takes place in chloroplasts containing the green pigment chlorophyll, which absorbs light energy. These chloroplasts are found in the mesophyll tissue of leaves, with stomata (pores) allowing CO₂ to enter and O₂ to exit during the process.
The process has two main stages. Light reactions occur in the thylakoid membrane where light energy is captured by photosystems I and II. Water is split, releasing oxygen, and energy is transferred through electron transport chains to produce ATP and NADPH. The Calvin cycle uses these energy carriers in the stroma to incorporate CO₂ into sugar molecules called G3P, requiring 9 ATP and 6 NADPH for each complete cycle.
Try This! Draw a simple diagram connecting the light reactions to the Calvin cycle, showing how ATP and NADPH move between them. This visual connection will help you understand how these processes work together.
2
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Cellular Respiration: Releasing Energy from Food
Cellular respiration is the exergonic process that breaks down glucose to release stored energy for cellular work. The equation C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP shows how oxygen helps extract maximum energy from sugar.
The process begins with glycolysis in the cell's cytosol, which splits glucose into two pyruvate molecules while generating 2 ATP and 2 NADH. The pyruvate molecules then enter mitochondria where they're converted to acetyl-CoA through a process that releases CO₂ and creates 2 more NADH.
Next, the citric acid cycle (Krebs cycle) in the mitochondrial matrix completely breaks down the carbons from acetyl-CoA into CO₂, producing more electron carriers (NADH and FADH₂). Finally, oxidative phosphorylation uses these electron carriers in the inner mitochondrial membrane. The electron transport chain pumps H⁺ ions across the membrane, creating a gradient that drives ATP production through ATP synthase, generating 30-36 ATP molecules.
Remember this! Aerobic respiration (with oxygen) produces up to 38 ATP total, while anaerobic respiration (without oxygen) produces only 2 ATP through fermentation processes like those that create alcohol or lactic acid.
3
of 3Sign up to see the content. It's free!
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Enzymes and Energy Concepts
All metabolic reactions can be categorized as either anabolic (building larger molecules from smaller ones using energy) or catabolic (breaking down larger molecules to release energy). Similarly, reactions are either exergonic (releasing energy) or endergonic (requiring energy input).
Enzymes are specialized proteins that dramatically speed up biochemical reactions by lowering the activation energy needed to start them. Each enzyme has a specific active site where the substrate fits like a lock and key. Enzymes can be affected by temperature, pH, and chemicals that alter their shape, potentially causing denaturation.
Enzyme activity can be regulated through allosteric regulation when molecules bind to locations other than the active site. Activators like cofactors (minerals) and coenzymes (vitamins) increase enzyme activity, while inhibitors decrease it. Feedback inhibition occurs when the product of a reaction stops the enzyme from making more product.
Make connections! Plants have evolved different photosynthetic adaptations: C3 plants perform normal photosynthesis, C4 plants separate processes spatially within the plant, and CAM plants separate processes temporally (at different times) to conserve resources in harsh environments.
We thought you’d never ask...
What is the Knowunity AI companion?
Our AI companion is specifically built for the needs of students. Based on the millions of content pieces we have on the platform we can provide truly meaningful and relevant answers to students. But its not only about answers, the companion is even more about guiding students through their daily learning challenges, with personalised study plans, quizzes or content pieces in the chat and 100% personalisation based on the students skills and developments.
Where can I download the Knowunity app?
You can download the app in the Google Play Store and in the Apple App Store.
Is Knowunity really free of charge?
That's right! Enjoy free access to study content, connect with fellow students, and get instant help – all at your fingertips.
Most popular content in AP Biology
9Most popular content
9Can't find what you're looking for? Explore other subjects.
Students love us — and so will you.
4.6/5App Store
4.7/5Google Play
The app is very easy to use and well designed. I have found everything I was looking for so far and have been able to learn a lot from the presentations! I will definitely use the app for a class assignment! And of course it also helps a lot as an inspiration.
Stefan SiOS user
This app is really great. There are so many study notes and help [...]. My problem subject is French, for example, and the app has so many options for help. Thanks to this app, I have improved my French. I would recommend it to anyone.
Samantha KlichAndroid user
Wow, I am really amazed. I just tried the app because I've seen it advertised many times and was absolutely stunned. This app is THE HELP you want for school and above all, it offers so many things, such as workouts and fact sheets, which have been VERY helpful to me personally.
AnnaiOS user