ap biology unit 3 notes

3 min read 01-02-2025

This comprehensive guide covers the key concepts of AP Biology Unit 3, focusing on cellular energetics. We'll delve into the intricate processes of energy conversion, exploring both cellular respiration and photosynthesis. Understanding these processes is crucial for success in the AP Biology exam.

Energy and Metabolism: The Foundation of Life

Before diving into the specifics of cellular respiration and photosynthesis, let's establish a foundational understanding of energy and metabolism.

What is Energy?

In the context of biology, energy is the capacity to do work. This work can involve various cellular processes, from building molecules (anabolism) to breaking them down (catabolism). Energy exists in different forms, including:

Kinetic Energy: Energy of motion (e.g., movement of molecules).
Potential Energy: Stored energy (e.g., chemical bonds in molecules like glucose).
Chemical Energy: A form of potential energy stored within the chemical bonds of molecules. This is the primary form of energy used by living organisms.

Metabolism: The Sum of Life's Reactions

Metabolism refers to the totality of an organism's chemical reactions. These reactions are highly organized and often involve enzyme-catalyzed pathways. Key metabolic processes include:

Catabolism: The breakdown of complex molecules into simpler ones, releasing energy. Examples include cellular respiration and digestion.
Anabolism: The synthesis of complex molecules from simpler ones, requiring energy input. Examples include protein synthesis and DNA replication.

Enzymes: The Catalysts of Life

Enzymes are biological catalysts that speed up the rate of metabolic reactions without being consumed in the process. Their activity is crucial for efficient energy transfer and metabolic regulation. Factors affecting enzyme activity include:

Temperature: Enzymes have optimal temperature ranges.
pH: Enzymes have optimal pH ranges.
Substrate Concentration: Enzyme activity increases with substrate concentration up to a point of saturation.
Inhibitors: Molecules that can reduce or block enzyme activity (competitive and non-competitive inhibition).

Cellular Respiration: Harvesting Energy from Glucose

Cellular respiration is a catabolic process that breaks down glucose to generate ATP (adenosine triphosphate), the primary energy currency of the cell. This process occurs in several stages:

Glycolysis: The First Step

Glycolysis takes place in the cytoplasm and involves the breakdown of glucose into pyruvate. This process produces a small amount of ATP and NADH (a reducing agent).

Pyruvate Oxidation: Preparing for the Citric Acid Cycle

Pyruvate, produced during glycolysis, is transported into the mitochondria, where it's converted into acetyl-CoA. This step releases carbon dioxide and generates NADH.

The Citric Acid Cycle (Krebs Cycle): Central Energy Hub

The citric acid cycle, located in the mitochondrial matrix, is a series of reactions that oxidize acetyl-CoA, releasing carbon dioxide and generating ATP, NADH, and FADH2 (another reducing agent).

Oxidative Phosphorylation: ATP Synthesis via Electron Transport Chain

The electron transport chain, embedded in the inner mitochondrial membrane, harnesses the energy from NADH and FADH2 to pump protons (H+) across the membrane, creating a proton gradient. This gradient drives ATP synthesis via chemiosmosis, a process known as oxidative phosphorylation, producing the majority of ATP in cellular respiration.

Photosynthesis: Capturing Light Energy

Photosynthesis is an anabolic process that converts light energy into chemical energy in the form of glucose. This process occurs in chloroplasts and involves two main stages:

Light-Dependent Reactions: Harnessing Light Energy

The light-dependent reactions occur in the thylakoid membranes of chloroplasts. Chlorophyll and other pigments absorb light energy, which is used to split water (photolysis), releasing oxygen and generating ATP and NADPH (another reducing agent).

Light-Independent Reactions (Calvin Cycle): Building Glucose

The light-independent reactions, or Calvin cycle, take place in the stroma of chloroplasts. ATP and NADPH produced during the light-dependent reactions are used to fix carbon dioxide, ultimately synthesizing glucose.

Connecting Cellular Respiration and Photosynthesis

Cellular respiration and photosynthesis are intricately linked. The products of one process are the reactants of the other, forming a cyclical exchange of energy and matter within ecosystems. Photosynthesis captures light energy and converts it into chemical energy in the form of glucose, which is then used by cells in cellular respiration to generate ATP.

This detailed overview provides a solid foundation for understanding AP Biology Unit 3. Remember to consult your textbook, class notes, and practice problems to reinforce your knowledge and prepare for the AP exam. Good luck!