Cellular Respiration and Photosynthesis Flashcards

Chapter 9 – Cellular Respiration

• Goal: Break down glucose into CO₂ and use the energy to make ATP.
• Happens in all eukaryotic cells and requires oxygen.
• We breathe in O₂ and breathe out CO₂ because of this process.

Three Main Steps:

1. Glycolysis (in cytoplasm)
   • Splits glucose (6C) → 2 pyruvate (3C)
   • Produces: 2 ATP, 2 NADH
   • Anaerobic (no O₂ needed)
2. Citric Acid Cycle (in mitochondrial matrix)
   • Pyruvate → Acetyl-CoA → CO₂ released
   • Produces: NADH, FADH₂, ATP
3. Oxidative Phosphorylation (in inner mitochondrial membrane)
   • NADH/FADH₂ → ETC → ATP
   • Uses O₂ as final electron acceptor → makes H₂O
   • Makes most of the ATP (~34)

Extra Notes:

• ATP & NADH = energy “money” (recycled, not made from scratch)
• Other nutrients (like proteins & fats) also enter this pathway
• We can’t digest cellulose (no enzyme for it)

Citric Acid Cycle – Key Summary

Where:

• Takes place in the mitochondrial matrix

What:

• A series of 8 enzyme-driven reactions
• Main job: break down carbon molecules to release energy

Important Concepts:

• Reactants/Substrates: Start with citrate, end with isocitrate; no need to memorize all names or structures
• Each step releases energy (called exergonic) → negative \DeltaG, meaning the process is spontaneous
• When enough energy is released in a step, it’s used to create:
  • ATP
  • NADH
  • FADH₂

Why the Energy Matters:

• Making 1 ATP = needs +7.3 kcal/mol
• Making 1 NADH = ~21.9 kcal/mol of energy released
• For a reaction to form NADH, it must release at least −21.9 kcal/mol
• Big steps like converting isocitrate → \alpha-ketoglutarate release lots of energy and CO₂, which increases entropy (randomness), driving the reaction forward

Comparing Energy Yields:

• NADH is worth 3 ATP
• FADH₂ is worth 2 ATP
• Some reactions release less energy, so they make FADH₂ or ATP instead of NADH
• Some steps don’t produce energy directly but set up future reactions that do

Substrate-Level Phosphorylation:

• A way to make ATP directly
• Happens when an enzyme uses energy from the reaction to build ATP
• The enzyme acts like a teeter-totter, flipping between substrates and ADP to combine them into ATP

Big Picture:

• ATP is produced in two ways:
  • A little directly in glycolysis & the citric acid cycle
  • Most come later in oxidative phosphorylation using NADH & FADH₂

Oxidative Phosphorylation – Fast Summary

Where:

• Inner mitochondrial membrane

What:

• Final step of cellular respiration
• Converts NADH and FADH₂ (from glycolysis and the citric acid cycle) into ATP
• Requires oxygen to work

How It Works:

1. Electron Transport Chain (ETC):

• NADH and FADH₂ donate electrons to a chain of proteins in the membrane
• As electrons move through the ETC, protons (H⁺) are pumped into the space between the inner and outer membranes
• This creates a high H⁺ concentration (acidic region) = potential energy

2. Chemiosmosis:

• H⁺ flows back into the matrix through a protein called ATP Synthase
• This flow powers the enzyme to synthesize ATP from ADP + P
• This process is called chemiosmosis

ATP Yield:

• 1 NADH → ~3 ATP
• 1 FADH₂ → ~2 ATP (enters chain lower, pumps fewer protons)

Oxygen’s Role:

• Final electron acceptor in the ETC
• Combines with electrons and H⁺ → forms H₂O
• Without oxygen, the ETC stops, causing a backup in the whole system
• That’s why we need oxygen to keep making ATP!

ATP Synthase:

• A complex enzyme with a quaternary structure
• Uses proton gradient like a waterwheel to generate ATP
• Converts potential energy (from H⁺ gradient) → chemical energy (ATP)
• It’s one of the most efficient “engines” in biology

Bottom Line:

• Oxidative phosphorylation = ETC + chemiosmosis
• Produces the majority of ATP in cellular respiration
• Needs oxygen to run—no oxygen, no ATP from this step

Fermentation Overview

• Fermentation is a backup process that kicks in when there’s no oxygen, preventing oxidative phosphorylation.
• It occurs in the cytoplasm and lets glycolysis continue by recycling NADH into NAD⁺.
• Though inefficient (produces only a small amount of ATP), it helps cells survive temporarily without oxygen.

Types of Fermentation

• Lactic Acid Fermentation: Happens in muscle cells and causes soreness.
• Alcohol Fermentation: Occurs in yeast; produces ethanol and CO₂.
• Both serve the same goal: keep glycolysis going by regenerating NAD⁺.

Why Fermentation Is Inefficient

• Most energy is lost as heat, not captured as ATP.
• That’s why you get hot and sweaty when exercising without enough oxygen losing calories as heat.

Metabolic Pathway Regulation

• Cells have “switches” that control which pathway is active, depending on oxygen and energy needs.
• These switches include:
  • Traffic jams (e.g. buildup of pyruvate and NADH when mitochondria are full)
  • Enzyme regulation (turning enzymes on or off)

Enzyme Inhibition

• Competitive Inhibitor: Blocks the enzyme’s active site.
• Non-competitive Inhibitor: Changes enzyme shape by binding elsewhere.
• Both stop the reaction, helping redirect resources to other pathways.

Chapter 10: Photosynthesis

What is Photosynthesis?

• Process where plants use light, CO₂, and water to make glucose and oxygen.
• Happens in chloroplasts (only in eukaryotic cells in this chapter).

Chloroplast Structure

• Double membrane
• Stroma: fluid where sugars are made (Calvin Cycle)
• Thylakoids: contain chlorophyll, where light is absorbed

Main Reaction (Reverse of Respiration)

• 6CO₂ + 6H₂O + light → C₆H₁₂O₆ + 6O₂

Two Stages of Photosynthesis

1. Light Reactions (in thylakoid membrane)

*   Use light to make ATP and NADPH
*   Water is split, and oxygen is released

2. Calvin Cycle (in stroma)

*   Uses ATP and NADPH to turn CO₂ into glucose
*   It does not need light directly

Extra Notes

• CO₂ enters through stomata
• Photosynthesis and cellular respiration are connected
• Chloroplasts and mitochondria both use electron transport chains and ATP synthase to make ATP

Light Reactions

Location

• Take place in the thylakoid membrane of chloroplasts.

Key Components

• Photosystem II (PSII) and Photosystem I (PSI) — named by discovery order.
• Both contain chlorophyll, which absorbs light energy.

Pigments

• Multiple pigments absorb light.
• Energy is funneled to chlorophyll.
• Some pigments protect the plant from damage (like sunscreen).

What Happens

1. Light excites chlorophyll → An electron is raised to a high energy level.
2. The excited electron is passed to a primary electron acceptor.
3. The electron transport chain (ETC) moves electrons through proteins.
4. Photosystem II:
   • Electron passed to PSI.
   • Energy is used to pump protons into thylakoid space → creates proton gradient.
   • Drives ATP synthesis.
5. Photosystem I:
   • Re-excites the electron with more light.
   • Electron is passed to NADP⁺, reducing it to NADPH.

Electron Source

• PSII replaces lost electrons by splitting H₂O:
  • Produces O₂ (released)
  • Adds more protons to the gradient
• PSI replaces its electron with one from PSII

Products of Light Reactions

• ATP (used in the Calvin Cycle)
• NADPH (used in the Calvin Cycle)
• Oxygen (as a byproduct)

Why Two Photosystems?

• Together they push the electron to a high enough energy level to reduce NADP⁺.
• Called the Z-Scheme based on energy diagram shape.

Regulation – Switch in Pathway

• Sometimes plants only need ATP, not NADPH (e.g., young growing leaves).
• They can switch to cyclic electron flow:
  • Only uses PSI
  • Electrons cycle back into the ETC
  • Produces ATP only, no NADPH

The Calvin Cycle (a.k.a. Dark Reactions)

• Named after Melvin Calvin, who studied it in the 1940s–50s in Berkeley.
• Called “dark reactions” because they don’t directly need light, though they usually happen alongside light reactions since they rely on ATP and NADPH made by the light reactions.

What Happens in the Calvin Cycle?

• The main goal is to make sugars (starting with a 3-carbon sugar, which can become glucose).
• ATP and NADPH (from light reactions) provide the energy and electrons to build these sugars.
• This is an endergonic process (requires energy), but it’s coupled with exergonic reactions (like ATP breakdown), making the overall reaction spontaneous (net negative \DeltaG).
• The enzyme rubisco is key—it’s the one that fixes CO₂ into an organic form.

Photorespiration – A Problem for C3 Plants

• Rubisco sometimes makes a mistake: when O₂ is high and CO₂ is low (common on hot, dry days when stomata close), rubisco uses O₂ instead of CO₂.
• This process is called photorespiration. It wastes energy and releases CO₂.
• Happens often in C3 plants (like most trees and plants in cooler climates).

C4 and CAM Plants – Their Solution to Photorespiration

• C4 plants (like grasses):
  • Fix CO₂ in mesophyll cells using a different enzyme.
  • Then send that carbon to rubisco in bundle sheath cells, separating CO₂ fixation from rubisco by space.
• CAM plants (like cacti):
  • Fix CO₂ at night and store it.
  • During the day, they release the stored CO₂ to rubisco.
  • This separates the process by time instead of space.

Big Picture

• Plants evolved different strategies (C3, C4, CAM) to adapt to their environment.
• It’s a good example of how molecular processes (like rubisco and photorespiration) affect entire ecosystems.