Unit 2 energy in Cells notes

Cellular Respiration – Notes

Purpose of Cellular Respiration

• Cellular respiration is the process by which cells release energy from sugars.

• The released energy is stored in ATP (adenosine triphosphate).

• ATP is the main energy currency of the cell and fuels cellular activities.

ATP (Adenosine Triphosphate)

• ATP is a high-energy molecule.

• It consists of:

  • Adenine

  • Ribose (a sugar)

  • Three phosphate groups

• Energy is released when the bond between phosphate groups is broken.

• When one phosphate is removed:

  • ATP → ADP (adenosine diphosphate) + energy

Types of Cellular Respiration

• Aerobic respiration

  • Requires oxygen

• Anaerobic respiration

  • Occurs without oxygen

  • Commonly called fermentation

Aerobic Respiration: Main Stages

Aerobic respiration occurs in three stages:

1. Glycolysis

   • Occurs in the cytoplasm

   • Glucose is broken down into smaller molecules

   • Produces a small amount of ATP

2. Krebs Cycle

   • Occurs in the mitochondria

   • Breaks down molecules from glycolysis

   • Releases carbon dioxide

   • Produces energy-carrying molecules

3. Electron Transport Chain

   • Occurs in the inner mitochondrial membrane

   • Uses oxygen

   • Produces most of the ATP

Key Takeaways

• Cells store energy as ATP

• Breaking ATP releases usable energy

• Aerobic respiration needs oxygen and is more efficient

• Anaerobic respiration does not require oxygen

Aerobic Respiration – Glycolysis

What is Glycolysis?

• Glycolysis is the process that breaks down glucose.

• One 6-carbon glucose molecule is split into two 3-carbon pyruvate molecules.

• It occurs in the cytosol (cytoplasm) of the cell.

• It does not require oxygen, so it can happen with or without O₂.

Stages of Glycolysis

1. Energy Investment Stage

• The cell uses energy to start the process.

• 2 ATP molecules are spent.

• ATP donates phosphate groups to glucose.

• This process is called phosphorylation.

• Phosphorylation:

  • Makes glucose more reactive

  • Helps prepare it to split into smaller molecules

2. Energy Payoff Stage

• The phosphorylated glucose is broken down into:

  • Two molecules of pyruvate

• Energy is released and captured as:

  • 4 ATP molecules

  • 2 NADH molecules (high-energy electron carriers)

ATP and Energy Yield

• ATP used: 2

• ATP produced: 4

• Net ATP gain: 2 ATP

• NADH produced: 2

Why NADH Matters

• NADH carries high-energy electrons.

• These electrons are later used in the electron transport chain to make more ATP.

Summary (Exam-Friendly)

• Location: Cytosol

• Oxygen required: No

• Input: Glucose, 2 ATP

• Output:

  • 2 Pyruvate

  • 2 ATP (net)

  • 2 NADH

Memory Tip

Think of glycolysis as:

Explanation (Plain but Precise)

All three stages of cellular respiration rely on energy and electron transfers.

• ATP ↔ ADP handles energy transfer

  • Breaking ATP releases energy

  • Rebuilding ATP stores energy

• NADH ↔ NAD⁺ handles electron transfer

  • NAD⁺ accepts electrons

  • NADH donates electrons later

NAD⁺ acts as an oxidizing agent, meaning it gains electrons. In cells, NAD⁺ commonly accepts two hydrogen atoms from an organic molecule like glucose. This produces:

• NADH (which carries high-energy electrons)

• A free H⁺ ion

Later, NADH gives up its electrons, reacting with an H⁺ to regenerate NAD⁺. This recycling is essential so respiration can continue.

The enzyme dehydrogenase removes hydrogen atoms from organic molecules. NAD⁺ works as a coenzyme, partnering with dehydrogenase to capture the electrons. This allows cells to break down glucose gradually, preventing energy loss as heat and enabling efficient ATP production.

Cellular Respiration – ATP & NADH Notes

Key Transformations

• ATP ⇄ ADP

  • Stores and releases energy

• NADH ⇄ NAD⁺

  • Transfers electrons

NAD⁺ and NADH

• NAD⁺

  • Oxidizing agent

  • Accepts electrons

  • Picks up 2 H atoms from organic molecules

• NADH

  • Reduced form

  • Carries high-energy electrons

  • Later donates electrons

Hydrogen Transfer Reaction

• NAD⁺ + 2H → NADH + H⁺

• Reverse reaction:

  • NADH + H⁺ → NAD⁺

Role of Dehydrogenase

• Enzyme that removes hydrogen atoms from organic molecules

• Enables oxidation of sugars

• Works with NAD⁺ as a coenzyme

• Transfers electrons safely and efficiently

Why This Matters

• Allows step-by-step energy release

• Prevents energy loss as heat

• Feeds electrons into the electron transport chain

• Essential for continued ATP production

One-Line Exam Summary

Explanation (Conceptual Understanding)

After glycolysis, pyruvate still contains a lot of usable energy, but it cannot enter the Krebs cycle directly.

1. Pyruvate enters the mitochondria.

2. It loses one carbon atom as CO₂.

3. The remaining 2-carbon fragment attaches to coenzyme A, forming acetyl-CoA.

   • This step links glycolysis to the Krebs cycle.

Once inside the cycle:

• Acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

• Citrate is gradually broken down:

  • From 6C → 5C → 4C

• Each carbon loss releases CO₂.

• By the end, oxaloacetate is regenerated, allowing the cycle to repeat.

Most of the energy released does not become ATP directly. Instead:

• Electrons are captured by NAD⁺ and FAD

• They become NADH and FADH₂

• These carriers transport electrons to the electron transport chain, where large amounts of ATP are made.

Krebs Cycle – Structured Notes

Location

• Occurs in the mitochondrial matrix

Preparation Step (Before the Cycle)

• Pyruvate (3C) → CO₂ + 2-carbon fragment

• 2-carbon fragment + Coenzyme A → Acetyl-CoA

Main Steps of the Cycle

• Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C)

• Citrate is broken down in stages:

  • 6C → 5C → 4C

• 2 CO₂ molecules released per cycle

• Oxaloacetate regenerated at the end

Energy Products (Per Cycle)

• 1 ATP

• 3 NADH

• 1 FADH₂

• 2 CO₂

⚠ Remember: one glucose produces two acetyl-CoA, so the cycle runs twice per glucose.

Role of NADH and FADH₂

• Act as electron carriers

• Store energy temporarily

• Deliver electrons to the electron transport chain

• Lead to massive ATP production later

Purpose of the Krebs Cycle

• Complete the breakdown of carbon compounds

• Capture high-energy electrons

• Supply the electron transport chain

• Regenerate oxaloacetate to keep the cycle running

One-Sentence Exam Summary

Memory Image

Think of the Krebs cycle as a roundabout 🔄:

• Acetyl-CoA enters

• CO₂ exits

• Electrons hop onto carriers

• The road resets for the next turn

Explanation (Big Picture)

The Electron Transport Chain is where most ATP is made during aerobic respiration.

• NADH and FADH₂ arrive carrying high-energy electrons from glycolysis and the Krebs cycle.

• The ETC is located in the inner mitochondrial membrane.

• Electrons move through a series of protein complexes, each one slightly more electronegative than the last.

• As electrons move “downhill,” their energy is used to pump hydrogen ions (H⁺) across the membrane.

• This creates a proton gradient.

• Hydrogen ions flow back through ATP synthase, driving the production of ATP.

• At the end of the chain, oxygen accepts the electrons and combines with H⁺ to form water.

• Without oxygen, the chain stops.

Electron Transport Chain – Notes

Location

• Inner mitochondrial membrane

• The folds (cristae) increase surface area for ATP production

Inputs

• NADH

• FADH₂

• Oxygen (O₂)

• ADP + Pi

Electron Flow

• Electrons pass through protein complexes

• Each transfer releases energy

• Energy is used to pump H⁺ ions into the intermembrane space

Chemiosmosis

• High concentration of H⁺ builds up outside the inner membrane

• H⁺ flows back through ATP synthase

• This flow powers ATP formation

Final Electron Acceptor

• Oxygen

• Combines with electrons and H⁺

• Forms water (H₂O)

ATP Yield

• Produces 32–34 ATP

• This accounts for most ATP made in aerobic respiration

Why the ETC Is So Important

• Maximizes energy extraction from glucose

• Prevents energy loss as heat

• Links electron transport to ATP synthesis

• Makes aerobic respiration highly efficient

One-Line Exam Summary

Memory Hook

Think of the ETC as a hydroelectric dam 💧:

• Electrons pump protons uphill

• Protons rush back down

• ATP is generated

• Oxygen drains the system safely

Chemiosmosis — Explained Simply

Chemiosmosis is how the Electron Transport Chain actually makes ATP.

Step 1: Electron energy pumps protons

• NADH and FADH₂ give electrons to the ETC.

• As electrons move through the chain, their energy is used to pump H⁺ ions:

  • From the mitochondrial matrix

  • Into the intermembrane space

Step 2: A proton gradient forms

Because of this pumping:

• The matrix loses H⁺ → becomes negatively charged

• The intermembrane space gains H⁺ → becomes positively charged

This creates:

• A concentration difference (many H⁺ vs few H⁺)

• A charge difference (positive vs negative)

Together, this is the proton gradient.

Step 3: Protons are trapped

• The inner mitochondrial membrane is impermeable to H⁺

• Protons cannot diffuse back freely

• Energy is now stored, like pressure behind a wall

Step 4: Proton motive force

• The proton gradient + charge difference =
  👉 proton motive force

• This force represents stored potential energy

Step 5: ATP synthase uses the force

• The inner membrane contains ATP synthase

• ATP synthase has a channel for protons

• When H⁺ flows back into the matrix:

  • ATP synthase rotates

  • Rotation provides energy to:

    • ADP + Pi → ATP

Every rotation = ATP made.

Chemiosmosis — Study Notes

Definition

• Chemiosmosis is the process by which ATP is synthesized using the energy stored in a proton gradient.

Where it occurs

• Inner mitochondrial membrane

Key Players

• NADH, FADH₂ → electron donors

• ETC → pumps protons

• H⁺ ions → create gradient

• ATP synthase → makes ATP

Proton Gradient

• High H⁺ in intermembrane space

• Low H⁺ in matrix

• Creates electrical and chemical imbalance

Proton Motive Force

• Combination of:

  • Proton concentration gradient

  • Electrical charge difference

• Drives ATP synthesis

ATP Formation

• Protons flow through ATP synthase

• ATP synthase rotates

• Rotation phosphorylates ADP

• ATP is produced

One-Sentence Exam Answer

Final Mental Image

Think wind-up toy 🌀

• ETC winds it up (pumps protons)

• ATP synthase lets it unwind

• Energy released = ATP

Explanation (Big Picture)

Prokaryotes do aerobic respiration, but they do it without mitochondria.

Because prokaryotic cells lack mitochondria, they can’t use an inner mitochondrial membrane for the ETC. Instead:

• Their plasma membrane does the job.

• The electron transport chain is embedded directly in the plasma membrane.

• Protons (H⁺) are pumped from the cytoplasm to the outside of the membrane.

• This creates a proton gradient across the plasma membrane.

• ATP synthase uses that gradient to make ATP, just like in eukaryotes.

The chemistry is the same.
The location is different.

Why Prokaryotes Can Make More ATP

Eukaryotes must transport electrons into mitochondria, which costs energy.

Prokaryotes:

• Have no mitochondria

• Do not need electron shuttles

• Lose no ATP during transport

Because of this efficiency:

• Prokaryotes can theoretically produce 38 ATP per glucose

• Eukaryotes produce about 34–36 ATP per glucose

Aerobic Respiration in Prokaryotes — Notes

Key Difference

• Prokaryotes do not have mitochondria

• ETC occurs in the plasma membrane

Where Each Stage Happens

• Glycolysis: cytoplasm

• Krebs cycle: cytoplasm

• Electron Transport Chain: plasma membrane

Proton Gradient

• Protons pumped out of the cytoplasm

• High H⁺ outside, low H⁺ inside

• Gradient stores energy

ATP Synthesis

• Protons flow back through ATP synthase

• ATP synthase rotates

• ATP is produced via chemiosmosis

ATP Yield

• Prokaryotes: ~38 ATP (theoretical)

• Eukaryotes: ~34–36 ATP

Quick Comparison Table

One-Sentence Exam Summary

Memory Hook

Prokaryotes are minimalist engineers 🛠
Same machine, fewer walls, less energy lost.

Common symbols for “no oxygen”

✅ O₂ ✕

(Oxygen absent)

✅ –O₂

(Read as “without oxygen”)

✅ Anaerobic conditions

Often written as:

• anaerobic

• anoxic

✅ O₂ = 0

Used in diagrams or notes

✅ ⛔ O₂

Visual shorthand (not formal, but clear for notes)

Most exam-safe option

👉 –O₂

Example:

• Anaerobic respiration (–O₂)

Sentence example

If you want, I can also give:

Fermentation — Explained Simply

When oxygen is absent (–O₂), cells cannot complete aerobic respiration because:

• The electron transport chain stops

• NADH cannot unload electrons

• NAD⁺ runs out

But glycolysis must continue, because it’s the only way to make ATP without oxygen.

What fermentation does:

• It does not make ATP directly

• Its job is to recycle NADH back into NAD⁺

• This allows glycolysis to keep running

Step-by-step under anaerobic conditions

1. Glycolysis still occurs

   • Glucose → 2 pyruvate

   • Produces 2 ATP

   • Produces 2 NADH

2. No oxygen present

   • Pyruvate cannot enter the Krebs cycle

   • ETC cannot function

3. Fermentation pathway begins

   • Pyruvate is reduced by NADH

   • NADH loses electrons → becomes NAD⁺

4. NAD⁺ is recycled

   • Glycolysis can start again

   • ATP production continues (slowly)

Fermentation — Study Notes

Definition

• Fermentation is an anaerobic pathway that allows glycolysis to continue by regenerating NAD⁺.

Key Characteristics

• Occurs –O₂

• Includes glycolysis + fermentation

• Produces 2 ATP per glucose (from glycolysis only)

Role of NADH / NAD⁺

• NADH donates electrons to pyruvate

• NADH → NAD⁺

• NAD⁺ reused in glycolysis

What Fermentation Does NOT Do

• ❌ Does not use oxygen

• ❌ Does not involve Krebs cycle

• ❌ Does not involve ETC

• ❌ Does not produce additional ATP

Why Fermentation Is Essential

• Prevents NAD⁺ depletion

• Maintains ATP production

• Allows survival in anaerobic environments

One-Sentence Exam Answer

Memory Hook

Fermentation is the reset button 🔄
Not more power, just enough to stay alive.

Types of Fermentation — Explained

Fermentation happens without oxygen (–O₂) and exists to recycle NAD⁺ so glycolysis can keep making ATP. There are two main types, depending on how pyruvate is handled.

1. Alcoholic Fermentation

This occurs mainly in yeast and some plants.

What happens:

• Pyruvate (3C) is converted in two steps

• CO₂ is released

• The final product is ethyl alcohol (ethanol)

Why it matters:

• CO₂ gas forms bubbles

• In bread dough, these bubbles make the dough rise

• Ethanol later evaporates during baking

Energy role:

• NADH → NAD⁺

• Allows glycolysis to continue

2. Lactic Acid Fermentation

This occurs in bacteria and animal muscle cells.

What happens:

• Pyruvate is reduced in one step

• Product is lactate (lactic acid)

• No CO₂ is released

Why it matters:

• Used by lactobacilli to make yogurt and other fermented foods

• In humans, occurs during vigorous exercise

Lactic Acid Fermentation in Humans

When you exercise intensely:

• Oxygen supply to muscles becomes limited

• ETC slows or stops

• Muscles rely on glycolysis + lactic acid fermentation

• Lactate builds up

This buildup:

• Changes muscle chemistry

• Contributes to the burning/soreness sensation

Once oxygen returns:

• Lactate is transported to the liver

• Converted back into usable molecules

Types of Fermentation — Study Notes

Alcoholic Fermentation

• Occurs in: yeast

• Products: ethanol + CO₂

• CO₂ released: ✅ Yes

• Used in: bread, beer, wine

Lactic Acid Fermentation

• Occurs in: bacteria, humans

• Product: lactate

• CO₂ released: ❌ No

• Used in: yogurt, cheese, muscle metabolism

Common Features

• Occur –O₂

• Follow glycolysis

• Regenerate NAD⁺

• Produce 2 ATP per glucose (from glycolysis only)

One-Sentence Exam Summary

Memory Trick

• Alcohol → bubbles 🍾

• Lactic → burn 🔥

Explanation (Concept Made Clear)

Glucose is the body’s main fuel.

• It is a simple sugar with the formula C₆H₁₂O₆
  (6 carbons, 12 hydrogens, 6 oxygens)

• Most carbohydrates we eat (like starch) are chains of glucose

• Before cells can use carbohydrates for energy, they must be broken down into glucose

Glucose travels through the bloodstream as blood sugar. Its normal concentration is 70–110 mg per 100 mL of blood. If this level stays too high, it can damage organs and lead to diabetes.

To extract energy from glucose, cells begin with glycolysis, the first step of cellular respiration. Glycolysis happens in all cells, with or without oxygen, and breaks glucose into two molecules of pyruvate.

Glycolysis — Structured Notes

Glucose

• Chemical formula: C₆H₁₂O₆

• Most common carbohydrate in the diet

• Main source of cellular energy

• Circulates in blood as blood sugar

Carbohydrates

• Provide >75% of the body’s energy needs

• Must be broken down into glucose before use

• Complex carbs (starch) are made of glucose units

Blood Glucose Levels

• Normal range: 70–110 mg / 100 mL

• High levels over time → diabetes

Glycolysis

• First stage of cellular respiration

• Also called the Embden–Meyerhof–Parnas (EMP) pathway

• Name origin:

  • glyco = sweet

  • lysis = breaking

Where & When

• Occurs in the cytoplasm

• Can occur with or without oxygen

What Happens

• One glucose (6C) → two pyruvate molecules (3C each)

Energy Changes

Two stages:

1. Energy investment stage

   • 2 ATP used

2. Energy payoff stage

   • 4 ATP produced

➡ Net gain: 2 ATP per glucose

One-Sentence Exam Summary

Memory Hook

Big-Picture Explanation (How it all fits together)

Pyruvate is the key crossroads molecule of cellular respiration.
It is formed when glucose is broken down during glycolysis.

Once pyruvate is made, the cell must decide what to do with it based on oxygen availability:

• If oxygen is present (+O₂) → pyruvate enters the Krebs (TCA / citric acid) cycle

• If oxygen is absent (–O₂) → pyruvate enters fermentation

Aerobic pathway (+O₂)

When oxygen is available:

1. Pyruvate enters the mitochondrion

2. It proceeds through the Krebs cycle

3. The Krebs cycle:

   • Produces very little ATP directly

   • Produces NADH and FADH₂, which carry high-energy electrons

4. These electrons move to the electron transport chain (ETC)

5. The ETC uses electrons to create a proton gradient

6. Chemiosmosis uses this gradient to make large amounts of ATP

➡ Most ATP is made in the ETC, not in glycolysis or Krebs

Anaerobic pathway (–O₂)

When oxygen is absent:

• The ETC stops

• NADH cannot unload electrons

• NAD⁺ becomes depleted

• Glycolysis would stop without NAD⁺

Fermentation solves this problem:

• NADH donates electrons to pyruvate

• NADH → NAD⁺

• Glycolysis continues

• ATP production continues (slowly)

Mitochondrial Structure (Why location matters)

• Outer membrane: smooth

• Inner membrane: folded into cristae

• Cristae:

  • Increase surface area

  • Contain the ETC and ATP synthase

• Intermembrane space:

  • Stores pumped H⁺ ions

• Matrix:

  • Site of the Krebs cycle

Chemiosmosis occurs across the inner membrane, using the cristae.

ATP: The Energy Currency

• ATP = adenosine triphosphate

• Contains three phosphate groups

• Energy is stored in the phosphate bonds

• Breaking a phosphate bond:

  • ATP → ADP + Pi + energy

• That energy powers cellular work

Role of NAD / NADH

• NAD⁺ = electron acceptor

• NADH = electron carrier

• NAD⁺ + H → NADH (reduction)

• NADH delivers electrons to:

  • ETC (aerobic)

  • Pyruvate (fermentation)

Recycling NAD⁺ is essential for survival.

Fermentation Types

Alcoholic fermentation

• Occurs in yeast

• Produces:

  • Ethanol

  • CO₂

• Used in bread making

Lactic acid fermentation

• Occurs in bacteria and humans

• Produces:

  • Lactate

  • No CO₂

• Seen in muscle cells during intense exercise

Lactate buildup contributes to muscle soreness.

Structured Study Notes

Pyruvate

• Product of glycolysis

• Central decision molecule

• Enters Krebs cycle (+O₂)

• Enters fermentation (–O₂)

Krebs Cycle

• Also called TCA or citric acid cycle

• Produces:

  • Small amount of ATP

  • Large amounts of NADH and FADH₂

• Links glycolysis to ETC

Electron Transport Chain

• Located in inner mitochondrial membrane

• Uses electrons from NADH/FADH₂

• Oxygen is final electron acceptor

• Produces maximum ATP

Fermentation

• Occurs without oxygen

• Regenerates NAD⁺

• Allows glycolysis to continue

• Produces either:

  • Ethanol + CO₂

  • Lactate

One-Sentence Exam Summary

Final Mental Map

Glycolysis → Pyruvate →
• +O₂ → Krebs → ETC → lots of ATP
• –O₂ → Fermentation → NAD⁺ → glycolysis continues

Cellular Respiration: The Big Picture

Cellular respiration is how cells convert glucose into ATP, the cell’s spendable energy.

It happens in three main stages:

1. Glycolysis (cytoplasm)

2. Krebs Cycle (mitochondrial matrix)

3. Electron Transport Chain + Chemiosmosis (inner mitochondrial membrane)

1. Glycolysis (Cytoplasm)

“Sugar splitting”

Purpose

• Break glucose (6C) into 2 pyruvate (3C)

• Produce ATP and NADH

• Does not require oxygen

Energy Investment Phase

The cell spends ATP to make glucose reactive.

1. Glucose → Glucose-6-phosphate
   Enzyme: Hexokinase
   ATP → ADP
   (Traps glucose in the cell)

2. Glucose-6-phosphate → Fructose-6-phosphate
   Enzyme: Phosphoglucose isomerase
   (Rearranges the molecule to allow symmetry later)

3. Fructose-6-phosphate → Fructose-1,6-bisphosphate
   Enzyme: Phosphofructokinase (PFK)
   ATP → ADP
   ⭐ Rate-limiting step (most regulated)

Splitting Phase

4. Fructose-1,6-bisphosphate → DHAP + G3P
   Enzyme: Aldolase

5. DHAP → G3P
   Enzyme: Triose phosphate isomerase
   (Now you have 2 G3P)

Energy Payoff Phase

Each step happens twice (once per G3P).

6. G3P → 1,3-bisphosphoglycerate
   Enzyme: G3P dehydrogenase
   NAD⁺ → NADH

7. 1,3-BPG → 3-phosphoglycerate
   Enzyme: Phosphoglycerate kinase
   ADP → ATP (substrate-level phosphorylation)

8. 3-PG → 2-PG
   Enzyme: Phosphoglycerate mutase

9. 2-PG → PEP
   Enzyme: Enolase

10. PEP → Pyruvate
    Enzyme: Pyruvate kinase
    ADP → ATP

Net Yield (per glucose)

• 2 ATP (net)

• 2 NADH

• 2 pyruvate

2. Pyruvate Oxidation (Link Reaction)

(Not always listed as part of Krebs, but AP loves it)

Location

• Mitochondrial matrix

Reaction

Pyruvate (3C) → Acetyl-CoA (2C) + CO₂

Enzyme complex: Pyruvate dehydrogenase

Per glucose:

• 2 NADH

• 2 CO₂

• 2 Acetyl-CoA

3. Krebs Cycle (Citric Acid Cycle)

“Molecular wheel of oxidation”

Purpose

• Fully oxidize acetyl-CoA

• Load electron carriers (NADH, FADH₂)

• Release CO₂

Location

• Mitochondrial matrix

One turn (per acetyl-CoA)

1. Acetyl-CoA + Oxaloacetate → Citrate
   Enzyme: Citrate synthase

2. Citrate → Isocitrate
   Enzyme: Aconitase

3. Isocitrate → α-ketoglutarate
   Enzyme: Isocitrate dehydrogenase
   NAD⁺ → NADH, CO₂ released

4. α-ketoglutarate → Succinyl-CoA
   Enzyme: α-ketoglutarate dehydrogenase
   NAD⁺ → NADH, CO₂ released

5. Succinyl-CoA → Succinate
   Enzyme: Succinyl-CoA synthetase
   GDP → GTP (ATP)

6. Succinate → Fumarate
   Enzyme: Succinate dehydrogenase
   FAD → FADH₂

7. Fumarate → Malate
   Enzyme: Fumarase

8. Malate → Oxaloacetate
   Enzyme: Malate dehydrogenase
   NAD⁺ → NADH

Yield per glucose (2 turns)

• 6 NADH

• 2 FADH₂

• 2 ATP (or GTP)

• 4 CO₂

4. Electron Transport Chain + Chemiosmosis

“Turning electrons into ATP”

Location

• Inner mitochondrial membrane

Electron Transport Chain

• NADH and FADH₂ donate electrons

• Electrons move through protein complexes

• Energy pumps H⁺ ions into the intermembrane space

Key complexes:

• Complex I (NADH dehydrogenase)

• Complex II (succinate dehydrogenase)

• Complex III

• Complex IV (cytochrome c oxidase)

Final electron acceptor: Oxygen

• O₂ + e⁻ + H⁺ → H₂O

Chemiosmosis

• H⁺ flows back through ATP synthase

• ADP + Pi → ATP

ATP Yield (approximate, AP Bio level)

• NADH ≈ 2.5 ATP each

• FADH₂ ≈ 1.5 ATP each

• Total per glucose: ~30–32 ATP

Final AP Bio Summary Table

One-Sentence AP Exam Master Line

Cellular respiration converts glucose into ATP through glycolysis, the Krebs cycle, and oxidative phosphorylation, using redox reactions and chemiosmosis to efficiently harvest energy.

If you want, next we can:

Cellular Respiration – Summary Notes (AP Biology)

Purpose of Cellular Respiration

• Animals obtain energy from food

• Cellular respiration harvests this energy and stores it as ATP

• ATP is the cell’s direct energy currency

• Cellular respiration can be:

  • Aerobic (with oxygen)

  • Anaerobic (without oxygen)

Aerobic Respiration (Oxygen Present)

Overview

Aerobic respiration consists of three main stages:

1. Glycolysis

2. Krebs (Citric Acid) Cycle

3. Electron Transport Chain (ETC) + Chemiosmosis

1. Glycolysis

• Location: Cytoplasm

• Oxygen: Not required

• Function:

  • Breaks 1 glucose (6C) into 2 pyruvate (3C)

• Products:

  • 2 ATP (net)

  • 2 NADH

  • 2 pyruvate

2. Krebs Cycle

• Location: Mitochondrial matrix

• Pyruvate enters after being converted to acetyl-CoA

• Function:

  • Breaks down acetyl-CoA

  • Releases CO₂

  • Transfers high-energy electrons to carriers

• Products (per glucose):

  • 2 ATP

  • 6 NADH

  • 2 FADH₂

  • 4 CO₂

3. Electron Transport Chain (ETC)

• Location: Inner mitochondrial membrane

• Function:

  • NADH and FADH₂ donate electrons

  • Energy from electrons pumps H⁺ ions across the membrane

• Oxygen:

  • Final electron acceptor

  • Forms water (H₂O)

Chemiosmosis

• Occurs during the ETC

• H⁺ ions accumulate, creating a proton motive force

• H⁺ flows back through ATP synthase

• ATP synthase:

  • Rotates as protons pass through

  • Converts ADP + Pi → ATP

• Produces most of the ATP in cellular respiration

Anaerobic Processes (No Oxygen)

Fermentation

• Occurs when oxygen is unavailable

• Purpose:

  • Regenerates NAD⁺ so glycolysis can continue

• ATP yield:

  • Only 2 ATP (from glycolysis)

Types of Fermentation

• Lactic acid fermentation

  • Occurs in animal muscle cells

  • Produces lactic acid

• Alcoholic fermentation

  • Occurs in yeast and plants

  • Produces ethanol + CO₂

One-Line AP Bio Wrap-Up

Cellular respiration converts the chemical energy in glucose into ATP through glycolysis, the Krebs cycle, and the electron transport chain, with chemiosmosis producing most of the ATP.