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C. Metabolic Processes · Metabolic Processes

Cellular Respiration Overview, the Mitochondrion and Glycolysis

C3.1 C3.4 C2.1 A1.12About 70 minutes
Learning goals
  • I can write the overall equation for aerobic cellular respiration and name its four stages and where each happens.
  • I can label the parts of a mitochondrion and relate each part to a stage of respiration.
  • I can describe the energy-investment and energy-payoff phases of glycolysis.
  • I can state the reactants and products of glycolysis and calculate its net ATP and NADH yield.

Words to know

aerobic cellular respiration
The breakdown of glucose using oxygen to produce CO₂, water, ATP and heat.
mitochondrion
A double-membrane organelle where pyruvate oxidation, the Krebs cycle and oxidative phosphorylation take place.
matrix
The protein-rich fluid inside the inner mitochondrial membrane; site of pyruvate oxidation and the Krebs cycle.
cristae
Folds of the inner mitochondrial membrane that increase its surface area for the electron transport chain and ATP synthase.
intermembrane space
The narrow space between the outer and inner mitochondrial membranes, where H⁺ ions accumulate.
glycolysis
The ten-step pathway in the cytosol that splits one glucose (6C) into two pyruvate (3C), with a net gain of 2 ATP and 2 NADH.
pyruvate
The three-carbon end product of glycolysis (the ion of pyruvic acid).
G3P (glyceraldehyde 3-phosphate)
A three-carbon sugar phosphate formed when fructose 1,6-bisphosphate splits; it also appears in the Calvin cycle.
phosphofructokinase (PFK)
The enzyme of glycolysis step 3, which adds a second phosphate; a key control point of respiration.

From breakfast to ATP

A slice of toast is mostly starch, which your digestive system breaks into glucose. Each glucose molecule that reaches a muscle cell will be taken apart in a series of more than 20 enzyme-controlled steps, and its energy will end up in about 30 ATP molecules. In this lesson you get the big picture of that process, then zoom into the first stage, glycolysis, which happens in every living cell on Earth, from yeast to sugar maples to you.

Remember from earlier grades

In Grade 9 you learned the overall equation for respiration and that it happens mostly in mitochondria. In Grade 11 Biology you saw that cells need ATP for active transport. In Lesson 3 you learned that respiration oxidizes glucose and reduces oxygen, with NAD⁺ and FAD as electron carriers.

The overall equation

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O ΔG = −2870 kJ/mol

In redox terms: glucose is oxidized to CO₂ and oxygen is reduced to H₂O. Part of the free energy is captured as ATP; the rest is released as heat.

Aerobic respiration happens in four stages:

  1. Glycolysis (cytosol): glucose → 2 pyruvate. No oxygen needed.
  2. Pyruvate oxidation (mitochondrial matrix): each pyruvate → acetyl-CoA + CO₂.
  3. The Krebs cycle (matrix): acetyl groups are oxidized completely to CO₂, loading up NADH and FADH₂.
  4. Electron transport and chemiosmosis (inner mitochondrial membrane): NADH and FADH₂ give their electrons to a chain of proteins that ends with oxygen; the energy pumps H⁺ ions, which then flow back through ATP synthase to make most of the ATP. This stage is called oxidative phosphorylation.

Stages 1 and 3 make a little ATP by substrate-level phosphorylation. Stage 4 makes most of it by oxidative phosphorylation.

cytosolmitochondrion1. Glycolysisglucose → 2 pyruvate2. Pyruvateoxidation3. Krebscycle4. ETC andchemiosmosis2 ATP (net)2 NADH2 NADH2 CO₂2 ATP6 NADH, 2 FADH₂4 CO₂about 26–28 ATPO₂ → H₂ONADH and FADH₂ carry electrons to the ETCSubstrate-level phosphorylation in stages 1 and 3; oxidative phosphorylation in stage 4
The four stages of aerobic respiration and where they happen. Glycolysis is in the cytosol; the other three stages are in the mitochondrion. Numbers are per glucose.

The mitochondrion: a power plant with two walls

A mitochondrion has two membranes, and each compartment has a job:

  • Outer membrane: smooth and quite permeable to small molecules (it has large channel proteins).
  • Intermembrane space: the thin space between the membranes. In stage 4, H⁺ ions are pumped into it and build up to a high concentration.
  • Inner membrane: folded into cristae, which greatly increase its surface area. It holds the electron transport chain proteins and ATP synthase. It is nearly impermeable to H⁺, which is essential for chemiosmosis.
  • Matrix: the fluid inside the inner membrane. It holds the enzymes of pyruvate oxidation and the Krebs cycle, plus mitochondrial DNA and ribosomes.

Cells that use a lot of energy (heart muscle, sperm cells, liver cells) have many mitochondria, and their mitochondria have densely packed cristae. Structure fits function!

outer membraneintermembrane spaceinner membranecrista (fold ofthe inner membrane)matrix(pyruvateoxidation,Krebs cycle)ETC and ATP synthase are in the inner membrane (not to scale)
A mitochondrion. The inner membrane is folded into cristae; it carries the electron transport chain and ATP synthase. The matrix holds the Krebs cycle enzymes. (Not to scale.)

Glycolysis: splitting sugar

Glycolysis means "sugar splitting". It is a ten-step pathway in the cytosol, and each step has its own enzyme. It does not need oxygen. You don't have to memorize all ten intermediates for this course, but you should know the two phases and the key events.

Phase 1: energy investment (steps 1 to 5). The cell spends 2 ATP.

  • Step 1: glucose is phosphorylated by ATP to make glucose 6-phosphate. This traps glucose inside the cell (the charged phosphate can't cross the membrane) and makes it more reactive.
  • Step 2: it is rearranged into fructose 6-phosphate.
  • Step 3: a second ATP adds another phosphate, making fructose 1,6-bisphosphate. The enzyme here, phosphofructokinase (PFK), is the main control point of glycolysis.
  • Steps 4 and 5: the 6-carbon sugar is split into two 3-carbon sugars, and both end up as glyceraldehyde 3-phosphate (G3P).

Phase 2: energy payoff (steps 6 to 10). Each G3P goes through these steps, so everything happens twice per glucose.

  • Step 6: G3P is oxidized by NAD⁺ (a dehydrogenase step) and gains a phosphate from Pᵢ. This makes NADH (1 per G3P, so 2 per glucose).
  • Steps 7 and 10: phosphates are transferred from substrates to ADP, making ATP by substrate-level phosphorylation: 2 ATP per G3P, so 4 per glucose.
  • The final product is pyruvate (3C), 2 per glucose.
glucose (6C)ATP → ADPglucose 6-phosphate (6C)fructose 6-phosphate (6C)ATP → ADPfructose 1,6-bisphosphate (6C)G3P (3C)G3P (3C)pyruvate (3C)pyruvate (3C)NAD⁺ → NADH2 ADP → 2 ATPNAD⁺ → NADH2 ADP → 2 ATPEnergy investmentphase: uses 2 ATPEnergy payoff phase(×2): makes 4 ATP,2 NADHNet per glucose:2 pyruvate, 2 ATP,2 NADH
Glycolysis in summary. The investment phase uses 2 ATP; the payoff phase (which happens twice, once for each G3P) makes 4 ATP and 2 NADH. Net: 2 ATP, 2 NADH and 2 pyruvate per glucose.

The glycolysis balance sheet

glucose + 2 ADP + 2 Pᵢ + 2 NAD⁺ → 2 pyruvate + 2 ATP + 2 NADH + 2 H⁺ + 2 H₂O

  • ATP: 4 made − 2 used = 2 net ATP.
  • 2 NADH, which can carry electrons to the electron transport chain if oxygen is present.
  • 2 pyruvate, which still hold most of glucose's energy.

Glycolysis captures only a small fraction of the energy in glucose (2 × 30.5 = 61 kJ of 2870 kJ, about 2%). The real payoff comes in the mitochondrion.

Why does every organism do it? Glycolysis needs no oxygen and no organelles. Earth's early atmosphere had almost no free oxygen, and the earliest cells were prokaryotes without mitochondria. Glycolysis is thought to be one of the oldest metabolic pathways, inherited by all living things today.

Worked example

Let's track carbon and energy in glycolysis

A muscle cell breaks down 5 molecules of glucose by glycolysis. How many pyruvate, net ATP and NADH are produced? How many carbon atoms are in all the pyruvate made, and what does this tell you?

  1. Per glucose: 2 pyruvate, 2 net ATP, 2 NADH.
  2. For 5 glucose: 5 × 2 = 10 pyruvate, 5 × 2 = 10 net ATP, 5 × 2 = 10 NADH.
  3. Carbon: 5 glucose × 6 C = 30 C. 10 pyruvate × 3 C = 30 C.
  4. All the carbon is still there: no CO₂ is released in glycolysis. The glucose has been split and partly oxidized, but not broken down to CO₂.

10 pyruvate, 10 net ATP and 10 NADH. All 30 carbon atoms are in pyruvate; CO₂ is first released in pyruvate oxidation.

Worked example

Let's reason about a poison

Arsenate (AsO₄³⁻) looks like phosphate. In step 6 of glycolysis, it can take the place of Pᵢ, and the product then falls apart without making ATP in step 7. Predict the net ATP yield of glycolysis if arsenate replaces phosphate in every step 6.

  1. Normally ATP is made in step 7 (2 per glucose) and step 10 (2 per glucose): 4 made, 2 used.
  2. If step 7 makes no ATP, only step 10 produces ATP: 2 per glucose.
  3. Investment is still 2 ATP.
  4. Net = 2 − 2 = 0.

Net yield = 0 ATP per glucose from glycolysis. Glycolysis keeps running (pyruvate is still made) but captures no net energy. This is one reason arsenic compounds are toxic.

Just for fun

Glycolysis has ten steps, ten enzymes and a two-phase plan: spend first, earn later. It's basically the cell's version of buying a lemonade stand.

Hands-on activity

Build glycolysis with paper clips

You need: 12 paper clips (carbon atoms), 6 small sticky notes labelled P (phosphate), 4 coins (ATP made), 2 bottle caps (NADH), 2 more sticky notes labelled ATP spent, a large sheet of paper.

Safety note: keep small items such as caps and clips away from any young children at home.

  1. Link 6 paper clips in a chain: one glucose. On the paper, draw a line down the middle: left = investment, right = payoff.
  2. Investment: stick a P on each end of the chain, and put the two ATP spent notes beside it. (Glucose is now fructose 1,6-bisphosphate.)
  3. Split the chain into two 3-clip pieces, each with one P. These are G3P.
  4. Payoff, for each G3P: add a bottle cap (NADH formed) and another P (from Pᵢ). Then remove both P's one at a time; each one becomes a coin (ATP). The 3-clip piece left is pyruvate.
  5. Count: coins made, ATP spent, NADH, pyruvate, and clips.

Check yourself:

  • Coins (ATP made): 4. ATP spent: 2. Net: 2.
  • NADH: 2. Pyruvate: 2, with 3 clips each: all 6 carbons are still there.
  • Repeat it without looking at the instructions, explaining each step aloud to someone at home.

Your turn

1

Write the overall equation for aerobic cellular respiration.

2

List the four stages of aerobic respiration and where each happens in a eukaryotic cell.

3

What is the function of the cristae?

4

Why is the first phase of glycolysis called the energy-investment phase?

5

How many ATP are made in total in glycolysis, and what is the net yield? Explain the difference.

6

Name the two products of glycolysis (besides ATP) that carry energy on to later stages.

7

Does glycolysis need oxygen? Explain why this is important.

More practice: warm-up

1

In which part of the cell does glycolysis take place?

2

How many carbon atoms are in pyruvate?

3

Which mitochondrial compartment holds the Krebs cycle enzymes?

4

What does the word glycolysis mean?

5

What type of phosphorylation makes ATP in glycolysis?

More practice: on your own

1

Explain why heart muscle cells contain many more mitochondria than skin cells.

2

In glycolysis, is G3P oxidized or reduced in step 6? Identify the oxidizing agent.

3

Why does phosphorylating glucose in step 1 help the cell?

4

A cell has plenty of glucose but runs out of NAD⁺. Which step of glycolysis stops, and what happens to ATP production?

5

What percentage of the free energy of glucose is captured as ATP in glycolysis? Show your calculation.

Challenge

1

Explain why the inner mitochondrial membrane must be nearly impermeable to H⁺ ions, using what you know about the function of the intermembrane space.

2

Mature human red blood cells have no mitochondria. Which stage(s) of respiration can they carry out, how much ATP do they make per glucose, and why does this make sense for their job?

3

Using a free-energy argument, explain why the cell spends ATP in the investment phase instead of just splitting glucose directly.

Check your understanding

Quiz

1.

Where does glycolysis occur in a eukaryotic cell?

2.

The net yield of glycolysis per glucose is:

3.

Which structure increases the surface area for the electron transport chain?

4.

In glycolysis, NADH is formed when:

5.

How many carbon atoms from one glucose are released as CO₂ during glycolysis?

6.

Which enzyme is a key control point of glycolysis?

For parents and teachers

Big idea: Respiration happens in four stages in two places (cytosol and mitochondrion). Glycolysis is the universal first stage: glucose → 2 pyruvate, net 2 ATP and 2 NADH, no oxygen needed (C3.1, C3.4).

How to help: Ask your teen to draw the four-stage overview from memory, with locations, then the glycolysis two-phase summary. The paper-clip model makes the carbon count concrete. Students do not need to memorize all ten intermediates, but should know glucose, G3P and pyruvate and what happens in each phase.

Common difficulties:

  • Saying glycolysis makes 4 ATP (that is the gross yield, not the net).
  • Putting glycolysis in the mitochondrion.
  • Forgetting that the payoff phase happens twice per glucose.
  • Thinking glycolysis releases CO₂. It doesn't; CO₂ first appears in pyruvate oxidation.