7.0 - Cellular Respiration & Metabolism

Last Updated
2026-01-01
Reading Time
3 minute(s)

Table of Contents

Ancestors

Siblings

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Atoms

High-Energy Bonds How ATP Works
A very weak bond that wants to form a stronger bond with another molecule.
Remember: Breaking bonds consumes energy, creating bonds releases energy.
  • Though a net chemical reaction between two or more molecules may either yield or consume more outside energy.

Molecules

NADH Energy Shuttle Bus
An electron carrier.
Picks up free electrons that would otherwise disrupt other atoms / cause unwanted chemical reactions.
FADH₂ Energy Shuttle Bus
An electron carrier.
Picks up free electrons that would otherwise disrupt other atoms / cause unwanted chemical reactions.

ATP is needed for active transport in cells.

Cellular Respiration & Metabolism

Photosynthesis Plants
Makes glucose from light energy.
Cellular Respiration Plants & Animals
Makes ATP from glucose.

Photosynthesis

_Plants

Converts energy into food.

Chlorophyll In Chloroplasts
Absorbs blue and red light best.
Reflects green light.
Stage 1 Light Reactions In Chloroplasts
Converts light energy into chemical energy.
Input
Sunlight + Water + ADP
Output
ATP + NADPH
Byproduct
Oxygen
Stage 2 Calvin Cycle In Chloroplasts
Converts chemical energy into glucose.
Input
CO₂ + ATP + NADPH
Output
Glucose + NADP⁺ + ADP

Think of these two as separate, distinct, reactions. But byproducts are intimately linked, concentration & rates less so. Inputs for each reaction is required. If something affects the output of one, it will affect the input of another. If something affects the input of one, the other may remain unaffected because the reaction caps out.

Plants can starve if they don't have enough light. Light energy produces glucose (their food). Plants do cellular respiration 24/7 and require that glucose to continuously turn into ATP.

Cellular Respiration

_Plants & Animals

Has 3 distinct steps for:

Stage 1 Glycolysis Cytoplasm No Oxygen
Uses a small amount of energy to break down glucose into pyruvate.
Input
1 Glucose (6 carbons)
Output
2 Pyruvate (3 carbons each) + 2 ATP + 2 NADH + 2 H₂O
Outputs 4 atp total, but 2 atp is used in breaking down glucose.
Stage 1.5 Pyruvate Processing Mitochondria Requires Oxygen
Uses a small amount of energy and oxygen to break down pyruvate into acetyl-CoA
Input
2 Pyruvate (from glycolysis)
Output
2 Acetyl-CoA + 2 CO₂ + 2 NADH
Stage 2 Krebs Cycle
Citric Acid Cycle Mitochondria Requires Oxygen
Harvesting electrons into energy carriers.
Input
2 Acetyl-CoA (from pyruvate processing)
Output
4 CO₂ + 2 ATP + 6 NADH + 2 FADH₂
Stage 3 Electron Transport Chain Inner Mitochondrial Membrane Requires Oxygen
Like a factory: Proton pumps use electrons carried from NADH and FADH₂ to pump H⁺ ions across the inner mitochondrial membrane.
H⁺ ions then seemingly combine with ADP to form ATP through the synthase factory.
Input
NADH + FADH₂ + Oxygen + ADP
Output
About 28-32 ATP, NAD, FAD
Byproducts
H₂O

Per glucose molecule:

Types of Respiration

Anerobic Respiration Without Oxygen
Get only 2 ATP per glucose.
Only Stage 1 (glycolysis) can run.
Excess pyruvate is fermented into lactic acid (muscle burn) or ethanol (in yeast).
Aerobic Respiration With Oxygen
Get ~32 ATP per glucose.
All three stages can run.
Why You Breathe Hard During Exercise

Your body's response: