Cell Structure » Mitochondria and Energy

What you'll learn this session

Study time: 30 minutes

The structure and function of mitochondria
How mitochondria produce energy through cellular respiration
The importance of ATP as the energy currency of cells
How mitochondrial structure relates to its function
Real-world applications and disorders related to mitochondrial function

Introduction to Mitochondria

Mitochondria are often called the "powerhouses" of the cell because they generate most of the cell's supply of energy. These tiny organelles are found in nearly all eukaryotic cells (cells with a nucleus) and are responsible for producing ATP (adenosine triphosphate), which cells use as their main energy currency.

Key Definitions:

Mitochondria: Membrane-bound organelles that generate most of the chemical energy needed to power the cell's biochemical reactions.
ATP: Adenosine triphosphate, the main energy currency of cells.
Cellular respiration: The process by which cells break down glucose to release energy in the form of ATP.
Matrix: The gel-like material inside the mitochondria where many biochemical reactions take place.
Cristae: Folds of the inner mitochondrial membrane that increase the surface area for ATP production.

🔥 Mitochondrial Structure

Mitochondria have a distinctive structure with two membranes:

Outer membrane: Smooth and permeable to small molecules and ions.
Inner membrane: Folded into cristae, contains proteins involved in electron transport and ATP synthesis.
Intermembrane space: The area between the outer and inner membranes.
Matrix: The space inside the inner membrane, containing enzymes, mitochondrial DNA and ribosomes.

⚡ Energy Production

Mitochondria produce energy through cellular respiration, which involves:

Glycolysis: Occurs in the cytoplasm, breaking down glucose into pyruvate.
Link reaction: Converts pyruvate to acetyl-CoA.
Krebs cycle: Occurs in the matrix, generating reduced coenzymes.
Electron transport chain: Located on the cristae, produces most of the ATP.

Cellular Respiration: The Energy-Making Process

Cellular respiration is the process by which cells convert the energy in glucose into ATP. This process takes place mainly in the mitochondria and can be summarised by the equation:

Glucose + Oxygen → Carbon dioxide + Water + Energy (ATP)

C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + ATP

Stages of Cellular Respiration

Cellular respiration occurs in several stages, each producing different amounts of ATP:

🟢 Glycolysis

Takes place in the cytoplasm (outside the mitochondria)

Glucose (6C) is split into two pyruvate (3C) molecules
Produces 2 ATP molecules
Doesn't require oxygen (can occur anaerobically)

🟡 Krebs Cycle

Occurs in the mitochondrial matrix

Pyruvate is converted to acetyl-CoA (2C)
Acetyl-CoA enters the cycle and is broken down
Produces 2 ATP directly per glucose molecule
Generates reduced coenzymes (NADH and FADH₂)

🔴 Electron Transport Chain

Located on the cristae of the inner membrane

Electrons from NADH and FADH₂ pass through protein complexes
Energy released pumps H⁺ ions across the membrane
Creates a concentration gradient that drives ATP synthesis
Produces about 34 ATP per glucose molecule

ATP: The Energy Currency

ATP (adenosine triphosphate) is often called the "energy currency" of cells because it stores energy in a form that can be quickly used by the cell. When a cell needs energy, it breaks the bond between the second and third phosphate groups in ATP, releasing energy and forming ADP (adenosine diphosphate).

💰 ATP Structure and Function

ATP consists of:

An adenine base
A ribose sugar
Three phosphate groups

The energy is stored in the bonds between phosphate groups. When ATP is broken down to ADP + Pi (inorganic phosphate), energy is released that can power cellular processes.

🔋 ATP Recycling

ATP is constantly being recycled in cells:

ATP → ADP + Pi + energy (for cell processes)
ADP + Pi + energy (from cellular respiration) → ATP

A typical cell may recycle its entire ATP supply 1,000-1,500 times per day!

Structure-Function Relationship

The structure of mitochondria is perfectly adapted to its function as the cell's powerhouse:

🔍 Structural Adaptations

Cristae: The folded inner membrane increases surface area, allowing more electron transport chains to fit, maximising ATP production.
Double membrane: Creates compartments necessary for establishing the proton gradient that drives ATP synthesis.
Matrix: Contains enzymes needed for the Krebs cycle and other metabolic reactions.
Own DNA and ribosomes: Allows mitochondria to produce some of their own proteins, supporting the theory that they evolved from bacteria (endosymbiotic theory).

🔬 Mitochondrial Distribution

Cells with higher energy needs have more mitochondria:

Muscle cells: Contain many mitochondria to provide energy for contraction.
Liver cells: Rich in mitochondria to power detoxification processes.
Heart muscle cells: Can contain up to 40% of their volume as mitochondria.
Red blood cells: Lack mitochondria completely as they need to maximise space for haemoglobin.

Case Study Focus: Mitochondrial Disorders

Mitochondrial disorders occur when the mitochondria fail to produce enough energy for the cell or organ to function properly. These disorders can be inherited or acquired during a person's lifetime.

Example: MELAS Syndrome (Mitochondrial Encephalomyopathy, Lactic Acidosis and Stroke-like episodes)

Caused by mutations in mitochondrial DNA
Affects cells with high energy demands, particularly in the brain and muscles
Symptoms include stroke-like episodes, headaches, seizures and muscle weakness
Demonstrates the critical importance of properly functioning mitochondria for overall health

Practical Applications and Research

Understanding mitochondria has important applications in medicine, aging research and biotechnology:

💊 Medical Applications

Mitochondrial replacement therapy: A technique to prevent mitochondrial diseases being passed from mother to child.
Drug development: Targeting mitochondria for treatments of cancer, neurodegenerative diseases and metabolic disorders.
Diagnostic tools: Measuring mitochondrial function to assess overall health and disease progression.

📈 Current Research

Aging research: Investigating the role of mitochondrial damage in aging.
Exercise physiology: Studying how exercise increases mitochondrial number and efficiency.
Nutrition: Exploring how different diets affect mitochondrial function.
Neurodegenerative diseases: Understanding the role of mitochondrial dysfunction in conditions like Parkinson's and Alzheimer's.

Summary: Why Mitochondria Matter

Mitochondria are essential organelles that:

Produce most of the cell's ATP through cellular respiration
Have a unique structure with two membranes and cristae that maximise energy production
Contain their own DNA and can reproduce independently within the cell
Vary in number depending on the energy needs of different cell types
Play crucial roles in cell metabolism, signalling and even programmed cell death
Are implicated in numerous diseases when they malfunction

Understanding mitochondria and cellular energy production is fundamental to biology and has far-reaching implications for medicine, aging research and our understanding of life itself.

🧠 Test Your Knowledge!