Concept 6.5: Mitochondria and Chloroplasts Change Energy from One Form to Another

This section explores the structure and function of mitochondria, chloroplasts, and peroxisomes—organelles essential for energy conversion and metabolism in eukaryotic cells.

Mitochondria: Chemical Energy Conversion

Mitochondria are membrane-bound organelles found in nearly all eukaryotic cells. They are the primary sites of cellular respiration, a process that uses oxygen to extract energy from organic molecules and generate ATP, the cell's main energy currency.

• Definition: Mitochondria are organelles responsible for converting chemical energy from food into ATP through aerobic respiration.

• Presence and Quantity: Most eukaryotic cells contain mitochondria. The number of mitochondria correlates with the cell's energy demand; cells with high aerobic activity have more mitochondria.

• Size: Typically 1–10 μm in length.

• Dynamic Behavior: Mitochondria can move, change shape, fuse, and divide within the cell.

Structure of Mitochondria

• Double Membrane:

  • Outer membrane: Smooth and encloses the organelle.

  • Inner membrane: Highly folded into structures called cristae, increasing surface area for ATP production.

• Internal Compartments:

  • Intermembrane space: Narrow region between the outer and inner membranes.

  • Mitochondrial matrix: Fluid-filled space inside the inner membrane, containing mitochondrial DNA, ribosomes, and enzymes for cellular respiration.

Functional Specialization

• Enzymes in the matrix: Catalyze steps of the citric acid cycle (Krebs cycle).

• Proteins in the inner membrane: Include components of the electron transport chain and ATP synthase, essential for oxidative phosphorylation and ATP generation.

• Cristae: The folds of the inner membrane increase the surface area, enhancing the organelle's capacity for ATP production.

Example: ATP Production in Mitochondria

• During cellular respiration, glucose and other fuels are oxidized, and the released energy is used to synthesize ATP.

• Key equation:

Chloroplasts: Capture of Light Energy

Chloroplasts are organelles found in plants and algae. They are the sites of photosynthesis, where solar energy is converted into chemical energy stored in sugars.

• Definition: Chloroplasts are organelles that use sunlight to synthesize organic molecules from carbon dioxide and water.

• Location: Found in leaves and other green tissues of plants and algae.

• Size: Typically 3–6 μm in diameter.

• Pigment: Contain chlorophyll, the green pigment essential for capturing light energy.

Internal Structure of Chloroplasts

• Double Membrane: Encloses the organelle.

• Stroma: Fluid-filled space inside the inner membrane, containing chloroplast DNA, ribosomes, and enzymes.

• Thylakoids: Flattened membranous sacs where the light-dependent reactions of photosynthesis occur.

• Grana: Stacks of thylakoids, resembling stacks of poker chips, increase the surface area for light absorption.

Functional Compartments of Chloroplasts

• Intermembrane space

• Stroma

• Thylakoid space

Dynamic Nature of Chloroplasts

• Chloroplasts are dynamic, mobile, and can change shape. They move along the cytoskeleton within plant cells.

Example: Photosynthesis in Chloroplasts

• Chloroplasts use sunlight to convert CO2 and H2O into glucose and oxygen.

• Key equation:

Plastids: The Family of Chloroplasts

Chloroplasts belong to the plastid family, a group of plant organelles with specialized functions.

• Amyloplasts: Colorless plastids that store starch, especially in roots and tubers.

• Chromoplasts: Plastids that store pigments, giving color to fruits and flowers.

Peroxisomes: Oxidation and Detoxification

Peroxisomes are small, membrane-bound organelles involved in various oxidative reactions. They play a key role in breaking down fatty acids and detoxifying harmful substances.

• Definition: Peroxisomes are single-membrane organelles containing enzymes that transfer hydrogen from substrates to oxygen, producing hydrogen peroxide (H2O2).

• Functions:

  • Break down fatty acids into smaller molecules for use in cellular respiration.

  • Detoxify alcohol and other harmful compounds, especially in liver cells.

  • Contain specialized forms, such as glyoxysomes in plant seeds, which convert fatty acids to sugars for seedling growth.

• Key Reaction: Hydrogen peroxide is then broken down by catalase:

Comparison Table: Mitochondria, Chloroplasts, and Peroxisomes

Key Terms and Definitions

• ATP (Adenosine Triphosphate): The primary energy carrier in cells.

• Cellular Respiration: Metabolic process that converts biochemical energy from nutrients into ATP.

• Photosynthesis: Process by which plants and algae convert light energy into chemical energy.

• Plastids: Family of plant organelles including chloroplasts, amyloplasts, and chromoplasts.

• Peroxisome: Organelle involved in oxidation reactions and detoxification.

• Cristae: Infoldings of the inner mitochondrial membrane that increase surface area for ATP production.

• Stroma: Fluid-filled space in chloroplasts surrounding the thylakoids.

• Thylakoid: Membranous sac in chloroplasts where light-dependent reactions occur.

• Grana: Stacks of thylakoids within chloroplasts.

Example Exam Questions

• What are the infoldings of the mitochondrial inner membrane called? Answer: Cristae

• Which structure is common to plant and animal cells? Answer: Mitochondrion

• Cyanide binds to at least one molecule involved in producing ATP. In a cell exposed to cyanide, most of the cyanide will be in: Answer: Mitochondria

• What pigment is found in chloroplast? Answer: Chlorophyll

• Which of the following is a common trait of chloroplasts and mitochondria? Answer: Both contain their own DNA

• Peroxisomes in liver cells detoxify alcohol by removing hydrogen atoms and combining them with which of the following molecules? Answer: With oxygen molecules to generate hydrogen peroxide

• Which cell organelle can break down fatty acids into sugar in plants? Answer: Glyoxysomes