AP Syllabus focus:
‘Mitochondria’s inner membrane and folded cristae support coordinated, enzyme-catalyzed reactions that capture energy from macromolecules.’
Mitochondria are specialized organelles that compartmentalise respiration so energy stored in food molecules can be efficiently transferred to ATP. Their membrane architecture creates distinct spaces that localise enzymes, substrates, and gradients.
Core Mitochondrial Architecture
Outer membrane: boundary and exchange
Outer mitochondrial membrane forms a boundary with the cytosol and contains transport proteins that allow many small molecules to pass.
This membrane helps regulate which metabolites can access mitochondrial compartments needed for respiration.
Inner membrane: the key respiratory surface
Inner mitochondrial membrane is selectively permeable and densely packed with proteins that carry out energy-capturing steps of respiration.
Extensive folding into cristae increases membrane surface area, allowing more membrane-bound enzyme complexes to operate simultaneously.
Schematic of mitochondrial structure highlighting the outer membrane, inner membrane, cristae (inner-membrane folds), and the matrix. Use it to connect the idea that cristae increase inner-membrane surface area with where respiration-related protein complexes are embedded. Source
Cristae: Infoldings of the inner mitochondrial membrane that increase surface area for membrane-associated reactions of cellular respiration.
Intermembrane space and matrix: separate chemical environments
Intermembrane space lies between the outer and inner membranes and acts as a distinct compartment where ions can be differentially concentrated.
Mitochondrial matrix is the internal fluid region containing enzymes and dissolved substrates required for several respiration reactions, plus mitochondrial DNA and ribosomes.
How Structure Supports “Coordinated, Enzyme-Catalyzed Reactions”
Compartmentalisation improves efficiency
Mitochondrial compartments keep sequential reactions near one another and maintain optimal local conditions (pH, ion concentrations, substrate availability) for enzyme-catalysed steps.
Enzymes in the matrix catalyse reactions that extract high-energy electrons from carbon-based molecules.
Protein complexes embedded in the inner membrane catalyse reactions that transfer energy through controlled redox steps.
Cristae increase throughput of energy capture
Because crucial respiration proteins are located in the inner membrane, cristae enable:
Higher density of membrane-bound enzymes per mitochondrion
Shorter diffusion distances between related enzyme complexes
Greater capacity to convert energy released from macromolecules into a form usable for ATP synthesis
Membrane organisation enables coupling of reactions
The inner membrane’s impermeability to many ions helps create separations in ion concentration between compartments.
Diagram of oxidative phosphorylation in the inner mitochondrial membrane: electron flow through complexes I–IV drives H+ pumping into the intermembrane space, creating an electrochemical gradient. ATP synthase then uses H+ flow back into the matrix to power ATP formation from ADP + Pi. Source
This separation allows mitochondria to link energy-releasing electron transfers to ATP formation by:
Maintaining distinct chemical conditions on each side of the inner membrane
Positioning enzyme complexes so products of one step become reactants for the next with minimal loss of usable energy
Mitochondria and Energy from Macromolecules
Respiring cells derive energy by breaking down macromolecules (especially carbohydrates and fats) into smaller molecules that can be oxidised.
Carbon-containing substrates are converted into intermediates that enter mitochondrial pathways.
Electron carriers are reduced during mitochondrial reactions and later used by inner-membrane protein complexes to drive ATP production.
Mitochondrial Variability Reflects Energy Demand
Mitochondrial structure correlates with cellular ATP needs.
Cells with high energy demand (e.g., muscle, liver) typically have more mitochondria and/or more developed cristae.
Changes in cristae abundance and organisation can alter the cell’s capacity for enzyme-catalysed respiration reactions without changing the basic pathway steps.
FAQ
Mitochondrial DNA encodes a small subset of respiratory proteins and RNAs.
Most respiration proteins are nuclear-encoded, synthesised in the cytosol, and imported into mitochondria.
They can increase inner membrane surface area by altering cristae abundance.
They can also increase the concentration of specific inner-membrane protein complexes.
The outer membrane is relatively permissive, but the inner membrane requires specific transporters.
Transport specificity helps control respiration rate by regulating substrate entry.
It is highly protein-rich and selectively permeable.
Its architecture concentrates the machinery that converts energy from substrates into ATP.
High-demand tissues often show greater mitochondrial density and more elaborate cristae.
Lower-demand tissues may have fewer mitochondria with less extensive inner-membrane folding.
Practice Questions
State two structural features of mitochondria and, for each, describe how it supports cellular respiration. (1–3 marks)
Outer membrane/transport proteins allow exchange of metabolites with cytosol (1)
Inner membrane contains respiration enzymes/protein complexes (1)
Cristae increase inner membrane surface area to increase capacity for respiration reactions (1) (any two features with correct role, max 3)
Explain how the compartmentalisation of the mitochondrion supports efficient ATP production during respiration. Refer to the inner membrane, cristae, matrix, and intermembrane space. (4–6 marks)
Identifies matrix as location containing enzymes for respiration reactions producing reduced carriers (1)
Identifies inner membrane as site of membrane-bound enzyme complexes involved in energy capture (1)
Explains cristae increase inner membrane surface area, increasing number of enzyme complexes/throughput (1)
Explains intermembrane space is a separate compartment enabling different ion concentrations from matrix (1)
Links inner membrane selective permeability to maintaining compartment differences that support ATP production (1)
Describes coordination/short diffusion distances between sequential steps due to organisation (1)
