Mitochondria and Plastids: Structure and Cellular Roles
Mitochondria: Structure and Functions
Mitochondrial Structure
Mitochondrial Matrix
Contains a semi-liquid material with the consistency of a gel. It includes:
- Mitochondrial DNA molecules: In most mammalian cells, this is circular, double-stranded DNA, distinct from nuclear DNA.
- Mitochondrial RNA molecules.
- Enzymes for mitochondrial DNA replication, transcription, and translation.
- Enzymes involved in the Krebs cycle and the beta-oxidation of fatty acids.
- Ions of Calcium (Ca), Phosphorus (P), and ribonucleoproteins.
Intermembrane Space
Located between the outer and inner membranes, containing enzymes that use ATP to phosphorylate AMP or other nucleotides.
Inner Mitochondrial Membrane
Features folds extending into the matrix called cristae. It contains approximately 20% lipids and 80% protein.
Outer Mitochondrial Membrane
Defines the boundary of the mitochondrion. Its structure is a double layer of lipids with associated proteins. It contains approximately 40% lipids (including cholesterol) and 60% protein.
Elementary Particles (F1 Particles)
Located on the inner side of the cristae, oriented towards the matrix. They consist of a spherical head or F1 complex (a globular protein), a stalk, and a hydrophilic base. These are involved in ATP synthesis.
Mitochondrial Functions
Mitochondria possess different enzyme complexes because each performs distinct functions:
Krebs Cycle (Citric Acid Cycle)
Takes place in the mitochondrial matrix and is a crucial process for cellular catabolism.
Respiratory Chain (Electron Transport Chain)
Electrons released during the Krebs cycle are passed along transporters located in the inner mitochondrial membrane, organized into complexes.
Oxidative Phosphorylation
Performed by the F1 particles on the mitochondrial cristae. The F1 head component phosphorylates ADP to form ATP.
Beta-oxidation of Fatty Acids
The enzymes responsible for this process are located in the mitochondrial matrix.
Concentration of Substances
Mitochondria concentrate various substances in their internal compartments, such as proteins, iron, dyes, lipids, etc.
Plastids: Plant Cell Organelles
Plastids are organelles unique to plant cells. They possess pigments (like chlorophylls and carotenoids) and synthesize and accumulate reserve substances (such as starch, oil, etc.). They are classified into two main groups:
Types of Plastids
- Leucoplasts: Lack pigments and store substances such as starch, fats, and proteins. They are found in non-photosynthetic plant tissues like roots and stem cells.
- Chromoplasts: Contain pigments that impart color. Those containing chlorophyll are green and called chloroplasts. Those containing phycoerythrin are red and called rhodoplasts.
Chloroplasts: Photosynthesis Hubs
Chloroplasts are biologically the most important plastids because they perform photosynthesis, transforming light energy into chemical energy. Their morphology varies; in higher plants, they are usually oval-shaped, but in some algae, they exhibit different forms (e.g., helical, cup-shaped). Each plant cell typically contains 20 to 40 chloroplasts, although extreme cases exist (e.g., Ricinus cells with up to 400,000 per mm²). Their size varies among species. They are located in the cytoplasm, though without a fixed position, moving with cytoplasmic streaming (cyclosis) and capable of amoeboid movement.
Chloroplast Structure
Chloroplasts are formed by a double membrane, an intermembrane space, and the stroma, within which lie the thylakoids.
Inner and Outer Membrane
Similar in structure to other cellular membranes. The outer membrane has higher permeability, while the inner membrane is almost impermeable.
Thylakoids
These are flattened sacs stacked upon each other, forming a membranous network. Each stack is called a granum (plural: grana). The space connecting different grana is called the intergrana or stroma lamellae. The light-dependent reactions of photosynthesis occur here.
Stroma
The fluid-filled space within the inner membrane. It contains a circular, double-stranded DNA molecule and ribosomes called plastoribosomes. The light-independent reactions (Calvin cycle) of photosynthesis are performed here.
Chloroplast Functions
Photosynthesis
Chloroplasts are responsible for carrying out photosynthesis, producing ATP and NADPH during the light-dependent reactions, which are then used to synthesize sugars in the Calvin cycle.
Fatty Acid Biosynthesis
Chloroplasts synthesize fatty acids using carbohydrates, NADPH, and ATP generated during photosynthesis.
Nitrate Reduction
They reduce nitrate to nitrite, and subsequently to ammonia, which serves as the nitrogen source for forming amino acids and nucleotides.