Cellular Organelles: Functions and Processes
The Endoplasmic Reticulum
The endoplasmic reticulum (ER) is a network of tubules and flattened sacs. Its membrane constitutes about 50% of all cell membranes, and it is continuous with the outer nuclear envelope. There are two types:
- Smooth Endoplasmic Reticulum (SER): Lacks ribosomes.
- Rough Endoplasmic Reticulum (RER): Has ribosomes attached to its cytosolic side.
Functions of the Smooth Endoplasmic Reticulum (SER)
- Lipid Synthesis: Manufactures lipids (phospholipids and cholesterol) and their derivatives, such as steroid hormones and bile acids.
- Detoxification: Detoxifies soluble substances in liver cells. Detoxifying enzymes convert these toxic substances into insoluble and soluble substances to be excreted in the urine.
- Calcium Regulation: Contains ATP-dependent calcium pumps that regulate processes like secretion, cell proliferation, and muscle contraction.
Functions of the Rough Endoplasmic Reticulum (RER)
- Synthesizes proteins and their derivatives (phospholipids and folding) using its attached enzymes.
The Golgi Apparatus
The Golgi apparatus consists of flattened sacs with dilated ends, called dictyosomes. It contains a large number of vesicles of various sizes. The Golgi has three distinct functional regions: the cis face, the medial region, and the trans face. Proteins and lipids synthesized in the ER are transported in vesicles to the Golgi.
Functions of the Golgi Apparatus
- Completes the glycosylation of proteins.
- Synthesizes glycolipids and sphingomyelin.
- In plant cells, it synthesizes complex polysaccharides of the cell wall (hemicellulose and pectin).
- Secretion: Manages the distribution and export of proteins to lysosomes, the plasma membrane, or outside the cell.
Constitutive Secretion Pathway
Occurs in all cells. Renews components of the plasma membrane and extracellular matrix.
Regulated Secretion Pathway
Exists only in specialized secretory cells, such as endocrine cells. Requires a regulatory signal or stimulus. Used to secrete hormones, neurotransmitters, and digestive enzymes.
Lysosomes: The Cell’s “Garbage Cans”
Lysosomes are a set of hydrolytic enzymes used for intracellular digestion. These enzymes are acid hydrolases that require a low pH. Lysosomes possess proton pumps and require ATP to pump H+ into their interior. The lysosome membrane also contains transport proteins.
Types of Lysosomes
- Primary Lysosomes (Lys 1st): Newly formed, containing enzymes but no active intracellular digestion. They are small.
- Secondary Lysosomes (Lys 2nd): Larger, actively digesting material. They form when primary lysosomes fuse with vesicles containing material to digest.
- Residual Bodies: Lysosomes that have finished digestion.
Autophagy
Recycling of the cell’s own organelles.
Heterophagy
Digestion of external materials.
Mitochondria: The Powerhouses of the Cell
Mitochondria are characteristic organelles of eukaryotic aerobic cells that produce energy in the form of ATP. They are the site of specific cellular respiration reactions. Mitochondria have two membranes: an outer membrane and an inner membrane.
- Outer Membrane: Contains porins, proteins that form channels allowing the passage of small molecules.
- Inner Membrane: Has a higher protein content than any other cell membrane. The intermembrane space has a low concentration of cardiolipin, which produces ATP synthase.
- Matrix: Contains multiple copies of a double-stranded, circular DNA molecule, as well as ribosomes.
Mitochondrial Physiology
Mitochondria carry out complex oxidation reactions involved in aerobic respiration, producing carbon dioxide. This process generates energy used for ATP synthesis.
- Matrix: Contains enzymes that catalyze oxidation processes, such as the Krebs cycle, which forms CO2.
- Inner Membrane: Site of the electron transport chain.
- Respiratory Chain: Collects electrons from oxidation reactions and transports them to their final acceptor. This electron transport chain releases energy, generating a proton flow from the matrix to the intermembrane space.
- Oxidative Phosphorylation: The process that produces ATP from ADP and phosphate, using the concentration gradient generated by the proton flow in the ATP synthase complex.
Chloroplasts: The Photosynthetic Organelles
Chloroplasts are organelles found in plant cells that contain chlorophyll. They have three membranes: an outer membrane, an inner membrane, and the thylakoid membrane. The space between the outer and inner membranes is the intermembrane space. The stroma contains ribosomes, enzymes, DNA, and RNA molecules. The thylakoid space is enclosed by the thylakoid membrane.
Chloroplast Physiology
Photosynthesis takes place in chloroplasts. The processes of photosynthesis are:
- Thylakoid Membrane: Site of light-dependent reactions. These reactions begin with the capture of solar energy by photosystems and the photolysis of water molecules, providing electrons. The electron transport chain in this membrane uses electrons from water to reduce NADP+ to NADPH. Electron transport generates an electrochemical proton gradient from the stroma into the thylakoid space, driving ATP synthesis from ADP. This process, known as photophosphorylation, occurs in the ATP synthase complex of the thylakoid membrane.
- Stroma: Site of light-independent reactions (dark phase). ATP and NADPH are used to reduce simple molecules like carbon dioxide and produce organic molecules (glucose).