Summary
Highlights
The nucleus, often called the 'brain of the cell,' is central to a cell's identity and function. It's enclosed by a double-layered nuclear envelope, featuring an outer layer with ribosomes for mRNA binding and an inner layer lined with lamins, crucial for nuclear structure and cell division. Nuclear pores regulate transport in and out of the nucleus. Inside, the nucleolus synthesizes ribosomal RNA (rRNA) and proteins to form ribosomes. Chromatin, composed of DNA and histone proteins, exists as euchromatin (loose, for gene expression) or heterochromatin (tight, near the inner membrane). The nucleus is vital for DNA replication and transcription, producing various RNA types (tRNA, mRNA, rRNA).
The endoplasmic reticulum comes in two forms: rough ER and smooth ER. The rough ER is studded with ribosomes, making it a primary site for protein synthesis, especially for proteins destined for lysosomes, membranes, or excretion. It also aids in protein folding and 'N-type' glycosylation (adding sugar residues). The smooth ER, lacking ribosomes, is crucial for lipid synthesis (fatty acids, phospholipids, cholesterol), detoxification (via CYP450 enzymes in organs like the liver), glucose-6-phosphate metabolism, and calcium storage. Both ER types package their products into vesicles that can be sent to the Golgi apparatus.
The Golgi apparatus acts as a processing and packaging center. It receives vesicles containing proteins and lipids from the rough and smooth ER at its 'cis' face. Within the Golgi, these molecules undergo further modification, including 'O-type' glycosylation (unique to the Golgi) and phosphorylation (a crucial reaction, implicated in diseases like I-cell disease). Finally, the Golgi packages these modified molecules into new vesicles at its 'trans' face, dispatching them to their final destinations, such as lysosomes, cell membranes, or for excretion from the cell.
The cell membrane serves as a selectively permeable barrier, regulating what enters and exits the cell. It's primarily composed of a phospholipid bilayer, with polar (hydrophilic) heads facing outwards and inwards, and non-polar (hydrophobic) fatty acid tails facing each other. Cholesterol, synthesized by the smooth ER, is embedded within the bilayer and controls membrane fluidity. Various integral and peripheral proteins within the membrane perform diverse functions like transport, enzymatic activity, and cell-to-cell linking. The membrane facilitates simple diffusion, facilitated diffusion, and vesicular transport.
Lysosomes are spherical organelles containing hydrolytic enzymes (proteases, nucleases, lipases, glucosidases) that break down macromolecules like proteins, nucleic acids, lipids, and carbohydrates. They are essential for degrading substances brought into the cell, recycling worn-out organelles through a process called autophagy, and facilitating autolysis (self-destruction) of severely damaged cells by releasing their enzymes.
Peroxisomes are spherical organelles rich in enzymes like catalase and oxidase. They play a vital role in detoxifying harmful free radicals by converting hydrogen peroxide into water and oxygen. Peroxisomes are also involved in fatty acid oxidation (both alpha and beta types, breaking down long-chain fatty acids into acetyl-CoA), lipid synthesis (including plasmalogen, important for brain white matter), cholesterol synthesis, and a small amount of ethanol metabolism.
Mitochondria are often called the 'powerhouse of the cell' due to their role in ATP synthesis. They have an outer membrane (smooth and highly permeable) and a folded inner membrane called cristae (less permeable). The mitochondrial matrix, the innermost compartment, contains enzymes for metabolic reactions and mitochondrial DNA (inherited from the mother). ATP is primarily generated via oxidative phosphorylation, a process driven by the electron transport chain located on the inner membrane. Other crucial metabolic pathways occurring in the mitochondria include the Krebs cycle, heme synthesis, the urea cycle, gluconeogenesis, and ketogenesis.
Ribosomes are responsible for protein synthesis (translation) from mRNA. They consist of a large (60S in eukaryotes) and a small (40S in eukaryotes) subunit, composed of rRNA and proteins. Ribosomes exist in two locations: membrane-bound on the rough ER, where they synthesize proteins for lysosomes, membranes, or secretion; and free-floating in the cytosol (cytosolic ribosomes), producing proteins that remain within the cell, such as enzymes for metabolic pathways.
The cytoskeleton provides structural support and facilitates movement within the cell. It comprises three main types of filaments: microfilaments (actin), intermediate filaments, and microtubules. Microfilaments (actin) are involved in muscle contraction (with myosin), cytokinesis during cell division (forming a constriction ring), and cell shape changes necessary for processes like diapedesis (white blood cell movement) and phagocytosis. Intermediate filaments are tough, high-tensile structures that act as anchors, connecting cells to each other, to the extracellular matrix, and stabilizing organelles within the cell. Microtubules, composed of alpha and beta tubulin, serve as intracellular transport railways, using motor proteins like dynein and kinesin (an ATP-dependent process) to move organelles and vesicles. They are also crucial for cell division (separating chromatids) and form the structural basis of cellular extensions like cilia and flagella, enabling beating or twisting motions for various functions.