Cell Organelles

Organelles (from Latin: "little organs") are specialised structures inside eukaryotic cells, each enclosed by its own membrane and each performing a specific function. Think of them like the organs of the body — but on a microscopic scale inside every cell. Just as the body needs a brain, heart, lungs, and liver all working together, a cell needs its nucleus, mitochondria, ribosomes, and ER all doing their jobs simultaneously. The existence of membrane-bound organelles is the defining feature of eukaryotic cells (prokaryotic bacteria do NOT have them). This compartmentalisation allows different chemical reactions to happen simultaneously in the same cell without interfering with each other — a remarkable organisational achievement.

The nucleus is the largest organelle in most cells and the control centre — it houses the cell's DNA and directs all cellular activity. Structure: - Surrounded by a double membrane called the nuclear envelope - The nuclear envelope has nuclear pores — channels that allow molecules to pass in and out (mRNA leaving, proteins entering) - Inside is the nucleoplasm (similar to cytoplasm but inside the nucleus) - The nucleolus is a dense region inside the nucleus where ribosomes are assembled What it does: The nucleus contains all the cell's DNA, organised into chromosomes. Human cells have 46 chromosomes (23 pairs) — about 3 billion base pairs of DNA total. This DNA holds the instructions for making every protein in the body. When a cell needs a protein, the relevant gene is transcribed into mRNA (messenger RNA) in the nucleus. The mRNA leaves through nuclear pores and travels to ribosomes where the protein is built. This is the central dogma: DNA → RNA → Protein. Clinical relevance: Many cancer drugs target the nucleus — interfering with DNA replication (chemotherapy agents like cyclophosphamide) or blocking enzymes involved in DNA synthesis (methotrexate). These drugs preferentially affect rapidly dividing cells (like cancer), but also affect normal dividing cells (which is why chemotherapy causes hair loss and gut problems — hair follicle cells and gut lining cells divide rapidly).

Mitochondria (singular: mitochondrion) are the energy factories of the cell. They produce ATP (adenosine triphosphate) — the universal energy currency — through a process called cellular respiration. Structure: - Two membranes: a smooth outer membrane and a folded inner membrane - The inner membrane folds are called cristae — these folds dramatically increase surface area (more surface = more ATP production) - The space inside the inner membrane is the matrix — this is where the Krebs cycle takes place - Mitochondria have their own DNA (mitochondrial DNA, mtDNA) — separate from nuclear DNA Why their own DNA? The endosymbiotic theory explains this: about 1.5 billion years ago, an early cell engulfed a bacterium but didn't digest it. The bacterium survived inside, providing energy in exchange for protection. Over generations, they became inseparable — the bacterium became the mitochondrion. This explains why mitochondria have bacterial-shaped double membranes and their own DNA. Mitochondrial DNA is inherited maternally — you get all your mitochondria from your mother's egg. This is used in forensic DNA testing and tracing human migration history. Mitochondria in disease: Cells with the highest energy demands have the most mitochondria — heart muscle cells, liver cells, and neurons. When mitochondria fail: - Heart failure — cardiac muscle cells lose ATP, cannot contract effectively - Neurodegeneration — neurons starve of energy; seen in Parkinson's and Alzheimer's - Cyanide poisoning — blocks the ETC, stopping all mitochondrial ATP production within minutes - Some inherited conditions (mitochondrial diseases) affect the mitochondria's own DNA, causing muscle weakness, nerve problems, and metabolic disorders

Ribosomes are tiny, non-membrane-bound molecular machines that build proteins. They read mRNA (a copy of the DNA instructions) and link amino acids together in the correct order — one protein at a time. Ribosomes are found in ALL cells (prokaryotic and eukaryotic), which is why they are one of the four universal cell features. The Endoplasmic Reticulum (ER) is a vast network of interconnected membranes extending from the nuclear envelope throughout the cytoplasm — like a cell-wide road system. There are two types: Rough ER: - Studded with ribosomes on its outer surface (giving it a "rough" appearance under the microscope) - The ribosomes here make proteins that are destined to be secreted outside the cell, inserted into the membrane, or sent to organelles - As proteins are made, they are threaded into the ER where they fold properly with the help of chaperone proteins - Misfolded proteins are detected and destroyed — quality control Smooth ER: - No ribosomes — hence "smooth" - Makes lipids (phospholipids and steroids) - Metabolises and detoxifies drugs and toxins (especially important in liver cells — liver cells are packed with smooth ER) - Stores calcium ions (Ca²⁺) — important for muscle contraction signals Clinical relevance: - The unfolded protein response (UPR) — when too many proteins misfold in the ER (due to stress or disease), the cell triggers protective responses. In some diseases (like type 2 diabetes and neurodegeneration), chronic ER stress contributes to cell death. - Many drugs are metabolised by enzymes in the liver's smooth ER — this is why some drugs interact with each other (they compete for the same detoxifying enzymes, like the cytochrome P450 system).

The Golgi Apparatus The Golgi is a stack of flattened membrane sacs — often described as the cell's "post office." It receives proteins from the rough ER, processes and modifies them (adding sugar molecules, cutting and reassembling chains), then packages them into vesicles and ships them to their final destination: the cell surface, a lysosome, or outside the cell (secretion). Clinical connection: Cells that secrete a lot (pancreatic cells making digestive enzymes, goblet cells making mucus, antibody-producing plasma cells) have very prominent Golgi apparatus — they need maximum shipping capacity. Lysosomes Lysosomes are membrane-bound sacs filled with about 50 types of digestive enzymes. They are the cell's recycling and disposal system. They digest: - Old, worn-out organelles (autophagy — "self-eating" — the cell recycles its own parts) - Bacteria and viruses engulfed by endocytosis - Cellular waste and debris The interior of a lysosome is acidic (pH ~5) — the enzymes only work at this low pH, which is a safety mechanism. If a lysosome ruptures, the enzymes are inactive at the cell's normal pH (7.2) and cannot digest the cell itself. Lysosomal storage diseases — rare inherited conditions where a specific enzyme is missing. Without it, the substance the enzyme normally breaks down accumulates. Examples: Tay-Sachs disease (GM2 ganglioside accumulates in neurons → neurodegeneration), Gaucher disease (glucocerebroside accumulates). Other important organelles: - Peroxisomes — break down fatty acids and neutralise toxic hydrogen peroxide (using catalase enzyme) - Cytoskeleton — not an organelle but a network of protein filaments (microfilaments, intermediate filaments, microtubules) that give the cell its shape, allow movement, and act as internal highways for transport - Centrosome — organises the spindle fibres that pull chromosomes apart during cell division