4.1 Cell Organelles
Key Takeaways
The nucleus stores most eukaryotic DNA, and the nucleolus inside it makes ribosomal RNA; the nucleolus is not a separate membrane-bounded organelle.
Ribosomes synthesize protein and have no membrane; free ribosomes usually make cytosolic proteins, while bound ribosomes on the rough ER usually make secreted and membrane proteins.
Rough ER folds secreted and membrane proteins; smooth ER makes lipids, helps detoxify some compounds, and stores calcium; the Golgi modifies, sorts, and ships.
Lysosomes hold hydrolytic enzymes for digestion and autophagy, and the plant central vacuole stores water and maintains turgor.
Mitochondria and chloroplasts have double membranes and their own DNA and descended from free-living prokaryotes; the cytoskeleton uses microtubules, microfilaments, and intermediate filaments, cilia and flagella are microtubule structures, and animal cells lack a cell wall.
4.1 Cell Organelles
A eukaryotic cell separates incompatible chemistry by housing it in organelles. An organelle is a subcellular structure with a defined job. A surrounding membrane lets the cell keep particular enzymes, and often a particular pH, inside that compartment. The useful question is where a process happens and what fails if that structure is absent.
Genetic instructions and protein synthesis
The nucleus and the nucleolus
The nucleus holds most of a eukaryotic cell's DNA. DNA is transcribed into RNA, and messenger RNA exits through pores in the nuclear envelope, a double membrane. Inside the nucleus, the nucleolus is a dense region that makes ribosomal RNA and starts the assembly of ribosomal subunits. The nucleolus is not a separate membrane-bounded organelle. It is a region of the nucleus organized around the genes for ribosomal RNA.
Free ribosomes and bound ribosomes
Ribosomes carry out protein synthesis by reading messenger RNA and joining amino acids. Each ribosome is built from ribosomal RNA and protein and has no membrane, so a ribosome is not a membrane-bounded organelle. Free ribosomes sit in the cytosol and usually make proteins that remain there. Bound ribosomes attach to the rough endoplasmic reticulum and usually make proteins that will be secreted, inserted into membranes, or sent through the endomembrane system.
Manufacturing, shipping, and recycling
Rough ER and smooth ER
The rough endoplasmic reticulum is a membrane network studded with bound ribosomes. Secreted proteins and membrane proteins enter it as they are made, then fold, and many receive carbohydrate groups. The smooth endoplasmic reticulum lacks ribosomes. It synthesizes lipids, including phospholipids and steroids; it helps liver cells detoxify certain drugs and poisons; and it stores calcium ions that muscle cells release to start contraction.
The Golgi apparatus
The Golgi apparatus is a stack of flattened membrane sacs. Vesicles from the ER arrive, and the Golgi modifies the cargo, sorts it by destination, and ships it in new vesicles. Some vesicles release proteins outside the cell. Some deliver proteins to the plasma membrane. Some become lysosomes. The rough ER makes the protein, and the Golgi ships it.
Lysosomes and vacuoles
Lysosomes are sacs of hydrolytic enzymes that break down proteins, lipids, nucleic acids, and carbohydrates. The lysosomal membrane keeps those enzymes in an acidic interior. Lysosomes digest material taken in from outside the cell. They also perform autophagy, recycling the cell's own worn-out organelles by enclosing them and breaking them down. Vacuoles are membrane-bounded storage compartments. In a mature plant cell, the central vacuole can occupy most of the volume. It stores water, ions, pigments, and some defensive compounds, and the water it holds maintains turgor pressure against the cell wall.
Organelles that convert energy
Mitochondria are the main sites of cellular respiration, which harvests energy from fuel and stores much of it in ATP. A mitochondrion has a double membrane: a smooth outer membrane and an inner membrane folded into cristae that add surface for ATP production. The matrix contains respiratory enzymes plus the mitochondrion's own DNA and ribosomes. Chloroplasts carry out photosynthesis in plants and algae, using light energy to build sugars. A chloroplast also has a double membrane and its own DNA. Animal cells have mitochondria and do not have chloroplasts. A plant cell typically has both.
Shape, movement, and the wall outside the membrane
The cytoskeleton is a protein-fiber network that supports the cell, moves organelles, and helps the cell change shape. Microtubules are hollow tubulin tubes. They act as tracks for moving cargo, shape the cell, and form the spindle. Microfilaments, or actin filaments, bear tension and help a cell crawl or pinch. Intermediate filaments are rope-like fibers that give mechanical strength, including keratin in animal skin. Cilia and flagella are microtubule structures that bend to move fluid or the cell. Cilia are short and numerous; flagella are longer and fewer. Eukaryotic cilia and flagella share an internal ring of microtubules. Bacterial flagella are a different structure and are not built from microtubules.
A cell wall lies outside the plasma membrane in plants, fungi, and many protists. It is a rigid extracellular layer. Plant walls are made mainly of cellulose. Fungal walls are made mainly of chitin. Protist walls vary. Animal cells lack a cell wall; the plasma membrane is their outer boundary, often reinforced by an extracellular matrix. The plant wall works with the central vacuole: turgor pushes outward, and the wall keeps the cell from bursting.
Why mitochondria and chloroplasts resemble prokaryotes
The endosymbiotic theory holds that mitochondria and chloroplasts descended from free-living prokaryotes that took up residence inside a host cell. The evidence is structural and genetic. Each organelle has its own circular DNA. Each has a double membrane. Each contains ribosomes similar to prokaryotic 70S ribosomes rather than the 80S ribosomes of the eukaryotic cytosol. In the standard introductory account, mitochondria came from an aerobic bacterium and chloroplasts from a photosynthetic cyanobacterium. The nucleus and the rest of the endomembrane system do not share this package of traits.
| Structure | Membrane boundary | Main function |
|---|---|---|
| Nucleus | Double membrane | Stores DNA; the nucleolus inside it makes ribosomal RNA |
| Ribosome | None | Synthesizes protein; free in the cytosol or bound to rough ER |
| Rough ER | Membrane network | Folds secreted proteins and membrane proteins |
| Smooth ER | Membrane network | Makes lipids, helps detoxify some compounds, stores calcium |
| Golgi apparatus | Flattened membrane sacs | Modifies, sorts, and ships proteins and lipids |
| Lysosome | Single membrane | Digests macromolecules and takes part in autophagy |
| Central vacuole | Single membrane | Stores water in plant cells and maintains turgor |
| Mitochondrion | Double membrane | Site of cellular respiration; has its own DNA |
| Chloroplast | Double membrane | Site of photosynthesis in plants and algae; has its own DNA |
Warning
Ribosomes make protein, but they are not surrounded by a membrane. The nucleolus makes ribosomal RNA, but it is a region of the nucleus, not an organelle with a membrane of its own.
Which structure synthesizes protein and is not surrounded by a membrane?
The ribosome, the complex of ribosomal RNA and protein that joins amino acids
The lysosome, the sac of hydrolytic enzymes used in autophagy
The mitochondrion, the double-membrane site of cellular respiration
The nucleus, which holds most of the cell's DNA
Which job belongs to the nucleolus?
Making ribosomal RNA inside the nucleus
Modifying, sorting, and shipping proteins that arrived from the endoplasmic reticulum
Digesting worn-out mitochondria during autophagy
Storing water and maintaining turgor pressure in a plant cell
Which set of features is the usual evidence for the endosymbiotic origin of mitochondria and chloroplasts?
A cellulose wall, chitin in the same organelle, and only 80S cytosolic ribosomes
A single membrane, no DNA, and a role limited to lipid synthesis
Identity with the nucleolus, treated as a separate membrane-bounded organelle
Their own circular DNA, a double membrane, and ribosomes similar to prokaryotic ribosomes
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