Lesson 1.6 · Unit 1 · HS-LS1-1, HS-LS1-2
The organelles, read as job assignments rather than a list
A cell has to make its own materials, package them, power itself, dispose of waste, hold its shape and move. Those are separate problems, and a cell solves them with separate structures. Learn the organelles as an answer to "who does what job here" and the list stops being a list.
- The plasma membrane is a phospholipid bilayer with embedded proteins. Phospholipid tails face inward away from water and heads face outward, which is why the membrane assembles itself. The proteins do the selective work: channels, carriers, pumps, receptors and identity markers.
- The nucleus holds the instructions and controls the cell. DNA stays inside, RNA carries copies out through nuclear pores, and the nucleolus builds ribosomes. A red blood cell has no nucleus and consequently cannot repair itself, which is why it lasts only about 120 days.
- Ribosomes build proteins. Free ribosomes make proteins for use inside the cell; ribosomes bound to the rough endoplasmic reticulum make proteins for export or for the membrane.
- The endoplasmic reticulum has two forms with different jobs. Rough ER is studded with ribosomes and processes proteins. Smooth ER makes lipids and steroids, stores calcium, and detoxifies drugs, which is why liver cells are full of it.
- The Golgi apparatus sorts, modifies and ships. It receives vesicles from the ER, finishes the product, tags it with a destination and sends it out. A cell that secretes heavily has a large Golgi.
- Mitochondria produce ATP by aerobic respiration. Their number tracks the cell's energy demand directly, which is the single most reliable prediction you can make from a micrograph.
- Lysosomes digest, and peroxisomes detoxify. Lysosomes hold hydrolytic enzymes that break down worn organelles, bacteria and debris. Peroxisomes neutralize free radicals and break down fatty acids.
- The cytoskeleton holds shape and generates movement. Microfilaments of actin, intermediate filaments for tension, and microtubules for transport tracks and for separating chromosomes. Cilia and flagella are built from microtubules.
Where students lose marks: saying mitochondria "make energy." They do not create energy; they transfer energy from the bonds of glucose into the bonds of ATP, and they do it with oxygen. Say "produce ATP" or "transfer energy to ATP," and expect a question that tests whether you know the difference.
The problem. A pancreatic acinar cell secretes large quantities of digestive enzymes into a duct. Trace one enzyme molecule from the gene that specifies it to the moment it leaves the cell, naming every organelle in order and what happens at each.
Step one: start in the nucleus. The gene for the enzyme is transcribed into messenger RNA. Transcription happens here because DNA never leaves the nucleus; only the copy travels.
Step two: exit through a nuclear pore. The mRNA passes through a pore in the nuclear envelope into the cytoplasm. The pore is the reason the envelope can be a barrier and still permit information out.
Step three: translate at a ribosome on the rough ER. A ribosome binds the mRNA and begins building the protein. Because this protein is destined for export, the first stretch of amino acids acts as an address label that docks the ribosome onto the rough endoplasmic reticulum, and the growing chain is threaded into the ER interior rather than released into the cytosol. This is the step that decides the protein's fate, and it happens early.
Step four: fold and modify inside the rough ER. The protein folds into shape and receives initial modifications such as sugar groups. Misfolded proteins are caught here and destroyed rather than shipped.
Step five: bud off in a transport vesicle and travel to the Golgi. A piece of ER membrane pinches off around the protein, forming a vesicle that carries it to the receiving face of the Golgi apparatus and fuses with it.
Step six: process and sort in the Golgi. The protein moves through the Golgi stack, receiving final modifications, and is then sorted according to its destination tag. Since this one is for export, it is packaged into a secretory vesicle at the far face.
Step seven: store, then release by exocytosis. Secretory vesicles accumulate near the apical surface of the cell, the end facing the duct. When the cell is signaled to secrete, the vesicle membrane fuses with the plasma membrane and the enzyme is released into the duct.
Step eight: check the prediction against the cell's appearance. This pathway predicts that an acinar cell should be packed with rough ER, have a prominent Golgi, and be full of secretory vesicles concentrated at one end. Under a microscope that is exactly what it looks like, and the polarized arrangement, with the nucleus at the base and the vesicles at the apex, is visible at low magnification. Structure again matches function, and the digestive enzymes in question return in lesson 10.4.
Questions 1 to 5 are recall. Questions 6 to 10 ask for reasoning.
- Name the organelle that produces ATP.
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The mitochondrion
- State the function of the Golgi apparatus.
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It modifies, sorts and packages proteins and lipids for delivery to their destinations
- Distinguish rough from smooth endoplasmic reticulum.
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Rough ER bears ribosomes and processes proteins; smooth ER lacks them and synthesizes lipids, stores calcium and detoxifies
- Which organelle contains hydrolytic enzymes for digestion within the cell?
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The lysosome
- Name the three components of the cytoskeleton.
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Microfilaments, intermediate filaments and microtubules
- Liver cells contain unusually large amounts of smooth endoplasmic reticulum. Explain
why.
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Smooth ER detoxifies drugs and other foreign chemicals and synthesizes lipids. The liver receives all blood draining the digestive tract through the hepatic portal vein, so everything absorbed from the gut, including alcohol and drugs, reaches it first and at high concentration. It is also the body's principal site of lipid and cholesterol synthesis. Both jobs are done by smooth ER, so the organelle is abundant in proportion to the work. The liver detoxifies absorbed substances and synthesizes lipids, and smooth ER performs both functions
- A cell is found to have a very large nucleolus. Predict what the cell is doing heavily.
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The nucleolus assembles ribosomal subunits. A large nucleolus therefore means the cell is producing many ribosomes, and a cell only needs many ribosomes if it is synthesizing large amounts of protein. Synthesizing protein in large quantity, which requires many ribosomes
- Explain why a mature red blood cell cannot repair damaged proteins.
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Repairing or replacing a protein requires making a new one, which requires transcribing the gene. A mature red blood cell ejects its nucleus during development, so it has no DNA to transcribe and no ribosomes to translate with. It runs on the proteins it had when it matured, and as those degrade the cell cannot renew them, which sets its lifespan at about 120 days. It has no nucleus and therefore no DNA to transcribe, so no new protein can be made
- A drug destroys lysosomal membranes in a cell. Predict the consequence and explain the
mechanism.
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Lysosomes contain powerful digestive enzymes, and the membrane is what keeps those enzymes separated from the rest of the cell. Rupturing the membranes releases the enzymes into the cytosol, where they begin digesting the cell's own proteins, membranes and organelles indiscriminately. The cell destroys itself. This also explains why lysosomal enzymes work best at an acidic pH: it is a second safeguard, since the neutral cytosol partially inhibits any enzyme that leaks out. The released enzymes digest the cell's own components and the cell dies
- Compare a skeletal muscle cell and a mature fat cell in terms of expected mitochondrial
density, and justify the comparison.
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Mitochondrial number tracks the cell's rate of ATP consumption. A skeletal muscle cell does mechanical work, and every crossbridge cycle consumes ATP, so its demand is high and highly variable, and it is densely packed with mitochondria, particularly in slow oxidative fibers. A mature fat cell is essentially a storage container: it holds a large lipid droplet and carries out little metabolic work of its own, so its ATP demand is low and it contains comparatively few mitochondria. The muscle cell has far more, because contraction consumes ATP continuously while fat storage does almost no work