Difficulty: Introductory | Prerequisites: Sections 6.1–6.2 (microscopy, prokaryote/eukaryote distinction).
Now that you know the difference between prokaryotic and eukaryotic cells, this material zooms into the eukaryotic cell to explore its internal compartments. The nucleus houses and protects DNA, and the endomembrane system (ER, Golgi, lysosomes, vacuoles) is the factory line that makes, modifies, packages, and ships proteins and lipids. Understanding how these organelles work together is essential before you study cell signalling, gene expression, or metabolism. If you are not yet comfortable with the prokaryote/eukaryote distinction, revisit Sections 6.1–6.2 first.
The nucleus stores DNA inside a double-membraned envelope, with pore complexes controlling what enters and exits, and a nucleolus where ribosomal RNA is made and ribosome subunits are assembled. The endomembrane system is a connected set of organelles (ER, Golgi, lysosomes, vacuoles, vesicles, plasma membrane) that synthesises proteins and lipids, modifies and sorts them, and handles intracellular digestion and recycling.
Nuclear envelope
A double membrane (two lipid bilayers) enclosing the nucleus. It separates the nuclear contents from the cytoplasm.
Pore complex
A protein structure lining each pore in the nuclear envelope. It regulates the entry and exit of proteins and RNA.
Nuclear lamina
A meshwork of protein filaments lining the inner side of the nuclear envelope. It helps maintain the shape of the nucleus.
Nuclear matrix
Protein fibres extending throughout the interior of the nucleus, providing structural support.
Chromatin
The complex of DNA and associated proteins that makes up each chromosome. Think of it as the "unpacked" form of chromosomes during interphase.
Nucleolus
A dense region within the nucleus where ribosomal RNA (rRNA) is synthesised and ribosome subunits are assembled.
Ribosomal RNA (rRNA)
RNA synthesised in the nucleolus that combines with imported proteins to form the large and small subunits of ribosomes.
Messenger RNA (mRNA)
RNA synthesised in the nucleus that carries the genetic instructions from DNA to the cytoplasm, where ribosomes translate it into protein.
Ribosome
A complex of rRNA and proteins that builds polypeptides (proteins). Not membrane-bound, so not classified as an organelle. Made of a small subunit and a large subunit.
Free ribosomes
Ribosomes suspended in the cytosol. The proteins they make mostly function within the cytosol.
Bound ribosomes
Ribosomes attached to the outside of the endoplasmic reticulum or nuclear envelope. They produce membrane proteins and secretory proteins.
Endomembrane system
A network of internal membranes that includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, vacuoles, and plasma membrane. These are related by direct physical continuity or by the transfer of membrane vesicles between them.
Endoplasmic reticulum (ER)
A network of membranous tubules (cisternae) that accounts for more than half the total membrane in a eukaryotic cell. The internal compartment is the ER lumen.
Smooth ER
ER without ribosomes on its surface. Responsible for lipid synthesis (oils, steroids, phospholipids), carbohydrate metabolism, detoxification of drugs and poisons, and storage of calcium ions.
Rough ER
ER studded with ribosomes on its cytoplasmic surface. Responsible for synthesising secretory proteins and glycoproteins, making membrane phospholipids, and serving as a membrane factory.
Glycoprotein
A protein with carbohydrates covalently bonded to it. Most secretory proteins are glycoproteins. In simple terms, it is a protein with a sugar coating.
Golgi apparatus
A stack of flattened membranous sacs (cisternae) that receives products from the ER, modifies them, manufactures some macromolecules, and ships them to their final destinations.
Cis face
The receiving side of the Golgi, located near the ER. Accepts transport vesicles from the ER.
Trans face
The shipping side of the Golgi, facing away from the ER. Vesicles pinch off here and travel to other sites in the cell or to the plasma membrane.
Lysosome
A membrane-bound sac of hydrolytic enzymes used to digest macromolecules. Operates best at acidic pH. Think of it as the cell's recycling and waste-disposal unit.
Phagocytosis
The process by which a cell engulfs a smaller organism or particle. The resulting food vacuole fuses with a lysosome for digestion.
Autophagy
The process by which a cell digests its own damaged organelles using lysosomes, recycling the organic components.
Food vacuole
A vacuole formed by phagocytosis, containing engulfed material for digestion.
Contractile vacuole
A vacuole that pumps excess water out of the cell, found in some freshwater protists.
Central vacuole
A large vacuole in mature plant cells formed by merging smaller vacuoles. Contains cell sap and stores inorganic ions, organic compounds, and sometimes defensive poisons. Plays a role in cell growth.
The nucleus contains most of the cell's genes and is enclosed by the nuclear envelope.
Nuclear envelope: a double membrane (two lipid bilayers). Each pore is lined by a pore complex that regulates traffic of proteins and RNA in and out.
Nuclear lamina: a protein meshwork on the inner surface of the envelope that maintains nuclear shape.
Nuclear matrix: protein fibres extending throughout the nuclear interior.
DNA organisation:
DNA is organised into chromosomes. Each chromosome consists of one DNA molecule and many associated proteins. This DNA-protein complex is called chromatin.
The nucleolus:
A dense region within the nucleus.
Where rRNA is synthesised.
Where imported proteins are assembled with rRNA into the large and small subunits of ribosomes.
Subunits exit through nuclear pores and assemble into a complete ribosome in the cytoplasm.
rRNA vs. mRNA:
rRNA is made in the nucleolus, combines with proteins to form ribosome subunits, exits through pores, and assembles into ribosomes.
mRNA is made in the nucleus, exits through nuclear pores to the cytoplasm, and is then translated by ribosomes into a polypeptide (a sequence of amino acids).
Made of rRNA and proteins. Small subunit on the bottom, large subunit on top.
Not membrane-bound, so ribosomes are not classified as organelles.
Their job: build proteins.
Free ribosomes: suspended in the cytosol. Proteins made here mostly function within the cytosol.
Bound ribosomes: attached to the ER or nuclear envelope. They make membrane proteins (e.g. for lysosomes) and secretory proteins.
An interconnected network of membranes that carries out protein synthesis, protein transport, lipid metabolism, and detoxification. Components: nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, vacuoles, plasma membrane. These are linked by direct physical continuity or by vesicle traffic.
Accounts for more than half the total membrane in a eukaryotic cell. It consists of cisternae (membranous tubules) whose interior is the ER lumen, separated from the cytosol.
Smooth ER:
Synthesises lipids (oils, steroids including sex hormones, phospholipids)
Metabolises carbohydrates
Detoxifies drugs and poisons (especially in liver cells, by adding hydroxyl groups to drug molecules, making them more soluble and easier to excrete)
Stores calcium ions
Drug tolerance: some drugs induce proliferation of smooth ER, increasing the release of detox enzymes, which raises tolerance
Rough ER:
Studded with ribosomes on its cytoplasmic surface
Synthesises secretory proteins: a polypeptide grows from a bound ribosome, is threaded into the ER lumen through a pore, folds into its functional shape inside the lumen, and leaves in a transport vesicle
Most secretory proteins are glycoproteins (proteins with covalently bonded carbohydrates)
Acts as a membrane factory: instead of entering the lumen, some proteins are inserted into the ER membrane itself and held by their hydrophobic portions
Makes membrane phospholipids using enzymes built into its membrane
Parts of it break off as transport vesicles heading for the Golgi
A stack of flattened membranous sacs (cisternae), often compared to a stack of pitta bread.
Cis face: the receiving side, near the ER. Accepts vesicles from the ER.
Trans face: the shipping side. Vesicles pinch off and travel to other destinations.
Functions:
Modifies glycoproteins (trims sugar monomers from their carbohydrate chains)
May alter membrane phospholipids
Manufactures some macromolecules (certain polysaccharides, but not cellulose)
Membrane-bound sacs of hydrolytic enzymes.
Enzymes and lysosomal membrane are made by rough ER and processed by the Golgi.
Enzymes work best at acidic pH (the lysosome interior is acidic). If released into the cytosol, they are far less active because the cytosol is nearly neutral.
The membrane and enzyme proteins are shaped so that the enzymes do not digest the lysosome itself.
Intracellular digestion (phagocytosis): the cell engulfs organisms or particles; the resulting food vacuole fuses with a lysosome. Digestion products (simple sugars, amino acids, other monomers) pass into the cytosol as nutrients.
Autophagy: a damaged organelle is wrapped in a double membrane. A lysosome fuses with the outer membrane and digests the inner membrane and enclosed material. Small organic compounds are released into the cytosol for reuse.
Large vesicles derived from the ER and Golgi.
Food vacuoles: formed by phagocytosis.
Contractile vacuoles: pump excess water out of the cell.
Hydrolytic vacuoles: found in plant cells, carry out enzymatic hydrolysis similar to lysosomes.
Central vacuole (plant cells): forms by merging smaller vacuoles. Contains cell sap, stores inorganic ions (potassium, chloride), organic compounds, and sometimes toxic compounds that protect the plant. Plays a role in cell growth.
The rough ER and Golgi pathway is how your body produces insulin: pancreatic beta cells synthesise insulin on rough ER ribosomes, fold it in the ER lumen, process it through the Golgi, and secrete it via vesicles. Lysosomal enzyme deficiencies cause lysosomal storage diseases (e.g. Tay-Sachs disease), where undigested material accumulates inside cells. The smooth ER's detoxification role in liver cells is why chronic alcohol consumption leads to liver damage and why drug tolerance develops with repeated use.
Students often think ribosomes are organelles. They are not. Ribosomes lack a membrane, so they are classified as complexes, not organelles.
Students confuse the ER lumen with the cytosol. The lumen is inside the ER, separated from the cytosol by the ER membrane. Proteins in the lumen are kept apart from cytosolic proteins.
Students sometimes think the Golgi only packages proteins. It also modifies them (trimming carbohydrates, altering phospholipids) and manufactures some macromolecules of its own.
A common error is thinking lysosomes will destroy the cell if they burst. The cytosol's near-neutral pH makes lysosomal enzymes far less active outside the lysosome, providing a safety margin.
⚠️ The pathway from ER to Golgi to plasma membrane (or lysosome) via vesicles is a very common exam question. Know the direction of flow and what happens at each stop.
⚠️ Be able to distinguish smooth ER functions from rough ER functions. A table comparing the two is a likely exam format.
⚠️ Know why lysosomal enzymes are safe in the cytosol (pH mismatch) and what autophagy is.
⚠️ The difference between free and bound ribosomes, and what each produces, is a classic multiple-choice item.
⚠️ Understand the structure of the nuclear envelope: double membrane, pore complex, nuclear lamina.
True or false: Ribosomes are membrane-bound organelles. (False, they have no membrane.)
The ______ face of the Golgi receives vesicles from the ER. (cis)
True or false: Lysosomal enzymes function best at neutral pH. (False, they work best at acidic pH.)
Secretory proteins are synthesised by ribosomes on the ______ ER. (rough)
The process of a cell digesting its own damaged organelles is called ______. (autophagy)
Q: Trace the path of a secretory protein from its synthesis to its export from the cell.
A: The protein is synthesised by a ribosome on the rough ER, threaded into the ER lumen, folded, and packaged into a transport vesicle. The vesicle travels to the cis face of the Golgi, where the protein is modified (e.g. carbohydrate trimming). It moves through the Golgi cisternae to the trans face, where it is packaged into another vesicle that fuses with the plasma membrane and releases the protein outside the cell.
Q: Why are lysosomal enzymes unlikely to damage the cell if a lysosome ruptures?
A: Lysosomal enzymes are optimised for acidic pH (~5). The cytosol has a nearly neutral pH (~7.2), so the enzymes would be far less active outside the lysosome.
Q: Compare the functions of smooth ER and rough ER.
A: Smooth ER synthesises lipids, metabolises carbohydrates, detoxifies drugs, and stores calcium ions. Rough ER synthesises secretory and membrane proteins, produces glycoproteins, makes membrane phospholipids, and generates transport vesicles.
Q: What is the role of the nucleolus?
A: The nucleolus synthesises ribosomal RNA and assembles it with imported proteins into the large and small subunits of ribosomes. These subunits then exit through nuclear pores and join in the cytoplasm.
Q: What is the difference between free ribosomes and bound ribosomes?
A: Free ribosomes are suspended in the cytosol and mostly produce proteins that function within the cytosol. Bound ribosomes are attached to the ER or nuclear envelope and produce membrane proteins and secretory proteins.
The endomembrane system connects directly to Chapter 7 (membrane structure and function), because the proteins and lipids made here end up in those membranes. Autophagy links to cell signalling and apoptosis (programmed cell death), covered later in the course. The rough ER and Golgi pathway is central to understanding how hormones and neurotransmitters are produced and secreted.
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