What counts as an organelle?

An organelle is a discrete cell subunit adapted to carry out one or more specific functions. “Discrete” is doing real work here: you should be able to point to the structure as a cell part, rather than a general region or a spread-out network.

Structure and function match closely in organelles. A ribosome is built for protein synthesis; a vesicle for transport; a nucleus for storing chromosomes and controlling access to genetic information. You see this pattern all over biology. Form isn’t decoration — it helps the function happen.

Some organelles are membrane-bound, but not all of them. Ribosomes count as organelles even though they have no membrane. Nuclei and vesicles count too. In this syllabus, the plasma membrane is also treated as an organelle because it is a defined cell structure with specific functions in transport, signalling and boundary formation.

Classification of selected cell structures by organelle status and membrane type.

Three important cell structures are not considered organelles here. A cell wall is an extracellular structure outside the plasma membrane that supports or protects the cell. A cytoskeleton is a cell-wide network of protein filaments that gives shape, organization and movement, but it is too spread out to be one discrete subunit. Cytoplasm is the material inside the plasma membrane but outside the nucleus, so it is a broad cell region containing many structures, not a single organelle.

How new techniques opened organelles up to study

A technique is a repeatable method that lets scientists collect evidence or manipulate material in a controlled way. This topic gives a neat nature-of-science example: scientists understood organelles better once they could separate them.

Cell fractionation is a separation technique that breaks cells open and separates their components into fractions for study. First, cells are placed in a cold, buffered solution. The cold slows enzyme activity and damage; the buffer helps maintain pH and osmotic conditions. The cells are then gently broken open to form a homogenate, a mixed suspension of cell contents.

The homogenate can be spun in a centrifuge. An ultracentrifuge is a high-speed centrifuge that separates very small cell components by spinning samples at extremely high acceleration. Larger or denser organelles form pellets at lower speeds or after shorter times than smaller organelles. In differential centrifugation, repeated spins at selected speeds separate cell components according to size and density. That answers one linking question about separation techniques used by biologists: cell fractionation and ultracentrifugation made it possible to isolate organelles and test what each one does.

Keeping transcription and translation apart

A nucleus is a double-membraned organelle that contains the cell’s chromosomes and separates them from the cytoplasm. The point IB wants here is more specific than “DNA is protected”, even though that’s true. The nucleus keeps transcription away from translation.

Transcription is the synthesis of an RNA copy from a DNA template. Translation is the synthesis of a polypeptide by ribosomes using the information in messenger RNA. Messenger RNA (mRNA) is a single-stranded nucleic acid transcript that carries coded information from DNA to ribosomes.

In eukaryotic cells, transcription happens in the nucleus. Translation happens at ribosomes in the cytoplasm or on the rough endoplasmic reticulum. Because the two processes are separated, the cell has time to process the mRNA before ribosomes use it. Post-transcriptional modification is the alteration of an RNA transcript after transcription and before translation, producing an mRNA molecule ready for use by ribosomes.

Prokaryotic cells do not have a nucleus. Their DNA and ribosomes share the same cytoplasmic compartment, so mRNA can meet ribosomes almost straight away, even while transcription is still finishing. That speed can be useful, but it takes away the eukaryotic chance to modify and check mRNA before translation begins.

Why compartments help cells work

Compartmentalization is how a cell is divided into separate spaces, so different biochemical processes can run under different local conditions. Eukaryotic cells are especially good at this because many of their organelles have membranes around them.

A membrane-bound compartment can gather the right molecules in one place. A metabolite is a small molecule that is used, produced or altered during metabolism. When the metabolites and enzymes for a pathway are kept in a small space, reactions run more efficiently than they would if those same molecules were spread through the whole cytoplasm.

Compartmentalization also keeps incompatible processes apart. Some reactions need conditions or chemicals that would disrupt other reactions. A lysosome is a single-membraned organelle containing hydrolytic enzymes that digest macromolecules and worn-out cell material. Those enzymes are useful inside a lysosome, but risky if they mix freely with the rest of the cytoplasm.

A phagocytic vacuole is a membrane-bound compartment formed after a cell engulfs solid material by phagocytosis. It can fuse with lysosomes, so the engulfed material gets digested in a controlled internal space. That’s the key point: the cell can carry out digestion without digesting itself.

Compartments also let the same cell maintain different pH values, ion concentrations or substrate concentrations at the same time. The cytoskeleton can move them around, and their membranes add extra surface area for membrane-based reactions. So, when an answer asks for advantages, don’t stop at “organization”. Say exactly what that organization achieves: concentration, separation, control and transport.

A mitochondrion is built around gradients and enzyme concentration

A mitochondrion is a double-membraned organelle that produces ATP by aerobic cell respiration. Adenosine triphosphate (ATP) is a nucleotide-based energy carrier; when its terminal phosphate bond is hydrolysed, it transfers energy to cell processes.

Start with the double membrane. The outer membrane separates the mitochondrion from the rest of the cytoplasm, while the inner membrane encloses the matrix and folds into cristae. Between them is a small intermembrane space.

That small intermembrane space matters. During aerobic respiration, protons can be pumped into it, allowing a steep concentration gradient to build quickly. The inner membrane also carries ATP synthase and electron transport proteins.

Cristae are folds of the inner mitochondrial membrane that increase the membrane surface area available for ATP production. With more cristae, there is more room for the membrane proteins used in oxidative phosphorylation, so cells with high ATP demand often have mitochondria with especially extensive inner membranes.

The matrix is the fluid-filled compartment inside the inner mitochondrial membrane that contains enzymes, substrates and products for parts of aerobic respiration. For the syllabus, remember that enzymes and substrates of the Krebs cycle are compartmentalized in the matrix. Holding them in one compartment raises their effective concentration and helps the pathway run efficiently.

A chloroplast separates light reactions from carbon fixation

A chloroplast is a double-membraned organelle in plants and algae that carries out photosynthesis. Its structure is a good structure–function example: the membrane system absorbs light and builds gradients, while the surrounding fluid contains the enzymes needed for carbon fixation.

A thylakoid is a flattened membrane sac inside a chloroplast that contains photosystems and other proteins used in the light-dependent reactions. Photosystems are pigment-protein complexes in thylakoid membranes that absorb light energy and pass excited electrons into photosynthetic electron transfer pathways.

Thylakoid membranes have a large surface area, so a chloroplast can pack in many photosystems. In many chloroplasts, thylakoids stack into grana, which are columns of thylakoids that increase membrane packing. More thylakoid membrane gives more light-harvesting capacity.

Inside each thylakoid is a small fluid compartment. Protons moved into this space during the light-dependent reactions can build a proton gradient quickly because the volume is small. Photosynthesis then uses that gradient to produce ATP.

The stroma is the fluid compartment surrounding the thylakoids inside a chloroplast. It contains enzymes and substrates of the Calvin cycle. Compartmentalization keeps the Calvin cycle molecules together, while ATP and reduced NADP made at the thylakoids are close enough to be supplied efficiently.

Pores large enough for traffic

The nuclear envelope is the double membrane around the nucleus, separating the nucleoplasm from the cytoplasm. Having two membranes helps because the nucleus needs separation, not a complete seal. Large molecules still have to move in and out under control.

A nuclear pore is a protein-lined opening through the nuclear envelope that controls movement of selected molecules between nucleus and cytoplasm. Some proteins made by cytoplasmic ribosomes need to enter the nucleus, such as proteins involved in chromosome structure or gene regulation. RNA molecules and ribosomal subunits made or assembled in the nucleus have to leave for the cytoplasm.

These aren’t tiny ions slipping through a channel. mRNA, tRNA and ribosomal subunits are large structures, so the nuclear envelope needs large, regulated pores. The double membrane can make pores where the inner and outer membranes join around an opening, with proteins around the rim controlling traffic.

Vesicles during nuclear division

The double nuclear membrane also helps during mitosis and meiosis. Mitosis is nuclear division that produces genetically identical nuclei. Meiosis is nuclear division that produces haploid nuclei for sexual reproduction.

During these divisions, the nuclear envelope has to break down so the spindle can move the chromosomes. It can break into vesicles, which are membrane-bound sacs. Later, these vesicles can fuse again around the separated chromosome sets to reform nuclear envelopes. That reversible break-up and reassembly makes more sense if you picture the envelope as membrane material that can vesiculate and then fuse, rather than as a rigid wall.

Ribosomes: same basic job, different destinations

A ribosome is a non-membrane-bound organelle made of rRNA and proteins that catalyses polypeptide synthesis from an mRNA template. In eukaryotes, the small subunit binds mRNA, while the large subunit helps position tRNA molecules and catalyses peptide bond formation.

A free ribosome is a ribosome not attached to a membrane, located in the cytoplasm. Free ribosomes synthesize proteins that stay in the cell. Many work in the cytoplasm itself; others are imported into organelles such as the nucleus, mitochondria or chloroplasts after synthesis.

The rough endoplasmic reticulum (rough ER) is a single-membraned organelle made of flattened sacs with ribosomes attached to its cytoplasmic surface. It looks “rough” because of those ribosomes. A cisterna is a flattened membrane-bound sac forming part of organelles such as the endoplasmic reticulum or Golgi apparatus. The inside space of the rough ER is the lumen, a membrane-enclosed internal space.

Ribosomes bound to the rough ER synthesize proteins for transport within the cell or secretion from the cell. As each polypeptide is made, it enters the rough ER lumen or becomes inserted into the ER membrane. From there, vesicles can carry it onwards, usually first to the Golgi apparatus.

So the difference is not that free ribosomes make “simple” proteins and rough ER ribosomes make “better” proteins. It’s about destination: free ribosomes mainly make proteins for retention in the cell; ribosomes on rough ER make proteins for the endomembrane system, organelles such as lysosomes, the plasma membrane or secretion.

Processing and secretion of protein

The Golgi apparatus is a single-membraned organelle made of stacked cisternae. It processes proteins that arrive from the rough ER, then packages them into vesicles. Keep the focus here narrow: the syllabus wants the Golgi’s role in processing and secretion of protein.

Proteins made on the rough ER travel to the Golgi in vesicles. Once inside the Golgi cisternae, enzymes modify them. Carbohydrate groups, for example, may be added to form glycoproteins. Other proteins may be changed so they fold, assemble or function correctly. You don’t need every chemical detail; what matters is that the Golgi changes proteins after synthesis.

The Golgi has a direction of travel. The cis face is the receiving side of the Golgi apparatus, usually nearest the rough ER. The trans face is the shipping side, where vesicles leave with processed proteins and carry them to their destinations.

During secretion, vesicles carrying processed proteins move from the trans face to the plasma membrane. The vesicles fuse with the membrane and release their contents outside the cell by exocytosis. This gives a clear structure–function correlation: stacked cisternae allow sequential processing, and vesicles allow delivery.

Vesicles are small moving compartments

A vesicle is a small, single-membraned sac that transports or stores substances within cells. Vesicles don’t sit still: they bud from membranes, travel through the cytoplasm and fuse with other membranes.

They move materials in two main ways. In some cases, the key cargo is inside the vesicle, such as a protein moving from the rough ER to the Golgi or from the Golgi to the plasma membrane for secretion. In other cases, the cargo is the membrane itself, including phospholipids and membrane proteins that must be delivered to another membrane.

Clathrin and vesicle formation

Clathrin is a coat protein that builds a lattice on the cytoplasmic side of a membrane and helps shape a budding vesicle. Textbooks often describe it as three-legged, because clathrin units form a cage-like coat.

In clathrin-mediated vesicle formation, clathrin molecules collect on the inner face of the membrane. As they assemble, they help the membrane curve inward. The bud then pinches off, forming a vesicle surrounded by a clathrin coat. After that, the vesicle can move to a target membrane and eventually fuse with it.

This completes the second linking question about separation techniques and the first about structure–function correlations. The same pattern runs through the topic: organelles work because their structures create the right spaces, surfaces, gradients and routes for movement.