What counts as an organelle?

An organelle is a discrete cell subunit adapted to carry out one or more specific functions. The word “discrete” is doing real work here: you should be able to point to it as a cell part, rather than describing 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. Biology uses this pattern again and again. Form is not decoration — it helps the function happen.

Some organelles are membrane-bound, but not all of them are. Ribosomes still 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 lies outside the plasma membrane; it is an extracellular structure that supports or protects the cell. A cytoskeleton is a cell-wide network of protein filaments giving shape, organization and movement, but it is too spread out to count as a discrete subunit. Cytoplasm means the material inside the plasma membrane but outside the nucleus, so it is a broad cell region containing many structures, not one 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 is a good nature-of-science example because progress in understanding organelles depended on scientists being able to separate them.

Cell fractionation is a separation technique in which cells are broken open and their components are separated into fractions for study. First, the 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 shorter times than smaller organelles. In differential centrifugation, repeated spins at selected speeds separate cell components according to size and density. That gives one answer to the linking question about separation techniques used by biologists: cell fractionation and ultracentrifugation made it possible to isolate organelles and then 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. For this syllabus point, the advantage isn’t just that “DNA is protected”, though that is true. The main idea is that the nucleus keeps transcription separate 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 these 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 have no nucleus. Their DNA and ribosomes share the same cytoplasmic compartment, so mRNA can meet ribosomes almost immediately, even while transcription is still finishing. That speed can be useful, but it removes the eukaryotic opportunity to modify and check mRNA before translation begins.

Why compartments help cells work

Compartmentalization means organizing a cell 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 happen more efficiently than they would if those same molecules were spread through the whole cytoplasm.

Separate compartments also keep 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 do their job inside a lysosome, but they would be dangerous if they mixed 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, allowing the engulfed material to be digested in a controlled internal space. That’s the neat point: the cell carries out digestion without digesting itself.

Compartments also let different pH values, ion concentrations or substrate concentrations exist at the same time in the same cell. The cytoskeleton can move them around, and their membranes add extra surface area for membrane-based reactions. So when you’re asked about 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 two membranes. The outer membrane separates the mitochondrion from the rest of the cytoplasm. The inner membrane encloses the matrix and folds into cristae, leaving a small intermembrane space between the two membranes.

That small intermembrane space matters. During aerobic respiration, protons can be pumped into it, so a steep concentration gradient builds quickly. The inner membrane also gives ATP synthase and electron transport proteins a surface to sit in.

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 involved 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, know that enzymes and substrates of the Krebs cycle are compartmentalized in the matrix. Keeping them together 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 useful structure–function example: the membrane system absorbs light and builds gradients, while the surrounding fluid contains the enzymes needed for carbon fixation.

Inside a chloroplast, a thylakoid is a flattened membrane sac containing photosystems and other proteins used in the light-dependent reactions. Photosystems are pigment-protein complexes in thylakoid membranes; they absorb light energy, then pass excited electrons into photosynthetic electron transfer pathways.

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

The fluid inside each thylakoid forms a small compartment. During the light-dependent reactions, protons are moved into this space. Because the volume is small, a proton gradient can develop quickly. 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. This compartmentalization keeps Calvin cycle molecules together, while ATP and reduced NADP produced at the thylakoids remain close enough for efficient supply.

Pores large enough for traffic

The nuclear envelope is the double membrane around the nucleus. It separates the nucleoplasm from the cytoplasm. The nucleus needs that separation, but it cannot be sealed off completely, because large molecules have to pass in and out in a controlled way.

A nuclear pore is a protein-lined opening through the nuclear envelope that controls the movement of selected molecules between the nucleus and cytoplasm. Proteins made by cytoplasmic ribosomes may 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 must move out to the cytoplasm.

These are not tiny ions slipping through a channel. mRNA, tRNA and ribosomal subunits are large structures, so the nuclear envelope needs large, regulated pores. Where the inner and outer membranes join around an opening, they can form pores, with proteins lining the rim to control 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 and 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 catalyse 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 internal 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. Vesicles can then carry it onwards, usually first to the Golgi apparatus.

The difference, then, is not that free ribosomes make “simple” proteins and rough ER ribosomes make “better” proteins. It is 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 receives proteins from the rough ER, processes them, then packages them into vesicles. Keep this section focused: the syllabus wants the Golgi’s role in processing and secretion of protein.

Proteins made on the rough ER reach the Golgi in vesicles. Inside the Golgi cisternae, enzymes modify them. Carbohydrate groups may be added to form glycoproteins, for example, or proteins may be changed so they fold, assemble or function correctly. You don’t need every chemical modification here; the key idea is that the Golgi changes proteins after synthesis.

The Golgi has a direction to it. 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 carrying processed proteins to their destinations.

For secretion, vesicles carrying processed proteins move from the trans face to the plasma membrane. They fuse with the membrane and release their contents outside the cell by exocytosis. Nice structure–function link: stacked cisternae allow sequential processing, while vesicles allow delivery.