Eukaryotic Cell Ultrastructure: Golgi, Ribosomes, Mitochondria and Lysosomes
Continuing our tour of eukaryotic organelles, this lesson focuses on the Golgi apparatus, ribosomes (both 80S and 70S), mitochondria, and lysosomes. These organelles are central to protein processing, protein synthesis, energy conversion, and intracellular digestion. OCR 2.1.1 (e)(ii) requires that you describe the ultrastructure of each and relate it to function.
Ribosomes
Ribosomes are the sites of translation (protein synthesis), where mRNA is decoded to assemble amino acids into polypeptides. They are not membrane-bound; they are considered organelles in the broad sense because they have a specific structural organisation and function.
Structure of Ribosomes
Ribosomes are ribonucleoproteins (RNP) composed of ribosomal RNA (rRNA) and ribosomal proteins. They exist in two sizes:
| Type | Large subunit | Small subunit | Total | Location |
|---|
| 80S ribosome | 60S | 40S | 80S | Eukaryotic cytoplasm (free or on RER) |
| 70S ribosome | 50S | 30S | 70S | Prokaryotes, mitochondria, chloroplasts |
Key Point: "S" (Svedberg unit) refers to the sedimentation rate in an ultracentrifuge, not a simple measure of mass. This is why 60S + 40S = 80S, rather than 100S — the units are not additive.
Where Are Ribosomes Found?
- Free in the cytoplasm — make proteins that remain inside the cytosol, such as many enzymes.
- Attached to the rough endoplasmic reticulum — make proteins that will be secreted, inserted into the membrane, or sent to lysosomes.
- Inside mitochondria (70S) — make proteins encoded by the mitochondrial genome, essential for oxidative phosphorylation complexes.
- Inside chloroplasts (70S, in plant cells) — make proteins encoded by the chloroplast genome.
Functional Sites on Ribosomes
Each ribosome has three tRNA binding sites:
- A site (aminoacyl) — accepts incoming charged tRNA.
- P site (peptidyl) — holds the tRNA carrying the growing polypeptide.
- E site (exit) — from which the empty tRNA leaves.
The large subunit contains the peptidyl transferase activity, which catalyses peptide bond formation. This activity is itself performed by rRNA (the ribosome is therefore a ribozyme).
Exam Tip: The existence of 70S ribosomes in mitochondria and chloroplasts is powerful evidence for the endosymbiotic theory, which proposes that these organelles evolved from free-living prokaryotes engulfed by an ancestral eukaryotic cell.
The Golgi Apparatus
The Golgi apparatus (also called the Golgi body or Golgi complex) consists of a stack of 4–8 flattened, membrane-bound cisternae resembling a stack of plates. It is usually located near the nucleus and the RER.
Structural Regions
- Cis face — the receiving side, closest to the RER. Transport vesicles from the RER fuse here.
- Medial cisternae — the middle region where most enzymatic modifications occur.
- Trans face — the dispatching side, furthest from the RER. Vesicles bud off from here to deliver their cargo.
Functions of the Golgi
The Golgi apparatus is the cell's sorting, modifying, and packaging centre. Its main functions are:
- Modifies proteins and lipids as they pass through the cisternae. Modifications include:
- Glycosylation — trimming and adding sugars to form mature glycoproteins and glycolipids.
- Phosphorylation — tagging lysosomal enzymes with mannose-6-phosphate so they are sorted to lysosomes.
- Proteolytic cleavage — cutting larger precursor proteins into their active forms (e.g., proinsulin → insulin).
- Sorts molecules according to their final destination (secretion, plasma membrane, lysosome).
- Packages finished molecules into vesicles that bud from the trans face.
- Synthesises certain polysaccharides, including plant cell wall components (e.g., pectin and hemicellulose).
- Forms lysosomes — specialised vesicles containing hydrolytic enzymes.
Vesicles from the Golgi
Three main types of vesicles leave the trans-Golgi network:
- Secretory vesicles — fuse with the plasma membrane to release contents outside the cell (exocytosis).
- Membrane vesicles — incorporate new proteins and lipids into the plasma membrane.
- Lysosomes — remain inside the cell for intracellular digestion.
Mitochondria
Mitochondria are often described as the powerhouses of the cell because they generate most of the cell's ATP through aerobic respiration. They are typically 1–10 µm long and 0.5–1 µm wide, visible with light microscopy as rods, but their ultrastructure is only revealed by TEM.
Structure
- Double membrane. The outer membrane is smooth and contains porin channels (permeable to small molecules). The inner membrane is highly folded into cristae, which drastically increase surface area for the electron transport chain.
- Matrix — the fluid-filled interior enclosed by the inner membrane. Contains:
- Enzymes of the Krebs cycle (also called the citric acid cycle or TCA cycle).
- Mitochondrial DNA — a small circular molecule, maternally inherited.
- 70S ribosomes — synthesise proteins encoded by mitochondrial DNA.
- Inorganic ions and substrates.
- Intermembrane space — the region between the outer and inner membranes. The electron transport chain pumps protons (H⁺) into this space, creating a gradient used for ATP synthesis.
Function: Aerobic Respiration