The ultrastructure of eukaryotic cells and the functions of organelles, the difference between prokaryotic and eukaryotic cells, and the fluid-mosaic model of the cell membrane.

What this dot point is asking

CCEA wants you to describe the ultrastructure of a eukaryotic cell and the function of each organelle, contrast prokaryotic and eukaryotic cells, and describe the cell surface membrane using the fluid-mosaic model. You should be able to recognise organelles on an electron micrograph and link each structure to its function.

Eukaryotic cell ultrastructure

The nucleus is surrounded by a double membrane (the nuclear envelope) with nuclear pores that let messenger RNA leave for the ribosomes. Inside, the nucleolus makes ribosomal RNA. Mitochondria have a folded inner membrane (cristae) that increases the surface area for the electron transport chain, and a fluid matrix where the Krebs cycle occurs. Cells with high energy demand, such as muscle and liver cells, have many mitochondria.

The secretory pathway links several organelles. Proteins are made on ribosomes attached to the rough endoplasmic reticulum, packaged into vesicles, and sent to the Golgi apparatus for modification (such as adding sugars to make glycoproteins). The Golgi then buds off secretory vesicles that fuse with the cell surface membrane to release the protein by exocytosis. Plant cells additionally have a cellulose cell wall, a large permanent vacuole filled with cell sap, and chloroplasts.

Prokaryotic versus eukaryotic cells

Eukaryotic cells are typically $10$ to $100\ \mu\text{m}$ across, while prokaryotic cells are around $1$ to $5\ \mu\text{m}$. Prokaryotes may also have a protective capsule, flagella for movement, and pili. Because mitochondria and chloroplasts are about the size of bacteria and contain their own circular DNA and 70S ribosomes, they are thought to have evolved from free-living prokaryotes (the endosymbiont theory).

The fluid-mosaic model

The membrane is a phospholipid bilayer with hydrophilic phosphate heads facing the watery surroundings and hydrophobic fatty acid tails pointing inwards. It is "fluid" because the phospholipids and many proteins move laterally within the layer, and a "mosaic" because proteins of different sizes are scattered through it. Cholesterol sits between phospholipids to control fluidity and stop the membrane becoming too leaky, glycoproteins and glycolipids act as receptors and in cell recognition, and channel and carrier proteins allow transport of ions and polar molecules that cannot cross the bilayer directly.

Examples in context

Example 1. A pancreatic acinar cell making digestive enzymes. These cells are packed with rough endoplasmic reticulum and Golgi because their whole job is to make and secrete protein-digesting enzymes. The enzyme amylase is synthesised on the RER, modified in the Golgi, and stored in secretory (zymogen) granules near the membrane until a hormonal signal triggers exocytosis into the pancreatic duct. This shows the secretory pathway working at full capacity, and explains why a cell adapted for secretion looks so different under the electron microscope from, say, a red blood cell.

Example 2. Cholesterol and membrane fluidity in cold-water fish. Fish living in cold seas adjust the amount of cholesterol and the proportion of unsaturated fatty acids in their membranes so that the bilayer stays fluid at low temperature. If the membrane became too rigid, transport proteins and receptors could not function. This is a real-world demonstration that the fluid-mosaic model is dynamic, with membrane composition tuned to keep the right level of fluidity.

Try this

Q1. State two structural differences between a prokaryotic and a eukaryotic cell. [2 marks]

• Cue. No nucleus or membrane-bound organelles in prokaryotes; smaller 70S ribosomes; peptidoglycan cell wall.

Q2. Explain why the cell membrane is described as a "fluid mosaic". [2 marks]

• Cue. Fluid because phospholipids move; mosaic because proteins are scattered through the bilayer.

Q3. Describe the pathway taken by a protein from its synthesis to its secretion from the cell. [4 marks]

• Cue. Made on ribosomes of the RER, packaged into vesicles, modified in the Golgi, packaged into secretory vesicles, released by exocytosis at the cell surface membrane.