Fundamentals of Molecular Biology. Part 1: Cell Molecular Biology - A. N. Ogurtsov 2011
Intracellular Transport
Membrane Trafficking in the Cell
The system of cytoplasmic vacuolar structures discussed above Functions as a single entity, with its individual components capable of transforming into one another. This process involves a regular, continuous flow of membrane components from The Endoplasmic reticulum, through the Golgi apparatus, to Cell/30.html">The Plasma Membrane.
Nevertheless, the membranes of each organelle exhibit a distinct Specificity, which is manifested, in particular, by the presence or absence of Enzymes that serve as markers for a given membrane type. This flux is referred to as "membrane flow differentiation".
Transport vesicles integrating into the plasma membrane would continuously expand its surface area were it not for compensatory endocytosis—a flow of vesicles budding off the plasma membrane and heading into The Cell interior. As a result, a continuous turnover of membrane components takes place within the cell, allowing the exact same phospholipid molecule to reside in A wide variety of membranes. The overall scheme of the different pathways of membrane turnover in the cell is illustrated in Figure 34.
In cases where exocytosis is not counterbalanced by compensatory endocytosis, or during rapid cell growth, an additional synthesis of membrane components becomes necessary.
Both Phospholipids and Cholesterol, which serve as the primary Building Blocks of all lipid bilayers, are synthesized on the membranes of the smooth endoplasmic reticulum. All Lipids produced within the cell, with the exception of Fatty acids and two mitochondrial phospholipids, are synthesized on these membranes.
Both the incorporation and modification of new lipids occur in the monolayer facing the Cytosol. The redistribution of newly synthesized lipids between the two bilayers is facilitated by the enzyme flippase, since spontaneous lipid translocation via the flip-flop diffusion mechanism is far too slow and fails to match the redistribution rates observed experimentally.
The smooth endoplasmic reticulum is often called the factory of cellular membranes. Bacteria lack internal membranes, and in their case, lipid synthesis takes place within the plasma membrane.
Only two mitochondrial lipids—phosphatidylglycerol and cardiolipin—are synthesized within the mitochondrial membranes. The remaining lipids are delivered to the Mitochondria from the smooth endoplasmic reticulum by specialized Water-soluble carrier Proteins. Each such protein recognizes only specific types of phospholipids and binds a single molecule. The protein "snatches" a phospholipid molecule from a membrane and departs with this lipid safely tucked away in its binding pocket. Upon encountering another membrane with suitable properties, the protein releases the bound phospholipid molecule into the new lipid bilayer.
Unlike mitochondria, all lipids required for the construction of chloroplast membranes are produced within the Plastids themselves.
1. Which membrane structures belong to The Vacuolar System of the cell?
2. What are the Similarities and differences between the smooth and rough endoplasmic reticulum?
3. What are the similarities and differences between the cis- and trans-Golgi regions?
4. What is constitutive exocytosis, and how does it differ from regulated exocytosis?
5. What are the Functions of the rough endoplasmic reticulum?
6. What are the functions of the smooth endoplasmic reticulum?
7. What is The Role of the signal recognition particle in Protein Synthesis within the rough endoplasmic reticulum?
8. What is the function of endosomes in the cell?
9. What are coated vesicles, and what is this coat formed from?
10. List the proteins that form coats on coated vesicles.
11. What is the composition of a clathrin triskelion?
12. What is the function of SNARE complexes in Vesicular Transport?
13. What are the similarities and differences between anterograde and retrograde vesicular transport?
14. What reaction does catalase catalyze?
15. To which category of Organelles do Peroxisomes belong?
16. What is membrane trafficking in the cell, and what functions does it perform?
Last update: 12/08/2026
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