Bulk Transport
Sections
BULK TRANSPORT
Endocytosis
- Ingestion of extracellular material
Exocytosis
- Delivery of intracellular material to the plasma membrane or extracellular space
Both processes work together to maintain the fluid balance and membrane balance of the cell
EXOCYTOSIS
- Proteins synthesized in the endoplasmic reticulum and trafficked to the Golgi apparatus
- From the Golgi, the proteins are sorted and can follow different paths:
3a) Constitutive secretory pathway
- Also called default pathway as it does not require a signal for cargo to enter this pathway
- Can transport soluble proteins or plasma membrane proteins
- Transport vesicles fuse with the plasma membrane and release their contents (plasma membrane proteins become accessible to the extracellular space)
3b) Regulated secretory pathway
- Found mainly in specialized cells that release molecules like hormones, neurotransmitters, etc.
- Secretory vesicles are blocked from fusing with the plasma membrane without a proper signal
- An extracellular ligand binds a cellular receptor which causes an intracellular signal that releases the block on the secretory vesicles
- Secretory vesicles then fuse with the plasma membrane and release their cargo
ENDOCYTOSIS
- Membrane invagination has solutes in it
- Vesicle formed from this invagination traffics to the early endosome
- Next the solutes are trafficked to the late endosome
- Solutes are then delivered to the lysosome where they are broken down for use by the cell
Early Endosome vs Late Endosome
- Differences in acidity (early endosome is slightly less acidic)
- Differences in protein composition
Three types of endocytosis:
- Pinocytosis
- All eukaryotic cells utilize this
- Vesicles are less than 150nm in diameter
- Phagocytosis
- Vesicles generally greater than 250nm in diameter (phagosomes)
- Phagosomes delivered directly to lysosomes
- Usually only specialized cells use phagocytosis
- Receptor-mediated endocytosis
- All eukaryotic cells utilize this
- Very specific for the molecules ingested
- Can concentrate solute molecules
- Can allow for transcytosis
Transcytosis
- Allows polarized cells to move molecules from one side of the cell to another
Clinical Correlation:
Antibodies in breast milk are absorbed by the intestinal epithelial cells, transcytosed, and ejected into the bloodstream.
FULL-LENGTH TEXT
- In this tutorial, we will learn about the bulk transport of materials across the cell membrane.
- To begin, let's start a table to learn some key features of bulk transport.
- Denote that it covers two types of transport:
- Endocytosis, which is the ingestion of extracellular material and
- Exocytosis, which is the delivery of intracellular material to the plasma membrane or extracellular space.
Let's illustrate an overview of exocytosis and endocytosis. Indicate that we will first describe endocytosis.
- Write "Enter" below and underline the "En" in both words as an easy way to remember the direction of endocytosis.
- Draw the plasma membrane with a bulge inward (aka invagination).
- Draw solutes above the plasma membrane and indicate they are brought in.
- As the invagination deepens, it begins to pinch off.
- Now draw a uniform plasma membrane.
- Below this plasma membrane, draw the solutes surrounded by a double membrane.
- Label this structure a vesicle and indicate that it separates from the plasma membrane and travels to a destination inside the cell.
Now indicate that we will describe exocytosis.
- Write "Exit" below and underline the "Ex" in both words as an easy way to remember the direction of exocytosis.
- Draw a uniform plasma membrane.
- Below this plasma membrane, draw a vesicle full of solutes and indicate that it has traveled from inside the cell to the plasma membrane.
- Now draw the membrane with a bulge to show that the vesicle fused with it.
- Draw the solutes outside the membrane and indicate that they were released.
- Although the two processes are not exactly reverses of one another, they do work to maintain the fluid balance and membrane balance of the cell.
Now let us look more in depth at these types of transport. Begin with exocytosis.
- On one side, draw the nucleus.
- Label the membrane surrounding the nucleus, the nuclear envelope.
- On the other side, draw and label the plasma membrane.
- Draw the endoplasmic reticulum as connected membrane compartments continuous with the nuclear envelope.
- It is a major biosynthetic factory in the cell.
- Now draw a series of flattened, membrane stacks.
- Label one stack, cisterna, and label the combined structure the Golgi apparatus, a site which sorts and sends off the products of the endoplasmic reticulum.
- Label the side closest to the nucleus "cis" and the side closest to the plasma membrane "trans".
- Indicate that proteins travel from the endoplasmic reticulum to the Golgi.
From here, the exocytic pathway can branch off.
- Indicate the first path is the constitutive secretory pathway which is always operating.
- It is also known as the default pathway because it does not require a particular signal for cargo to enter this pathway.
- Draw a vesicle with soluble proteins for secretion.
- Draw plasma membrane proteins through the vesicle's membrane.
- Indicate that this vesicle is traveling from the Golgi to the plasma membrane.
- Now draw the vesicle fusing to the plasma membrane.
- This allows the soluble proteins to be released and the plasma membrane proteins to be available to the extracellular space.
- Indicate that the other secretory pathway is called the regulated secretory pathway.
- This pathway is found mainly in specialized cells that release molecules like hormones, neurotransmitters, etc.
- Draw a vesicle with a lot of one type of protein.
- Label this vesicle, secretory vesicle.
- Indicate that this vesicle has come from the Golgi apparatus.
- Indicate that the vesicle is traveling towards the plasma membrane but is not able to fuse with it until it receives an outside signal - hence regulated pathway.
- Draw a receptor on the plasma membrane with a bound ligand.
- Indicate the binding of receptor and ligand causes an intracellular signal to remove the block on the secretory vesicle.
- Now draw the vesicle fusing with the plasma membrane, releasing its contents.
Now let us explore the endocytic pathway.
- Begin at the plasma membrane by drawing an invagination with solutes in it.
- The vesicle that forms is transported to the early endosome for the first stage of sorting.
- Indicate this by drawing the early endosome with the solutes.
- From here, the material is transported to the late endosome.
- Draw the late endosome.
- Indicate that differences between the early endosome and late endosome include internal acidity (early endosome is slightly less acidic) and protein compositions.
- The solute is then trafficked to the lysosome, which is full of powerful enzymes used to break macromolecules down into simpler parts useful to the cell.
- Draw the lysosome and indicate that the digested material is released to the cytoplasm.
- Now there are three main forms of endocytosis:
- pinocytosis
- phagocytosis, and
- receptor-mediated endocytosis (textual variation exists which includes receptor-mediated endocytosis as a specific type of pinocytosis).
- Make a table so we can explore the differences between these three.
- Indicate that pinocytosis utilizes vesicles of less than 150nm in diameter and all eukaryotic cells use this form.
- Indicate that phagocytosis utilizes vesicles which are generally greater than 250nm in diameter called phagosomes, these phagosomes are delivered directly to the lysosome, and usually only specialized cells can phagocytose.
- Finally, indicate that like pinocytosis, all eukaryotic cells utilize receptor-mediated endocytosis.
- However, solutes bind to receptors on the cell surface and are then endocytosed which means receptor-mediated endocytosis is much more specific than pinocytosis.
- Indicate that this also allows a cell to concentrate macromolecules without requiring a large fluid intake.
- Once inside the cell, the macromolecules follow the normal endocytic pathway, except in the case of transcytosis.
- Indicate that transcytosis allows polarized cells to move molecules from one side of the cell to another.
- As a clinical correlation, this is how antibodies in breast milk enter the bloodstream of an infant.
- They are absorbed by the intestinal epithelial cells, transcytosed, and ejected into the bloodstream.
UNIT CITATIONS:
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- Lodish, H., Berk, A., Kaiser, C. A., Krieger, M., Scott, M. P., Bretscher, A., Ploegh, H. & Matsudaira, P. Molecular Cell Biology, 6th ed. (W. H. Freeman and Company, 2008).
- Seifter, J., Sloane, D. & Ratner, A. Concepts in Medical Physiology. (Lippincott Williams & Wilkins, 2005).
- Marieb, E. N. & Hoehn, K. Human Anatomy & Physiology, 10th ed. (Pearson, 2016).
- Roberts, M., Reiss, M. J. & Monger, G. Advanced Biology. (Nelson Thornes, 2000).
- Lewin, B. Cells. (Jones & Bartlett Learning, 2007).
- Segev, N. (edited by). Trafficking Inside Cells: Pathways, Mechanisms and Regulation. (Springer Science & Business Media, 2010).