Principles of Membrane Transport
- Discuss the structure and semipermeable nature of biological membranes
- Delineate principles involved in transport of solutes and solvent/water
- Distinguish types of membrane transport based on mechanism of solute transfer
The Envelope of Life
The human body is essentially a water-based system. Various solutes, small and large, dissolved (or suspended) in water bound by a non-leaky covering make up the chemistry of our cells.
In the previous session, we looked at basic principles used by cell organelles to sort molecules and divide cellular tasks among themselves.
In this session, we will look at the mechanism of sorting at the molecular level with special focus on biological membranes as the site of sorting.
We begin by taking a closer look at the composition and structure of biological membranes.
Every cell is defined by its boundary—the plasma membrane—a covering that protects from as well as interacts with the world outside of the cell.
Within a cell, every cell organelle too has a membrane that facilitates compartmentalization and allows docking of enzymes, receptors, etc.
The membranes that surround living cells and their organelles are all made of the same molecules – amphipathic lipids.
Let us take a look at the prototype structure of an amphipathic lipid molecule – A PHOSPHOLIPID.
Note: The hydrophilic groups at one end of the molecule are labelled as “head”; the hydrophobic groups clustered at the other end are labelled as “tail”.
When mixed with water, a phospholipid molecule can form one of the following structures.
The type of aggregate formed depends on the concentration of phospholipid in solution as well as the presence of other amphipathic and hydrophobic molecules.
You will learn more about the structure of membrane lipids in the sessions on lipid chemistry. For now, just remember the following:
- Lipids that form biological membranes are amphipathic in nature.
- Phospholipids and glycolipids are most common membrane lipids.
- The hydrophilic groups in a membrane lipid are together referred to as the “head” region of the molecule (and hence, represented by a circle in membrane diagrams).
- The hydrophobic groups in a membrane lipid are together referred to as the “tail” region of the molecule (and hence, represented by straight or zig-zag lines arising from the head in membrane diagrams).
The activity below will help you better understand lipid aggregation.
A liposome-like covering consists of two sandwich-like layers of lipid molecules; hence, the cell membrane is often referred to as a lipid bilayer.
The vesicles formed in the body for transport of cellular material between cell organelles or two different cells are also comparable to liposomes.
Artificial liposomes synthesized in laboratories are now used as drug delivery systems to target drug molecules to specific infected/injured cells without the risk of dilution in blood or toxicity to healthy cells.
You will learn more about the use of micelles in the body when we look at lipid digestion in the small intestine and circulation of lipid-protein complexes in blood.
Sorting Solute Or Solvent?
We now know the chemical nature of the barrier that separates the interior of a cell from its exterior—a lipid bilayer with polar groups facing the exterior (and interior) of the cell and a hydrophobic water-repelling core.
This barrier is semi-permeable, i.e., it allows only selective movement of molecules across its core. Even water molecules, present in abundance inside and outside the cell, cannot pass freely through this membrane. Thus we can say that the cell membrane sorts molecules between the interior and exterior of the cell.For convenience, we will abbreviate the word semi-permeable membrane as SPM.
Here, recall two important terms from high school chemistry with respect to study of SPMs: osmosis and diffusion.
- Osmosis refers to the movement of solvent molecules across an SPM that is impermeable to solute molecules. In living organisms, the solvent is water and the SPM is the lipid bilayer.
- Diffusion refers to the movement of solute molecules across an SPM that is impermeable to solvent molecules. In living organisms, solutes include ions, metabolites, nutrients etc. and SPM is the lipid bilayer.
Did you note – the lipid bilayer can act as a barrier to both the solute and the solvent. So how does the cell membrane decide what to sort and how?
We will look at some possible answers in the next activity.
Remember: Maintaining a constant osmolarity across fluid compartments is the key to simplifying the transport of solutes and solvent across cell membrane.
Movement of Solute
Let us look at the molecular model of the cell membrane and the various types of transport occurring across it. The same mechanisms are used for transport across cell organelle membranes.
To know more about the mechanism and examples of each type of transport, refer your physiology lesson on membrane transport.
The Solute Carriers
Living cells carry out diverse activities essential to life, central to which is their ability to differentially distribute solutes across SPMs.
The proteins that carry solutes across cell and cell organelle membranes are called transporters. The entire set of genes encoding various transport proteins accounts for ~10% of the human genome. Of these, a subset of genes called the solute carrier (SLC) genes encodes classical transporter proteins (facilitative transporters and secondary active transporters). Currently, there are 52 families of SLC genes consisting of over 400 transporter genes.
The SLC genes and their products are implicated in various genetic diseases such as Hartnup’s disease, cystinuria, hemochromatosis, glucose-galactose malabsorption etc. You will come across several genetic disorders with an underlying defect in the synthesis or regulation of one of the SLC family of transport proteins.
The drug development process for treatment / management of these genetic diseases relies on thorough analysis of the SLC gene family expression and regulation in a tissue-specific manner.
To know more about the SLC gene family, click here.
You can use the concepts learnt in this lesson in your study of anatomy and physiology.
In anatomy, when you learn about the epithelial membrane (a layer of cells that partition two spaces inside the body), observe carefully the histology of an epithelial membrane. Compare and contrast the structure and function of epithelial membrane in sorting with that of the cell membrane.
In physiology, when you learn the mechanisms of nutrient absorption through the digestive tract or action of ion channels in neurotransmission or formation and regulation of urine output in kidneys, remember to distinguish osmosis from diffusion and apply the concepts of solute transport learnt here.
Define carbohydrates and enlist their functions in the body. (2M)
|Solute carrier gene family||Database of solute carrier gene families and their expressed proteins with the cellular locations and functions||The SLC Tables by Bioparadigms. Web link accessed on Jul 12, 2017.|
|Activities on membrane transport||Three simple HTML-based activities that clarify the various aspects of membrane transport||Wiley’s interactive module on membrane transport. Accessed on Jul 14, 2017.
Wisc-Online interactive animation on passive diffusion and passive osmosis by Barbara Liang. Last updated on Oct 15, 2015. Accessed on Jul 14, 2017.
|Membrane channels||Explore the effect of gated membrane channels on solute composition across the cell membrane using this simulation activity.||Membrane Channels, PhET Interactive Simulations, University of Colorado, Boulder. Accessed here on Nov 29, 2016.|
Quick Retrieve – A Recap of Key Concepts
Recall the key points regarding membrane transport by watching the video below.
Sample exam questions related to this session
- Compare and contrast – diffusion and osmosis [2M]
- Explain the difference between facilitated diffusion and primary active transport. [2M]