Simplifying Microbiology One Post Every Week + Helpful Glossary

#1- Cells: Nature’s Building Blocks

Cells = Bricks as Organisms = Houses. Simple.

To understand the most complex mechanisms at a molecular level, you must understand what a cell is and how they work.

“The key to every biological problem must finally be sought in the cell; for every living organism is, or at some time has been, a cell.”

- E.B. Wilson, pioneering cell biologist

Figure 1- A sketch of the cork cells in two different sections drawn by Robert Hooke from the book Micrographia in 1665.

The cell, a microscopic factory capable of making every organism around you alive (yourself included!) possesses the ability to replicate into two identical cells when required. Hence, it is the basic structural, functional, and biological unit of every known organism.

The term ‘cell’ was coined by Robert Hooke while observing slices of cork tissue under an early compound microscope made by Christopher Cook mentioned in his book Micrographia, published in 1665 (See Fig. 1)

If you compare a cell from a spinach plant to a cell from your inner cheek lining using a compound light microscope (See Fig. 2), they will look structurally a bit different. But, both perform the same functions like replicating, making proteins, getting rid of waste products, etc. However, there will be some noticeable differences between the cells if you use the electron microscope (See Fig. 3) to search for the teeny-tiny organs that do all the hard work (discussing in the second post). For example, plant cells from leaves appear green due to chloroplasts which are not seen in humans but present in some tiny organisms.

Figure 2- A modern binocular compound microscope with a light bulb as a source of light to view the specimen on the stage. It doesn’t require a lot of space on a lab bench.

Figure 3- A large electron microscope. It has a lot of fine-tuning knobs, etc. to view a much smaller specimen. It has its own computer where images can be viewed and saved and other software to adjust the microscope’s parameters. Instead of a light source, an electron beam is used to view the specimen. This instrument definitely requires a separate room and a lot of care!

Let us begin by looking at the very exterior of the cell known as the plasma membrane, shall we?

The Plasma Membrane

Ah yes, the plasma membrane, the outer, jiggly shell of the cell that protects the internal structures and is known for its semi-permeability, i.e., it allows only a select few substances to enter the cell.

The membrane has two essential parts — a thin lipid bilayer (about 5 nanometers in thickness) and membrane protein molecules, of which there are many different kinds, held mainly by non-covalent interactions. It is a dynamic structure of cells, permitting the movement of most molecules within the membrane, which gives it its name: The Fluid Mosaic Model, a description proposed by Seymour Jonathan Singer and Garth L. Nicholson in 1972 (See Fig. 8). Therefore, it is known as the Singer and Nicholson model.

Figure 4- An electron micrograph taken from an electron microscope showing a distinct lipid bilayer of a cellular body. The arrows pinch to indicate that the membrane is approximately less than 5 nanometers thick.

It is time to check out the membrane lipids and then the membrane proteins!

Membrane Lipids

Looking closely at the lipid molecules, like one molecule (See Fig. 5 and See Fig. 8), you will notice that it has a round hydrophilic (hydro- water, philic- loving) head and two hydrophobic (hydro- water, phobic- fearing) tails. These hydrophilic heads face outwards to hide the hydrophobic tails all the time, even when made artificially at the laboratory. To be more specific, these molecules are called phospholipids, and they are the most abundant type. Furthermore, all phospholipids in cellular membranes are amphiphilic since these structures contain both the polar, hydrophilic head and nonpolar, hydrophobic tails.

Figure 5- A simplified, and chemical formula versions of a type of phosphoglyceride named phosphatidylcholine. Source- The Molecular Biology of the Cell, by Alberts et al., 6th Edn.

Taking this to another level, the most common type of phospholipid in animal cell membranes are phosphoglycerides, which possess a 3-carbon glycerol backbone in the head region of this molecule (See Fig. 5). By altering the contents in the head and hydrophobic tail groups, cells can produce types of phosphoglycerides.

Figure 6- (A) and (B) showing the chemical and simplified structure of a cholesterol molecule. Source- The Molecular Biology of the Cell, by Alberts et al., 6th Edn.

The most predominant phosphoglycerides in membranes of mammalian cells are phosphatidylethanolamine, phosphatidylserine, and phosphatidylcholine.

Additionally, cholesterol is found mainly in eukaryotic cells at a frequency of one cholesterol molecule per phospholipid. This kind of molecule orients itself like the phospholipids — polar heads facing the watery environments inside and outside the cell and hiding their hydrophobic tails deep within the lipid bilayer (See Fig. 6 and Fig. 8).

Membrane Proteins

You have the lipids that are essential for making membranes of all cells. But, having the membrane proteins are crucial as well.

Did you know that around 30% of the proteins from an animal genome are just proteins that belong to the cellular membrane? These proteins play a vital role in keeping the cell alive and perfectly functioning and interact with its environment. In my opinion, that is a LOT of proteins.

In a regular plasma membrane, about 50% of its weight is protein. All cells in our body have varying proportions of lipids and proteins depending on their function and various proteins that carry out specific roles and interact with the membrane differently (See Fig. 7).

Like the lipid molecules around them, the proteins are amphiphilic too, i.e., containing both hydrophobic and hydrophilic portions.

In Figure 7, you may notice two main types of membrane proteins; the integral and peripheral types. Integral/transmembrane proteins pass right through the membrane while their hydrophobic middle regions remain buried within the membrane where they interact with the lipids’ hydrophobic tails. Some of them act as channels due to their hollowed centres, called pores, to allow the passage of water and a bunch of ions like sodium ions, calcium ions, and so on.

On the other hand, peripheral membrane/membrane-associated proteins do not mingle with the hydrophobic interior of the membrane, sort of like a floating iceberg. Lastly, small glycolipids (carbohydrate + fatty acid) and glycoproteins (carbohydrate + protein) are present on the membrane and some of the proteins, too.

Figure 7- 1–8 depict the different forms of proteins usually seen in a plasma membrane. Examples 1–3 are transmembrane proteins. Some dangle and face the cell’s cytosol by interacting only with the cytosolic monolayer of lipids either with (4) an amphiphilic alpha-helix or by (5) 1 or more covalently attached lipid chains (5). (6) Some completely hang outside the cell and are connected to the exterior monolayer of lipids via complex sugars to a lipid anchor. (7,8) Peripheral proteins. The green interactive portions face either outside or inside the cell and not at all interact with the hydrophobic region of the membrane but with other membrane proteins via non-covalent bonds. Source- The Molecular Biology of the Cell, by Alberts et al., 6th Edn.

Figure 8- The fluid mosaic model of the plasma membrane. It contains a lipid bilayer and a variety of membrane proteins which are integral and peripheral protein types. Additional molecules are found dotted around the bilayer. Cytoskeleton filaments are seen beneath the membrane for structural support. Source- LadyofHats Mariana Ruiz. Wikipedia link: https://en.wikipedia.org/wiki/Cell_(biology)

That is quite enough, for now, my peeps! I shall put some information about the tiny organs of the cell in the second post, and more will come eventually.

Thank you for reading, and I will be happy to receive some comments and constructive feedback on my work 😊

Glossary

Lipid- A science-y word for a class of large molecules, including fatty acids; mainly used in molecular biology studies (more in future posts)

Bilayer- In layers of two

Nanometer or nm- A unit of measuring size, mainly things which are absolutely not visible to the naked eye

Molecules- Very small structures made of atoms like, carbon and hydrogen atoms make up hydrocarbon molecules

Non-covalent interaction- An interaction involving the lack of sharing electrons between 2 or more molecules but involves other forms of weak molecular interactions

Phospholipid- A fatty acid molecule containing a phosphate group

Polar- To contain either positive or negative charge; the opposite is non-polar

Eukaryotic cell or eukaryote- Organisms or cells that contain a well-defined nucleus (More in future posts)

Cytosol- The watery environment within the cell, excluding the tiny cell organs

Extracellular- Outside the cell; opposite is intracellular

Cytoskeleton- Literally meaning cell skeleton; protein filaments that give the cells a shape

Sources

• Part I: INTRODUCTION TO THE CELL, Chapter 1: Cells and Genomes. From the textbook Molecular Biology of the Cell, by Alberts et al., 6th Edn.
• Part IV: INTERNAL ORGANIZATION OF THE CELL, Chapter 10: Membrane Structure. From the textbook Molecular Biology of the Cell, by Alberts et al., 6th Edn.