Some people say a bacterial cell is just a simple bag of enzymes. This couldn’t be further from the truth! Learn about the structure and function of the bacterial cell membrane, what’s in the cytoplasm, and how membrane surface area impacts bacterial size.
When was the last time you went swimming? Was it in a pool? The ocean? Maybe a pond? Depending on your swimming hole of choice, the water could have been clear, clean, and chlorinated, or dark, weedy, and teeming with organisms.
You probably swam around in this water without ever considering what your skin just did for you. Your skin kept the chemicals, germs, and water on the outside while keeping your blood and organs safe and secure. Your skin represents a very complex external membrane.
But, what about bacteria? They are far too small to have something as complex as your skin. So, what keeps their internal components separate from the environment?
The answer is the cell membrane. The cell membrane is a phospholipid bilayer that completely surrounds a bacterial cell. The word ‘completely’ is important here because any break in the bilayer will lead to the death of the bacteria. In fact, some of our favorite antibacterial cleaning products kill bacterial cells by destroying or making holes in the cell membrane, allowing the bacterial cell contents to spill out.
The phospholipid bilayer structure has both hydrophilic, or water-loving, and hydrophobic, or water-fearing, components. The hydrophilic components line up on the external and internal surface of the membrane in contact with the environment on the outside and the internal contents of the cell on the inside. The hydrophobic parts of the membrane orient towards the interior of the bilayer, stabilizing and contributing to the structure. In images and drawings, it may look like the cell membrane is a rigid structure, but in reality, it has the consistency of olive oil. So, the membrane, while stable, moves like a fluid.
Just look at a bottle of salad dressing – the hydrophobic oil stays separate from the water, but if you tip the bottle, you can see the oil layer is fluid and moves freely.Going back to our refreshing swim, when you executed your perfect cannonball into the pool, you likely shut your mouth. You didn’t want a mouthful of pool water to spoil your dive. But, after that grueling swim, you might have climbed out of the pool and headed straight for your water bottle. You selected to let bottled water into your body but selected not to allow in the pool water.
Likewise, the bacterial cell membrane is a highly selective barrier. This barrier prevents materials from simply diffusing into and out of the cell. This allows the cell to take up chemicals and nutrients needed for survival while keeping the vital cell components separated from the environment.Now, before we said that the cell membrane is simply a phospholipid bilayer. While this is true, the story is much more complicated.
Isn’t everything in biology?! The cell membrane is not a smooth structure, like the surface of a balloon. Instead, poking out all around the cell are proteins, sugars, and complex structures the bacteria needs for life. In fact, the cell membrane is about 40% phospholipid and 60% protein! These proteins are embedded into the membrane and are crucial for transporting nutrients in and waste products out. Some of these proteins are also crucial enzymes needed for cell metabolism. And, other additional proteins serve as anchor points for extracellular appendages like flagella and pili. So, there are many different types of proteins found associated with the cell membrane.
We keep saying that the cell membrane keeps the ‘inside’ of the cell apart from the ‘outside’. So, what is actually on the inside of the cell and why is it so important?Cytosol is the water-like fluid found in bacterial cells. The cytosol contains all the other internal compounds and components the bacteria needs for survival. The fluid and all its dissolved or suspended particles is called the cytoplasm of the cell. Proteins, amino acids, sugars, nucleotides, salts, vitamins, enzymes, DNA, ribosomes, and internal bacterial structures all float around the cell in the cytoplasm.
All of these components are vital to the life of the cell and are contained by the cell membrane.
Surface Area and Volume
Bacterial cells are tiny. The average bacterial cell is only about 1 micrometer by 3 micrometers. The size of the cell influences several aspects of bacterial survival. For example, how fast the cell can take up food and excrete waste is inversely proportional to the cell size.Wait – what does that mean?! That means that as the cell gets larger, the rate of food uptake and waste excretion gets slower.
The average bacterial cell has a large cell membrane surface area and a small internal cytoplasm volume. All that exposed cell membrane surface area can efficiently take up nutrients and deliver them to the cytoplasm where they’re needed.But, what happens if the cell gets larger? There are in fact some bacterial species that have evolved incredibly large cells, almost visible to the naked eye! Well, as a cell gets larger, the volume increases more quickly than the surface area.
So now, food uptake is more difficult because there is less cell membrane surface area relative to cell volume. But have no fear; the bacteria have a trick up their sleeves to deal with this problem, and it all relies on the cell membrane.Remember that the cell membrane is responsible for the surface area. Well, what if the cell could increase that surface area? A bacterial cell can increase surface area of the cell membrane with only a small change in the volume of the cytoplasm by making many invaginations of the cell membrane. ‘Invagination’ is just a fancy word for ‘folding the membrane.’ By making tiny folds in the cell membrane, the surface area increases dramatically without much change in the cytoplasm volume, allowing a larger surface for uptake of nutrients.To imagine how this works, try channeling your internal 1980s jazzercise persona, including the leg warmers! But, let’s say we get a little crazy and decide to keep adding leg warmers.
To do this, we would scrunch up the first leg warmer and add a new one below it, scrunch that one up, and add another one, until we can’t fit any more! Now, we have the same volume (our leg) covered by, instead of one flat, smooth leg warmer, many folded and scrunched leg warmers. We have greatly increased the surface area (all those folds) while covering the same volume.
Let’s recap this lesson. We learned three important things. First, we learned that the cell membrane is a phospholipid bilayer containing many proteins that acts as a selective barrier to keep the environment outside and the vital cell components inside.Second, we learned that the gelatinous material inside of the cell is called the cytoplasm and is a water-based fluid called cytosol that contains all the important vitamins, nutrients, and machinery of the cell.Finally, we learned that the surface area of the cell is crucial for the ability of the cell to absorb sufficient nutrients and that if a cell grows too large it can increase the membrane surface area through many tiny folds called invaginations.
After you’ve finished with this lesson, you’ll be able to:
- Restate the structure and function of a bacterial cell membrane
- Mention the makeup of the cytoplasm of a bacterial cell
- Report on the importance of the cell membrane’s surface area
- Determine how and why a bacterial cell membrane can increase its surface area