30 4.4 Plasma Membrane

Created by: CK-12/Adapted by Christine Miller

Figure 4.4.1 Simple cut-away model of an animal cell. 
Figure 4.4.2 Jello molds containing fruit.

A Bag Full of Jell-O

The simple cut-away model of an animal (Figure 4.4.1) shows that a cell resembles a plastic bag full of Jell-O. Its basic structure is a filled with . Like Jell-O containing mixed fruit (Figure 4.4.2), the cytoplasm of the cell also contains various structures, including a and other . Your body is composed of trillions of cells, but all of them perform the same basic life functions. They all obtain and use , respond to the environment, and . How do your cells carry out these basic functions and keep themselves — and you — alive? To answer these questions, you need to know more about the structures that make up cells, starting with the .

What is the Plasma Membrane?

The is a structure that forms a barrier between the inside the and the environment outside the cell. Without the plasma membrane, there would be no cell. Although it is very thin and flexible, the plasma membrane protects and supports the cell by controlling everything that enters and leaves it. It allows only certain substances to pass through, while keeping others in or out. To understand how the plasma membrane controls what passes into or out of the cell, you need to know its basic structure.

Phospholipid Bilayer

The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol. The phospholipids in the plasma membrane are arranged in two layers, called a . As shown in the simplified diagram in Figure 4.4.3, each individual  phospholipid molecule has a phosphate group head (in red) and two fatty acid tails (in yellow). The head “loves” water () and the tails “hate” water (). The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outward, toward either the cytoplasm (intracellular) or the fluid that surrounds the cell (extracellular).

molecules can easily pass through the if they are small enough, because they are water-hating like the interior of the membrane. molecules, on the other hand, cannot pass through the plasma membrane — at least not without help — because they are water-loving like the exterior of the membrane.

Image shows a diagram of a phospholipid bilayer. The bilayer is made up of two sheets of phospholipids, with the fatty acid tails facing towards the center, and the phosphate heads on the two external surfaces.
Figure 4.4.3 The phospholipid bilayer is made up of two sheets of phospholipids, with the fatty acid tails facing the centre.

Other Molecules in the Plasma Membrane

The plasma membrane also contains other molecules, primarily other and . The yellow molecules in the diagram here, for example, are the cholesterol. Molecules of the steroid lipid cholesterol help the plasma membrane keep its shape. Proteins in the plasma membrane (shown blue in Figure 4.4.4) include: transport that assist other substances in crossing the cell membrane, receptors that allow the cell to respond to chemical signals in its environment, and cell-identity markers that indicate what type of cell it is and whether it belongs in the body.

Image shows a diagram of a plasma membrane. The lipid bilayer contains embedded molecules including proteins, glycoproteins, glycolipids, and cholesterol.
Figure 4.4.4 The plasma membrane contains many molecules embedded in the lipid bilayer.

Additional Functions of the Plasma Membrane

The may have extensions, such as whip-like (singular flagellum) or brush-like (singular cilium), shown below (Figure 4.4.5), that give it other functions. In single-celled organisms, these membrane extensions may help the organisms move. In multicellular organisms, the extensions have different functions. For example, the cilia on human lung cells sweep foreign particles and mucus toward the mouth and nose, while the flagellum on a human sperm cell allows it to swim.

Image shows a scanning electron microscope image of three human sperm on a porous surface.
Figure 4.4.5 Human sperm with their long, whip-like flagella.
Image shows a scanning electron microscope image of the interior surface of bronchi. The cells lining the interior of this tube have clumps of cilia.
Figure 4.4.6 Brush-like cilia on lung epithelial cells.

Feature: My Human Body

If you smoke or use e-cigarettes (vaping) and need another reason to quit, here’s a good one. We usually think of lung  as the major disease caused by smoking. But smoking and vaping can have devastating effects on the body’s ability to protect itself from repeated, serious respiratory infections, such as bronchitis and pneumonia.

4.4.7 Adverse Affects of Vaping
Figure 4.4.7 Airways of “healthy” vapors are abnormal – results of vaping.

are microscopic, hair-like projects on cells that line the respiratory, reproductive, and digestive systems. Cilia in the respiratory system line most of your airways, where they have the job of trapping and removing dust, germs, and other foreign particles before they can make you sick. Cilia secrete mucus that traps particles, and they move in a continuous wave-like motion that sweeps the mucus and particles upward toward the throat, where they can be expelled from the body. When you are sick and cough up phlegm, that’s what you are doing.

Smoking prevents cilia from performing these important functions. Chemicals in tobacco smoke paralyze the cilia so they can’t sweep mucus out of the airways. Those chemicals also inhibit the cilia from producing mucus. Fortunately, these effects start to wear off soon after the most recent exposure to tobacco smoke. If you stop smoking, your cilia will return to normal. Even if prolonged smoking has destroyed cilia, they will regrow and resume functioning in a matter of months after you stop smoking.

4.4 Summary

  • The is a structure that forms a barrier between the inside the cell and the environment outside the cell. It allows only certain substances to pass in or out of the cell.
  • The is composed mainly of a molecules. It also contains other molecules, such as the steroid cholesterol, which helps the membrane keep its shape, and transport proteins, which help substances pass through the membrane.
  • The plasma membranes of some cells have extensions that have other functions, like flagella to help sperm move, or cilia to help keep our airways clear.

4.4 Review Questions

  1. What are the general functions of the plasma membrane?
  2. Describe the phospholipid bilayer of the plasma membrane.
  3. Identify other molecules in the plasma membrane. State their functions.
  4. Why do some cells have plasma membrane extensions, like flagella and cilia?
  5. Explain why hydrophilic molecules cannot easily pass through the cell membrane. What type of molecule in the cell membrane might help hydrophilic molecules pass through it?
  6. Which part of a phospholipid molecule in the plasma membrane is made of fatty acid chains? Is this part hydrophobic or hydrophilic?
  7. The two layers of phospholipids in the plasma membrane are called a phospholipid ____________.
  8. Steroid hormones can pass directly through cell membranes. Why do you think this is the case?
  9. Some antibiotics work by making holes in the plasma membrane of bacterial cells. How do you think this kills the cells?
  10. What is the name of the long, whip-like extensions of the plasma membrane that helps some single-celled organisms move?

4.4 Explore More

Insights into cell membranes via dish detergent – Ethan Perlstein, TED-Ed, 2013.

Inside the cell membrane, by The Amoeba Sisters, 2018.

Attributions

Figure 4.4.1

Animal Cell Unannotated, by Kelvin Song on Wikimedia Commons is used under a CC0 1.0 (https://creativecommons.org/publicdomain/zero/1.0/deed.en) public domain dedication license.

Figure 4.4.2

Jello mold at the mexican bakery photo by Aimée Knight on Flickr is used under a CC BY 2.0 (https://creativecommons.org/licenses/by/2.0/) license.

Figure 4.4.3

Phospholipid_Bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 4.4.4

Lipid bilayer by OpenStax on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0) license.

Figure 4.4.5

Spermatozoa-human-3140x by No specific author on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.4.6

Cilia/ Bronchiolar epithelium 3 – SEM by Charles Daghlian on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

Figure 4.4.7

Adverse effects of vaping (raster) by Mikael Häggström on Wikimedia Commons is released into the public domain (https://en.wikipedia.org/wiki/Public_domain).

References

Amoeba Sisters. (2018, February 27). Inside the cell membrane. YouTube. https://www.youtube.com/watch?v=qBCVVszQQNs&feature=youtu.be

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.3 Phospolipid Bilayer [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Betts, J.G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E.. Womble, M., DeSaix. P. (2013, April 25). Figure 3.4 Cell Membrane [digital image]. In Anatomy and Physiology. OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/3-1-the-cell-membrane

Ghosh, A., Coakley, R. C., Mascenik, T., Rowell, T. R., Davis, E. S., Rogers, K., Webster, M. J., Dang, H., Herring, L. E., Sassano, M. F., Livraghi-Butrico, A., Van Buren, S. K., Graves, L. M., Herman, M. A., Randell, S. H., Alexis, N. E., & Tarran, R. (n.d.). Chronic E-Cigarette Exposure Alters the Human Bronchial Epithelial Proteome. American Journal of Respiratory and Critical /Care Medicine198(1), 67–76. https://doi-org.ezproxy.tru.ca/10.1164/rccm.201710-2033OC

TED-Ed. (2013, February 26). Insights into cell membranes via dish detergent – Ethan Perlstein. YouTube. https://www.youtube.com/watch?v=yAXnYcUjn5k&feature=youtu.be

 

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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