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Structure and function of the plasma membrane and cytoplasm of cells. Also covers the phospholipid bilayer and microvilli.
Introduction
What’s a cell? Well, on some level, it's a bag of goo. The plasma membrane—the outer boundary of the cell—is the bag, and the cytoplasm is the goo.
Of course, a cell is ever so much more than just a bag of goo. It's a complex, highly organized unit, the basic building block of all living things. And the plasma membrane and cytoplasm are actually pretty sophisticated.
The membrane is a delicate, two-layered structure of lipids and proteins, and it controls what can enter and exit the cell. Similarly, the cytoplasm of a eukaryotic cell consists not only of cytosol—a gel-like substance made up of water, ions, and macromolecules—but also of organelles and the structural proteins that make up the cytoskeleton, or "skeleton of the cell."
In this article, we’ll take a closer look at the plasma membrane and cytoplasm.
The plasma membrane
Both prokaryotic and eukaryotic cells have a plasma membrane, a double layer of lipids that separates the cell interior from the outside environment. This double layer consists largely of specialized lipids called phospholipids.
A phospholipid is made up of a hydrophilic, water-loving, phosphate head, along with two hydrophobic, water-fearing, fatty acid tails. Phospholipids spontaneously arrange themselves in a double-layered structure with their hydrophobic tails pointing inward and their hydrophilic heads facing outward. This energetically favorable two-layer structure, called a phospholipid bilayer, is found in many biological membranes.
As shown below, proteins are also an important component of the plasma membrane. Some of them pass all the way through the membrane, serving as channels or signal receptors, while others are just attached at the edge. Different types of lipids, such as cholesterol, may also be found in the cell membrane and affect its fluidity.
The plasma membrane is the border between the interior and exterior of a cell. As such, it controls passage of various molecules—including sugars, amino acids, ions, and water—into and out of the cell. How easily these molecules can cross the membrane depends on their size and polarity. Some small, nonpolar molecules, such as oxygen, can pass directly through the phospholipid portion of the membrane. Larger and more polar, hydrophilic, molecules, such as amino acids, must instead cross the membrane by way of protein channels, a process that is often regulated by the cell. You can learn more about cellular transport in the membranes and transport section.
The surface area of the plasma membrane limits the exchange of materials between a cell and its environment. Some cells are specialized in the exchange of wastes or nutrients and have modifications to increase the area of the plasma membrane. For instance, the membranes of some nutrient-absorbing cells are folded into fingerlike projections called microvilli, singular, microvillus. Cells with microvilli cover the inside surface of the small intestine, the organ that absorbs nutrients from digested food. The microvilli help intestinal cells maximize their absorption of nutrients from food by increasing plasma membrane surface area.
People with celiac disease have an immune response to gluten—a protein found in wheat, barley, and rye. This immune response damages the microvilli of the small intestine. Because of the damage, intestinal cells cannot absorb nutrients normally, leading to malnutrition, cramping, and diarrhea. Fortunately, a gluten-free diet prevents the immune response from taking place, allowing intestinal cells to remain healthy and structurally intact.
The cytoplasm
The part of the cell referred to as cytoplasm is slightly different in eukaryotes and prokaryotes. In eukaryotic cells, which have a nucleus, the cytoplasm is everything between the plasma membrane and the nuclear envelope. In prokaryotes, which lack a nucleus, cytoplasm simply means everything found inside the plasma membrane.
One major component of the cytoplasm in both prokaryotes and eukaryotes is the gel-like cytosol, a water-based solution that contains ions, small molecules, and macromolecules. In eukaryotes, the cytoplasm also includes membrane-bound organelles, which are suspended in the cytosol. The cytoskeleton, a network of fibers that supports the cell and gives it shape, is also part of the cytoplasm and helps to organize cellular components.
Even though the cytosol is mostly water, it has a semi-solid, Jello-like consistency because of the many proteins suspended in it. The cytosol contains a rich broth of macromolecules and smaller organic molecules, including glucose and other simple sugars, polysaccharides, amino acids, nucleic acids, and fatty acids. Ions of sodium, potassium, calcium, and other elements are also found in the cytosol. Many metabolic reactions, including protein synthesis, take place in this part of the cell.
Attribution
This article is a modified derivative of “Eukaryotic cells” by OpenStax College, Biology, CC BY 3.0. Download the original article for free at http://cnx.org/contents/185cbf87-c72e-48f5-b51e-f14f21b5eabd@9.85:18/Biology.
The modified article is licensed under a CC BY-NC-SA 4.0 license.
Additional references
Cooper, G. M. "Structure of the plasma membrane." In The cell: A molecular approach. 2nd ed. Sunderland, MA: Sinauer Associates, 2000. http://www.ncbi.nlm.nih.gov/books/NBK9898/.
"Cytoplasm." Scitable. 2014. http://www.nature.com/scitable/definition/cytoplasm-280.
Kaiser, G. E. "The Prokaryotic Cell: Bacteria." BIOL 230 Lecture on the Web. March 2014. http://faculty.ccbcmd.edu/courses/bio141/lecguide/unit1/prostruct/procyto.html.
OpenStax College, Biology. "Components and Structure." OpenStax CNX. Last modified May 27, 2016. http://cnx.org/contents/GFy_h8cu@10.53:QOGUelqL@9/Components-and-Structure.
Raven, P. H., G. B. Johnson, K. A. Mason, J. B. Losos, and S. R. Singer. "All cells exhibit basic structural similarities." In Biology, 62-63. 10th ed. AP ed. New York, NY: McGraw-Hill, 2014.
Reece, J. B., L. A. Urry, M. L. Cain, S. A. Wasserman, P. V. Minorsky, and R. B. Jackson, R. B. "A tour of the cell." In Campbell Biology, 92-123. 10th ed. San Francisco, CA: Pearson, 2011.
Tobin, A. J. and J. Dusheck. "The cytosol is the cytoplasm that lies outside the organelles." In Asking about life, 71. 3rd ed. Belmont, CA: Thomson Learning. 2005.
Log in Sualeha 9 years agoPosted 9 years ago. Direct link to Sualeha's post “What is meant by fluidity...” What is meant by fluidity of cell membrane? How is it regulated? • (29 votes) Matt B 9 years agoPosted 9 years ago. Direct link to Matt B's post “Nice question!A membrane...” Nice question! (63 votes) Tayyiaba Kalsoom 9 years agoPosted 9 years ago. Direct link to Tayyiaba Kalsoom's post “what is chemistry behind ...” what is chemistry behind each function of cytoplasm? • (8 votes) hamzasonic345 8 years agoPosted 8 years ago. Direct link to hamzasonic345's post “Colloidal solution” Colloidal solution (3 votes) Varad Asawa 8 years agoPosted 8 years ago. Direct link to Varad Asawa's post “how do plasma membrane di...” how do plasma membrane differ from cell membrane? • (10 votes) Sukhada 10 months agoPosted 10 months ago. Direct link to Sukhada's post “While cell membrane cover...” While cell membrane covers the entire components of a cell, plasma membrane covers only the cell's organelles. Some main differences between the two are the fact that the plasma membrane encloses the organelles, whereas the cell membrane encloses the entire cell. (2 votes) Elizabeth R. 8 years agoPosted 8 years ago. Direct link to Elizabeth R.'s post “How does cholesterol effe...” How does cholesterol effect fluidity the of the membrane? • (8 votes) Liu, Andrew 8 years agoPosted 8 years ago. Direct link to Liu, Andrew's post “Is cytoplasm the same thi...” Is cytoplasm the same thing as cytosol? • (4 votes) richajon 8 years agoPosted 8 years ago. Direct link to richajon's post “Cytosol is the specific l...” Cytosol is the specific liquid within the cytoplasm. Cytosol is a component of the cytoplasm like lysozymes and cellular structures are part of the cytoplasm. (12 votes) Mango a year agoPosted a year ago. Direct link to Mango's post “Why do polar molecules ha...” Why do polar molecules have more trouble getting through the phospholipid bilayer than non-polar molecules? Wouldn't the hydrophilic head mean that it attracts polar molecules? • (5 votes) Michael Wade a year agoPosted a year ago. Direct link to Michael Wade's post “The hydrophilic head does...” The hydrophilic head does interact with polar molecules, but the double layer of non-polar lipids between the heads prevents the polar molecules from passing through easily. (5 votes) Moiz Asim 4 years agoPosted 4 years ago. Direct link to Moiz Asim's post “Do double membrane bound ...” Do double membrane bound Organelles have two phospholipid Bilayers? • (6 votes) Baron rojo 7 years agoPosted 7 years ago. Direct link to Baron rojo's post “are the organelus full of...” are the organelus full of the same cytosol as the rest of the cell? • (5 votes) Ivana - Science trainee 5 years agoPosted 5 years ago. Direct link to Ivana - Science trainee's post “Partially. The ion ratios...” Partially. The ion ratios are usually the same (except for sarcoplasmic reticulum which stores Ca and results in high concentrations of Ca inside). Also it depends what the function of organelle is. Peroxisomes are known to have lower pH and lots of peroxidase enzymes since they catalyze oxidation reactions. (2 votes) Pranaav a year agoPosted a year ago. Direct link to Pranaav's post “Do prokaryotic cells have...” Do prokaryotic cells have microvilli? • (4 votes) Mason Smith a year agoPosted a year ago. Direct link to Mason Smith's post “No, prokaryotic cells do ...” No, prokaryotic cells do not have microvilli. (4 votes) mohammadmuslim.qasemi 2 years agoPosted 2 years ago. Direct link to mohammadmuslim.qasemi's post “Hi;I have problem with t...” Hi; • (4 votes) kunling802 2 years agoPosted 2 years ago. Direct link to kunling802's post “A plasma membrane is a la...” A plasma membrane is a layer around a cell that prevents the cytoplasm from getting all mixed up with the outside environment. Plasma membrane (cell membrane) is made of two phospholipid layers, or a type of lipid with hydrophilic heads on the outside and hydrophobic tails inside. It is also known as a phospholipid bilayer. There are also protein channels which allow certain molecules to go in and out of the cell and sugar embedded in the membrane. (4 votes)Want to join the conversation?
A membrane is not a rigid structure. It is fluid and is able to move which actually gives it more stability. As an analogy, think of your skin that is solid but is still able to move.
As for regulation: there are far too many pathways and factors to list. Heat and pressure for example can increase the fluidity, drugs and medication can do the same, cholesterol can be a bidirectional regulator, etc...
I have problem with the main answer of a plasma membrane?
And what's the phospholipid?
