Structure and function of the plasma membrane and cytoplasm of cells. Also covers the phospholipid bilayer and microvilli.
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.
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.
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- What is meant by fluidity of cell membrane? How is it regulated?(28 votes)
- Nice question!
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...(59 votes)
- what is chemistry behind each function of cytoplasm?(8 votes)
- how do plasma membrane differ from cell membrane?(10 votes)
- Is cytoplasm the same thing as cytosol?(4 votes)
- 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)
- are the organelus full of the same cytosol as the rest of the cell?(5 votes)
- 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)
- 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?(4 votes)
- 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.(4 votes)
- If any of you guys know freeze fracturing, can you help me:
Why are the pits and mounds of the membrane interior significant in understanding the structure of the membrane(3 votes)
- The cell membrane is composed of 3 types of lipids. One of the chemicals which make up phospholipid heads is alcohol glycerol.
That glycerol is so important in case of the freezing cell because it acts as antifreeze.
So why, do our cells freeze, why humans die if left in cold environment?
Because our cells have minimal content of glycerol, not enough to protect us. Usually, ice crystals form in our cells from all the water content in it.(5 votes)
- Does nucleoid have a membrane ?(4 votes)
- No, the nucleoids of prokaryotic cells do not have membranes. In fact, prokaryotic cells do not have membrane-bound organelles whatsoever.(2 votes)