Question

Moats helped protect a castle from unwanted intruders, while drawbridges allowed easy passage to and from the castle. Similarly, prokaryotic cells and eukaryotic cells have a structure called a plasma membrane that controls what may pass into and out of their internal environments. What are the components of the plasma membrane? What is its function? Most importantly, how does the plasma membrane maintains the cell’s homeostasis? As you think about these questions, list what you know about the plasma membrane and what you would like to know.

Answer 1

The cell membrane is composed of molecules like phospholipids, cholesterol, and proteins. The cell membrane functions are isolation, protection, transport, communication. Through transport, the cell keeps homeostasis.

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MEMBRANE COMPOSITION

The cell membrane is composed of two lipidic bilayers, cholesterol, proteins, and glucans incrusted in between.

⇒ Lipids

• Phospholipids are amphipathic molecules.

• They have hydrophilic heads facing the exterior and the interior of the cells and hydrophobic tails that arrange against each other in the interlayer space.

• Lipids can easily change places with other lipids by lateral diffusion in the same layer.

• They can diffuse transversally to the other layer but not as easily as lateral diffusion.

• There are also other lipidic movements as rotational diffusion that imply the rotation of the molecule.

⇒ Cholesterol

These molecules are incrusted in the membrane between the hydrophobic tails of lipids.

⇒ Proteins

• Among the proteins, we can find integral proteins and peripheric proteins.

• Integral proteins are permanently associated with the membrane. They accomplish many different functions such as substances transport, cellular receptors, cellular adhesion, among others.

According to how they are incrusted in the lipidic bilayer, integral proteins might be,

→ Transmembrane proteins ⇒  span the entire membrane                                              

→ Monotypic integral proteins ⇒ attached to only one of the lipidic layers

Integral proteins provide structure to the plasmatic membrane.

• Periferic proteins are in the internal or external surface but not incrusted in the membrane.

MEMBRANE FUNCTION

The cell membrane accomplish many different functions. Among them, we can mention,

• Isolation  ⇒  It separates the intracellular content from the

                              extracellular space.

• Protection  ⇒  It protects the interior of the cell from any damaging

                                 external factor that might affect is correct functioning.

• Transport  ⇒  It allows the selective pass of certain molecules or ions.    

                                Substances interchange

   → Passive transport → Simple diffusion  →  through lipids

                                    → Facilitated diffusion → Assited by channel proteins

                                    → Does not need energy

                                    → Depends on the molecule size, concentration at each side of the membrane, and the ions charges.

                                    → Occurs in favor of the electrochemical gradient

    → Active transport  → It does energy to occur

                                    → Mediated by carrier proteins

                                    → Transport big-sized molecules

                                    → Against the electrochemical gradient

       

     → Endocytosis/Exocytosis →  Active transport mediated by vesicles                        

• Communication ⇒ By emision and reception of substances and  

                                         molecules,  cell can communicate with other cells.  

                                         This process is always mediated by membranes

                                         and their transport  proteins or receptor proteins.

HOMEOSTASIS

Homeostasis refers to stability or equilibrium between intracellular and extracellular space.

Since the extracellular content is in constant change, the cell needs to keep a dynamic equilibrium between the interior and the exterior.

The main way of doing so is by substances transport.

Cells need to equilibrate substances concentrations in both sides of the membrane, so by transport, they take molecules in and out of the cell until they reach equilibrium.

In this way, the cell ensures its correct functioning.

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