Cell Membrane Structure, Function, and Transport Mechanisms

Posted on Dec 10, 2024 in Biology

The Plasma Membrane

The plasma membrane is a biological membrane that separates the interior of a cell from the extracellular environment. It allows interactions and is selective.

Composition

  • Lipids: These are amphipathic molecules. The main ones are phospholipids (which have hydrophobic tails and polar heads), cholesterol (found only in animals, interspersed between phospholipids), and glycolipids (occurring in the outer monolayer; in plants, they appear more often).
  • Proteins: These fulfill the biological functions of the membrane. They are associated with carbohydrates, forming glycoproteins. According to their localization in the bilayer, there are two types: intrinsic (embedded in the bilayer) and extrinsic (adhering to the membrane).

Membrane Models

  • Unit Membrane Model: The layer of fatty acids is clear, and the protein layer is dark. This model is fulfilled in all biological plasma membranes.
  • Fluid Mosaic Model: This model establishes four points:
    • The structural basis of the membrane is a lipid bilayer of amphipathic lipids (phospholipids).
    • Cholesterol (in animals) and proteins are associated with the bilayer.
    • Biological membranes are fluid because lipids and proteins can rotate.
    • The rest of the oligosaccharides form the glycocalyx.

Membrane Functions

  • Defense.
  • Selective permeability and nutrient transport.
  • Receives signals from the environment and communicates them from the exterior to the interior.
  • Allows communication between cells.
  • Allows cell recognition.

Cell Junctions

  • Tight Junctions: These take place between tightly bound cells. They are very strong and seal the space between cell membranes. They occur in epithelial cells.
  • Desmosomes: These occur in epithelial cells and are very flexible. Physical contact is made by proteins (cadherins). Desmosomes consist of dense plaques (desmoplakin).
  • Adherens Junctions: These are flexible and provide stability to epithelial tissue. They are also mediated by cadherins, but these are attached to actin, and catenins are involved.
  • Gap Junctions: These are found in most animal cells and allow direct connection between the cytoplasm of adjacent cells. They are formed by two connexons, each composed of six connexins.

Types of Nucleotides

The phosphate groups of nucleotides are linked by high-energy bonds. Nucleotide triphosphates are used as energy carriers in the cell because the hydrolysis of these bonds releases energy. The most important energy carrier is adenosine triphosphate (ATP).

  1. Coenzyme A: Transports acyl groups in fatty acid metabolism.
  2. Cyclic AMP: An important second messenger in the response of cells to various hormones.
  3. Flavin Adenine Dinucleotide (FAD) and Flavin Mononucleotide (FMN).
  4. Nicotinamide Adenine Dinucleotide (NAD): NAD-NADH.
  5. Nicotinamide Adenine Dinucleotide Phosphate (NADP): NADP-NADPH.

DNA

DNA is a nucleic acid comprising chains of deoxyribonucleotides. Its nitrogenous bases are adenine, guanine, cytosine, and thymine.

Structures of DNA

  1. Primary Structure: This is a sequence of deoxyribonucleotides chained together, containing the genetic information.
  2. Secondary Structure: This is a double helix structure that explains the storage and behavior of DNA. It was proposed in 1953 by Watson and Crick.
  3. Tertiary Structure or Supercoiled DNA: The double helix is compacted and coils on itself, occupying less space. In prokaryotes, mitochondria, and chloroplasts, it is folded as a supercoiled circle and is associated with proteins.

Chargaff’s Rules

  1. The amount of adenine is equal to that of thymine (A = T).
  2. The amount of guanine is equal to that of cytosine (G = C).
  3. The proportion of purine bases is equal to that of pyrimidine bases. Therefore, the relationship between the two base types is equal to 1: (A + G) / (T + C) = 1.

Properties of the DNA Double Helix

  1. The two helices are held together by hydrogen bonds, where adenine pairs with thymine via two hydrogen bonds, and guanine pairs with cytosine via three hydrogen bonds.
  2. Base pairing can only occur when the orientation of the helices is antiparallel (one 5′-3′, and the other 3′-5′) due to complementarity.
  3. The diameter of the double helix is 20 Å. The nucleobases are stacked over each other at a distance of 3.4 Å. Every 34 Å (ten pairs), there is a complete turn.

Types of DNA

  1. Double-Stranded Circular: A double helix that covalently closes to form a circular dsDNA ring. It is found in prokaryotic cells, mitochondria, and chloroplasts.
  2. Double-Stranded Linear: This is the DNA of many eukaryotic cells and viruses. It appears associated with histones.
  3. Single-Stranded Linear or Circular: This is very common and is present in some viruses.

RNA

Ribonucleic acid (RNA) is composed of chains of ribonucleotides. Its nitrogenous bases are adenine, guanine, cytosine, and uracil. Most RNA molecules are single-stranded.

Types of RNA

  1. Messenger RNA (mRNA): This is a single-stranded linear chain of polyribonucleotides whose base sequence is complementary to a DNA fragment (gene). The DNA fragment that is transcribed into mRNA is translated in the cytoplasm into proteins. Its function is to transfer the genetic information from DNA to the cytoplasm for the synthesis of a specific protein. There is a specific mRNA for each protein.
  2. Nucleolar RNA: This is part of the nucleolus of eukaryotic cells and is synthesized from DNA regions of the nucleolar organizer (the piece of DNA that is transcribed to give rise to the nucleolus).
  3. Ribosomal RNA (rRNA): This is a double helix chain of polyribonucleotides that associates with proteins to form ribosomes. In prokaryotes, ribosomes are 70S, and in eukaryotes, ribosomes are 80S.
  4. Transfer RNA (tRNA): This is a small polyribonucleotide chain containing between 75 and 90 nucleotides, characterized by having a large number of unusual nucleotides. It has a cloverleaf structure. It is found in the cytoplasm and is responsible for joining amino acids and placing them in the exact spot indicated by mRNA during translation. Two important regions:
    1. Central Loop: It has a sequence of three bases (triplet) called the anticodon, which specifically pairs with three other bases (codon) in mRNA.
    2. 3′ OH End: Where the amino acid binds.

Transmembrane Transport of Ions and Small Molecules

Passive Transport

Performed along the concentration gradient, i.e., from the area of highest to lowest concentration, and requires no energy expenditure. It can be carried out by different mechanisms:

  • Simple Diffusion: This is the simplest mechanism by which molecules can cross the cell membrane. It occurs with gases like CO2 or oxygen and small hydrophobic, polar but uncharged molecules, which dissolve in the lipid bilayer and diffuse rapidly through it until the concentration is balanced on both sides of the plasma membrane. No membrane proteins are involved.
  • Facilitated Diffusion: It differs from simple diffusion in that the transported molecules do not dissolve in the lipid bilayer, but the traffic is mediated by proteins that allow the transported molecules to pass through the membrane without interacting directly with its hydrophobic interior. Therefore, it allows polar and charged molecules such as glucose, amino acids, nucleosides, and ions like H+, Na+, K+, Cl-, etc., to penetrate the plasma membrane. It is slower than simple diffusion since it takes place through a limited number of proteins, which can also become saturated. Two types of proteins are involved in this type of transport: channel proteins and carrier proteins.

Channel Proteins

Channel proteins form open pores in the membrane, allowing small molecules and appropriate charges to pass freely through the lipid bilayer. The best-characterized channel proteins are ion channels, which are present in all cells and are involved in the transport of ions across the plasma membrane. These channels are not permanently open, are highly selective, and facilitate very fast transport. Many cells also contain in their membranes channel proteins called aquaporins, through which water molecules can pass much faster than by diffusing through the lipid bilayer.

Carrier Proteins

Carrier proteins are responsible for transporting sugars, amino acids, and nucleosides in most cells. These proteins bind to molecules that have to be transported on one side of the membrane, then undergo a reversible conformational change that allows the molecules to pass through the membrane and then release them to the other side.

Active Transport

In many cases, the cell must transport molecules against a concentration gradient or electrochemical gradient by an active transport mechanism that requires energy expenditure by the cell. To perform active transport of ions across the membrane, all cells have membrane proteins called ion pumps that derive their energy from ATP hydrolysis. There are different types of pumps, including the Na+-K+ ATPase, also called the Na+-K+ pump.

Vesicular Transport of Particles of High Molecular Weight

If macromolecules and large particles are transported, they need to distort the plasma membrane since these substances will enter through vesicles surrounded by the membrane. There are three types of vesicular transport:

Endocytosis

It is the entry of particles into the cell by vesicles formed from the plasma membrane. There are several forms of endocytosis:

  • Phagocytosis: If microorganisms or cellular debris are taken in through the issuance of expansions of the cytoplasm and membrane called pseudopodia, which surround the particle and incorporate it into a vesicle called a phagosome, which subsequently merges with a lysosome for digestion of the particle.
  • Pinocytosis: If extracellular fluids or macromolecules are taken in, it is carried out by the invagination of the plasma membrane, forming depressions called caveolae that are subsequently strangled and become pinocytotic vesicles.
  • Receptor-Mediated Endocytosis: This is a selective entry mechanism of specific extracellular macromolecules, referred to as ligands, that bind to specific receptors on the cell surface. These receptors accumulate in coated pits with a filamentous protein called clathrin. These depressions invaginate to form small clathrin-coated vesicles, which in the cell detach from the lining and form vesicles called endosomes. These fuse with lysosomes to degrade their contents or send them to the plasma membrane for recycling. An example is the incorporation of cholesterol transported in the form of LDL.

Exocytosis

This is the expulsion of waste substances or metabolites from the cell, previously enclosed in a vesicle called an exocytic vesicle, which moves to the cell surface, binds to the plasma membrane, and fuses with it, spilling its contents to the outside.

Transcytosis

This is a process that occurs in cells with polarization, such as epithelial cells, and involves the entry of a particle through one pole of the cell and its exit through the opposite pole after crossing the cytoplasm.