Carbohydrates, Polysaccharides, and Biological Membranes

Posted on May 12, 2024 in Biology

Sept. 05 – B1OA

Carbohydrates

The simplest carbohydrates are composed of a molecule or monomer and are called monosaccharides. These are solid, white, crystalline, very soluble in water but insoluble in nonpolar solvents. Most of them have a sweet taste. They consist of a single unit of polyhydroxyaldehyde or polyhydroxyketone and have the empirical formula (CH2O)n, where n = 3 to 8. The carbon skeleton of the current monosaccharides is branched, and all carbon atoms except one have a hydroxyl group (-OH). In the remaining carbon atom, carbonyl oxygen exists, which, as we shall see, is frequently combined to form an acetal bond. If the carbonyl group (–C=O) is at the end of the chain, the monosaccharide is an aldehyde derivative and is called aldose; if it is in the second position, the monosaccharide is a ketone derivative called a ketose.

According to the number of carbon atoms they possess, they are called, and we will mention the most important: Trioses (3), Tetroses (4), Pentoses (5), Hexoses (6), and Heptoses (7).

Polysaccharides

Polysaccharides are high molecular weight substances and present two characteristic biological functions: as reserve substances or as structural molecules. Those performing a structural function link β-glycosidic bonds, and those performing a function of energy reserves have the α-glycosidic bond.

Homopolysaccharide Starch

Starch is the plant energy reserve, especially abundant in seeds, tubers, cereals, etc. It is formed by the union of monomers of α-D-glucose by O-glycosidic bonds (1→4) and α (1→6). Starch is synthesized during photosynthesis and is stored in starch granules within the cell, either inside or in the chloroplasts of the amyloplast.

Cellulose

Cellulose is the structural homopolysaccharide of plants, in which it is the main component of their cell wall. It is a linear polymer of beta-D-glucose molecules attached via β (1→4).

Plasma Membrane

The plasma membrane is composed of lipids, proteins, and oligosaccharides. Here they are covalently attached to proteins and lipids, forming glycoproteins and glycolipids, and are located preferably on the extracellular side, constituting the glycocalyx in animal cells.

The functions of the glycocalyx are varied: molecular recognition, protection against the action of proteolytic enzymes, and regulation of cellular uptake, varying permeability, and allowing the union of cells in tissue formation.

B2OA – Golgi Apparatus

The Golgi apparatus is a membranous organelle consisting of discoidal and flattened saccules, delimited by a unit membrane slightly dilated at the ends where vesicles appear to emerge. These sacs are grouped in stacks of 5 to 10 units called dictyosomes, which usually have a concave and a convex surface. The cavities are bounded by a unit membrane and filled with fluid. Different dictyosomes are interconnected. The Golgi apparatus is often found surrounding the nucleus or centrosome.

The Golgi apparatus has a close relationship between structure and function and, in turn, is related to the endoplasmic reticulum, as they shape their expense, and their functions are complementary. The reticulum and Golgi apparatus are called the GERL complex. The two surfaces or faces of the Golgi define two spaces in it:

  • Cara cis (external or training) is the concave side of the saccules surrounded by endoplasmic reticulum cisterns that, by budding, emit vesicles. These vesicles are called transition vesicles and are loaded with products stored in the endoplasmic reticulum. Several of them are merged with the Golgi saccules, being first in the same space. From here, new vesicles are given off that flow to the convex side, leading into the second compartment of the Golgi apparatus.
  • Cara trans (internal or ripening): Arriving at the convex side of the dictyosomes, they fragment into secretory vesicles. Several of these vesicles can merge and form secretory granules. The latter can remain in the cytoplasm or go to the extracellular space by exocytosis.

Lysosomes

Lysosomes are membrane-bound organelles that contain hydrolytic enzymes capable of degrading all kinds of biological polymers.

Cellular digestion is to break complex molecules into simpler molecules. It is carried out by lysosomes and may be of two types:

  • Intracellular: The substrates to digest can be:
    • External heterophagy: is a function of nutrition or defense against infections (e.g., leukocytes phagocytize bacteria), cleaning (e.g., macrophages phagocytose debris), resorption, destruction of substances, etc.
    • Internal: autophagy: the substrate is a cellular constituent (lots of other organelles, vacuoles, etc.). They serve to destroy damaged or unnecessary areas of the cell.
  • Extracellular: lysosomes spill their contents outside the cell by exocytosis.

The degradation of the materials incorporated into the cell by endocytosis (either phagocytosis or pinocytosis) is carried out within the lysosomes in which hydrolytic or digestive enzymes remain. First, the substance is engulfed by endocytosis with the formation of a phagocytic vesicle or phagosome. Primary lysosomes caused by the Golgi apparatus or smooth endoplasmic reticulum join the phagosome with the consequent formation of a secondary lysosome called heterophagosome or vacuole. Primary lysosomes discharge their contents, and enzymatic degradation of the incorporated substance occurs. After digestion, the resulting products cross the secondary lysosome membrane and are partly released into the hyaloplasm and partly expelled to the outside of the cell by exocytosis, the reverse of endocytosis. The chemicals in the hyaloplasm can be used for the synthesis of cell constituents that are or may be catabolized anaerobically or aerobically for energy.

Endoplasmic Reticulum

The general function of the endoplasmic reticulum is related to the synthesis and transport of molecular components, most notably those of biological membranes, proteins, and lipids. However, at the functional level, we also distinguish between rough and smooth endoplasmic reticulum:

  • Rough: Protein synthesis in ribosomes that are attached to its membrane. These synthesized proteins are discharged within the endoplasmic reticulum and are stored or transported to other organelles or cell sites. Some proteins are part of the membrane of the endoplasmic reticulum and may well become part of other cell membranes (the plasma membrane or other organelles). The beginning of protein glycosylation occurs within the endoplasmic reticulum and is completed in the Golgi apparatus.
  • Smooth: This relates to the synthesis, storage, and transport of lipids, particularly phospholipids and cholesterol. It detoxifies harmful substances for the cell from outside or inside the cell.

B3OA – Plasma Membrane Structure

Fluid Mosaic Model

At present, the most accepted model for the structure of the plasma membrane is the proposed by Singer and Nicolson in 1972. According to this model, lipid membranes have proteins and oligosaccharides that are formed into a configuration of lower free energy.

Lipids form a lipid bilayer that provides the basic structure to the membrane and acts as a relatively impermeable barrier to the flow of most water-soluble molecules. The molecules are oriented so that the polar groups are directed toward the aqueous phase, i.e., the outer layer of the membrane into the extracellular environment and the inner layer to the cytoplasm. In addition to its properties of self-assembly and self-sealing, lipid bilayers have another characteristic that makes them an ideal structure for cell membranes: their fluidity. This allows lipid molecules to move freely through the membrane, either by lateral movement or rotation about themselves.

Membrane Proteins

The location of proteins in the lipid bilayer is a function of their amphipathic character. According to their position in the membrane, there are two types of proteins: transmembrane integral or intrinsic, which are interspersed or embedded in the lipid bilayer, and peripheral or extrinsic proteins, which are usually associated with the cytoplasmic surface of the membrane. Proteins mediate the various functions of the membrane.

Glycocalyx

The oligosaccharides form the glycocalyx on the outer surface of the membrane. The vast majority are covalently linked to lipids or proteins, forming glycolipids and glycoproteins, respectively.

Integral Membrane Proteins

Integral membrane proteins are synthesized in the endoplasmic reticulum, particularly in the ribosomes attached to its external face. These proteins carry out functions in various membranes; some are carriers of substances, others are involved in cell recognition processes, while others act as receivers of various substances (hormones, metabolites, etc.).

Cholesterol

Cholesterol is responsible for regulating the fluidity of the bilayer since it interferes with the hydrocarbon chains of fatty acids and gives them stiffness, while preventing the chains from coming together and aggregating.

B4OA – The Genetic Code

The genetic code is the key or “dictionary” by which DNA, with its four constituent nucleotides, encodes each protein of, at most, twenty different amino acids. The genetic code includes all the information stored in DNA. Each of the 64 possible codons or triplets of bases identifies the 20 protein amino acids and several initiation and termination signals.

The general characteristics of the genetic code for all cell types, with no differences, are the following:

  1. Collinearity principle: three nucleotides encode an amino acid. As the number of nucleotides is four and the number of amino acids is twenty, it is impossible to have a one-to-one correspondence with two nucleotides encoding an amino acid, as the number of combinations is 16, still leaving four amino acids without encoding. Therefore, with three nucleotides per amino acid, there are 64 different combinations, i.e., 64 triplets that encode the 20 amino acids of proteins.
  2. It is degenerate, i.e., being composed of 64 codons, several triplets encode the same amino acid. Almost all have the first two nucleotides in common, providing variability in the third.
  3. The triplets do not overlap, i.e., the triplets are interpreted one after another in the 5′-3′ direction, and a nucleotide cannot belong simultaneously to two consecutive codons.
  4. The reading of the code is without commas, i.e., the sequence of nucleotides or bases starts from a point counting by threes bases without a comma to ensure an accurate reading, so if you start at a wrong point, the entire sequence will shift to the end.
  5. It has signs of the beginning and end of the reading, which are encoded by initiation (AUG) and termination (UAG, UAA, and UGA) codons.
  6. It is universal; the same triplets have the same meaning in all types of cells.

Therefore, the genetic code establishes the relationship between the sequence of nucleotides of genes and the amino acid sequence of proteins. The process that led to deciphering the code was based on the assumption stated by Beadle and Tatum in 1941 that a gene encodes the formation of an enzyme, i.e., a polypeptide chain.

In general, the discovery of the genetic code is an example of scientific progress and collaboration between different research groups. Some milestones in the process were:

Semiconservative DNA Replication

Semiconservative DNA replication was proposed by Watson and Crick and demonstrated experimentally by Meselson and Stahl in 1957. DNA replication takes place during the S-phase of the cell cycle or interface and is semiconservative because the two chains of nucleotides that form the DNA double helix are preserved and serve as a template for the synthesis of two complementary strands. Thus, replication results in two DNA molecules in which each chain is preserved old, and the other is new.

Replication begins at a place that DNA enzymes responsible for initiation recognize. The two DNA strands are unwound through the action of enzymes known as helicases, forming a replication fork. Replication starts from here in two directions, i.e., bidirectionally.

DNA Polymerases

DNA polymerases that carry out DNA replication activity have exonuclease activity, which allows them to hydrolyze phosphodiester bonds, thereby acting in DNA repair processes.

DNA Repair Mechanisms

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