The Role of Macromolecules in Supporting a Cell

Primarily, living cells are comprised of water. Besides water, the cells also act as home to myriad other molecules. The most critical of these are the macromolecules. These are large molecules that not only populate a cell but also give it the most crucial functions for life ( Pisetsky, 2007 ). For instance, macromolecules give a cell the much needed structural support, acts as the source of stored fuel, an allow the cell to store as well as retrieve information. Further, macromolecules play a crucial role in speeding biochemical reactions. There are four principal types of macromolecules are lipids, proteins, nucleic acids, and carbohydrates. The importance of these macromolecules in sustaining a cell’s life cannot be overstated. Macromolecules are not only large but are also made up of large numbers of atoms. Often, they are made up of long chains of atoms that exist as repetitive units, which are referred to as polymers ( Pisetsky, 2007 ). However, not all macromolecules are classified as polymers. They also play different roles in a living organism. Against this backdrop, this paper is aimed at exploring the role of macromolecules in supporting cells. 

Macromolecules: A Background 

Within human cells, small organic molecules are joined together leading to the formation of larger molecules. The resultant large macromolecules may be comprised of thousands of atoms that are covalently bonded. There are four main classes of macromolecules. These include proteins, nucleic acids, lipids and carbohydrates ( MacKerell Jr, 2004 ). These molecules form chainlike molecules that are referred to as polymers. A polymer refers to a long molecule that is comprised of many identical building blocks which are linked together using covalent bonds. The repeated units that makeup polymers are tiny molecules called monomers. While some molecules act as monomers, they also have their functions. However, the chemical mechanisms that the cells use in making or breaking the polymers are the same for the different classes of macromolecules ( MacKerell Jr, 2004 ). The covalent bonds that connect monomers form via loss of a molecule of water. The reaction via which water is lost is referred to as a dehydration or condensation reaction. Once a bond is formed between two monomers, each of the monomer involved contributes part of the molecule of water that is lost. The cells involved invest energy in the execution of dehydration reactions. This process is made possible by enzymes. 

The covalent bonds that connect monomer to form a polymer are disassembled using hydrolysis ( Bustamante et al., 2010 ). This reaction is the reverse of dehydration. During hydrolysis, bonds are broken through the addition of molecules of water. In the process, a hydrogen atom is attached to a monomer while a hydroxyl group is attached to the adjacent monomer. At the point of ingestion, food takes the form of organic polymers which implies that it is too large for the human cells to absorb it. While food is still in the digestive tract, numerous enzymes facilitate the hydrolysis of specific polymers. The monomers that result from this process are then absorbed by the cells that line the gut and are subsequently transported to the bloodstream for ease of distribution to the body cells. The body cells consequently use dehydration reaction for purposes of assembling monomers into new polymers which then execute functions that are specific to the given cell type. Various polymers are capable of being built from a set of monomers. On the other hand, each cell is comprised of numerous types of macromolecules. Often, these molecules may vary among the cells of the same person. Likewise, the molecules vary even more among unrelated individuals. This diversity is occasioned by the various combinations of common monomers including those whose occurrence is rare ( MacKerell Jr, 2004 ) . These monomers may be connected in numerous combinations hence the resultant diversity. 

Carbohydrates 

Carbohydrates are formed from sugars, also referred to as monosaccharides, and their polymers ( MacKerell Jr, 2004 ). In this case, the monosaccharides bond together and in the process lead to the formation of polysaccharides. Polysaccharides can be conceptualized as the polymers of carbohydrates. The most common monosaccharide is glucose. Glucose also forms one of the most important sugars for all plants and animals. Carbohydrates act as the energy source which is used for structure and storage for all living things. In plants, starch acts as the main source of energy while cellulose provides support and structure. In animals, the source of energy is glycogen while chitin offers support and structure. 

Carbohydrates comprise of sugars and their polymers. Monosaccharides form the simplest carbohydrates and are also referred to as simple sugars. Polysaccharides, or double sugars, on the other hand, comprise of two monosaccharides that are joined together via a condensation reaction. Polysaccharides are thus polymers that are made up of many monosaccharides. All sugars, which are the smallest carbohydrates act as fuel as well as a source of carbon. In particular, monosaccharides, especially glucose act as the primary fuel for cellular work. They act as raw materials in the synthesis of such monomers as fatty acids and amino acids ( MacKerell Jr, 2004 ). When two monosaccharides join with glycosidic linkage, they form a disaccharide using the process of dehydration. For instance, malt sugar or maltose is built following the joining of two molecules of glucose while table sugar or sucrose is formed when fructose and glucose join. Sucrose also acts as the dominant form via which sugar in plants is transported. Lastly, milk sugar or lactose is formed following the joining of galactose and glucose. Polysaccharides are polymers that are comprised of hundreds of thousands of monosaccharides which are joined using glycosidic linkages. Some polysaccharides act storage and when sugars are needed, they are hydrolyzed. Other polysaccharides act as building materials for not only cells but also the entire organism. Starch acts as a storage polysaccharide that is composed entirely of glucose monomers. Majority of these monomers are joined using one to four linkages between the glucose molecules. Amylose is the simplest form of starch. It is unbranched and leads to the formation of a helix. However, such branched types of starch as amylopectin are often more complex. 

Lipids 

Lipids take three distinct forms. These include phospholipids, steroids, and fats. The core function of lipids is insulation and energy. Fats may either be unsaturated or saturated. They are also insoluble, hence buoyant. Saturated fats are solids at room temperature and are often found in animals. The unsaturated fats, on the other hand, are mostly found in plants and are either oils or liquids at room temperature ( Simons & Vaz , 2004 ). Further, lipids that take the form of phospholipids are vital elements in membranes. Compared to other macromolecules, lipids rarely form polymers. A unifying attribute for all lipids is that they bear little or no affinity for water. This is primarily because they are mostly composed of hydrocarbons which often form covalent bonds that are nonpolar. Lipids are highly diverse in not only form but also function. While fats are not polymers strictly, they take the form of large molecules that are assembled from smaller molecules using dehydration reactions ( Tribet & Vial , 2008) . Fat is created from two different kinds of smaller molecules, notably fatty acids and glycerol. Glycerol refers to three-carbon alcohol that has a hydroxyl grouped attached to each of its carbon. A fatty acid comprises a carboxyl group which is attached to a long skeleton of about sixteen to eighteen carbons. Fats separate from water owing to the fact that the water molecules hydrogen often bond to one another and consequently exclude the fats. 

In the making of a fat, three fatty acids are joined to glycerol using an ester linkage leading to the creation of a triglyceride or triacylglycerol. The three fatty acids may be different or the same. Fatty acids may have varying lengths based on the number of carbons. The variation may also be due to the locations and number of double bonds. When the fatty acid lacks carbon-carbon double bonds, the molecule is referred to as a saturated fatty acid ( Tribet & Vial , 2008 ). In this case, the fatty acid is saturated with hydrogens. However, if the fatty acid has a single or multiple carbon-carbon double bonds, then the molecule is deemed an unsaturated fatty acid. While a saturated fatty acid comprises a straight line, an unsaturated fatty acid is characterized by a kink anytime there is a double bond. Fats made from saturated fatty acids are referred to as saturated fats while those made from unsaturated fatty acids form unsaturated fats. Most animal fats are saturated and are solid at room temperature. 

Proteins 

Proteins are vital macromolecules and boast numerous functions and structure levels. Proteins are core components of each cell in the human body. Likewise, protein is found in most bodily fluids. Protein is also found in muscles, human skin, glands, and organs. Proteins aid the body in not only repairing cells but also in making new ones ( Strick et al., 2002 ). They are also critical energy and dietary requirements particularly for expectant mothers and growing adolescents. Protein takes up more than 50% of a cell's dry mass and is instrumental in all the functions of an organism. Proteins offer such functions as transport, movement, structural support, cellular support, transportation, as well as defense against various foreign substances. The most important function is that protein enzymes act as catalysts in cells and thus help regulate metabolism by accelerating chemical reactions selectively without being consumed in the process. 

There are tens of thousands of distinct proteins in the human body, and each has a specific function and structure. Proteins are the most complex molecules structurally ( Bergmann & Peppas , 2008 ). Each form of protein boasts a three-dimensional conformation or shape whose complexity cannot be overstated. All protein polymers are formed using a similar set of twenty amino acid monomers. Protein polymers are referred to as polypeptides. A protein comprises one of more polypeptides which are coiled and folded into a particular conformation. On the other hand, the monomers that act as the basis for proteins are referred to as amino acids ( Strick et al., 2002 ). Amino acids comprise the organic molecules that have both amino and carboxyl groups. The center of an amino acid contains an asymmetric carbon atom referred to as an alpha carbon. This alpha carbon features four key components. The four include a carboxyl group, a hydrogen atom, a side chain referred to as a variable R group, and lastly, an amino group. 

Nucleic Acids 

Notable nucleic acids include ribonucleic acid (RNA) and deoxyribonucleic acid DNA (Bustamante et al., 2010). The deoxyribonucleic acid acts as the blueprint for the genetic development of all life-forms. In this regard, it's responsible for holding all the information that is necessary for efficient synthesis of proteins. On the other hand, RNA acts as a carrier for this information and subsequently takes it to the protein production sites. The body is comprised of proteins that occur in hundreds of thousands. Each of these proteins acts in a specific way for it to function effectively. This is made possible by nucleic acids, which contain all the information required for the protein to not only develop but also act as is expected of them. A polypeptide's amino acid sequence is programmed using a gene which acts as a unit of inheritance. The gene consists of DNA which is a polymer. Both DNA and RNA help living organisms in reproducing their complex components from one generation to another. The DNA offers directions for its replication and directs the synthesis of RNA. Likewise, through RNA, it controls the synthesis of proteins.

Organisms inherit DNA from their parents. Each DNA is long and contains hundreds or thousands of genes. A cell reproduces by dividing itself (Bustamante et al., 2010). However, before this takes place, copies of its DNA are passed over to the next generation of the cells. The DNA is used in encoding the information used in programming all activities of the cell. Despite this, the DNA is not involved in the cell’s operations on a day-to-day basis. Proteins are used in the implementation of all the instructions that are contained in the DNA. Each gene that is found along a DNA molecule offers direction in the synthesis of a particular type of messenger RNA molecule (mRNA) (Saha et al., 2016). Interactions take place between the mRNA molecule and the protein-synthesizing machinery of the cell. In the process, it directs the ordering of the amino acids contained in a polypeptide. Genetic information flows from the DNA to the RNA and finally to the protein (Saha et al., 2016). The synthesis of proteins takes place on cellular structures referred to as ribosomes. In the eukaryotes, the DNA is located in the cell’s nucleus. However, most ribosomes are located in the cytoplasm. The mRNA acts as an intermediary in which case it moves directions and information from the nucleus to the cell’s cytoplasm. Prokaryotes, on the other hand, lack nuclei (Saha et al., 2016). Nevertheless, they use RNA as an intermediary for carrying messages to the ribosomes from the DNA. Nucleic acids act as polymers which are made of nucleotide monomers.

Different Macromolecules and their C ontribution to M embrane Composition and Function 

Cell membranes are vital components of any cell. This is because they act as physical barriers between a cell from its environment as well as other cells (Simons & Vaz, 2004). While biological membranes vary in composition, they enjoy various similarities with regard to properties and general activities. The roles of membranes are fourfold (van Vliet et al., 2014; Simons & Vaz, 2004). Firstly, the plasma membrane which surrounds the cell is used in defining the cell’s boundary. It also acts as a barrier to permeability and thus restricts the movement of various substances in and out of the cell. A eukaryotic cell contains both a plasma membrane and others that are used in defining such organelles as nucleus and mitochondria. These membranes are also used as permeability barriers implying that the contents of different organelles are incapable of mixing freely with all the cytoplasm contents (Lipkow & Odde, 2008). Secondly, membranes are used in compartmentalizing and organizing specific activities around and within the cell (van Vliet et al., 2014). This is achieved through the association between the membranes and proteins. This association is characterized by distinct activities that take place in different regions of the plasma and organelle membranes. For instance, some enzymes may be embedded in the endoplasmic reticulum (ER) membrane (van Vliet et al., 2014). These enzymes are used in modifying protection through the addition of polysaccharides. The third function of membranes is the regulation of molecule transportation in and out of a cell, between the cytoplasm and organelles (van Vliet et al., 2014; Lipkow & Odde, 2008). Regulation of this role is carried out by particular proteins that are embedded in the cell membrane and allows only selective movement of glucose, ions, and other small molecules. The last role of the plasma membrane is the reception of signals from the environment and other cells. Often, these act as extracellular signals which take the form of proteins or small molecules. The signals are detected using specific receptors that are embedded in the membrane and which lead to changes in the cell. The process via which chemical signals are received and transmitted to the cell is known as signal transduction.

The key components of a biological membrane are proteins and lipids. All cell membranes take the form of a lipid bilayer (Simons & Vaz, 2004). Subsequently, proteins are associated with and embedded in this bilayer. Also present are carbohydrates though they are less abundant. These carbohydrates take the form of sugars and are linked to either the proteins or lipids in the cell membrane. Three major forms of lipids act as basis of the lipid bilayer. These are sterols, glycolipids, and phospholipids (Simons & Vaz, 2004). Phospholipids act as the primary type of lipid. They have both long-chain hydrocarbon tails and polar heads, a characteristic that aids in the spontaneous formation of a bilayer in water. Lipids in the bilayer boast a tail-to-tail configuration. The polar heads are exposed to water on both the intracellular and extracellular sides of the membrane. Also shielded from water are the long-chain fatty acids. Cholesterol is one of the most common lipid bilayer component (Simons & Vaz, 2004). It occurs in the animal cells’ plasma membrane. It is worth noting that different membranes boast different proportions of various lipids. For example, in a liver cell, the plasma membrane contains a higher proportion of cholesterol compared to mitochondria membranes. Conversely, membranes of the mitochondria boast a higher proportion of phosphatidylethanolamine compared to the plasma membrane.

While the lipid bilayer defines a membrane’s basic structure, it is the proteins linked to the membrane that facilitate execution of a membrane’s specific functions (Simons & Vaz, 2004). For example, a cell’s plasma membrane contains proteins which act as receptors for the extracellular signals and facilitate the transmission of signals to the cell's interior. The mitochondria’s inner membranes are comprised of different protein complexes which play a role in adenosine triphosphate (ATP) synthesis, particularly during oxidative phosphorylation. There is a wide range of proteins that are linked with various membranes. However, a protein can only associate with the membrane in a few ways (Lipkow & Odde, 2008). For instance, the integral membrane proteins are embedded partially in the lipid bilayer. Transmembrane proteins, on the other hand, traverse the lipid bilayer either in one or more passes. Proteins don’t need to be inserted into the lipid bilayer for them to be associated directly. Lipid-anchored proteins boast a lipid that is covalently linked. This lipid is inserted on a single lipid bilayer. On the bilayer’s inner side, the protein is linked covalently to a prenyl or fatty acid. However, on the extracellular side, it is linked covalently to a glycosylphosphatidylinositol (GPI). Lastly, the peripheral proteins, though juxtaposed to the lipid’s bilayer, they don’t interact directly often choosing to instead bind to a different membrane protein.

Prokaryotes and Eukaryotes : Targeting and Distribution of Different Components, Diffusion, and Energy Synthesis 

Cells are classified into two broad categories namely eukaryotic and prokaryotic ( Freeman, 2017 ). The prokaryotes and single-celled and are the basis of such Archaea and Bacteria. Eukaryotes, on the other hand, include fungi, protists, and plant and animal cells. All cells have four key components which include a plasma membrane, cytoplasm, DNA, and lastly ribosomes ( Freeman, 2017 ). Despite this similarity, eukaryotic and prokaryotic cells differ significantly in various ways. For instance, a prokaryotic cell is not only simple but is also a unicellular and single-celled organism. It lacks a nucleus as well as any other organelle that is membrane-bound. This is as opposed to eukaryotes. The DNA of prokaryotes is located at the center of the cell. This region is referred to as a nucleoid and is dark in color. However, as opposed to eukaryotes and archaea, bacteria boast a wall that is made up of peptidoglycan ( Freeman, 2017 ). The peptidoglycan are comprised of amino acid, and majority boast a polysaccharide capsule. This cell wall offers an extra layer of protection and aids in maintaining the cell shape as well as preventing dehydration. The capsule allows the cell to attach to surfaces that occur in its environment. 

Prokaryotes have such components as pili, flagella, and fimbriae. The pili are used in exchanging genetic material during conjugation or reproduction while flagella promote locomotion. The relationship between function and form in nature is apparent the different levels. This narrative is true for cells as is true for other organisms. Eukaryotic cells have a nucleus that is membrane-bound as well as other membrane-bound compartments. The latter are referred to as organelles or sacs and have special functions. The term ‘eukaryotic’ alludes to availability of membrane-bound nucleus in the cells. Organelles are smaller than organs and bear specialized functions as is the case for other body organs. Prokaryotes boast a cellular barrier referred to as cytoplasmic membrane ( Spitzer & Poolman, 2013; Ren & Paulsen, 2007; Pohl et al., 2005). This barrier offers selective entry and exit of substances and comprises two layers, also referred to as a phospholipid bilayer. The cytoplasmic membrane is not only selectively permeable but also has a concentration gradient. These attributes play a crucial role in facilitating transport. 

Transport in prokaryotes may be passive or active ( Pohl et al., 2005). While active transport requires energy, passive transport does not. The latter entails such processes as simple and facilitated diffusion and osmosis. The cell membranes of the eukaryotes adhere to the fluid mosaic model that the prokaryotic membrane has. In the eukaryotes, this membrane takes the form of a dynamic structure that governs the passage of particles and dissolved molecule in and out of the cytoplasm. Nevertheless, it lacks the enzymes that the prokaryotic cell has. It also does not function in DNA replication. For both cell types to communicate, movement of materials via the cell membrane is requisite. This movement is made possible by various mechanisms. Notable among these is diffusion ( Schavemaker et al., 2018; Mika & Poolman , 2011). Diffusion refers to the movement of molecules from a high concentration region to a low concentration one. ( Schavemaker et al., 2018; Pohl et al., 2005). This takes place due to the constant collision of molecules and their net movement from the high concentration region. This movement is random and is made possible by the concentration gradient. In particular, the molecules move down this concentration gradient. Facilitated diffusion, on the other hand, is made possible by specific proteins in the cell membrane ( Schavemaker et al., 2018; Mika & Poolman , 2011). These proteins only allow specific molecules to pass through the membrane besides encouraging movement from a high to a low region of concentration. Osmosis is a form of diffusion which facilitates movement of water from a high concentration region to one of low concentration ( Schavemaker et al., 2018). Osmosis takes place across a semi-permeable membrane, implying that the membrane allows molecules to pass through selectively. However, osmosis involves water only. 

Since prokaryotes are single-celled, they do not contain a nucleus or membrane-bound organelles ( Freeman, 2017 ; Mika & Poolman , 2011). In contrast, eukaryotes have organelle. Cellular respiration occurs in the mitochondria leading to the conversion of nutrients into ATP, which is the unit for energy storage in a cell ( Lane & Martin , 2015; Atteia et al., 2013; Inoue et al., 2003). Prokaryotes also produce ATP. However, in this case, the enzymes involved are attached to the cellular membrane. This membrane folds or adjusts in a bid to promote the production of ATP via these enzymes if necessary. Prokaryotes are either heterotrophic or autotrophic. Autotrophs are capable of synthesizing their own food for energy production. Conversely, heterotrophs have to consume food for use in producing energy. 

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