Structural carbohydrates in non-plants have amino acids or contain amino acid sequencesas monomers. Plant cell walls contain relatively little protein or peptide.Carbohydrates with -glycosidic linkages can be found in some invertebrates such asinsects, shrimp, or lobster. Their exoskeleton contains chitin, which is the polymer of Nacetyl- -D-glucosamine, a monosaccharide with an amine group added onto the sugar. Inchitin, individual strands are held together by hydrogen bonds as in cellulose. Accordingly,chitin has a structural function. It is also found in the cell walls of yeasts, fungi and algae. -Glycosidic linkages also connect the two amine-group-containing monomers in bacterialcell walls: N-acetyl- -D-glucosamine and N-acetylmuramic acid. The strands are crosslinkedby amino acid residues, forming a peptidoglycan. Peptidoglycans form a strongstructure that is the target of certain antibiotic agents.
Glycogen
Glycogen is found in granules in certain types of cells in animals and humans, like liverand muscle cells, but not normally in heart and brain cells in the human organism. -Glycosidic linkages connect the glucose molecules in glycogen. As in starch, the -glycosidic linkages allow for glycogen’s function in energy storage because glucose canreadily be cleaved off.
Fig. 2.3 The peptide bond (from Campbell,
1999)
The tertiary structure of proteins adds to their actual three-dimensional structure with thehelp of covalent disulfide bonds between sulfide-containing amino acid side chains,hydrogen bonding between amino acid side chains, electrostatic forces of attraction andhydrophobic interactions. Proteins can exhibit a rod-like fibrous or a compact globularconformation, depending not only on the bonding forces mentioned above but also on theconditions under which the protein is formed (sections 4.2.2. and 4.2.3.).Proteins can also have a quaternary structure, which involves several different polypeptidechains. The bonds involved to hold this protein structure together are noncovalent.The conformation of a protein is specific to it and determines its function and itsfunctional ability.
Lipid clusters can take on either micelle or membrane-like forms to allow sequestering of
their hydrophobic portions from watery surroundings. The micelle form occurs, for
instance, when lipids are taken up into and transported in the body. Micelles have a single
layer of lipids in their structure. They get a high degree of complexity in low-density
lipoprotein (LDL) particles, for instance, in which a mosaic of cholesterol and
phospholipids bound to a protein (apoprotein B-100) forms the outer structure around
many molecules of cholesteryl esters. LDL particles play an important role in the transport
of cholesterol in the blood stream.
The triacylglycerols (triglycerides) are the energy storage form of lipids and accumulate as
fat globules in the cells of adipose tissue (see section 5.2.1.).
All lipids except for triglycerides can be found as components of membranes.
Membranes: Lipids are necessary for the formation of membranes because of their
hydrophobic property, and the consequent clustering that occurs. Membranes are bilayers
Fig. 2.4 The micelle and membrane forms of lipids. ( indicates the hydrophilic
backbone of the lipid, indicates the hydrophobic tail of the lipid). The micelle can
be large and have an inner space that is sequestered from water in which lipids can
be transported, as in LDL particles. Membranes are actually also round structures and
enclose an inner space, the interior of cells or cell compartments.
of lipids. They divide water into compartments, which is essential for the functioning of all
organisms. In single-cell organisms it makes the formation of organelles possible and
separates the organism from its (mostly aqueous) environment.
Membranes are semi-permeable mainly as a result of the presence of proteins, which
function as channels in between the lipids. This allows for the transportation of
compounds across the membrane and ensures the connection between the watery milieu
inside the membrane with that outside the membrane. Semi-permeable membranes allow
for the possibility of a different intracellular milieu from the environment. This makes the
single cell into an organism. In multi-cell organisms, membranes make differentiated
functioning possible. Cells can have different functions and yet have the same
extracellular environment. Metabolism mainly occurs in the intracellular milieu. The extracellular
milieu has an important role in transportation of metabolic substances and
connecting the cells in the organism to form a whole. In vertebrates, the separation of
intracellular and extra-cellular milieu makes functions such as the contraction of muscles
and conduction of electrical impulses in the nervous system possible.
The presence of the fused-ring lipid, cholesterol, which is rather rigid in its structure, and
of unsaturated fatty acids, which have kinks in their tail portion, influences the fluidity of
membranes in opposite ways. The membranes of prokaryotes (such as bacteria) are the
most supple of all since they hardly contain steroids. Plant membranes contain
phytosterols (a steroid similar to cholesterol) and have less fluidity. The many unsaturated
fatty acids in plant membranes make them more fluid than membranes in animals and
humans, which contain cholesterol.
Lipids (possibly except for the triacylglycerols) occur in specific structured forms in
organisms. Lipid structures such as micelles and membranes could perhaps be seen as the
polymer form of lipids. The defining interaction in these structures is the spontaneously
occurring hydrophobic interaction, supported by weak and changeable van der Waals
bonds. Hydrophobic interactions induce the relative immobility of the lipid components
toward each other.
However, all components of membranes are in flux, as the seemingly constant shape of
organisms is always in movement.
Summary and conclusion
Carbohydrates and proteins can occur as polymers, which can be broken down to
monomers. Lipids do not have polymer forms but form clusters in the watery milieu of the
organism.
Glycogen
Glycogen is found in granules in certain types of cells in animals and humans, like liverand muscle cells, but not normally in heart and brain cells in the human organism. -Glycosidic linkages connect the glucose molecules in glycogen. As in starch, the -glycosidic linkages allow for glycogen’s function in energy storage because glucose canreadily be cleaved off.
Fig. 2.3 The peptide bond (from Campbell,
1999)
Proteins
When proteins are hydrolyzed this results ina large number of amino acids. Unlike themonomers of polysaccharides, the aminoacids in proteins are of many differentforms. Twenty different amino acids arefound in human protein in varyingquantities and combinations. They arelinked together by peptide bonds to formthe primary structure of proteins. Peptidebonds in proteins are also specializedcovalent bonds, like the glycosidic bonds incarbohydrates. And, like glycosidiclinkages, peptide bonds inhibit rotation ofspecific molecules in the amino acidsaround each other and therefore play a rolein the final shape of proteins.However, the final shape of proteins is not exclusively determined by the sequence ofamino acids and the peptide bonds linking them together (the protein’s primary structureof covalent bonds). The conformation of proteins is also subject to intricate foldingprocesses connected to different types of bonds such as hydrogen bonds and disulfidebonds. The primary structure of proteins, though, determines their ability to form asecondary and tertiary structure, which is required for proteins to be biologically active inthe organism.The secondary structure of proteins is based on the hydrogen-bonded arrangement of theprotein’s amino acid backbone. It is responsible for -helix and -pleated sheet sections inthe protein chain. Hydrogen bonds are important in the final structure of collagen, astructural protein (see section 4.2.2.). The tertiary structure of proteins adds to their actual three-dimensional structure with thehelp of covalent disulfide bonds between sulfide-containing amino acid side chains,hydrogen bonding between amino acid side chains, electrostatic forces of attraction andhydrophobic interactions. Proteins can exhibit a rod-like fibrous or a compact globularconformation, depending not only on the bonding forces mentioned above but also on theconditions under which the protein is formed (sections 4.2.2. and 4.2.3.).Proteins can also have a quaternary structure, which involves several different polypeptidechains. The bonds involved to hold this protein structure together are noncovalent.The conformation of a protein is specific to it and determines its function and itsfunctional ability.
Lipids
Acetyl-CoA can be considered an important common component of lipids. In the group oflipids, fatty acids and cholesterol are both ultimately synthesized from acetyl-CoA. Theopen chain lipids contain one or more of the fatty acids, and the fused ring lipids, thesteroids, are conversions of cholesterol. Fatty acids are also oxidized to acetyl-CoA.Cholesterol derivatives are not broken down in the human body but are excreted (section5.4.1.).Lipids have the tendency to form clusters in the watery milieu of the body because theyhave long nonpolar tails which are hydrophobic. The hydrophobic fatty acid tails are”hidden” inside the cluster, sequestered from water, and at the periphery of the cluster thewater-soluble hydrophilic side, connected to the head group, is exposed (see fig. 2.4.).Hydrophobic interactions occur spontaneously in aqueous surroundings. In terms ofthermodynamics, this type of interaction does not require added energy whenhydrophobic side chains or tails are present in the watery milieu of the body. This is incontrast to the types of bonding described earlier, which do require extra energy to occur(see section 1.1.). Lipid clusters can take on either micelle or membrane-like forms to allow sequestering of
their hydrophobic portions from watery surroundings. The micelle form occurs, for
instance, when lipids are taken up into and transported in the body. Micelles have a single
layer of lipids in their structure. They get a high degree of complexity in low-density
lipoprotein (LDL) particles, for instance, in which a mosaic of cholesterol and
phospholipids bound to a protein (apoprotein B-100) forms the outer structure around
many molecules of cholesteryl esters. LDL particles play an important role in the transport
of cholesterol in the blood stream.
The triacylglycerols (triglycerides) are the energy storage form of lipids and accumulate as
fat globules in the cells of adipose tissue (see section 5.2.1.).
All lipids except for triglycerides can be found as components of membranes.
Membranes: Lipids are necessary for the formation of membranes because of their
hydrophobic property, and the consequent clustering that occurs. Membranes are bilayers
Fig. 2.4 The micelle and membrane forms of lipids. ( indicates the hydrophilic
backbone of the lipid, indicates the hydrophobic tail of the lipid). The micelle can
be large and have an inner space that is sequestered from water in which lipids can
be transported, as in LDL particles. Membranes are actually also round structures and
enclose an inner space, the interior of cells or cell compartments.
of lipids. They divide water into compartments, which is essential for the functioning of all
organisms. In single-cell organisms it makes the formation of organelles possible and
separates the organism from its (mostly aqueous) environment.
Membranes are semi-permeable mainly as a result of the presence of proteins, which
function as channels in between the lipids. This allows for the transportation of
compounds across the membrane and ensures the connection between the watery milieu
inside the membrane with that outside the membrane. Semi-permeable membranes allow
for the possibility of a different intracellular milieu from the environment. This makes the
single cell into an organism. In multi-cell organisms, membranes make differentiated
functioning possible. Cells can have different functions and yet have the same
extracellular environment. Metabolism mainly occurs in the intracellular milieu. The extracellular
milieu has an important role in transportation of metabolic substances and
connecting the cells in the organism to form a whole. In vertebrates, the separation of
intracellular and extra-cellular milieu makes functions such as the contraction of muscles
and conduction of electrical impulses in the nervous system possible.
The presence of the fused-ring lipid, cholesterol, which is rather rigid in its structure, and
of unsaturated fatty acids, which have kinks in their tail portion, influences the fluidity of
membranes in opposite ways. The membranes of prokaryotes (such as bacteria) are the
most supple of all since they hardly contain steroids. Plant membranes contain
phytosterols (a steroid similar to cholesterol) and have less fluidity. The many unsaturated
fatty acids in plant membranes make them more fluid than membranes in animals and
humans, which contain cholesterol.
Lipids (possibly except for the triacylglycerols) occur in specific structured forms in
organisms. Lipid structures such as micelles and membranes could perhaps be seen as the
polymer form of lipids. The defining interaction in these structures is the spontaneously
occurring hydrophobic interaction, supported by weak and changeable van der Waals
bonds. Hydrophobic interactions induce the relative immobility of the lipid components
toward each other.
However, all components of membranes are in flux, as the seemingly constant shape of
organisms is always in movement.
Summary and conclusion
Carbohydrates and proteins can occur as polymers, which can be broken down to
monomers. Lipids do not have polymer forms but form clusters in the watery milieu of the
organism.
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