Carbon Compounds in Cells
Carbon has unique quantum properties that make it the backbone of all
organic compounds. It cycles through the atmosphere and the biomass of living organisms
through various biochemical pathways. Carbon is cool.
- Properties of Organic Compounds
- An organic compound consists of carbon and one or more
additional elements, covalently bonded to one another.
- Effects of Carbons Bonding Behavior
- Oxygen, hydrogen, and carbon are the most abundant elements in living matter.
- Much of the H and O are linked as water.
- Carbon can share pairs of electrons with as many as four other atoms to form organic
molecules of several configurations.
- A carbon atom can rotate freely around a single covalent bond.
- A double covalent bond restricts rotation.
- Such interactions help give rise to the three-dimensional shapes and functions of
- Hydrocarbons and Functional Groups
- In hydrocarbons, only hydrogen atoms are attached to the
carbon backbone; these molecules are quite stable.
- Functional groups are atoms or groups of atoms covalently bonded to a carbon
backbone; they convey distinct properties, such as solubility and chemical reactivity, to
the complete molecule.
- How Cells Use Organic Compounds
- Five Classes of Reactions
- Enzymes are a special class of proteins that mediate five categories of
- functional-group transfer from one molecule to another,
- electron transfer stripped from one molecule and given to another,
- rearrangement of internal bonds converts one type of organic molecule to
- condensation of two molecules into one,
- cleavage of one molecule into two.
- In a condensation reaction, one molecule is stripped of its
H+, another is stripped of its OH- ;
then the two molecule fragments join to form a new compound and the H+ and OH form
- Hydrolysis is the reverse: one molecule is split by the addition of H+ and
OH (from water) to the components.
- The Simple Sugars
- Monosaccharides one sugar unit are the simplest carbohydrates.
- They are characterized by solubility in water, sweet taste, and several OH
- Ribose and deoxyribose (five-carbon backbones) are building blocks for nucleic acids.
- Glucose and fructose (six-carbon backbones) are used in assembling larger carbohydrates.
- Other important molecules derived from sugar monomers include glycerol and vitamin C.
- Short-Chain Carbohydrates
- An oligosaccharide is a short chain of two or more sugar
- Disaccharidestwo sugar unitsare the simplest.
- Lactose (glucose + galactose) is present in milk.
- Sucrose (glucose + fructose) is a transport form of sugar used by plants and
harvested by humans for use in food.
- Maltose (two glucose units) is present in germinating seeds.
- Oligosaccharides with three or more sugar monomers are attached as short side chains to
proteins where they participate in membrane function.
- Complex Carbohydrates
- A polysaccharide is a straight or branched chain of
hundreds or thousands of sugar monomers.
- Starch is a plant storage form of energy, arranged as unbranched coiled chains,
easily hydrolyzed to glucose units.
- Cellulose is a fiber-like structural materialtough, insolubleused in
plant cell walls.
- Glycogen is a highly branched chain used by animals to store energy in muscles
- Chitin is a specialized polysaccharide with nitrogen attached to the glucose
units; it is used as a structural material in arthropod exoskeletons and fungal cell
Lipids are greasy or oily non-polar compounds that function in energy
storage, membrane structure, and coatings.
- Fatty Acids
- A fatty acid is a long chain of mostly carbon and hydrogen atoms with a COOH group
at one end.
- When they are part of complex lipids, the fatty acids resemble long, flexible tails.
- Unsaturated fats are liquids (oils) at room temperature because one or more double bonds
between the carbons in the fatty acids permits "kinks" in the tails.
- Saturated fats (triglycerides) have only single CC bonds in their fatty acid tails
and are solids at room temperature.
- Triglycerides (Neutral Fats)
- These are formed by the attachment of one (mono-), two (di-), or three (tri-) fatty
acids to a glycerol.
- They are a rich source of energy, yielding more than twice the energy per weight basis
- They are formed by attachment of two fatty acids plus a phosphate group to a glycerol.
- They are the main structural material of membranes where they arrange in bilayers.
- Sterols and Their Derivatives
- Sterols have a backbone of four carbon rings but no fatty acid tails.
- Cholesterol is a component of cell membranes in animals and can be modified to form sex
hormones (testosterone and estrogen) and vitamin D.
- They are formed by attachment of long-chain fatty acids to long-chain alcohols or carbon
- They serve as coatings for plant parts and as animal coverings.
Proteins are composed of amino acids. They function as enzymes, in
cell movements, as storage and transport agents, hormones, antibodies, and structural
- Structure of Amino Acids
- Amino acids are small organic molecules with an amino
group, a carboxyl group, and one of twenty varying R groups.
- All of the parts of an amino acid molecule are covalently bonded to a central carbon
- Primary Structure of Proteins
- Primary structure is defined as ordered sequences of amino acids each linked
together by peptide bonds to form polypeptide chains.
- There are 20 kinds of amino acids available in nature.
- The sequence of the amino acids is determined by DNA and is unique for each kind of
- Fibrous proteins have polypeptide chains organized as strands or sheets; they
contribute to the shape, internal organization, and movement of cells.
- Globular proteins, including most enzymes, have their chains folded into compact,
- The Three-Dimensional Structure of Proteins
Three-dimensional structure is determined by how amino acid sequences
present their atoms for hydrogen bonding.
- 2° Protein Structure
- Secondary structure refers to the helical coil
(as in hemoglobin) or sheet-like array (as in silk) that results from hydrogen bonding of
side groups on the amino acid chains.
- The peptide bonds between the amino acids of primary structure allow slight bending to
permit secondary structure.
- 3° Protein Structure
- Tertiary structure is the result of folding due
to interactions among R groups along the polypeptide chain.
- The result is a more compact, globular shape in complex proteins.
- 4° Protein Structure
- Quaternary structure describes the complexing of
two of more polypeptide chains.
- Hemoglobin is a good example of four interacting chains that form globular proteins; keratin and collagen are complex
- Glycoproteins and Lipoproteins
- Lipoproteins have both lipid and protein components; they transport fats and
cholesterol in the blood.
- Glycoproteins consist of oligosaccharides covalently bonded to proteins; they are
abundant on the exterior of animal cells, as cell products, and in the blood.
- Structural Changes by Denaturation
- High temperatures or changes in pH can cause a loss of a proteins normal
three-dimensional shape (denaturation).
- Normal functioning is lost upon denaturation, which is often irreversible (for example,
a cooked egg).
- Nucleotides and the Nucleic Acids
Nucleic acids are polymers of nucleotides.
Nucleotides and nucleic acids are the molecular source of what we
know as life. They perpetuate themselves by a self-replicating process that began on our
planet about 3.5 billion years ago. Since they have achieved relatively complex systems
(cells, organelles, organisms
) for preserving themselves, demonstrating the ultimate
example of teleology. Everything from our self-sustaining metabolic pathways to our social
behavior participates intimately in insuring their own molecular integrity.
- Nucleotides with Roles in Metabolism
- Each nucleotide consists of a five-carbon sugar (ribose or deoxyribose), a
nitrogen-containing base, and a phosphate group.
- Adenosine phosphates are chemical messengers (cAMP) or
energy carriers (ATP).
- Nucleotide coenzymes transport hydrogen atoms and electrons (examples: NAD+ and FAD).
- Nucleic Acids - DNA and RNA
The two most important nucleic acids are DNA and RNA.
- Four different kinds of nucleotides are strung together to form large single or
- Each strands backbone consists of joined sugars and phosphates with nucleotide
bases projecting toward the interior.
- DNA is a double-stranded helix that stores encoded
- RNA is single stranded and functions in translating the
code to build proteins.