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In triglycerides (fats and oils), long carbon chains known as fatty acids may contain double bonds, which can be in either the cis or trans configuration, illustrated in [link] . Fats with at least one double bond between carbon atoms are unsaturated fats. When some of these bonds are in the cis configuration, the resulting bend in the carbon backbone of the chain means that triglyceride molecules cannot pack tightly, so they remain liquid (oil) at room temperature. On the other hand, triglycerides with trans double bonds (popularly called trans fats), have relatively linear fatty acids that are able to pack tightly together at room temperature and form solid fats. In the human diet, trans fats are linked to an increased risk of cardiovascular disease, so many food manufacturers have reduced or eliminated their use in recent years. In contrast to unsaturated fats, triglycerides without double bonds between carbon atoms are called saturated fats, meaning that they contain all the hydrogen atoms available. Saturated fats are a solid at room temperature and usually of animal origin.

Oleic acid and eliadic acid both consist of a long carbon chain. In oleic acid the chain is kinked due to the presence of a double bond about half way down, while in eliadic acid the chain is straight.
These space-filling models show a cis (oleic acid) and a trans (eliadic acid) fatty acid. Notice the bend in the molecule cause by the cis configuration.


Enantiomers are molecules that share the same chemical structure and chemical bonds but differ in the three-dimensional placement of atoms so that they are mirror images. As shown in [link] , an amino acid alanine example, the two structures are non-superimposable. In nature, only the L-forms of amino acids are used to make proteins. Some D forms of amino acids are seen in the cell walls of bacteria, but never in their proteins. Similarly, the D-form of glucose is the main product of photosynthesis and the L-form of the molecule is rarely seen in nature.

Molecular models of D-and L-alanine are shown. The two molecules, which contain the same number of carbon, hydrogen, nitrogen atoms, are mirror images of one another.
D-alanine and L-alanine are examples of enantiomers or mirror images. Only the L-forms of amino acids are used to make proteins.

Functional groups

Functional groups are groups of atoms that occur within molecules and confer specific chemical properties to those molecules. They are found along the “carbon backbone” of macromolecules. This carbon backbone is formed by chains and/or rings of carbon atoms with the occasional substitution of an element such as nitrogen or oxygen. Molecules with other elements in their carbon backbone are substituted hydrocarbons .

The functional groups in a macromolecule are usually attached to the carbon backbone at one or several different places along its chain and/or ring structure. Each of the four types of macromolecules—proteins, lipids, carbohydrates, and nucleic acids—has its own characteristic set of functional groups that contributes greatly to its differing chemical properties and its function in living organisms.

A functional group can participate in specific chemical reactions. Some of the important functional groups in biological molecules are shown in [link] ; they include: hydroxyl, methyl, carbonyl, carboxyl, amino, phosphate, and sulfhydryl. These groups play an important role in the formation of molecules like DNA, proteins, carbohydrates, and lipids. Functional groups are usually classified as hydrophobic or hydrophilic depending on their charge or polarity characteristics. An example of a hydrophobic group is the non-polar methane molecule. Among the hydrophilic functional groups is the carboxyl group found in amino acids, some amino acid side chains, and the fatty acids that form triglycerides and phospholipids. This carboxyl group ionizes to release hydrogen ions (H + ) from the COOH group resulting in the negatively charged COO - group; this contributes to the hydrophilic nature of whatever molecule it is found on. Other functional groups, such as the carbonyl group, have a partially negatively charged oxygen atom that may form hydrogen bonds with water molecules, again making the molecule more hydrophilic.

Table shows the structure and properties of different functional groups. Hydroxyl groups, which consist of OH attached to a carbon chain, are polar. Methyl groups, which consist of three hydrogens attached to a carbon chain, are nonpolar. Carbonyl groups, which consist of an oxygen double bonded to a carbon in the middle of a hydrocarbon chain, are polar. Carboxyl groups, which consist of a carbon with a double bonded oxygen and an OH group attached to a carbon chain, are able to ionize, releasing H+ ions into solution. Carboxyl groups are considered acidic. Amino groups, which consist of two hydrogens attached to a nitrogen, are able to accept H+ ions from solution, forming H3+. Amino groups are considered basic. Phosphate groups consist of a phosphorous with one double bonded oxygen and two OH groups. Another oxygen forms a link from the phosphorous to a carbon chain. Both OH groups in phosphorous can lose a H+ ion, and phosphate groups are considered acidic.
The functional groups shown here are found in many different biological molecules.

Hydrogen bonds between functional groups (within the same molecule or between different molecules) are important to the function of many macromolecules and help them to fold properly into and maintain the appropriate shape for functioning. Hydrogen bonds are also involved in various recognition processes, such as DNA complementary base pairing and the binding of an enzyme to its substrate, as illustrated in [link] .

Molecular models show hydrogen bonding between thymine and adenine, and between cytosine and guanine. These four DNA bases are organic molecules containing carbon, nitrogen, oxygen, and hydrogen in complex ring structures. Hydrogen bonds between the bases hold them together.
Hydrogen bonds connect two strands of DNA together to create the double-helix structure.

Section summary

The unique properties of carbon make it a central part of biological molecules. Carbon binds to oxygen, hydrogen, and nitrogen covalently to form the many molecules important for cellular function. Carbon has four electrons in its outermost shell and can form four bonds. Carbon and hydrogen can form hydrocarbon chains or rings. Functional groups are groups of atoms that confer specific properties to hydrocarbon (or substituted hydrocarbon) chains or rings that define their overall chemical characteristics and function.

Art connections

[link] Which of the following statements is false?

  1. Molecules with the formulas CH 3 CH 2 COOH and C 3 H 6 O 2 could be structural isomers.
  2. Molecules must have a double bond to be cis - trans isomers.
  3. To be enantiomers, a molecule must have at least three different atoms or groups connected to a central carbon.
  4. To be enantiomers, a molecule must have at least four different atoms or groups connected to a central carbon.

[link] C

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Questions & Answers

what is cell?
V.S.Nikhil Reply
The smallest structure and functional unit
Hydra reproduce through which process
Saint Reply
which is smallest organ in our body
pineal gland
Yh in the ears...
why you hand plam is sweating in everytime
who is the father of mycology
Sagar Reply
Heinrich Anton de Bary
describe the similarities and differences between cytokinesis mechanism found in animal cells versus in plant cells
hiro Reply
are humans beings considered to have the eukaryotic cells
success Reply
eukaryotes are organisms that possess cells with a nucleus enclosed in a membrane, humans, and all complex organisms are eukaryotes.
so humans and animals also have cell membranes.... cause I did this test prep and they said plants...I just want to be sure
and thank you for your reply it was helpful👍✌
eu= "perfect", "good", karyon= nut, amound, nucleus
you're welcome. Plants are also eukaryotes.
plants, like animals, possess a nucleus bound by a membrane.
similarities and differences between cytokinesis mechanism found in animal cell vs cell division
Raymark Reply
what is the name of a male flower?
Ikeomu Reply
staminate means flower containing only stamen
what is the definition of evolution in a population?
Homero Reply
the slow changing of a species to adapt to any changes in the environment or how it feeds/hunts. im not good at explaining things lol.
the organ which is sensitive to light in euglena
Fatimah Reply
the organ which is sensitive to light in euglena is
all chlorophyll containing motile cells are sensitive to light
there is no more other chapter
Sandeep Reply
Give tow examples for nutritional deficiency Diseases-
Singampalli Reply
How does a plant cell look like
Sang Reply
in a sleepers form
what do you mean ? I could not understand
they have a regular shape and a large vacoule
I thought it looked like rectangle
a stage in mitosis wherein in spindle fibers begin to shorten to pu the sister chromatids away from each other towards the opposite ends of the cell
Earl Reply
a stage in interphase where chromosome s are duplicated
What is biodiversity
Sp Reply
Name two secretions of Golgi apparatus
Daniel Reply

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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