Chapter 5: An Introduction to Carbohydrates
Functions of Carbohydrates
Energy source- glucose is rapidly metabolized
Energy storage- starch and glycogen
Structural support- cellulose, chitin and cartilage
Transport of energy source- sucrose (plants), lactose (milk)
Cell surface signals- cell communication and cell-cell recognition
An organ is identified as belonging to the body by carbohydrates
Monosaccharides- are single sugars (simple sugars)
Ribose- present in RNA
Deoxyribose- present in DNA
Glucose- present in starch, cellulose and glycogen
Galactose- present in cartilage (cartilage is found in fish and part of the human body)
The carbonyl group that serves as one of the monosaccharide’s distinguishing features can
be found either at the end of the molecule, forming an aldehyde sugar (aldose), or within the
carbon chain, forming a ketone sugar (ketose). The presence of the carbonyl group along with
multiple hydroxyl groups provides an array of functional groups in sugars. Glucose and
Galactose are both 6-carbon sugars that only differ by their variation in the spatial orientation
of their hydroxyl groups (however, they are not mirror images).
Fig. 5.3 Sugars Exist in Linear and Ring Forms
Although the linear form of glucose is rare, in solution almost all glucose molecules
spontaneously bend into one of two ring structures, called the alpha and beta forms of glucose.
The difference between the two forms lies in whether the hydroxyl group on carbon #1 is above
or below the plane of the ring. The two forms exist in equilibrium, but the beta form is more
common because it is slightly more stable than the alpha form.
The ring structure forms when the oxygen from the hydroxyl group bonds with the
carbon from the carbonyl group.
Alpha glucose- OH group at carbon number 1 is below the plane of the ring
Beta glucose- OH group at carbon number 1 is above the plane of the ring
Fig. 5.4 Monosaccharides Polymerize through Formation of Glycosidic Linkages
A Glycosidic linkage occurs when hydroxyl groups on two monosaccharides undergo a
condensation reaction to form a bond. The linkage between carbon 1 and carbon 4 (for both
maltose and lactose), the formation of different monosaccharides form these new
disaccharides, creating alpha- 1,4- Glycosidic linkages.
Disaccharides Are double sugars that consist of two monosaccharides, which are joined through a
condensation (dehydration) reaction.
GLUCOSE + GLUCOSE=MALTOSE & GLUCOSE + FRUCTOSE=SUCROSE & GLUCOSE +
The liquid form of sucrose is now used as a type of sweetner; it is good for the quick
production of food and for making candy.
Be sure to know which carbons the Glycosidic linkages form between.
Real World Examples
Lactose Intolerance and Galactosemia (Box 5.1)
Lactose intolerance is the inability for people to digest lactose, which is common in milk,
cheese, yogurt, ice cream, candy, cake, etc.
It is an autosomal recessive trait
Symptoms: gas, flatulence, bloating, diarrhea, dehydration and cramps in the stomach.
Diagnosis: hydrogen breath test, blood glucose test, small bowel biopsy, etc.
Ways to prevent these symptoms are: to not eat any products with lactose or to take
lactase pills 30 minutes prior to eating dairy.
Supplementary information: growth hormones were given to cows for mass milk
production but they turned out being responsible for students developing a lot of acne.
When humans are born they are able to digest lactose (breast feeding, mammals) because
they produce the enzyme lactase which allows them to break down lactose into glucose and
galactose and digest them. However, many lose this ability to produce the enzyme as they get
older, even if they could digest it as young adults (not preventable).
Summary Table 5.1 Polysaccharides Differ in Structure
Polysaccharide Function Chemical Structure Three-
Starch Used for energy Amylose
Example: storage in plant cells. (unbranched
Glycogen Used for energy Highly
storage in animal cells branched
(liver and muscles). helices Cellulose Used for structural Parallel
Example: support in cell walls strands joined
celery, fiber of plants and many by hydrogen
Chitin Used for structural Parallel
Example: crab, support in the cell strands joined
mushrooms walls of fungi and the by hydrogen
external skeletons of bonds
Peptidoglycan Used for structural Parallel
support in bacteria strands joined
cell walls. by peptide
Are polymers that consist of many sugar monomers
Hand sanitizer kills off the bacteria that make up peptidoglycan
Fig. 5.6 Cellulose, Chitin and Peptidoglycan can Form Tough Fibers or Sheets
Note the diagrams of what each carbohydrate looks like under a microscope.
Fig. 5.7 Carbohydrates are an Identification for Cells
Glycoproteins contain sugar groups that project from the surface of the plasma
membrane. These sugar groups have distinctive structures that identify the type or species of
the cell. Chapter 6: Lipids, Membranes and the First Cells
More Classes and Functions of Lipids
Phospholipids, glycolipids and some steroids (cholesterol)- membrane structure and
Other steroids- hormones and regulators (involved in development)
Triglycerides- food storage
Waxes- bee hives and plant leaf surfaces for the protection against 2 O loss (shedding of
water to not way down the plant).
Fig. 6.3 Fats are One Type of Lipid found in Cells
Fats- consist of glycerol and 3 fatty acids, linked by ester linkages
When glycerol and a fatty acid react, a water molecule leaves (dehydration reaction)
Lipids are defined by a physical property- their solubility
Fig. 6.4 Amphipathic Lipids Contain Hydrophilic and Hydrophobic Elements
Amphipathic means in terms of membranes, that a molecule has a polar or hydrophilic
and a nonpolar or hydrophobic side.
Steroids- a family of lipids distinguished by the four-ring structure; they differ from each
other by their functional or side group which are attached to those rings.
Phospholipids- consist of a glycerol that is linked to a phosphate group and to either two
chains of isoprene or two fatty acids.
Both steroids and phospholipids contain a polar and nonpolar component
All steroids have a distinctive four-ring structure all phospholipids consist of a glycerol
that is linked to a phosphate group and to either two chains of isoprene or two fatty acids.
Fig. 6.5 Phospholipids Form Bilayers in Solution
Phospholipids spontaneously form bilayers in water
Nonpolar- hydrophobic tails
Polar- hydrophilic heads
Why are Membranes Important?
Membranes allow for compartmentalization
Compartments may contain: different enzymes (important for reactions), different
products and may increase metabolic capacity.
Functions of the Plasma Membrane (outermost membrane of the cell)
Functions as a boundary, which separates a living cell’s internal environment from its
external nonliving environment. It separates “life” from “nonlife.”
Acts as a selective barrier allowing only certain molecules to pass through and controls
the movement of materials in and out of the cell (nutrients in and waste products out) The enclosed volume serves to increase reaction efficiently and concentrates reactants
Why is Membrane Fluidity Important?
Permits useful nutrients to be transported into the cell
Permits harmful wastes to be transported out of the cell (prevent toxicity).
What influences Fluidity?
Unsaturated/ unsaturated fatty acids
Number of carbons in fatty acid tail
Ambient (surrounding) temperature
Fig 6.9 Unsaturated Hydrocarbons Contain Carbon-Carbon Double Bonds
A double bond in a hydrocarbon chain produces a “kink,” which only unsaturated
hydrocarbons have. This allows them to have better membrane permeability and fluidity since
these “kinks” make them more flexible allowing things to go through the membrane. Finally,
these kinks are caused by double bonds in the unsaturated hydrocarbon.
There are 3 characteristics:
1. The Presence of Unsaturated or Saturated Fatty Acids
- unsaturated hydrocarbon (fatty acid) tails have one or more double bonds
- each double bond forms a kink (bend), which results I the fatty acid tails spaced apart and
polar heads loosely packed
- this decreases the strength of interactions between adjacent hydrocarbon tails resulting in a
-saturated hydrocarbon tails have no double bonds and therefore do not have kinks or bends
- hydrocarbon tails are closely packed allowing for inter