The Chemistry of Life

Elements

Water

The acid test, pH

Organic molecules

Elements

The Elements of Life of the 92 natural elements, about 25% are essential elements that an organism needs to live a healthy life and reproduce.

The essential elements are similar among organisms, but there is some variation—for example, humans need 25 elements, but plants need only 17. Just four elements—oxygen (O), carbon (C), hydrogen (H), and nitrogen (N)—make up 96% of living matter. Calcium (Ca), phosphorus (P), potassium (K), sulfur (S), and a few other elements account for most of the remaining 4% of an organism’s mass.

Trace elements are required by an organism in only minute quantities. Some trace elements, such as iron (Fe), are needed by all forms of life; others are required only by certain species. For example, in vertebrates (animals with backbones), the element iodine (I) is an essential ingredient of a hormone produced by the thyroid gland.

Q1. Is a trace element an essential element? Explain.

Q2. In humans, iron is a trace element required for the proper functioning of hemoglobin, the molecule that carries oxygen in red blood cells. What might be the effects of an iron deficiency?

Subatomic Particles

The atom is the smallest unit having the properties

of an element.

Subatomic particles: scientists have split the atom into more

than a hundred types of subatomic particles, but only three

kinds of subatomic particles are relevant here: neutrons,

protons, and electrons.

Protons and electrons are electrically charged. Each proton

has one unit of positive charge, and each electron has one

unit of negative charge. A neutron, as its name implies, is

electrically neutral.

Protons and neutrons are packed together tightly in a

atomic nucleus, at the center of an atom; protons give the

nucleus a positive charge. The electrons are negative

charge around the nucleus.

Figure shows two commonly used models of the structure of

the helium atom as an example.

Figure: Simplified models of a helium (He) atom.

The helium nucleus consists of 2 neutrons (brown)

and 2 protons (pink). Two electrons (yellow) exist

outside the nucleus. These models are not to scale;

they greatly overestimate the size of the nucleus in

relation to the electron cloud.

Atomic Number and Atomic Mass

The neutron and proton are almost identical in mass,

neutrons and protons have masses close to 1

Dalton. Because the mass of an electron is only about

1/2,000 that of a neutron or proton, we can ignore

electrons mass of an atom.

All atoms of a particular element have the same number of

protons in their nuclei. This number of protons, which is

unique to that element, is called the atomic number and is

written as a subscript to the left of the symbol for the element.

The mass number is written as a superscript to the left of an

element’s symbol.

Isotopes: same atoms have the same number of

protons but differ in the number of neutrons in the

nucleus.

If an atom is neutral in electrical charge, which means:

the number of protons = the number of electrons.

the atomic number = the number of protons = the number of

electrons

the atomic mass number = protons + neutrons

2H2 +O2

1. A lithium atom has 3 protons and 4 neutrons. What is its atomic mass in Daltons?

2. A nitrogen atom has 7 protons, and the most common isotope of nitrogen has 7 neutrons. A radioactive

isotope of nitrogen has 8 neutrons. Write the atomic number and mass number of this radioactive nitrogen

as a chemical symbol with a subscript and superscript.

3. A radioactive isotope of hydrogen, tritium (3H). Tritium differs from the more common from of hydrogen

because

(A) It contains two neutrons and one proton in its nucleus

(B) It contains one neutrons and two proton in its nucleus

(C) It differs by its atomic number

(D) It is radioactive and therefore gives off one electron

4. Which of the following is correct about isotopes of carbon?

(A) They are all radioactive

(B) It contains same number of neutrons and a different number of protons in its nucleus

(C) They contain the same number of electrons but they are chemically different because they have the

different number of neutrons

(D) They are chemically identical because they have the same number of electrons

1. A lithium atom has 3 protons and 4 neutrons. What is its atomic mass in Daltons? 7

2. A nitrogen atom has 7 protons, and the most common isotope of nitrogen has 7 neutrons. A radioactive

isotope of nitrogen has 8 neutrons. Write the atomic number and mass number of this radioactive nitrogen

as a chemical symbol with a subscript and superscript.

3. A radioactive isotope of hydrogen, tritium (3H). Tritium differs from the more common from of hydrogen

because

(A) It contains two neutrons and one proton in its nucleus

(B) It contains one neutrons and two proton in its nucleus

(C) It differs by its atomic number

(D) It is radioactive and therefore gives off one electron

4. Which of the following is correct about isotopes of carbon?

(A) They are all radioactive

(B) It contains same number of neutrons and a different number of protons in its nucleus

(C) They contain the same number of electrons but they are chemically different because they have the

different number of neutrons

(D) They are chemically identical because they have the same number of electrons

A water molecule contains one oxygen and two hydrogen atoms that are

connected by covalent bonds.

Although most of this water is in liquid form, water is also present on earth

as a solid (ice) and a gas (water vapor). Water is the only common

substance to exist in the natural environment in all three physical states of

matter.

Model of hydrogen bonds (1)

between molecules of water

The unique properties of water:

1. Water is polar and can dissolve other polar substances.

2. Water has cohesive property and has a strong tendency to stick

together. Hydrogen bonding is also responsible for water’s surface

tension.

3. Water has adhesive property, a high surface tension and like to stick to

other substances.

4. Water has a high heart capacity, water’s ability to resist temperature

changes to keep a constant body temperature for us.

5. Water expands when it freezes, so ice floats.

Water

Model of hydrogen bonds (1)

between molecules of water

The unique properties of water:

1. Water is polar and can dissolve other polar substances.

Water

Adhesion, cohesion and tension help to transport water from roots to leaves against gravity. For this to work, xylem vessel must be continuous of water and contains no bubbles 1. Water evaporates from the leaves at top of xylem through transpiration system.2. This creates tension which pulls more water into the leaf (think of a straw).3. Water molecules are adhesion and cohesion so a column of water moves upwards through the xylem

Model of hydrogen bonds (1)

between molecules of water

4. Water has a high heart capacity, water’s ability to resist temperature

changes to keep a constant body temperature for us.

5. Water expands when it freezes, so ice floats.

Water

Q1. Describe how properties of water contribute to the upward movement of water in a

tree.

Q2. Explain the saying “It’s not the heat; it’s the humidity.”

Q3. A water strider (which can walk on water) has legs that are coated with a hydrophobic

substance. What might be the benefit? What would happen if the substance were

hydrophilic?

Q4.All of the following are characteristics of water EXCEPT

(A) Water has a relatively high boiling point

(B) Water molecules have little attraction for each other

(C) Water is a universal solvent

(D) Ice is less dense than water

Q5. Which of following does NOT describe a way that the exceptional characteristics of

water are used in nature for life?

(A) The high heat capacity of water prevents lakes and steams from rapidly changing

temperature and freezing completely solid in winter.

(B) The surface tension and cohesiveness of water facilitates capillary action in plants.

(C) The low polarity of water prevents dissolution of cells and compounds.

(D) The high intermolecular forces of water, such as hydrogen bonding, results in a boiling

point which exceeds the tolerance of most life on planet.

Q1. Describe how properties of water contribute to the upward movement of water in a tree.

Q2. Explain the saying “It’s not the heat; it’s the humidity.”

Q3. A water strider (which can walk on water) has legs that are coated with a hydrophobic

substance. What might be the benefit? What would happen if the substance were

hydrophilic?

Q4.All of the following are characteristics of water EXCEPT

(A) Water has a relatively high boiling point

(B) Water molecules have little attraction for each other

(C) Water is a universal solvent

(D) Ice is less dense than water

Q5. Which of following does NOT describe a way that the exceptional characteristics of water

are used in nature for life?

(A) The high heat capacity of water prevents lakes and steams from rapidly changing

temperature and freezing completely solid in winter.

(B) The surface tension and cohesiveness of water facilitates capillary action in plants.

(C) The low polarity of water prevents dissolution of cells and compounds.

(D) The high intermolecular forces of water, such as hydrogen bonding, results in a boiling

point which exceeds the tolerance of most life on planet.

2D

The pH ScaleThe pH scale compresses the range of H+ and OH+ concentrations

by employing logarithms. The pH of a solution is defined as the

negative logarithm (base 10) of the hydrogen ion concentration:

pH = -log [H+]

For a neutral aqueous solution, [H+] is 10-7 M, giving us

Water pH = -log 10-7 = -(-7) = 7

Notice that pH declines as H+ concentration increases. The pH of

a neutral aqueous solution at 25°C is 7, the midpoint of the pH

scale. A pH value less than 7 denotes an acidic solution; the lower

the number, the more acidic the solution. The pH for basic

solutions is above 7. Most biological fluids are within the range pH

6–8. There are a few exceptions, however, including the strongly

acidic digestive juice of the human stomach, which has a pH of

about 2.

Remember that each pH unit represents a 10-fold difference in H+

and OH+ concentrations. A solution of pH 3 is not twice as acidic as

a solution of pH 6, but a thousand times (10 x10 x10) more acidic.

When the pH of a solution changes slightly, the actual

concentrations of H+ and OH+ in the solution change substantially.

pH=-[10-7]=7

3c. pH=4=-log[10-4]

4a. a pH 10, b pH4, c pH7, d pH-1

Acids and Bases

What would cause an aqueous solution to have an imbalance in hydrogen ions (H+) and hydroxide ions (OH-) concentrations? When acids dissolve in water, they donate additional H+ to the solution.

An acid is a substance that increases the H+ concentration of a solution. For example, when hydrochloric acid (HCl) is added to water, H+ dissociate from chloride ions:

This source of H+ (dissociation of water is the other source) results in an acidic solution—one having more H+ than OH-.

A substance that reduces the H+ concentration of a solution is called a base. Some bases reduce the H+ concentration directly by accepting H+. Ammonia (NH3), for instance, acts as a base when it attracts a H+ from the solution, resulting in an ammonium ion NH4+ .

1. Compared with a basic solution at pH 9, the same volume of an acidic solution at pH 4 has ____ times as many hydrogen ions (H).

2. HCl is a strong acid that dissociates in water:

What is the pH of 0.01 M HCl?

3. Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake?

a. 4.0 M

b. 10-10M

c. 10-4 M

d. 104 M

e. 4%

1. Compared with a basic solution at pH 9, the same volume of an acidic solution at pH 4 has _105 times as many hydrogen ions (H).

2. HCl is a strong acid that dissociates in water:

What is the pH of 0.01 M HCl?

pH 2= -log[0.01] = -log[10-2]=2

3. Measurements show that the pH of a particular lake is 4.0. What is the hydrogen ion concentration of the lake?

a. 4.0 M

b. 10-10M

c. 10-4 M

d. 104 M

e. 4%

pH 4 = -log[0.0001] = -log[10-4]

Molecules of Life

Carbohydrates

Lipids

Proteins

Nucleic acids

Because they are huge in size and could be over

100Kda, therefore are called macromolecules

Macromolecule polymers are made from monomers

Carbohydrates, proteins and nucleic acids are chain-like molecules called polymers. But lipids is monomers.

Q1. What are the four main classes of large biological

molecules? Which class does not consist of polymers? Why?

The synthesis and breakdown of polymers

(a) Dehydration reaction: When a covalent bond forms between two monomers, each monomer contributes part of water that is released during the reaction: one provides a hydroxyl group(-OH), the other gives a hydrogen (-H).

(b) Hydrolysis (catabolism): The bond between two monomers is broken by addition of a water molecule, with hydrogen attached to one monomer and hydroxyl group attached to adjacent monomer.

Q1. How many molecules of water are needed to complete

hydrolyze a polymer that is ten monomers long?

Q2. Catabolism refers to breaking down complex macromolecules into their basic components, many biological processes use hydrolysis for catabolism. Hydrolysis of proteins could directly result in:

A. free water; B. adenine; C. cholesterol; D. Dipeptides

Dipeptides =two amino acids joined by a single peptide bond

Carbohydrates

• Serve as energy and building material

• Monosaccharides are simple sugars

• Disaccharides are double sugars

• Polysaccharides are composed with many simple sugar that linked by covalent bonds

Three classes of carbohydrates: monosaccharides, disaccharides and polysaccharides

Sugar molecules that are not immediately used in these ways are generally incorporated as monomers into disaccharides or polysaccharides

Monosaccharides (single sugar) generally

have molecular formulas that are some

multiple of the unit CH2O. Glucose

(C6H12O6), the most common

monosaccharide, is of central importance in

the chemistry of life.

Q1. Write the formula for a monosaccharide that has three carbons.

Q2. Maltose is a disaccharide formed by the linking of two molecules of glucose. Also known as malt sugar, maltose is an ingredient used in brewing beer. What would the formula be for maltose?

C12H22O11=C12H24O12-H2O

A.

Q1. Maltotriose is a trisaccharide consisting of three glucose molecules linked with α-1,4 glycosidic bonds. It is most commonly produced by the digestive enzyme alpha-amylase on amylose in starch. What would the formula be for maltotriose?

A. C18H36O18; B. C18H10O2; C. C18H32O16; D. C3H6O3

Q3. Based on the chart below, a basic unit of malt sugar could be made similar to a basic unit of corn syrup through the process of

A. Condensation

B. Dehydration synthesis

C. Hydrolysis

D. Hydrogen bonding

Q1. Maltotriose is a trisaccharide consisting of three glucose molecules linked with α-1,4 glycosidic bonds. It is most commonly produced by the digestive enzyme alpha-amylase on amylose in starch. What would the formula be for maltotriose?

A. C18H36O18; B. C18H10O2; C. C18H32O16; D. C3H6O3

Ans C: C18H36O18-2xH2O

Q2. Maltose is a disaccharide formed by the linking of two molecules of glucose. Also known as malt sugar, maltose is an ingredient used in brewing beer. What would the formula be for maltose?

C12H22O11=C12H24O12-H2O

Q3. Based on the chart below, a basic unit of malt sugar could be made similar to a basic unit of corn syrup through the process of

A. Condensation

B. Dehydration synthesis

C. Hydrolysis

D. Hydrogen bonding

Notice the numbering of carbons in the rings. The numbering always begins to the right of the oxygen.

C6H12O6 C6H12O6 C12H22O11 H2O

An ingredient used in producing beer

glucose glucose maltose water

The sugar present in milk

glucose galactose lactose water

The sugar present Plants

glucose fructose sucrose water

monosaccharide + monosaccharide disaccharide + water

A disaccharide consists of two monosaccharides joined by a glycosidiclinkage, a covalent bond formed between two monosaccharides by a dehydration reaction and release of one molecule of water. Hydrolysis is breakdown of a disaccharide by adding water

Dehydration synthesis

hydrolysis

Maltose is a disaccharide formed by the linking of two molecules of glucose. Maltose is an ingredient used in brewing beer.

Sucrose, is a disaccharide formed by linking of glucose and fructose. The most prevalent disaccharide is sucrose, which is table sugar. Plants generally transport carbohydrates from leaves to roots and other

nonphotosynthetic organs in the form of sucrose.

Lactose, is a disaccharide formed by linking of glucose and galactose. the sugar present in milk.

1-4 glycosidic linkage

PolysaccharidesPolymer = monomer + monomer + monomer…. So on..

Polysaccharides are macromolecules, polymers with a few hundred to a few thousand monosaccharides joined by glycosidic linkages. Some polysaccharides serve as storage material, hydrolyzed as needed to provide sugar for cells. Other polysaccharides serve as building material for structures that protect the cell or the whole organism. The architecture and function of a polysaccharide are determined by its sugar monomers and by the positions of its glycosidic linkages.

Structural Storage

In Plants Cellulose Starch

Makes up plant cell walls Two forms: amylose and amylopectin

In Fungi, arthropods, animals and humans

Chitin Glycogen

Component of the cell walls of fungi; Makes up the exoskeleton in arthropods (insects, spiders….)

Stored in liver and skeletal muscle of animal and human

Structural and Storage Polysaccharides

Q1. Which of the following is stored in human liver for energy? A. Glycine; B. Glycogen; C. Glycerol; D. glucagon

Structural and Storage Polysaccharides

Chitin

Structural Polysaccharides

PolysaccharidesPolymer = monomer + monomer + monomer…. So on..

Storage Polysaccharides

• glycogen - found in animals. glycogen has more glucose units than amylopectin, glycogen - branching occurs every 8 to 12 glucose units

• amylopectin – found in plants and branches separated by 12 to 20 glucose units

Both plants and animals store sugars for later use in the form of storage polysaccharides. Plants store starch. Most animals, including humans, also have enzymes that can hydrolyze plant starch, making glucose available as a nutrient for cells. Potato tubers and grains—the fruits of wheat, maize (corn), rice, and other grasses—are the major sources of starch in the human diet.

Animals lack of enzymes to digest cellulose. Some

microorganisms can digest cellulose, breaking it down into

glucose monomers. A cow harbors cellulose digesting prokaryotes

and protists in its stomach. These microbes hydrolyze the

cellulose of grass and convert the glucose to other compounds

that nourish the cow. Similarly, a termite, which is unable to digest

cellulose by itself, has prokaryotes or protists living in its gut that

can make a meal of wood. Some fungi can also digest cellulose,

thereby helping recycle chemical elements within Earth’s

ecosystems.

Q1. Which of the following categories includes all others

in the list?

a. monosaccharide

b. carbohydrate

c. disaccharide

d. polysaccharide

e. starch

Q2. After a cow is given antibiotics to treat

an infection, a vet gives the animal a drink

of “gut culture” containing various

prokaryotes. Why is this necessary?

Q1. Which of the following categories includes all others

in the list?

a. monosaccharide

b. carbohydrate

c. disaccharide

d. polysaccharide

e. starch

Q2. After a cow is given antibiotics to treat

an infection, a vet gives the animal a drink

of “gut culture” containing various

prokaryotes. Why is this necessary?

PROTEIN

• Proteins are large macromolecules, consisting of one or more long chains of polypeptides.

• In the human genome, 20 amino acids are created to build proteins.

9 hydrophobic amino acids

11 hydrophilic amino acids

The repeating sequence in purple is called polypeptide backbone.

A peptide has a N-terminus (a amino end, left) as well as C- terminus (a carboxyl end, right).Peptide bonds are formed by dehydration of between the carboxyl group of one amino acid and the amino group of next).

The chemical nature property of a polypeptide is determined by the sequence of the side chains.

Q1. Catabolism refers to breaking down complex macromolecules into their basic components, Many biological processes use hydrolysis for catabolism. Hydrolysis of proteins could directly result in: A. free water; B. adenine; C. cholesterol; D. dipeptides

Protein Structure and Function (1)

• Polypeptides =Protein ? No

• A function protein is one or more polypeptide(s) twisted, folded, coiled into a unique shape molecule. The specific activity of a protein is determined by its three dimensional structure.

• The amino acid sequences of polypeptides determine the protein’s three dimensional structure in normal cellular conditions.

Protein Structure and Function (2)

The Protein’s Specific Structure Determines How It Works

The function of a protein is dependent on its ability to recognize and bind to another molecule.

Four Levels of Protein Structure

• Every protein has primary, secondary and tertiary structures.

• A protein has the quaternary structure if it consists two or more polypeptides.

Secondary structure is the result of

hydrogen bonds between of the

polypeptide backbone (not the side

chains)

• Tertiary structure is the overall shape of a polypeptide resulting from interactions between side chains (R group) of various amino acids. Such as 1.hydrophobic interaction and van der Waals' interaction; 2. disulfide bridges; 3. ionic bond; 4. hydrogen bond.

The van der Waals forces (van der Waals' interaction) are the residual attractive or repulsive forces between molecules or atomic groups that do not arise from a covalent bond, or electrostatic interaction.

Quaternary structure is the overall structure of a protein that contains two or more polypeptides.

The structural level of a protein least affected by a disruption in hydrogen bonding is the

a. primary level.

b. secondary level.

c. tertiary level.

d. quaternary level.

e. all structural levels are equally affected.

if a protein consists one polypeptide, which level of protein structure is most directly related to specificity? If

a protein consists four polypeptides?

a. primary level.

b. secondary level.

c. tertiary level.

d. quaternary level.

e. all structural levels are equally.

The structural level of a protein least affected by a disruption in hydrogen bonding is the

a. primary level.

b. secondary level.

c. tertiary level.

d. quaternary level.

e. all structural levels are equally affected.

if a protein consists one polypeptide, which level of protein structure is most directly related to specificity? If

a protein consists four polypeptides?

a. primary level.

b. secondary level.

c. tertiary level (a protein consists one polypeptide).

d. quaternary level.

e. all structural levels are equally.

A slight change in primary structure can alter the shape and function of the protein.

A slight change in primary structure can alter the shape and function of the protein.

A slight change in primary structure can alter the shape and function of the protein.

Effect of temperature on proteins

What Determines Protein Structure?

• Amino acids sequence of the polypeptides of a protein.

• Physical and chemical conditions of protein’s environment.

• Denaturation: If the pH, salt concentration, temperature, or other aspects of its environment are altered, the weak chemical bonds and interactions within a protein may be destroyed, causing the protein lose its native shape and biological function.

Amino acids are the basic molecular unis which comprise proteins. Which labeled components of

the amino acid structure of phenyalanine shown below will vary from amino acid to amino acid? (A);

(B); (C); (D)

Which of the following functional groups( I, II and III) combine to form peptide bond?

a. I and II

b. I and III

c. II and III

d. I, II and III

I II III

Amino acids are the basic molecular unis which comprise proteins. Which labeled components of the amino

acid structure of phenyalanine shown below will vary from amino acid to amino acid? (A); (B); (C); (D)

Which of the following functional groups( I, II and III) combine to form peptide bond?

a. I and II

b. I and III

c. II and III

d. I, II and III

II I III

Lipids are a diverse group of hydrophobic

molecules

• The most biologically important types of lipids are

1. Solid fats,

2. Liquid oils,

3. Phospholipids and

4. Steroids.

• Lipids are the one class of large biological molecules

that does not include true polymers.

• The compounds of lipids mix poorly with water. The

hydrophobic behavior of lipids is based on their

molecular structure. Lipids are varied in form and

function.

A fat/an oil is constructed from two kinds of

smaller molecules: glycerol and fatty acids.

Glycerol is an alcohol; each of its three

carbons bears a hydroxyl group.

A fatty acids are hydrophobic, has a long

carbon skeleton, usually 16 or 18 carbon

atoms in length. The carbon at one end of

the skeleton is part of a carboxyl group (-

COOH).

In making a fat, three fatty acid molecules

are each joined to glycerol by an ester

linkage, a bond between a hydroxyl group

and a carboxyl group. The resulting fat, also

called a Triacylglycerol (triglyceride), thus

consists of three fatty acids linked to one

glycerol molecule. One water molecule is

removed for each fatty acid joined to the

glycerol. The fatty acids in a fat can be the

same, or they can be of two or three different

kinds, as in Figure (b).

The terms saturated fats and unsaturated fats are

commonly used in the context of nutrition. These terms

refer to the structure of the hydrocarbon chains of the fatty

acids.

If there are no double bonds between carbon atoms

composing a chain, such a structure is called saturated

fatty acid (Figure (a)). A fat made from saturated fatty

acids is called a saturated fat. Most animal fats are

saturated. Saturated animal fats—such as lard and

butter—are solid at room temperature.

An unsaturated fatty acid has one or more double

bonds, with one fewer hydrogen atom on each double-

bonded carbon. Nearly all double bonds in naturally

occurring fatty acids are cis double bonds, which cause a

kink in the hydrocarbon chain wherever they occur (Figure

(b). The fats of plants and fishes are generally

unsaturated, meaning that they are built of one or more

types of unsaturated fatty acids. Usually liquid at room

temperature, plant and fish fats are referred to as oils—

olive oil and cod liver oil are examples.

Trans fats (the hydrogenated vegetable oils) on food labels means that unsaturated fats have

been synthetically converted to saturated fats by adding hydrogen. Peanut butter, margarine,

and many other products are hydrogenated to prevent lipids from separating out in liquid (oil) form.

A diet rich in saturated fats is one of several factors that may contribute to the cardiovascular

disease.

Studies have shown that the process of hydrogenating vegetable oils produces not only saturated

fats but also unsaturated fats with trans double bonds. Because trans fats are especially common

in baked goods and processed foods, the U.S.Department of Agriculture requires nutritional labels

to include information on trans fat content.

Certain unsaturated fatty acids must be supplied in the human diet because they cannot be

synthesized in the body. These essential fatty acids include the omega-3 fatty acids (a double

bond at the third carbon-carbon bond from the end of the hydrocarbon chain) which are required

for normal growth in children and appear to protect against cardiovascular disease in adults. Fatty

fish and certain nuts and vegetable oils are rich in omega-3 fatty Acids.

Phospholipids are essential for cells

because they make up cell membranes.

A phospholipid is similar to a fat molecule but

has only two fatty acids attached to glycerol

rather than three. The third hydroxyl group

of glycerol is joined to a phosphate group,

which has a negative electrical charge in the

cell.

The two ends of phospholipids show different

behavior toward water. phospholipid has a

hydrophilic (polar) head and two hydrophobic

(nonpolar) tails. Phospholipid diversity is

based on differences in the two fatty acids

and in the groups attached to the phosphate

group of the head. When phospholipids are

added to water, they self assemble into

double-layered structures called

“bilayers,” shielding their hydrophobic

portions from water.

Phospholipids

At the surface of a cell, phospholipids are arranged in a

similar bilayer. The hydrophilic heads of the molecules are on

the outside of the bilayer, in contact with the aqueous solutions

inside and outside of the cell. The hydrophobic tails point

toward the interior of the bilayer, away from the water. The

phospholipid bilayer forms a boundary between the cell and its

external environment; in fact, cells could not exist without

phospholipids.

Cholesterol is a crucial molecule in animals.

in vertebrates, cholesterol is synthesized in

the liver and obtained from the diet. A high

level of cholesterol in the

blood may contribute to heart diseases ( such

as: atherosclerosis). In fact, both saturated fats

and trans fats exert their negative impact on

health by affecting cholesterol levels.

Steroids are lipids characterized by a carbon

skeleton consisting of four fused rings.

Different steroids, such as cholesterol and the

vertebrate sex hormones (Estradiol and

Testosterone), are distinguished by the

particular chemical groups attached to this

ensemble of rings. The natural steroid

hormones are generally synthesized

from cholesterol. They can pass through the

cell membrane as they are fat-soluble.The natural steroid hormones

USA Biology Olympiad(USABO)2011,3C

Q1. Compare the structure of a fat (triglyceride) with that

of a phospholipid.

Q2. Why are human sex hormones considered lipids?

Sex hormones (Steroids) are made from cholesterol.

Q3. Suppose a membrane surrounded an oil

droplet, as it does in the cells of plant seeds. Describe

and explain the form it might take.

21

Q4. Which of the following contain both hydrophilic and

hydrophobic properties and are often found in cell plasma

membranes?

A. Nucleotides; B. Phospholipids; 3. Water; 4. Amino acids.

Q5. Which of the following statements concerning unsaturated

fats is true?

a. They are more common in animals than in plants.

b. They have double bonds in the carbon chains of their fatty

acids.

c. They generally solidify at room temperature.

d. They contain more hydrogen than do saturated fats having

the same number of carbon atoms.

e. They have fewer fatty acid molecules per fat molecule.

Q4. Which of the following contain both hydrophilic and hydrophobic

properties and are often found in cell plasma membranes?

A. Nucleotides; B. Phospholipids; 3. Water; 4. Amino acids.

Q5. Which of the following statements concerning unsaturated

fats is true?

a. They are more common in animals than in plants.

b. They have double bonds in the carbon chains of their fatty

acids.

c. They generally solidify at room temperature.

d. They contain more hydrogen than do saturated fats having

the same number of carbon atoms.

e. They have fewer fatty acid molecules per fat molecule.

Q6. According to the chart below, the lipid membrane-spanning port of

a transmembrane protein is most likely to be made up of

(1). Alanine

(2). Arginine

(3). Isoleucine

A. 1 only

B. 2 only

C. 1 and 3 only

D. 2 and 3 only

Q6. According to the chart below, the lipid membrane-spanning port of

a transmembrane protein is most likely to be made up of

(1). Alanine

(2). Arginine

(3). Isoleucine

A. 1 only

B. 2 only

C. 1 and 3 only

D. 2 and 3 only

OH

DNA RNA

DNA RNA

cytosine cytosine

adenine adenine

guanine guanine

thymine uracil

Nitrogenous base

DNA RNA

deoxyribose ribose

Pentose sugar

Nucleic acids are polymers, essential for all known forms of life. Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from monomers known as nucleotides.

Each nucleotide has three components: a nitrogenous base, a 5-carbon sugar, and a phosphate group.

If the sugar is deoxyribose the polymer is DNA. If the sugar is ribose, the polymer is RNA. When all three components are combined, they form a nucleic acid.

DNA RNA

Base cytosine, adenine, guanine, thymine

cytosine, adenine, guanine, uracil

Sugar deoxyribose ribose

Phosphate same same

Nucleic Acids

Q1. The Figure, for the nucleotides, number all the carbons in the sugars,

name the nitrogenous bases, and name the phosphates.

Q2. In a DNA double helix, a region along one DNA strand has this

sequence of nitrogenous bases: 5’-TAGGCCT-3’. Copy this sequence,

and write down its complementary strand, clearly indicating the 5’and 3’

ends of the complementary strand.

Q3. Suppose a substitution occurred in one DNA strand of the double helix

in question 2, resulting in

5-TAAGCCT-3

3-ATCCGGA-5

(a) Copy these two strands, and circle and label the mismatched

bases.

(b) If the modified top strand is used by the cell to construct a

complementary strand, what would that matching strand be?

Q1. The Figure, for the nucleotides, number all the carbons in the sugars,

name the nitrogenous bases, and name the phosphates.

Q2. In a DNA double helix, a region along one DNA strand has this

sequence of nitrogenous bases: 5’-TAGGCCT-3’. Copy this sequence,

and write down its complementary strand, clearly indicating the 5’and 3’

ends of the complementary strand.

5’-TAGGCCT-3’

3’-ATCCGGA-5’

Q3. Suppose a substitution occurred in one DNA strand of the double helix

in question 2, resulting in

5-TAAGCCT-3

3-ATCCGGA-5

(a) Copy these two strands, and circle and label the mismatched

bases.

(b) If the modified top strand is used by the cell to construct a

complementary strand, what would that matching strand be?