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MCAT Biology Review > Chapter 3: Biologically Important Molecules > Flashcards

Chapter 3: Biologically Important Molecules Flashcards

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1
Q

Name seven examples of protein function/purpose in the body?

A
  1. Enzymes
  2. Hormones
  3. Receptors
  4. Channels
  5. Transporters
  6. Support structures inside and outside cells
  7. Antibodies
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2
Q

What is responsible for protein uniqueness in role and structure?

A

The composition and sequence of amino acids in the polypeptide chain

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3
Q

What are the building blocks of proteins?

A

Amino Acids

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4
Q

What are the four main components of the Amino Acid Structure?

A

α-Amino Group (H2N)

Tetrahedral α-carbon

Variable R Group

α- carboxyl group (COOH)

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5
Q

What’s a Carboxyl Group?

A

A combination of two functional groups attached to a single carbon atom

Hydroxyl (single bonded OH) + Carbonyl (double bonded O) = carboxyl group

Includes Carboxylic Acids and Amino Acids

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6
Q

What is Hydroxyl?

A

A functional group -OH that consists of one atom of hydrogen covalently bonded to one oxygen

Is neutral or negatively charged

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7
Q

What’s a Carbonyl ?

A

Functional group composed of a carbon atom double bonded to an oxygen atom

C=O

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8
Q

What is Carboxylic Acid ?

A

An organic acid containing a carboxyl group (C(=O)OH) attached to an R group

General formula R-COOH or R-CO2H

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9
Q

Define Organic Acid

A

An organic compound with acidic properties

The most common organic acids are carboxylic acids, whose acidity comes from the carboxyl group

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10
Q

What is a functional group in chemistry?

A

A functional group or moiety is a specific group of atoms within a molecule that is responsible for characteristic chemical reactions of that molecule

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11
Q

Which part of the structure do amino acids share, and which is unique?

A

Amino acids share the same Nitrogen- Carbon- Carbon backbone

The unique feature is the side chain/ Variable R-group which give it the physical and chemical properties that distinguish it

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12
Q

What are the 4 important properties of amino acid side chains that affect an amino acids ability to act as acids or bases?

A
  1. Shape
  2. Charge
  3. Ability to Hydrogen Bond
  4. Ability to act as acids or bases
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13
Q

What are the two common structures of Hydrophobic (Nonpolar) Amino Acids?

A

Aliphatic (straight chain) or Aromatic (ring) side chains

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14
Q

Which hydrophobic non-polar amino acids are aliphatic?

A
  • Glycine
  • Alanine
  • Valine
  • Leucine
  • Isoleucine
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15
Q

Which hydrophobic non-polar amino acids are aromatic?

A
  • Phenylalanine
  • Tyrosine (neutral, polar)
  • Tryptophan
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16
Q

Where are hydrophobic non-polar amino acids found?

A

They are found on the interior of folded globular proteins, because they are being repelled away from water

Hydrophobic residues associate more with each other than with water

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17
Q

The larger the hydrophobic group, the _____ (greater or less) the hydrophobic force

A

Greater

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18
Q

List the Hydrophobic non-polar amino acids

A

Aliphatic: Glycine, Alanine, Valine, Leucine, Isoleucine

Aromatic: Phenylalanine, Tryptophan

Methionine, Proline

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19
Q

What are the characteristics of the R group in Polar amino acids?

A
  • polar enough to form hydrogen bonds with water
  • NOT polar enough to act as an acid or base
  • hydrophillic, water and R group are attracted to each other
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20
Q

Which 3 Polar amino acids stand out from the rest and why?

A
  1. Serine
  2. Threonine
  3. Tyrosine
  • They have an attachment of a phosphate group by the regulatory enzyme kinase. The attached phosphate group is very hydrophilic.
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21
Q

Name the polar, Neutral Amino Acids

A

Serine
Threonine
Tyrosine
Asparagine
Glutamine
Cysteine

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22
Q

Which Amino Acids are Polar Acidic?

A

Aspartic Acid (Aspartate: anionic unprotonated form)
Glutamic Acid ( Glutamate: “””)

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23
Q

What makes polar Acidic amino acids Acidic?

A

They have Carboxylic Acid Functional groups (pKa ≈ 4) in their side chains –> acidic

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24
Q

Which 3 functional groups in Acidic Amino acids may act as acids or bases?

A

The two backbone groups and the R group

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25
Q

What makes Basic Amino Acids Basic?

A

They have basic R-group Side chains

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26
Q

What are the Polar Basic Amino Acids and their pKa’s ?

A

Lysine. pKa= 10
Arginine. pKa= 12
Histidine. pKa= 6.5

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27
Q

Which of the polar basic amino acids are unique and why?

A

Histidine

  • Unique because it’s close to physiological pH
  • at pH 7.5 it can be either pronated or depronated
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27
Q

Which of the polar basic amino acids are unique and why?

A

Histidine

  • Unique because it’s close to physiological pH
  • at pH 7.5 it can be either protonated or de-protonated
  • It is classified as basic but can often act as acid too (is a readily available proton acceptor or donor)
  • very prevalent at protein active sites

HIS GOES BOTH WAYS

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28
Q

a) Amino acids containing _____ are ALWAYS anionic at physiological pH
b) Amino Acids containing _____ are ALWAYS cationic at physiological pH

A

a) –COOH (RCOO-)
b) –NH2 side chains (RNH3+)

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29
Q

What are the two sulfur containing amino acids? How do their structures differ from the rest?

A

a) Cysteine: Contains a thiol/ sulfhydryl (an alcohol with an S atom instead of O) and is fairly polar

b) Methionine: Contains a thioether (an ether with an S atom instead of O) and is fairly NON-polar

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30
Q

What is Proline and why is it unique in its group?

A

It is an amino acid: categorized as non-polar hydrophobic

It is unique because its amino group is bound covalently to a part of the side chain creating a secondary a-amino group and a distinctive ring structure

This effects protein folding

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31
Q

What are the 8 essential amino acids? What does this mean?

A
  1. Valine
  2. Leucine
  3. Isoleucine
  4. Phenylalanine
  5. Tryptophan
  6. Methionine
  7. Threonine
  8. Lysine

These are amino acids that cannot be synthesized by adult humans and must be obtained from diet

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32
Q

What are the two common covalent bonds between amino acids in proteins?

A

Peptide bonds and Disulfide Bridges

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33
Q

What are Peptide Bonds?

A

Link amino acids together into polypeptide chains

Formed between the carboxyl group of one amino acid and the a-amino group of another amino acid with the loss of water

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34
Q

What are Disulfide Bridges ?

A

Exist Between Cysteine R groups

A covalent sulfur- sulfur bond

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35
Q

Amino Acids linked with peptide bonds form what?

A

polypeptides

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36
Q

What is the polypeptide backbone?

A

The N-C-C-N-C-C chain pattern

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37
Q

What is an individual amino acid called when its part of a polypeptide?

A

Residue

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38
Q

What is the amino terminus?

A

The first end made during polypeptide synthesis

The amino terminal residue is always written first

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39
Q

What is the carboxy Terminus?

A

The last end made during polypeptide synthesis

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40
Q

Phe- Glu- Gly- Ser- Ala
a) What is the Amino Terminus?
b) What is the Carboxy Terminus?

A

a) Phe
b) Ala

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41
Q

What is thermodynamically favoured but kinetically slow?

A

Hydrolysis of peptide bonds

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42
Q

Define Proteolysis/ Proteolytic Cleavage

A

Hydrolysis of a protein by another protein

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43
Q

Describe how peptide bonds are formed and maintained between amino acids.

A

During protein synthesis, stored energy is used to force the formation of peptide bonds

Once formed, destruction of peptide bonds is thermodynamically favoured. However, the bond remains stable because the activation energy for hydrolysis is too high.

Hydrolysis is thermodynamically favoured but kinetically slow.

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44
Q

Describe how proteolysis or proteolytic cleavage works?

A

The protein that cuts the bond is called the proteolytic enzyme, or protease

Some enzymes only cleave peptide bonds adjacent to specific AAs

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45
Q

Protease

A

Also known as proteolytic enzyme

Is the protein that cuts the peptide bonds of proteins in proteolysis (the hydrolysis of a protein)

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46
Q

How does Disulfide Bonding work?

A

Occurs in amino acids [cysteine] with a reactive thiol (sulfhydryl, SH)

Thiol of one cysteine reacts with the thiol of another to produce bond

This bonding can occur within the same, or different polypeptide chains

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47
Q

Why are disulfide bonds important?

A

They stabilize tertiary protein structure

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48
Q

What do we call cysteine once it becomes disulfide bonded?

A

Cystine

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49
Q

What is Cystine ?

A

This is what we call cysteine once it becomes disulfide bonded

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50
Q

Would the sulfur in cystine be more or less oxidized than cysteine ?

A

The sulfur in cystine is more oxidized because it is bonded to a carbon and a sulfur, whereas cysteine is bonded to a hydrogen and a carbon

SO. When a carbon is bound to a heteroatom (any atom that is not carbon or hydrogen), this increases oxidization.

” Heteroatoms such as oxygen and nitrogen are more electronegative than carbon, so when a carbon atom gains a bond to a heteroatom, it loses electron density and is thus being oxidized. Conversely, hydrogen is less electronegative than carbon, so when a carbon gains a bond to a hydrogen, it is gaining electron density, and thus being reduced.”

Therefore, in cystine, the sulfur will be more oxidized because its bonded to a carbon and a heteroatom. But in cysteine, its bonded to a carbon and hydrogen thus decreasing oxidization.

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51
Q

Where in the body are Disulfide Bridges found?

A

These bridges are only found in extracellular polypeptides

In other environments, more reducing environments, the S-S group is reduced to two S-H groups. In extracellular environments the S-S group wont be reduced.

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52
Q

What two important proteins in the body are held together by disulfide bridges?

A

Antibodies and the hormone insulin

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53
Q

What can cause a protein to become nonfunctional?

A

Denaturation or if they are improperly folded

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54
Q

How many levels of protein folding are there? What is each level of structure dependent on?

A

4 levels
Each level is dependent on a specific type of bond

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55
Q

Define Denaturation

A

Disruption of a proteins shape without breaking peptide bonds

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56
Q

What causes denaturation of proteins?

A

Proteins are denatured by urea (disrupts H bonding interactions) through 3 acts:
1. Extremes of pH
2. Extremes of temperature
3. Changes in salt concentration (Tonicity)

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57
Q

Define tonicity

A

Tonicity is a measure of the effective osmotic pressure gradient

The water potential of two solutions separated by a partially-permeable cell membrane

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58
Q

What is the simplest level of protein structure?

A

Primary Structure, The Amino Acid Sequence

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59
Q

What is the primary structure of Proteins?

A

The linear ordering of amino acid residues

The order of amino acids are bonded in the polypeptide chain

The peptide bond is the characteristic bond that determines primary structure as it links amino acids

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60
Q

What is secondary protein structuring?

A

Hydrogen Bonds between backbone groups

The Initial folding of polypeptide chains into shapes stabilized by hydrogen bonds between backbone NH and CO groups

2 common structures: a-helix and B pleated sheet (parallel and antiparallel)

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61
Q

What are the two common secondary structures of proteins?

A
  1. alpha helix
  2. Beta pleated sheets
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62
Q

What are the two subtypes of Beta pleated sheets? How do they differ?

A

a) Parallel Beta pleated sheet
- Adjacent polypeptide strands run in the SAME direction

b) Anti-parallel Beta pleated sheet
- Polypeptide strands run in OPPOSITE directions

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63
Q

What is tertiary structure of proteins?

A

Hydrophobic/ Hydrophilic interactions (depending on the amino acid)

Concerns interactions between amino acid residues located more distantly from each other in the polypeptide chain

The folding of secondary structures into 3D structures

Driven by interactions of R-groups with each other and with solvent (Water)

64
Q

How do amino acids with hydrophobic R groups fold in tertiary protein structuring?

A

Often fold into the interior of the protein, away from the solvent

65
Q

How do amino acids with hydrophilic R groups fold in tertiary protein structuring?

A

These tend to be exposed to water on the surface of the protein

66
Q

What determines how a protein folds in tertiary structure?

A

The forces that drive hydrophobic avoidance and hydrogen bonding fold polypeptides spontaneously into the correct conformation, the lowest energy conformation

67
Q

What is Quaternary Protein Structure?

A

Various Bonds between Separate Chains; The highest level of protein structure

The arrangement of subunits (a single polypeptide chain as part of a large complex containing many subunits) in a multi-subunit complex

68
Q

What forces stabilize Quaternary and generally are the same as those stabilizing Tertiary?

A

Non Covalent interactions
Van der Waals forces
H bonds
Disulfide bonds
Electrostatic interactions

69
Q

What bond is NOT involved in quaternary protein structure, but is key in primary structures?

A

Peptide bonds

70
Q

Whats the difference between a disulfide bridge involved in quaternary structure and one involved in tertiary structure?

A

In quaternary structure, bonds form between chains that are separate, they arent linked by peptide bonds

In tertiary structure, bonds occur between residues in the same polypeptide

71
Q

Oxidation of carbohydrates means what? What is the result?

A

The combustion or burning to break down carbohydrates into CO2

The process releases large amounts of energy

72
Q

What is the principle energy source for cellular metabolism?

A

carbohydrates

73
Q

What is a monosaccharide?

A

A single carbohydrate molecule AKA a simple sugar

“a single sweet unit”

74
Q

What is the general chemical formula for a monosaccharide?

A

CnH2nOn

75
Q

3 examples of monosaccharides

A

Fructose
Glucose
Ribose

76
Q

What are Disaccharides?

A

Two monosaccharides bonded together

77
Q

What is an Oligosaccharide?

A

A few monosaccharides bonded together

78
Q

What is a polysaccharide?

A

Many monosaccharides bonded together

79
Q

How are 2 sugar molecules bonded together?

A

Glycosidic linkage: a covalent bond formed in a dehydration reaction that requires enzymatic catalysis

79
Q

How are 2 sugar molecules bonded together?

A

Glycosidic linkage: a covalent bond formed in a dehydration reaction that requires enzymatic catalysis

80
Q

What is Glycosidic linkage?

A

The bond between 2 sugar molecules

Covalent bond formed in a dehydration reaction that requires enzymatic catalysis

81
Q

Glucose + Fructose =?

A

Sucrose

82
Q

Galactose + Glucose = ?

A

Lactose

83
Q

What 2 monosaccharides form Sucrose?

A

Glucose and Fructose

84
Q

What 2 monosaccharides form Lactose?

A

Galactose and Glucose

85
Q

Name 4 common disaccharides

A
  1. Sucrose
  2. Lactose
  3. Maltose
  4. Cellobiose
86
Q

Define polymer

A

A polymer is a substance or material consisting of very large molecules called macromolecules, composed of many repeating subunits

87
Q

Together, what do disaccharides form? Give broad term and examples.

A

Disaccharides form important polymers
- Glycogen
- Starch
- Cellulose

88
Q

What is glycogen?

A

energy storage carbohydrate composed of thousands of glucose units in animals

89
Q

What is Starch?

A

The same as glycogen, an energy storage carbohydrate composed of thousands of glucose units in animals. Starch however, serves the same purpose in plants

90
Q

What is cellulose?

A

A polymer of Cellobiose (a disaccharide)

Cellobiose does not exist freely in nature, it only exists in its polymerized cellulose form.
You would have to obtain it through enzymatic or acidic hydrolysis of cellulose.

91
Q

What are lipids?

A

Oily/ fatty substances

92
Q

What 3 physiologic roles do lipids play?

A

1) In adipose cells, triglycerides (fats) store energy

2) In cellular membranes, phospholipids constitute a barrier between intracellular and extracellular environments

3) Cholesterol is a special lipid that serves as the building block for hydrophobic steroid hormones

93
Q

How does polarity effect reaction with water?

A

Water is very polar. Polar substances dissolve well in water. These polar substances are “water loving” or “hydrophilic”

94
Q

How does non-polarity effect reaction with water?

A

C-C bonds and C-H bonds are non polar
They do not dissolve well in water and are therefore hydrophobic

95
Q

How do hydrophobic substances react with lipids?

A

Hydrophobic= lipophilic (lipid loving )

96
Q

How do hydrophilic substances react with lipids?

A

Hydrophilic= lipophobic (lipid hating)

97
Q

Define Alkanes

A

Any series of saturated hydrocarbons including methane, ethane, and propane

98
Q

What is the fatty acid structure?

A

Composed of long unsubstituted alkanes ending in a carboxylic acid

Chains are usually 14-18 carbons long and always even numbered in human cells

99
Q

Describe the structure of saturated fatty acids.

A

A fatty acid with no C-C double bonds, because every carbon is covalently bound to the maximum number of Hydrogen

100
Q

Describe the structure of an unsaturated fatty acid.

A

Contain one or more double bonds in the tail. These double bonds are almost always (Z) (or cis)

Unsaturated fatty acids are bent or “kinked” at the Cis double bond

101
Q

How do hydrophobic chains react to water?

A

Long fatty acid hydrophobic chains will interact with each other to minimize contact with water, exposing their charged carboxyl group to the aqueous environment

When interacting in aqueous solutions, fatty acids form structures called micelles. A force called the hydrophobic interaction drives the tails into the centre of the micelle.

102
Q

Define Micelle

A

The structure formed by fatty acids when they interact with aqueous solutions

103
Q

Define hydrophobic interaction forces

A

The force that drives the tails of fatty acids into the centre of the micelle, exposing the carboxyl groups to the aqueous environment

104
Q

How are fatty acids stored?

A

As fats, or Triacyglycerol/ triglycerides

105
Q

What are triglycerides?

A

They are fats that fatty acids are stored as

106
Q

How are triglycerides formed?

A

3 fatty acids esterified [process of combining an organic acid (RCOOH) to an alcohol (ROH)] to a glycerol molecule (a simple triol compound , meaning 3 -OH compounds)

107
Q

Define Triol

A

A chemical compound containing 3 hydroxyl groups

108
Q

What is glycerol? How is it formed?

A

glycerol is a simple triol compound.

A 3 carbon triol, formula HOCH2-CHOH-CH2OH

  • has 3 hydroxyl groups (R-O-H) esterified to fatty acids

glycerol forms the background of lipids known as glycerides

109
Q

Why do we need to store fatty acids in relatively inert forms?

A

Because free fatty acids are reactive chemicals

110
Q

Define Esterification

A

Compound production by reaction between acids and alcohols with the elimination of water

111
Q

Define lipases. What is their function

A

Lipases are enzymes that hydrolyze fats

Necessary because triacyglycerols are stored in fat cells as an energy source

112
Q

Which is the more efficient energy storage molecule? Carbs, or Fats? Why? (2 reasons)

A

Fats
1. Packing: fat hydrophobicity lets them pack together closer than carbohydrates
- Carbs carry large amounts of water-of-solvation (water molecules H-bonded to their hydroxyl groups)

  1. Energy Content: Fat molecules also store more energy than carbs
    - No matter what its dissolved in, fat has more energy per carbon than a carb
113
Q

What are membrane lipids? What are they composed of?

A

Membrane lipids are lipids involved in forming the structure of biological membranes

Consist primarily of phospholipids, glycolipids, and cholesterol derived from diacylglycerol phosphate or DG-P (a second messenger signaling lipid)

114
Q

What is a phospholipid?

A

A class of lipids whose molecule has a hydrophilic head containing a phosphate group and 2 hydrophobic tails derived from fatty acids joined by an alcohol residue

115
Q

What happens when phospholipids interact with water?

A

They minimize their interactions with water by forming a structure : a lipid bilayer

116
Q

Define lipid bilayer

A

A thin polar membrane made of two layers of lipid molecules

These membranes are flat sheets that form a continuous barrier around all cells

The lipid bilayer is self-assembled and stabilized by the so called hydrophobic effect (lipid molecules unable to hydrogen bond with water aggregate to prevent their hydrophobic portions from being exposed to water

117
Q

What stabilizes the lipid bilayer?

A

Van der Waals forces between the long tails

118
Q

Would a saturated or unsaturated fatty acid residue have more Van der Waals interactions with neighbouring alkyl chains in a bilayer membrane ?

A

Phospholipids composed of saturated fatty acids make the membrane more solid

The bent shape of unsaturated fatty acids means it doesn’t fit in as well and has less contact with neighbouring groups to form van der waals interactions

Double bonds in unsaturated fatty acids tend to increase membrane fluidity and prevent membrane from solidifying by disrupting the orderly packing of hydrophobic lipid tails (The right amount of fluidity is essential for function)

119
Q

What affects membrane fluidity in the lipid bilayer and how?

A

Double bonds (unsaturation) in fatty acids increase membrane fluidity and prevent membrane from solidifying by disrupting the orderly packing of hydrophobic lipid tails (The right amount of fluidity is essential for function)

Decreasing length of fatty acids also increases fluidity

Cholesterol: Cholesterol is known as membrane antifreeze. At low temperatures it increases fluidity in the same way as kinks in fatty acid tails. At high temperatures, cholesterol attenuates (reduces) membrane fluidity

120
Q

What is cholesterol’s role in phospholipid bilayer membrane fluidity?

A

Cholesterol is known as membrane antifreeze

At low temperatures it increases fluidity in the same way as kinks in fatty acid tails. At high temperatures, cholesterol attenuates (reduces) membrane fluidity

Just remember, that cholesterol keeps fluidity at an optimum level

121
Q

What are the 3 structural determinants of lipid bilayer membrane fluidity?

A
  1. Degree of saturation
  2. Tail length
  3. Amount of cholesterol
122
Q

What is a terpene?

A

A member of a broad class of compounds built from isoprene units (C5H8) with the general formula (C5H8)n

Is formally a simply hydrocarbon (compound of H and C)

123
Q

In what two forms can terpenes come in?

A

Linear and cyclic

124
Q

How are terpenes classified?

A

Classified by the number of isoprene units they contain

125
Q

What is a monoterpene?

A

2 isoprene units

126
Q

What is a sesquiterpene?

A

3 isoprene units

127
Q

What is a diterpene?

A

4 isoprene units

128
Q

What is a Squalene or triterpene?

A

6 isoprene units

129
Q

What is squalene (Triterpene) and why is it an important compound?

A

Contains 6 isoprene units

Important because it is biosynthetically used in the manufacture of steroids.

Also is a component of earwax.

130
Q

What is a terpenoid?

A

Essentially functionalized terpenes

Natural and synthetically derived species built from isoprene skeleton and functionalized with other elements such as O, S, N, etc.

eg. Vitamin A is a terpenoid

130
Q

What is a terpenoid?

A

Essentially functionalized terpenes

Natural and synthetically derived species built from isoprene skeleton and functionalized with other elements such as O, S, N, etc.

eg. Vitamin A is a terpenoid

131
Q

How are steroids similar to fats? How do they differ

A

Similarity: hydrophobicity

Differences: steroids are otherwise unique and unlike fats

132
Q

What structure do steroids have ?

A

All steroids have the basic tetracyclic ring system which is based on the structure of cholesterol

133
Q

How is cholesterol obtained?

A

diet

134
Q

How is cholesterol synthesized?

A

Liver

135
Q

How are lipoproteins produced?

A

Cholesterol is carried in the blood and packaged with fats and proteins to produce lipoproteins

Cholesterol + fats + Protein = Lipoproteins

136
Q

Name 2 cholesterol derived steroid hormones

A
  1. Estrogen/ Estradiol
  2. Testosterone
137
Q

What is an inorganic acid ?

Name an example.

A

An acid derived from inorganic compounds.

These acids do not contain carbon.

They form hydrogen ions and conjugated base ions when dissolved in water.

Ex. Phosphoric Acid

138
Q

What is pKa?

A

A number that describes the acidity of a particular molecule

It measures the strength of an acid by how tightly a proton is held by a bronsted acid.

The lower the pKa, the stronger the acid and the greater its ability to donate its protons

139
Q

What happens to phosphoric acid at physiological pH?

A

Phosphoric acid is significantly dissociated and exists largely in anionic form.

140
Q

What is acid dissociation?

A

The process in which molecules separate or split into other things such as atoms, ions, radicals

141
Q

What is phosphate also known as?

A

orthophosphate

142
Q

When 2 orthophosphates bind, what bond is created and what does it form?

A

They bind via anhydride linkage and form pyrophosphate

143
Q

What is an example of a high energy phosphate bond?

A

The P-O-P (anhydride) bond in pyrophosphate, meaning hydrolysis of pyrophosphate is extremely favourable

144
Q

Why do anhydride bonds store so much energy? (3 reasons)

A
  1. When phosphates link together, their negative charges repel each other strongly
  2. Orthophosphate has more resonance forms (Theres another way of drawing its Lewis dot structure that satisfies the octet rule)
  3. Orthophosphate has more favourable interaction with biological solvent (H2O) than linked phosphates
145
Q

What analogy are linked phosphates given?

A

Linked phosphates are like compressed springs, waiting to fly open and provide energy for an enzyme to catalyze reaction

146
Q

What are the building blocks of nucleic acids?

A

Nucleotides

147
Q

What are the three components of nucleotides?

A
  1. a Ribose (or deoxyribose) sugar group
  2. A Purine or pyrimidine base joined to carbon #1 of the ribose ring
  3. 1, 2, or 3 phosphate units joined to carbon #5 of the ribose ring
148
Q

What is ATP? Why is it important

A

Adenosine Triphosphate

An important nucleotide, the central role in cellular metabolism and is an RNA precursor

ATP is the universal short term energy storage molecule

149
Q

How does ATP store and release energy?

A

Energy is extracted from oxidation of foodstuff and immediately stored in the phosphoanhydride bonds of ATP

Energy is later used to power cellular processes and synthesize glucose or fats (longer term energy storage molecules)

This applies to all living organisms (bacteria, humans)

150
Q

Why is ATP known as a “high energy” structure at neutral pH?

A

It exhibits a large decrease in free energy when it undergoes hydrolytic reactions

151
Q

Which of the following best describes the secondary structure of a protein?

a) Various folded polypeptide chains joining together to form a larger unit
b) The amino acid sequence of the chain
c) The polypeptide chain folding upon itself due to hydrophobic/ hydrophilic interactions
d) Peptide bonds hydrogen bonding to one another to create a sheet like structure

A

D: Because a) is quaternary, b) is primary, c) is tertiary

152
Q

Phenylketonuria (PKU) is an autosomal recessive disorder that results from a deficiency of the enzyme phenylalanine hydroxylase. This enzyme normally converts phenylalanine into tyrosine. PKU results in intellectual disability, growth retardation, fair skin, eczema and a distinct musty body odour. Which of the following is most likely true?

a) Treatment should include a decrease in tyrosine in the diet
b) The musty body odour is likely caused by a disorder in aromatic amino acid metabolism
c) Patients with PKU should increase the amount of phenylalanine in their diet.
d) PKU can be acquired by consuming too much aspartame (an artificial sweetener that contains high levels of phenylalanine)

A

a) wrong: A defect in phenylalanine hydroxylase would result in a build up of phenylalanine leading to an excess of phenylalanine byproducts like phenylacetate, phenyllactate, and phenylpyruvate, and a decrease in tyrosine. Patients with PKU should increase the amount of tyrosine in their diet (it becomes an essential amino acid in this condition; choice A is wrong.

b) Correct: Phenylalanine and its derivatives are aromatic amino acids and high levels of these compounds lead to the distinct musty body odour

c) Wrong: Patients have a build up of phenylalanine and they should eliminate phenylalanine from their diet

d) Wrong: As stated in the question, PKU is an autosomal recessive disorder (genetically acquired) and thus it is not by consuming to much phenylalanine

153
Q

A genetic regulator is found to contain a lysine residue that is important for its binding to DNA. If a mutation were to occur such that a different amino acid replaces the lysine at that location, which of the following resulting amino acids would likely be the least harmful to its ability to bind DNA?

a) Glycine
b) Glutamate
c) Aspartate
d) Arginine

A

Answer: D

Knowing the structures of amino acids is unlikely to be important but know which amino acids are basic (Arginine, histidine, lysine) and which are acidic (Glutamate, aspartate).

Since lysine is basic it is therefore best at binding the negatively charged DNA.

You can assume that a mutation resulting in another basic amino acid (lysine –> arginine) would cause the least change in its ability to bind DNA. Therefore a mutation from lysine to arginine would cause the least harm.

b+c) Wrong: A mutation from lysine to glutamate or aspartate would likely cause the most harm to its ability to bind DNA.

a) Wrong: Glycine is a neutral amino acid.

154
Q

Increasing the amount of cholesterol in a plasma membrane would lead to an increase in:

a) Membrane permeability
b) Atherosclerotic plaques
c) Membrane fluidity at low temperatures but a decrease in membrane fluidity at high temperatures
d) Membrane fluidity at high and low temperatures

A

Answer: C

Plasma membranes can be up to 50% composed of sterols which help stabilize the membrane at both spectrums of the temperature

At low temps, cholesterol increases fluidity because the ring structure of cholesterol doesn’t allow for tight phospholipid tail packing.

At high temperatures, cholesterol decreases membrane fluidity (The OH group of cholesterol prevents phospholipid dispersion

a) Wrong: Cholesterol decreases permeability of membranes by filling in the holes between the fatty acid tails

b) Wrong: The formation of atherosclerotic plaques is due to high levels of blood cholesterol not membrane cholesterol

155
Q

Glycogen is to humans as ___ is to plants

A

Starch

Glycogen serves as n energy storage carbohydrate in animals and is composed of thousands of glucose units

Starch is the same as glycogen, except that the branches are a little different, and serves the same purpose in plants

156
Q

A human space explorer crash lands on a planet where the native inhabitants are entirely unable to digest glycogen but are able to digest cellulose. consequently, they make their clothing out of glycogen based material. The starving space explorer eats one of the native inhabitants’ shirts and the natives are amazed. Based on this information, which of the following is/are true ?

I. The explorer cann digest a- glycosidic linkages
II. The native inhabitants can digest a-glycosidic linkages
III. The native inhabitants can digest starch

A) I only
b) I and III only
c) II and III only
d) I, II, and III

A

A

Item I is true: humans can digest a- glycosidic linkages, such as those found in glycogen. If the natives’ shirts are made of glycogen, our space explorer should have no trouble consuming and digesting them (choice C can be eliminated).

Item II is false: Cellulose contains B-glycosidic linkages. If the natives can digest cellulose but not glycogen, then they cannot digest a- glycosidic linkages (choice D can be eliminated)

Item III is false: starch also contains a a-glycosidic linkages. If the natives cannot digest glycogen, then they likely cannot digest starch either (choice B can be eliminated, and choice A is true)

MCAT Biology Review (3 decks)

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