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Questions and Answers
Which of the following are major types of organic molecules in biology?
Lipids are classified as true polymers.
False
What process forms polymers from monomers?
Dehydration reaction
What is the basic building block of carbohydrates?
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What type of carbohydrates consist of two monosaccharides?
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What is the storage form of carbohydrates in plants?
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What is the primary function of glycogen?
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Which of the following are functions of triglycerides?
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What are the main structural components of a phospholipid?
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What determines the specific functions of proteins?
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What are the building blocks of nucleic acids?
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DNA is single-stranded.
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In DNA, adenine pairs with ______.
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What is the primary purpose of ATP in cells?
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Study Notes
Organic Molecules
- There are 4 major types of organic molecules (macromolecules) in biology: lipids, carbohydrates, proteins, and nucleic acids (RNA & DNA)
- Macromolecules such as proteins and carbohydrates are polymers that are made of many similar or repeating units called monomers.
- Polymers are formed by adding monomers through a dehydration reaction (loss of a water molecule).
- Polymers are broken down by removing monomers through hydrolysis (addition of a water molecule).
- Sugars, proteins, and nucleic acids are true polymers, while lipids are not.
Carbohydrates
- Carbohydrates consist of carbon, hydrogen, and oxygen. The term "carbohydrates" means "hydrated carbon" (C + H2O).
- The basic building blocks of carbohydrates are simple sugars called monosaccharides.
Monosaccharides
- Monosaccharides generally have molecular formulas that are multiples of nCH2O (n = number of carbon atoms).
- Examples of monosaccharides:
- Trioses (n = 3)
- Tetroses (n = 4)
- Pentoses (n = 5): Ribose and Deoxyribose (found in nucleic acids)
- Hexoses (n = 6): Glucose, Fructose, Galactose (C6H12O6)
- Monosaccharides are classified as either ketoses or aldoses:
- Ketoses: Contain a ketone group (e.g., Fructose)
- Aldoses: Contain an aldehyde group (e.g., Glucose, Galactose)
Disaccharides
- Disaccharides consist of two monosaccharides joined together through a dehydration reaction, forming a glycosidic linkage (bond).
- Examples of disaccharides:
- Glucose + glucose = maltose (malt sugar)
- Glucose + galactose = lactose (milk sugar)
- Glucose + fructose = sucrose (table sugar, cane sugar)
Polysaccharides
- Polysaccharides are polymers of glucose; they are very large (thousands of linked monomers), insoluble, good for storage, lack sweetness, and store high levels of energy.
- Two main types of polysaccharides:
- Starch: Storage form of polysaccharide in plants (we consume); consists entirely of glucose monomers.
- Glycogen: Storage form of polysaccharide in animals (liver and muscles); also made of glucose monomers; more extensively branched than starch.
Functions of Carbohydrates
- Provide easy-to-use source of energy: When we eat carbohydrates, they are digested in the small intestine and absorbed as monomers (glucose). This glucose then goes to the blood, where part of it goes to cells for energy production (broken down into H2O, CO2, and ATP), and another part is stored as glycogen in the liver and muscles. If we don't eat for a few hours or if glucose content in food is low, glycogen from the liver is degraded into glucose, which then goes to the blood and cells for energy production.
- Structural and functional purposes: A small amount of carbohydrates is used for structural and functional purposes in our cells and tissues, making up about 1-2% of cell mass.
Lipids
- Lipids represent a unique group of hydrophobic molecules with diverse structures and functions in both plants and animals.
- They consist mostly of hydrocarbons (hydrogen and carbon atoms with few oxygen atoms), making them less oxidized than sugars and resulting in high chemical energy.
- Most lipids are insoluble in water (hydrophobic) but dissolve in organic solvents like alcohol and acetone.
Types of Lipids
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Triglycerides (most abundant in the body): Include fat (in animals) and oil (in plants). They are large molecules (not polymers) constructed from two kinds of molecules:
- 1 glycerol (3-carbon alcohol with 3 OH groups) + 3 fatty acids (long hydrocarbon chain with carboxyl group)
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Phospholipids: Important structural components of cell membranes. They have a polar "head" (phosphorous-containing group) and a nonpolar "tail" (two fatty acid chains) attached to a glycerol backbone.
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Steroids: Characterized by the presence of a carbon skeleton consisting of four interconnected rings. Cholesterol is a precursor of all steroid hormones (e.g., sex hormones) and is also present in cell membranes where it regulates membrane fluidity.
Fatty Acids
- Fatty acids are long hydrocarbon chain molecules that contain a polar carboxyl head group attached to a nonpolar hydrocarbon tail. (Head = hydrophilic; Tail = hydrophobic).
- Saturated fatty acids: No double bonds between carbons; solid at room temperature; found in animal fat.
- Unsaturated fatty acids: Contain one or more double bonds within the chain; liquid at room temperature; found in plants.
Functions of Triglycerides
- Compact energy storage: Triglycerides are a major energy storage form.
- Insulation (subcutaneous fat): A layer of subcutaneous fat insulates the body.
- Cushions internal organs: Triglycerides cushion internal organs.
Trans Fats
- Trans fats are oils that have been solidified by the addition of hydrogen atoms at the sites of double bonds (e.g., margarines).
Omega-3 Fatty Acids
- Omega-3 Fatty acids are found in cold-water fish.
- They have been shown to decrease the risk of heart disease.
Phospholipids in water
- Phospholipids spontaneously assemble into micelles and phospholipid bilayers (and liposomes) in water.
- The nonpolar, hydrophobic tails tuck away from contact with water, while the polar, hydrophilic heads face the aqueous environment.
- Cell membranes are made of phospholipids and are also bilayers.
Proteins
- Proteins are polymers formed by monomers called amino acids.
- Proteins have diverse structures and functions.
Types and Functions of Proteins
- Structural proteins: Provide support (e.g., silk, collagen, keratin).
- Storage proteins: Store nutrients (e.g., ovalbumin in eggs, zeins in corn seeds, casein in milk).
- Transport proteins: Carry substances (e.g., O2 by hemoglobin, ion transporters in cell membranes).
- Hormonal proteins: Coordinate organism's activities (e.g., insulin, glucagon).
- Receptor proteins: Allow cells to respond to chemical stimuli (e.g., neurotransmitter receptors, hormone receptors)
- Contractile proteins: Involved in movement (e.g., actin and myosin).
- Defense proteins: Protect against disease (e.g., antibodies).
- Enzymatic proteins: Most crucial; catalyze (speed up) biochemical reactions.
Amino Acids
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Amino acids consist of an asymmetric carbon bonded to four different covalent partners:
- Amino group: basic part
- Carboxyl group: acidic part
- Hydrogen atom
- R (side chain group)
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All amino acids are identical except for the R group, giving rise to the 20 different amino acids found in proteins.
Types of Proteins based on Shape/Function
- Fibrous proteins: Long fibers; structural roles (e.g., collagen).
- Globular proteins: Round, compact shapes; functional roles (e.g., hemoglobin).
Enzymes
- Enzymes are globular proteins that function as biological catalysts in biochemical reactions.
- A catalyst is a substance that increases the rate of a reaction without being affected by the reactants or products.
- Enzymes are highly specific and highly efficient.
- They are not consumed in the reaction.
- Enzyme activity depends on the presence of an active site where a substrate (reactant) binds.
Naming Enzymes
- Enzymes are named according to the type of reaction they catalyze:
- Hydrolase → hydrolysis reaction
- Polymerase → polymerization reactions
- Phosphatase → removes a phosphate group
Enzyme Activation and Inactivation
- Enzymes in our bodies stay inactive until needed and can be activated or inactivated by complex mechanisms.
Enzyme-Substrate Reactions
- Enzymes and their substrates temporarily bind to form an enzyme-substrate complex (E-S complex).
- This complex undergoes internal rearrangements that form the product, and the enzyme releases the product.
Nucleic Acids
- Nucleic acids encode genetic information (i.e., the primary structure of proteins).
- The flow of information proceeds from DNA to RNA to protein, a process known as the "central dogma."
Types of Nucleic Acids
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Deoxyribonucleic acid (DNA):
- Deoxyribose sugar
- Double-stranded (helix)
- Contains thymine rather than uracil.
-
Ribonucleic acid (RNA):
- Ribose sugar
- Single-stranded
- Contains uracil instead of thymine
- Three varieties: mRNA, rRNA, tRNA
Structure of Nucleic Acids
- The building block of nucleic acid is a nucleotide.
- A nucleotide consists of:
- Pentose sugar (ribose or deoxyribose)
- Nitrogen base
- Phosphate group (PO4-)
Nitrogen Bases
- Nitrogenous bases come in two types:
- Purines: Two-ring structure (e.g., Adenine (A), Guanine (G)).
- Pyrimidines: One-ring structure (e.g., Cytosine (C), Thymine (T), Uracil (U)).
Base pairing in DNA
- In DNA, a purine can only pair with a pyrimidine:
- Thymine (T) pairs with Adenine (A) (A=T; 2 hydrogen bonds)
- Cytosine (C) pairs with Guanine (G) (C≡G; 3 hydrogen bonds)
- Note: A base sequence of ATGA on one chain is bonded to a complementary base sequence TACT on the other strand.
Functional Differences between DNA and RNA
- DNA is the genetic material; genes consist of DNA.
- RNA mainly serves as an intermediate language during the translation of DNA (genetic) language into protein.
Adenosine Triphosphate (ATP)
- ATP is the primary energy currency of cells.
- Glucose metabolism (cellular respiration) produces ATP.
- ATP drives cellular work:
- Chemical work (energy-absorbing reactions)
- Transport work (solute transport across membranes)
- Mechanical work (muscle contraction)
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