Biological macromolecules are the building blocks of life. Carbohydrates, lipids, proteins, and nucleic acids play crucial roles in energy storage, structural support, and information transfer within cells. Understanding these molecules is key to grasping how life functions at the molecular level.
Each macromolecule has a unique structure that determines its function. From the energy-storing glycogen to the information-carrying DNA, these molecules work together to maintain life processes. Their interactions and transformations form the basis of metabolism and cellular communication.
Biological macromolecules and their monomers
Types and composition of macromolecules
- Four major classes of biological macromolecules carbohydrates, lipids, proteins, and nucleic acids
- Carbohydrates composed of monosaccharide monomers (glucose, fructose, galactose)
- Proteins made up of 20 standard amino acid monomers
- Nucleic acids (DNA and RNA) built from nucleotide monomers
- Nucleotides consist of sugar, phosphate group, and nitrogenous base
- Lipids lack true monomers but composed of fatty acids and glycerol or other alcohol molecules
Polymerization and breakdown processes
- Joining monomers to form macromolecules called dehydration synthesis
- Involves removal of water molecule between monomers
- Breaking down macromolecules into monomers called hydrolysis
- Involves addition of water molecule to break bonds
- These processes essential for macromolecule synthesis and digestion in living organisms
Structure and function of macromolecules
Carbohydrates
- Classified as monosaccharides, disaccharides, and polysaccharides
- Functions include energy storage and structural support
- Energy storage examples glycogen in animals, starch in plants
- Structural support examples cellulose in plant cell walls, chitin in arthropod exoskeletons
- Monosaccharides serve as quick energy source for cells (glucose in blood)
- Disaccharides formed by two monosaccharides (sucrose, lactose, maltose)
- Polysaccharides can be branched or unbranched chains of monosaccharides
Lipids
- Include triglycerides, phospholipids, and steroids
- Serve roles in energy storage, cell membrane structure, and hormone signaling
- Triglycerides primary form of energy storage in adipose tissue
- Phospholipids form bilayers in cell membranes due to amphipathic nature
- Hydrophilic heads face aqueous environments, hydrophobic tails face each other
- Cholesterol, a steroid lipid, modulates membrane fluidity
- Also serves as precursor for steroid hormones (testosterone, estrogen)
- Waxes provide waterproofing and protection (plant cuticles, insect exoskeletons)
Proteins
- Complex 3D structures determined by four levels of organization
- Primary structure amino acid sequence
- Secondary structure alpha helices and beta sheets
- Tertiary structure overall 3D shape of a single polypeptide
- Quaternary structure interaction of multiple polypeptide subunits
- Protein functions include
- Enzymatic catalysis (amylase, DNA polymerase)
- Structural support (collagen, keratin)
- Transport (hemoglobin, ion channels)
- Signaling (insulin, growth factors)
- Defense (antibodies)
- Shape of protein directly related to its function
- Enzyme active sites shaped to fit specific substrates
Nucleic acids
- Store and transmit genetic information
- DNA typically double-stranded with double helix structure
- RNA usually single-stranded, can form complex secondary structures
- Central dogma of molecular biology describes flow of genetic information
- DNA transcribed to RNA, RNA translated to proteins
- DNA replication ensures genetic continuity during cell division
- RNA types include mRNA, tRNA, and rRNA, each with specific functions in protein synthesis
Role of macromolecules in life
- Carbohydrates serve as primary energy source for cellular respiration
- Glucose broken down through glycolysis, citric acid cycle, and electron transport chain
- Lipids provide long-term energy storage in the form of triglycerides
- Fatty acids can be oxidized for energy when carbohydrates are scarce
- Proteins function as enzymes, catalyzing biochemical reactions
- Regulate metabolic pathways by controlling reaction rates
- ATP, a nucleotide, serves as universal energy currency in cells
Structural support and protection
- Structural proteins provide support and protection at cellular and organismal levels
- Collagen gives strength to connective tissues (tendons, ligaments)
- Keratin forms protective structures (hair, nails, feathers)
- Carbohydrates contribute to structural integrity
- Cellulose provides rigidity to plant cell walls
- Chitin forms exoskeletons in arthropods and cell walls in fungi
- Lipids in cell membranes maintain cellular integrity and regulate selective permeability
- DNA stores genetic information in the form of nucleotide sequences
- DNA replication ensures genetic continuity during cell division
- Transcription produces mRNA from DNA template
- Translation uses mRNA to synthesize proteins on ribosomes
- RNA molecules play diverse roles in gene expression
- mRNA carries genetic information from DNA to ribosomes
- tRNA transports amino acids to ribosomes during translation
- rRNA forms structural component of ribosomes
Cellular communication and regulation
- Proteins act as receptors and signaling molecules in cell communication
- Hormone receptors detect and respond to endocrine signals
- Neurotransmitter receptors facilitate nerve impulse transmission
- Lipids participate in signal transduction pathways
- Phospholipids can be cleaved to produce second messengers
- Post-translational modifications of proteins regulate function and cellular signaling
- Glycosylation addition of carbohydrate groups to proteins
- Phosphorylation addition of phosphate groups to alter protein activity
Structure vs function of macromolecules
Carbohydrate structure-function relationships
- Branched structure of glycogen allows rapid mobilization of glucose units
- Multiple end points for enzymes to act on simultaneously
- Alpha and beta glycosidic linkages determine digestibility and structural properties
- Alpha linkages in starch easily digested by humans
- Beta linkages in cellulose indigestible by humans, provide structural rigidity to plants
- Ring structure of monosaccharides allows for diverse bonding patterns
- Enables formation of linear and branched polysaccharides
Protein structure-function relationships
- Hydrophobic core and hydrophilic surface of globular proteins enable function in aqueous environments
- Allows proteins to maintain stable structure while interacting with cellular components
- Enzyme active sites have specific shapes and chemical properties
- Determines substrate specificity and catalytic function
- Lock-and-key model and induced fit model explain enzyme-substrate interactions
- Fibrous proteins have repeating structural motifs related to their function
- Alpha-helical coiled coils in keratin provide strength and flexibility to hair
Nucleic acid structure-function relationships
- Complementary base pairing in DNA allows for accurate replication and transcription
- A-T and G-C base pairs maintain genetic fidelity
- Flexibility of RNA's single-stranded structure enables complex secondary structures
- Essential for diverse functions like catalysis in ribozymes
- Double-helix structure of DNA provides stability and protection for genetic material
- Allows for efficient packaging of DNA in chromosomes
Lipid structure-function relationships
- Amphipathic nature of phospholipids directly relates to bilayer formation in cell membranes
- Hydrophilic heads face aqueous environments, hydrophobic tails face inward
- Fatty acid composition affects membrane fluidity
- Unsaturated fatty acids increase fluidity, saturated fatty acids decrease fluidity
- Cholesterol's rigid ring structure modulates membrane fluidity
- Prevents crystallization of phospholipids at low temperatures
- Reduces membrane fluidity at high temperatures