Biological Molecules and Polymers

Monomers and Polymers

Biological macromolecules are often polymers, which are large molecules made by joining smaller units called monomers. The process of forming and breaking these polymers is fundamental to cell biology.

• Monomer: A small molecule that can join with others to form a polymer (e.g., amino acids, nucleotides, monosaccharides).

• Polymer: A large molecule made up of repeating monomer units (e.g., proteins, nucleic acids, polysaccharides).

• Hydrolysis: A chemical reaction that breaks bonds between monomers by adding water.

• Dehydration (Condensation): A reaction that forms a bond between monomers by removing water.

• Disassociation: The process of breaking apart molecules into smaller units.

Example: Proteins are polymers made of amino acid monomers; cellulose is a polymer of glucose.

Types of Biological Polymers

• Proteins: Polymers of amino acids.

• Carbohydrates: Polymers of monosaccharides (e.g., cellulose is made of glucose).

• Nucleic Acids: Polymers of nucleotides (e.g., RNA, DNA).

• Lipids: Not true polymers; fatty acids are not linked in long chains like other macromolecules.

Key Point: Not all biological molecules are polymers (e.g., fatty acids).

Carbohydrates: Structure and Classification

Carbohydrates are classified by the number of sugar units and the position of the carbonyl group.

• Monosaccharide: Single sugar unit (e.g., glucose, fructose).

• Disaccharide: Two monosaccharides joined together (e.g., sucrose).

• Polysaccharide: Many monosaccharides joined (e.g., starch, glycogen, cellulose).

• Aldose: Monosaccharide with the carbonyl group at the end of the carbon chain.

• Ketose: Monosaccharide with the carbonyl group within the carbon chain.

Example: Glucose is an aldose; fructose is a ketose.

Starch and Glycogen

• Amylose: Unbranched form of starch found in plants.

• Glycogen: Highly branched polysaccharide found in animals.

• Key Difference: Amylose is found in plants; glycogen in animals.

Lipids and Membrane Structure

Phospholipids and Membrane Properties

Phospholipids are major components of cell membranes, consisting of a hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails.

• Phospholipid: Molecule with a phosphate group and fatty acid tails; forms bilayers in water.

• Saturated Fat: Fatty acid chains with no double bonds; solid at room temperature.

• Unsaturated Fat: Fatty acid chains with one or more double bonds; liquid at room temperature.

• Trans Fat: Unsaturated fat with trans double bonds; associated with health risks.

Example: Cell membranes are composed of a phospholipid bilayer.

Membrane Fluidity

Membrane fluidity is influenced by temperature, fatty acid composition, and cholesterol content.

• Temperature: Higher temperatures increase fluidity; lower temperatures decrease it.

• Unsaturated Fatty Acids: Increase membrane fluidity due to kinks in the tails.

• Cholesterol: Modulates fluidity; stabilizes membrane at high temperatures and prevents rigidity at low temperatures.

Example: Dehydrogenating fatty acid chains (removing double bonds) lowers melting temperature and increases fluidity.

Membrane Proteins

Proteins embedded in the membrane perform various functions, including transport, signaling, and maintaining gradients.

• Carrier Proteins: Transport specific molecules across the membrane.

• Pump Proteins: Use energy (often ATP) to move substances against their concentration gradient.

• Receptor Proteins: Receive and transmit signals across the membrane.

Membrane Permeability

• Semi-permeable Membrane: Allows some molecules to pass while blocking others (e.g., allows water but not ions).

• Hydrophobic Molecules: Pass through membranes easily.

• Charged or Large Molecules: Require transport proteins to cross the membrane.

Cell Structure and Organelles

Major Organelles and Their Functions

• Nucleus: Contains genetic material (DNA); control center of the cell.

• Ribosomes: Synthesize proteins.

• Endoplasmic Reticulum (ER): Smooth ER synthesizes lipids; rough ER has ribosomes and synthesizes proteins.

• Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.

• Mitochondria: Site of cellular respiration; produces ATP.

• Peroxisomes: Break down fatty acids and detoxify harmful substances.

• Lysosomes: Digest macromolecules and old organelles (autophagy).

• Centrosome: Organizes microtubules; important in cell division.

• Chloroplasts: Site of photosynthesis in plant cells.

• Cell Wall: Provides structure and protection in plant cells.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic Cells: Lack membrane-bound organelles; DNA is not enclosed in a nucleus.

• Eukaryotic Cells: Have membrane-bound organelles, including a nucleus.

Cellular Structures

• Cristae: Folds of the inner mitochondrial membrane; increase surface area for ATP production.

• Nuclear Lamina: Network of proteins that maintains the shape of the nucleus.

• Chromatin: DNA and protein complex in the nucleus.

Transport Across Membranes

Types of Transport

• Passive Transport: Movement of molecules down their concentration gradient without energy input (e.g., diffusion, facilitated diffusion).

• Active Transport: Movement of molecules against their concentration gradient using energy (usually ATP).

• Osmosis: Diffusion of water across a semi-permeable membrane.

Osmotic Solutions

• Isotonic: Equal solute concentration inside and outside the cell; no net water movement.

• Hypertonic: Higher solute concentration outside the cell; water moves out, cell shrinks.

• Hypotonic: Lower solute concentration outside the cell; water moves in, cell swells.

Enzymes and Metabolism

Enzyme Function

Enzymes are biological catalysts that speed up chemical reactions by lowering the activation energy required.

• Active Site: Region on the enzyme where the substrate binds.

• Activation Energy: The minimum energy required to start a chemical reaction.

• Competitive Inhibitors: Molecules that bind to the active site and decrease enzyme activity.

• Allosteric Regulation: Regulatory molecule binds at a site other than the active site, affecting enzyme function.

• Cooperativity: Binding of one substrate molecule increases the likelihood of additional substrate molecules binding.

Catabolic Pathways

• Catabolic Pathways: Break down molecules and release energy.

• Anabolic Pathways: Build complex molecules and consume energy. (Additional info: Not explicitly mentioned but important for context.)

Thermodynamics in Biology

First Law of Thermodynamics

• First Law: Energy cannot be created or destroyed, only transformed.

• Entropy: A measure of disorder in a system; the universe tends toward increased entropy.

• Enthalpy ($H$): Total energy of a system.

Equation:

$\Delta G = \Delta H - T\Delta S$

Where $\Delta G$ is the change in free energy, $\Delta H$ is the change in enthalpy, $T$ is temperature in Kelvin, and $\Delta S$ is the change in entropy.

Table: Comparison of Cell Structures and Functions

Key Terms and Definitions

• Hydrophilic: Attracted to water.

• Hydrophobic: Repelled by water.

• Phagocytosis: Engulfing of particles by the cell.

• Autophagy: Digestion of old organelles within the cell.

• Centrosome: Organelle that organizes microtubules.

• Cristae: Folds in the inner mitochondrial membrane.