General Biology: Biomolecules, Cell Structure, and Membrane Function Study Guide

Biological Molecules

Polymers and Dehydration Synthesis

Biological macromolecules such as carbohydrates, proteins, and nucleic acids are polymers, which are large molecules made by joining smaller subunits called monomers. The process of forming polymers is called dehydration synthesis (or condensation reaction), where a water molecule is removed to form a new bond.

• Polymer: A large molecule composed of repeating monomer units.

• Dehydration Reaction: A chemical reaction that joins two molecules by removing a water molecule.

• Hydrolysis: The breakdown of polymers into monomers by adding water.

• Example: Formation of a peptide bond between two amino acids releases water.

Carbohydrates

Carbohydrates are organic molecules consisting of carbon, hydrogen, and oxygen, typically with a 1:2:1 ratio. They serve as energy sources and structural components.

• Monosaccharides: Simple sugars (e.g., glucose, fructose).

• Disaccharides: Two monosaccharides joined by a glycosidic linkage (e.g., sucrose, lactose).

• Polysaccharides: Long chains of monosaccharides (e.g., starch, cellulose, glycogen).

• Function: Energy storage (starch in plants, glycogen in animals) and structural support (cellulose in plants).

Type	Example	Function
Monosaccharide	Glucose	Immediate energy
Disaccharide	Sucrose	Transported energy
Polysaccharide	Starch, Cellulose	Storage, Structure

Lipids

Lipids are hydrophobic molecules including fats, oils, phospholipids, and steroids. They are not true polymers but are assembled from smaller molecules.

• Triglyceride: Composed of glycerol and three fatty acids, joined by ester bonds.

• Phospholipid: Glycerol, two fatty acids, and a phosphate group; major component of cell membranes.

• Steroid: Four fused carbon rings (e.g., cholesterol).

• Saturated Fatty Acid: No double bonds; solid at room temperature.

• Unsaturated Fatty Acid: One or more double bonds; liquid at room temperature.

Lipid Type	Structure	Function
Triglyceride	Glycerol + 3 fatty acids	Energy storage
Phospholipid	Glycerol + 2 fatty acids + phosphate	Membrane structure
Steroid	4 carbon rings	Hormones, membrane fluidity

Proteins

Proteins are polymers of amino acids linked by peptide bonds. They perform a wide range of functions including catalysis, structure, transport, and signaling.

• Amino Acid: Monomer of proteins; contains amino group, carboxyl group, hydrogen, and R group.

• Peptide Bond: Covalent bond between amino acids formed by dehydration synthesis.

• Levels of Structure: Primary (sequence), secondary (alpha helix, beta sheet), tertiary (3D folding), quaternary (multiple polypeptides).

• Hydrophilic/Hydrophobic Nature: Determined by R group; affects protein folding and function.

General Structure of an Amino Acid:

• Amino group (-NH2)

• Carboxyl group (-COOH)

• Hydrogen atom

• R group (side chain, variable)

Types of Amino Acids:

• Nonpolar (hydrophobic)

• Polar (hydrophilic)

• Electrically charged (acidic or basic)

Nucleic Acids

Nucleic acids (DNA and RNA) store and transmit genetic information. They are polymers of nucleotides.

• Nucleotide: Monomer consisting of a phosphate group, a five-carbon sugar (deoxyribose or ribose), and a nitrogenous base.

• Phosphodiester Bond: Covalent bond linking nucleotides in a nucleic acid chain.

• DNA: Stores genetic information; double helix structure.

• RNA: Involved in protein synthesis; usually single-stranded.

Cell Structure and Organelles

Major Organelles and Their Functions

• Nucleus: Contains genetic material (DNA); controls cell activities.

• Nuclear Envelope: Double membrane surrounding the nucleus.

• Nucleolus: Site of ribosome synthesis.

• Ribosome: Site of protein synthesis; can be free or bound to ER.

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

• Vesicle: Small membrane-bound sac for transport.

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

• Lysosome: Contains digestive enzymes; breaks down waste.

• Vacuole: Storage; large central vacuole in plants.

• Mitochondria: Site of cellular respiration; produces ATP.

• Chloroplast: Site of photosynthesis in plants.

• Peroxisome: Breaks down fatty acids and detoxifies.

• Cytoskeleton: Network of fibers for support and movement.

Comparisons and Definitions

• Magnification vs. Resolution: Magnification is the increase in apparent size; resolution is the ability to distinguish two points as separate.

• Prokaryotic vs. Eukaryotic Cells: Prokaryotes lack a nucleus and membrane-bound organelles; eukaryotes have both.

• Free vs. Bound Ribosomes: Free ribosomes float in cytosol; bound ribosomes are attached to ER.

Endomembrane System

The endomembrane system includes the nuclear envelope, ER, Golgi apparatus, lysosomes, vesicles, and plasma membrane. It is involved in synthesis, modification, and transport of cellular products.

Cytoskeleton

• Microtubules: Hollow tubes; maintain cell shape, aid in cell division, and form cilia/flagella.

• Microfilaments: Thin rods of actin; support cell shape, involved in movement.

• Intermediate Filaments: Provide mechanical support.

Cell Wall and Extracellular Matrix

• Plant Cell Wall: Rigid structure outside plasma membrane; made of cellulose.

• Extracellular Matrix (ECM): Network of proteins and carbohydrates outside animal cells; provides support and signaling.

Intercellular Junctions

• Plasmodesmata (plants): Channels between plant cells.

• Tight Junctions (animals): Prevent leakage between cells.

• Desmosomes (animals): Anchor cells together.

• Gap Junctions (animals): Allow communication between cells.

Membrane Structure and Function

Fluid-Mosaic Model

The plasma membrane is described by the fluid-mosaic model: a flexible bilayer of phospholipids with embedded proteins, cholesterol, and carbohydrates.

• Phospholipids: Amphipathic molecules with hydrophilic heads and hydrophobic tails; form bilayers.

• Membrane Proteins: Integral (span membrane) and peripheral (surface-associated); functions include transport, signaling, and support.

• Glycolipids/Glycoproteins: Carbohydrates attached to lipids/proteins; involved in cell recognition.

• Cholesterol: Maintains membrane fluidity and stability.

Phospholipid Structure

• Glycerol backbone

• Two fatty acid tails (hydrophobic)

• Phosphate group head (hydrophilic)

Membrane Properties

• Amphipathic Nature: Molecules with both hydrophilic and hydrophobic regions (e.g., phospholipids) form bilayers in water.

• Saturated vs. Unsaturated Fatty Acids: Unsaturated fatty acids increase membrane fluidity; saturated fatty acids decrease it.

• Cholesterol: Buffers membrane fluidity across temperature changes.

Membrane Proteins and Carbohydrates

• Integral Proteins: Span the membrane; amphipathic.

• Peripheral Proteins: Loosely attached to membrane surface.

• Glycolipids/Glycoproteins: Involved in cell-cell recognition.

• Extracellular Matrix: Attached to outside of plasma membrane in animal cells.

Membrane Synthesis

New membrane sections are made inside the cell, primarily in the ER, and transported via vesicles to the plasma membrane.

Functions of the Plasma Membrane

• Selective barrier for substances entering/exiting the cell

• Cell communication

• Attachment to cytoskeleton and ECM

Membrane Transport

Diffusion and Osmosis

• Diffusion: Movement of molecules from high to low concentration.

• Osmosis: Diffusion of water across a selectively permeable membrane.

• Facilitated Diffusion: Passive transport via membrane proteins.

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

Gradients

• Concentration Gradient: Difference in concentration across a space.

• Charge Gradient: Difference in electrical charge across a membrane.

• Electrochemical Gradient: Combined effect of concentration and charge gradients.

Solution Tonicity

• Hypertonic: Higher solute concentration outside the cell; cell loses water.

• Hypotonic: Lower solute concentration outside; cell gains water.

• Isotonic: Equal solute concentration; no net water movement.

• Crenation: Shrinking of animal cells in hypertonic solution.

• Plasmolysis: Shrinking of plant cell cytoplasm in hypertonic solution.

Bulk Transport

• Endocytosis: Uptake of large particles by engulfing them in vesicles.

• Exocytosis: Release of substances from cell via vesicles.

Additional info: Some explanations and examples have been expanded for clarity and completeness based on standard General Biology curriculum.