Biological Molecules, Cell Structure, and Membrane Function: Study Guide for Exam 2

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Biological macromolecules are assembled and disassembled through two key chemical reactions: dehydration synthesis and hydrolysis.

Dehydration Synthesis (Condensation Reaction): This process joins two monomers by removing a water molecule, forming a covalent bond between them.
Hydrolysis: The reverse process, where a water molecule is added to break the bond between monomers, resulting in smaller molecules.
Equation Example:
- Dehydration:
- Hydrolysis:

The ATP/ADP cycle is central to cellular energy transfer.

ATP (Adenosine Triphosphate): The primary energy carrier in cells.
ADP (Adenosine Diphosphate): Formed when ATP loses a phosphate group, releasing energy.
Chemical Equation:
- ( = inorganic phosphate)
ATP is regenerated from ADP by the addition of a phosphate group during cellular respiration.

Cells contain four major classes of large biological molecules, each with distinct structures and functions.

Carbohydrates
- Monomer: Monosaccharides (e.g., glucose)
- Polymer: Polysaccharides (e.g., starch, cellulose, glycogen)
- Function: Energy storage, structural support
- Identification: Contain C, H, O in a 1:2:1 ratio (e.g., C6H12O6)
Proteins
- Monomer: Amino acids
- Polymer: Polypeptides (fold into functional proteins)
- Function: Enzymes, structural support, transport, signaling
- Identification: Contain C, H, O, N (sometimes S); have amino and carboxyl groups
Nucleic Acids
- Monomer: Nucleotides (sugar, phosphate, nitrogenous base)
- Polymer: DNA and RNA
- Function: Store and transmit genetic information
- Identification: Contain C, H, O, N, P; phosphate backbone
Lipids
- Monomer: Fatty acids and glycerol (not true polymers)
- Polymer: Triglycerides, phospholipids, steroids
- Function: Energy storage, membrane structure, signaling
- Identification: Hydrophobic, mostly C and H, few O

Protein function is determined by its structure, which is organized into four levels:

Primary Structure: Linear sequence of amino acids in a polypeptide chain.
Secondary Structure: Local folding into alpha-helices and beta-pleated sheets, stabilized by hydrogen bonds.
Tertiary Structure: Overall 3D shape of a polypeptide, formed by interactions among R groups (side chains).
Quaternary Structure: Association of multiple polypeptide subunits to form a functional protein.
Example: Hemoglobin has quaternary structure, consisting of four polypeptide subunits.

Cells are classified as prokaryotic or eukaryotic based on structural differences.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	No (nucleoid region)	Yes (membrane-bound)
Membrane-bound organelles	No	Yes
Size	Smaller (1-10 μm)	Larger (10-100 μm)
Examples	Bacteria, Archaea	Plants, Animals, Fungi, Protists

Eukaryotic cells contain specialized organelles, each with distinct functions:

Nucleus: Contains genetic material (DNA); controls cell activities.
Mitochondria: Site of cellular respiration; produces ATP.
Chloroplasts: Site of photosynthesis (plants and algae).
Endoplasmic Reticulum (ER):
- Rough ER: Studded with ribosomes; synthesizes proteins.
- Smooth ER: Synthesizes lipids, detoxifies toxins.
Golgi Apparatus: Modifies, sorts, and packages proteins and lipids.
Lysosomes: Contain digestive enzymes; break down waste.
Peroxisomes: Break down fatty acids and detoxify harmful substances.
Ribosomes: Synthesize proteins (found in all cells).
Vacuoles: Storage (large central vacuole in plants).
Cytoskeleton: Provides structural support, cell movement.

The plasma membrane is described by the fluid mosaic model, which explains its structure and dynamic nature.

Phospholipid Bilayer: Forms the basic structure; hydrophilic heads face outward, hydrophobic tails inward.
Proteins: Embedded within the bilayer; serve as channels, receptors, enzymes.
Carbohydrates: Attached to proteins and lipids; involved in cell recognition.
Cholesterol: Maintains membrane fluidity (in animal cells).

Substances move across cell membranes via several mechanisms:

Passive Transport: Does not require energy; moves substances down their concentration gradient.
- Simple Diffusion: Movement of small, nonpolar molecules (e.g., O2, CO2).
- Facilitated Diffusion: Movement via transport proteins (e.g., glucose, ions).
- Osmosis: Diffusion of water across a selectively permeable membrane.
Active Transport: Requires energy (ATP); moves substances against their concentration gradient.
- Example: Sodium-potassium pump ( out, in per ATP hydrolyzed).
Bulk Transport: Movement of large molecules via vesicles (endocytosis, exocytosis).

Tonicity describes the ability of a surrounding solution to cause a cell to gain or lose water.

Isotonic Solution: No net water movement; cell volume remains stable.
Hypotonic Solution: Water enters the cell; cell may swell or burst (lysis).
Hypertonic Solution: Water leaves the cell; cell shrinks (crenation or plasmolysis in plants).
Application: Intravenous fluids must be isotonic to prevent cell damage.

Cell junctions connect adjacent cells and facilitate communication and adhesion.

Tight Junctions: Seal cells together, preventing leakage of extracellular fluid.
Desmosomes: Anchor cells together, providing mechanical stability.
Gap Junctions: Allow ions and small molecules to pass directly between cells (animal cells).
Plasmodesmata: Channels between plant cells for transport and communication.