Study Guide: Chemistry Comes Alive & Cells—The Living Units (CH 02-03)

CH 02. Chemistry Comes Alive

Proteins: Structure and Function

Proteins are essential macromolecules in all living organisms, serving structural, enzymatic, and regulatory roles. Understanding their building blocks, structure, and function is fundamental to cell biology.

• Building Blocks: Proteins are polymers of amino acids, which contain an amino group, carboxyl group, hydrogen atom, and a variable R group attached to a central carbon.

• General Structure: Amino acids are linked by peptide bonds to form polypeptide chains, which fold into specific three-dimensional shapes.

• Biological Functions: Proteins function as enzymes, structural components, signaling molecules, transporters, and antibodies.

• Example: Hemoglobin is a protein that transports oxygen in the blood.

Levels of Protein Structure and Denaturation

Proteins have four levels of structural organization, each critical for their function. Denaturation disrupts these structures, affecting protein activity.

• Primary Structure: Linear sequence of amino acids.

• Secondary Structure: Local folding into alpha-helices and beta-sheets, stabilized by hydrogen bonds.

• Tertiary Structure: Overall three-dimensional shape formed by interactions among R groups.

• Quaternary Structure: Association of multiple polypeptide chains.

• Denaturation: Loss of protein structure due to heat, pH changes, or chemicals, leading to loss of function.

• Impact: Denatured proteins cannot perform their biological roles (e.g., enzymes lose catalytic activity).

CH 03. Cells: The Living Units

Major Regions of a Generalized Cell

Cells are the basic units of life, each with specialized regions performing distinct functions.

• Plasma Membrane: Outer boundary, regulates entry and exit of substances.

• Cytoplasm: Contains cytosol and organelles, site of most cellular activities.

• Nucleus: Control center, stores genetic material (DNA).

Chemical Composition and Function of the Plasma Membrane

The plasma membrane is a selectively permeable barrier composed mainly of lipids and proteins.

• Phospholipid Bilayer: Provides fluidity and selective permeability.

• Proteins: Serve as channels, receptors, and enzymes.

• Carbohydrates: Involved in cell recognition and signaling.

• Selective Permeability: Allows certain molecules to pass while restricting others.

Cell Junctions: Tight Junctions, Desmosomes, and Gap Junctions

Cell junctions connect adjacent cells and facilitate communication and adhesion.

• Tight Junctions: Seal adjacent cells, preventing passage of substances between them.

• Desmosomes: Anchor cells together, providing mechanical strength.

• Gap Junctions: Allow direct communication between cells via channels.

Gradients Across the Plasma Membrane

Gradients drive the movement of substances across membranes.

• Concentration Gradient: Difference in solute concentration across a membrane.

• Electrical Gradient: Difference in charge across a membrane.

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

Membrane Transport Processes

Substances move across the plasma membrane via passive or active transport mechanisms.

• Passive Transport: No energy required; includes simple diffusion, facilitated diffusion, and osmosis.

• Active Transport: Requires energy (ATP); includes primary and secondary active transport.

• Direction: Passive transport moves substances down their gradients; active transport moves them against gradients.

• Example: Sodium-potassium pump is a primary active transport mechanism.

Osmosis and Effects of Tonicity

Osmosis is the movement of water across a semipermeable membrane, influenced by solute concentration.

• Osmotic Pressure: Pressure required to prevent water movement by osmosis.

• Isotonic Solution: No net water movement; cell volume remains constant.

• Hypertonic Solution: Water leaves the cell; cell shrinks (crenation).

• Hypotonic Solution: Water enters the cell; cell swells and may burst (lysis).

Vesicular Transport: Endocytosis and Exocytosis

Cells use vesicles to transport large molecules across the plasma membrane.

• Endocytosis: Uptake of materials into the cell; includes pinocytosis (fluid), phagocytosis (solids), and receptor-mediated endocytosis (specific molecules).

• Exocytosis: Release of materials from the cell.

• Direction: Endocytosis brings substances in; exocytosis expels substances out.

Cytosol and Cytosolic Organelles

The cytosol is the fluid component of the cytoplasm, containing dissolved substances and organelles.

• Mitochondria: Site of ATP production via cellular respiration.

• Ribosomes: Sites of protein synthesis.

• Endoplasmic Reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids and detoxifies chemicals.

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

• Lysosomes: Contain digestive enzymes for breakdown of waste.

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

• Proteasomes: Degrade unneeded or damaged proteins.

Cytoskeleton, Cilia, and Flagella

The cytoskeleton provides structural support and facilitates movement.

• Microfilaments, Intermediate Filaments, Microtubules: Components of the cytoskeleton with roles in shape, support, and transport.

• Cilia: Short, hair-like structures that move substances across cell surfaces.

• Flagella: Long, whip-like structures for cell movement (e.g., sperm cells).

Nucleus: Structure and Function

The nucleus is the control center of the cell, containing genetic material and directing cellular activities.

• Nuclear Envelope: Double membrane that encloses the nucleus.

• Nucleolus: Site of ribosomal RNA synthesis and ribosome assembly.

• Chromatin: DNA-protein complex; condenses to form chromosomes during cell division.

• DNA Replication: Process by which DNA is copied before cell division.

Protein Synthesis: Transcription and Translation

Protein synthesis occurs in two main phases, involving DNA, mRNA, tRNA, and rRNA.

• Transcription: DNA is used as a template to synthesize mRNA in the nucleus.

• Translation: mRNA is decoded by ribosomes in the cytoplasm to assemble amino acids into proteins.

• mRNA Codons: Triplets of nucleotides that specify amino acids.

• tRNA Anticodons: Complementary to mRNA codons; bring specific amino acids to the ribosome.

• rRNA: Structural and catalytic component of ribosomes.