General Biology Exam 2 Study Notes: Enzymes, Cell Structure, Membranes, and Transport

Enzymes and Chemical Reactions

How Enzymes Work

Enzymes are biological catalysts, typically proteins, that speed up chemical reactions by lowering the activation energy required for the reaction to proceed.

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

• Mechanism: Enzymes bind to reactants (substrates), physically manipulating them to facilitate the reaction. This may involve straining substrate bonds or orienting substrates optimally.

• Result: The chemical reaction proceeds faster and with less energy input.

Example: The enzyme sucrase catalyzes the breakdown of sucrose into glucose and fructose.

Enzyme Regulation

Enzyme activity can be regulated to ensure proper cellular function.

• Inhibitors: Molecules that decrease enzyme activity. They can be reversible or irreversible.

• Competitive Inhibition: A regulator binds to the active site, blocking substrate binding. Usually reversible and competes with the substrate.

• Non-competitive Inhibition: The inhibitor binds elsewhere on the enzyme, changing its shape so the substrate can no longer bind effectively.

Example: Many drugs act as enzyme inhibitors to block specific metabolic pathways.

Cell Theory and Cell Types

Cell Theory

• All living organisms are made up of one or more cells.

• All cells arise from other pre-existing cells.

Basic Cell Structure

• Genetic Material: DNA or RNA that carries hereditary information.

• Cell Membrane: A lipid bilayer that encloses the cell, controlling the movement of substances in and out.

• Cytoplasm: The jelly-like substance within the cell membrane containing organelles.

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic or eukaryotic based on their structure.

Feature	Prokaryotic Cells	Eukaryotic Cells
Nucleus	Absent	Present
Organelles	Few (no membrane-bound)	Many (membrane-bound)
Examples	Bacteria, Archaea	Animals, Plants, Fungi, Protists

Animal vs. Plant Cells: Plant cells have chloroplasts, cell walls, and large central vacuoles; animal cells have centrioles and lysosomes.

Endosymbiosis

Endosymbiosis is a theory explaining the origin of mitochondria and chloroplasts in eukaryotic cells. It proposes that these organelles originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Mitochondria and Chloroplasts: Both have their own DNA and double membranes, supporting the endosymbiotic theory.

Cell Membranes and Transport

Types of Membrane Transport

• Simple Diffusion: Movement of small, non-polar molecules (e.g., gases, small lipids) directly through the membrane. Does not require channel proteins.

• Facilitated Diffusion: Movement of larger or charged molecules across the membrane via channel or carrier proteins. Requires channel proteins.

• Active Transport: Movement of substances from low to high concentration (against the concentration gradient). Requires energy (usually ATP).

Types of Active Transport

• Primary Active Transport: Uses pump proteins and ATP directly (e.g., sodium-potassium pump).

• Secondary Active Transport: Uses concentration gradients created by primary active transport to move other substances.

Bulk Transport

• Endocytosis: Bringing materials into the cell via vesicles.

• Phagocytosis: "Cell eating"; engulfing large particles.

• Pinocytosis: "Cell drinking"; engulfing fluids and dissolved substances.

• Exocytosis: Expelling materials from the cell via vesicles.

Osmosis and Tonicity

Osmosis is the diffusion of water across a selectively permeable membrane.

• Hypotonic Solution: Lower solute concentration outside the cell; water enters the cell, causing it to swell or burst.

• Hypertonic Solution: Higher solute concentration outside the cell; water leaves the cell, causing it to shrink.

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

The Nucleus and Genetic Material

Structure of the Nucleus

• Chromatin: Thin fibers of DNA and proteins carrying genetic information.

• Nucleolus: Site of ribosome subunit assembly.

• Nuclear Membrane: Double membrane with nuclear pores for transport of molecules in and out of the nucleus.

The Cytoskeleton

Types of Cytoskeletal Fibers

• Microtubules: Hollow tubes that transport materials and help in cell division. Also form cilia and flagella for movement.

• Intermediate Filaments: Rope-like fibers providing structural support and strength.

• Microfilaments: Solid, actin-based fibers involved in cell movement and contraction.

Functions: Support cell shape, enable intra-cellular transport, and facilitate cell movement.

Additional Info

• ATP (Adenosine Triphosphate): The primary energy currency of the cell. Energy is released when the bond between the last two phosphate groups is broken: $\text{ATP} \rightarrow \text{ADP} + \text{P}_i + \text{energy}$

• Photosynthesis: The process by which plants convert sunlight, water, and carbon dioxide into glucose and oxygen: $6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

• Cellular Respiration: The process by which cells convert glucose and oxygen into ATP, carbon dioxide, and water: $\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$