General Biology 1: Core Concepts and Practice Questions (Midterm Study Guide)

Nucleic Acids

DNA Structure and Complementarity

DNA is a double-stranded molecule with antiparallel strands, meaning the two strands run in opposite directions. Each strand has a 5' (five-prime) end and a 3' (three-prime) end, referring to the carbon numbers in the DNA's sugar backbone.

• Antiparallel Orientation: One strand runs 5' → 3', and its complementary strand runs 3' → 5'.

• Base Pairing: Adenine pairs with thymine, and cytosine pairs with guanine.

• Example: If one strand is 5'–ATCG–3', the complementary strand is 3'–TAGC–5'.

Biological Membranes and Lipids

Phospholipid Diversity

Phospholipids are major components of cell membranes. In Archaea, phospholipids have isoprenoid tails instead of fatty acids, which is a key adaptation for survival in extreme environments.

• Bacteria: Have fatty acid-based phospholipids.

• Archaea: Have isoprenoid-based phospholipids.

Microscopy

Types of Microscopes

Microscopes are essential tools for studying cells and viruses. The electron microscope provides the highest resolution for viewing detailed structures such as viruses.

• Light Microscope: Used for general cell observation.

• Electron Microscope: Used for detailed ultrastructure, including viruses.

Cell Structure

Prokaryotic vs. Eukaryotic Cells

Eukaryotic cells contain membrane-bound organelles such as the nucleus, mitochondria, and (in plants) chloroplasts. Prokaryotic cells (bacteria and archaea) lack these structures.

• Plant Cells: Have a nucleus, mitochondria, and chloroplasts.

• Animal Cells: Have a nucleus and mitochondria, but no chloroplasts.

Eukaryotic Cell Organelles

Key organelles include mitochondria (energy production), chloroplasts (photosynthesis in plants), and the endoplasmic reticulum (protein and lipid synthesis).

Organelle	Function
Mitochondria	ATP production via cellular respiration
Chloroplast	Photosynthesis (plants and algae)
Endoplasmic Reticulum	Protein and lipid synthesis

Endomembrane System and Protein Targeting

Proteins destined for the nucleus contain a nuclear localization signal. If this signal is missing, the protein remains in the cytoplasm.

Digestive Organelles

Lysosomes contain digestive enzymes that break down waste and cellular debris.

Endosymbiotic Theory

Secondary endosymbiosis occurs when a eukaryotic cell engulfs another eukaryotic cell that has already undergone primary endosymbiosis.

Cytoskeleton and Mitosis

The mitotic apparatus, responsible for chromosome movement during cell division, is primarily composed of microtubules.

Plant Cell Walls

The secondary cell wall contains cellulose, hemicellulose, and lignin, providing structural support.

Biological Membranes

Membrane Fluidity

Membrane fluidity is influenced by lipid composition and temperature.

• Increases with: Unsaturated phospholipids, high temperature, low protein content.

• Decreases with: High protein content.

Concentration Gradients and Diffusion

A concentration gradient refers to the difference in solute concentration across a membrane, driving diffusion.

Membrane Permeability

Small, nonpolar molecules (e.g., O2, CO2) diffuse easily; large or polar molecules (e.g., glucose) require transport proteins.

Passive vs. Active Transport

Feature	Passive Transport	Active Transport
Energy Requirement	No ATP required	ATP required
Direction	Down concentration gradient	Against concentration gradient

Osmosis

Osmosis is the movement of water across a semipermeable membrane. When slugs and snails are sprinkled with salt, water leaves their cells, causing dehydration and death.

Facilitated Diffusion

• Carrier Proteins: Bind and transport molecules across the membrane.

• Channel Proteins: Form pores for molecules to diffuse through.

Active Transport

The sodium-potassium pump maintains high potassium and low sodium inside the cell, powered by ATP.

Endocytosis

• Phagocytosis: Engulfment of large particles or dead cells.

Energy and Metabolism

Chemical Energy in Molecules

The C-H bonds in glucose store potential energy, which can be released during cellular respiration.

Laws of Thermodynamics

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

• Second Law: Some energy is lost as heat during metabolic processes.

Gibbs Free Energy

A spontaneous reaction has a negative Gibbs free energy ($riangle G < 0$).

Energy Coupling

Energy released by exergonic reactions is used to drive endergonic reactions.

Enzymes

Enzyme Activity and Temperature

• Optimal Temperature: Enzymes function best within a specific temperature range; too high or too low can denature or inactivate them.

Enzyme Activation Energy

Enzymes lower the activation energy required for biological reactions, increasing reaction rates.

Label	Description
A	Transition state
B	Activation energy without enzyme
C	Activation energy with enzyme

Enzyme Binding Models

• Induced Fit Model: Substrate binding induces a conformational change in the enzyme, enhancing catalysis.

Additional info: The lock-and-key model suggests a rigid fit, while the induced fit model allows for flexibility and better explains enzyme specificity.