General Biology Exam 1 Review: Cell Structure, Membrane Function, and Macromolecules

Cell Structure and Function

Distinguishing Prokaryotic and Eukaryotic Cells

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

• Prokaryotic cells lack a nucleus and membrane-bound organelles.

• Eukaryotic cells possess a nucleus and various membrane-bound organelles.

• Key features found only in eukaryotes include the nucleus and cytoskeleton.

Example: Only eukaryotic cells have a nucleus and cytoskeleton, while both cell types have a plasma membrane.

Endosymbiotic Theory

The endosymbiotic theory explains the origin of certain organelles in eukaryotic cells.

• Mitochondria and chloroplasts are believed to have originated from free-living prokaryotes engulfed by ancestral eukaryotic cells.

• Both organelles contain circular DNA, similar to prokaryotes.

Example: Prokaryotes, mitochondria, and chloroplasts have circular DNA; nuclei do not.

Major Eukaryotic Organelles and Their Functions

Eukaryotic cells contain specialized organelles that perform distinct functions.

• Lysosome: Digests cellular waste and macromolecules.

• Chloroplast: Site of photosynthesis in plant cells.

• Mitochondrion: Produces ATP via cellular respiration.

• Smooth Endoplasmic Reticulum (ER): Synthesizes lipids and detoxifies chemicals.

Example: Lipids are synthesized in the smooth ER.

Organelle Abundance and Cell Type Function

The abundance of specific organelles varies depending on cell function.

• Heart muscle cells have many mitochondria to meet high energy demands.

Example: Mitochondria are abundant in muscle cells due to their role in ATP production.

Defective Organelles and Cellular Consequences

Defects in organelles can lead to cellular dysfunction.

• Peroxisomes break down hydrogen peroxide (H2O2); their malfunction leads to toxic accumulation.

Example: Excess H2O2 suggests defective peroxisomes.

Protein Secretion Pathway

Proteins destined for secretion follow a specific pathway:

• Produced in the rough ER

• Transported to the Golgi apparatus

• Packaged into vesicles for secretion

Example: Secreted proteins: Rough ER → Golgi → Vesicle → Plasma membrane

Macromolecules and Elements in Cells

Common Elements in Cells

Cells are primarily composed of four elements:

• Carbon (C)

• Hydrogen (H)

• Oxygen (O)

• Nitrogen (N)

Classes of Macromolecules and Their Monomers

Cells contain four major classes of macromolecules, each built from specific monomers.

Macromolecule	Monomer
Proteins	Amino acids
Nucleic acids	Nucleotides
Carbohydrates	Monosaccharides
Lipids	Fatty acids & glycerol (not true polymers)

Example: Proteins are polymers of amino acids.

Lipids: Structure and Properties

Lipids differ from other macromolecules as they are not true polymers and are largely hydrophobic.

• Composed mainly of hydrocarbon chains.

• Serve as energy storage, membrane structure, and signaling molecules.

Chemical Bonds and Molecular Properties

Electronegativity and Bond Types

Electronegativity measures an atom's tendency to attract electrons in a bond.

• Oxygen is the most electronegative among C, N, H, and O.

• Bond types:

  • Polar covalent: Unequal sharing of electrons (e.g., O-H bond)

  • Nonpolar covalent: Equal sharing of electrons (e.g., C-H bond)

  • Ionic: Transfer of electrons (e.g., Na+Cl-)

Example: $\Delta EN = |EN_{A} - EN_{B}|$ (difference in electronegativity)

Solubility and Molecular Behavior in Water

Polarity affects solubility in water.

• Molecules with many C-O or O-H bonds are hydrophilic and dissolve in water.

• Molecules with many C-H or C-C bonds are hydrophobic.

Hydrophobic, Hydrophilic, and Amphipathic Molecules

Molecules are classified based on their interaction with water:

• Hydrophobic: Repel water (e.g., hydrocarbons)

• Hydrophilic: Attract water (e.g., sugars, salts)

• Amphipathic: Contain both hydrophobic and hydrophilic regions (e.g., phospholipids)

Example: Fatty acids are amphipathic; hydrocarbons are hydrophobic.

Water Molecule Structure and Interactions

Water molecules are polar and form hydrogen bonds.

• Bond within water: Polar covalent (O-H)

• Bond between water molecules: Hydrogen bond

Example: $\text{H}_2\text{O}$ forms hydrogen bonds with other water molecules.

Covalent, Ionic, and Hydrogen Bonds

Types of chemical bonds:

• Covalent: Shared electrons (strongest)

• Ionic: Transferred electrons (moderate strength)

• Hydrogen: Attraction between partial charges (weakest)

Bond Type	Example	Relative Strength
Covalent	O-H in water	Strong
Ionic	Na+Cl-	Moderate
Hydrogen	Between water molecules	Weak

Membrane Structure and Function

Phospholipids and Membrane Behavior

Phospholipids form the basic structure of cell membranes.

• Polar head: Hydrophilic

• Nonpolar tail: Hydrophobic

• Form bilayers in water, creating cell membranes.

Example: Fatty acid tails are hydrophobic; heads are hydrophilic.

Cell Membrane Components

The cell membrane consists of four primary components:

• Phospholipids

• Proteins

• Cholesterol

• Carbohydrates

Membrane Permeability

Substances cross membranes at different rates depending on their properties.

Substance	Relative Permeability
Ions	Least permeable
Small polar molecules	Moderately permeable
Large polar molecules	Less permeable
Small nonpolar molecules	Most permeable

Example: Oxygen (O2) crosses membranes easily; ions do not.

Membrane Fluidity and Saturation

Membrane fluidity depends on the saturation of phospholipid tails.

• Unsaturated phospholipids: Increase fluidity and permeability.

• Saturated phospholipids: Decrease fluidity and permeability.

Example: Membranes with unsaturated phospholipids are more permeable.

Cholesterol and Membrane Permeability

Cholesterol modulates membrane fluidity and permeability.

• At warm temperatures: Cholesterol decreases permeability.

• At cold temperatures: Cholesterol increases fluidity and permeability.

Transport Across Membranes

Osmosis and Tonicity

Osmosis is the movement of water across membranes in response to solute concentration.

• Hypertonic solution: Higher solute concentration outside cell; cell loses water and shrivels.

• Hypotonic solution: Lower solute concentration outside cell; cell gains water and may burst.

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

Example: Seawater is hypertonic to human cells.

Water Movement in Plant Cells

Plant cells in hypotonic solutions absorb water and may burst if not for the cell wall.

• Cell wall prevents bursting; instead, cells become turgid.

Active and Passive Transport

Transport across membranes can be passive or active.

• Passive transport: Moves substances down concentration gradient; no energy required.

• Active transport: Moves substances against concentration gradient; requires ATP.

Example: $\text{Active transport:} \quad \text{Low} \rightarrow \text{High}$ (requires ATP)

Transport Proteins: Carrier, Channel, and Pump

Membrane proteins facilitate transport:

• Channels: Allow passive movement of ions/molecules.

• Carriers: Bind and transport specific molecules, may require energy.

• Pumps: Use ATP to move substances against gradients.

Protein Type	Energy Required?	Example
Channel	No	Ion channel
Carrier	Sometimes	Glucose transporter
Pump	Yes	Sodium-potassium pump

Example: Pumps generally require ATP; carriers may or may not.

Additional info: Academic context and explanations have been expanded for clarity and completeness.