Cell Structure, Membrane Function, and Energy Processes: Study Guide for General Biology

Cell Structure and Function

Differences Between Eukaryotes and Prokaryotes

The distinction between eukaryotic and prokaryotic cells is fundamental in biology. Eukaryotes include animals, plants, fungi, and protists, while prokaryotes are bacteria and archaea.

• Eukaryotic cells have membrane-bound organelles, including a nucleus.

• Prokaryotic cells lack a nucleus and most organelles; their DNA is located in the nucleoid region.

• Plant cells have cell walls, chloroplasts, and large central vacuoles; animal cells do not.

• Fungal cells have cell walls made of chitin.

Example: Escherichia coli is a prokaryote; Homo sapiens cells are eukaryotic.

Properties of the Cell Membrane

Membrane Composition, Permeability, and Function

The cell membrane is a selectively permeable barrier composed mainly of a phospholipid bilayer with embedded proteins.

• Composition: Phospholipids, proteins, cholesterol (in animal cells), and carbohydrates.

• Permeability: Allows passage of small, nonpolar molecules; restricts ions and large polar molecules.

• Function: Protects the cell, mediates transport, and facilitates communication.

Hypotonic, Hypertonic, and Isotonic Solutions

These terms describe the relative concentration of solutes in solutions compared to the cell.

• Hypotonic: Lower solute concentration outside the cell; water enters the cell.

• Hypertonic: Higher solute concentration outside the cell; water leaves the cell.

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

Example: Red blood cells in a hypotonic solution swell; in a hypertonic solution, they shrink.

Transport Across the Cell Membrane

Types of Transport

• Active transport: Requires energy (ATP); moves substances against their concentration gradient (e.g., Na+/K+ pump).

• Passive transport: Does not require energy; includes diffusion, facilitated diffusion (via channels like aquaporins), and osmosis.

Bulk Transport Processes

• Exocytosis: Export of materials out of the cell via vesicles.

• Endocytosis: Import of materials into the cell via vesicles.

Biochemical Reactions and Enzyme Function

Types of Biochemical Reactions

• Spontaneity: Spontaneous reactions occur without energy input; non-spontaneous require energy.

• Redox reactions: Involve transfer of electrons; oxidation is loss, reduction is gain.

Enzyme Properties and Function

• Enzymes are biological catalysts that speed up reactions by lowering activation energy.

• Enzyme activity can be regulated by inhibitors (competitive/noncompetitive) and activators.

Example: Catalase breaks down hydrogen peroxide into water and oxygen.

Cellular Respiration and Photosynthesis

Overview and Major Steps

Both processes are essential for energy transformation in cells.

• Cellular respiration: Converts glucose and oxygen into ATP, CO2, and water.

• Photosynthesis: Converts light energy, CO2, and water into glucose and oxygen.

Cellular Respiration Steps

• Glycolysis: Occurs in cytoplasm; breaks glucose into pyruvate.

• Krebs Cycle (Citric Acid Cycle): Occurs in mitochondria; processes pyruvate, produces NADH and FADH2.

• Electron Transport Chain: Uses NADH/FADH2 to produce ATP via oxidative phosphorylation.

Equation:

$\text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2 \rightarrow 6\text{CO}_2 + 6\text{H}_2\text{O} + \text{ATP}$

Photosynthesis Steps

• Light reactions: Occur in thylakoid membranes; convert light energy to chemical energy (ATP, NADPH).

• Calvin Cycle: Occurs in stroma; uses ATP and NADPH to fix CO2 into glucose.

Equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} + \text{light energy} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Regulation and Energy Transfer

• Enzymes regulate each step of respiration and photosynthesis.

• Electron carriers (NADH, FADH2, NADPH) transfer energy between steps.

Differences Between C3 and C4 Plants

C3 and C4 plants differ in their photosynthetic pathways, especially in how they fix carbon dioxide.

• C3 plants: Use the Calvin Cycle directly; most common; less efficient in hot, dry climates.

• C4 plants: Use an additional set of reactions to minimize photorespiration; more efficient in high light and temperature.

Example: Wheat is a C3 plant; maize (corn) is a C4 plant.

Table: Comparison of C3 and C4 Plants

Feature	C3 Plants	C4 Plants
First product of CO2 fixation	3-carbon compound (3-phosphoglycerate)	4-carbon compound (oxaloacetate)
Photorespiration	High	Low
Efficiency in hot climates	Low	High
Examples	Wheat, rice	Corn, sugarcane

Additional info: Some details about enzyme regulation and electron carriers were expanded for clarity.