ap bio unit 2.1

Absolutely! Let’s expand AP Bio Unit 2.1 with more depth while still keeping it strictly within the College Board curriculum. I’ll include definitions, mechanisms, and examples for better understanding.

AP Biology Unit 2 — Topic 2.1: Cell Structure & Function (In-Depth)

1. Fundamental Features of All Cells

All living cells, regardless of type, share four common components:

  1. Plasma membrane – Semi-permeable barrier that separates internal contents from the external environment.

  2. Cytosol – The aqueous fluid inside the cell where chemical reactions occur.

  3. Ribosomes – Non-membrane-bound structures where protein synthesis occurs.

  4. Genetic material (DNA) – Contains instructions for cell growth, reproduction, and metabolism.

Key point: Even the simplest prokaryotic cell carries out all basic life functions: growth, reproduction, energy processing, and response to environment.

2. Plasma Membrane Structure & Function

  • Composition: Phospholipid bilayer with embedded proteins, cholesterol, and carbohydrates.

  • Properties:

    • Hydrophilic heads face outward (toward water)

    • Hydrophobic tails face inward (away from water)

    • Creates a fluid mosaic model – the membrane is dynamic, not static.

Functions:

  • Regulates movement of substances in and out of the cell (selective permeability).

  • Facilitates cell signaling via receptor proteins.

  • Anchors cytoskeleton and extracellular matrix for structural support.

3. Selective Permeability & Transport

Passively crosses:

  • Small, nonpolar molecules: O₂, CO₂

  • Lipid-soluble molecules

Needs assistance (facilitated or active transport):

  • Polar molecules: H₂O (via aquaporins)

  • Ions: Na⁺, K⁺, Cl⁻

  • Large molecules: glucose, amino acids

Mechanisms:

  1. Passive transport – no energy, moves down concentration gradient (diffusion, osmosis, facilitated diffusion).

  2. Active transport – requires energy (ATP), moves molecules against gradient.

  3. Bulk transport – endocytosis (into cell) and exocytosis (out of cell).

4. Surface Area-to-Volume Ratio (SA:V)

  • Importance:

    • Determines efficiency of diffusion, nutrient uptake, and waste removal.

    • Larger cells → lower SA:V → slower exchange with environment → limits cell size.

  • Adaptations to increase SA:V:

    • Membrane folding: cristae in mitochondria, thylakoids in chloroplasts, microvilli in intestinal cells.

    • Flattened or elongated cell shapes.

5. Prokaryotic vs. Eukaryotic Cells

Feature

Prokaryotic

Eukaryotic

Nucleus

No

Yes

Membrane-bound organelles

No

Yes

DNA structure

Circular, free-floating in nucleoid

Linear, enclosed in nucleus

Size

Smaller (1–10 μm)

Larger (10–100 μm)

Examples

Bacteria, Archaea

Plants, Animals, Fungi, Protists

Key concept: Compartmentalization in eukaryotes allows specialized environments for specific reactions.

6. Compartmentalization

  • Membrane-bound organelles create distinct local environments, optimizing metabolic efficiency.

  • Examples of compartments and functions:

    • Nucleus – protects DNA, site of transcription

    • Mitochondria – ATP synthesis, oxidative phosphorylation

    • Chloroplasts – photosynthesis

    • Lysosomes – acidic environment for hydrolytic enzymes

    • Endoplasmic reticulum – protein and lipid synthesis

7. Endosymbiotic Theory

  • Explains origin of mitochondria and chloroplasts in eukaryotes.

  • Hypothesis: Early eukaryotic cells engulfed free-living prokaryotes.

  • Evidence:

    • Double membranes

    • Circular DNA (like bacteria)

    • Binary fission independent of host

    • Ribosomes resemble prokaryotic ribosomes

Significance: Demonstrates evolutionary relationships and the origin of complex cells.