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AP Biology 2.2 Cell Size Study Notes

AP Biology 2.2 Cell Size Study Notes - New Syllabus Effective 2025

AP Biology 2.2 Cell Size Study Notes- New syllabus

AP Biology 2.2 Cell Size Study Notes – AP Biology –  per latest AP Biology Syllabus.

LEARNING OBJECTIVE

Explain the effect of surface area-to-volume ratios on the exchange of materials between cells or organisms and the environment.

Key Concepts: 

  • Cells & Surface Area

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AP Biology Exam Questions MCQs and FRQs

2.2.A – Cell Size & Surface Area-to-Volume Ratio

📏 What is Surface Area-to-Volume Ratio (SA:V)?

  • Surface area is the outer area of a cell (like the membrane).
  • Volume is how much space the inside (cytoplasm, organelles) takes up.
  • The SA:V ratio compares how much surface area a cell has relative to its volume.

📈 Why SA:V Ratio Matters

  • Cells exchange materials (like nutrients, oxygen, and waste) across the cell membrane (surface area).
  • A higher SA:V ratio = more efficient exchange of materials.
  • A lower SA:V ratio = exchange becomes slower and harder.

🧫 Smaller Cells Are More Efficient

  • Small cells have a larger SA:V ratio → quicker diffusion of gases, nutrients, waste.
  • As a cell grows, volume increases faster than surface area → SA:V decreases.
  • When a cell becomes too large, it can’t efficiently transport materials, so it may:
    • Divide (mitosis)
    • Develop adaptations (e.g., long thin shape, folds in membrane)

🌿 Real-Life Examples

Organism/CellAdaptation to Increase SA:V
Intestinal cellsMicrovilli (increase surface area)
Alveoli (lungs)Thin walls & lots of surface area
Root hairsMaximize nutrient absorption
FlatwormsFlat shape for more surface area

🧠 Key Concept:

Cells must maintain a high SA:V ratio to support efficient exchange of materials and to avoid becoming too slow at transporting nutrients and waste.

2.2.A.1 – Surface Area-to-Volume Ratio & Material Exchange

🔍 Why SA:V Ratio Matters in Biology

  • SA:V ratio directly impacts how efficiently a cell or organism:
    • Takes in nutrients
    • Gets rid of wastes
    • Exchanges gases
    • Transfers heat
  • A high SA:V ratio = more efficient exchange with the environment.

📊 How It Affects Life Processes

ProcessWhy SA:V Ratio Matters
Nutrient absorptionCells must absorb nutrients through the membrane. More surface area = more entry points.
Waste removalWastes must exit efficiently. Low SA:V slows this.
Gas exchangeEx: O₂ & CO₂ move through membranes. Larger volume = more demand, but less area for exchange.
Heat transferSmaller organisms lose/gain heat faster due to higher SA:V ratio.

🧮Geometry Equations

📦 Volume Equations:

  • Sphere: \( V = \frac{4}{3} \pi r^3 \)
  • Cube: \( V = s^3 \)
  • Rectangular Solid: \( V = l \times w \times h \)
  • Cylinder: \( V = \pi r^2 h \)

📐 Surface Area Equations:

  • Sphere: \( SA = 4 \pi r^2 \)
  • Cube: \( SA = 6s^2 \)
  • Rectangular Solid: \( SA = 2(lw + lh + wh) \)
  • Cylinder: \( SA = 2 \pi r h + 2 \pi r^2 \)

🧠 Key Insight:

  • Smaller cells or organisms have greater SA:V ratios, which enhances efficiency.
  • As a structure grows in size, volume increases faster than surface area, reducing efficiency.

📌 Summary:

  • SA:V ratio determines how well a cell exchanges materials & heat with its environment.
  • Biological structures rely on optimal SA:V to support life functions.
  • Shapes, sizes, and internal adaptations (like membrane folds) help maintain proper SA:V.

2.2.A.2 – SA:V Ratio, Cell Size & Organism Metabolism

🧫 Why Cell Size Is Limited by Surface Area

  • The plasma membrane must be big enough to handle all the import/export needs of the cell.
  • Too big a cell = too little surface area per volume = inefficient.

1. SA:V Ratio Can Limit Cell Size

  • Smaller cells have a higher SA:V ratio → better exchange of materials.
  • Larger cells = low SA:V → slower transport, buildup of waste, shortage of nutrients.

2. As Cell Volume Increases…

  • SA:V ratio ↓
  • Internal demand ↑
  • Eventually, the membrane can’t keep up, so the cell must:
    • Divide
    • Stop growing
    • Change shape

3. Cells Evolve Internal Folds for Efficiency

  • Membrane folds increase surface area without increasing volume.
  • Examples: Mitochondrial cristae, intestinal microvilli.

 4. Organism Size Affects Heat Exchange

  • Smaller organisms:
    • High SA:V → lose heat faster
    • Need high metabolism to stay warm
  • Larger organisms:
    • Low SA:V → lose heat slower
    • Can conserve heat better
  • 🧊 That’s why a mouse eats more food per gram of body mass than an elephant.

🔥 v. Metabolism & SA:V Relationship

SizeSA:V RatioMetabolic Rate per Gram
Small organisms (e.g., mouse)HighHigh
Large organisms (e.g., whale)LowLow

🧠 This is because smaller bodies exchange heat faster and need to burn more energy to maintain body temp.

📌 Summary:

  • Small cell = high SA:V = efficient exchange.
  • Large organism = low SA:V = slower exchange, but better at conserving heat.
  • Metabolic rates per gram drop as body size increases.
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