#🧬 Cell Membrane & Walls: Your Night-Before Review

Hey! Let's get you feeling super confident about cell membranes and walls. We'll make sure everything's crystal clear for tomorrow.

#🧱 Membrane Structure & Permeability

#Selective Permeability: The Gatekeeper 🚪

• The cell membrane is selectively permeable, meaning it controls what goes in and out. Think of it like a bouncer at a club, not everyone gets in!

• This is all thanks to the phospholipid bilayer: hydrophilic (water-loving) heads face outwards, and hydrophobic (water-fearing) tails face inwards.

  

  Image: The phospholipid bilayer with hydrophilic heads and hydrophobic tails.

#What Can Pass Through? 🤔

• Freely Cross (No Help Needed): Small, nonpolar molecules like O2, CO2, and N2. They slip right through the hydrophobic region.
• Need Help (Transport Proteins):
  • Large molecules
  • Polar (hydrophilic) molecules
  • Ions (charged particles)
  • Even small, polar, uncharged molecules like water need help (aquaporins) to cross in large quantities.

#Diffusion Across the Membrane

• Diffusion: Movement of molecules from an area of high concentration to an area of low concentration. It's like a drop of dye spreading out in water.
• This movement is passive, meaning it doesn't require the cell to use energy.

#Quick Table

#🧱 Cell Walls

#Structural Support 💪

• Cell walls provide structural support and protection for cells. Think of it like a rigid outer shell.

• They are found in plants, protists, fungi, and bacteria, but NOT in animal cells.

  

  Image: A plant cell with its cell wall.

#Composition Varies

• Plants: Cell walls are primarily made of cellulose, a type of carbohydrate (sugar). 🌿
• Fungi: Cell walls are made of chitin, a polysaccharide. 🍄
• Bacteria: Cell walls are made of peptidoglycan.

#🎯 Final Exam Focus

#High-Priority Topics

• Membrane Permeability: Understand how the structure of the phospholipid bilayer dictates what can pass through. Focus on the role of hydrophobic tails.
• Transport Proteins: Know the difference between passive and active transport, and the role of transport proteins (like aquaporins).
• Cell Walls: Understand the function of cell walls and their composition in different organisms.

#Common Question Types

• Multiple Choice: Expect questions that ask you to identify which substances can cross the membrane freely, or which require transport proteins.
• Free Response: Be prepared to discuss the role of the cell membrane in maintaining homeostasis, and how it relates to the movement of molecules across the membrane.

#Last-Minute Tips

• Time Management: Don't spend too long on any one question. If you're stuck, make a note and come back to it later.
• Common Pitfalls: Avoid confusing diffusion with active transport. Remember that diffusion is passive and doesn't require energy.
• Strategies: Read each question carefully, underline key terms, and use process of elimination to narrow down your choices.

#📝 Practice Questions

Practice Question

Multiple Choice Questions

1. Which of the following molecules would MOST likely require a transport protein to cross a cell membrane? (A) Oxygen (O2) (B) Carbon dioxide (CO2) (C) Glucose (C6H12O6) (D) Nitrogen (N2)

2. The primary function of the cell wall is to: (A) Regulate the movement of molecules in and out of the cell. (B) Provide structural support and protection. (C) Synthesize proteins. (D) Store genetic material.

Free Response Question

Describe the structure of the cell membrane and explain how its structure relates to its function in regulating the movement of molecules across the membrane. In your explanation, include a discussion of the types of molecules that can freely cross the membrane and those that require transport proteins. (8 points)

Scoring Breakdown

• (1 point): Correctly identifies the cell membrane as a phospholipid bilayer.
• (1 point): Correctly states that the bilayer has hydrophilic heads and hydrophobic tails.
• (2 points): Explains that the hydrophobic tails create a barrier to polar and charged molecules.
• (1 point): Identifies small, nonpolar molecules as able to freely cross the membrane.
• (2 points): Explains that large, polar, and charged molecules require transport proteins.
• (1 point): Mentions aquaporins as a specific type of transport protein for water.