#Cell Membrane Transport: Your Ultimate AP Bio Review 🚀

Hey, future AP Bio master! Let's break down cell membrane transport, the 'public transit' system of the cell. This is a **** that pops up everywhere on the exam, so let's make sure you've got it down cold.

#The Cell Membrane: The Gatekeeper 🚪

The cell membrane (or plasma membrane) is like the cell's security system. It's a flexible barrier that controls what enters and exits.

• Phospholipid Bilayer: Imagine a sandwich 🥪 with two layers of phospholipids.
  • Hydrophilic heads (water-loving) face outwards.
  • Hydrophobic tails (water-fearing) face inwards.
  • This creates a barrier that only certain molecules can cross.
• Proteins: Embedded within the bilayer, these act like gates, channels, and ID cards.
  • Integral proteins: Span the entire membrane.
  • Peripheral proteins: Sit on the membrane surface.

#Transport Mechanisms: Moving In and Out 🚌

There are four main ways substances move across the cell membrane:

1. Active Transport: Requires energy (ATP) to move substances against their concentration gradient. Think of it as pushing a ball uphill. ⛰️
2. Passive Transport: Doesn't require energy; substances move down their concentration gradient. Like rolling a ball downhill. 🏞️
3. Endocytosis: The cell membrane engulfs substances, bringing them into the cell.
4. Exocytosis: The cell membrane expels substances out of the cell.

#Active Transport: The Energy Drink ⚡

Active transport is essential for maintaining concentration gradients. It's like a pump that moves things where they're not naturally inclined to go.

• Primary Active Transport: Uses ATP directly.
  • Example: The sodium-potassium pump. This is a **