AP BIO UNIT 2 : CELLS (Cell Membrane Foldable & Informational Slides)

Phospholipid

The primary component of cell membranes; an amphipathic molecule consisting of a polar head group and two nonpolar fatty acid tails.

Phospholipid Head

Property: Polar and hydrophilic (water-loving)

• Components: Phosphate group and choline

• Orientation: Points outward toward the aqueous environment

Phospholipid Tails

Property: Nonpolar and hydrophobic (water-fearing)

• Components: Two fatty acid chains:

  • Saturated tail: Straight chain

  • Unsaturated tail: Bent chain

Amphipathic

Describing a single molecule that contains both hydrophilic (water-attracting) and hydrophobic (water-repelling) regions.

Cholesterol

General Function: Gives the membrane fluidity and stability

• At High Temperatures: Reduces fluidity by restricting the movement of phospholipids

• At Low Temperatures: Increases fluidity by preventing fatty acids from forming a non-fluid gel

Phospholipid Movement

Definition: - Lateral Movement:

• Frequency: Rapid (~107107 times per second)

• Mechanism: Adjacent phospholipids switch positions side-to-side

  • Flip-Flop Movement:

• Frequency: Rare (~ once per month)

• Mechanism: Phospholipid flips across to the opposite membrane layer

Membrane Fluidity & Protein Mobility

Bilayer Stabilization: Held together by weak hydrophobic interactions

• Protein vs. Lipid Speed:

  • Lipids: Move rapidly via lateral drift

  • Proteins: Larger than lipids and move more slowly

• Immobile Proteins: Some are nearly immobile because they are attached to the cytoskeleton

Integral Protein

Definition: - Alternative Name: Also known as intrinsic proteins

• Structure: Has at least one region embedded within the hydrophobic core of the lipid bilayer

• Variations:

  • Transmembrane Proteins: Span the entire bilayer (most typical type)

  • Single-Side Associated: Embedded in one side of the membrane without spanning it entirely

Peripheral Protein (Extrinsic Protein)

• What it is: A protein attached strictly to the inner or outer surface of the cell membrane.

• Logic / Core Rule: It stays on the outside edges and never goes into the hydrophobic core (middle layer).

• Easy Memory Hack: Think of it like a hat on a head—it sits on top or underneath, but doesn't pierce through the middle.

Cholesterol in Cell Membranes

• What it is: A lipid found in cell membranes (animal cells only, not plant/bacterial) that acts as a temperature buffer for membrane fluidity.

• Structure Logic: It has a hydrophilic end pointing toward the polar surface and a hydrophobic end pointing inside the non-polar core.

• Temperature Logic:

  • Hot Temp: Restricts phospholipid movement $\rightarrow$ keeps membrane from getting too runny/fluid.

  • Cold Temp: Prevents phospholipids from packing tightly $\rightarrow$ keeps membrane from freezing into a non-fluid gel.

• Easy Memory Hack: Think of cholesterol as the membrane's thermostat—it keeps things stable whether it's hot or cold.