phospholipid key properties

semi-permeable

allows small non-polar molecules through (O₂, CO₂), blocks large or charged ones unless

transported

fluid and flexible

lipids move laterally (fluid mosaic model)

self reparing

phospholipids rearrange if membrane disrupted

asymmetric

inner & outer leaflets differ in composition & function

Outer (Noncytosolic) Monolayer

faces the world

Inner (Cytosolic) Monolayer

Faces the inside of the cell (the cytosol)

phosphatidylcholine and sphingomyelin

-outer leaflet

-bulk structural lipids

glycolipids

• always on the outside

• have sugar heads for cell recognition

Phosphatidylserine and Phosphatidylethanolamine

• inner leaflet

• ofter have a negative charge

Phosphatidylinositols

• inner leaflet

• Crucial for cell signaling

cholesterol

• both inner and outer leaflet

• distributed

Integral Membrane Proteins

Proteins that are permanently attached to the lipid bilayer; can span the entire membrane (transmembrane) or be anchored to one side. Examples: Ion channels, receptors, and transporters.

Peripheral Membrane Proteins

Proteins that associate with the membrane through weak, noncovalent interactions with other proteins. They do NOT penetrate the lipid bilayer. Example: Signal transduction proteins.

Transmembrane Protein Structure

A type of integral protein that crosses the bilayer as alpha-helices or beta-barrels. They can protrude from one or both sides of the cell.

Membrane Protein Functions (General)

1. Transport (carriers/channels)

2. Receptors (sensing signals)

3. Cell adhesion (sticking to neighbors)
4. Recognition (ID tags

Rules for Simple Diffusion

Molecules move faster if they are:

1. Small (Large molecules like glucose can't cross alone).

2. Hydrophobic/Nonpolar (Ions and charged molecules can NEVER cross alone).

Passive Transport

Moves "downhill" (High to Low concentration). No energy/ATP needed.

active transport

Moves solute against their concentration (Low to High concentration). Requires energy/ATP and a special proteins called "Pumps."

The Na+ / K+ Pump (Sodium-Potassium Pump)

An active transporter that uses ATP to push Na+ OUT of the cell and pull K+ IN. It maintains the cell's negative "resting membrane potential."

no matter how small this is, it cannot cross the lipid bilayer (alone)

ions

these types of molecules can barley cross

large uncharged polar molecules (like glucose)

these can diffuse across but only if it’s the right size

small uncharged polar molecules

Resting Membrane Potential

The electrical charge of a cell when it is not stimulated. It is always negative, typically between -20 and -200 mV.

how to solutes cross the membrane

either passive or active transport

Most Abundant Intracellular Cation

Potassium (K+). It is found in high concentrations inside the cell.

Most Abundant Extracellular Cation

sodium (NA+) It is found in high concentrations outside the cell.

Extracellular Ions (The "Outside" Group)

Besides Sodium, the ions Magnesium Mg2+, Calcium Ca2+, and Chloride (Cl-) are also most abundant outside the cell.

Electrochemical Gradient

the combined influence of a solute's concentration gradient and the membrane potential (electrical charge) on its movement.

Na+ Pump function

Generates a Steep Concentration Gradient of Na+ Across the Plasma Membrane

Ca2+ Pump function

Keep the Cytosolic Ca2+ Concentration Low