Cell Membrane: Lipid Composition and Membrane Proteins

Plasma Membrane: The Cell's Outer Wall (Big Picture)

What it is: The plasma membrane is like the skin of the cell. It's the boundary that separates the inside of the cell from its outside environment.
Its jobs (Functions):
- Protects the cell: It acts as a barrier, controlling what goes in and out, keeping the cell's contents safe and maintaining its shape.
- Communicates: It receives signals from outside the cell, helping the cell react to its environment.
- Connects: It helps cells stick together and recognize each other.
- Controls flow: It's "semi-permeable" (meaning it only lets certain things pass through), regulating the cell's water and other important substances.

Plasma Membrane Structure: The Basic Building Blocks

How thick is it? It's super thin, about 7.5-10\,\text{nm} (nanometers, which are tiny!).
Main components: It's mostly made of two layers of phospholipids (a type of fat molecule), with proteins embedded in or attached to these layers, and sugars (carbohydrates) often attached to the outside.
Looking closely: Under powerful microscopes, it looks like two dark lines (the hydrophilic heads or water-loving parts) separated by a lighter space (the hydrophobic tails or water-fearing parts). Each dark line is about 2.5\,\text{nm} thick, and the gap is about 3\,\text{nm}.

Lipid Bilayer: The Phospholipid Sandwich

Phospholipids: These are special fat molecules with two parts:
- A "head" (hydrophilic): This part loves water and faces outwards, towards the watery environments inside and outside the cell.
- Two "tails" (hydrophobic): These parts fear water and tuck inwards, forming the middle, oily layer of the membrane, away from water.
The "bilayer" (two layers) arrangement:
- The water-loving heads of the phospholipids face the watery outside and inside of the cell.
- The water-fearing tails hide in the middle, forming a barrier that water-soluble (things that dissolve in water) substances can't easily cross.
Structural support: This arrangement gives the cell its shape and acts as a barrier.

Lipid Composition: Different Fats, Different Jobs

Different types of phospholipids: The two layers (leaflets) are not exactly the same!
- Outer (exoplasmic) layer (faces outside the cell): Mainly contains phosphatidylcholine, sphingomyelin, and some phosphatidylethanolamine. Also has cholesterol and glycolipids (lipids with sugars attached) here, with their sugar parts poking out.
- Inner (cytosolic) layer (faces inside the cell): Mainly contains phosphatidylserine, phosphatidylinositol, and more phosphatidylethanolamine. The heads of these lipids often have negative charges, making the inside surface of the membrane slightly negative.
Cholesterol's role: This molecule acts like a "fluidity regulator" for the membrane.
- It prevents the membrane from becoming too stiff or too runny.
- At high concentrations, it stops the lipid tails from packing too tightly (crystallizing), helping maintain the right level of fluidity.
- It also makes the membrane less permeable (harder for small water-soluble molecules to pass through).
Lipid rafts: Think of these as special "mini-platforms" on the membrane.
- They are fatter (thicker) and stiffer (less fluid) areas.
- They are rich in cholesterol and glycosphingolipids (a type of glycolipid).
- Important signaling proteins (proteins that send messages) often gather together in these rafts.

Proteins in the Membrane: The Workers of the Cell Surface

What they do: Proteins in the membrane are crucial for almost all the membrane's functions. They act as:
- Pumps: Move substances across the membrane.
- Channels: Form tunnels for substances to pass through.
- Receptors: Catch signals from outside the cell.
- Linkers: Connect the cell to its surroundings or other cells.
- Enzymes: Speed up chemical reactions.
- Structural support: Help maintain the cell's shape.
Two main types:
- Integral membrane proteins (Deeply embedded):
  - These are stuck right into the lipid bilayer.
  - Many span the entire membrane (called transmembrane proteins), going from one side to the other.
  - They often have several alpha-helical segments (spiral shapes) that pass through the oily middle layer.
  - The parts sticking outside the cell are usually glycosylated (have sugars attached).
  - The parts inside the cell often connect to the cell's internal skeleton (cytoskeleton).
- Peripheral membrane proteins (Loosely attached):
  - These are not embedded in the lipid layers.
  - They attach indirectly to the membrane, usually by binding to other membrane proteins.

Glycocalyx and Membrane Charge: The Cell's Sugary Coat and Electrical Field

Glycocalyx (The 'sugar coat'): This is a fuzzy layer on the outside surface of the cell membrane.
- It's made of glycoproteins (proteins with sugars) and glycolipids (lipids with sugars), all their sugar parts facing outwards.
- Importance:
  - Immune recognition: Helps the body recognize its own cells and spot invaders.
  - Cell adhesion: Helps cells stick to each other and to the surrounding material.
  - Protection: Shields the cell from damage.
Membrane charge: The outer surface of the plasma membrane usually has a net negative charge.
- This negative charge mainly comes from sialic acid residues (a type of sugar) found on the glycoproteins and glycolipids of the glycocalyx. This negative charge is important for how cells interact and respond to signals.

Quick Exam-Style Points (Application)

Q: A 32-year-old man with a mutation in a voltage-gated Na+ channel in skeletal muscle membrane. How would you describe the basic structure of this membrane protein?
- Answer: A) It's an amphipathic molecule (meaning it has both water-loving and water-fearing parts) that is embedded within the lipid bilayer and spans the membrane multiple times. This is typical for integral, transmembrane proteins like channels.
Q: The resting surface charge of the plasma membrane is mainly negative due to which component?
- Answer: Glycoproteins (specifically their sialic acid residues) on the glycocalyx.

Based on the provided notes, here are the key learning objectives about the plasma membrane:

1. Understand What the Plasma Membrane Is and Its Core Functions

Definition: The plasma membrane is the cell's outer boundary, separating the cell's inside from its outside environment.
Main Jobs (Functions):
- Protection: It acts as a barrier, controlling what enters and leaves the cell, protecting its contents, and maintaining its shape.
- Communication: It receives signals from outside to help the cell react to its surroundings.
- Connection: It helps cells stick together and recognize each other.
- Flow Control: It's "semi-permeable" (only lets certain things through), managing the flow of water and other important substances.

2. Describe the Basic Structure and Components of the Plasma Membrane

Thickness: It's very thin, about 7.5-10\, \text{nm}.
Main Parts: It's primarily made of two layers of phospholipids (fat molecules), with proteins either stuck in or attached to these layers, and sugars (carbohydrates) often on the outside.
Appearance under a Microscope: It looks like two dark lines (water-loving "heads") separated by a lighter space (water-fearing "tails"), each dark line about 2.5\, \text{nm} thick with a 3\, \text{nm} gap.

3. Explain the Lipid Bilayer Arrangement

Phospholipids: Each has a water-loving "head" that faces outwards (towards water) and two water-fearing "tails" that tuck inwards.
Bilayer Structure: The water-loving heads face the watery inside and outside of the cell, while the water-fearing tails form the oily middle layer, acting as a barrier for water-soluble substances.
Support: This structure helps maintain cell shape and acts as a barrier.

4. Understand Lipid Composition and Specialized Areas

Different Layers (Leaflets): The outer layer of the membrane (facing outside) has phosphatidylcholine, sphingomyelin, cholesterol, and glycolipids (sugars attached to fats). The inner layer (facing inside) has phosphatidylserine, phosphatidylinositol, and sometimes has negative charges.
Cholesterol's Role: It acts as a "fluidity regulator," preventing the membrane from becoming too stiff or too runny. It also makes the membrane less permeable to small water-soluble molecules.
Lipid Rafts: These are special, thicker, and stiffer "mini-platforms" in the membrane, rich in cholesterol and glycosphingolipids, where important signaling proteins often gather.

5. Identify the Types and Functions of Membrane Proteins

Protein Functions: Membrane proteins are crucial workers that act as:
- Pumps (move substances)
- Channels (form tunnels)
- Receptors (catch signals)
- Linkers (connect cells)
- Enzymes (speed up reactions)
- Structural support (maintain cell shape)
Two Main Types:
- Integral Membrane Proteins (Deeply Embedded): Stuck directly into the lipid layers. Many span the entire membrane (transmembrane proteins), often with spiral alpha-helical segments. Parts outside are usually glycosylated (have sugars), and parts inside connect to the cell's cytoskeleton.
- Peripheral Membrane Proteins (Loosely Attached): Not embedded, but attach indirectly to other membrane proteins.

6. Know About the Glycocalyx and Membrane Charge

Glycocalyx (The 'Sugar Coat'): A fuzzy layer on the outside of the cell membrane, made of glycoproteins (proteins with sugars) and glycolipids (lipids with sugars).
Glycocalyx Importance: Crucial for immune recognition, cell adhesion (sticking together), and protection.
Membrane Charge: The outer surface typically has a net negative charge, mainly from sialic acid residues on the glycoproteins and glycolipids of the glycocalyx. This is important for cell interactions and responses to signals.