Plasma membrane

I. What Is the Plasma Membrane?

AKA: Cell membrane or phospholipid bilayer
Main idea: It’s the protective skin of the cell that controls what goes in and out, helps cells communicate, and interacts with the environment.

1. Components of the Plasma Membrane

• Phospholipid bilayer: Know that it has hydrophilic (water-loving) heads facing outside/inside and hydrophobic (water-fearing) tails inside, creating a barrier.

• Proteins: Understand the difference between transmembrane (integral) proteins that go through the membrane and peripheral proteins on the surface. Know they help with transport, communication, and structure.

• Carbohydrates: Attached to proteins or lipids on the outer surface, important for cell recognition and communication.

• Cholesterol: Helps keep the membrane flexible and stable.

2. Fluid Mosaic Model

• The membrane is fluid (parts move around, not rigid) and a mosaic (made of many different molecules like lipids, proteins, carbohydrates).

• This model explains the membrane’s structure and how it works dynamically.

3. Functions of the Plasma Membrane

• Selective permeability: Controls what enters and exits the cell.

• Communication: Receptors detect signals (like hormones), allowing the cell to respond.

• Protection and identification: Carbohydrates help the cell recognize itself and others.

• Support and adhesion: Helps cells stick together and maintain shape.

• Enzymatic activity: Some proteins speed up chemical reactions.

• Passive transport (no energy):

  • Simple diffusion (small nonpolar molecules like O₂, CO₂)

  • Facilitated diffusion (uses protein channels for larger/polar molecules like glucose)

  • Osmosis (movement of water)

• Active transport (needs energy/ATP):

  • Protein pumps (like Na⁺/K⁺ pump) move substances against the concentration gradient

  • Bulk transport:

    • Endocytosis (cell takes in materials via vesicles)

    • Exocytosis (cell expels materials via vesicles)

    • Phagocytosis and Pinocytosis are types of endocytosis ("cell eating" and "cell drinking").

IV. Types of Membrane Transport

How things get across the membrane:

✅ 1. Primary Active Transport → b

"Moves solutes against gradient with ATP"

• Active = Needs energy (ATP)

• "Against the gradient" = from low to high concentration

• Example: Sodium-potassium pump (Na⁺/K⁺ pump)

✅ 2. Facilitated Diffusion → a

"Moves solutes down gradient with the help of a protein"

• "Facilitated" means it uses a protein (like a channel or carrier)

• No energy needed

• Down the gradient = high to low concentration

• Example: Glucose transport into cells

✅ 3. Simple Diffusion → e

"Moves solutes down gradient directly across the cell membrane"

• No protein needed

• Happens with small, nonpolar molecules (like oxygen or carbon dioxide)

• Just slips through the membrane

✅ 4. Exocytosis → c

"Bulk secretion of solutes from the cell"

• "Exo" = exit

• Cell packages stuff into vesicles and pushes it out

• Used for things like hormones, neurotransmitters, or waste

✅ 5. Osmosis → d

"Moves solvent down solvent gradient"

• Solvent = water

• Osmosis = water movement across a membrane

• Water moves toward a higher solute concentration to balance it out

• Down water's gradient = from more water to less water

5. Tonicity and Osmosis

• Know the effects of:

  • Hypotonic solution: Water moves into the cell → cell swells, may burst

  • Hypertonic solution: Water moves out of the cell → cell shrinks

  • Isotonic solution: No net water movement → cell stays the same

  Functions of the Plasma Membrane

  1. Controls what enters and exits (semi-permeable)
     
     

  2. Sends signals inside the cell (second messenger systems)
     
     

  3. Speeds up reactions (enzymes)
     
     

  4. Receives signals from outside (receptors)
     
     

  5. Identifies the cell (identity markers)
     
     

  6. Helps cells stick together (adhesion)

     The functions of proteins can include

     ✅ a. cell suicide (This refers to apoptosis, a process regulated by proteins.)
     ✅ b. cellular reproduction (Proteins like cyclins and kinases regulate the cell cycle.)
     ✅ c. cell migration (Proteins help with movement, especially cytoskeletal and signaling proteins.)