Chapter 5

Introduction to the Plasma Membrane

The plasma membrane is a semi-permeable barrier that separates the inside of the cell from its external environment. It controls what enters and leaves the cell to maintain homeostasis (stable internal conditions).

Why is the plasma membrane important?

• Protects the cell from harmful substances.

• Regulates the exchange of materials (nutrients, oxygen, waste).

• Allows communication between cells using receptors.

• Maintains cell shape and structure.

Structure of the Plasma Membrane – Fluid Mosaic Model

The plasma membrane follows the Fluid Mosaic Model, which means:

1. "Fluid" = The membrane is flexible, not rigid.

2. "Mosaic" = Made of many different molecules working together.

Major Components of the Plasma Membrane

1. Phospholipid Bilayer

2. Cholesterol

3. Membrane Proteins

4. Carbohydrates

1. Phospholipid Bilayer – The Foundation

A phospholipid is a molecule with:

• A hydrophilic ("water-loving") head → Faces outward, toward water.

• Two hydrophobic ("water-hating") tails → Face inward, avoiding water.

Since cells live in watery environments (inside and outside), the phospholipids automatically arrange into a bilayer:

• Heads face outward (toward water)

• Tails face inward (away from water)

🔑 Key Features: ✔ Acts as a barrier – prevents water-soluble molecules from easily passing through.
✔ Keeps the membrane flexible and self-repairing.

2. Cholesterol – The Stability Factor

Cholesterol molecules are embedded within the phospholipid bilayer. They:

• Prevent the membrane from becoming too rigid or too fluid.

• Fill gaps between phospholipids, making it harder for small molecules to slip through.

🔑 Key Features: ✔ Helps maintain fluidity (especially in changing temperatures).
✔ Keeps the membrane strong but flexible.

3. Membrane Proteins – The Functional Units

Proteins in the membrane perform critical functions, and they come in two main types:

A. Integral Proteins (Embedded in the Membrane)

• Also called transmembrane proteins.

• Pass through the entire bilayer.

• Functions:

  • Transport Proteins: Move substances across the membrane (like doors).

  • Receptor Proteins: Detect signals from the environment (like an antenna).

  • Enzymes: Speed up chemical reactions.

B. Peripheral Proteins (On the Surface)

• Attached to the inner or outer surface of the membrane.

• Functions:

  • Cell signaling – Help in communication.

  • Cell structure – Connect to the cytoskeleton (inner framework of the cell).

  • Recognition – Help immune cells identify "self" vs. "non-self".

🔑 Key Features: ✔ Proteins allow communication, transport, and support.
✔ Without proteins, the membrane wouldn’t function properly.

4. Carbohydrates – The Identification Tags

Carbohydrates in the membrane attach to lipids (glycolipids) or proteins (glycoproteins).

• Act as "name tags" for cells.

• Allow cells to recognize and interact with each other.

• Help immune cells distinguish "self" from invaders.

🔑 Key Features: ✔ Essential for cell-cell recognition (important in immune response).
✔ Help in cell adhesion (holding cells together in tissues).

Transport Across the Membrane

The membrane is selectively permeable, meaning some molecules can pass freely while others need help.

A. Passive Transport (No Energy Needed)

Molecules move from high concentration to low concentration (down their gradient).

1. Diffusion

• Random movement of molecules from high to low concentration.

• No energy required.

• Example: Oxygen & carbon dioxide diffuse across the membrane.

2. Osmosis (Diffusion of Water)

• Water moves from low solute concentration (more water) to high solute concentration (less water).

• Uses aquaporins (water channel proteins) to speed up water movement.

• Example: Water moving into a dehydrated cell.

3. Facilitated Diffusion

• Large or charged molecules cannot pass through the membrane easily.

• Transport proteins help them move without energy.

• Example: Glucose entering the cell using a transport protein.

B. Active Transport (Requires Energy – ATP)

Molecules move against the concentration gradient (from low to high concentration), using energy.

1. Protein Pumps

• ATP-powered proteins pump molecules against the gradient.

• Example: Sodium-Potassium Pump (important for nerve signals).

2. Bulk Transport – Moving Large Molecules

Cells use vesicles (small membrane sacs) to move large molecules.

• Exocytosis (Moving Out)

  • Vesicles fuse with the membrane and release materials outside the cell.

  • Example: Releasing hormones or neurotransmitters.

• Endocytosis (Moving In)

  • The membrane folds around a substance and brings it inside.

  • Types of Endocytosis:

    1. Phagocytosis ("Cell Eating") – Engulfs large particles (like bacteria).

    2. Pinocytosis ("Cell Drinking") – Takes in liquids with dissolved substances.

    3. Receptor-Mediated Endocytosis – Uses receptors to bring in specific molecules (e.g., cholesterol uptake).

Tonicity – Water Balance in Cells

Water movement affects cell size.

1. Isotonic Solution:

   • Equal solute concentration inside and outside.

   • No net water movement → Cell stays the same size.

2. Hypotonic Solution:

   • Lower solute outside, higher inside.

   • Water enters the cell → Cell swells and may burst.

   • Plant cells love this (they become turgid).

3. Hypertonic Solution:

   • Higher solute outside, lower inside.

   • Water leaves the cell → Cell shrivels.

   • Causes dehydration in animal cells.

Energy & Chemical Reactions in Cells

Cells need energy (ATP) to function. This energy comes from chemical reactions.

First & Second Laws of Thermodynamics

1. First Law: Energy cannot be created or destroyed, only transformed.

2. Second Law: Every energy transfer increases disorder (entropy).

Types of Reactions

1. Exergonic Reactions = Release energy (Example: cellular respiration).

2. Endergonic Reactions = Require energy (Example: photosynthesis).

Role of Enzymes in Cellular Reactions

• Enzymes are proteins that speed up reactions.

• They work by lowering activation energy (the energy needed to start a reaction).

• How Enzymes Work:

  1. Substrate binds to enzyme's active site.

  2. Enzyme changes shape (induced fit).

  3. Reaction happens, and products are released.

Example: Digestive enzymes breaking down food.