Overview
This lecture covers the movement of substances across the cell membrane, focusing on types of diffusion, the roles of transport proteins, and how different molecules cross the phospholipid bilayer.
Types of Diffusion
- Diffusion is the net movement of particles from higher to lower concentration due to random motion.
- Simple diffusion refers to the movement of nonpolar molecules (like oxygen, carbon dioxide, fatty acids, and steroids) directly through the phospholipid bilayer.
- Polar molecules (such as water, ions, glucose, and amino acids) cannot easily cross the bilayer and require facilitated diffusion.
- Both simple and facilitated diffusion are examples of passive transport, meaning they do not require ATP.
Facilitated Diffusion
- Facilitated diffusion involves the movement of substances across the membrane through transport proteins.
- This process is necessary for polar molecules that cannot pass directly through the phospholipid bilayer.
- Particles still move down the concentration gradient without energy input.
Transport Proteins
- There are two main types of transport proteins: channel proteins and carrier proteins.
Channel Proteins
- Channel proteins form hydrophilic pores (water-filled channels) that allow specific ions to pass through.
- Each channel protein is highly specific (e.g., sodium ion channels only allow sodium ions).
- Channel proteins are not just two halves; they have a central hollow passage when open, permitting diffusion of ions.
Carrier Proteins
- Carrier proteins have binding sites complementary to specific substances (but these are not active sites like in enzymes).
- When a substance binds, the carrier protein changes shape, transports the molecule across the membrane, and then returns to its original shape.
- Carrier proteins are specific—for example, different proteins for glucose and amino acids.
Key Terms & Definitions
- Phospholipid bilayer — double layer of phospholipids forming the core of cell membranes.
- Simple diffusion — movement of nonpolar particles directly through the phospholipid bilayer.
- Facilitated diffusion — movement of polar particles via transport proteins down the concentration gradient.
- Transport proteins — membrane proteins assisting the movement of substances across the membrane.
- Channel protein — a protein forming a pore for specific ions to cross the membrane.
- Carrier protein — a protein that changes shape to move specific molecules across the membrane.
Action Items / Next Steps
- Review diagrams showing channel and carrier protein mechanisms.
- Memorize the specificity of channel and carrier proteins for different substances.
- Prepare for upcoming content on ion channels in nerve coordination (Chapter 15).