Overview
This lecture covers passive transport in cells, focusing on facilitated diffusion and the roles of channel and carrier proteins in helping molecules cross the membrane without using energy.
Simple Diffusion Recap
- Simple diffusion allows small, nonpolar, noncharged molecules (like O₂ and CO₂) to cross cell membranes easily.
- Water molecules can pass through the membrane due to their small size, but their polarity makes it harder.
- Charged particles (ions) have difficulty crossing the membrane due to interactions with the phosphate heads of phospholipids.
Facilitated Diffusion
- Facilitated diffusion is a type of passive transport where proteins assist molecules in crossing the cell membrane.
- This process still moves substances down their concentration gradient and does not require cellular energy (ATP).
Channel Proteins
- Channel proteins form tunnels in the membrane, providing a passageway for specific molecules or ions.
- Aquaporins are channel proteins specifically for water, allowing quick water transport (osmosis) across the membrane.
- Other channel proteins are specific for ions like sodium (Na⁺), allowing them to cross without interacting with the phospholipid bilayer.
- Some channel proteins are gated, opening or closing in response to certain conditions, which is critical in nerve cells.
Carrier Proteins
- Carrier proteins bind to specific molecules on one side of the membrane and change shape to transport them to the other side.
- This process is still passive and depends entirely on moving substances down their concentration gradient.
- Carrier proteins are essential for molecules that cannot diffuse directly or through channels.
Key Terms & Definitions
- Passive Transport — movement of substances across a membrane without energy, down their concentration gradient.
- Simple Diffusion — passive movement of small, nonpolar, or noncharged molecules through the membrane.
- Facilitated Diffusion — passive transport helped by proteins, allowing substances to cross the membrane more easily.
- Channel Protein — membrane protein forming a tunnel for specific molecules or ions to pass through.
- Aquaporin — a channel protein specialized for water transport.
- Carrier Protein — membrane protein that changes shape to move molecules across the membrane.
Action Items / Next Steps
- Review the roles of channel and carrier proteins in passive transport.
- Prepare for upcoming content on gated channels in nerve cell signaling.
- Ensure understanding of how concentration gradients drive passive transport.