Passive transport and selective permeability | Biology | Khan Academy

Summary

This video script explores passive transport of molecules across cellular membranes without energy expenditure. It emphasizes the role of concentration gradients and membrane permeability, highlighting how small, non-charged molecules like CO2 and O2 can easily diffuse. It contrasts this with charged ions like sodium and potassium, which face difficulty due to their attraction to hydrophilic heads. The script also discusses the intermediate case of water molecules and anticipates future discussions on facilitated diffusion mechanisms like channel proteins and aquaporins.

Takeaways

• 🚶‍♂️ Passive transport is the movement of molecules across a cellular membrane without the need for energy.
• 📉 Molecules move down their concentration gradient, meaning from areas of higher concentration to lower concentration.
• 🧽 The cellular membrane is selectively permeable, allowing different types of molecules to pass through at varying rates.
• 💧 Small, non-charged molecules like carbon dioxide and molecular oxygen can easily diffuse across the membrane.
• 🔄 The movement of molecules is influenced by their size and charge, with smaller, non-charged molecules having an easier time.
• 🚫 Charged molecules, such as sodium and potassium ions, have difficulty diffusing passively due to their attraction to the hydrophilic heads of the membrane.
• 💧 Water molecules, with their partial charges, can slowly diffuse through the membrane due to their size and polarity.
• 🛑 Large molecules, like proteins, face significant challenges in diffusing through the membrane due to their size and potential for both hydrophobic and hydrophilic interactions.
• 🔄 The process of diffusion is bidirectional, but there is a net movement from areas of higher concentration to areas of lower concentration.
• 🔬 Future videos will discuss facilitated passive transport, which involves proteins like channel proteins and aquaporins that assist in the transport of molecules.

Q & A

What is passive transport across a cellular membrane?

Passive transport is the movement of molecules across a cellular membrane without the need for energy, following their concentration gradient.

How does the size of a molecule affect its ability to passively transport across a membrane?

Small molecules can more easily fit between the gaps of the phospholipid bilayer, aiding in passive transport and diffusion across the membrane.

Why are non-charged molecules like carbon dioxide and molecular oxygen able to diffuse well across cellular membranes?

These molecules are small and non-charged, allowing them to pass through the hydrophobic tails without being repelled and to move freely between the hydrophilic heads.

What role does the selective permeability of the cellular membrane play in passive transport?

Selective permeability means the membrane is more or less permeable to different types of molecules, allowing certain small and non-charged molecules to pass through more easily.

How does the concentration gradient influence the passive transport of molecules?

Molecules will naturally move from an area of higher concentration to an area of lower concentration, following their concentration gradient.

Why do charged molecules like sodium and potassium ions have difficulty diffusing passively across the membrane?

Charged molecules are attracted to the hydrophilic heads of the phospholipids but find no attraction in the hydrophobic tails, making it difficult for them to pass through the membrane.

What is the significance of water molecules in passive transport across cellular membranes?

Water molecules, despite having a partial charge, can slowly diffuse through the membrane due to their small size and the fact that their charge is not strong enough to prevent passage.

How do hydrophilic heads of the phospholipids interact with water molecules?

The hydrophilic heads are attracted to water molecules due to their polarity, but the water molecules can still pass through the membrane given enough interactions.

What are aquaporins and how do they relate to the passive transport of water?

Aquaporins are channel proteins that provide tunnels through the membrane, facilitating the passive transport of water molecules by bypassing the phospholipid bilayer.

Why do large molecules have trouble diffusing through the cellular membrane?

Large molecules may not fit through the gaps in the phospholipid bilayer and may also have parts that are incompatible with either the hydrophobic or hydrophilic regions of the membrane.

What is facilitated passive transport and how does it differ from simple passive transport?

Facilitated passive transport involves the use of transport proteins to help molecules move along their concentration gradient without energy input, unlike simple passive transport which occurs directly through the membrane.