Membrane Transport

Explain how the chemical properties of the plasma membrane influence whether molecules can diffuse through the plasma membrane or not.

The plasma membrane is selectively permeable due to its phospholipid bilayer structure, which contains a hydrophobic (non-polar) core.

This hydrophobic core allows small, lipid-soluble (hydrophobic) molecules to diffuse directly through the membrane.

In contrast, large, hydrophilic or charged molecules are repelled by the hydrophobic core and cannot diffuse freely, requiring transport proteins to cross.

What is the difference between penetrating and non penetrating molecules?

Penetrating molecules are small and/or hydrophobic molecules able to pass through the hydrophobic core of the phospholipid bilayer

Non penetrating molecules are large, charged or hydrophilic molecules that cannot pass through the fatty core of the phospholipid bilayer without the assistance of transport proteins

What are examples of penetrating molecules?

• Gases (o2, Co2) (small & hydrophobic)

• Water and ethanol (Hydrophilic but small enough to pass through)

What are examples of non- penetrating molecules?

• Ions (small, strong charge/very hydrophilic)

• Glucose and proteins (Large and hydrophilic)

 

What is passive membrane transport?

Passive transport is the movement of substances across a membrane without the use of ATP. It occurs down the concentration gradient, from high to low concentration.

What is active transport?

Active transport requires ATP to move molecules against their concentration gradient, from an area of low concentration to high concentration. 

What are the types of Passive transport?

• Simple diffusion

• Facilitated diffusion

• Osmosis

What is simple diffusion?

Where small/uncharged/hydrophobic (penetrating) molecules can diffuse directly through the plasma membrane unaided. These include gases such as oxygen (O₂) and carbon dioxide (CO₂), move down their concentration gradient by simple diffusion.

What is facilitated diffusion?

Where charged/large/hydrophilic (non-penetrating) molecules require aid from membrane transport proteins, such as carrier proteins or channel proteins, to cross the plasma membrane.

What is osmosis?

Osmosis is the passive movement of water across a semipermeable membrane from an area of lower solute concentration (low osmolarity) to an area of higher solute concentration (high osmolarity). 

What is osmolarity?

The total concentration of all solutes dissolved in a solution

Describe the difference between simple and facilitated diffusion

Simple diffusion involves the movement of small, lipid-soluble molecules directly through the hydrophobic core of the plasma membrane. In contrast, facilitated diffusion involves the movement of non-penetrating molecules through membrane proteins.

Both processes occur down the concentration gradient and do not require ATP.

Explain how osmosis impacts on the movement of
water across a semipermeable membrane

Osmosis causes water to move across a semipermeable membrane from an area of low solute concentration to an area of high solute concentration. This happens because the membrane allows water to pass but restricts non-penetrating solutes, so water moves to balance the concentration difference. This movement continues until equilibrium is reached.

What is tonicity?

The effect that a solution has on cell size and shape due to water movement or osmosis

What is a Hypotonic solution?

A hypotonic solution has a lower osmolarity (lower concentration of non-penetrating solutes) outside the cell compared to inside the cell. As a result, water moves into the cell by osmosis, causing the cell to swell.

What is a hypertonic solution?

A hypertonic solution has a higher osmolarity (higher concentration of non-penetrating solutes) inside the cell compared to outside the cell. This causes water to move out of the cell by osmosis, resulting in cell shrinkage.

What is an isotonic solution?

In an isotonic solution, the concentration of non-penetrating solutes is equal on both sides of the membrane. As osmolarity is the same there is no net movement of water across the membrane, therefore there is no change in cell size and shape.

Where is water found?

• Intracellular fluid (in cells)

• Extracellular fluid (plasma and interstital fluid)

What is active transport?

Active transport uses ATP to move molecules against their concentration gradient, from an area of lower concentration to higher concentration. 

What are the two types of Active transport?

• primary active transport

• secondary active transport

What is primary active transport?

It directly uses energy from the breakdown of ATP

What is secondary active transport?

What is an example of a primary active transport?

Sodium pottasium pump