Diffusion

What is meant by the ‘steepness’ of the concentration gradient in diffusion?

It’s the difference in concentration of a substance on the two sides of a surface or membrane. A steeper gradient means a larger difference in concentration.

How does the steepness of the concentration gradient affect the rate of diffusion?

The greater the difference in concentration, the faster the rate of diffusion, as more molecules move from the higher to the lower concentration side.

How does temperature affect the rate of diffusion?

• At higher temperatures, molecules and ions have more kinetic energy.

• They move faster, so diffusion occurs more rapidly.

How does surface area affect the rate of diffusion?

• A larger surface area allows more molecules or ions to cross at once, increasing the diffusion rate.

• Cells increase membrane surface area by folding (e.g. microvilli or cristae).

Why does the surface area to volume ratio affect diffusion in cells?

• As a cell increases in size, its surface area to volume ratio decreases.

• This slows diffusion because substances have to travel a greater distance through the cell.

how does thickness of exchange surface affect the rate of diffusion

• the thinner the exchange surface the shorter the distance the particles travel and so the faster the rate

How do the properties of molecules or ions affect diffusion rate?

• Large molecules diffuse more slowly because they need more energy to move.

• Uncharged, non-polar molecules diffuse directly through the phospholipid bilayer.

• Non-polar molecules diffuse faster than polar ones because they are soluble in the non-polar bilayer.

Which types of molecules diffuse directly through the phospholipid bilayer?

Uncharged and non-polar molecules such as O₂ and CO₂ 

Why are non-polar molecules soluble in the phospholipid bilayer?

• The centre of the bilayer is non-polar due to fatty acid tails.

• Non-polar molecules are soluble in non-polar substances (“like dissolves like”).

• Therefore, they can dissolve in and diffuse through the bilayer easily.

why can water move through the phospholipid bilayer even though it’s polar

• because it’s small enough to

Define diffusion in biological terms.

Diffusion is the net movement of molecules or ions, resulting from their random motion, from a region of higher concentration to a region of lower concentration, down a concentration gradient, until equilibrium is reached.

• This movement occurs due to the natural kinetic energy of the particles.

• It is a passive process, meaning it does not require ATP.

Where does diffusion occur in cells and what is its function?

Diffusion occurs across cell membranes and within the cytoplasm to allow essential molecules (e.g. oxygen, carbon dioxide, lipid-soluble substances) to move in or out of cells.
It enables gas exchange, absorption of nutrients, and waste removal in both prokaryotic and eukaryotic cells.

What happens to molecules over time during diffusion?

Given sufficient time, molecules or ions become evenly distributed within a given volume — this is known as dynamic equilibrium, where molecules continue to move randomly but there is no net movement in any direction.

What 6 factors influence the rate of diffusion across membranes?

• Concentration gradient – steeper gradient → faster diffusion.

• Temperature – higher temperature → more kinetic energy → faster diffusion.

• Surface area – larger area → faster rate.

• Diffusion distance (membrane thickness) – thinner membrane → faster diffusion.

• Molecular size – smaller molecules diffuse faster.

• Nature of molecule – lipid-soluble molecules (non-polar) diffuse more easily than polar molecules.

Which molecules can and cannot diffuse directly through the phospholipid bilayer?

✅ Can diffuse freely:

• Small, non-polar molecules (e.g. O₂, CO₂, lipid-soluble molecules).
  🚫 Cannot diffuse freely:

• Large polar molecules (e.g. glucose, amino acids).

• Charged ions (e.g. Na⁺, Cl⁻).
  These require facilitated diffusion through transport proteins.

Define facilitated diffusion and explain how it differs from simple diffusion.

Facilitated diffusion is the passive movement of molecules or ions across a membrane down their concentration gradient, using channel or carrier proteins.

• It still does not require ATP.

• It enables polar or charged substances to cross the hydrophobic core of the phospholipid bilayer.

what factors affect facilitated diffusion?

• same as simple diffusion factors but also the number of carrier proteins and channel proteins

Describe the structure and function of channel proteins.

• Channel proteins form water-filled pores in the membrane.

• They allow specific charged ions (e.g. Na⁺, Cl⁻, K⁺) to diffuse through.

• Most are gated, meaning they can open or close to regulate ion exchange.

• Movement through channel proteins is always down the concentration gradient.

• so channel proteins provide hydrophillic pathways for ions

How do carrier proteins enable diffusion across membranes?

• Carrier proteins bind to a specific molecule on one side of the membrane.

• The protein changes shape ( requires energy but facilitated diffusion is still a passive process overall) to transport the molecule to the opposite side. of the membrane.

• This process is reversible and passive.

• Each carrier protein is specific to one molecule or ion.

• carrier molecules carry two molecules at a time as they have two binding sites

How does facilitated diffusion differ from simple diffusion?

Feature	Simple Diffusion	Facilitated Diffusion
Pathway	Through phospholipid bilayer	Through channel or carrier proteins
Molecules	Small non-polar (O₂, CO₂)	Large/polar (glucose, ions)
Requires ATP?	❌ No	❌ No
Saturation limit?	❌ No	✅ Yes – limited by number of transport proteins

Why doesn’t facilitated diffusion require energy even though carrier proteins change shape?

The conformational change in carrier proteins occurs spontaneously when the molecule binds — it is driven by the kinetic energy of the particles, not ATP.
No energy is used because molecules move down the concentration gradient.