Ch 6 Interactions Between Cells - Lecture cards.

What is diffusion?

The movement of molecules from an area of high concentration to an area of low concentration.

Does diffusion require energy?

No — diffusion is passive; no energy is needed.

What is the overall function of the plasma membrane?

It regulates what enters and exits the cell.

What direction is "net movement" in diffusion?

From the region of high concentration to the region of low concentration.

In the two-compartment diffusion example (A = high concentration, B = low concentration), which direction is net diffusion occurring?

From A to B.

True or False: At equilibrium (equal concentrations), all molecular movement stops.

False — molecules continue moving in both directions equally; only the net movement stops.

What condition must be met for net diffusion to stop?

Equal concentration of the molecule on both sides of the membrane (equilibrium).

In a diffusion diagram, if the arrow pointing A→B is thicker/darker than B→A, what does this indicate?

More net movement is occurring from A to B — this is the direction of net diffusion.

At equilibrium, are the two directional arrows (in a diffusion diagram) equal or unequal in size?

Equal — equal movement in both directions means no net diffusion.

Why does O2 move into the cell?

Because O2 concentration is higher in the extracellular environment than inside the cell (high → low).

Why does CO2 move out of the cell?

Because CO2 concentration is higher inside the cell than in the extracellular environment (high → low).

What happens when a molecule has a concentration gradient favoring diffusion, but it's too large to pass through the membrane's pores?

The molecule cannot diffuse across, even though it "wants to" — the membrane physically blocks it, preventing movement despite the gradient.

In the dialysis membrane example, why can't the protein exit the artificial cell even though its concentration is higher inside?

The protein molecule is too large to fit through the tiny holes of the dialysis membrane.

What does it mean for a molecule to move "down the gradient" (or "with the gradient")?

It means the molecule is moving from an area of high concentration to an area of low concentration.

What are the factors that affect the rate of diffusion? (5)

Magnitude of the concentration gradient: — the size of the concentration difference across a barrier; the bigger the difference, the faster diffusion occurs (e.g., higher concentration outside the cell = faster diffusion into the cell).

Permeability of the membrane: — how easily a molecule can pass through the membrane.

Temperature: — the heat of the environment; affects molecule speed.

Surface area of the membrane: — the amount of membrane space available for diffusion.

Size of the molecule: — how large or small a molecule is.

How does the magnitude (steepness) of a concentration gradient affect the rate of diffusion?

The steeper/larger the concentration gradient, the faster the rate of diffusion.

in this case, more X’s outside (steeper magnitude) = faster and higher rate of diffusion.

If more molecules are added to the outside of a cell (increasing the concentration difference), what happens to the rate of diffusion into the cell?

The rate of diffusion increases, because the gradient became steeper.

What are the three factors that affect diffusion rate that are independent of the cell?

Magnitude/steepness of the concentration gradient, temperature, and size (of the molecule).

How does increasing temperature affect the rate of diffusion?

It increases the rate of diffusion.

How does decreasing temperature affect the rate of diffusion?

It decreases the rate of diffusion.

What is the relationship between temperature and rate of diffusion?

Directly proportional — as temperature increases, diffusion rate increases; as temperature decreases, diffusion rate decreases.

How does the size of a molecule affect its rate of diffusion?

Inversely — the larger the molecule, the slower the rate of diffusion.

The smaller the molecule, the faster the rate of diffusion.

Why does molecule size affect the rate of diffusion?

Larger molecules interact more with their environment and generate more friction, which slows movement; smaller molecules generate less friction and move faster.

What is the relationship between molecule size and rate of diffusion?

Inversely proportional — as size increases, diffusion rate decreases; as size decreases, diffusion rate increases.

What are the two factors affecting diffusion rate that are controlled by the cell membrane (not cell-independent)?

Permeability of the membrane and surface area of the membrane.

What does it mean for a membrane to be "permeable" to a molecule?

It means the membrane allows that molecule to pass through it.

How does membrane permeability affect the rate of diffusion?

Directly — the more permeable the membrane is to a molecule, the faster the rate of diffusion; the less permeable, the slower the rate of diffusion.

How does surface area of the cell membrane affect the rate of diffusion?

Directly — the greater the surface area, the faster the rate of diffusion; the smaller the surface area, the slower the rate of diffusion.

In the columnar cell example, why does the cell with microvilli (Cell 2) have a faster rate of diffusion than the cell without microvilli (Cell 1)?

Microvilli increase the surface area of the cell membrane, and more membrane surface area allows molecules to diffuse in faster.

• So technically the more surface area = the more space for diffusion = the higher rate of diffusion because more molecule can enter at a time

What is the relationship between surface area of the membrane and rate of diffusion?

Directly proportional — as surface area increases, diffusion rate increases; as surface area decreases, diffusion rate decreases.

Summarize the 5 factors affecting rate of diffusion and their relationships:

Factor	Cell-dependent?	Relationship to diffusion rate
Magnitude of concentration gradient	No	Gradient ↑ → Diffusion ↑
Temperature	No	Temp ↑ → Diffusion ↑ / Temp ↓ → Diffusion ↓
Size of molecule	No	Size ↑ → Diffusion ↓ (inverse — friction)
Permeability of membrane	Yes	Permeability ↑ → Diffusion ↑
Surface area of membrane	Yes	Surface area ↑ → Diffusion ↑ (e.g., microvilli)

What does it mean that the plasma membrane is "selectively permeable"?

It means the membrane allows certain molecules to cross while preventing others from crossing.

Is the plasma membrane a barrier to lipid molecules?

No — lipids can cross the membrane freely as long as there is a concentration gradient.

Is the plasma membrane a barrier to non-lipid molecules?

Yes, generally — unless the molecule meets two criteria: it is both small AND uncharged (no positive or negative charge).

What two criteria must a non-lipid molecule meet to cross the plasma membrane despite not being a lipid?

It must be small and uncharged (no charge).

What are the two main categories of movement across the plasma membrane?

Carrier-mediated transport and non-carrier mediated transport.

What is a "carrier" in the context of membrane transport?

A helper protein that allows molecules to cross the membrane.

Which molecules need a carrier to cross the membrane?

Molecules that are not permeable to the membrane (not a lipid, and not both small and uncharged).

Which molecules do NOT need a carrier to cross the membrane?

Molecules that are permeable to the membrane (lipids, or small uncharged molecules).

What is non-carrier mediated transport?

Movement of molecules across the membrane by diffusion, without needing a helper protein (carrier).

What is simple diffusion?

The diffusion of a molecule across the membrane without the need for a carrier — a type of non-carrier mediated transport.

In the example where molecule X is a lipid with higher concentration outside the cell, which direction does it move and by what mechanism?

It moves into the cell (down its gradient) via simple diffusion, since lipids don't need a carrier to cross the membrane.