MS (movement of substances)

Diffusion

Osmosis

Active Transport

In order to survive, the cell must be able to:

obtain useful substances required

remove waste substances produced

Passive process

Passive process

Active process

The particles move down a concentration gradient


No energy from the cell required

Substances move according to their own natural tendency without an input of energy

The net movement of substances from a region of higher concentration to a region of lower concentration down a concentration gradient, until equilibrium is reached.

Diffusion continues until the particles are uniformly distributed throughout on both sides of the membrane or when concentration in all regions are equal.This is called equilibrium

Hence, the particles themselves are still moving, and moving across regions even when there is no more observable diffusion
But no net movement of particles in a particular direction

Equilibrium definition

At equilibrium, concentration is equal in both regions.
Particles will still constantly move across regions.
But no net / overall movement of particles
(Particles move in both directions at equal rates)

Concentration

As particles move, they tend to collide with each other and spread out from regions of high concentration to regions of low concentration

definition:Concentration refers to the proportion of a solute in a particular volume of solution.
(Recall that air is a solution too!)

The steeper the concentration gradient, the faster the rate of diffusion.
and vice versa

Diffusion

Factors affecting rate of diffusion

Surface area to volume ratio (SA:Vol) – covered next

  • higher SA:Vol = faster rate of diffusion

Temperature

  • Higher temp = faster rate of diffusion

Concentration gradient – covered next

  • steeper gradient = faster rate of diffusion

Size of the particles

  • smaller size = faster rate of diffusion

Distance over which diffusion occurs

  • Shorter distance = faster rate of diffusion

Medium in which diffusion occurs

  • gas > liquid > solid

in cells

Cells need to maintain a large surface area to volume ratio to ensure efficient exchange of materials across the cell membrane via fast diffusion.

Cells depend on the surface area of the cell membrane for diffusion of food nutrients, oxygen, waste products

rate of diffusion in cells depends on:

Size
The smaller the cell, the larger the surface area to volume ratio, the higher the rate of diffusion.


Shape of specialised cells (recall!)
Certain shapes increase the surface area to volume ratio, thus increases the rate of diffusion.
E.g. root hair cell – long and narrow extension
E.g. red blood cell – biconcave shape

in living systems

Root hair cells are found on roots
Some mineral salts diffuse from the outside (soil) into the root hair cells

Oxygen and carbon dioxide diffuse through the stomata of the leaves.

The exchange of gases take place in the lungs
Oxygen diffuses into the red blood cell from lungs, carbon dioxide diffuses out of the red blood cells

Net movement of water molecules from a region of higher water potential to a region of lower water potential across a partially permeable membrane.

Water potential: Water potential is the measure of the tendency of water to move from one place to another.

Dilute solution

Lower proportion of dissolved solute

Higher proportion of water as the solvent

Hence, higher water potential

Concentrated solution

Higher proportion of dissolved solutes

Lower proportion of water as the solvent

Hence, lower water potential

Same concentration: Equal water potential


Water potential gradient
Difference in water potential between two regions

Examples:

Lowest
eg. concentrated salt or sugar solutions

In between

  • Have to compare the solutions- eg dilute solutions

Highest
eg. pure water, distilled water

Answering Technique - Explaining

A has a higher water potential than B. (identify)

Hence, the water molecules from A will move to B (direction)

by osmosis. (process)

Thus, the water level increases in B and decreases in A. (results if applicable)

Plants take in water from the soil through root hair cells by osmosis.

The large central vacuole is responsible for turgor pressure in a plant cell. (Turgid cell or Plasmolysed cell)


Solution vs cell cytoplasm

Solution has lower water potential

Solution has higher water potential

Plant cells

animal cells

Shrink and shrivel / crenate

Plasmolysed

animal cell

plant cell

Swell and burst / lyse

Turgid

Root hair cells use active transport to take in some mineral salts that are present in lower concentrations in the soil than in the cell.


Active transport is the movement of substances across a membrane from a region of lower concentration to a region of higher concentration i.e. against a concentration gradient with the expenditure of energy in the form of ATP by the cell.

The villi cells in the small intestines carry out active transport to take in digested food from the blood stream into the cells.

importance

It allows cells to take up nutrients even when their concentrations outside the cells are lower than inside the cells.

It enables cells to remove waste products when their concentrations outside the cell are lower than those inside the cells.