Movement of Substances

definition: diffusion

the net movement of ions / molecules from a region of higher solute concentration to a region of lower solute concentration (down a concentration gradient).

definition: facilitated diffusion

passive transport that allows substances to cross membranes with the assistance of special transport proteins such as ion channel proteins and carrier proteins.

definition: osmosis

the net movement of water molecules from a region of lower solute concentration (/ higher water potential) to a region of higher solute concentration (/ lower water potential), against a concentration gradient across a partially permeable membrane.

definition: active transport

the net movement of ions / molecules from a region of lower solute concentration to a region of higher solute concentration (against a solute concentration gradient) through a partially permeable membrane with the expenditure of energy from cellular respiration.

definition: concentration

the amount of particles per unit volume

definition: concentration gradient

the difference in concentration between a region of higher concentration of a substance and a region of lower concentration of that substance

definition: water potential

the measure of the tendency of water molecules to move from one region to another. the more free water molecules, the higher the water potential.

definition: hypertonic

higher concentration of solute (lower water potential)

definition: isotonic

equal water potential

definition: hypotonic

lower concentration of solute (higher water potential)

factors affecting rate of diffusion: SA:V ratio

the larger the SA:V; the faster the rate of diffusion

factors affecting rate of diffusion: concentration difference / difference in concentration gradient

the steeper the concentration gradient → the greater the concentration difference between two regions; the faster the rate of diffusion

factors affecting rate of diffusion: temperature

the higher the temperature → the higher the kinetic energy of the molecules; the faster the rate of diffusion

factors affecting rate of diffusion: thickness of membrane

the thicker the membrane → more time taken for molecules to move across it; the slower the rate of diffusion

factors affecting rate of diffusion: diffusion distance

the longer the distance between two regions → more time taken for molecules to travel between the regions; the slower the rate of diffusion

factors affecting rate of diffusion: pressure difference

the larger the pressure difference → the molecules move faster; the higher the rate of diffusion (from region of higher to lower pressure)

factors affecting rate of diffusion: solubility

the more soluble a molecule; the faster the rate of diffusion

when will diffusion stop?

• when the two regions have the same concentration (same no. of solute molecules), there will be no diffusion (no net movement)

• system has reached dynamic equilibrium - the particles are still in random and constant motion, and there is the same no. of these particles everywhere.

• solutes distribute themselves by diffusion, uniformly and independently of each other

effects of diffusion on plant cell treated with heat

• heat denatures the proteins on the cell membrane

• the cell membrane becomes porous, and thus particles / molecules move across the cell membrane faster and more easily

• the rate of diffusion of the solute particles increases

effects of diffusion on plant cell treated with alcohol

• alcohol dissolves the membrane lipids and denatures the membrane proteins

• the cell membrane becomes even more porous, and thus particles move across the cell membrane even faster and more easily

• the rate of diffusion of the solute particles increases

movement of water through a cell membrane

the actual movement of water through a cell membrane is the result of two processes:

1. by simple diffusion of water (aka. osmosis)

2. bulk flow by facilitated diffusion (protein carrier is called aquaporin)

• the cell membrane possesses integral proteins; the protein carrier aquaporin is involved in water transport

• it serves as a water-filled pipe across the membrane. the flow through this channel is accomplished by submicroscopic bulk flow (facilitated diffusion of water).

movement of water through a cell membrane: facilitated diffusion

• some molecules and ions such as glucose, sodium ions and chloride ions are unable to pass through the lipid bilayer of cell membranes and requires the help of transport proteins to help them cross the membranes.

• through the use of ion channel proteins and carrier proteins that are embedded in the cell membrane, these substances can be transported into the cell. ion channel proteins allow specific ions to pass through the protein channel.

how do the membrane proteins bring about facilitated diffusion?

• the ion channels are regulated by the cell and are either open or closed to control the passage of substances into the cell.

• carrier proteins bind to specific molecules, change shape, then deposit the molecules across the membrane. once the transaction is complete, the proteins return to their original position.

movement of water through a cell membrane: osmosis

• the size of a water molecule permits it to pass through the cell membrane (phospholipid bilayer). this would be a simple diffusion movement (osmosis).

• water molecules can pass through the bilayer by zipping between phospholipid molecules and through protein carriers called aquaporins.

effect of osmosis on animal cells: hypertonic

loses water, shrinks in size, and becomes crenated

effect of osmosis on animal cells: hypotonic

gains water, becomes bloated, becomes lysed / bursts

effect of osmosis on plant cells: hypertonic

plant cell cytoplasm & cell sap loses water, the vacuole decreases in size and volume, the cell membrane shrinks away from the cell wall, becomes plasmolysed

effect of osmosis on plant cells: hypotonic

plant cell cytoplasm & cell sap gains water, the vacuole increases in size and volume, the cell membrane pushes against from the cell wall, nucleus gets pushed to the side, becomes bloated and turgid

turgor pressure

• water may enter a plant cell by osmosis. as water enters the cell, the vacuole and cytoplasm increases in volume.

• the water molecules in the cytoplasm and vacuoles presses against the cell wall → creates an outward pressure

this outward pressure is called turgor pressure

• to prevent over-expansion of the cell, the cell wall exerts an opposite pressure to prevent entry of more water

turgid

plant cells in this state are said to be turgid cells

turgor

the turgidity of the cell with water is called turgor

importance of turgor pressure in plants

1. maintains the shape of soft tissues in plants; keeps stem upright

2. controls the opening and closing of stomata

• guard cells lose water → shrinks → curved less → stomata closes

• guard cells gain water → swells → curved more → stomata opens

3. opens up the leaf and keeps it flat

• larger surface area exposed to sunlight so as to trap more sunlight for photosynthesis

4. opens up the petals in flowers

5. prevent plants from wilting

why is osmosis considered a special type of diffusion?

• diffusion is the movement …

• osmosis is similar to diffusion as it also deals with movement of particles from high concentration to low concentration

• osmosis is a special type of diffusion as it deals with only water molecules, and it requires a partially permeable membrane unlike diffusion, which can occur with or without one.

active transport

• solute particles will pass through specific protein carriers on the cell membrane

• energy is stored in the form of ATP (adenosine triphosphate). - energy is used to change the shape of the carrier protein in order to bring in the nutrients from lower concentration to higher concentration.

• (A-P-P-P) → breaks a bond to release energy, forming A-P-P

required conditions for active transport to take place

1. solute particles / molecules

2. lower solute concentration → higher solute concentration

3. partially permeable membrane

4. energy released by the process of cellular respiration (mitochondria)

difference between facilitated diffusion and active transport

energy is used to change the shape of the carrier protein in order to bring in the nutrients from lower concentration to higher concentration