Diffusion, Osmosis, Active Transport

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Diffusion Diffusion is the net movement of ions or molecules

from a region of higher concentration to a region of lower concentration, down a concentration gradient

In liquids and gases, molecules possess kinetic energy and constantly move about

As this movement is random, an equilibrium is reached when the molecules are evenly spread out, where there is no more net change – dynamic equilibrium reached

The steeper the gradient for a substance, the faster the rate of diffusion

No energy input is required for diffusion because the particles already are in motion.

Hence, it is know as passive transport

Diffusion

Key

potassium iodide ions

copper sulphate ions

permeable membrane

20 potassium iodide ions

20 copper sulphate ions

10 potassium iodide ions, 10 copper sulphate ions

10 copper sulphate ions, 10 potassium iodide ions

water

Factors affecting diffusion

What are some factors affecting the rate of diffusion?

Concentration gradient Temperature Pressure size of particles surface area to volume ratio

Osmosis

Osmosis is the movement of water molecules from a solution of higher water potential to a solution of lower water potential through a partially permeable membrane

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

A water potential gradient is established when a partially-permeable membrane separates two solutions of different water potentials.

A dilute solution contains more water molecules per unit volume than a concentrated solution so it has a higher water potential than a concentrated solution

Osmosis

Osmotic pressure is the pressure that must be applied to a solution to prevent the inward flow of water across a partially permeable membrane .

Osmotic pressure is the opposite of water potential.

A partially permeable membrane is a membrane that will allow certain molecules or ions to pass through it by diffusion.

The cell membrane is partially permeable. What other structures or organs are partially

permeable?

Osmosis

5% sucrose solution (more water, less sucrose)

8 sucrose molecules, 9 water molecules

4 sucrose molecules, 18 water molecules

partially permeable membrane

rise in level

drop in level

A B A A B B

Key

sucrose molecule

water molecule

Net movement of water molecules

water molecules diffuse from B to A.

8 sucrose molecules, 18 water molecules

4 sucrose molecules, 9 water molecules

Hypotonic, Isotonic, Hypertonic

(animal cells only)

Lower water potentialEqual water potentialHigher water potential

Plant cell in high water potential

solution As water enters the cell, the vacuole increases

in size and pushes the cell contents against the cellulose cell wall. The cell wall prevents over expansion of the cell by exerting an opposing pressure preventing the entry of more water

A plant in this state becomes turgid

The turgidity of the cell with water is called turgor pressure

The plant cell does not burst because the cell wall is strong and relatively inelastic

Animal cell in high water potential

solution Animal cell will swell and may bust in a

hypotonic solution because it does not have a cell wall

in a solution with high water potential

Plant cell in low water potential solution

The water potential of the cell sap is higher than that of the solution outside the cell. Osmosis takes place and water moves out of the cell

As water is lost from the cell, the vacuole decreases in size and the cytoplasm shrinks away from the cellulose cell wall.

This shrinkage of the cytoplasm away from the cell wall when plant cells are immersed in a solution of low water potential is known as plasmolysis

The cells are said to be plasmolysed. The cells can be restored by placing them in water or solution of high water potential

Plant cell in low water potential solution

water leaves by osmosis

Cytoplasm shrinks away from cell wall

in a solution with low water potential

As the cell loses water, the vacuole decreases in size.

The cell becomes plasmolysed

Animal cell in low water potential

solution Animal cells in hypertonic solution: it will lose

water and the cell membrane will form little spikes as water is lost and the cell shrinks, a process called crenation. The cell dies eventually

in a solution with low water potential

Spikes form

Turgor in plants

Turgor is very important in maintaining the shape of soft tissues in plants

Young stems and most leaves, esp. those of herbaceous or non-woody plants, can remain firm and erect because of the turgor pressure within their cells.

When there is a high rate of evaporation of water from the cells, they lose their turgidity and the plant wilts

The movement of certain plants are due to changes in turgor

Eg opening and closing of the stomata, folding of leaflets of the Mimosa plant

Active Transport

Active transport is the process in which energy is used to move the particles of a substance against a concentration gradient from a region of lower concentration to a region of higher concentration

Occurs in living cells because only living cells respire

Active transport

The energy used in cells is called “ATP”.

Active transport requires:

Protein transporters ATP (or energy)

Active transport occurs in:

Absorption of mineral salts by root hair cells. Absorption of glucose and amino acids by cells in

the small intestine. Na-K pump

Example of Active Transport

Absorption of mineral salts by root hair cells

The soil solution has to have a higher water potential than the root hair cell in order for water to move into the root hair cell by osmosis.

This means that the soil solution has a lower concentration of ions (ie. mineral salts) than the root hair cell.

Therefore, active transport is used to transport ions into the root hair cell.

Specific transporter proteins

Energy expended.

Transport of large molecules

Some substances are too large to cross the plasma membrane by diffusion, facilitated diffusion and osmosis

Other means of transport are necessary:

Endocytosis Exocytosis

Surface area: Volume ratio

The greater the area of cell surface membrane, the faster will be the rate of diffusion of a substance for a given concentration gradient

Surface area: Volume ratio

The greater the surface area to volume ratio, the faster the rate of diffusion.

This explains why cells are so small: nutrients, oxygen and waste substances have to diffuse into and out of cells quickly.

Eg.

Question

Ranee Mohan

Four clear agar blocks were placed into solutions of methylene blue as shown. Which agar block would be first to become completely stained?

2mm

2mm2mm

4mm

2mm2mm

6mm

2mm2mm

4mm

4mm4mm

A B C D

Question

Ranee Mohan

Volume of the liquid in tube X

Concentration of the sugar

solution

A Decreases Decreases

B Decreases Increases

C Increases Decreases

D Increases Increases

Partially permeable membrane

Tube X

An experiment to investigate osmosis is set up as shown. What happens?

Question A plasmolysed cell is shown. Which numbered

structures are partially-permeable?

A. 1 only

B. 2 only

C. 1 and 2 only

D. 1 and 3 only

1

3

2

Question

Ranee Mohan

Stage 1 Stage 2

A Boiled in water for 10 min

Placed in 10% salt solution

B Boiled in water for 10 min

Placed in distilled water

C Unboiled Placed in 10% salt solution

D Unboiled Placed in distilled water

Four identical pieces of potato are treated in two stages, as shown. Which piece of potato will be the largest after two hours?

Question Which one of the following enters plant cells by

active transport?

A. Carbon dioxide

B. Nitrate ions

C. Water

D. Oxygen