Bio - Movement of Substances

What is a concentration gradient?

Concentration gradient is the difference in concentration between two regions

What is diffusion?

Diffusion is the net movement of particles (atoms,molecules, ions) from a region where they are of higher concentration to a region where they are of lower concentration, that is, down a concentration

How do the particles get diffused?

The particles of fluids possess movement (kinetic energy) which are continuously moving about. As their movement is random, particles will collide with one another, causing the particles to diffuse till they reach equilibrium

What factors affect the rate of diffusion?

1. Concentration gradient: steeper the gradient, faster the rate of diffusion

2. Diffusion distance: as the distance increases, the rate of diffusion decreases (longer time taken)

3. Surface-area-to-volume ratio: greater ratio, faster rate of diffusion (smaller cell → faster rate of diffusion)

4. Temperature: Higher thermal energy, increased KE, causing it to collide faster, faster rate of diffusion

Why do living things undergo diffusion?

Living things need to regulate movement of substances (nutrients, water, waste products, gases) ← diffusion is the process that controls the exchange

What is osmosis?

It is the net movement of water molecules down a water potential gradient from a region of higher water potential to a region of lower water potential, across a partially permeable membrane, to reach equilibrium

What is a hypotonic solution?

High water potential, diluted solution

What is an isotonic solution?

water potential of cell is equivalent to water potential of solution

What is a hypertonic solution?

low water potential, concentrated solution

What happens to a plant cell when it is placed in an hypotonic solution?

When a cell is placed in a solution with a higher water potential, water molecules will move from a region of higher water potential into a region of lower water potential (into the cell from the solution) and hence:

1. plant cell will expand or swell

2. Since, plant cells are protected by ineleastic cell wall, plants are supported by the pressure of water inside the cells pressing outwards on the cell wall

How does a cell become turgid?

1. When a plant cell is placed in a hypotonic solution, the cell sap has lower water potential than of the solution outside the cell

2. By osmosis, water molecules enter the cell through the partially permeable cell membrane

3. The cell expands/swells and the cell becomes turgid

4. As water molecules enter the cell, the vacuole increases in size and pushes the cytoplasm against the cell wall and the cell does not burst because it is protected by the inelastic cell wall

5. The turgidity of the cell with water is called turgor, and the pressure exerted by the water in the vacuole is the turgor pressure

Why is turgor important for plants?

1. Maintains shape of soft tissues: young stems and leaves remain firm and erect

2. Movement of certain plant parts due to changes in turgor: opening and closing of stomata is caused by change in turgor of guard cell

3. Protect the plant from too much fertiliser: when there is too much fertiliser, soil is very concentrated and water molecules move out of the root hair cells via osmosis → inability to absorb water and continued evaporation of water from leaves will cause the plants to wilt unless sufficient water is added to dilute the soil solution, and may cause plant to die eventually

What is lysis?

When an animal cell is placed in a hypotonic solution, the net movement of water molecules is entering the animal cell, causing the cell to swell and burst as it does not have a cell wall to protect it

What is crenation?

When an animal cell is placed in a hypertonic solution, the cytoplasm has a higher water potential that outside solution, causing the water molecules to leave the cell and enter the solution via osmosis, and hence, the cell shrinks and spikes appear → cells will become dehydrated and eventually die

What is plasmolysis?

When a plant cell is placed in a hypertonic solution, water molecules will move from the plant cell of higher water potential to the solution of lower water potential via osmosis, causing the plant cell to decrease in size and become limp

How does a cell become plasmolised?

1. When a plant cell is in a hypertonic solution, the cell sap has a higher water potential than the solution outside the cell

2. By osmosis, water molecules from the vacuole and cytoplasm leave the cell through the partially permeable cell membrane

3. The cell decreases in size and becomes flaccid or limp

4. As the cell loses water, the vacuole decreases in size, and cytoplasm shrinks away from the cell wall. The shrinkage of the cytoplasm and the cell membrane away from the cell wall is called plasmolysis

5. A plasmolysed cell can be restored to its original state by placing it in water/hypotonic solution

What are the factors that affect osmosis?

1. Water potential gradient: steeper gradient → higher rate of osmosis

2. Distance over which the molecules need to move: greater distance → slower the rate of osmosis

3. Surface area-to-volume ratio: greater ratio, higher rate of osmosis

What is active transport?

It is the process in which energy is used to move the particles of a substance across a membrane against its concentration gradient from a region of lower concentration to a region of higher concentration, in the presence of energy

Why does active transport only occur in living cells?

1. Only living cells respire and release energy which is needed for active transport

2. Examples:

• Human → uptake of glucose by the microvilli of the epithelial cells in the small intestine, and from kidney tubules into blood capillaries

• Root hair cells → Take in mineral salts from soil solution via diffusion/active transport (depending on conc. of mineral ions in salt solution

How do diffusion and active transport differ?

Diffusion: Net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient

Active Transport: Net movement of particles from a region of lower concentration to a region of higher concentration, against a concentration gradient

Diffusion: Energy from respiration not required

Active Transport: Energy from respiration required

Diffusion: Cell membrane not required

Active Transport: Cell membrane required

What type of particles do they move?

Diffusion: Molecules, ions of substances in liquid and gaseous state

Osmosis: Water molecules

Active Transport: Molecules and ions of substances in liquid state