Diffusion Osmosis And Active Transport

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Notes

Diffusion

**Diffusion** is the net movement of molecules from a region of higher concentration to a region of lower concentration (down a concentration gradient).
Movement occurs due to the **random motion** (Brownian motion) of particles; energy comes from their kinetic energy.
The **cell membrane** is **partially permeable** – it allows some molecules to cross easily but restricts others (e.g., based on size).
Diffusion helps organisms obtain requirements (e.g., oxygen, carbon dioxide) and remove waste products (e.g., carbon dioxide).
Examples: oxygen diffusing into cells for respiration; carbon dioxide diffusing out of cells; carbon dioxide diffusing into leaf cells for photosynthesis.

Factors that Influence Diffusion

**Surface area to volume ratio**: larger surface area speeds up diffusion; cells like root hair cells and ileum cells have folded surfaces to increase area.
**Distance**: shorter distance increases rate; e.g., capillary and alveolus walls are one cell thick.
**Temperature**: higher temperature gives particles more kinetic energy, increasing diffusion rate.
**Concentration gradient**: steeper gradient increases rate because more collisions occur on the high-concentration side.

Osmosis

**Osmosis** is the diffusion of water molecules from a dilute solution (high water potential) to a concentrated solution (low water potential) across a **partially permeable membrane**.
Water moves down its concentration gradient; the membrane allows small water molecules through but not larger solute molecules.
**Water potential** describes the tendency of water to move: dilute solutions have high water potential; concentrated solutions have low water potential.

Osmosis Experiments

Common practical: cut cylinders of potato or radish, weigh, place in distilled water or sucrose solutions of increasing concentration, leave 20–30 min, dry and reweigh.
If mass **increases**: water moved into tissue by osmosis; external solution is more dilute (higher water potential) than the tissue.
If mass **decreases**: water moved out of tissue; external solution is more concentrated (lower water potential) than the tissue.
If mass **unchanged**: no net movement; internal and external concentrations are equal.

Osmosis in Animals & Plants

**Plant cell in dilute solution**: water enters by osmosis, pushes membrane against cell wall, cell becomes **turgid** (increased turgor pressure).
**Plant cell in concentrated solution**: water leaves, cell becomes **flaccid**; if severe, **plasmolysis** occurs (membrane pulls away from cell wall).
**Animal cell in dilute solution**: water enters, cell swells and may **burst** (no cell wall).
**Animal cell in concentrated solution**: water leaves, cell **crenates** (shrivels).

Active Transport

**Active transport** is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration (against a concentration gradient).
Requires **energy from respiration** (ATP).
Enables cells to take up substances even when they are in low concentration outside.

Importance of Active Transport

Uptake of **glucose** by epithelial cells in the villi of the small intestine and by kidney tubules in the nephron.
Uptake of **ions** from soil water by root hair cells in plants.

Plant cell structure – note the cell wall, chloroplasts and large vacuole.

Particle arrangement in solid, liquid and gas – diffusion occurs fastest in gases due to greater particle movement.

Practice questions

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1.Osmosis is:
Easy
- AThe active movement of molecules from a region of their lower concentration to a region of their higher concentration.
- BThe movement of water through a partially permeable membrane from a more concentrated to a more dilute solution.
- CParticles from a region of lower concentration to a region of higher concentration using energy from respiration.
- DThe movement of water through a partially permeable membrane from a more dilute to a more concentrated solution.
2.How does a water vapour molecule move out of the air space of a leaf into the atmosphere on a dry day?
Easy
- AActive transport
- BDiffusion
- CEvaporation
- DTranspiration
3.What causes the diffusion of carbon dioxide out of a living cell?
Easy
- AActive transport
- BRandom movement of molecules
- CRespiration
- DPhotosynthesis
4.Which of the structures below is best adapted for diffusion?
Easy
- Aileum
- Bbronchioles
- Catria
- Dcapillary
5.Which of the following statements would be correct for both diffusion and osmosis?
Medium
- Adoes not require energy from the cell ✓; molecules must be separated by a partially permeable membrane ✓; molecules move from a dilute to a more concentrated solution ✗
- Bdoes not require energy from the cell ✗; molecules must be separated by a partially permeable membrane ✗; molecules move from a dilute to a more concentrated solution ✓
- Cdoes not require energy from the cell ✓; molecules must be separated by a partially permeable membrane ✗; molecules move from a dilute to a more concentrated solution ✗
- Ddoes not require energy from the cell ✓; molecules must be separated by a partially permeable membrane ✗; molecules move from a dilute to a more concentrated solution ✓
6.A number of red blood cells were immersed in a pure solution of water before being observed under a microscope. There was a dramatic change in their appearance. Which row of the table below correctly describes and explains what happened to the red blood cells?
Medium
- Adirection of water movement: in; from higher to lower concentration: ✓; from higher to lower water potential: ✗; effect: cells burst
- Bdirection of water movement: in; from higher to lower concentration: ✗; from higher to lower water potential: ✓; effect: cells burst
- Cdirection of water movement: out; from higher to lower concentration: ✓; from higher to lower water potential: ✗; effect: cells shrink
- Ddirection of water movement: out; from higher to lower concentration: ✓; from higher to lower water potential: ✓; effect: cells shrink
7.A plant absorbs a number of substances into its roots. Which row of the table below correctly shows the substances absorbed by the roots and how these substances are absorbed?
Medium
- Aminerals: active transport; oxygen: diffusion; water: osmosis
- Bminerals: active transport; oxygen: not absorbed; water: transpiration
- Cminerals: diffusion; oxygen: diffusion; water: osmosis
- Dminerals: active transport; oxygen: not absorbed; water: osmosis
8.The image below shows two solutions separated by a partially permeable membrane. Which of the statements below would be true in relation to the image?
Medium
- AThere is no net movement of water from W to X
- BThere would be movement of water molecules from W to X
- COsmosis of sugar molecules would occur from the dilute solution (W) to the concentrated solution (X)
- DSolution X is more dilute and therefore has a higher water potential than solution W

Slides

Diffusion

Diffusion is the net movement of molecules from a region of higher concentration to a region of lower concentration.
Molecules move down a concentration gradient due to their random movement (Brownian motion).
The cell membrane is partially permeable, allowing some molecules to cross easily but not others.

What is diffusion?

Movement of molecules from low to high concentration using energy
Net movement of molecules from high to low concentration
Movement of water through a partially permeable membrane
Movement of ions against a concentration gradient

Factors that Influence Diffusion

Surface area to volume ratio: larger surface area increases diffusion rate.
Distance: shorter distance increases diffusion rate (e.g., one-cell-thick walls of capillaries and alveoli).
Temperature: higher temperature increases kinetic energy and diffusion rate.
Concentration gradient: steeper gradient increases diffusion rate.

Which of these increase the rate of diffusion?

Increasing temperature
Decreasing surface area
Increasing distance
Steeper concentration gradient

Osmosis

Osmosis is the diffusion of water molecules from a dilute solution (high water concentration) to a concentrated solution (low water concentration) across a partially permeable membrane.
Water moves down its concentration gradient.
The partially permeable membrane allows small molecules like water through but not larger solute molecules.

Osmosis is the movement of water through a partially permeable membrane from:

A more concentrated to a more dilute solution
A more dilute to a more concentrated solution
Low water potential to high water potential
High solute concentration to low solute concentration

Osmosis Experiments

Common practical: place potato cylinders in distilled water and sucrose solutions of increasing concentration.
Measure mass before and after 20-30 minutes.
If mass increases, water moved into the tissue (solution more dilute than tissue).
If mass decreases, water moved out (solution more concentrated than tissue).
No change means concentrations are equal (no net movement).

If a potato cylinder gains mass after being placed in a solution, the solution is more ____ than the potato cells.

Osmosis in Plants

In a dilute solution (high water potential), water enters plant cells, pushing the membrane against the cell wall → turgid cell.
In a concentrated solution (low water potential), water leaves plant cells → flaccid cell.
Severe water loss causes plasmolysis: cell membrane pulls away from the cell wall.

Which statements about plant cells in solutions are true?

In a dilute solution, plant cells become turgid.
Plasmolysis occurs when water enters the cell.
A flaccid cell has lost water by osmosis.
The cell wall prevents the cell from bursting.

Osmosis in Animals

Animal cells lack a cell wall, so they are more affected by osmosis.
In a concentrated solution, animal cells lose water and become crenated (shrivelled).
In distilled water, animal cells gain water and burst (lysis).

What happens to red blood cells placed in pure water?

They shrink
They burst
They become turgid
No change

Active Transport

Active transport is the movement of particles across a cell membrane from a region of lower concentration to a region of higher concentration.
It requires energy from respiration.
It moves substances against their concentration gradient.

Active transport requires energy from $____ to$ move substances against a concentration gradient.

Importance of Active Transport

Uptake of glucose by epithelial cells in the small intestine and kidney tubules.
Uptake of mineral ions from soil by root hair cells in plants.
Essential for absorbing nutrients when concentrations are lower outside the cell.

Which processes use active transport?

Uptake of glucose by villi
Movement of oxygen into a cell
Uptake of ions by root hair cells
Movement of water by osmosis

Which row correctly describes active transport?

Movement from high to low concentration, no energy needed
Movement from low to high concentration, energy needed
Movement from high to low concentration, energy needed
Movement from low to high concentration, no energy needed

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