Transport: diffusion, osmosis and active transport

- Diffusion is the net movement of particles from a region of high concentration to a region of low concentration, down a concentration gradient.
- Diffusion is a consequence of the random movement of individual particles.

- Diffusion is passive and does not require energy - ATP
- Diffusion continues till equilibrium is reached. This is when there's no longer a concentration gradient; particles will still be moving.

What increases the Rate of Diffusion?

1) Increasing temperature
2) Increasing surface area
3) Increasing concentration gradient
4) Decreasing the diffusion distance

- Diffusion happens because molecules are always moving and bumping into each other. These are called random collisions. When a molecule collides with another, it moves further away, allowing more liquid to spread out.
- When gases/liquids are more concentrated, they diffuse faster. This is because molecules are closer and hit each other more often.
- As diffusion proceeds, the rate decreases. Because molecules are less concentrated, they hit each other less.
- As the temperature rises, the molecules move faster, gain kinetic energy, and collide more frequently, increasing the rate of diffusion.

Diffusion in and out of Cells:

- Cell membranes are partially permeable. Water, carbon dioxide, and oxygen can pass through, but starch can't.
- Carbon dioxide and oxygen are the gases exchanged in the lungs. Alveoli's large surface area, short diffusion distance, and maintenance of a steep concentration gradient ensure efficient gas exchange (by diffusion).
- Carbon dioxide diffuses into a leaf via the stomata. During the day, oxygen will diffuse out of the leaf. At night, carbon dioxide doesn't diffuse into the leaf.
- Absorption of digested food happens in the small intestines. It is adapted to ensure absorption is digested well (by diffusion), as it has lots of villi. The large surface area and short diffusion distances maintain a steep concentration gradient.

Osmosis

- Osmosis: The net movement of water particles from an area of high water potential to an area of low water potential across a partially permeable membrane.
- Osmosis is a special type of diffusion and only involves the random movement of water molecules.
- Osmosis occurs across partially permeable membranes, which have holes that only allow small molecules to pass through. Cell membranes are partially permeable.

- The diluted sugar solution exhibits a high water potential. It contains a large number of water molecules and a relatively small number of solute molecules.
- A concentrated solution has a low water potential.
- In osmosis, water will move from a dilute solution to a concentrated solution.

Comparison:

Osmosis	Diffusion
Particles move randomly.	Particles move randomly.
Water moves down a concentration gradient.	Substances move from an area of high concentration to low concentration.
Involves a partially permeable membrane.	Does not require a membrane.
Needs water for movement.	Does not need water for movement.
Does not require ATP (energy).	Does not require ATP (energy).

Experiments

- Visking tubing is an artificial, partially permeable membrane.
- Its microscopic holes allow the passage of smaller molecules such as glucose and water.
- Larger molecules like starch and sucrose cannot pass through.

1) Water partially submerges the visking tubing, causing the liquid to rise.

- The sucrose solution is hypertonic to the water—it's more concentrated.
- Osmosis makes the water outside move to the sucrose solution inside the visking tubing. This makes the liquid level inside the capillary tube rise.

2) The Viking tubing is half full of water, and the liquid level falls.

- A less concentrated solution is hypotonic to a more concentrated solution, while two solutions at the same concentration are isotonic.

Osmosis in Plants

- Plants rely on the movement of water as root hair cells take in water from the soil by osmosis.

Turgid Cells:

- When water enters a cell and the cell wall prevents it from bursting, we refer to the cell as turgid.
- This indicates that the vacuole is exerting pressure on the cell wall. This is beneficial because it provides support to the stems, resulting in an upright plant.

Flaccid and Plasmolysed Cells:

- When water moves out of the cell vacuole by osmosis, it becomes flaccid. 
- In a flaccid cell, the vacuole is much smaller and takes up less space.
- When a significant amount of water exits the cell, the cytoplasm begins to separate from the cell, a process known as plasmolysis.

Osmosis in Animal Cells

- Animal cells don't have a cell wall. 

Active Transport

- Some substances enter cells by active transport.
- Active transport is the movement of substances into/out of a cell against a concentration gradient using energy (ATP).
- ATP comes from respiration, and since active transport is against the concentration gradient, it goes from low to high concentration.
- For example, plants absorb nitrate ions through their root hairs from the soil water. The concentration of nitrate ions is typically lower in the soil.
- The greater the rate of cellular respiration, the more energy is available for active transport.

- Using active transport, cells can absorb different molecules at different rates.
- They can also select which ones they need and which ones they don't.
- Special carrier proteins in the cell membrane move the substances into/out of the cells using ATP. Different carriers take different substances. A carrier that moves glucose will not move calcium ions.

3) Then, using energy, the protein rotates in the membrane and releases the molecule inside the cell.
4) Then the protein rotates back again using energy.

Transport Glossary

- Concentration gradient: The variation in a substance's concentration from one region to another.
- Equilibrium: At this point, particles or water molecules move at equal rates in both directions, ending net diffusion or osmosis.
- Turgid: A plant cell that has absorbed as much water as possible through osmosis, with the cell wall resisting further uptake.
- Turgor pressure: Pressure acting against an inelastic cell wall.
- Flaccid: A plant cell that has lost some water by osmosis.
- Plasmolysed: A plant cell that has lost lots of water by osmosis, and the cytoplasm shrinks and moves away from the cell wall.
- Lysis: An animal cell undergoes this process when it takes up water by osmosis, swells, and bursts.
- Crenated: An animal cell becomes this when it loses water by osmosis.