Topic 3 [Movement In and Out of Cells]

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There are two main processes that substances can undergo to move across a membrane:

: Does NOT require energy from breaking down of ATP

: Requires energy from breaking down of ATP

The of from a region of to a region of down a through a until .

**Note that 'higher concentration' and 'lower concentration' are comparative words.**

in of a in compared to .

The concentration of a particular substance is the same in and there is no more of molecules.

Diffusion happens as a result of the kinetic energy that come from the random movement of particles.

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- Gaseous exchange in the alveoli

- Gaseous exchange in photosynthesis

- Absorption of digested food through the small intestine

- Absorption of oxygen by unicellular organisms

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: The steeper the concentration gradient, the higher the rate.

: The shorter the distance, the higher the rate.

: The higher the temperature, the higher the rate.

: The bigger the surface area, the higher the rate.

: The from a region of to a region of , down through a until is achieved.

> Higher water potential = Diluted Solution

> Lower water potential = Concentrated Solution

:

of in a .

: Solute concentration is the same in both regions, so there the water potential is the same and there is no net movement of molecules.

: Solute concentration is lower (In the solution that is being discussed), so it has higher water potential. It is therefore a dilute solution.

: Solute concentration is higher (In the solution that is being discussed), so it has lower water potential. It is threfore a concentrated solution.

:

:

- The solution has the same water potential as inside the cell.

- The cell is in equilibrium, so there is no net movement of water across the cell membrane, and therefore no change in the size of the cell.

:

- The solution has higher water potential than inside the cell.

- The cell is not in equilibrium and water diffuses into the cell by osmosis down water potential gradient. This causes an increase in the size of the cell.

- The cell swells and eventually bursts, which is called .

:

- The solution has lower water potential than inside the cell.

- The cell is not in equilibrium and water diffuses out of the cell by osmosis down water potential gradient. This causes a decrease in the size of the cell.

- The cell shrinks and eventually becomes .

:

:

- The solution has the same water potential as inside the cell.

- Water diffuses into and out of the cell at equal rates, causing no net movement in water across the cell membrane. There will be no change in the size of the cell.

:

- The solution has higher water potential than inside the cell.

- Water diffuses into the cell by osmosis down water potential gradientThe cytoplasm pushes against the cell wall, and turgor pressure increases, causing the plant cell to be .

:

- The solution has lower water potential than inside the cell.

- Water diffuses out of the cell by osmosis down water potential gradient. The cytoplasm pulls away from the cell wall, and the cell becomes , or rather it undergoes .

:

- Molecules or ions move concentration gradient.

- (Molecules/Ions) move from a region of lower concentration to a region of higher concentration.

- Active transport requires enegy from ATP, which is synthesised by respiration.

- Active transport involves carrier proteins in the cell membrane.

E.g Ion uptake by root hairs and uptake of glucose by epithelial cells of villi and kidney tubules.

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- A particular molecule attaches itself to the binding site of a carrier protein for that olecule.

**Note that active transport is specific.**

- Energy is provided to the carrier protein in the form of ATP molecules.

- The carrier protein and delivers the molecule across the cell membrane .

(In root hair of plants)

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- Inside the root hair vacuole there is a more concentrated solution of sugars and other dissolved substances. The cells actively transport mineral ions into the root hair cell, and across cell to cell .

- This causes a lower concentration of water molecules inside the root hair cells as there is now a higher concentration of minerals and sugars. Water from the soil then passes into the root hair cell, and across cell to cell by .

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