MEMBRANE PERMEABILITY

PERMEABILITY

The membrane must allow water molecules to diffuse through. It is permeable to water.

• If a concentrated solution is separated from a dilute solution by a suitable membrane, water will pass from the dilute to the concentrated solution.

In fact, water passes both ways but faster from the dilute to the concentrated solution.

The membrane allows only molecules of a certain size to diffuse through it, it is called selectively permeable.

The cell membrane functions as a selectively permeable membrane.

The cell sap and cytoplasm function as fairly concentrated solutions.

In a plant cell, the cell membrane acts as a selectively permeable membrane

The cell wall is freely permeable to water The vacuole contains a solution of salts and sugars If there is water outside the cell, it will diffuse by

osmosis into the vacuole The vacuole will expand, pushing the cytoplasm

outwards against the cell wall

Most common type of passive transport.

Diffusion – is the random movement of particles (atoms, ions, molecules) from a region of high concentration to low concentration, down a concentration gradient.

Molecules diffuse down a concentration gradient.

Diffusion stops when molecules dispersed evenly (with no concentration gradient), and a state of equilibrium is reached.

Diffusion

Process of diffusion

Dissolved substance diffuse throughout liquid in which they are dissolved.

Diffusion is important for:

Gaseous exchange (oxygen, carbon dioxide) during respiration and photosynthesis

Excreting waste products e.g. ammonia, water, mineral salts

Absorption of digested food into blood through walls of small intestine.

Enables animals to detect food by smell.

Why is diffusion important?

Osmosis

A form of passive transport process

Osmosis – diffusion/movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.

A partially/selectively permeable membrane only allows certain molecules to pass through it but not others.

What is the difference between diffusion and osmosis?

Osmosis is the net movement of water molecules across a Partially-permeable membrane.

Water molecules move randomly with a certain amount of kinetic energy…

Distilled water separated by a partially-permeable membrane:

Water molecules are moving from one side of the

membrane to the other but there is no net osmosis

If a substance is dissolved in water, the kinetic energy of the water

molecules is lowered.

This is because some water molecules aggregate on the

surfaces of the other molecules…

Osmosis demonstration

The osmotic potential of a cell is known as its WATER

POTENTIAL. For animal cells, the water potential is the osmotic potential of the

cytoplasm.

Osmosis and Plant cells

In plant cells, cell sap contains dissolved salts and sugar. If cell sap has lower water potential than that of surrounding solution, water enters by osmosis.

Plant cell will swell and become firm / turgid.

Plant cell walls prevent cells from bursting.

Turgor pressure - outward pressure which cell sap exerts against inside wall of cell.

Turgor helps to support soft tissues in plants

Osmosis and Plant cells

If cell sap has higher water potential than surrounding solution, water moves out of the vacuole and cytoplasm shrinks away from the cell wall.

Cell loses its turgor, shrinks and becomes flaccid or soft. The cell becomes plasmolysed.

Plasmolysis - shrinkage of cytoplasm away from the cell wall when plant cells are immersed in a solution of low water potential.

Plasmolysis causes land plants to wilt, in non-woody parts of plants e.g. leaves, shoots

Hypotonic - Dilute solution A ( higher water potential) compared to concentrated sugar solution B ( lower water potential)

Hypertonic - Solution B has water potential compared to solution A

Isotonic - when both solutions have the same water potential (‘iso’: same as; ‘tonicity’: strength of solution).

Dilute vs Concentrated solutions

A B

Osmosis in plant and animal cells

Osmosis and Plant cells

Plasmolysis

Plasmolysis is the loss of water from the cell by osmosis,and this is evident when the cell contents pull away from the rigid cell wall as the water moves out.

Steps of Plasmolysis

Wilting due to loss of water!!

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