Biology - Chapter 13: Diffusion and Osmosis

diffusion

the movement of molecules from an area of high concentration to an area of low concentration (until the concentrations are equal)

passive process

no energy is required and is just a natural process

examples of diffusion

lungs: gas exchange in the alveoli when oxygen is absorbed and co2 is released

leaf: where oxygen and co2 move in an out of the leaf through tiny openings called stomata, allowing for photosynthesis and respiration

osmosis

the movement of water across a selectively permeable membrane from an area of high water concentration to an area of low water concentration. It is also a passive process

examples of osmosis

• amoeba - osmoregulation

• nephron in the kidney - water reabsorption

• plant roots - water absorption

osmoregulation in ameobas

this is how it controls the balance of water inside the cell. since amoebas live in freshwater, water constantly flows into the cell by osmosis since inside, it has a high salt concentration. To prevent the cell from bursting, they have a special organelle called a contractile vacuole, which collects excess water and regularly pumps it out of the cell to help the amoeba maintain a stable water balance

experiment to demonstrate osmosis

• soak 2 lengths of visking tubing in water

• take it out, open it up and then tie a know at one end of the visking tubing

• fill the tubing with concentrated sucrose solution and the other with distilled or tap water (control) and then tie a know at the other end of the visking tubing

• dry it and weight it out, then record

• place the tubings in a beaker of distilled water and leave it for about 30 minutes

• remove the bags and then dry them

• note and record the fullness and mass of each bag and record the results

visking tubing

a selectively permeable membrane, where only water can pass freely and not sucrose

results of osmosis experiment

the visking tubing containing the sucrose solution will have filled with water so that it has gained mass and is more full. This is due to water entering the tubing as a result of osmosis.

the visking tubing containing the water shows no change in mass

osmosis in plant cells

if placed in pure water, the plant cell vacuole will swell with water. the swollen vacuole will push against the cell wall and the plant becomes turgid

if placed in a salty solution, it will lose water from its vacuole, the cell membrane shrinks, the cell becomes plasmolysed and the plant becomes limp

observing a turgid and plasmolysed onion cell

Prepare a sample of dark onion epidermis and view under a microscope at medium power. Then add a drop of 5% salt solution and put a coverslip on top. Observe the cell membrane pulling away from the cell wall due to it being plasmolysed

plasmolysation

where the cell’s water content is lost due to osmosis and occurs when a cell is placed in a highly concentrated solution

hypotonic solution

low concentration of solute compared to the cell, causing water to enter the cell, leading to swelling.

isotonic solution

equal concentration of solute compared to the cell

hypertonic solution

high concentration of solute compared to the cell, causing the cell to lose water and shrink

when hypotonic?

when hypertonic?

cells become turgid

cells become plasmolysed

osmosis in animal cells

• animal cells must be isotonic to be balance

• in hypotonic solution, the cells will swell and burst

• in hypertonic solution, the cells will shrivel and shrink

• e.g. RBC in pure water (hypotonic) and in a strong salt solution (hypertonic)

food preservation using high salt and sugar concentration

• salt and sugar is used to preserve food

• therefore, when microorganisms lands on the food, it loses water by osmosis, making them dehydrated and then die

• the water content outside the microorganism is lower than its insides, so it loses water through osmosis

active transport

the movement of molecules from an area of low concentration to an area of high concentration

this process uses energy in the form of ATP

e.g. reabsorption of important nutrients in the kidney through the nephron

selectively permeable

only allows certain molecules to enter and exit the cell