Diffusion and osmosis

A membrane is said to be permeable to a substance if the substance can pass through it and impermeable if it cannot pass through.

A selectively permeable membrane allows some but not all substances to pass through e.g. Visking tubing is an artificial membrane used in the lab, it is selectively/semi-permeable.

Membranes allow molecules such as water, oxygen, and carbon dioxide to pass through freely. They do not, however, allow sugars, proteins, and salts to pass through easily. Cells have specialised mechanisms in their membranes to allow these substances to pass through when necessary. This movement is facilitated by ACTIVE TRANSPORT.

There are two ways in which substances move in and out of cells:A) DiffusionB) Active transport

DIFFUSION is the spreading out of molecules from a region of high concentration to a region of low concentration.Diffusion is said to take place along a concentration gradient. It is caused by the kinetic energy of molecules. This movement does not require external energy and is said to be passive.

e.g. O2 levels are high in the lungsBlood from the pulmonary arteries is low in oxygenWhen the blood from the pulmonary artery reaches the alveoli in the lungs, the high levels of oxygen diffuses from the lungs to the blood. Similarly, the high levels of CO2 from the blood passes into the alveoliAdaptations to improve gas exchange:a) Alveoli have thin wallsb) Alveoli are numerousc) Alveoli are moistd) Capillaries have thin wallse) Capillaries are numerousThe rate of diffusion in the lungs depends ona) large, moist surface areab) Concentration gradient By increasing the surface area, more substances can diffuse across it (numerous alveoli in the lungs) The difference in the concentration gradient of a substance will affect the movement of the substance (In the lungs, the volume of O2 is high, whereas in the blood it is very low - O2 diffuses rapidly

Solvent --> A solvent is a liquid that dissolves other substances. Water is the most common biological solvent.Solute --> A solute is a substance that has been dissolved. Salt or sugar dissolved in water are both solutes.Solution --> The mixture of the solvent and the solute is a solution (e.g. salty water)

Osmosis is the diffusion of water molecules across a semi-permeable membrane from a region of high water concentration to a region of low water concentration.Osmosis is a special type of diffusion. Like diffusion, it is also passive.e.g. Absorption of water by plantsRe-absorption of water by the proximal and distal convaluted tubules of the kidneys.The turgor pressure of plant cells gives support to the non-woody cellsOpening and closing of stomotaAbsorption of water by the stomach, small intestine, and the colonFlow of water in the amoeba.

Hypotonic solution --> higher concentration of water than the other solutionHypertonic solution --> lower concentration of water than the other substanceIsotonic solution --> equal concentration of water to the other solution

OSMOSIS AND ANIMAL CELLS- Animal cells in an isotonic solutionWater will move in and out through the cell at the same rate (volume remains the same). Seawater has the same concentration as the cytoplasm of many animals that live in the sea. The cells of most land animals are surrounded by tissue fluid that has the same concentration as the cells. In the same way, one of the functions of the kidneys is to ensure that plasma, the liquid portion of out blood, has the same concentration as our blood cells.-Animal cells in a hypotonic solutionGains water due to osmosis. The cell will enlarge and may burst and die. Amoeba --> contractile vacuole. Amoeba species that live in seawater do not have contractile vacuoles. This is because the seawater has the same concentration as their cells (isotonic)-Animal cells in a hypertonic solutionWater moves out of the cell into the external environment causing the cell to shrivel and die (crenation)

Active transport allows larger molecules to enter and leave cells, it differs from diffusion in that it requires energy and goes against a concentration gradient

OSMOSIS AND PLANT CELLS-Plant cells in a hypotonic solutionPlant cells gain water by osmosis. This is how plant roots absorb water from the soil. Water enters the vacuole and the plant cell swells slightly, but the cell wall is relatively rigid and prevents the cell from swelling too much. The vacuole will continue to swell until the cell is fully turgid i.e. it has turgor pressure Turgor pressure in the pressure of the cytoplasm and vacuole against the cell wall of a plant.House plants wilt as a result of lack of turgor pressure. Plants that do not have wood get their strength because their cells are fully turgid.-Plant cells in a hypertonic solutionPlant cells lose water due to osmosis. The vacuole and cytoplasm shrivel and the cell membrane moves away from the cell wall. The cell loses turgor pressure. As a result, it is not as strong as it was and is said to be flaccid (limp) When all the cells are plasmolysed, the plant wilts. Reversal of plasmolysis involves placing the plant cells in a hypotonic solution --> a process referred to as deplasmolysis.

Osmosis and food preservationBacteria - has a polysaccharide / protein cell wallFungi - Has a cell wall made of chitinOsmosis can be used to prevent the growth of these organisms and thus prevent food spoilage e.g. salting bacon/fish or preserving fruit or jam by adding sugarThe high concentration of sugar or salt means that water moves out of the bacteria/fungi by osmosis. The lack of water inside these organisms means that their enzymes can no longer function --> death of the microbe. This ensure that the food remains free from microbial contamination.

Diffusion

Osmosis

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