How substances get in and out of cells
The cell needs food materials for energy and water and salts for chemicals reactions. Also needs to get rid of other substances (like CO2). There are two ways for pass through the Cell Membrane:
- Needs energy
- Goes from the less concentrated place, to the high concentrated place.
- Eg: Exocytosis, Endocytosis
Passive Transport :
- Does not need energy.
- Goes from the high concentrated place, to the low concentrated place.
- Eg: Diffusion, Osmosis
· Molecules of gas, liquid or a dissolved solution will move from a region where there are a lot of them to regions where there are few of them UNTIL the concentration everywhere is equal.
· When a cell uses up all the O2 molecules, its concentration fall so diffusion takes place and the concentration will be the same again. The same happens with CO2: during Respiration, it’s produced and the concentration goes up. So, by diffusion, the amount of Carbon Dioxide will be the same. Diffusion can explain how takes in oxygen and get rid of CO2.
· The speed with which a substance diffuses depend on:
o The distance it has to diffuse.
o Its concentration inside and outside the cell.
o The size of its molecules or ions.
o The cell walls (in plants) can vary in their thickness and permeability. The thicker the wall, the slower is the rate of diffusion (generally speaking).
o The bigger the difference in concentration, the faster it will diffuse. The difference is called “Concentration gradient” or “Diffusion gradient”.
o The larger the molecules or ions, the slower they diffuse.
o The greater the surface area, the faster is the total diffusion.
§ “Microvillis” (tiny projections which form a “free” surface membrane) increase the absorbing surface.
§ The shape will also affect the surface area.
· The process where water pass through a partially permeable membrane from the dilute solution (weak solution) to the concentrated solution (strong solution) is known as OSMOSIS.
· A partially permeable membrane is a porous membrane that allows water to pass through it more rapidly than dissolved substances (like sugar molecules).
· In living cells, the cell membrane is partially permeable and the cytoplasm and vacuole (in plants cells) contains dissolved substances, so osmosis takes place and water diffuses into the cells (if there are surrounded by a weak solution, if not, water will tend to diffuse out of it).
· When sugar (for example) is dissolved into water, the sugar molecules attract the water ones and reduce the amount of water molecules. So water will diffuse more rapidly from one side (the one which have more free molecules) to the other (the one which more water molecules were “captured” by the sugar molecules). Sugar molecules CAN diffuse opposite as the water does, but as they are bigger and are surrounded by water molecules, they diffuse slower than the water.
· The water potential of a solution is a measure of whether it’s likely to lose or gain water molecules from another solution. Eg: A dilute solution will lose water molecules because they will flow to a concentrated solution (from a high potential to a low potential). Pure water has the highest possible water potential because water will flow to any other solution.
· In some cases, substances are taken into or expelled from the cell against the concentration gradient. This process is described as “Active Transport”. The lack of oxygen or glucose (C6O12H6), prevents active transport taking place. Active transport needs a supply of energy from respiration.
· As the concentration of water outside is greater, it diffuses inside the cell by osmosis
· Water entering the cell will make it swell up, and unless the extra water is expelled in some way, the cell will burst.
· If the cells are surrounded by a solution which is more concentrated than the cytoplasm, water will pass out of the cell by osmosis and the cell will shrink.
· Excessive uptake or loss of water by osmosis may damage cells.
· The water molecules move into and out of the cell. Because there are more of them on the outside, they will move in faster than they move out, and vice versa.
· The cytoplasm and the cell sap in vacuole contain salts, sugars and proteins which reduce the amount of free water molecules.
· The Cell Wall is Freely Permeable to water and dissolved substance but the cell membrane is partially permeable.
· As in animal cells, if the solution outside the cell is more dilute than the one inside, the water molecules will pass into the vacuole by osmosis. So the vacuole will expand and press outwards on the cytoplasm and the cell wall.
· The cell wall of a mature plant cell cannot be stretched much, so there comes a time when the inflow of water is resisted by the cell wall.
· A plant cell with a vacuole pushing out on the cell wall is said to be turgid and the vacuole is exerting turgor pressure.
· If the vacuole loses water, the cells will lose their turgor and become flaccid. The cell is said to be PLASMOLYSED when it shrinks but the cell wall does not, so the GAPS between the cell wall and the cell membrane are filled with the water that goes in by osmosis. If the cell wall shrinks too, the cell is said to be a WILTED CELL.