Movement of substances into and out of cells

Specification:

2.12 recall simple definitions of diffusion, osmosis and active transport

2.13 understand that movement of substances into and out of cells can be by diffusion, osmosis and active transport

2.15 understand the factors that affect the rate of movement of substances into and out of cells to include the effects of surface area to volume ratio, temperature and concentration gradient

2.16 describe simple experiments on diffusion and osmosis using living and non-living systems

Specification:

2.12 recall simple definitions of diffusion, osmosis and active transport 

Diffusion: this is the net movement of fluid molecules from a region of high concentration to a region of low concentration. i.e down a concentration gradient. The steeper the gradient, as in the bigger the difference in concentration, the faster diffusion will occur. Examples include if you spray perfume in a corner of the room, it will slowly spread out until you can smell it from the opposite side of the room. This is because the particles move randomly and continuously collide with each other until they spread out evenly. In our body, diffusion occurs when oxygen from the alveolus in the lungs diffuses into the red blood cells which have a lower concentration of oxygen, since it transports it to body cells and 'gives it away'.

Osmosis: this is like diffusion, except it involves water molecules. So again, water molecules move via osmosis from a region of high concentration to a region of low concentration, through a partially permeable membrane. In our body cells, the cell surface membrane is partially permeable, its function is to control what is allowed to enter the cell.

Sometimes you read a definition on osmosis regarding water potential. And this confuses a lot of people. People think that it's from a region of low water potential to high water potential, since a region without much water should have water going to it right? Wrong.

Water potential is a measure of the tendency of water to move from one place to another. So a place without much water would have a low water potential, it's not going anywhere. So the definition is still the same, just switch 'concentration' for 'water potential'.

So osmosis is the movement of water molecules from high to low water potential i.e. down a water potential gradient, through a partially permeable membrane.

So a high concentration of water would be a dilute salt solution, and a low concentration of water would be a concentrated salt solution.

Likewise, a dilute salt solution has a higher water potential, and a concentrated salt solution has a lower water potential.

Active transport: This is the active uptake of molecules against a concentration gradient using ATP (adenosine triphosphate) or if you don't think you can remember this, just say using energy. Against a concentration gradient just means from a region of low concentration to a region of high concentration-unlike diffusion/osmosis. Carrier proteins transport the molecules from one side of the membrane to the other side .

This occurs in root hair cells when they actively uptake mineral ions from the soil even though there is a greater concentration in the root hair cells.

2.13 understand that movement of substances into and out of cells can be by diffusion, osmosis and active transport

Diffusion: e.g. oxygen diffusing into red blood cells, or carbon dioxide diffusing into leaves for photosynthesis

Osmosis: e.g. when water diffuses into plant cells it makes the cells turgid, which provides the plant with support so it can stand upright. If water diffuses out of the cell, it becomes flaccid and wilts. The cell is turgid because the water entering the cell makes the cytoplasm and the vacuole push against the cell wall, exerting turgor pressure. In animal cells, there isn't a cell wall so if too much water enters the cell, it may burst-called lysis. 

Active transport: e.g. root hair cells actively uptaking mineral ions such as magnesium for chlorophyll. In humans, in our kidneys, salts are actively uptaken into the blood.

2.15 understand the factors that affect the rate of movement of substances into and out of cells to include the effects of surface area to volume ratio, temperature and concentration gradient

Surface area to volume ratio: root hair cells have a high surface area to volume ratio, so it increases the rate of diffusion/osmosis.

Temperature: temperature increases the kinetic energy of the particles, so diffusion occurs quicker.

Concentration gradient: The steeper it is, i.e. the greater the difference in concentration between 2 regions, the faster the rate of diffusion/osmosis.

2.16 describe simple experiments on diffusion and osmosis using living and non-living systems

BlogThis!