Diffusion, Osmosis and Active Transport Flashcards
What is diffusion?
Diffusion is the random net movement of particles form as area of high concentration to an area of lower concentration, along the concentration gradient
What is osmosis?
The net movement of water molecules from an area of high water potential to an area of lower water potential, through a partially permeable membrane, down the water potential gradient (from dilute to a concentrated solution)
What is a partially permeable membrane?
- A selectively permeable membrane is a membrane that allows only certain materials to cross it
- Materials pass through the pores in the membrane (only tiny molecules can pass through these small holes like water and not bigger molecules e.g. sucrose
- A cell membrane is a partially permeable membrane
Describe how water moves into and out of cells by osmosis
- Tissue fluid surround the cells in the body, it is basically just water with oxygen and glucose dissolved in it. It is squeezed out of the blood capillaries to supply the cells with everything they need
- The tissue fluid will have a different concentration to the fluid inside to cell. This means that the water will either move into the cell from the tissue fluid or out of the cell, by osmosis
- If a cell is short of water, the solution inside it will become quite concentrated. This usually means the solution outside is more dilute, and so water will move into the cell by osmosis
- If a cell has lots of water, the solution inside it will be more dilute, and water ill be drawn out of the cell and into the fluid outside by osmosis
Describe how plants are supported by turgid cells
- When a plant is well watered its cells will draw water in by osmosis and become plump and swollen. When the cells are like this, they are said t be turgid
- The contents of the cell push against the cell wall, this is called turgor pressure. Turgor pressure helps support the plant tissue
- If there is no water in the soil, a plant starts to wilt. This is because the cells start to lose water and so lose their turgor pressure. The cells are then said to be flaccid
- The plant does not completely lose its shape though, because the inelastic cell wall keeps things in position. It just droops a bit
Where does diffusion happen?
In both liquids and gases, because the particles in these substances are free to move about
What affects diffusion?
- The bigger the difference in concentration, the faster the diffusion rate
- Larger SA to volume ratio, diffusion will be quicker
- Temperature
What are examples of diffusion?
- CO2 into alveoli
- Oxygen into blood
- Glucose from blood to cells
What is special about cell membranes?
Only very small molecules can diffuse through cell membranes, like glucose, amino acids, water and oxygen, big molecules like starch and proteins can’t fit through the membrane
How can you investigate diffusion in a non-living system?
- First make up some agar jelly with phenolphthalein (indicator) and dilute sodium hydroxide, this will make the jelly pink
- Then fill a beaker will some dilute HCL and using a scalpel, cut out a few cubes from the jelly and put them in the beaker of acid
- If you leave the cubes for a while they will eventually turn colourless as the acid diffuses into the agar jelly and neutralises the sodium hydroxide
- You can investigate the rate of diffusion by using different sized cubes of agar jelly and timing how long it takes for each cube to go colourless. The cube with the largest surface area to volume ratio will lose its colour quickest
How can you investigate osmosis in a living system?
- Cut up potatoes into identical cylinders and get some beakers with different sugar solutions in them
- One should be pure water another should be very concentrated sugar solution and then have a few other with concentrations in between
- Measure the length of each cylinder, then leave a few cylinders in each beaker for half an hour
- Then take them out and measure their lengths again
- If the cylinders have drawn in water by osmosis, they will be a bit longer and if water has been drawn out they have shrunk a bit
- Keep volume of solution and time experiment run for the same to be a fair test
How can you investigate osmosis in a non-living system?
- Tie a piece of wire around one end of some Visking tubing and put a glass tube in the other end (Visking tubing is a partially permeable membrane), fix the tubing around it with wire
- Then pour some sugar solution down the glass tube int the Visking tubing
- Put the Visking tubing into a beaker of pure water, measure where the sugar solution comes up to on the glass tube
- Leave the tubing overnight, then measure where the liquid is in the glass tube. Water should be drawn into the Visking tubing by osmosis and this will force the liquid up the glass tube
In the potato practical how do you work out percentage change?
end mass-initial mass / initial mass x 100
-Important to use percentage change as starting mass for potatoes would not have been the same
What are examples of osmosis?
-Plant roots transporting water
What is active transport?
The movement of particles against the concentration gradient, from an area of lower concentration to an area of higher concentration using energy released during respiration (ATP)
