Topic 1b-Transport in Cells Flashcards
What is diffusion?
The movement of substances into and out of cells across the cell membranes.
What is the definition of diffusion?
DIFFUSION is the SPREADING OUT of PARTICLES of any substance in solution, or particles of gas, resulting in a net movement from an area of HIGHER CONCENTRATION to an area of LOWER CONCENTRATION.
What does diffusion occur in?
Diffusion happens in both SOLUTIONS and GASES-that’s because the particles in these substances are free to MOVE ABOUT randomly.
What is the simplest type of diffusion?
The simplest type is when different GASES diffuse through each other.
What factors affect the rate of diffusion?
1) The difference in concentrations (CONCENTRATION GRADIENT). The bigger the concentration gradient, the faster the diffusion rate. This is because the net movement from one side is greater.
2) The TEMPERATURE. A higher temperature will also give a faster diffusion rate because the particles have more kinetic energy, so move around faster.
3) The SURFACE AREA of the membrane. The larger the surface area (e.g. of the cell membrane), the faster the rate of diffusion. This is because more particles can pass through at once.
What substances in the human body are transported by diffusion?
Some of the substances transported in and out of cells by diffusion are oxygen and carbon dioxide in gas exchange, and of the waste product urea from cells into the blood plasma for excretion in the kidney. (on spec!!)
How does a cell membrane help in the process of diffusion?
1) They let stuff move both IN and OUT of the cell.
2) Dissolved substances can move in and out of the cells by DIFFUSION.
3) Only very SMALL molecules can DIFFUSE through cell membranes-things like OXYGEN (needed for respiration), GLUCOSE, AMINO ACIDS and WATER.
4) BIG molecules like STARCH and PROTEINS can’t fit through the membrane.
5) Just like with diffusion in air, particles flow through the cell membrane from where there’s a HIGHER CONCENTRATION to where there’s a LOWER CONCENTRATION.
6) They’re only moving about RANDOMLY of course (gas particles), so they go BOTH ways-but if there are a lot MORE particles on one side of the membrane, there’s a NET (overall) movement FROM that side.
7) The LARGER the SURFACE AREA of the membrane, the FASTER the diffusion rate, because more particles can pass through at once.
How is a single-celled organism adapted for diffusion?
A single-celled organism has a relatively large surface area to volume ratio. This allows sufficient transport of molecules into and out of the cell to meet the needs of the organism.
Why are exchange surfaces important?
-All living organisms rely on exchanges with the environment to survive.
However, diffusion only works efficiently if the distance over which the substances have to diffuse is small and the organism has a large surface area compared to its volume. This is the case for small organisms.
Why are multicellular organisms, surfaces and organ systems specialised for exchanging material?
For larger, more complex organisms – which have a small surface area:volume ratio and a bigger distance from the surface to the cells inside the body - diffusion alone is insufficient to meet the needs of all cells.
As larger organisms evolved, specialised organ systems - with surfaces across which substances could be exchanged efficiently - also evolved. These specialised organ systems were needed in order to transport substances around the organisms.
How are multicellular organisms, surfaces and organ systems specialised for exchanging material?
1) Having a LARGE SURFACE AREA for greater exchange (achieved by having a folded surface.)
2) Having a THIN exchange surface for a SHORT DIFFUSION distance. E.g. a membrane that is thin, to provide a short diffusion path
Animals also further maximise the efficiency of exchange by:
3) Exchange surfaces in ANIMALS have LOTS OF BLOOD VESSELS, to get stuff into an out of the blood quickly.
4) Maintaining VENTILATION of the surface (at gas exchange surfaces) through breathing - this always ensures that a concentration gradient is maintained.
Is energy needed for diffusion to occur?
It is ‘easy’ for molecules to move down the concentration gradient so this is a PASSIVE process, which means that it does not need any energy from respiration.
Define osmosis…
OSMOSIS is the MOVEMENT OF WATER MOLECULES across a PARTIALLY PERMEABLE MEMBRANE from a region of HIGHER WATER CONCENTRATION (a dilute solution) to a region of LOWER WATER CONCENTRATION (a concentrated solution).
What’s a partially permeable membrane?
A PARTIALLY PERMEABLE membrane (like the cell membrane) does not let all types of particles through.
It has small pores/gaps within it which control the movement of particles from one side to the other. It is sometimes called SELECTIVELY PERMEABLE MEMBRANE since it selects which particles can pass through.
The pores are so small, that only tiny MOLECULES (like water) can pass through them, and bigger molecules (e.g. sucrose) can’t.
In which direction do the water molecules pass through the membrane?
The water molecules actually pass BOTH WAYS through the membrane during osmosis. This happens because water molecules MOVE ABOUT RANDOMLY all the time.
Does osmosis require energy?
Osmosis is a type of diffusion, so it is a PASSIVE PROCESS (it does not need energy to occur).
What happens if a cell is put in a solution with a higher water concentration than itself?
If a cell is put in a solution with a higher water concentration than itself, water moves into the cell and it bursts.
What happens if a cell is put into a solution with the same water concentration as itself?
if a cell is put into a solution with the same water concentration as itself, water moves in and out at an equal rate so the cell remains the same.
What happens if a cell is put into a solution with a lower water concentration than itself?
If a cell is put into a solution with a lower water concentration than itself, water moves out of the cell and it shrivels up.
Practical-Observing the effect of sugar solutions on plant tissue
What is the method?
N.b:
1) Osmosis ONLY involves water molecules-nothing else. Osmosis is water diffusion.
2) Osmosis occurs across a barrier such as a cell membrane which is ‘selectively permeable’, which means it only lets through small water molecules.
1) First cut up a POTATO into IDENTICAL CYLINDERS.
And get some beakers with DIFFERENT SUGAR SOLUTIONS in them. One should be PURE WATER and another should be a VERY CONCENTRATED SUGAR SOLUTION (e.g. 1 mol/dm3). Then you can have a few others with concentrations IN BETWEEN (e.g. 0.2 mol/dm3, 0.4 mol/dm3, 0.6 mol/dm3, etc).
2) You measure the MASS of the cylinders, then leave one cylinder in each beaker for 24 hours or so .
3) Then you take them out, DRY them with a paper towel and measure their masses AGAIN.
What does the final mass indicate?
How can you calculate the percentage change?
-If the cylinders have drawn in water by osmosis, they’ll have INCREASED IN MASS. If water has been drawn out, they’ll have DECREASED IN MASS.
You can calculate the PERCENTAGE CHANGE IN MASS, then plot a GRAPH.
- Percentage change=final value - original value
______________________ x 100
original value
What do the results conclude?
The water molecules enter through the partially permeable membrane down their CONCENTRATION GRADIENT. The potato will swell up with the extra water, and so get bigger-and heavier. It’s also really firm, because all its vacuoles are swollen with water. This is called TURGID.
Eventually there will be no difference between the concentrations outside and inside.
The opposite will happen if the potato is put into a concentrated solution, like really syrupy sugar solution-the potato cells lose their water- so the potato is FLACCID.
What are the dependent and independent variable of this experiment?
The DEPENDENT VARIABLE is the CHIP MASS.
The INDEPENDENT VARIABLE is the CONCENTRATION of the sugar solution. All OTHER variables must be kept the SAME in each case or the experiment won’t be a FAIR TEST.
What were the risks involved in this experiment?
Risk assessment:
If you’re using a knife, make sure you cut away from yourself and that the blade is clean and sharp.
What errors could occur?
You need to be aware of how the errors may arise. Sometimes they may occur when CARRYING OUT THE METHOD, e.g. if some potato cylinders were not FULLY DRIED, the excess water would give a HIGHER MASS, or if water EVAPORATED from the beakers, the CONCENTRATIONS of the sugar solutions would change.
