BIOLOGY GE osmosis兩條問題

1. when there's equilibrium, the rate of in-flux of water = rate of out-flux of water. net flux of water is zero.
the difference in water potential (due to dissolved solute) will drive water to go into the dialysis tubing -- however, the weight of portion of solution above beaker's water level will drive the water inside tubing to go out. these two force will balance after some time.

for water level to fall, there's net outflux of water. but at equilibrium, there's no net (out-)flux of water. therefore, water level will NOT fall.


2. osmosis determines the water potential and concentration of solutes inside cells, thus controlling the rate of metabolism of cells.
also, human and plants absorb water by means of osmosis. water potential inside cells is lower than the water inside intestine / inside soil, thus water will flow into the cells, and transported by blood / via xylem.

active transport is important to transfer materials efficiently, especially when the concentration gradient is small or even reversed.
in small intestine, nutrients are actively transported into blood stream, increasing rate of absorption. otherwise, the absorption would be too slow and inefficient.

active transport is also important in transferring substances from places of low concentration to high concentration. for example, nutrients like glucose and amino acids are reabsorbed after ultrafiltration, though the concentration of these substances in the filtrate is higher than that in blood.



2010-10-13 22:41:02 補充：
你有問入面係sucrose 時會點咩? 我當你"... hydrostatic force 咩?" 之後的句子係比較詳細的說明, 我先淨答後半部咋.

2010-10-13 22:42:17 補充：
不過既然下面的仁兄已經做過補充, 我亦無謂拾人牙慧. 祝好!

I don't think the gentleman above answer the question...

I presume that you understand the basic of osmosis, osmosis is about the diffusion of water.

What are the factors that affect osmosis?

firstly, the osmotic gradient, i.e the different in concentration of the separated solution. When the different is high, the rate of osmosis is high, when the different is small, the diffusion rate is low. There are other term to describe this factor, eg osmotic potential different, osmotic pressure etc.

Secondly, the permeability of tubing, high permeability = high diffusion rate
low permeability = low rate...

for your question...
1) when you vertically hold the tubing about water level, it will generate hydrostatic pressure, what will happen is the water will pass though the barrier to the tube by osmosis and at the same time force back to the beaker by hydrostatic pressure, an equilibrium will reach when the
osmotic pressure = hydrostatic pressure.

if the tube is open at the top, if you add more sucrose to the tube, the solution level will rise, and suck up more water from the beaker!

2) in the biological system, osmosis and active transport play import role in
i) absorption of nutrients
ii) ensure nervous transmission
iii) maintain ionic, pH, water balance of the body

it is important in order to sustain LIFE!