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Osmosis and Animal Cells

The cytoplasm inside the cell isa a fairly concentrade solution .The protein and many other substances dissolved in it are too large to get through the cell membrane .Water molecules ,though ,can get through .The concentrated solution is the cytoplasm and the partiallly permeable membrane is the cell surface membrane .Therefore ,osmosis will occur .

Water molecules will diffuse from the dilute solution into the concentrated solution . As more and more water enters the cell ,it swells .The cell membrane has to stretch as the cell gets bigger ,until eventually the strain is too much ,and the cell bursts .And animal cell in a concentrated solution .if this solution is more concentrated than the cytoplasm ,then water molecules will diffuse out of the cell .As the water molecules go out through the cell membrane ,the cytoplasm shrinks .The cell shrivels up .

Osmosis and Plant Cells

Plant cells do not burst in pure water .Plants cell are surrounded by a cell wall .This is fully permeable ,which means that it will let any molecules go through it .
Although it is not easy to see ,a plant cell also has a cell surface membrane just like an animal cell . The cell membrane is partiallly permeable .A plant cell in pure water will take in water by osmosis through its partially permeable cell membrane in the same way as an animal cell .As the water goes in ,the cytoplasm and vacuole will swell .
However ,the plant cell has a very strong cell wall around it .The cell wall is much stronger than the cell membrane and it stops the plant cell from bursting .The cytoplasm presses out againts the cell wall ,but the wall resist and presses back on the contents .

Enzymes Have Certain Properties

Enzymes are specific ,this means that each kind of enzymes will only catalyse one kind of chemical reaction .Most chemical reactions happen faster at higher temperatures .This is because the molecules have more kinetic energy -  they are moving around faster ,so they bump into each other more frequently .This means that at higher temperatures an enzyme is likely to bump into its substrate more often than at lower temperatures .They will also hit each other with more energy ,so the reaction ia more likely to take place .



Chlorophyll Absorbs Sunlight

However ,sunlight shining into water and carbon dioxide still will not make them  react together to make glucose .The sunlight energy has to be trapped ,and then used in the reaction .Green plants have a substance which does this .It is called Chlorophyll .
Chlorophyll is the pigment which makes plants look green .It is kept inside the chloroplasts of plants cells .When sunlight falls on a chlorophyll molecule ,the energy is absorbed .The chlorophyll molecule then releases the energy .The energy makes carbon dioxide combine with water ,with the help of enzymes inside the chloroplast .The glucose that is made contains energy that was originally in the sunlight .So ,in this process ,light energy is converted to chemical energy .

Used of Glucose

One of the first carbohydrates to be made in photosynthesis is glucose .There are several things that may then happen to it .Energy may be released from glucose in the leaf .All cells need energy ,which they obtain by the process of respiration .Some of the glucose which a leaf makes will be broken down by respiration ,to release energy .

Iodine Solution can Stain Starch in Leaves

Iodine solution is used to test for starch .A blue-black colour shows that starch is present .However ,if you put iodine solution into a leaf which contains starch ,it will not immediately turn black .This is because the starch is inside the chloroplasts in the cells .The iodine solution cannot get through the cell membranes to reach the starch and react with it .
Another difficulty is that the green colour of the leaf and the brown iodine solution can look black together .



Sumber : Mary Jones and Geoff Jones .IGCSE Biology Coursebook Second Edition .  
              Cambridge University Press .