[STPM] [BIOLOGY] [NOTES] CHAPTER 1 : BASIC CHEMISTRY OF A CELL (PART 2)
Thursday, May 8, 2014 | 2:30 AM | 0 comments
Here comes our Part 2 guys!!!
In Part 1 we discuss about water, and in Part 2 we're going to learn the second subtopic, that is carbohydrates.
Okay, let's introduce our dear carbohydrates or simplify name carbs.
Well, actually carbohydrates are substance which contain element C(carbon) H(hydrogen) and O(oxygen) with a certain ratio.
So, carbohydrates can be found in 3 forms :
First is monosaccharide, which is the simple sugar.
Second is disaccharide, which is 2 simple sugars combine through process condensation.
Third is polysaccharide, and this one is the complex sugar.
After that, let us take a look at our monosaccharide. Generally, monosaccharide is the monomer for carbohydrates. They can be categorized using 2 methods.
First, by determining the number of carbon atoms. (Like what we learnt in SPM Chemistry)
Example is like triose for three carbon atoms, pentose for five carbon atoms and hexose for six carbon atoms.
Secondly, is by determining the carbonyl groups. Carbonyl groups are either aldehyde or keto group. Well, what's the difference again between these two groups? Aldehyde group is CHO, while keto group is CO. And both of them have reducing properties, making all monosaccharides reducing sugars.
Reducing means that when these sugars are tested using Benedict's solution, the copper (II) ions in this solution is reduced to copper(I) ions. Blue Benedict's solution turns brick red.
The general formula for monosaccharide is :
Pentose or hexose sugar can form ring structure which is more stable. And, ***do remember, only ring structure can be used to make disaccharides and polysaccharides.
Usually first carbon atom in aldose sugar attached to carbonyl group while in ketone sugar is attached to second carbon atom. What does that mean?
After that, it's about the importance or functions of monosaccharides.
We've done with monosaccharide, let's continue with our disaccharide.
These are formed when two monosaccharides are condensed together. One monosaccharide loses an H atom from carbon atom number 1 and the other loses an OH group from carbon 4 to form the bond.
The reaction, which is called a condensation reaction, involves the loss of water (H2O) and the formation of an 1,4-glycosidic bond. Depending on the monosaccharides used, this can be an α-1,4-glycosidic bond or a β-1,4-glycosidic bond.
The reverse of this reaction, the formation of two monosaccharides from one disaccharide, is called hydrolysis reaction and requires one water molecule to supply the H and OH to the sugars formed.
Examples of Disaccharides
Sucrose: glucose + fructose,
Lactose: glucose + galactose,
Maltose: glucose + glucose.
Sucrose is used in many plants for transporting food reserves, often from the leaves to other parts of the plant. Lactose is the sugar found in the milk of mammals and maltose is the first product of starch digestion and is further broken down to glucose before absorption in the human gut.
Finally, it's our last one, polysaccharide!!!
Examples of polysaccharides:
1.)Starch
2.)Glycogen
3.)Cellulose
Let's introduce the starch first :)
Finally, it's our last one, polysaccharide!!!
Examples of polysaccharides:
1.)Starch
2.)Glycogen
3.)Cellulose
Let's introduce the starch first :)
Function of starch:Main storage polysaccharide in plants.
Structure of starch:Made of 2 polymers - amylose and amylopectin.
Amylose: a polymer of glucoses joined by α-1,4-glycosidic bonds. Forms a helix with 6 glucose molecules per turn and about 300 per helix.
Amylopectin: a polymer of glucoses joined by α-1,4-glycosidic bonds but with branches of α-1,6-glycosidic bonds. This causes the molecule to be branched rather than helical.
Function of glycogen:Main storage polysaccharide in animals and fungi
Structure of glycogen:Similar to amylopectin but with many more branches which are also shorter.
Function of cellulose:Main structural constituent of plant cell walls
Structure of cellulose:Adjacent chains of long, unbranched polymers of glucose joined by β-1,4-glycosidic bonds hydrogen bond with each other to form microfibrils.
Functions of carbohydrates
- Substrate for respiration (glucose is essential for cardiac tissues).
- Intermediate in respiration (e.g. glyceraldehydes).
- Energy stores (e.g. starch, glycogen).
- Structural (e.g. cellulose, chitin in arthropod exoskeletons and fungal walls).
- Transport (e.g. sucrose is transported in the phloem of a plant).
- Recognition of molecules outside a cell (e.g. attached to proteins or lipids on cell surface membrane).
Notes by Gemini
Image from Google
