Revision:Carbohydrates, Lipids and Proteins

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Draw the basic structure of a generalised amino acid

COOH

|

C H

|

NH₂

Draw the ring structure of an alpha-D glucose

13

Draw the basic structure of glycerol and a generalised fatty acid

CH₂0H

    |

CH₂OH

    |

CH²OH

Fatty Acid

C_xH_(2x+1)-COOH (saturated)

C_somethingH_bigsomethingCOOH (saturated)

Outline the role of condensation and hydrolysis in the relationships between monosaccharides and disaccharides; fatty acids, glycerol and trigylcerides, amino acids dipeptides and polypeptides

• Monosaccharide ---> Condensation Reaction ---> Disaccharide (Condensation, loss of a single water molecule)
• Disaccharide ---> Hydrolysis ---> Monosaccharide (Hydrolysis gain of single water molecule)

• Fatty Acids + Glycerol ---> Condensation Reaction ---> Fats

• Amino Acid (carboxyl group) + Amino Acid(amino group) ---> Condensation Reaction ---> Dipeptide

• Dipeptides + Amino Acids ---> Condensation Reaction ---> Polypeptide

Draw the structure of a generalised dipeptide, showing the peptide linkage

                                         H   H

                                          \  /

                                              N

                                           |

                                          R₁-C-H

                                           |

                                       OC

                                            |

                                        H-N

                                            |

                                       R₂-C-H

                                             |

                                            C

                                          // \

                                         O     O-H

Explain the relative solubility of carbohydrates, lipids, and proteins in water

• Most lipids are insoluble because they have a long hydrocarbon chain (hydrophobic)
• Carbohydrates are soluble because of the presence of OH groups
• Proteins are also soluble because of OH groups and other groups and charges
• Solubility decreases with size

Compare the energy content of carbohydrates, lipids, and proteins

Lipids have about twice the energy of carbohydrates and proteins in x mass.

List two examples of monosaccharides, disaccharides, and polysaccharides

• Monosaccharides
  • Glucose
  • Ribose
  • Fructose
  • Galactose
• Disaccharides
  • Sucrose
  • Lactose
  • Maltose
• Polysaccharides
  • Glycogen
  • Cellulose
  • Chitin
  • Starch

Polysaccharides: Structure and Function

Polysaccharides are polymers. This means that they are long chains made of several smaller units (monomers; in this case, monosaccharides).

Polysaccharides are formed by condensation: the monomers form (glycosidic) bonds and lose water in the process. This can be reversed by adding water to the polysaccharide (hydrolysis). Polysaccharides come in two varieties: homopolysaccharides (which contain only one kind of monomer) and heteropolysaccharides (which contain two or more kinds of monomer).

The bullet points below describe the structure of three common polysaccharides and explain how structure relates to function in each case.

• Starch is a storage carbohydrate found in plant cells. It is a polymer of the monosacchairde, alpha-glucose. Starch is formed from two main sub-units: amylose (which forms a helix shape) and amylopectin (which forms branches). Starch is suited to its functions in several ways. Firstly, because it is so large, it is insoluble in water. This means that it does not affect the water potential of the cell, and thus does not cause any change to the osmotic movement of water between the cell and its environment. Its large size also means that it can be kept within the cell easily; it does not pass through the membrane. The branched nature of amylopectin makes starch an ideal energy store. The alpha-glucose molecules on these branches can easily be removed by hydrolytic enzymes and used as a substrate in respiration to release energy.

• Glycogen is a storage carbohydrate found in animal cells. It has a branched structure, similar to the amylopectin found in starch, but with more branches. This, again, makes it an ideal energy store.

• Cellulose is a structural carbohydrate found in the cell wall of plant cells. It is a polymer of beta-glucose, unlike starch and glycogen. The beta-glucose forms straight chains, or microfibrils, which come together by hydrogen bonding to form the larger cellulose fibres which run the length of the cell wall. The fibres are aligned in different directions, giving the cell wall both strength and a degree of elasticity. This allows plant cells to swell (become turgid) without bursting when they fill with water by osmosis. There are gaps between the cellulose fibres which allow even the largest molecules to pass through. The cell wall is thus said to be "freely permeable".

State three functions of lipids

1. Energy Source
2. Energy Storage
3. Insulation
4. Protection
5. Waterproofing
6. Cell Membranes (Phospholipids are a major component of)

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