Student’s Name

Professor’s Name




1.Classify proteins with suitable examples based on their biological functions and levels of organization of their structures.

This classification of protein is based on shape or structure and composition. They are classified into three types; fibrous, globular and derived protein.

(a). Fibrous protein: They are elongated or finger like protein with an axial ratio which is more than ten, they are also static in nature with simple structure and they are mostly present in animals. Fibrous protein they are further classified as- simple and conjugated fibrous protein. Simple fibrous protein include scleoprotein which make the animal skeleton and they are water soluble while conjugated fibrous proteins include pigments present in chicken feather.

(b). Globular proteins: They are spherical in shape with an axial ratio which always less than ten, there are dynamic in nature with high degree of complexity in structure. They include enzymes, hormones etc. Globular protein is further classified on the basis of composition which are; simple or homo globular proteins composing of amino acids only and complex or hetero globular proteins which are always linked by non- protein moiety to become functional. So they are composed of both protein and non- protein component known as prosthetic group.

(c) Derived protein: These proteins are derivatives of either simple or complex proteins resulting from action of heat, enzymes and chemicals. They are classified as primary derived protein and secondary derived protein, examples of primary derived proteins are; proteans, metaprotein and coagulated protein while examples of secondary derived proteins are proteosis.

Proteins are structurally organized into four level; primary structure, secondary structure, tertiary structure and quaternary structure.

(a) Primary structure: it refers to the sequence and arrangement of amino acids in polypeptide chain where carboxyl group (R- COOH) of one group of the amino acids is linked with amino group (R-NH2) of other amino acid by peptide bond. The peptide bond links successive amino acids in polypeptide chain. In polypeptide chain carboxyl group and amino group of most amino acids are involved in formation of peptide bond. However, two amino acids which are situated at either end of polypeptide chain have R-COOH free or RNH2 group free. The edge at which -NH2 is free is called N-terminal and other edge which -COOH group is free is called C- terminal. Since, most of R-COOH and R-NH2 group formed peptide bond, they are not available for other bonding except Hydrogen bond. The peptide bond occur in trans configuration and have partial body character.

(b) Secondary structure of protein: Formation of secondary structure involves local folding of polypeptide chain. They include; Alpha helix, Beta sheets and Beta bends.

(c) Tertiary structure of protein: It refers to the overall folding of a polypeptide chain to form a final three- dimensional structure for example, a globular protein which are larger than 200 amino acids units forms two or more domains by folding of polypeptide chain by either Alpha helix or Beta pleated sheet or Beta bend. Finally, these domains associates with each other to form 3D structure.

(d) Quaternary structure of protein: Some proteins are composed of more than one polypeptide chain. Each polypeptide chain in such protein are called sub units thus the quaternary structure refer to the interaction between these sub units to large final 3D structure. Therefore, quaternary structure is interaction between different polypeptide chains of multi chain protein. Quaternary structure is found only in protein which are composed of more than one polypeptide chains such as Hemoglobin. Bonds such as H- bonds, hydrophobic interactions, helps to form quaternary structure.

The biological functions of proteins are classified into 9 groups which include;

(a) Catalytic proteins which include the enzymes and they are used to speed up the reaction of biochemical reactions.

(b) Structural proteins; they make the various structural components of living beings for example the collagen makes the bone.

(c) Nutrient proteins; they have nutritional value and provide nutrition when consumed for example casein in milk.

(d) Regulatory proteins; They regulate metabolic and cellular activities in cells and tissues they include Hormones.

(e) Defense protein; they provide defensive mechanism against pathogens and they include the antibodies and complement protein.

(f) Transport protein; They transport nutrients and other molecules from one organ to the other for example, hemoglobin.

(g) Storage proteins; they store various molecules and ions in cells and they include Ferritin store iron.

(h) Contractile or mobile proteins; they help in movement of various body parts for examples are; Actins, myosin and Tublins.

(i) Toxic proteins ; this are proteins which can damage the tissues for example; Snake venom and bacterial exotoxins.

2. Explain how this level of organization occur and stabilized. (20 marks)

Protein structure are made by condensation of amino acids forming peptide bonds. The sequence of amino acid in a protein is called its primary structure. The secondary structure is determined by dihedral angles of the peptide bonds, the tertiary structure by the folding of protein chains in space. Association of folded polypeptide molecules to complex functional proteins results in quaternary structure. Folded proteins are stabilized by thousands of noncovalent bonds between amino acids. In addition, chemical forces between a protein and its immediate environment contribute to protein shape and stability.

3. Give an overview of how DNA is capable of replicating and repairing itself. (15marks)

DNA unwinds at the origin of replication new bases are added to the complementary parental strands. The matching of free nucleotides to the parental strands is accomplished by an enzymes called DNA polymerase. Primers are removed, new DNA nucleotides are put in place of the primers and the backbone is sealed by DNA ligase.

DNA polymerase can make mistakes while adding nucleotides. It’s edits the DNA by proofreading every newly added base. Incorrect bases are removed and replaced by the correct base and then polymerization continues.

4. Explain active and passive transport across cell membrane. (10 marks)

Active transport moves molecules and ions from a region of lower concentration to a region of higher concentration with the help of energy in the form of Adenosine trio-phosphate it is a dynamic and rapid process which transport all molecules such as proteins, large cells, complex sugars and ions. During the active transport process, carrier proteins are required and the process is influenced by temperature. Active transport process include; Exocytosis, Endocytosis and Sodium- potassium pump.

On the other hand, passive transport moves ions and molecules from a region of higher concentration to a region of lower concentration without any energy thus it is a physical and comparatively slow process. The main aim is to transport all soluble molecules including oxygen, water, carbon dioxide, lipids and sex hormones. This process include; osmosis, diffusion and facilitated diffusion.

5. How do structures of hemoglobin and myoglobin relate to their functions? (5 marks)

Hemoglobin; It transport both oxygen and carbon dioxide. This function occurs because of hemoglobin unique shape, which is globular and made of four sub units of proteins surrounding an iron group. Hemoglobin undergoes changes to its shspe to help make it more efficient in its function of carrying oxygen.

Myoglobin; it has a globular structure and contains a heme group which is responsible for carrying of oxygen molecules to muscle tissues. Myoglobin can exist in the oxygen free form , de- oxymyoglobin, or in a form in which the oxygen molecule is bound , called oxymyoglobin.

Work cited

Gichaga, 04, biology