Cell Biology: Structure, Function, and Processes
Classification of Proteins
Holoproteins
Filamentous, insoluble in H2O, polypeptides arranged in only one dimension as parallel fibers.
- Collagen: Found in connective tissue, cartilage, and bone, provides resistance to stretch.
- Keratin: Synthesized and stored in the cells of the epidermis, forms hair.
- Elastin: Flexible, fibrous proteins found in tendons and blood vessels, allows them to recover their shape after stretching.
- Myosins: Actively involved in muscle contraction.
Globular Proteins
Complex globular structure, most are soluble in water or polar solvents.
- Protamines: Basic proteins associated with DNA in the sperm nucleus, facilitate DNA compaction.
- Histones: Low molecular mass, high proportions of basic amino acids, associated with DNA in all cells except sperm.
- Prolamins: Water-insoluble, found in vegetable seeds (e.g., zein in maize).
- Glutenin: Water-insoluble, soluble in dilute acids or bases.
- Albumin: Large proteins, function as amino acid pool and transport molecules.
- Globulins: Large molecular weight, soluble in salt solutions, perfect globular form.
Heteroproteins
- Chromoproteins: Contain a non-protein pigment.
- Glycoproteins: Prosthetic group is a carbohydrate.
- Lipoproteins: Prosthetic group is a lipid, found in the cytoplasmic membrane and blood plasma.
- Phosphoproteins: Contain a phosphate group.
- Nucleoproteins: Associations of histones and protamines with DNA, form chromatin fibers in the cell nucleus.
Glycosidic Bonds
O-Glycosidic Bond
Formed between two hydroxyl groups. Two types: 1) Monocarbonyl, between the carbonyl carbon and a non-anomeric carbon. 2) Dicarbonyl, between two carbonyl carbons. Disaccharides with a free carbonyl group can reduce Fehling’s reagent.
N-Glycosidic Bond
Formed between a hydroxyl group and an amino group (aminoglycosides).
The Cell Theory
Postulates of Schleiden and Schwann:
- The cell is the morphological unit of all living things.
- The cell is the physiological unit of life.
- All cells come from pre-existing cells by cell division.
- The cell is the autonomous genetic unit of life.
Structure and Function of the Cytosol
An aqueous solution (85% water) containing dissolved molecules and colloidal dispersions of proteins, carbohydrates, lipids, nucleosides, nucleotides, metabolic products, and salts. Water content can vary, resulting in two physical states: viscous fluid (sol) and gel-like (gel).
The cytosol acts as a regulator of intracellular pH and is rich in enzymes. It is the site of major cellular metabolic reactions, including:
- Gluconeogenesis (synthesis of glucose)
- Glycogen synthesis and breakdown (glycogenesis and glycogenolysis)
- Amino acid biosynthesis
- Protein modification
- Lipogenesis (synthesis of fatty acids)
- Lactic acid fermentation
- Reactions involving ATP and tRNA
The Plasma Membrane
A thin film (75Å thick) that surrounds the cell and separates it from the external environment. According to the fluid mosaic model, it consists of a lipid bilayer with associated protein molecules. The lipid bilayer is composed of amphipathic molecules (phospholipids and glycolipids) that form a polar and nonpolar region. These molecules can rotate and move laterally within their monolayer, creating membrane fluidity. Cholesterol modulates membrane fluidity, especially in brain cells. Proteins are embedded in the bilayer and interact with the lipids. Based on their position, proteins are classified as:
- Integral proteins: Partly or completely embedded in the bilayer.
- Extrinsic proteins: Attached to the surface of the bilayer and are soluble.
The plasma membrane has two key properties:
- Dynamic structure: Molecules can move laterally, and the membrane can undergo exocytosis and endocytosis.
- Asymmetric structure: Oligosaccharide chains of glycolipids and glycoproteins are found only on the outer surface, forming the glycocalyx. The glycocalyx is involved in cell recognition (e.g., sperm-egg interaction, virus-host cell interaction, tissue formation).
Membrane Functions
- Separates the intracellular environment from the extracellular environment.
- Performs endocytosis and exocytosis.
- Regulates the entry and exit of molecules.
- Regulates the entry and exit of ions.
- Enables cell recognition.