Topic_2-Cell_Chemistry

Cell Chemistry and Physiology

Carbohydrates

Composition and Importance

• Carbohydrates make up approximately 1% of body weight, yet they play crucial roles in energy storage and supply.

• They are composed of carbon (C), hydrogen (H), and oxygen (O), typically with a 2:1 ratio of H to O, which is characteristic of their classification as "hydrates of carbon."

• The term "carbohydrate" is derived from the fact that they can be viewed as carbon atoms bonded to water molecules.

Types of Carbohydrates

1. Monosaccharides:

   • The simplest form of carbohydrates, consisting of a single sugar unit made up of 5-6 carbon atoms.

   • Common examples include:

     • Glucose: Known as blood sugar, it is the primary energy source for cells.

     • Galactose: Part of lactose, it is not commonly found alone in nature.

     • Fructose: Found in fruits, it is sweeter than glucose and used for energy production.

     • Ribose: A component of RNA, it plays a critical role in genetic function.

2. Disaccharides:

   • Formed by the combination of two monosaccharides through a glycosidic bond.

   • Key examples include:

     • Maltose: Comprised of two glucose units, it occurs during the digestion of starch.

     • Sucrose: Common table sugar made up of glucose and fructose; a primary source of energy for many organisms.

     • Lactose: Found in milk, composed of glucose and galactose; significant for infant nutrition.

3. Polysaccharides:

   • Long chains of monosaccharide units that serve as energy storage and structural components.

   • Notable examples are:

     • Starch: The main energy storage form in plants, can be broken down into glucose during metabolism.

     • Glycogen: The energy reserve in animal cells, particularly stored in the liver and muscles, converted back to glucose during fasting to maintain blood sugar levels.

Role and Functions of Sugars

• Glycogen acts as a crucial energy reservoir due to the chemical bonds between glucose molecules, which can be swiftly hydrolyzed into glucose when needed, especially during periods of fasting or physical activity to maintain stable blood glucose levels.

• Glucose, often referred to as "blood sugar," is essential for cellular respiration, providing energy via metabolic pathways.

Lipids

General Characteristics

• Lipids primarily consist of hydrogen and carbon atoms bonded through nonpolar covalent bonds, resulting in their nonpolar nature and low solubility in water.

Subclasses of Lipids

1. Fatty Acids:

   • These are long hydrocarbon chains characterized by a carboxyl group, which can be classified into:

     • Saturated Fatty Acids: No double bonds; typically solid at room temperature (e.g., butter).

     • Unsaturated Fatty Acids: Contain one or more double bonds; generally liquid at room temperature (e.g., olive oil).

2. Triglycerides:

   • The predominant form of stored fat in the body, made up of one glycerol molecule bonded to three fatty acids; found in adipose tissue, serving as a significant energy reserve.

3. Phospholipids:

   • Comprised of glycerol, two fatty acids, and a phosphate group; they are vital for forming the lipid bilayer of cell membranes, facilitating the formation of cellular boundaries.

4. Steroids:

   • Lipids structured with four interconnected carbon rings; include essential components like cholesterol, which contributes to membrane structure, and hormones such as cortisol, estrogen, and testosterone, which regulate various physiological processes.

Proteins

Significance

• Proteins constitute approximately 50% of organic material in the body and 17% of total body weight, making them integral to structural and functional roles in living organisms.

• Composed of carbon, hydrogen, oxygen, nitrogen, and trace amounts of other elements, proteins participate in various physiological functions crucial for survival and homeostasis.

Major Categories and Functions

• Proteins take on numerous roles:

  • Gene expression regulators: They synthesize RNA from DNA.

  • Transporters: Proteins that mediate the movement of solutes across cell membranes.

  • Enzymes: Biological catalysts that speed up reactions (e.g., lipase for fat digestion, amylase for starch digestion).

  • Cell signaling proteins: Molecules that bind to hormones or neurotransmitters to relay signals within and between cells.

  • Motor proteins: Facilitate muscle contraction and movement (e.g., myosin, kinesin).

  • Structural proteins: Provide mechanical support and shape to cells and tissues (e.g., collagen in connective tissues, elastin in elastic tissues).

  • Defense proteins: Protect the body against pathogens (e.g., antibodies as part of the immune system).

Amino Acids and Polypeptides

Amino Acid Structure

• Amino acids are the building blocks of proteins, each containing:

  • An amino group (-NH2)

  • A carboxyl group (-COOH)

  • A hydrogen atom

  • A variable side chain (R-group) that defines the specific properties of the amino acid.

• There are 20 standard amino acids, each with unique side chains contributing to protein structure and function.

Polypeptide Formation

• Amino acids are linked together through peptide bonds via dehydration reactions, forming polypeptides. Peptide bonds can be broken down by hydrolysis to release individual amino acids.

• Polypeptides consist of chains of amino acids; chains with 50 or fewer amino acids are typically referred to as peptides.

Protein Structure

Levels of Structure

• Proteins have four distinct levels of structural organization:

  • Primary Structure: The unique sequence of amino acids in the polypeptide chain.

  • Secondary Structure: Local folded structures stabilized by hydrogen bonds, forming alpha helices and beta-pleated sheets.

  • Tertiary Structure: The three-dimensional conformation of a polypeptide, resulting from interactions among side chains, leading to a functional protein.

  • Quaternary Structure: The assembly of multiple polypeptide chains into a single functional complex (e.g., hemoglobin, which consists of two alpha and two beta chains).

Nucleic Acids

Composition and Function

• Nucleic acids account for about 2% of body weight and are pivotal in storing, expressing, and transmitting genetic information critical for life.

• There are two main types:

  • DNA (Deoxyribonucleic acid): A double-helix structure containing deoxyribose sugar and nitrogenous bases adenine (A), thymine (T), cytosine (C), and guanine (G).

  • RNA (Ribonucleic acid): A single-stranded molecule composed of ribose sugar and nitrogenous bases adenine (A), uracil (U—replacing thymine), cytosine (C), and guanine (G).

• Nucleotides, the building blocks of nucleic acids, consist of a phosphate group, a sugar, and a nitrogenous base.

Protein Synthesis

• The process of protein synthesis involves two main stages:

  • Transcription: The genetic information from DNA is transcribed to messenger RNA (mRNA) in the nucleus.

  • Translation: mRNA is translated into a polypeptide chain at ribosomes, leading to protein formation.

  • RNA polymerase binds to the DNA template, unwinding it to create mRNA.

Roles of Proteins

• Proteins serve vital functions including enzymatic activity, structural integrity, transport mechanisms, cell signaling pathways, and immune responses.

Enzymatic Reactions

Role of Enzymes

• Enzymes act as catalysts, significantly lowering the activation energy required for chemical reactions to occur, thus increasing reaction rates.

• Enzyme-substrate complexes form transiently during reactions, enabling the conversion of substrates into products.

Chemical Reactions

• Involve the breaking and forming of chemical bonds, leading to the transformation of reactants into products.

Factors Affecting Rates

• The rates of enzymatic reactions can be influenced by:

  • Concentration of reactants

  • Temperature

  • Presence of catalysts, such as enzymes themselves.

Binding Sites and Regulation

• Ligands: Molecules that bind to proteins, inducing structural changes that affect protein activity.

• Affinity: The strength of the binding interaction; critical for the effectiveness of therapeutic agents.

• Allosteric Modulation: Regulatory binding at sites distinct from the active site can alter the functional conformation of proteins; phosphorylation is a common example of this modulation.

Clinical Case Study

• A clinical case involving a man who developed muscle damage linked to the influence of a statin medication interacting negatively with high grapefruit juice intake illustrates the complexities of drug metabolism and interactions in biological systems, highlighting the importance of understanding biochemical interactions in clinical settings.