Cellular Chemical Components

• The chemical components of cells are classified into organic and inorganic groups

Organic Compounds

• Includes water and minerals

Inorganic Compounds

• Includes nucleic acids, carbohydrates, lipids, and proteins
• Most cellular compounds are lipids and large molecules called macromolecules
• Macromolecules are composed of monomers linked by covalent bonds

Important Polymers

• Nucleic acids: composed of nucleotide associations (4 types)
• Polysaccharides: long chain carbohydrates linked by glycosidic bonds
• Proteins: made up of amino acids
• Polysaccharides are composed of monomers
• Proteins are composed of basic units called amino acids

Water and Minerals

• Water acts as a natural solvent for ions and as a colloidal dispersion medium
• Found in cells in two forms: free and bound
• Water molecules are bipolar or dipolar, enabling them to bind to other molecules
• Water eliminates substances from cells
• Water absorbs heat, preventing drastic temperature changes in cells

Salts

• The concentration of ions differs inside and outside the cell
• Dissociated salts maintain osmotic pressure and acid-base balance in cells
• Some inorganic ions act as enzyme cofactors
• Certain minerals are non-ionized (no charge)
• For normal cellular activity, trace amounts of magnesium, copper, cobalt, iodine, selenium, nickel, molybdenum, and zinc are indispensable
• Iodine is a component of thyroid hormones

Nucleic Acids

• Nucleic acids are macromolecules with great biological importance
• DNA: Integrated into chromosomes in the nucleus
• RNA: In the nucleus and cytoplasm

Fundamental Differences Between DNA and RNA

• DNA contains deoxyribose and thymine, while RNA contains ribose and uracil
• DNA is usually double-stranded, while RNA is single-stranded

Types of RNA

• Messenger RNA (mRNA): Contains the genetic information that determines the amino acid sequence in proteins
• Ribosomal RNA (rRNA): Essential for protein synthesis and ribosome construction
• Transfer RNA (tRNA): Identifies and transports amino acids to ribosomes

Carbohydrates

• Compounds made of carbon, hydrogen, and oxygen
• Major energy source for cells and are important in cell membranes and the extracellular matrix

Monosaccharides

• Simple sugars classified as trioses, tetroses, pentoses, and hexoses
• General formula: Cn(H2O)n

Disaccharides

• Sugars formed by the combination of two hexoses, with the loss of water
• General formula: C12H22O11

Oligosaccharides

• Bound to lipids and proteins, forming glycolipids and glycoproteins

Polysaccharides

• Combination of many hexose monomers
• General formula: (C6H10O5)n
• Glycosaminoglycans: polysaccharides composed of repeating disaccharide units containing a uronic acid and often an amino sugar

Lipids

• Lipids are insoluble in water but soluble in organic solvents due to their hydrophobic structures
• Common cellular lipids include triacylglycerols, phospholipids, glycolipids, steroids, and polyisoprenoids

Phospholipids

• Include glycerophospholipids (two fatty acids linked to a glycerol molecule) and sphingophospholipids

Triacylglycerols

• Triesters of fatty acids and glycerol, serving as energy reserves

Glycolipids

• Cerebrosides (glucose or galactose linked to ceramide) and gangliosides (oligosaccharide integrated by various monomers and ceramide)
• Diacylglycerols: result when carbons are linked to fatty acids
• Lipids bound to sugars are called glycolipids
• Proteins have primary, secondary, tertiary, and quaternary structures, with the quaternary structure involving associations of multiple subunits
• Enzymes are proteins that catalyze reactions

Cellular Energy

• Most of the energy used by the cell comes from ATP
• ADP is ATP without energy
• ATP comes from the mitochondria
• Cells deficient in mitochondria have altered functions

ATP Functions

• Role in phosphate bond function
• Role in cellular metabolism
• Instability (permits energy release)
• Essential (without ATP, the cell dies)

ATP Hydrolysis

• Process in which ATP breaks down into ADP and inorganic phosphate, releasing energy
• ATP has three phosphate groups linked by high-energy bonds
• These bonds are negatively charged and repel each other, making ATP unstable
• When cells need energy, the ATase enzyme breaks the terminal phosphate bond using water producing energy

ATP Synthesis

• Process by which ADP and inorganic phosphate reform into ATP

Substrate-Level Phosphorylation

• Occurs when an enzyme directly transfers a phosphate group from a substrate molecule to ADP (e.g., in glycolysis)
• Quick process but produces less ATP

Oxidative Phosphorylation

• The main form of ATP production in aerobic cells; occurs in the mitochondria, using electrons from NADH and FADH2 to generate a proton gradient
• Protons flow through ATP synthase to generate ATP

Photophosphorylation

• In plants, sunlight drives ATP formation in the chloroplasts during photosynthesis
• To ensure a continuous energy supply, cells store glucose and fatty acids in the cytosol

Glycolysis

• Glucose breaks down into two smaller pyruvate molecules in the cytoplasm
• If oxygen is present, pyruvate enters the mitochondria for further processing
• If oxygen is absent, pyruvate is converted to lactic acid (in muscles) or ethanol and CO2 (in yeast)

Oxidative Decarboxylation of Pyruvate

• Converts pyruvate into acetyl-CoA; a key molecule for energy production in the Krebs cycle
• Occurs in the mitochondrial matrix and is catalyzed by the pyruvate dehydrogenase complex
• Carbon is removed as CO2
• NADH is produced
• Pyruvate is converted to acetyl-CoA

Other pathways to obtain Acetyl-CoA

• Beta-oxidation of fatty acids: fatty acids are broken down into acetyl-CoA within the mitochondria
• Catabolism of amino acids: some amino acids can be converted into Krebs cycle intermediates, allowing energy production

Krebs Cycle

• A series of reactions in the mitochondrial matrix that extract energy from acetyl-CoA
• NADH and FADH2 transport electrons to the electron transport chain, generating ATP

Electron Transport Chain

• The final stage of ATP production, occurring on the inner mitochondrial membrane
• NADH and FADH2 deliver electrons to a series of protein complexes called enzymatic complexes
• Complex I (NADH dehydrogenase): receives electrons from NADH and pumps protons (H+)
• Complex II (Succinate Dehydrogenase): receives electrons from FADH2 but does not pump protons
• Complex III (Cytochrome bc1): receives and pumps more protons
• Complex IV (Cytochrome C oxidase): receives electrons and transfers them to oxygen, forming water

Chemiosmosis

• Protons accumulated in the intermembrane space return to the mitochondrial matrix through ATP synthase, generating ATP

Mitochondria Structure

• Double membrane

Outer membrane

• Permeable to small molecules due to porin proteins

Inner membrane

• Impermeable, highly folded to increase the surface area for ATP production

Intermembrane space

• Protons accumulate here during electron transport, creating a gradient

Mitochondrial matrix

• Contains Krebs cycle enzymes and mitochondrial DNA (mtDNA)

Mitochondrial DNA

• Contains essential genes for mitochondrial function
• Does not undergo recombination and is useful for studying evolution and genetic diseases