Chapter 4: Cellular metabolism

Metabolic reactions 2 major types: Anabolism & Catabolism Anabolism Provides biochemicals required for cell growth & repair Ex. Cells join many monosaccharides into a chain to form larger molecules of glycogen using an anabolic process called dehydration synthesis Ex. Links glycerol & fatty acid molecules in fat (adipose) cells to form triglycerides Ex. Join amino acid molecules to build protein molecules; water molecule forms & a peptide bond joins the two amino acid molecules together Two bound amino acids form a dipeptide & many joined in a chain form a polypeptide Polypeptides with specific functions consisting of ~ 100 or more amino acid molecules is called a protein

Catabolism Physiological processes that break larger molecules into smaller ones Hydrolysis is an example Decomposes carbohydrates, lipids, and proteins & splits a water molecule in the process Hydrolysis of a disaccharide like sucrose yields two monosaccharides  glucose & fructose Hydrolysis occurs during digestion Breaks down carbohydrates into monosaccharides Fats into glycerol & fatty acids Proteins into amino acids Nucleic acids into nucleotides

Control of Metabolic reactions Metabolic reactions require energy to happen The temperature in cells is usually too mild to promote the reactions required to support life  enzymes make these reactions possible Enzymes are almost always proteins & promote chemical reactions within cells by lowering the amount of energy required to start these reactions Enzymes speed the rates of metabolic reactions (catalysis) Enzymes are required in small quantities because if they are not completely consumed they can be recycled

Each enzyme only acts on a particular chemical (substrate) Ex. Catalase’s substrate is hydrogen peroxide During an enzyme –controlled reaction, parts of the enzyme molecule combine with portions of the substrate Forms an enzyme-substrate complex The interaction between the molecules distorts or strains the chemical bonds within the substrate  this increases the likelihood that the reaction will occur Many enzymatic reactions are reversible

Factors that alter enzymes Almost all enzymes are proteins so they can be denatured by exposure to heat, radiation, electricity, certain chemicals or fluids with extreme pH values Temperature also affects enzymes Many become inactive at 45 oC and nearly all are denatured at 55 oC Poisons can denature enzymes Cyanides interfere with respiratory enzymes, this impairs a cell’s ability to release energy from nutrient molecules

Energy for metabolic reactions Energy is the capacity to change or move matter; the ability to do work Common forms of energy are heat, light, sound, electrical energy, mechanical energy & chemical energy

Release of Chemical Energy Most metabolic processes use chemical energy It is held in the bonds between atoms and is released when the bond breaks Oxidation Cells burn glucose molecules This powers the reactions of cellular metabolism Oxidation inside cells differs from the burning of substances outside cells ‘Burning’ requires a large amount of energy to begin; most of the energy released escapes as heat or light Enzymes reduce the amount of energy required for oxidation in cells

Glycolysis Enzymes break down glucose in the cytosol into 2 three-carbon pyruvic acid molecules Does not require O2 so it is the anaerobic phase of cellular respiration Requires some energy  more is released than consumed This excess energy is used to synthesize ATP

Aerobic Respiration Second phase following glycolysis Oxygen must be present for this phase to happen Happens inside the mitochondria & transfers considerably more energy to ATP molecules When glucose breakdown is complete, CO2 & H2 remains CO2 diffuses out of the cell as waste H2 combine with O2 to form water

ATP Molecules For each glucose molecule that is decomposed completely, 38 molecules of ATP can be produced 2 from glycolysis, the rest from aerobic respiration ATP molecules break apart to release energy for a variety of functions Muscle contraction, active transport, synthesis of various compounds ATP molecules that are broken apart becomes an ADP molecule ADP can convert back into ATP by capturing energy & a phosphate

Describe the metabolic pathways of carbohydrates, lipids, & proteins. What are the products fro each of the metabolic pathways above? Explain how DNA & RNA store and carry genetic information. Explain how genetic information controls cellular processes. How do DNA molecules replicate?

RNA Molecules Messenger RNA (mRNA) – carries the information in a gene’s nucleotide sequence from the nucleus to the cytoplasm Synthesis of mRNA begins when the enzyme RNA polymerase associates w/ the DNA base sequence at the beginning of a gene Other enzymes begin to unwind and pull apart the double stranded DNA molecule This exposes the first portion of a gene RNA polymerase then moves along this strand Exposes other portions of the gene & stringing together mRNA from nucleotides Complimentary to those along the unwound DNA strand

Rna molecule EX) If the DNA bases sequence is: A T G C G T A A C A The complimentary bases in the mRNA molecule will be: U A C G C A U U G U RNA polymerase somehow knows which of the two DNA strands contains this information It also knows the correct direction to read DNA (Just like reading a sentence in a book)

RNA molecule RNA polymerase continues to move along the DNA strand exposing the gene until it gets to a specific DNA base sequence that represents the end of a gene Called the termination signal The mRNA molecule is released by RNA polymerase & leaves the DNA Transcription: process of copying DNA information into the structure of mRNA

RNA molecule Because an amino acid is encoded by a particular sequence of 3 nucleotides in a DNA molecule, the transcribed mRNA represents the complementary set of 3 nucleotides of the amino acid Codon: a triplet of nucleotides in mRNA that specifies a particular amino acid Translation: when mRNA molecules associate with ribosomes & act as patterns or templates for the synthesis of protein molecules