Cellular Metabolism Anatomy Chapter 4 Intro Cells require

Cellular Metabolism Anatomy Chapter 4

Intro � Cells require energy and information to build bodies. � Many chemical reactions occur in the body which break down nutrients to release energy and also build molecules to store energy. � Cells also carry genetic information that encodes the amino acid sequences of proteins. � Enzymes control the actions of metabolism. � Genes carry the information that instructs a cell to manufacture particular proteins.

Metabolic Reactions � Two major types of metabolic reactions: ◦ Anabolism: buildup of larger molecules from smaller ones; which requires energy. ◦ Catabolism: breakdown of larger molecules into smaller ones; which releases energy

Anabolism � Provides the biochemicals required for cell growth and repair. � Dehydration synthesis: anabolic process combining many simple sugar molecules into a chain to form larger molecules of glycogen. ◦ Hydroxyl group from one monosaccharide and hydrogen atom from another react to release a water molecule.

Catabolism � Physiological process that break larger molecules into smaller ones. � Hydrolysis: decomposes carbohydrates, lipids, and proteins, and splits a water molecule in the process. ◦ Occurs during digestion, where it breaks down carbohydrates into monosaccharides, fats into glycerol and fatty acids, proteins into amino acids, and nucleic acids into nucleotides.

Control of Metabolic Reactions � Specialized cells, such as nerve, muscle, or blood cells, carry out distinctive chemical reactions. � BUT…all cells perform certain basic reactions, such as buildup and breakdown of carbohydrates, lipids, proteins, and nucleic acids.

Enzyme Action � Metabolic reactions require energy to proceed. � Temperature promotes metabolic reactions but the temperature inside the cell is inadequate. � Enzymes inside the cell promote metabolic reactions to proceed. � Enzyme: complex molecules, almost always proteins, that promote chemical reactions within cells by lowering the amount of energy, called the activation energy, required to start these reactions.

Enzyme Action � Enzymes speed up reaction rates; acceleration called catalysis, and the enzyme called a catalyst. � Enzymes are used in very small quantities because, as they function, they are not consumed and can therefore be recycled.

Enzyme Action � Enzymes are VERY specific. � Each enzyme only acts on a particular chemical, called a substrate. ◦ Example: the substrate of the enzyme catalase is hydrogen peroxide, a toxic by-product of certain metabolic reactions. ◦ The enzyme’s only function is to speed up the breakdown of hydrogen peroxide into water and oxygen. ◦ Catalase helps prevent an accumulation of hydrogen peroxide, which might damage cells.

Enzyme Action � Specific enzymes catalyze each of the hundreds of different chemical reactions comprising cellular metabolism. � Each cell contains hundreds of different enzymes, and each enzyme must recognize its specific substrate. � The ability to identify its substrate depends on the shape (conformation) of the enzyme molecule. � Think lock and key!

Enzyme Action � During enzyme-catalyzed reaction, parts of the enzyme molecule called active sites temporarily combine with portions of the substrate, forming an enzyme-substrate complex. � This makes the opportunity for a reaction to occur more likely. � If/when a reaction occurs, the substrate and enzyme change shape and then return to their original shape.

Enzyme Action � Make sure YOU LEARN the enzyme-catalyzed reaction summarized on page 77. � The reverse reaction occurs in many, but not all cases. � Speed depends on the number of enzyme and substrate molecules in the cell. ◦ Generally, if there are higher concentrations of the enzyme/substrate, the reaction will occur more quickly. ◦ Rates of this process are different in each enzyme/substrate molecule.

Factors that Alter Enzymes � Almost all enzymes are proteins, and can be denatured by exposure to heat, radiation, electricity, certain chemicals, or fluids with extreme p. H values.

Cofactors and Coenzymes � Some enzymes are inactive until they combine with a nonprotein component. � Such a substance, called a cofactor, may be an ion of an element, such as copper, iron, and zinc, or a small organic molecule, called a coenzyme. � Many coenzymes are vitamin molecules. ◦ Example is coenzyme A, which takes part in cellular respiration.

Energy for Metabolic Reactions � Energy is the capability to change or move matter; that is, energy is the ability to do work. � Common forms of energy: ◦ ◦ ◦ Heat Light Sound Electrical energy Mechanical energy Chemical energy- used in most metabolic processes

Release of Chemical Energy � Chemical energy is held in the bonds between the atoms of molecules and is released when these bonds are broken, as in burning. � As the chemical burns, bonds break, and energy escapes as heat and light. � In cellular form, glucose is “burned” through a process called oxidation. ◦ The energy released in oxidation of glucose powers the reactions of cellular metabolism. ◦ However, the oxidation of biochemicals inside cells and the burning of substances outside cells differ in some ways.

Release of Chemical Energy � Burning usually requires a relatively large amount of energy to begin, and most of the energy released escapes as heat or light. � In cells, enzymes reduce the activation energy required for oxidation that occurs in reactions of cellular respiration. � It also occurs by transferring energy to special energy-carrying molecules, cell can capture about half of the energy released form breaking chemical bonds. The rest escapes as heat, which helps maintain body temperature.

Cellular Respiration � Cellular Respiration occurs in three distinct, yet interconnected, series of reactions: ◦ Glycolysis ◦ Citric Acid Cycle ◦ Electron Transport Chain � The products of these reactions include: ◦ CO 2 ◦ Water ◦ Energy � Most energy is lost as heat, almost half is captured in the form of high-energy electrons that the cell can use through the synthesis of ATP (adenosine triphosphate).

ATP � Each ATP molecule includes a chain of three chemical groups called phosphates. � As energy is released during cellular respiration, some of it is captured in the bond of the end phosphate. � When energy is required for a metabolic reaction, this terminal phosphate bond breaks, releasing the stored energy. � The cell uses ATP for a variety of functions, including active transport and synthesis of various compounds.

ATP � When ATP loses its terminal end, it becomes an ADP molecule. � ADP can be converted back into ATP by the addition of energy and a third phosphate.

Glycolysis � Cellular respiration begins with glycolysis, literally means “the breaking of glucose” � Glycolysis occurs in the cytosol (liquid portion of the cytoplasm), and it does not require oxygen, it is sometimes referred to as the anaerobic phase of cellular respiration.

Aerobic Respiration � If oxygen is present in significant quantities, the pyruvic acid generated by glycolysis can enter the pathways of aerobic respiration. � These reactions occur in the mitochondria and include the citric acid cycle and the electron transport chain. � Because the reactions of the electron transport chain add phosphates to form ATP, they are also known as oxidative phosphorylation. � The aerobic reactions yield up to 36 ATP molecules per glucose.

Aerobic Respiration � For each glucose molecule that is decomposed completely, up to 38 molecules of ATP can be produced. � Two of these ATP molecules are the result of glycolysis, and the rest form during the aerobic phase. � About half the energy released goes to ATP synthesis, while the rest ends up as heat.

Aerobic Respiration � In addition to releasing energy, the complete oxidation of glucose produces carbon dioxide and water. � Carbon dioxide is inhaled; water becomes part of the internal environment. � In humans, the amount of water produced by metabolism is far less than our daily water needs. We must drink water to survive. � In contrast, a small desert rodent can survive entirely on the water produced by aerobic respiration.

Metabolic Pathways � Like cellular respiration, anabolic and catabolic reactions in general have a number of steps that must occur in a particular sequence. � Enzymes control the rates of these reactions and must be in a specific sequence so they have control of the reaction. � The enzymes responsible for aerobic respiration are located in tiny, stacked particles in the membranes (cristae) within the mitochondria. � Metabolic pathway: a sequence of enzymecontrolled reactions.

Metabolic Pathways � The rate of enzyme-controlled reaction usually increases if either the number of substrate molecules or the number of enzyme molecules increases. � However, the rate of metabolic pathways if often determined by a regulatory enzyme responsible for one of its steps. � This regulatory enzyme is present in limited quantities. � If there is a saturation of this substrate concentration, it can increase the number reactions occurring.

Metabolic Pathways �A rate-limiting enzyme is the first enzyme in a series. � This position is important because some intermediate chemical in the pathway might accumulate in an enzyme occupying another location in the sequence if it were rate limiting. � Remember, lipids and proteins, as well as glucose, can be broken down to release energy for ATP synthesis. ◦ Final process in all cases is aerobic respiration, and the most common entry point is into the citric acid cycle as acetyl coenzyme A (acetyl Co. A)

Nucleic Acids � DNA molecules hold such information in the form of genetic code to instruct the cell as to how to synthesize enzymes and other specific protein molecules.

Genetic Information � We resemble our parents because of inherited traits. � What actually passes from parents to child is genetic information in the form of DNA molecules from the parents’ sex cells. � The portions of DNA molecules that contain the genetic information for making particular proteins are called genes. � Traits determined by the genes carried on in the sex cells.

Genetic Information � As an offspring develops, mitosis passes the information from cell to cell and tells the body how to construct different protein molecules. � Genes instruct cells to synthesize the enzymes that control metabolic pathways. � All of the DNA in a cell constitutes the genome. ◦ Only about 2% of the human genome encodes protein. ◦ Much of the rest of the human genome controls when and where genes are used to guide protein synthesis.

DNA Molecules � Building blocks of nucleic acids are nucleotides joined so that the sugar and phosphate portions alternate. � They form a long “back-bone” to the polynucleotide chain. � The nitrogenous bases project form the backbone and bind weakly to the bases of the second strand. � Resulting shape looks like a ladder

DNA Molecules � Organic ◦ ◦ bases of a DNA nucleotide: Adenine Thymine Cytosine Guanine � Both strands of a DNA molecule consist of nucleotides in a particular sequence. � Sequence of bases along one of these strands encodes the genetic information that specifies a particular protein’s amino acid sequence.

DNA Molecules � Complementary ◦ A pairs with T ◦ C pairs with G � DNA base pairing: molecule twists to form a double helix composed of millions of base pairs. � Length of DNA molecules may seen quite a challenge to copy, or replicate, when a cell divides, but a contingent of enzymes accurately and rapidly carries out this process.

DNA Replication � DNA molecules are replicated during interphase in the cell cycle. � Each new cell receives a copy of the existing cell’s genetic information so that the new cell can synthesize the proteins necessary for life, to build new cell parts, and metabolize. � Review DNA Replication

Protein Synthesis � DNA replication enables a cell to retain genetic instructions at the same time that it accesses that information to synthesize proteins. � Manufacturing proteins is a multi-step, enzyme-catalyzed process.

The Genetic Code: Instructions for Making Proteins � Cells can synthesize specific proteins because the sequence of nucleotide bases in the DNA of genes specifies a particular sequence of amino acid building blocks of a protein molecule. � Correspondence of gene and protein building blocks is called the genetic code.

The Genetic Code: Instructions for Making Proteins � Each of the 20 types of amino acids is represented in a DNA molecule by a particular sequence of three nucleotides. � The sequence of nucleotides in a DNA molecule denotes the order of amino acids of a protein molecule, as well as where to start or stop that protein’s synthesis.

Transcription � Because DNA molecules are within a cell’s nucleus and protein synthesis occurs in the cytoplasm, genetic information must be carried from the nucleus into the cytoplasm. � Messenger RNA (m. RNA) are synthesized in a process called transcription. ◦ RNA is a nucleic acid, and m. RNA is the type that carries a gene’s message out of the nucleus.

Transcription � RNA molecules differ from DNA molecules in several ways: ◦ RNA is single-stranded ◦ RNA sugar is ribose ◦ RNA contains uracil � Specific DNA sequences outside the actual genes signal RNA as to which of the two DNA strands contains the information to build a protein. � RNA polymerase also knows where the gene begins, where it stops, and the correct direction to read the DNA.

Translation � Each amino acid in a protein corresponds to three contiguous bases in the DNA sequence. � These three amino acids are known as codons. � These codons code for a specific amino acid in a protein chain. � To complete protein synthesis, m. RNA must leave the nucleus and associate with a ribosome. � There the series of codons on m. RNA are translated into a special “language” of amino acids called translation.

Translation � Building a protein molecule requires the correct amino acids in the cytoplasm and positioning them in the order specified along a strand of m. RNA. � Transfer RNA (t. RNA) correctly aligns amino acids, which are then linked by an enzyme to form proteins. � Because there are twenty different types of amino acid forms, there at least twenty different types of t. RNA molecules. � t. RNA serves as the anticodon and the t. RNA carries the amino acid to a correct position on the m. RNA strand. This action occurs on a ribosome.

Translation � As protein synthesis occurs, a ribosome binds to the m. RNA molecule so the t. RNA has a place to bind the anticodon. � The ribosome moves along the m. RNA molecule and t. RNA molecules attach themselves to the appropriate location. � With each t. RNA molecule, the amino acid chain lengthens until the STOP codon is reached. � As proteins form, it folds into its unique shape and is then released to complete a certain task.