Chapter 9: Energy in a Cell

9.1 ATP in a Molecule

Cell Energy Energy is essential to life All living organisms must be able to produce energy from the environment in which they live, store energy for future use, and use energy in a controlled manner

Work and need for energy Several cell processes require energy There is a molecule in your cells that is a quick source of energy for any organelle in the cell that needs it. this energy is stored in the chemical bonds of the molecule called adenosine triphosphate (ATP) composed of an adenosine molecule with three phosphate groups attached adenosine is made up of adenine bonded to a ribose

Forming and Breaking Down ATP Phosphate groups are positively charged Bonding phosphate groups to adenosine requires considerable energy When only one phosphate group is attached… A small amount of energy required Stores a small amount of energy Called adenosine monophosphate

When two phosphate groups are attached… More energy required to force two phosphate groups together Stores more energy Called adenosine diphosphate (ADP) When the third phosphate group is attached… A tremendous amount of energy is required Wants to break away from the other positively charged phosphate groups When this bond is broken a great amount of energy is released

How cells tap into the energy stored in ATP The energy of ATP becomes available when the molecule is broken down into ADP which can then bond with another phosphate group and form another ATP molecule. How cells tap into the energy stored in ATP When ATP is broken down and the energy is released, cells must have a way to capture that energy Cellular proteins have a specific site where ATP can bind

Uses of Cell Energy Making new molecules is one way that cells use energy Cells use energy to maintain homeostasis

9.2 Photosynthesis: Trapping the Sun’s Energy

Trapping Energy from Sunlight Plant cells must trap light energy and store it in a form that is readily usable by cell organelles – that form is ATP Photosynthesis = the process plants use to trap the sun’s energy and build carbohydrates, called glucose, that store energy

General equation = 6CO2 + 6H2O  C6H12O6 +6O2 Happens in two phases Light-dependent reactions convert light energy into chemical energy The molecules of ATP produced in the light-dependent reactions are then used to fuel the light-independent reactions that produce glucose. General equation = 6CO2 + 6H2O  C6H12O6 +6O2

The chloroplast and pigments Chloroplast is the organelle where photosynthesis occurs Light-dependent reactions take place in the thylakoid discs Contain molecules that absorb specific wavelengths of sunlight called pigments Chlorophyll is the most common pigment Appears green because that is the only wavelength that chlorophyll does not absorb, the green light is reflected

Light-Dependent Reactions As sunlight strikes the chlorophyll the energy in the light is transferred to electrons which become highly energized Excited electrons are passed from chlorophyll to an electron transport chain: a series of proteins embedded in the thylakoid membrane

At each step along the transport chain, the electron loses energy Passed down the first electron transport chain, then a second chain, and then passed to the stroma NADP+ is the electron carrier molecule that transports the electron to the stroma Combines with a hydrogen to become NADPH Transfers the energy to another series of reactions in the stroma

Restoring electrons to chlorophyll Many electrons leave with the NADPH To replace the lost electrons, molecules of water are split and each molecule produces two electrons This reaction is called photolysis

Light-Independent Reactions The Calvin cycle is a series of reactions in the stroma that use carbon dioxide to form carbohydrates One of the last molecules formed is also one of the molecules needed for the first reaction of the cycle. It takes six cycles to form one sugar molecule (6 carbon molecules) because only one CO2 molecule is added to each cycle

9.3 Getting Energy to Make ATP

Cellular Respiration The process by which mitochondria break down food molecules to produce ATP is called cellular respiration Anaerobic = no oxygen required Aerobic = oxygen required

Glycolysis Anaerobic process A series of chemical reactions in the cytoplasm of a cell that break down glucose (6 carbon compound) into two molecules of pyruvic acid (3 carbon compound) Two molecules of ATP used to start the reactions Four ATP molecules are produced Uses an electron carrier called NAD+ which forms NADH when carrying an electron

The citric acid cycle Pyruvic acid molecules move into the mitochondria where they are exposed to oxygen The citric acid cycle is a series of chemical reactions that breaks down glucose One molecule of ATP is produced for every turn of the cycle Two different electron carriers are used NAD+ FAD Electron carriers pass the energized electrons along to the electron transport chain in the inner membrane of the mitochondria

The electron transport chain The electron transport chain in the inner membrane of the mitochondria is very similar to the electron transport chain of the thylakoid membrane NADH and FADH2 pass energized electrons from protein to protein in the membrane Slowly releasing small amounts of energy

The final acceptor in the chain is oxygen Results in a high concentration of H+ in the inner membrane creating in both a chemical and electrical gradient The inner membrane forms ATP from this gradient The final acceptor in the chain is oxygen Reacts with four hydrogen ions to form two water molecules Without oxygen the transport chain would become blocked Produces a total of 32 ATP molecules: very effective

Fermentation When cells are without oxygen for a short period of time the anaerobic process of fermentation follows glycolysis and provides ATP until oxygen is available again.

Two types Lactic acid fermentation Alcoholic fermentation Two molecules of pyruvic acid use NADH to form two molecules of lactic acid Releases NAD+ to be used in glycolysis Two ATP molecules formed for each glucose molecule The liver changes lactic acid back into pyruvic acid Alcoholic fermentation Used by yeast cells to produce CO2 and ethyl alcohol

Comparing Photosynthesis an Cellular Respiration Both photosynthesis and cellular respiration use electron carriers and a cycle of chemical reactions to form ATP Photosynthesis produces high energy carbohydrates and oxygen from the sun’s energy, whereas cellular respiration uses oxygen to break down carbohydrates to form ATP and compounds of lower energy. CO2 is produced by cellular respiration, but the beginning products for photosynthesis. Table 9.2 in Figure 9.14