Cool Fires Attract Mates and Meals Fireflies use light instead of chemical signals to send a message to potential mates Females can also use light to attract males of other firefly species, as meals not mates The light comes from a set of chemical reactions, the luciferin-luciferase system Fireflies make light energy from chemical energy Life is dependent on energy conversions
ENERGY AND THE CELL Living cells are compartmentalized by membranes Membranes are sites where chemical reactions can occur in an orderly manner Living cells process energy by means of enzyme-controlled chemical reactions - e.g., the firefly uses such reactions to control the flashing of its abdomen 5.1 Energy is the capacity to perform work Energy is defined as the capacity to do work All organisms require energy to stay alive Energy makes change possible (e.g., growth)
Kinetic energy is energy that is actually doing work Figure 5.1A Potential energy is stored energy Figure 5.1B 5.2 Two laws govern energy conversion First Law of Thermodynamics - Energy can be changed from one form to another - Energy cannot be created or destroyed, only converted - e.g., the energy in the covalent bonds of gasoline molecules is converted to kinetic energy to move the car Figure 5.2A
Second Law of Thermodynamics - Energy changes are not 100% efficient - Energy conversions increase energetic disorder, called entropy - Some energy is always lost as heat Figure 5.2B 5.3 Chemical reactions either store or release energy Cells carry out thousands of chemical reactions The sum of these reactions constitutes cellular metabolism
There are two types of chemical reactions: Endergonic reactions absorb energy and yield products rich in potential energy Potential energy of molecules Reactants Products Amount of energy INPUT Figure 5.3A Exergonic reactions release energy and yield products that contain less potential energy than their reactants Potential energy of molecules Reactants Products Amount of energy OUTPUT Figure 5.3B
5.4 ATP shuttles chemical energy within the cell Adenosine triphosphate (ATP) is our body s cellular energy currency ATP powers nearly all forms of cellular work ATP molecules are the key to energy coupling (connecting exer- and endergonic reactions) When the covalent bond joining a phosphate group to an ATP molecule is broken by hydrolysis, the reaction produces ADP and supplies energy for cellular work Energy is also set free when the remaining phosphate bond of ADP is broken, forming AMP Phosphate groups Adenine Ribose Adenosine triphosphate Hydrolysis Adenosine diphosphate (ADP) Energy Figure 5.4A
The ATP cycle connects released (kinetic) energy and stored (potential) energy This cycle connects a cell s energy use and energy storage Energy from exergonic reactions Dehydration synthesis Hydrolysis Energy for endergonic reactions Figure 5.4C HOW ENZYMES WORK 5.5 Enzymes speed up the cell s chemical reactions by lowering energy barriers For a chemical reaction to begin, reactants must absorb some energy - This energy is called the energy of activation (E A ) - This represents the energy barrier that prevents molecules from breaking down spontaneously - E A provides our cells with control over which reactions should take place and which shouldn t
A protein catalyst called an enzyme can lower E A and therefore decrease the energy barrier - When E A is lowered, the reaction can proceed Energy barrier (E A ) Enzyme Reactants Reactants 1 Products 2 Products Figure 5.5A Reactants E A with enzyme E A without enzyme Net change in energy Products Figure 5.5B
5.6 A specific enzyme catalyzes each cellular reaction Enzymes are highly selective - Enzymes operate like a lock-and-key mechanism - Each enzyme will function only in catalyzing one specific molecular reaction - This selectivity determines which chemical reactions occur in a cell, and when they occur - Enzyme-controlled reactions take place only when both the enzyme and its substrate are present - Enzymes generally end in -ase (e.g., luciferase) How an enzyme works Glucose 4 Products are released Enzyme (sucrase) Fructose Active site 1 Enzyme available with empty active site Substrate (Sucrose) 3 Figure 5.6 Substrate is converted to products Hydrolysis The enzyme is unchanged and can repeat the process 2 Substrate binds to enzyme
5.7 The cellular environment affects enzyme activity Enzyme activity is influenced by - temperature high temperature denatures enzymes by altering their 3-dimensional shape (sometimes this is irreversible) - salt concentration salt ions can interfere with chemical bonding properties of enzymes - ph can change the 3-dimensional enzyme structure Enzymes often require helper molecules - inorganic helpers = cofactors; organic helpers = coenzymes 5.8 Enzyme inhibitors block enzyme action Inhibitors interfere with enzymes A competitive inhibitor takes the place of a substrate in the active site A noncompetitive inhibitor alters an enzyme s function by changing its shape Competitive inhibitor Substrate Enzyme Active site NORMAL BINDING OF SUBSTRATE Noncompetitive inhibitor ENZYME INHIBITION Figure 5.8
MEMBRANE STRUCTURE AND FUNCTION 5.10 Membranes organize the chemical activities of cells Membranes organize the chemical reactions making up metabolism Membrane layers!! Cytoplasm Figure 5.10 Membranes are selectively permeable Membrane chemistry permits access to the cell by some molecules (e.g., water) while tightly controlling the flow of other substances Membranes hold teams of enzymes that function in metabolism
5.11 Membrane phospholipids form a bilayer Phospholipids are the main structural component of membranes Head They each have a hydrophilic (water-accepting) head and two hydrophobic (waterrejecting) tails Symbol Tails Figure 5.11A In water, phospholipids form a stable bilayer The hydrophilic heads face outward and the hydrophobic tails face inward Hydrophilic heads Water Hydrophobic tails Water Figure 5.11B
5.12 The cell membrane is a fluid mosaic of phospholipids and proteins A model of a fluid mosaic membrane: Imagine a baby pool filled with ping-pong balls (phospholipid heads) You can place a basketball (protein molecule) or a floating noodle (cholesterol) into the layer of ping-pong balls without interrupting the layer Styrofoam balls with little flags (glycoproteins plus carbohydrates) act as identification tags The cell membrane of an animal cell Fibers of the extracellular matrix Glycoprotein plus carbohydrate (styrofoam ball with flag) Glycolipid Phospholipid (ping-pong ball) Figure 5.12 Microfilaments of the cytoskeleton Proteins (basketballs) CYTOPLASM Cholesterol ( noodle )
5.13 Proteins make the membrane a mosaic of function Some membrane proteins form cell junctions Others transport substances across the membrane Figure 5.13 Transport Many membrane proteins are enzymes Some proteins function as receptors for chemical messages from other cells The binding of a messenger to a receptor may trigger signal transduction Messenger molecule Receptor Activated molecule Figure 5.13 Enzyme activity Signal transduction
5.14 Passive transport is diffusion across a membrane In passive transport, substances diffuse through membranes without work by the cell to achieve chemical equilibrium Molecule of dye Membrane EQUILIBRIUM They spread from areas of high concentration to areas of lower concentration Figure 5.14A & B EQUILIBRIUM 5.15 Osmosis is the passive transport of water In osmosis, water travels from an area of lower solute concentration (hypotonic) to an area of higher solute concentration (hypertonic) Selectively permeable membrane Hyptonic solution HYPOTONIC SOLUTION Water molecule Hypertonic solution Solute molecule HYPERTONIC SOLUTION Selectively permeable membrane Solute molecule with cluster of water molecules NET FLOW OF WATER Figure 5.15
5.16 Water balance between cells and their surroundings is crucial to organisms Osmosis causes cells to shrink in a hypertonic solution and swell in a hypotonic solution The control of water balance is called osmoregulation ISOTONIC SOLUTION HYPOTONIC SOLUTION HYPERTONIC SOLUTION ANIMAL CELL (1) Normal (2) Lysing (3) Shriveled Cell membrane PLANT CELL Figure 5.16 (4) Flaccid (5) Turgid (6) Shriveled 5.17 Transport proteins facilitate diffusion across membranes Small nonpolar molecules diffuse freely through the phospholipid bilayer Many other kinds of molecules pass through selective protein pores by facilitated diffusion Solute molecule Figure 5.17 Transport protein
5.18 Cells expend energy for active transport Transport proteins can move solutes across a membrane against a concentration gradient This is called active transport Active transport requires ATP 5.19 Exocytosis and endocytosis transport large molecules To move large molecules or particles through a membrane a transport vesicle may fuse with the membrane and expel its contents (exocytosis) FLUID OUTSIDE CELL Figure 5.19A CYTOPLASM
or the membrane may fold inward, trapping material from the outside by creating a transport vesicle (endocytosis) Figure 5.19B Three kinds of endocytosis a cell engulfs solid food via phagocytosis a cell absorbs liquids via pinocytosis receptor-lined membrane pits accept specific molecules that trigger receptor-mediated endocytosis Pseudopod of amoeba Food being ingested Cell membrane Material bound to receptor proteins pit Cytoplasm Figure 5.19C