CHAPTER 5 The Working Cell Overview: Energy Def Laws Chemical Reactions ATP Enzymes Def Activity Membrane Structure Function Transport (passive, active, Osmosis, exo- and endocytosis)

–What is energy? Energy is defined as the capacity to do work All organisms require energy to stay alive Energy makes change possible

Kinetic energy is the energy of motion Potential energy is stored energy

First law of thermodynamics: Energy can be changed from one form to another –However, energy cannot be created or destroyed Two laws govern energy conversion Second law of thermodynamics: Energy changes are not 100% efficient –Energy conversions increase disorder, or entropy –Some energy is always lost as heat

There are two types of chemical reactions: –Endergonic reactions absorb energy and yield products rich in potential energy –Exergonic reactions release energy and yield products that contain less potential energy than their reactants

Cellular respiration –Is the energy-releasing chemical breakdown of fuel molecules –Provides energy for the cell to do work

Cells carry out thousands of chemical reactions –The sum of these reactions constitutes cellular metabolism Chemical reactions either store or release energy

In cellular respiration, some energy is stored in ATP molecules ATP powers nearly all forms of cellular work ATP molecules are the key to energy coupling ATP shuttles chemical energy within the cell The chemical energy of organic molecules is released in cellular respiration to make ATP in the mitochondria

ATP (adenosine triphosphate) The Structure of ATP –Consists of adenosine plus a tail of three phosphate groups –Is broken down to ADP, accompanied by the release of energy When the bond joining a phosphate group to the rest of an ATP molecule is broken by hydrolysis, the reaction supplies energy for cellular work ATP can energize other molecules by transferring phosphate groups

For a chemical reaction to begin, reactants must absorb some energy –This energy is called the energy of activation (EA) –This represents the energy barrier that prevents molecules from breaking down spontaneously Enzymes speed up the cell’s chemical reactions by lowering energy barriers ENZYMES Activation energy –Is the energy that activates the reactants –Triggers a chemical reaction

Enzymes –Lower the activation energy for chemical reactions

Enzymes are selective –This selectivity determines which chemical reactions occur in a cell A specific enzyme catalyzes each cellular reaction Each enzyme recognizes a specific substrate –The active site fits to the substrate, and the enzyme changes shape slightly –This interaction is called induced fit

Enzyme activity is influenced by –temperature –concentration –pH Some enzymes require nonprotein cofactors –Some cofactors are organic molecules called coenzymes The cellular environment affects enzyme activity

Enzyme inhibitors Enzyme Inhibitors –Can inhibit a metabolic reaction –Bind to the active site, as substrate impostors Other inhibitors –Bind at a remote site, changing the enzyme’s shape –In some cases, this is called feedback regulation

Membranes organize the chemical reactions making up metabolism Membranes organize the chemical activities of cells MEMBRANE STRUCTURE AND FUNCTION Cytoplasm  

Membranes are selectively permeable –They control the flow of substances into and out of a cell Membranes can hold teams of enzymes that function in metabolism

Phospholipids are the main structural components of membranes They each have a hydrophilic head and two hydrophobic tails Membrane phospholipids form a bilayer

In water, phospholipids form a stable bilayer –The heads face outward and the tails face inward

Phospholipid molecules form a flexible bilayer –Cholesterol and protein molecules are embedded in it –Carbohydrates act as cell identification tags The membrane is a fluid mosaic of phospholipids and proteins

Some membrane proteins form cell junctions Others transport substances across the membrane Proteins make the membrane a mosaic of function

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

In passive transport, substances diffuse through membranes without work by the cell –They spread from areas of high concentration to areas of lower concentration Passive transport is diffusion across a membrane

Diffusion is one result of the movement of molecules –Molecules tend to spread into the available space –Diffusion is passive transport; no energy is needed Another type of passive transport is facilitated diffusion, the transport of some substances by specific transport proteins that act as selective corridors

In osmosis, water travels from an area of lower solute concentration to an area of higher solute concentration Osmosis is the passive transport of water

Osmosis causes cells to shrink in a hypertonic (has a higher concentration of solute) solution and swell in a hypotonic (has a lower concentration of solute) solution The control of water balance (osmoregulation) is essential for organisms Water balance between cells and their surroundings is crucial to organisms

Small nonpolar molecules diffuse freely through the phospholipid bilayer Many other kinds of molecules pass through selective protein pores by facilitated diffusion Transport proteins facilitate diffusion across membranes

Active transport requires energy to move molecules across a membrane against a concentration gradient Active Transport: the Pumping of Molecules Across Membranes Active transport in two solutes across a membrane

To move large molecules or particles through a membrane –a vesicle may fuse with the membrane and expel its contents (exocytosis: Secretes substances outside of the cell) Exocytosis and endocytosis transport large molecules

–or the membrane may fold inward, trapping material from the outside (endocytosis: takes material into the cell)

Three kinds of endocytosis Receptor-mediated endocytosis –Is triggered by the binding of external molecules to membrane proteins

In phagocytosis (“cellular eating”) a cell engulfs a particle and packages it within a food vacuole In pinocytosis (“cellular drinking”) a cell “gulps” droplets of fluid by forming tiny vesicles Food being ingested Pseudopod of amoeba

Enzymes and membranes are central to the processes that make energy available to the cell Chloroplasts carry out photosynthesis, using solar energy to produce glucose and oxygen from carbon dioxide and water Mitochondria consume oxygen in cellular respiration, using the energy stored in glucose to make ATP Chloroplasts and mitochondria make energy available for cellular work

Nearly all the chemical energy that organisms use comes ultimately from sunlight Chemicals recycle among living organisms and their environment