CHAPTER 5The Working Cell

Overview: Energy

DefLawsChemical ReactionsATP

EnzymesDefActivity

MembraneStructureFunctionTransport (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

Pseudopodof 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