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
Two laws govern energy conversion
• First law of thermodynamics:
Energy can be changed from one form to another
– However, energy cannot be created or destroyed
• 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
Chemical reactions either store or
release energy
• Cells carry out thousands of chemical reactions
– The sum of these reactions constitutes cellular
metabolism
ATP shuttles chemical energy within the cell
• The chemical energy of organic molecules is
released in cellular respiration to make ATP in the
mitochondria
• 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
The Structure of ATP
• ATP (adenosine triphosphate)
– 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
ENZYMES
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 (EA)
– This represents the energy barrier that prevents
molecules from breaking down spontaneously
• Activation energy
– Is the energy that activates the reactants
– Triggers a chemical reaction
• Enzymes
– Lower the activation energy for chemical reactions
A specific enzyme catalyzes each
cellular reaction
• Enzymes are selective
– This selectivity determines which chemical reactions
occur in a cell
• 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
The cellular environment affects
enzyme activity
• Enzyme activity is influenced by
– temperature
– concentration
– pH
• Some enzymes require nonprotein cofactors
– Some cofactors are organic molecules called
coenzymes
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
MEMBRANE STRUCTURE AND
FUNCTION
Membranes organize the chemical activities of cells
• Membranes organize the chemical reactions
making up metabolism


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
Membrane phospholipids form a bilayer
• Phospholipids are
the main structural
components of
membranes
• They each have a
hydrophilic head
and two
hydrophobic tails
• In water, phospholipids form a stable bilayer
– The heads face outward and the tails face inward
The membrane is a fluid mosaic of
phospholipids and proteins
• Phospholipid molecules form a flexible bilayer
– Cholesterol and protein molecules are embedded in it
– Carbohydrates act as cell identification tags
Proteins make the membrane a
mosaic of function
• Some membrane proteins form cell junctions
• Others transport substances across the membrane
• 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
Passive transport is diffusion
across a membrane
• In passive transport,
substances diffuse
through membranes
without work by the
cell
– They spread from
areas of high
concentration to areas
of lower concentration
• 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
Osmosis is the passive transport of water
• In osmosis, water
travels from an
area of lower
solute
concentration to an
area of higher
solute
concentration
Water balance between cells and their surroundings is crucial to
organisms
• 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
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
Active Transport: the Pumping of
Molecules Across Membranes
• Active transport requires energy to move
molecules across a membrane against a
concentration gradient
• Active
transport in
two solutes
across a
membrane
Exocytosis and endocytosis transport
large molecules
• 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)
– 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
Pseudopod
of amoeba
Food being ingested
Chloroplasts and mitochondria make
energy available for cellular work
• 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
• Nearly all the chemical
energy that organisms
use comes ultimately
from sunlight
• Chemicals recycle
among living organisms
and their environment