chapter 5 gasesclasspages.warnerpacific.edu/sramos/bio102 lectures... · –a molecule that accepts...

© Cengage Learning 2016

Chapter 5

Ground Rules of

Metabolism


5.1 A Toast to Alcohol Dehydrogenase

• In the liver, the enzyme alcohol

dehydrogenase breaks down ethanol

• Ethanol breakdown interferes with normal

processes of metabolism

– Harms liver cells

• Fats tend to accumulate as large globules in

the tissues of heavy drinkers

– Can lead to alcoholic hepatitis and cirrhosis of

the liver


Alcoholic Liver Disease


5.2 Energy in the World of Life

• Sustaining life’s organization requires

ongoing energy inputs

• Energy is the capacity to do work

– One form of energy can be converted to another

– Familiar forms of energy include light, heat,

electricity, and motion (kinetic energy)

– Energy in chemical bonds is a type of potential

energy, because it can be stored


Energy Disperses

• First law of thermodynamics

– Energy is neither created nor destroyed, but can

be transferred from one form to another

• Second law of thermodynamics

– Entropy (a measure of dispersal of energy in a

system) increases spontaneously

– The entropy of two atoms decreases when a

bond forms between them

– Every time energy transfers, bits of it disperse


Energy Flows in Life

A Energy In

Sunlight reaches environments on

Earth. Producers in those environments

capture some of its energy and convert

it to other forms that can drive cellular

work.

PRODUCERS

B Some of the

energy captured by

producers ends up in

the tissues of

consumers.

CONSUMERS

C Energy Out

With each energy transfer, some energy

escapes into the environment, mainly as

heat. Living things do not use heat to

drive cellular work, so energy flows

through the world of life in one direction

overall.


5.3 Energy in the Molecules of Life

• Reaction

– A chemical change that occurs when atoms,

ions, or molecules interact

• Reactant

– Atoms, ions, or molecules that enter a reaction

• Product

– Atoms, ions, or molecules remaining at the end

of a reaction


Equations Represent Chemical

Reactions


Chemical Bond Energy

• All cells store and retrieve energy in

chemical bonds of the molecules of life

• By comparing the bond energies of

reactants with those of products, we can

predict whether a reaction requires or

releases energy

– Endergonic (“energy in”)

• Reactions that require a net input of energy

– Exergonic (“energy out”)

• Reactions that end with a net release of energy


Energy in Chemical Reactions

6

carbon

dioxide

CO2

water

H2O

6

oxyge

n O2

energy

out

energy in

Fre

e

En

erg

y

glucose

C6H12O6

6 6

carbon

dioxide

CO2

water

H2O

6

oxyge

n O2

glucose

C6H12O6

6


Why the Earth Doesn’t Go Up in

Flames

• Activation energy

– The minimum amount of energy needed to get a

reaction started

– Some reactions require a lot of activation

energy; others do not


Activation Energy

Reactants:

2H2 O2

Activation energy

Difference between free energy

of reactants and products

Products: 2H2O

Time

Fre

e e

nerg

y


Energy In, Energy Out

energy

out

organic

compounds

(carbohydrates

, fats, proteins)

organic

compounds

(carbohydrates

, fats, proteins)

small

molecules

(e.g., carbon

dioxide,

water)

small

molecules

(e.g., carbon

dioxide,

water)

endergonic reactions

exergonic reactions

energy

in


5.4 How Enzymes Work

• In a process called catalysis, an enzyme

makes a specific reaction occur much faster

than it would on its own

– Enzymes are not consumed or changed by

participating in a reaction

– Most are proteins; some are RNA

• Substrate

– The specific reactant acted upon by an enzyme


The Transition State

• Catalysis lowers activation energy

– Brings on the transition state, where substrate

bonds break and reactions run spontaneously

• Active sites

– Locations on the enzyme where substrates bind

and reactions proceed

– Complementary in shape, size, polarity and

charge to the substrate


Mechanisms of

Enzyme-Mediated Reactions

• Binding at enzyme active sites may bring on

the transition state by four mechanisms

– Bringing substrates physically together

– Orienting substrates in positions that favor

reaction

– Inducing a fit between enzyme and substrate

(induced-fit model)

– Excluding water molecules


Enzyme Activity

• Raising the temperature boosts reaction

rates by increasing free energy

– But very high temperatures denature enzymes

• Each enzyme has an optimum pH range

– In humans, most enzymes work at pH 6 to 8

• Salt levels affect the hydrogen bonds that

hold enzymes in their three-dimensional

shape


5.5 Metabolism—Organized, Enzyme-

Mediated Reactions

• Molecules interact in organized pathways of

metabolism (activities by which cells acquire

and use energy)

• A metabolic pathway is any series of

enzyme-mediated reactions

– Cells build, rearrange, or break down an

organic substance

– Linear pathways run from reactant to product

– Cyclic pathways regenerate a molecule from the

first step


Controls Over Metabolism

• Concentrations of reactants or products can

make reactions proceed forward or

backward

• Mechanisms can adjust enzyme production,

or activate or inhibit them

– Feedback inhibition decreases or stops the

activity

– Regulatory molecules can activate or inhibit

• Bind directly to active site

• Bind outside active site in allosteric regulation


intermediate

reactant

enzyme 1

enzyme 2

intermediate

enzyme 3

product

X

Stepped Art


Feedback Inhibition

reactant

enzyme 1

intermediate

enzyme 2

intermediate

enzyme 3

product

reactant

intermediate

product

intermediate

enzyme

2

enzyme 1

enzyme 3


Redox Reactions

• Oxidation-reduction reactions

– A molecule that gives up electrons is oxidized

– A molecule that accepts electrons is reduced

– Coenzymes can accept molecules in redox

reactions (also called electron transfers)

• Electron transfer chain

– Series of membrane-bound enzymes and other

molecules that give up and accept electrons in

turn


glucose

1

carbon

dioxide

+

water

oxygen

+

2

3

e–

e–

H+


5.6 Cofactors in Metabolic Pathways

• Most enzymes require helper molecules

– Cofactors

• Atoms or molecules (other than proteins) that are

necessary for enzyme function

– Coenzymes

• Organic cofactors such as vitamins

• May become modified during a reaction


ATP—A Special Coenzyme

• Energy in ATP drives many endergonic

reactions

• ATP (adenosine triphosphate)

– A nucleotide with three phosphate groups

– Transfers a phosphate group and energy to

other molecules

• Phosphorylation

– A phosphate-group transfer

– ADP binds phosphate in an endergonic reaction

to replenish ATP (ATP/ADP cycle)


ATP/ADP Cycle Couples Endergonic

and Exergonic Reactions

reduced

coenzyme

s

small molecules(e.g., carbon dioxide,

water)

oxidized

coenzyme

s

organic compounds(e.g., carbohydrates, fats,

proteins)

ADP + Pi

AT

P


5.7 A Closer Look at Cell Membranes

• A cell membrane is organized as a lipid

bilayer with many proteins embedded in it

and attached to its surfaces

• Phospholipid molecules in the plasma

membrane have two parts

– Hydrophilic heads interact with water molecules

– Hydrophobic tails interact with each other,

forming a barrier to hydrophilic molecules


The Fluid Mosaic Model

• Describes the organization of cell

membranes

– Phospholipids drift and move like a fluid

– The bilayer is a mosaic mixture of

phospholipids, steroids, proteins, and other

molecules


Cell Membrane Structure

one layer

of lipids

one layer

of lipids


Proteins Add Function

• Cell membrane function begins with the

many proteins associated with the lipid

bilayer

• Peripheral membrane proteins temporarily

attach to the lipid bilayer’s surfaces by

interactions with lipids or other proteins

• Integral membrane proteins permanently

attach to a bilayer


Membrane Proteins


5.8 Diffusion and Membranes

• Diffusion

– The net movement of molecules down a

concentration gradient

– Moves substances into, through, and out of

cells

– A substance diffuses in a direction set by its

own concentration gradient


The Rate of Diffusion

• Depends on five factors

– Size

– Temperature

– Steepness of the concentration gradient

– Charge

– Pressure


Semipermeable Membranes

• Selective permeability

– The ability of a cell membrane to control which

substances and how much of them enter or

leave the cell

– Allows the cell to maintain a difference between

its internal environment and extracellular fluid

– Supplies the cell with nutrients, removes

wastes, and maintains volume and pH

– Water diffuses across membranes by osmosis


Selective Permeability of Lipid Bilayers

gasesglucose and

other polar

molecules;

ions

lipid

bilayerwater


Tonicity

• Relative concentrations of solutes in two

fluids separated by a selectively permeable

membrane can differ (tonicity)

• When separated by a membrane, solutions

are either:

– Isotonic with the same solute concentration

– Hypotonic solution with a lower solute

concentration

– Hypertonic solution with a higher solute

concentration


Tonicity in Red Blood Cells

a Red blood cells in an isotonic

solution (such as the fluid portion of

blood) have a normal, indented disk

shape.

b Water diffuses out of red

blood cells immersed in a

hypertonic solution, so they

shrivel up.

C Water diffuses into red blood cells

immersed in a hypotonic solution, so

they swell up. Some of these have

burst.

2 µm


Turgor

• The pressure exerted by a volume of fluid

against a surrounding structure (membrane,

tube, or cell wall), which resists volume

change

• Osmotic pressure

– The amount of turgor that can stop water from

diffusing into cytoplasmic fluid or other

hypertonic solutions

– Keeps walled cells plump


5.9 Membrane Transport Mechanisms

• Many types of molecules and ions can cross

a lipid bilayer only with the help of transport

proteins

– Transport proteins allow a specific substance to

cross

• Ions and large polar molecules require other

mechanisms to cross the cell membrane

– Passive transport

– Active transport


Passive Transport

• Requires no energy input

– Driven entirely by concentration gradient

• Facilitated diffusion is a specific type of

passive transport

– A gated passive transporter changes shape

when a specific molecule binds to it


Passive Transport of Glucose


Active Transport

• Requires energy input (usually ATP)

– Moves a solute against its concentration

gradient, to the concentrated side of the

membrane

K+

K+

Na+

Na+

Na+

P P

P

Na+

Na+

Na+

Na+

Na+

Na+AT

PAD

P

Cytoplas

m

Extracellular

Fluid

K+

K+

K

+

K+


5.10 Membrane Trafficking

• By processes of endocytosis and

exocytosis, cells take in and expel particles

that are too big for transport proteins, as

well as substances in bulk

• Requires formation and movement of

vesicles formed from membranes

• Involves motor proteins and ATP


Exocytosis and Endocytosis

• Exocytosis

– The fusion of a vesicle with the cell membrane,

releasing its contents to the surroundings

• Endocytosis

– The formation of a vesicle from the cell

membrane, enclosing materials near the cell

surface and bringing them into the cell


Exocytosis and Endocytosis –

Illustrated

B P inoc yt os i s . A pit in the plasma membrane traps any fluid, solutes,

and particles near the cell’s surface in a vesicle as it sinks into the

cytoplasm.

C R e c e p t o r-m e d ia t e d endoc yt os i s . Cell surface receptors (green)

bind a target molecule and trigger a pit to form in the plasma membrane.

The target molecules are trapped in a vesicle as the pit sinks into the

cytoplasm. This mode is more selective about what is taken into the cell

than pinocytosis.

A E x o c yt o s i s . A vesicle in cytoplasm fuses with the plasma

membrane. lipids and proteins of the vesicle’s membrane become part

of the plasma membrane as its contents are expelled to the

environment.


Three Pathways of Endocytosis

• Pinocytosis

• A pathway that brings a drop of extracellular fluid along with suspended particles into cells

• Receptor-mediated endocytosis

• Specific molecules bind to surface receptors, which are then enclosed in an endocytic vesicle

• Phagocytosis

• Larger target particles such as microbes or cellular debris are engulfed by pseudopods, which merge as a vesicle


Recycling Membrane

• Exocytosis and endocytosis continually

replace and withdraw patches of the plasma

membrane

• New membrane proteins and lipids are

made in the ER, modified in Golgi bodies,

and form vesicles that fuse with plasma

membrane


Forming a New Plasma Membrane


Points to Ponder

• What factors are involved in how much

alcohol each individual can process?

• Why is heat is the most common form of

entropy?

• What would happen if our body pH rose to

9.5?

chapter 5 gasesclasspages.warnerpacific.edu/sramos/bio102 lectures... · –a molecule that accepts...

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