CHAPTER 5The Working Cell

Cool “Fires” Attract Mates and Meals

Fireflies use light to send signals to potential mates instead of using chemical signals like most other insects

They use a pattern on flashing lights to communicate with the opposite sex.

The light comes from a set of chemical reactions That occur in light-producing organs at the rear of the insect. Light is a form of energy; it requires an enzyme controlled

chemical reaction in the cells membrane.

◦ Females of some species Produce a light pattern that attracts males of other

species, which are then eaten by the female

ENERGY AND THE CELL5.1 Energy is the capacity to perform work

All organisms require energy which is defined as: “the capacity to do work”

What kind of work does a cell do? Breaks down polymers into monomers Builds polymers Transports nutrients

Kinetic energy is the energy of motion. Heat (thermal energy) is the kinetic energy associated with the movement of

molecules. Light is another kind of kinetic energy that can be harnessed to do

work(photosynthesis). Potential energy is stored energy that can be converted to kinetic energy.

Figure 5.1A–C

Chemical Energy

Chemical energy is the potential energy of molecules:The arrangement of atoms and their

charges provides potential energy that can do work for the cello.

Life depends on the fact that energy can be converted from one form to another.

How Can An Object At Rest Have

Energy? It can have potential energy as a result

of its location.

5.2 Two laws govern energy transformations

Thermodynamics. is the study of energy transformations

The First Law of ThermodynamicsAccording to the first law of thermodynamics:

○ Energy can be changed from one form to another○ Energy cannot be created or destroyed

Figure 5.2A

The Second Law of ThermodynamicsThe second law of thermodynamics

○ Energy transformations increase disorder or entropy, and some energy is lost as heat.

○ Heat is a disordered form of energy, and its release makes the universe more random, more disorganized.Example the flow of energy in an ecosystem:

Figure 5.2B

Heat

Chemical reactions

ATP ATP

Glucose

+

Oxygen water

Carbon dioxide

+

Energy for cellular work

Web/CD 5A

Describe the energy transformation that occur when you climb to the top of a stairway.

You convert chemical energy (food that you have consumed) into Kinetic energy (climbing the stairs)

At the top of the stairs, some of the energy has been stored as Potential Energy (higher elevation).

The rest is converted to Heat

5.3 Chemical reactions either store or release energy

Endergonic reactions requires input of energy.

○ Absorb energy and yield products rich in potential energy

Figure 5.3A

Pot

entia

l ene

rgy

of m

olec

ules

Reactants

Energy required

Products

Amount of energy

required

The energy gained Is stored in covalentbonds.

e.g. Photosynthesis

Exergonic reactions - releases energy○ Release energy and yield products that contain less

potential energy than their reactants○ Web/CD 5B

Figure 5.3B

Reactants

Energy released

Products

Amount of energy

released

Po

ten

tial e

ne

rgy

of

mo

lecu

les

Cellular RespirationCellular respiration uses oxygen to convert

chemical energy (glycogen, fats, protiens,etc) to energy that does work for the cell.

Cells carry out thousands of chemical reactions (endergonic and exergonic)the sum of which constitutes cellular metabolism.

Energy coupling○ Uses exergonic reactions to fuel endergonic

reactions.

Cellular respiration is an exergonic process. Remembering that energy must be conserved, what becomes of the energy extracted from food during cellular respiration?

Some is stored as ATP The rest is released as heat.

5.4 ATP shuttles chemical energy and drives cellular work

ATP powers nearly all forms of cellular work

The energy in an ATP molecule lies in the bonds between its phosphate groups○ By using HYDROLYSIS – the Phosphate bonds are broken and

energy is released. (i.e. it is EXERGONIC).○ Web/CD structure of ATP 5C

Phosphategroups

ATP

EnergyP P PP P PHydrolysis

Adenine

Ribose

H2O

Adenosine diphosphateAdenosine Triphosphate

++

ADP

Figure 5.4A

* The three bonds in the phosphate group are unstable and can readily be broken by hydrolysis

ATP drives endergonic reactions by phosphorylation○ Transferring a phosphate group to make molecules

more reactive○ ATP is a renewable resource that cell generate.

Figure 5.4B

ATP

Chemical work Mechanical work Transport work

P

P

P

P

P

P

P

Molecule formed Protein moved Solute transported

ADP +

Product

Reactants

Motorprotein

Membraneprotein Solute

+

There are threeforms of work: Chemical Mechanical Transport

ATP

ADP + P

Energy forendergonicreactions

Energy fromexergonicreactions

Pho

spho

ryla

tion

Hydrolysis

Cellular work can be sustained○ Because ATP is a renewable resource that cells

regenerate

Figure 5.4C

Explain how ATP transfers energy from exergonic to endergonic process in the cell

By phosphorylation:1. Exergonic reaction add (phosphorylates) a

phosphate group to ADP (Adenosine Di-Phosphate) to make ATP (Adenosine Tri-Phosphat.e).

2. ATP transfers energy (Endergonic reaction) by releasing a phosphate (phosphorylating) to another molecule.

(ATP → ADP)

5.5 HOW ENZYMES FUNCTION

Enzymes speed up the cell’s chemical reactions by lowering energy barriers

An enzyme is a protein molecule that functions as a biological catalyst, increasing the rate of a reaction without itself being changed into a different molecule.

For a chemical reaction to begin○ Reactants must absorb some energy, called the

energy of activation

Figure 5.5A

EA barrier

Reactants

Products1 2E

nzy

me

A protein catalyst called an enzyme○ Can decrease the energy of activation needed to begin

a reaction

Figure 5.5B

Reactants

EA withoutenzyme

EA withenzyme

Net changein energy

Products

Ene

rgy

Progress of the reaction

Why can’t an enzyme change an endergonic reaction into an exergonic reaction?

An enzyme has no effect on the reactants , it effects the rate of reaction by lowering the energy required to activate the reaction.

5.6 A specific enzyme catalyzes each cellular reaction

Enzymes have unique three-dimensional shapes that

determine which chemical reactions occur in a cell.Enzymes are proteins with a unique three

dimensional shape.

○ REVIEW: Reactants – the starting material in a chemical reaction Products – the ending material in chemical reaction

○ NEW TERM: Substrate – a specific substance (reactant) on which an

enzyme reacts

Figure 5.6

Enzyme(sucrase)Glucose

Fructose

Active site Substrate(sucrose)

H2O

1 Enzyme availablewith empty activesite

2 Substrate binds to enzyme with induced fit

4 Products arereleased

3 Substrate is converted to products

The catalytic cycle of an enzyme: Web/CD 5D

What is meant by induced fit?1. The change in shape of the active site

on an enzyme as the substrate attaches to the binding site.

2. This change in shape provides the right environment for the the reaction occur.

5.7 The cellular environment affects enzyme activity

An enzymes structure and shape are essential to its function:○ Its shape is effected by the changes in the

environmentTemperatureSalt concentrationpH influence enzyme activity

Some enzymes require non-protein cofactors

Such as metal ions or organic molecules called coenzymes

Temperature:High temperatures denature the enzymeOptimal temperature 35° - 40° C

Salt ConcentrationSalt ions can interfere with some chemical bonds

pHOptimal range = 6 – 8CONNECTION: lakes are influenced by acid

precipitation which effect enzyme activity in organism that live there.

Many Enzymes will not function without non-protein helpers:Cofactors (inorganic molecules)

○ Iron○ Zinc Copper

Coenzymes (organic molecules)○ Vitamins

A few enzymes work best at very low pH, about 2. Where in the body do you think these enzymes are located?

The Stomach

5.8 Enzyme inhibitors block enzyme action.

Chemicals that interfere with an enzymes activity are called Inhibitors

Two types of Inhibitors:Competitive inhibitor

Non-competitive inhibitor

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

Figure 5.8

Substrate

Enzyme

Active site

Normal binding of substrate

Enzyme inhibition

Noncompetitiveinhibitor

Competitiveinhibitor

• Inhibitors are notalways harmful. Some act as feedbackinhibition (i.e. the onand off switch)

What is the advantage of feedback inhibition to a cell?

It prevents the cell from wasting energy by synthesizing more of a product than is necessary

CONNECTION: 5.9 Many poisons, pesticides, and drugs are enzyme inhibitors

ExampleCyanide – inhibits an enzyme involved with the

production of ATP during cellular respirationSerine – blocks the active site on

acetylcholinesterase ( an enzyme vital to nerve transmission)

Some antibiotics – inhibit enzymes necessary for bacteria to survive.

How does the antibiotic penicillin work?

It interferes with an enzyme used in the making of cell wall in bacteria.

5.10 MEMBRANE STRUCTURE AND FUNCTION

Membranes organize the chemical activities of cells

Membranes provide structural order for metabolism

Membranes form most of the cell’s organelles, partitioning the cell into functional compartments that contain specific enzymes.

A. Membranes separate cells from the outside environments, including, in multi-cellular organisms, the environment in other cells that perform different functions.

B. Membranes control the passage of molecules from one side of the membrane to the other.

C. In eukaryotes, membranes partition function into organelles

D. Membranes provide reaction surfaces, and organize enzymes and their substrates.

E. Membranes are selectively permeable, which means some substances can pass through a membrane more easily than other substances. Compare ethanol (as a fixative) to glucose.

The plasma membrane of the cell is selectively permeable

○ It controls the flow of substances into or out of the cell

Figure 5.10

Cytoplasm

Outside of cell

TE

M 2

00,0

00

5.11 Membrane phospholipids form a bilayer phospholipids

○ Have a hydrophilic head and two hydrophobic tails

○ Are the main structural components of membranes

Figure 5.11A

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH

CH

CH2

CH2

CH2

CH2

CH2

CH2

CH2

CH3

CH2CH2

CH3

CH

3

CH3N+

O

O O–P

O

CH2CHCH2

C O C O

O O

Phosphategroup

Symbol

Hydrophilic head

Hydrophobic tails

Phospholipids form a two-layer sheet○ Called a phospholipid bilayer, with the heads facing

outward and the tails facing inward

Figure 5.11B

Water

Water

Hydrophilicheads

Hydrophobictails

Why do phospholipids tend to organize into a bilayer in an aqueous environment?

This structure shields the hydrophobic tail from water, while exposing the hydrophilic heads to water, making the membrane selectively permeable.

5.12 The membrane is a fluid mosaic of phospholipids and proteins

A membrane is a fluid mosaic with proteins and other molecules embedded in a phospholipid bilayer.

Web Activity 5E

Figure 5.12

Fibers of the extracellular matrix Carbohydrate

(of glycoprotein)

Glycoprotein

Microfilamentsof cytoskeleton

Phospholipid

CholesterolProteins

Plasmamembrane

Glycolipid

Cytoplasm

Why are cellular membranes described as a fluid mosaic?

5.13 Proteins make the membrane a mosaic of function

1. Membrane proteins have a variety of diverse functions.

2. Different cells have different sets of membrane proteins

3. Many membrane proteins function

as enzymes

Enzymes: catalyzing intracellular

and extracellular reactions

Figure 5.13A

4. Other membrane proteins function as receptors for chemical messages from other cells:

Receptors trigger cell activity when a messenger molecule attaches. This is also known as Signal Transduction.

Figure 5.13B

Messenger molecule

Receptor

Activatedmolecule

Example: hormones

Web/CD Activity 5F

Membrane proteins also function in transport○ Moving substances across the membrane

Figure 5.13C

ATP

.

CO2 & O2 are small non-polarAnd molecules that can pass freely through the lipid bi-layer;Other essential molecules such asglucose and water cannot pass freely,they need assistance.

Example: Transporters of hydrophilic molecules

5G

5.14 Passive transport is diffusion across a membrane◦ In passive transport, substances diffuse through membranes

without work by the cell Spreading from areas of high concentration to areas of

low concentration Web/CD Activity 5H

EquilibriumMembraneMolecules of dye

Equilibrium

Figure 5.14B

Figure 5.14A

Different moleculesdiffuse independently of one another

Small nonpolar molecules such as O2 and CO2

○ Diffuse easily across the phospholipid bilayer of a membrane○ Ions (charged atoms) and polar molecules (water, glucose) can diffuse if they

move down their concentration gradients and if they have a transport protein to escort them into the cell.

○ Passive transport is an extremely important way for small molecules to get into and out of cells. For example, O2 moves into red blood cells and CO2 moves out of these cells by this process in the lungs. The reverse process takes place in the tissue because the concentration gradients have reversed.

5.15 Facilitated Diffusion

A.Facilitated diffusion occurs when a pored protein, spanning the membrane bilayer, allows a solute to diffuse down a concentration gradient.

B.The cell does not expend energy.

C.The rate of facilitated diffusion depends on the number of such transport proteins, in addition to the strength of the concentration gradient.

Transport proteins may facilitate diffusion across membranes

Many kinds of molecules do not diffuse freely across membranes

For these molecules, transport proteins provide passage across membranes through a process called facilitated diffusion.

Figure 5.15

Solutemolecule

Transportprotein

Web 5IFacilitated diffusion occurs when a pored protein, spanning the membrane bi-layer, allows a solute to diffusedown a concentrationgradient.

Daily Planner

5.16 Osmosis is the diffusion of water across a membrane.

In osmosis water travels from a solution of lower solute concentration to one of higher solute concentration

Figure 5.16

Lowerconcentration

of solute

Higherconcentration

of solute

Equalconcentration

of solute

H2OSolutemolecule

Selectivelypermeablemembrane

Watermolecule

Solute molecule withcluster of water molecules

Net flow of water

Osmosis Cont.,

A. Water travels from area of low solute concentration to area of high solute concentration.

B. The direction of osmosis is determined only by the difference in total solute concentrations.

C. Two solutions equal in solute concentrations are isotonic to each other; therefore, osmosis does not occur.

D. However, even in isotonic solutions separated by a selectively permeable membrane, water molecules are moving in both directions at equal rates.

5.17 Water balance between cells and their surroundings is crucial to organisms.

◦Cell membranes act as selectively permeable membranes between the cell contents and its surroundings (Figure 5.17). The propensity of a cell to gain or lose water with its surroundings is referred to as tonicity.

Figure 5.17

Plantcell

H2O

H2O H2O

H2O

H2O

H2O

H2O

H2OPlasma

membrane

(1) Normal (2) Lysed (3) Shriveled

(4) Flaccid (5) Turgid(6) Shriveled (plasmolyzed)

Isotonic solution Hypotonic solution Hypertonic solution

Animalcell

Tonicity

◦Osmosis causes cells to:◦ shrink in hypertonic solutions◦swell in hypotonic solutions◦ In isotonic solutions Animal cells are normal,

but plant cells are limp.The control of water balance

○ Is called osmoregulation

◦WEB 5J Osmosis and Water Balance in Cells

Explain the function of the contractile vacuole in the Paramecium cell in Fig. 4.12B in terms of what you have learned about water balance in cells.

L M

6 5 0

Nucleus

Contractilevacuoles

Pond water is HYPOTONIC to theParamecium; therefore water movesInto the vacuole. The ability of theVacuoles to contract, expels the waterso the cell does not explode.

PP PProtein

changes shapePhosphatedetaches

ATPADPSolute

Transportprotein

Solute binding1 Phosphorylation2 Transport3 Protein reversion4

5.18 Cells expend energy for active transport◦Transport proteins can move solutes against a

concentration gradient Through active transport, which requires ATP Web 5K (optional)

Figure 5.18

Fluid outside cell

Cytoplasm

Protein

Vesicle

5.19 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)○ Web 5L Exocytosis and Endocytosis

Figure 5.19A

Membranes may fold inward○ Enclosing material from the outside (endocytosis)

Figure 5.19B

Vesicle forming

Endocytosis can occur in three ways○ Phagocytosis – taking in food particles○ Pinocytosis – cellular drinking○ Receptor-mediated endocytosis – highly specific, the

cell forms a pit and forms a vesicle that will carry the molecule to the cytoplasm.

Pseudopodium of amoebaFood being ingested

Phagocytosis Pinocytosis Receptor-mediated endocytosis

Material bound to receptor proteins

PIT

Cytoplasm

Plasma membrane

TE

M 5

4,00

0

TE

M 9

6,50

0

LM 2

30

Figure 5.19C

CONNECTION

A. Cholesterol is carried in the blood by low-density lipoprotein (LDL) particles.

B. In people with normal cholesterol metabolism, excess LDL-bound cholesterol in the blood is eliminated by receptor-mediated endocytosis by liver cells.

C. In people with a genetic condition that results in increased levels of cholesterol (hypercholesterolemia), fewer or no such receptor sites exist, and the people accumulate LDL-bound cholesterol. These people are at high risk for developing heart disease.

5.20 CONNECTIONFaulty membranes can overload the blood with cholesterol

Harmful levels of cholesterol can accumulate in the blood if membranes lack cholesterol receptors

LDL particle

Protein

Phospholipid outer layer

CytoplasmReceptorprotein

Plasmamembrane

Vesicle

Cholesterol

Figure 5.20

5.21 Chloroplasts and mitochondria make energy available for cellular work

Chloroplasts carry out photosynthesis○ Using solar energy to produce glucose and

oxygen from carbon dioxide and waterMitochondria consume oxygen in cellular

respiration○ Using the energy stored in glucose to make

ATP

Web 5M Build a Chemical Cycling System