Chapter 5

The working cell

Cellular energy• Forms

– Kinetic– Potential

• Energy of cells is ATP– Energy lies in covalent bonds between P groups

Energy

• Exergonic– Releases energy

• Endergonic– Intakes energy (required energy)

These reactions are coupled in “energy coupling” where exergonic reactions drive endergonic reactions

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

+

ATP• Drives reactions via

phosphorylation– Transfer of a

phosphate group to make molecules more reactive

Figure 5.4B

ATP• P groups are negatively charged and crowded

together– Repulsion creates unstable bonds– Broken via hydrolysis reaction– As bonds break, 1 P leaves and ATP becomes ADP

& energy is released

• Released P goes to another molecule– Transfer of P = phosphorylation

Begin a chemical reaction• Reactants must absorb some energy

– Energy of activation (EA)

– Enzymes can lower EA

EA barrier

Reactants

Products1 2En

zym

e

Enzymes help begin chemical reactions• Enzymes that lower EA are catalysts

– Decrease the EA needed to begin a reaction

Reactants

EA withoutenzyme

EA withenzyme

Net changein energy

Products

Ener

gy

Progress of the reaction

Specificity

• Specific enzymes catalyze specific reactions– Shape determines which chemical reaction an

enzyme catalyzes

• Substrate– Specific reactant an enzyme acts upon

• Active site– Region of enzyme where substrate fits

Active sites fit only specific substrates

Figure 5.6

Enzyme(sucrase)

Glucose

Fructose

Active site

Substrate(sucrose)

H2O

1

Enzyme availablewith empty activesite

Substrate bindsto enzyme with induced fit

4

Products arereleased

3

Substrate is converted to products

How Enzymes Catalyze Reactions

Factors Influence Enzymes• Temperature, pH, salinity

– Affect shape of the enzyme– Optimal temperature

• Cofactors– Non-protein “helper”– Inorganic

• Ex: zinc, iron, copper• Coenzyme

– Organic• Ex: vitamin

Inhibitors Interfere with an Enzyme’s Activity

• Competitive inhibitor

• Noncompetitive inhibitor

• Feedback inhibition

Substrate

Enzyme

Active site

Normal binding of substrate

Enzyme inhibition

Noncompetitiveinhibitor

Competitiveinhibitor

Phospholipid bilayer• Selectively permeable• Imbedded proteins

– Fluid mosaic of phospholipids and proteins

Water

Water

Hydrophilicheads

Hydrophobictails

Membrane Proteins

• Structural• Cell-cell recognition• Junction forming• Receptors• Enzymes• Signal transduction• Transport

Functions of membrane proteins

Messenger molecule

Receptor

Activatedmolecule ATP

Enzymes Receptors for messages Transport of substances

The membrane is a fluid mosaic of phospholipids and proteins

Figure5.12

Fibers of the extracellular matrix

Carbohydrate(of glycoprotein)

Glycoprotein

Microfilamentsof cytoskeleton

Phospholipid

CholesterolProteins

Plasmamembrane

Glycolipid

Cytoplasm

In and Out of Cells• Diffusion• Passive transport

– Concentration gradientEquilibriumMembraneMolecules of dye

Equilibrium

Plasma membrane (lipid bilayer)

Lipids

In and Out of Cells• Facilitated diffusion

– Small nonpolar molecules diffuse easily across the membrane

– Larger or polar molecules do not easily diffuse • Transport proteins provide passage across membranes

Solutemolecule

Transportprotein

Plasma membrane

Fructose

Transport protein

In and Out of Cells•Osmosis

–Diffusion of water across a membrane• Water travels from a solution of lower solute concentration to

one of higher solute concentration

Selectivelypermeablemembrane

H2O

Solutemolecule

Watermolecule

Net flow of water

Solute molecule withcluster of water molecules

Plasma membrane

Transport protein

Water molecules

BalanceWater balance between cells and their surroundings is crucial to organisms– Osmosis causes cells to shrink in hypertonic solutions

and swell in hypotonic solutions

Figure 5.17

Plantcell

H2O

H2OH2O

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

PP PProtein

changes shapePhosphatedetaches

ATPADPSolute

Transportprotein

Solute binding1 Phosphorylation2 Transport3 Protein reversion4

In and Out of Cells

Cells expend energy for active transport– Transport proteins can move solutes against

concentration gradient

Figure 5.18

Fluid outside cell

Cytoplasm

Protein

Vesicle

In and Out of CellsExocytosis and endocytosis transport large molecules

– Exocytosis• Export material from cell

– Endocytosis• Take into cell

Figure 5.19A

Types of Endocytosis

• Phagosytosis– Cellular “eating”

• Pinocytosis– Cellular “drinking”

• Receptor-mediated endocytosis– Specific molecules

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