How Cells Work

Chapter 4

Beer, Enzymes and Your Liver

• Alcohol is toxic

• Cells in liver break down alcohol to nontoxic compounds

• Breakdown is accelerated by enzymes

• Heavy drinking damages liver and other organs

Impacts, Issues Video

Alcohol, Enzymes, and Your LiverAlcohol, Enzymes, and Your Liver

Energy Laws

• Energy: the capacity to do work

• Total amount of energy in the universe is constant

• Energy flows from higher to lower energy forms

ENERGY LOSTWith each conversion, there is a one-way flow of a bit of energy back to the environment.

ENERGY GAINEDSunlight energy reaches environments on Earth. Producers secure some and convert it to stored forms of energy. They and all other organisms convert stored energy to forms that can drive cellular work.

ENERGY LOSTEnergy continually flows from the sun.

Fig. 4-1, p.59

ATP

3 phosphate groups

nucleotide base (adenine)

sugar (ribose)

• Main energy carrier in cells

• Can give up phosphate group to another molecule

• Phosphorylation energizes molecules to react

The Cell’s Energy Currency

• ATP couples energy inputs and outputs

• ATP/ADP cycle regenerates ATP

Energy Input(from nutrientslike glucose) ADP + Pi

ATP

Energy Output (to allow muscle contraction)

reactions that

release energy

reactions that

require energy

cellular work

(e.g., synthesis,breakdown, or rearrangement of substances;contraction of muscle cells; active transport across a cell membrane)

ATP

ADP + Pi

ATP

Fig. 4-2, p.59

base

sugar

three phosphategroups

Structure of ATP

The Role of ATP

Energy Changes

• Endergonic reactions require energy– Synthesis of glucose from carbon dioxide

and water during photosynthesis

• Exergonic reactions release energy– Breakdown of glucose to carbon dioxide

and water by aerobic respiration

glucose (product)

energyin

starting substances

+ 6

+ 6O2

6

Fig. 4-3a, p.60

Energy in Glucose

Photosynthesis

energy out

products

glucose + 6O2

(starting substances)

Fig. 4-3b, p.60

+ 66

Energy in Glucose

This energy helps make ATP

Metabolic Pathways

• Biosynthetic (anabolic) pathways– Require energy inputs– Assemble large molecules from subunits– Photosynthesis

• Degradative (catabolic) pathways– Release energy– Breakdown large molecules to subunits– Aerobic respiration

Enzymes

• Catalyze (speed up) reactions

• Are proteins

Factors Influencing Enzyme Activity

Coenzymes and cofactors

Allosteric regulators

Temperature

pH

Salt concentration

Effect of Temperature

• Small increase in temperature increases molecular collisions, reaction rates

• High temperatures disrupt bonds and destroy the shape of active site

Enzymes and temperatureEnzymes and temperature

Effect of TemperatureEffect of Temperature

Concentration Gradient

• Different numbers of molecules or ions in different regions

• Substances tend to move down gradient - from higher to lower concentration

oxygen, carbon dioxide, and other small, nonpolar molecules; some water molecules

glucose and other large, polar, water-soluble molecules; ions (e.g.,H+, Na+, K+, Ca++, Cl–)

Selective Permeability

Cell Membranes Show Selective Permeability

Selective permeability

Fig. 4-12, p.65

dye

dye

water

Diffusion

• Net movement of molecules or ions down a concentration gradient

DiffusionDiffusion

Diffusion of dye in water

Factors Affecting Diffusion Rate

• Steepness of concentration gradient– Steeper gradient, faster diffusion

• Molecular size– Smaller molecules, faster diffusion

• Temperature– Higher temperature, faster diffusion

• Electrical or pressure gradients

Passive and Active Transport

• Doesn’t require energy inputs

• Solutes diffuse through a channel inside the protein’s interior

• Net movement is down concentration gradient

Passive Transport Active Transport• Requires ATP

• Protein is an ATPase pump

• Pumps solute against its concentration gradient

Which Way Will Water Move?

• Water diffuses across cell membranes

Osmosis

• Diffusion of water molecules across a

selectively permeable membrane,

down the water concentration gradient

• Higher solute concentration = lower

water concentration

water molecules protein molecules

semipermeable membranebetween two compartments

Fig. 4-15, p.68

Osmosis

Solute concentration and osmosis

Membrane Traffic

• Exocytosis– Vesicle fuses with membrane, releasing

substance into intracellular fluid

Membrane Traffic

• Endocytosis– Membrane forms vesicle, bringing

substance into cell

Endocytosis (vesicles in) Exocytosis (vesicles out)

Fig. 4-20, p.71

Phagocytosis

Phagocytosis

Fig. 4-18, p.70

a

b

exocytosis(out from cytoplasm)

endocytosis(into cytoplasm)