Essentials of BiologySylvia S. Mader

Chapter 5Lecture Outline

Prepared by: Dr. Stephen EbbsSouthern Illinois University Carbondale

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

5.1 What Is Energy

• Energy is the capacity to do work.

• There are two basic forms of energy.– Potential energy is stored energy.

– Kinetic energy is energy of motion.

• Energy is constantly being exchanged between these two forms.

5.1 What Is Energy (cont.)

Measuring Energy

• Energy can be measured in different units.– Most forms of energy are measured in joules.– Food energy is measured in calories.

• A calorie is the amount of heat required to raise the temperature of 1 gram of water 1 degree Celsius.

• Food labels list the caloric value of food in kilocalories (1,000 calories).

Two Energy Laws

• Energy laws describe the principles of energy flow and energy conversion.

• The law of conservation of energy says that energy cannot be created or destroyed, but can change from one form to another.

Two Energy Laws (cont.)

• The second energy law says that energy cannot be changed from one form to another without a loss of usable energy.

• Most of the energy lost during energy interconversions is lost as heat.

Entropy

• Another interpretation of the second energy law says that every energy transformation leads to more disorder.

• The degree of disorder or disorganization is referred to as entropy.

• All energy transformations, including those in cells, increase the entropy of the universe.

Entropy (cont.)

Entropy (cont.)

5.2 ATP: Energy for Cells

• ATP (adenosine triphosphate) is the energy currency of cells.

• ATP is used to drive nearly all cellular activities.

Structure of ATP

• ATP is a nucleotide, similar to the monomers of DNA and RNA.

• The ATP molecule contains three parts.– The sugar ribose– The nitrogenous base adenine– Three phosphate groups

• The energy of ATP is stored in the phosphate groups.

Structure of ATP (cont.)

Use and Production of ATP

• The continual breakdown and regeneration of ATP is the ATP cycle.

• Because of its instability, ATP provides only short term storage of energy.

• Carbohydrates and fats are more stable energy storage molecules that, when degraded, are used to generate ATP.

Use and Production of ATP (cont.)

Use and Production of ATP (cont.)

• The production of ATP has several benefits for cells.– ATP can be used for many different types of

chemical reactions.– When ATP is split to release energy, the

amount of energy released is sufficient for most reactions without being wasteful.

– The breakdown of ATP can be coupled to energy-requiring reactions.

Coupled Reactions• Coupled reactions occur in the same place

at the same time.

• The energy-releasing reaction provides the energy to drive the energy-requiring reaction, as in the example below.

Coupled Reactions (cont.)

The Flow of Energy

• The cycling of molecules between the chloroplasts and mitochondria is responsible for the flow of energy through the biosphere.

• Chloroplasts use solar energy to convert water and carbon dioxide to carbohydrates.

• Cellular respiration in the mitochondria breaks down carbohydrates to yield energy (ATP), releasing carbon dioxide and water.

The Flow of Energy (cont.)

The Flow of Energy (cont.)

• Humans also contribute to the flow of energy from the sun and through the biosphere.

• Humans release carbon dioxide and water that plants can use for photosynthesis.

• The carbohydrates and nutrients in foods are broken down in the mitochondria of human cells to produce ATP needed for cellular activities.

The Flow of Energy (cont.)

5.3 Metabolic Pathways and Enzymes

• In living organisms, chemical reactions are often linked together in series to form metabolic pathways.

• The reactants, or substrates, are the chemicals that enter the metabolic pathway.

• Enzymes are protein molecules that function as organic catalysts to speed up a chemical reaction.

5.3 Metabolic Pathways and Enzymes (cont.)

• A metabolic pathway can be represented by a simple diagram.

– The letters A-G indicate substrates.– The letters E1-E6 represent enzymes.– A is the substrate for E1, B is the substrate for E2, and so on.

E1 E2 E3 E4 E5 E6

A B C D E F G

Energy of Activation

• Molecules often must be activated before a chemical reaction can occur.

• The energy needed to cause a molecule to react with another molecule is called the energy of activation (Ea).

• Enzymes help catalyze reactions by lowering the energy of activation for a reaction.

Energy of Activation (cont.)

An Enzyme’s Active Site

• The active site of an enzyme is the point where a substrate binds like a key in a lock.

• According to the induced fit model, the active site may undergo a slight change to accommodate a substrate.

• Once bound to the active site, the enzyme facilitates the conversion of substrate to product.

• The product is then released from the active site.

An Enzyme’s Active Site (cont.)

Enzyme Inhibition

• Enzyme inhibition occurs when an active enzyme is prevented from binding to a substrate by an inhibitor.

• Some inhibitors are poisonous to living organisms.– Cyanide is an inhibitor that blocks ATP

synthesis.– Penicillin inhibits a specific bacterial enzyme.

Enzyme Inhibition

• Another type of inhibition, called feedback inhibition, is used to control metabolic pathways.

• In feedback inhibition, production of sufficient product shuts the synthesis pathway off.

• There are several other complex mechanisms by which products provide feedback inhibition to pathways.

Enzyme Inhibition (cont.)

5.4 Cell Transport

• The plasma membrane regulates the transport of molecules into and out of the cell.

• The plasma membrane is differentially permeable, which means that some substances move freely across the membrane but others are restricted.

5.4 Cell Transport (cont.)

• Substances can enter cells in three ways.– Passive transport

– Active transport

– Bulk transport

Passive Transport: No Energy Required

• Simple diffusion occurs when the solute (a substance dissolved in a liquid solvent) moves from a higher concentration to a lower concentration.

• Simple diffusion occurs until equilibrium is reached.

• Simple diffusion is passive because it does not require energy.

Passive Transport: No Energy Required (cont.)

• Small, uncharged molecules such as oxygen, carbon dioxide, and water cross membranes by simple diffusion.

• Ions and polar molecules cross membranes by facilitated diffusion.– Facilitated diffusion is also passive transport.– Membrane proteins assist the movement of

the molecule across the membrane.

Passive Transport: No Energy Required (cont.)

Osmosis

• Diffusion of water across a differentially permeable membrane is called osmosis.

• Osmosis is a type of passive diffusion where the solvent (water) moves across the membrane, rather than the solute.

Osmosis (cont.)

The Effect of Osmosis on Cells

• Osmosis can affect the size and shape of cells, depending on differences in water concentration across the membrane.

• Cells placed in an isotonic solution do not change because the concentration of water on both sides of the membrane is the same.

The Effect of Osmosis on Cells (cont.)

• Cells placed in a hypotonic solution gain water (and may lyse) because the concentration of water is higher outside the cell and water rushes in.

• Cells placed in a hypertonic solution lose water because the concentration of water is higher inside the cell and water rushes out.– An animal cell in a hypertonic solution shrinks.– A plant cell in a hypertonic solution undergoes

plasmolysis (shrinking of the cytoplasm).

The Effect of Osmosis on Cells (cont.)

Active Transport: Energy Required

• During active transport, molecules move against their concentration gradient.

• Active transport requires a membrane protein and energy to move the molecule.

• The energy for active transport is generally provided by the mitochondria.

Active Transport: Energy Required (cont.)

• Proteins engaged in active transport are often called pumps.

• The sodium-potassium pump is an example of an active transport process critical to nerve conduction.

Active Transport: Energy Required (cont.)

Bulk Transport

• Macromolecules are too large to move with membrane proteins and must be transported across membranes in vesicles.

• The transport of macromolecules out of a cell in a vesicle is called exocytosis.

• The transport of macromolecules into a cell in a vesicle is called endocytosis.

Bulk Transport (cont.)

Bulk Transport (cont.)

Bulk Transport (cont.)

• If the material taken up by endocytosis is a large particle it is called phagocytosis.

• If the material taken up by endocytosis is a liquid or small particle it is called pinocytosis.

• Receptor-mediated endocytosis is a selective, highly efficient form of endocytosis.

Bulk Transport (cont.)