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Chapter 5Dynamic Activities

of Cells

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Living Things Transform Energy

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5.1 Energy makes things happen

Energy - the capacity to do work Chemical energy present in organic molecules

and is the direct source of energy for living things

Potential energy is stored energy Kinetic energy is energy in action

Calorie - the amount of energy to raise the temperature of 1 g of water by 1° Celsius

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Figure 5.1 Potential energy versus kinetic energy

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5.2 Two laws apply to energy and its use

1st law of thermodynamics—the law of conservation of energy Energy cannot be created or destroyed, but it

can be changed from one form to another

2nd law of thermodynamics Energy cannot be changed from one form to

another without a loss of usable energy Entropy - disorder increases because it is difficult

to use heat to perform more work

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Figure 5.2 Flow of energy from the sun to an animal that eats a plant

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5.3 Cellular work is powered by ATP

ATP (Adenosine TriPhosphate) - the energy currency of cells Cells use ATP to perform nearly all activities

Exergonic reactions Energy exits the reaction

Endergonic reactions Energy enters the reaction

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Figure 5.3A The ATP cycle

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5.4 ATP breakdown is coupled to energy-requiring reactions

Coupled reactions occur in the same place, at the same time energy-releasing (exergonic) reaction drives an

energy-requiring (endergonic) reaction

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Figure 5.4 Muscle contraction occurs when it is coupled to ATP breakdown

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Enzymes Speed Chemical Reactions

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5.5 Enzymes speed reactions by lowering activation barriers

Enzyme - usually a protein molecule that functions as an organic catalyst to speed a chemical reaction without itself being affected by the reaction Enzymes lower energy of activation

(Ea)

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Figure 5.5 The energy of activation (Ea) is lower when an enzyme is involved

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5.6 An enzyme’s active site is where the reaction takes place

Each enzyme is specific to its reaction Substrate(s) – reactants in an enzymatic

reaction Substrates combine with an enzyme, forming an

enzyme-substrate complex

The enzyme’s active site, binds with the substrate(s) Induced fit model the enzyme is induced to

undergo a slight alteration to achieve optimum fit with the substrate

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Figure 5.6 Enzymatic action

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5.7 Enzyme speed is affected by local conditions

Substrate Concentration More substrate means more chance encounters

between substrate molecules and the enzyme Temperature

warmer temperatures cause more effective encounters between enzyme and substrate

pH A change in pH can change the enzyme’s shape and

disrupt normal interactions Cofactors

The presence of molecules like coenzymes or vitamins allow enzymes to be active

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Figure 5.7A The effect of temperature on the rate of an enzymatic reaction

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5.8 Enzymes can be inhibited noncompetitively and competitively

Metabolic pathway - series of linked reactions Enzyme inhibition occurs when a molecule (the

inhibitor) binds to an enzyme and decreases its activity

Noncompetitive inhibition – inhibitor binds to the enzyme at a location other than the active site

Competitive inhibition – inhibitor and the substrate compete for the same active site

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Figure 5.8 Metabolic pathways and noncompetitive inhibition. In the pathway, A–E are substrates, E1–E5 are enzymes, and F is the end product of the pathway.

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

5.9 Enzyme inhibitors can spell death

Cyanide can be fatal because it binds to a mitochondrial enzyme necessary for the production of ATP

Sarin is a chemical that inhibits an enzyme at neuromuscular junctions, where nerves stimulate muscles

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The Plasma Membrane Has Many and Various Functions

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5.10 The plasma membrane is a phospholipid bilayer with embedded proteins

Plasma membrane is a fluid phospholipid bilayer Polar head of a phospholipid is hydrophilic, while the

nonpolar tails are hydrophobic Fluid-mosaic model – proteins embedded in

the membrane form a mosaic within the phospholipid bilayer Cholesterol molecules are steroids that lend support

to the membrane Glycolipids and glycoproteins – lipids and proteins

carrying carbohydrate chains

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Figure 5.10 Fluid-mosaic model of plasma membrane structure

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5.11 Proteins in the plasma membrane have numerous functions

Channel proteins have channels that let molecules to move across the membrane

Carrier proteins combine with a molecule to help it move across the membrane Differentially permeable - only certain substances

can pass through Receptor proteins have a binding site for a

specific molecule Enzymatic proteins carry out metabolic

reactions Junction proteins form various types of

junctions between cells

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Figure 5.11 Functions of plasma membrane proteins

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APPLYING THE CONCEPTS—HOW BIOLOGY IMPACTS OUR LIVES

5.12 Malfunctioning plasma membrane proteins can cause

human diseases Diabetes Type 2 - insulin binds to receptor protein, but

the number of carriers sent to the plasma is not enough Too much glucose in the blood, which spills over into the urine

Color Blindness – Usually three types of photopigment proteins in plasma membrane within photoreceptor cells Some people lack of functional red or green photopigment

Cystic Fibrosis (CF) - Usually, chloride ions pass easily through a plasma membrane channel protein When not regulated, a thick mucus appears in the lungs and

pancreas damaging the lungs and contributing to an early death

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Figure 5.12 Cystic fibrosis is due to a defective CF gene and defective CF channel proteins

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The Plasma Membrane Regulates the Passage of Molecules

Into and Out of Cells

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5.13 Simple diffusion across a membrane requires no energy

Simple diffusion – molecules move down concentration gradient until equilibrium is achieved and they are distributed equally

Dissolved gases can diffuse readily through the phospholipid bilayer How oxygen enters and carbon dioxide exits

cells How oxygen enters blood, and carbon dioxide

leaves the blood from air sacs in the lungs

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Figure 5.13 Some molecules can simply diffuse across a membrane

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5.14 Facilitated diffusion requires a carrier protein but no energy

Facilitated diffusion – certain molecules cross membranes by combining with a carrier protein The carrier proteins are thought to be specific.

Figure 5.14 During facilitated diffusion, a carrier protein assists solute movement across the membrane

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5.15 Osmosis can affect the size and shape of cells

Osmosis - diffusion of water across a differentially permeable membrane due to concentration differences Solution contains both a solute and a

solvent Solvents dissolve solutes

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Figure 5.15A During osmosis, net movement of water is toward greater solute concentration

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How Osmosis Affects the Size and Shape of Cells

Isotonic solutions (iso, same as) - concentration of water is the same on both sides of the membrane

Hypotonic solution (hypo, less than) - Outside the cell, the concentration of solute is

less, and the concentration of water is greater, than inside the cell

hypertonic solution (hyper, more than) - Outside the cell, the concentration of solute is more, and the concentration of water is less, than inside the cell

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Figure 5.15B Osmosis in animal and plant cells

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5.16 Active transport requires a carrier protein and energy

Active transport, molecules or ions move through the plasma membrane, accumulating on one side of the cell Movement of molecules against their

concentration gradients requires both a carrier protein and ATP

Sodium-potassium pump undergoes a change in shape when it combines with ATP allowing it to combine alternately with sodium ions and potassium ions

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Figure 5.16 During active transport, a substance moves contrary to its concentration gradient

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5.17 Bulk transport involves the use of vesicles

Bulk transport occurs when fluid or particles are brought into or out of a cell by vacuole formation, called endocytosis or by evagination, called exocytosis Phagocytosis occurs when the material taken in is

large Pinocytosis occurs when vesicles form around a

liquid or around very small particles Receptor-mediated endocytosis - receptors for

particular substances are found at one location in the plasma membrane

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Figure 5.17 Bulk transport into the cell is by endocytosis

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Connecting the Concepts: Chapter 5

Energy is the ability to do work, to bring about change, and to make things happen

A cell is dynamic because it carries out enzymatic reactions

Exchanges across the plasma membrane allow the cell to continue to perform its usual reactions Few reactions occur without enzymes, which are proteins,

because they lower the energy of activation with their substrate ATP, the universal energy “currency” of life, get energy-

requiring (endergonic) reactions going The plasma membrane is called the gatekeeper of the

cell because its numerous proteins allow only certain substances to enter or exit