• Energy
• capacity to do work
• to move matter in a direction it would no move if left alone
• required by all organisms to survive
• Energy exists in two states
• kinetic energy
• energy of motion
• moving objects perform work by causing matter to move
• potential energy
• stored energy
• objects that are not actively moving but have capacity to do so possess energy
• potential energy vs kinetic energy
• boulder perched on a hilltop has potential energy
• as it begins to roll down hill some of energy is converted into kinetic energy
• energy exists in many forms
• mechanical energy
• heat
• sound
• electric current
• light
• radioactive radiation
• chemical energy
• potential energy of molecules
• most important type of energy for living organisms
• many ways to measure energy
• most convenient is heat
• measure of random motion of molecules
• all other forms of energy can be converted into heat
• Life depends on fact that energy can be converted from one form to another
• thermodynamics
• study of energy transformations
• Laws of Thermodynamics
• set of universal laws that govern all energy changes in the universe
• concerns amount of energy in universe
• energy can be changed from one form to another but can neither be created or destroyed
• total amount of energy in universe remains constant
• in any living system, potential energy can be shifted to other molecules, stored in different chemical bonds, convert into other forms
• during each conversion some energy dissipates into environment in form of heat
• although amount of energy in universe remains constant
• energy available to do work decreases as progressively more of it dissipates as heat
• concerns transformation of potential energy into heat, or random molecular motion
• disorder (or entropy) in universe is continuously increasing; disorder is more likely than order
• entropy is a measure of disorder of a system
• heat is one form of disorder
• entropy increases
• when universe was formed it had all potential energy it will ever have
• has become more disordered ever since
• every energy exchange has increased amount of entropy in universe

Chemical reactions either store or release energy

• Chemical reactions, including those within cells, are of two types
• endergonic reactions
• "energy in"
• require a net input of energy
• exergonic reactions
• "energy out"
• release energy
• endergonic reactions
• yield products rich in potential energy
• start with reactants molecules that have little potential energy
• absorb energy from surroundings as reaction occurs
• products store more energy that reactants did
• don’t proceed spontaneously
• requires input of energy
• example is photosynthesis
• exergonic reactions
• reactants store more energy than products
• energy is released to surroundings as reaction proceeds
• tend to proceed spontaneously
• does not require an input of energy
• example is cellular respiration

• Cellular metabolism
• sum of exergonic and endergonic reactions carried out by working cells

• Energy coupling
• using energy released from exergonic reactions to drive essential endergonic reactions
• usable energy released from most exergonic reactions is stored in ATP
• energy used in most endergonic reactions comes from ATP
• ATP powers nearly all forms of cellular work

Energy and the Cell

• ATP (adenosine triphosphate)
• has 3 parts, connected by covalent bonds
• adenine = a nitrogenous base
• ribose = a 5-carbon sugar
• phosphate groups = a chain of 3 phosphate groups

• ATP cycle (ATP is renewable resource)
• hydrolysis of ATP to ADP + P
• removes a phosphate
• exergonic reaction
• releases energy for endergonic reactions
• dehydration synthesis of ADP + P to ATP
• adds a phosphate
• endergonic reaction
• requires energy from exergonic reactions

How Enzymes Work
Energy of activation (E_A)

• amount of energy that reactants must absorb to start a chemical reaction
• can be thought of as an energy barrier
• since most reactions require energy to get started
• illustrated with "jumping bean" (JB) analogy

• solution for speeding up a reaction lies in enzymes
• protein molecules that serve as biological catalysts
• increase rate of a reaction without being changed into a different molecule
• does not add energy to a cellular reaction
• speeds up reaction be lowering the Energy of activation (E_A), or energy barrier
• without enzymes, many reactions would occur too slowly to sustain life

• enzyme lowers energy barrier

• leading to speeding up of reaction
• can lower E_A by holding reactant molecules in a particular position
• selective in which reactions they catalyze
• have a unique 3-dimensional shape (since it’s a protein) that determines specificity
• recognize only substrate(s) of reaction it catalyzes
• substrate is substance enzyme acts on

• catalyzing a reaction
• enzyme binds to its substrate
• at active site
• pocket or groove on surface of enzyme
• active site fits only one substrate molecule
• while joined, substrate changes into product
• enzyme releases products

• enzyme emerges from reaction unchanged
• active site now ready for another substrate molecule and another cycle
• single enzyme molecule may act upon thousands or millions of substrate molecules per second

• Activity of an enzyme is affected by its environment
• conditions under which it is most effective
• any chemical or physical factor that alters an enzyme’s three-dimensional shape can affect its ability to catalyze a reaction

• Factors affecting enzyme activity
• temperature
• at optimum temperatures (35-40°C)
• highest rate of contact occurs between enzyme’s reactive site and substrate
• because temperature affects molecule motion
• at high temperatures
• enzyme can be denatured, lose its 3-dimensional shape, and lose its function

• Factors affecting enzyme activity
• pH and salinity
• cause variations in number of salt and hydrogen ions
• can interfere with some of chemical bonds that maintain protein structure
• optimum pH = 6-8
• optimum salinity = cell salinity

• Factors affecting enzyme activity
• presence of non-protein helpers called cofactors
• required by some enzymes
• may be inorganic molecules, called cofactors
• may be organic molecules, called coenzymes

• inhibitor
• chemical that interferes with an enzyme’s activity
• two types of inhibitors
• competitive inhibitor
• non-competitive inhibitor

• competitive inhibitor
• resembles enzyme’s normal substrate
• competes with substrate for enzyme’s active site
• non-competitive inhibitor
• does not compete with active site
• binds to enzyme outside of active site
• binding causes shape of enzyme to change

Membrane Structure and Function
Many metabolic reactions occur simultaneously in a cell

• teams of enzymes function like assembly lines
• right enzymes have to be in right place at right time
• Membranes provide the structural basis for metabolic order

Membrane Structure and Function

• For all types of cells
• plasma membrane is edge of life
• forming boundary between living cell and its surroundings
• For most eukaryotic cells
• membranes form
• most organelles
• compartments within cells that contain enzymes
• All membranes are selectively permeable
• control passage of molecules into and out of cell (or organelle)
• takes up substances needed by cell
• disposes of cell waste

• Plasma membrane (cell membrane)
• very thin
• 20 times too small to be seen by light microscope
• can be seen by electron microscope
• Plasma membrane (cell membrane) is composed mainly of phospholipids
• phospholipid molecule has two parts, which interact oppositely with water
• "head"
• glycerol and phosphate group
• polar = hydrophilic
• " tail"
• two fatty acid tails
• non-polar = hydrophobic

• Structure of phospholipids is suited to their role in membranes
• in water, they spontaneously form a stable two-layer sheet, a phospholipid bilayer
• hydrophilic (polar) heads face outwards towards the water
• hydrophobic tails point inward, shielded from the water
• membranes are selectively permeable
• partly due to hydrophobic interior of bilayer
• nonpolar, hydrophobic molecules are soluble in lipids
• polar, hydrophilic molecules are not soluble in lipids
• Structure of plasma membrane is described as a fluid mosaic
• mosaic = surface made of small fragments
• fluid = moveable
• most of the protein and phospholipid molecules can drift laterally w/in membrane
• two surfaces of plasma membrane are different
• outer surface (exterior of cell)
• has carbohydrates covalently bonded to proteins and lipids in membrane
• inner surface (interior of cell)

Diffusion and Osmosis

• Diffusion
• tendency for particles of any kind to spread out spontaneously to regions where they are less concentrated
• requires no work, results from
• random motion (kinetic energy)
• universal tendency of order to deteriorate into disorder (entropy)

diffusion of molecules across a biological membrane is called

• passive transport
• concentration affects direction in which a substance diffuses across a membrane
• substance moves from area of higher concentration to area of lower concentration until equilibrium is reached
• a substance diffuses down its concentration gradient
• at equilibrium, there is no net change in concentration on either side of membrane
• Passive transport
• different substances diffuse independently of one another
• is extremely important to all cells
• Osmosis
• special case of passive transport
• involving diffusion of water molecules across a selectively permeable membrane
• water molecules move down their concentration gradient
• plays role because cells contain and are surrounded by aqueous solution
• solution contains solutes
• solutes also diffuse down their concentration gradient

• aqueous solution on either side of membrane can be described on basis of concentrations of their solutes
• solution with higher concentration of solutes, hypertonic (hyperosmotic)
• solution with lower concentration of solutes, hypotonic (hypoosmotic)
• when solutions on both sides of membrane have same concentration of solutes, isotonic (isoosmotic)
• as solutes diffuse
• from hypertonic solution (area of higher concentration) across membrane into hypotonic solution (area of lower concentration)
• water molecules will move via osmosis
• from hypotonic solution (area of higher concentration of water molecules) across membrane into hypertonic solution (area of lower concentration of water molecules)
• If an animal cell is immersed in
• isotonic solution
• cell’s volume remains constant
• gains water at same rate it loses it
• hypotonic solution (lower solute conc than cell)
• cell gains water (loses solutes), swells, may lyse (pop)
• hypertonic solution (higher solute conc than cell)
• cell loses water (gains solutes), shrivels, may die
• If a plant cell is immersed in
• isotonic solution
• cell is flaccid, plant wilts
• If a plant cell is immersed in
• hypotonic solution (lower solute conc than cell)
• cell is turgid, plant is healthiest
• hypertonic solution (higher solute conc than cell)
• cell loses water (gains solutes), shrivels, may die

Active Transport

• Substances can be moved across a membrane against its concentration
• process called active transport
• requires a cell to expend energy
• usually in the form of ATP
• transport protein actively pumps a substance across the membrane against substance’s concentration gradient

The End