The Working Cell

The Working Cell

Ch. 5

A.Forms of Energy

•1.Energy is capacity to do work; cells continually use energy to develop, grow, repair, reproduce, etc.

•2.Kinetic energy is energy of motion; all moving objects have kinetic energy.

•3.Potential energy is stored energy.

•4. Food is chemical energy; it contains potential energy.

•5. Chemical energy can be converted into mechanical energy, e.g., muscle movement.

Two Laws of Thermodynamics

First law of thermodynamics (also called the law of conservation of energy)

•a.Energy cannot be created or destroyed, but it can be changed from one form to another.

•b. In an ecosystem, solar energy is converted to chemical energy by the process of photosynthesis; some of the chemical energy in the plant is converted to chemical energy in an animal, which in turn can become mechanical energy or heat loss.

continued

•c. Neither the plant nor the animal create energy, they convert it from one form to another.

•d. Likewise, energy is not destroyed; some becomes heat that dissipates into the environment.

Second law of thermodynamics

•a.Energy cannot be changed from one form into another without a loss of usable energy.

•b.Heat is a form of energy that dissipates into the environment; heat can never be converted back to another form of energy.

Cells and Entropy

•1.Every energy transformation makes the universe less organized and more disordered; entropy is the term used to indicate the relative amount of disorganization.

•2.When ions distribute randomly across a membrane, entropy has increased.

•3.Organized/usable forms of energy (as in the glucose molecule) have relatively low entropy; unorganized/less stable forms have relatively high entropy.

continued

•4.Energy conversions result in heat; therefore, the entropy of the universe is always increasing.

•5.Living things depend on a constant supply of energy from the sun, because the ultimate fate of all solar energy in the biosphere is to become randomized in the universe as heat; the living cell is a temporary repository of order purchased at the cost of a constant flow of energy.

Metabolic Reactions and Energy Transformations

•1.Metabolism is the sum of all the biochemical reactions in a cell.

•2.In the reaction A + B = C + D, A and B are reactants and C and D are products.

•3.Free energy (G) is the amount of energy that is free to do work after a chemical reaction.

continued

•4.Change in free energy is noted as G; a negative G means that products have less free energy than reactants; the reaction occurs spontaneously.

•5.Exergonic reactionshave a negative G and energy is released.

•6.Endergonic reactions have a positive G; products have more energy than reactants; such reactions can only occur with an input of energy.

ATP: Energy for Cells

•1.Adenosine triphosphate (ATP) is the energy currency of cells; when cells need energy, they “spend” ATP.

•2.ATP is an energy carrier for many different types of reactions.

•3.When ATP is converted into ADP + P, the energy released is sufficient for biological reactions with little wasted.

continued

•4.ATP breakdown is coupled to endergonic reactions in a way that minimizes energy loss.

•5.ATP is a nucleotide composed of the base adenine and the 5-carbon sugar ribose and three phosphate groups.

•6.When one phosphate group is removed, about 7.3 kcal of energy is released per mole.

Coupled Reactions

•1.Coupled reactions are reactions that occur in the same place, at the same time, and in a way that an exergonic reaction is used to drive an endergonic reaction.

•2.ATP breakdown is often coupled to cellular reactions that require energy.

•3.ATP supply is maintained by breakdown of glucose during cellular respiration.

Metabolic Pathways and Enzymes

•1.Enzymes are catalysts that speed chemical reactions without the enzyme being affected by the reaction.

•2.Every enzyme is specific in its action and catalyzes only one reaction or one type of reaction.

•3.Ribozymes are made of RNA rather than proteins and also serve as catalysts.

•4.A metabolic pathway is an orderly sequence of linked reactions; each step is catalyzed by a specific enzyme.

•5.Metabolic pathways begin with a particular reactant, end with a particular end product(s), and may have many intermediate steps.

•6.In many instances, one pathway leads to the next; since pathways often have one or more molecules in common, one pathway can lead to several others.

•7.Metabolic energy is captured more easily if it is released in small increments.

•8.A reactant is the substance that is converted into a product by the reaction; often many intermediate steps occur.

Energy of Activation

•1.A substrate is a reactant for an enzymatic reaction.

•2.Enzymes speed chemical reactions by lowering the energy of activation (Ea) by forming a complex with their substrate(s) at the active site.

•a.An active site is a small region on the surface of the enzyme where the substrate(s) bind.

•b.When a substrate binds to an enzyme, the active site undergoes a slight change in shape that facilitates the reaction. This is called the induced fit model of enzyme catalysis.

•3.Only a small amount of enzyme is needed in a cell because enzymes are not consumed during catalysis.

•4.Some enzymes (e.g., trypsin) actually participate in the reaction.

•5.A particular reactant(s) may produce more than one type of product(s).

•a.Presence or absence of enzyme determines which reaction takes place.

•b.If reactants can form more than one product, the enzymes present determine which product is formed.

Factors Affecting Enzymatic Speed

•1.Substrate concentration.

•Because molecules must collide to react, enzyme activity increases as substrate concentration increases; as more substrate molecules fill active sites, more product is produced per unit time.

2.Optimal pH

a.Every enzyme has optimal pH at which its rate of reaction is optimal.

b. A change in pH can alter the ionization of the R groups of the amino acids in the enzyme, thereby disrupting the enzyme’s activity.

3. Temperature

•As temperature rises, enzyme activity increases because there are more enzyme-substrate collisions.

•Enzyme activity declines rapidly when enzyme is denatured at a certain temperature, due to a change in shape of the enzyme.

4.Enzyme cofactors

•a.Many enzymes require an inorganic ion or non-protein cofactor to function.

•b.Inorganic cofactors are ions of metals.

•c.A coenzyme is an organic cofactor, which assists the enzyme (i.e., it may actually contribute atoms to the reaction).

•d.Vitamins are small organic molecules required in trace amounts for synthesis of coenzymes; they become part of a coenzyme’s molecular structure; vitamin deficiency causes a lack of a specific coenzyme and therefore a lack of its enzymatic action.

5. Enzyme inhibition

•a. Enzyme inhibition occurs when a substance (called an inhibitor) binds to an enzyme and decreases its activity; normally, enzyme inhibition is reversible.

• b.In noncompetitive inhibition,the inhibitor binds to the enzyme at a location other than the active site (the allosteric site), changing the shape of the enzyme and rendering it unable to bind to its substrate.

• c.In competitive inhibition, the substrate and the inhibitor are both able to bind to the enzyme’s active site.

Organelles and the Flow of Energy

Photosynthesis

•1.Photosynthesis uses energy to combine carbon dioxide and water to produce glucose in the formula:

•6 CO2 + 6 H2O + energy = C6H12O6 + 6 O2

•2.Oxidation is the loss of electrons.

•3.Reduction is the gain of electrons.

•4. When hydrogen atoms are transferred to carbon dioxide from water, water has been oxidized and carbon dioxide has been reduced.

•5.Input of energy is needed to produce the high-energy glucose molecule.

•6.Chloroplasts capture solar energy and convert it by way of an electron transport system into the chemical energy of ATP.

•7.ATP is used along with hydrogen atoms to reduce glucose; when NADP+(nicotinamide adenine dinucleotide phosphate) donates hydrogen atoms (H+ + e‑) to a substrate during photosynthesis, the substrate has accepted electrons and is therefore reduced.

•8.The reaction that reduces NADP+ is:

•NADP+ + 2e‑ + H+ = NADPH

Cellular Respiration

•1.The overall equation for cellular respiration is opposite that of photosynthesis:

•C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy

•2.When NAD removes hydrogen atoms (H+ + e-) during cellular respiration, the substrate has lost electrons and is therefore oxidized.

•3.At the end of cellular respiration, glucose has been oxidized to carbon dioxide and water and ATP molecules have been produced.

•In metabolic pathways, most oxidations involve the coenzyme NAD+ (nicotinamide adenine dinucleotide); the molecule accepts two electrons but only one hydrogen ion: NAD+ + 2e‑ + H+ = NADH

Electron Transport Chain

•1.Both photosynthesis and respiration use an electron transport chain consisting of membrane‑bound carriers that pass electrons from one carrier to another.

•High‑energy electrons are delivered to the system and low‑energy electrons leave it.

•The overall effect is a series of redox reactions; every time electrons transfer to a new carrier, energy is released for the production of ATP.

ATP Production

•1.ATP synthesis is coupled to the electron transport system.

•2.Peter Mitchell received the 1978 Nobel Prize for his chemiosmotic theory of ATP production.

•3.In both mitochondria and chloroplasts, carriers of electron transport systems are located within a membrane.

•4.H+ ions (protons) collect on one side of the membrane because they are pumped there by specific proteins.

•5.The electrochemical gradient thus established across the membrane is used to provide energy for ATP production.

•6.Enzymes and their carrier proteins, called ATP synthase complexes, span the membrane; each complex contains a channel that allows H+ ions to flow down their electrochemical gradient.

•7.In photosynthesis, energized electrons lead to the pumping of hydrogen ions across the thylakoid membrane; as hydrogen ions flow through the ATP synthase complex, ATP is formed.

•8.During cellular respiration, glucose breakdown provides energy for a hydrogen ion gradient on the inner membrane of the mitochondria that also couples hydrogen ion flow with ATP formation.

Fluid-Mosaic Model

•1.The fluid-mosaic model describes the plasma membrane.

•2.The fluid component refers to the phospholipids bilayer of the plasma membrane.

•3.Fluidity of the plasma membrane allows cells to be pliable.

•4.Fluidity is affected by cholesterol molecules in the plasma membrane.

•5.The mosaic component refers to the protein content in the plasma membrane.

•6.Protins bond to the ECM and/or cytoskeleton to prevent movement in the fluid phospholipid bilayer

. Permeability of the Plasma Membrane

The plasma membrane is differentially (selectively) permeable; only certain molecules can pass through.

•a.Small non-charged lipid molecules (alcohol, oxygen) pass through the membrane freely.

•b.Small polar molecules (carbon dioxide, water) move “down” a concentration gradient, i.e., from high to low concentration.

•c.Ions and charged molecules cannot readily pass through the hydrophobic component of the bilayer and usually combine with carrier proteins.

Both passive and active mechanisms move molecules across membrane.

•a.Passive transport moves molecules across membrane without expenditure of energy; includes diffusion and facilitated transport.

•b.Active transport requires a carrier protein and uses energy (ATP) to move molecules across a plasma membrane; includes active transport, exocytosis, endocytosis, and pinocytosis.

•3. The presence of a membrane channel protein called an aquaporin allows water to cross membranes quickly.

•4.Substances enter or exit a cell through bulk transport.

Passive Transport Across a Membrane

•1.Diffusion is the movement of molecules from higher to lower concentration (i.e., “down” the concentration gradient).

Diffusion continued

•a.A solution contains a solute, usually a solid, and a solvent, usually a liquid.

•b.In the case of a dye diffusing in water, the dye is a solute and water is the solvent.

•c.Once a solute is evenly distributed, random movement continues but with no net change.

Diffusion continued

•d.Membrane chemical and physical properties allow only a few types of molecules to cross by diffusion.

•e.Gases readily diffuse through the lipid bilayer; e.g., the movement of oxygen from air sacs (alveoli) to the blood in lung capillaries depends on the concentration of oxygen in alveoli.

•f. Temperature, pressure, electrical currents, and molecular size influence the rate of diffusion.

1.Osmosis is the diffusion of water across a differentially (selectively) permeable membrane

•a.Osmosis is illustrated by the thistle tube example:

•1)A differentially permeable membrane separates two solutions.

•2)The beaker has more water (lower percentage of solute) and the thistle tube has less water (higher percentage of solute).

•3)The membrane does not permit passage of the solute; water enters but the solute does not exit.

•4)The membrane permits passage of water with a net movement of water from the beaker to the inside of the thistle tube.

•b.Osmotic pressure is the pressure that develops in such a system due to osmosis.

•c.Osmotic pressure results in water being absorbed by the kidneys and water being taken up from tissue fluid.

2.Tonicity is strength of a solution with respect to osmotic pressure.

•a.Isotonic solutions occur where the relative solute concentrations of two solutions are equal; a 0.9% salt solution is used in injections because it is isotonic to red blood cells (RBCs).

•b.A hypotonic solution has a solute concentration that is less than another solution; when a cell is placed in a hypotonic solution, water enters the cell and it may undergo cytolysis(“cell bursting”).

•c.Swelling of a plant cell in a hypotonic solution creates turgor pressure; this is how plants maintain an erect position.

•When a plant cell is placed in a hypotonic solution, it is turgid.

•A hypertonic solution has a solute concentration that is higher than another solution; when a cell is placed in a hypertonic solution, it shrivels (a condition called crenation).

•Plasmolysis is shrinking of the cytoplasm due to osmosis in a hypertonic solution; as the central vacuole loses water, the plasma membrane pulls away from the cell wall.

•In a hypotonic solution, an animal cell will lyse.

3.Facilitated Transport

•a.Facilitated transport is the transport of a specific solute “down” or “with” its concentration gradient (from high to low), facilitated by a carrier protein; glucose and amino acids move across the membrane in this way.

Active Transport Across a Membrane

•A.Active transport is transport of a specific solute across plasma membranes “up” or “against” (from low to high) its concentration gradient through use of cellular energy (ATP).

•1.Iodine is concentrated in cells of thyroid gland, glucose is completely absorbed into lining of digestive tract, and sodium is mostly reabsorbed by kidney tubule lining.

•2.Active transport requires both carrier proteins and ATP; therefore cells must have high number of mitochondria near membranes where active transport occurs.

•3.Proteins involved in active transport are often called “pumps”; the sodium‑potassium pump is an important carrier system in nerve and muscle cells.

•4.Salt (NaCl) crosses a plasma membrane because sodium ions are pumped across, and the chloride ion is attracted to the sodium ion and simply diffuses across specific channels in the membrane.

Bulk Transport

•1.In exocytosis, a vesicle formed by the Golgi apparatus fuses with the plasma membrane as secretion occurs; insulin leaves insulin‑secreting cells by this method.

•2.During endocytosis, cells take in substances by vesicle formation as plasma membrane pinches off by either phagocytosis, pinocytosis, or receptor-mediated endocytosis.

•In phagocytosis, cells engulf large particles (e.g., bacteria), forming an endocytic vesicle.

•a.Phagocytosis is commonly performed by ameboid‑type cells (e.g., amoebas and macrophages).

•b.When the endocytic vesicle fuses with a lysosome, digestion of the internalized substance occurs.

•4.Pinocytosis occurs when vesicles form around a liquid or very small particles; this is only visible with electron microscopy.

5.Receptor‑mediated endocytosis, a form of pinocytosis, occurs when specific macromolecules bind to plasma membrane receptors.

•a.The receptor proteins are shaped to fit with specific substances (vitamin, hormone, lipoprotein molecule, etc.), and are found at one location in the plasma membrane.

•b.This location is a coated pit with a layer of fibrous protein on the cytoplasmic side; when the vesicle is uncoated, it may fuse with a lysosome.

•c.Pits are associated with exchange of substances between cells (e.g., maternal and fetal blood).

•d.This system is selective and more efficient than pinocytosis; it is important in moving substances from maternal to fetal blood.

•e.Cholesterol (transported in a molecule called a low-density lipoprotein, LDL) enters a cell from the bloodstream via receptors in coated pits; in familial hypocholesterolemia, the LDL receptor cannot bind to the coated pit and the excess cholesterol accumulates in the circulatory system.

Modification of Cell Surfaces

•A.Cell Surfaces in Animals

•1.The extracellular matrix is a meshwork of polysaccharides and proteins produced by animal cells.

•Collagen gives the matrix strength and elastin gives it resilience.

•Fibronectins and laminins bind to membrane receptors and permit communication between matrix and cytoplasm; these proteins also form “highways” that direct the migration of cells during development.

•Proteoglycans are glycoproteins that provide a packing gel that joins the various proteins in matrix and most likely regulate signaling proteins that bind to receptors in the plasma protein.

Junctions Between Cells are points of contact between cells that allow them to behave in a coordinated manner.

•Anchoring junctions mechanically attach adjacent cells.

•In adhesion junctions, internal cytoplasmic plaques, firmly attached to cytoskeleton within each cell are joined by intercellular filaments; they hold cells together where tissues stretch (e.g., in heart, stomach, bladder).

•In desmosomes, a single point of attachment between adjacent cells connects the cytoskeletons of adjacent cells.

•In tight junctions, plasma membrane proteins attach in zipper-like fastenings; they hold cells together so tightly that the tissues are barriers (e.g., epithelial lining of stomach, kidney tubules, blood-brain barrier).

A gap junction allows cells to communicate; formed when two identical plasma membrane channels join

•They provide strength to the cells involved and allow the movement of small molecules and ions from the cytoplasm of one cell to the cytoplasm of the other cell.

•Gap junctions permit flow of ions for heart muscle and smooth muscle cells to contract.