Chapter 5

Biology and Society: Harnessing Cellular Structures

           Cells control their chemical environment using

          energy,

          enzymes, and

          the plasma membrane.

           Cell-based nanotechnology may be used to power microscopic robots.

 

SOME BASIC ENERGY CONCEPTS

           Energy makes the world go around.

           But what is energy?

 

Conservation of Energy

           Energy is defined as the capacity to cause change.

          Some forms of energy are used to perform work.

          Energy is the ability to rearrange a collection of matter.

           Kinetic energy is the energy of motion.

           Potential energy is stored energy. It is energy that an object has because of its

          location or

          structure.

           Machines and organisms can transform kinetic energy to potential energy and vice versa.

           In all such energy transformations, total energy is conserved.

          Energy cannot be created or destroyed.

          Energy can be converted from one form to another.

          This is the principle of conservation of energy.

 

Entropy

           Every energy conversion releases some randomized energy in the form of heat.

           Heat is a

          type of kinetic energy and

          product of all energy conversions.

Entropy

           Scientists use the term entropy as a measure of disorder, or randomness, in a system.

           All energy conversions increase the entropy of the universe.

 

Chemical Energy

           Molecules store varying amounts of potential energy in the arrangement of their atoms.

           Organic compounds are relatively rich in such chemical energy.

           Chemical energy

          arises from the arrangement of atoms and

          can be released by a chemical reaction.

           Living cells and automobile engines use the same basic process to make chemical energy do work.

           Cellular respiration is

          the energy-releasing chemical breakdown of fuel molecules and

          the storage of that energy in a form the cell can use to perform work.

           Humans convert about 34% of the energy in food to useful work, such as the contraction of muscles.

           About 66% of the energy released by the breakdown of fuel molecules generates body heat.

 

Food Calories

           A calorie is the amount of energy that can raise the temperature of one gram of water by 1 degree Celsius.

           Food Calories are kilocalories, equal to 1,000 calories.

           The energy of calories in food is burned off by many activities.

 

ATP AND CELLULAR WORK

           Chemical energy is

          released by the breakdown of organic molecules during cellular respiration and

          used to generate molecules of ATP.

           ATP

          acts like an energy shuttle,

          stores energy obtained from food, and

          releases it later as needed.

 

The Structure of ATP

           ATP (adenosine triphosphate)

          consists of an organic molecule called adenosine plus a tail of three phosphate groups and

          is broken down to ADP and a phosphate group, releasing energy.

 

Phosphate Transfer

           ATP energizes other molecules by transferring phosphate groups.

           This energy helps cells perform

          mechanical work,

          transport work, and

          chemical work.

 

The ATP Cycle

           Cellular work spends ATP continuously.

           ATP is recycled from ADP and a phosphate group through cellular respiration.

           A working muscle cell spends and recycles up to 10 million ATP molecules per second.

 

ENZYMES

           Metabolism is the total of all chemical reactions in an organism.

           Most metabolic reactions require the assistance of enzymes, proteins that speed up chemical reactions.

           All living cells contain thousands of different enzymes, each promoting a different chemical reaction.

 

Activation Energy

           Activation energy

          activates the reactants and

          triggers a chemical reaction.

           Enzymes reduce the amount of activation energy required to break bonds of reactant molecules.

 

Induced Fit

           An enzyme is very selective in the reaction it catalyzes.

           Each enzyme recognizes a substrate, a specific reactant molecule.

          The active site fits to the substrate, and the enzyme changes shape slightly.

          This interaction is called induced fit because the entry of the substrate induces the enzyme to change shape slightly.

 

           Enzymes can function over and over again, a key characteristic of enzymes.

           Many enzymes are named for their substrates, but with an ase ending.

 

Enzyme Inhibitors

           Enzyme inhibitors can prevent metabolic reactions by binding

          to the active site or

          near the active site, resulting in changes to the enzymes shape so that the active site no longer accepts the substrate.

           Some products of a reaction may inhibit the enzyme required for its production.

          This is called feedback regulation.

          It prevents the cell from wasting resources.

           Many beneficial drugs work by inhibiting enzymes.

          Penicillin blocks the active site of an enzyme that bacteria use in making cell walls.

          Ibuprofen inhibits an enzyme involved in sending pain signals.

          Many cancer drugs inhibit enzymes that promote cell division.

 

MEMBRANE FUNCTION

           Cells must control the flow of materials to and from the environment.

           Membrane proteins perform many functions.

           Transport proteins

          are located in membranes and

          help move substances across a cell membrane.

 

Passive Transport: Diffusion across Membranes

           Molecules contain heat energy that causes them to vibrate and wander randomly.

           Diffusion is the movement of molecules so that they spread out evenly into the available space.

           Some substances do not cross membranes spontaneously or cross slowly.

          These substances can be transported via facilitated diffusion.

          Specific transport proteins act as selective corridors.

          No energy input is needed.

 

Osmosis and Water Balance

            The diffusion of water across a selectively permeable membrane is osmosis.

           Compared to another solution,

          a hypertonic solution has a higher concentration of solute,

          a hypotonic solution has a lower concentration of solute, and

          an isotonic solution has an equal concentration of solute.

 

Water Balance in Animal and Plant Cells

           Osmoregulation is the control of water balance within a cell or organism.

           Plants have rigid cell walls and are healthiest in a hypotonic environment, which keeps their walled cells turgid.

           As a plant cell loses water,

          it shrivels and

          its plasma membrane may pull away from the cell wall in the process of plasmolysis, which usually kills the cell.

 

Active Transport: The Pumping of Molecules across Membranes

           Active transport requires that a cell expend energy to move molecules across a membrane.

 

Exocytosis and Endocytosis: Traffic of Large Molecules

           Exocytosis is the secretion of large molecules within transport vesicles.

           Endocytosis takes material in via vesicles that bud inward from the plasma membrane.

           In the process of phagocytosis (cellular eating), a cell engulfs a particle and packages it within a food vacuole.

           Other times a cell gulps droplets of fluid into vesicles.

           Endocytosis can also be triggered by the binding of certain external molecules to specific receptor proteins built into the plasma membrane.

 

The Role of Membranes in Cell Signaling

The plasma membrane helps convey signals

          between cells and

          between cells and their environment.

           Receptors on a cell surface trigger signal transduction pathways that

          relay the signal and

          convert it to chemical forms that can function within the cell.

          self-assemble into simple membranes.