NE

The Working Cell

  • Introduction
    • Some organisms use an energy-converting reaction to produce light in a process called bioluminescence
    • Many marine invertebrates and fishes use bioluminescence to hide themselves from predators
    • Scientists estimate that 90% of deep-sea marine life produces bioluminescence
    • The light is produced from chemical reactions that convert chemical energy into visible light
    • Bioluminescence is an example of the multitude of energy conversions that a cell can perform
    • Many of the cell’s reactions
    • Take place in organelles
    • Use enzymes embedded in the membranes of the these organelles
  • Membranes are fluid mosaics of lipids and proteins with many functions
    • Membranes are composed of
    • A bilayer of phospholipids with embedded and attached proteins in a structure biologists call a fluid mosaic
    • Membrane proteins perform many functions
    • Some proteins help maintain cell shape and coordinate changes inside and outside the cell through their attachment to the cytoskeleton and extracellular matrix
    • Some proteins function as receptors for chemical messengers from other cells
    • Some membrane proteins function as enzymes
    • Some membrane glycoproteins are involved in cell-cell recognition
    • Membrane proteins may participate in the intercellular junctions that attach adjacent cells to each other
    • Membranes may exhibit selective permeability, allowing some substances to cross more easily than others
  • Membranes form spontaneously, a critical step in the origin of life
    • Phospholipids spontaneously self-assemble into simple membranes
    • The formation of membrane-enclosed collections of molecules was a critical step in the evolution of the first cells
  • Passive transport is diffusion across a membrane with no energy investment
    • Diffusion is the tendency of particles to spread out evenly in available spaces
    • Particles move from an area of more concentrated particles to an area where they are less concentrated
    • This means that particles diffuse down their concentration gradient
    • Eventually, the particles reach equilibrium where the concentration of particles is the same throughout
    • Diffusion, across a cell membrane does not require energy, so it is called passive transport
    • The concentration gradient itself represents potential energy for diffusion
  • Osmosis is the diffusion of water across a membrane
    • One of the most important substances that crosses membranes is water
    • The diffusion of water across a selectively permeable membrane is called osmosis
    • If a membrane is permeable to water, but not a solute, separates two solutions with different concentrations of solute
    • Water will cross the membrane, moving down its own gradient until the solute concentration on both sides is equal
  • Water balance between cells and their surrounding is crucial to organisms
    • Tonicity is a term that describes the ability of a solution to cause a cell to gain or lose water
    • Tonicity mostly depends on the concentration of solute on both sides of the membrane
    • How will animal cells be affected when placed into solutions of various tonicities?
    • Isotonic solution
      • The concentration of solute is the same on both sides of the membrane
      • The cell volume will not change
    • Hypotonic solution
      • The solute concentration is lower outside the cell
      • Water molecules move into the cell and the cell will expand
    • Hypertonic Solution
      • The solute concentration is higher outside the cell
      • Water molecules move out of the cell and the cell will shrink
    • For an animal cell to survive in a hypotonic or hypertonic environment, it must engage in osmoregulation
    • The control of water balance
    • The cell walls of plant cells, prokaryotes, and fungi make water balance issues somewhat different
    • The cell wall of a plant cell exerts pressure that prevents the cell from taking in too much water and bursting when placed in a hypotonic environment
    • In a hypertonic environment, plant and animal cells both shrivel
  • Transport proteins can facilitate diffusion across membranes
    • Hydrophobic substances easily diffuse across a cell membrane
    • Polar or charged substances do not easily cross cell membranes, and, instead, move across membranes with the help of specific proteins in a process called facilitated diffusions
    • Does not require energy
    • Relies on the concentration gradients
    • Some proteins function by becoming a hydrophilic tunnel for the passage of ions or other molecules
    • Other proteins bind their passenger, change shape, and release their passenger on the other side
    • Because water is polar, its diffusion through a membrane’s hydrophobic interior is relatively slow
    • The very rapid diffusion of water into and out of certain cells is made possible by a protein channel called an aquaporin
  • Research on another membrane protein led to the discovery of aquaporins
    • Dr. Peter Agre received the 2003 Nobel Prize in chemistry for his discovery of aquaporins
    • His research on the Rh protein used in blood typing led to this discovery
  • Cells expend energy in the active transport of a solute
    • In active transport, a cell must expend energy to move a solute against its concentration gradient
  • Exocytosis and endocytosis transport large molecules across membranes
    • A cell uses two mechanisms to move large molecules across membranes
    • Exocytosis
      • Used to export bulky molecules, such as proteins or polysaccharides
    • Endocytosis
      • Import substances useful to the livelihood of the cell
    • In both cases, material to be transported is packaged within a vesicle that fuses with the membrane
  • Three kinds of endocytosis
    • Phagocytosis
    • Engulfment of a particle by wrapping cell membrane around it, forming a vacuole
    • Pinocytosis
    • Same thing as phagocytosis except fluids are taken into small vesicles
    • Receptor-mediated endocytosis
    • Uses receptors in a receptor-coated pit to interact with a specific protein, initiating the formation of a vesicle
  • Cells transform energy as they perform work
    • Cells are small units, a chemical factory, housing thousands of chemical reactions
    • Cells uses these reactions for cell maintenance, manufacturing of cellular parts, and cell replication
    • Energy is the capacity to cause change or to perform work
    • There are two kinds of energy
    • Kinetic energy
      • Energy of motion
    • Potential energy
      • Energy that matter possesses as a result of its location or structure
    • Heat, or thermal energy, is a type of kinetic energy associated with the random movement of atoms or molecules
    • Light is also a type of kinetic energy, and can be harnessed to power photosynthesis
    • Chemical energy is the potential energy available for release in a chemical reaction
    • It is the most important type of energy for living organisms to power the work of the cell
    • Thermodynamics is the study of energy transformations that occur in a collection of matter
    • Scientists use the word
      • System for the matter under study
      • Surroundings for the rest of the universe
    • Two laws govern energy transformations in organisms
    • First law of thermodynamics
      • Energy in the universe is constant
    • Second law of thermodynamics
      • Energy conversions increase the disorder of the universe
    • Entropy
    • The measure of disorder, or randomness
    • Cells use oxygen in reactions that release energy from fuel molecules
    • In cellular respiration, the chemical energy stored in organic molecules is converted to a form that the cell can use to perform work
  • Chemical reactions either release or store
    • Chemical reactions either release energy (exergonic reactions) or require an input of energy and store energy (endergonic reactions)
    • Exergonic reactions release energy
    • These reactions release the energy in covalent bonds of the reactants
    • Burning woods releases energy in glucose as heat and light
    • Cellular respiration involves many steps, releases energy slowly, and uses some of the released energy to produce ATP
    • An endergonic reaction
    • Requires an input of energy and yields products rich in potential energy
    • Endergonic reactions
    • Begin with reactant molecules that contain relatively little potential energy
    • End with products that contain more chemical energy
    • Photosynthesis is a type of endergonic process
    • Energy-poor reactants, carbon dioxide, and water are used
    • Energy is absorbed from sunlight
    • Energy-rich sugar molecules are produced
    • A living organism carries out thousands of endergonic and exergonic chemical reactions
    • The total of an organism’s chemical reaction is called metabolism
    • A metabolic pathway is a series of chemical reactions that either
    • Builds a complex molecule
    • Breaks down a complex molecule into simpler compounds
    • Energy coupling uses the energy release from exergonic reactions to drive essential endergonic reactions, usually using the stored ATP molecules
  • ATP drives cellular work by coupling exergonic and endergonic reactions
    • Adenosine triphosphate
    • Powers nearly all forms of cellular work
    • ATP consists of
    • Nitrogenous bases adenine
    • Five carbon sugar ribose
    • Three phosphate groups
    • Hydrolysis of ATP releases energy by transferring the third phosphate from ATP to some other molecule in a process called phosphorylation
    • Most cellular work depends on ATP energizing molecules by phosphorylating them
    • There are three main types of cellular work
    • Chemical
    • Mechanical
    • Transport
    • ATP drives all three of these types of work
    • ATP is a renewable source of energy for the cell
    • In the ATP cycle, energy released in an exergonic reaction, such as the breakdown of glucose, is used in an endergonic reaction to generate ATP
  • Enzymes speed up the cell’s chemical reactions by lowering energy barriers
    • Although biological molecules possess much potential energy, it is not released spontaneously
    • An energy barrier must be overcome before a chemical reaction can begin
      • This energy is called the activation energy
    • We can think of the activation energy as the amount of energy needed for a reactant molecule to move uphill to a higher energy but an unstable state, so that the downhill part of the reaction can begin
    • One way to speed up a reaction is to add heat, which agitates atoms so that bonds break more easily and reactions can proceed, but could kill a cell
    • Enzymes
    • Function as a biological catalyst by lowering the activation energy needed for a reaction to begin
    • Increase the rate of a reaction without being consumed by the reaction
    • Are usually proteins
      • Although some RNA molecules can function as enzymes
  • A specific enzyme catalyzes each cellular reaction
    • An enzyme is very selective in the reaction it catalyzes and has a shape that determines the enzyme’s specificity
    • The specific reactant that an enzyme acts on is called the enzyme’s substrate
    • A substrate fits into a region of the enzyme called the active site
    • Enzymes are specific substrate molecules
    • For every enzyme, there are optimal conditions under which it is most effective
    • Temperature affects molecular motion
    • An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site
    • Most human enzymes work best at 35-40℃
    • Many enzymes require nonprotein helpers called cofactors, which bind to the active site and function in catalysis
    • Some cofactors are inorganic
    • Ex: zinc, iron, copper
    • If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme
  • Enzyme inhibitors can regulate enzyme activity in the cell
    • A chemical that interferes with an enzyme’s activity is called an inhibitor
    • Competitive inhibitors
    • Block substrates from entering the active site
    • Reduce the enzyme’s productivity
    • Noncompetitive inhibitors
    • Bind to the enzyme somewhere other than the active site
    • Change the shape of the active site
    • Prevent the substrate from binding
    • Enzyme inhibitors are important in regulating cell metabolism
    • In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it
    • This is called feedback inhibition
  • Many drugs, pesticides, and poisons are enzyme inhibitors
    • Many beneficial drugs act as enzyme inhibitors
    • Ibuprofen
      • Inhibiting the productions of prostaglandins
    • Some blood pressure medications
    • Some antidepressants
    • Many antibiotics
    • Protease inhibitors used to fight HIV
    • Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare