chapter 06

Energy is the ability to do and exists in two states: energy and energy.

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Objects that have the capacity to `move` but are not moving have potential energy that can be converted into kinetic energy.	Objects in `motion` have kinetic energy. Metabolism involves the transformation of potential energy into kinetic energy; some potential energy is lost as heat in the process.

Energy flows from the sun to and in most ecosystems.
- • Energy flow.
  Photosynthetic organisms such as plants are producers in an ecosystem.
  They convert energy from sunlight into carbohydrates.
  Animals are consumers that digest the carbohydrates and convert the potential energy in the form of covalent bonds into kinetic energy for metabolism.
  Energy flow overview:
Living systems transfer energy in reactions which follow the laws of , or energy change.
- The law of thermodynamics ( of energy) states that energy can neither be nor , only changed from one form to another.
  - • First law of thermodynamics.
    - Energy can neither be created nor destroyed.
    - Energy can be changed from one form to another, always releasing heat in the process.
    Cellular processes that transform potential energy into kinetic energy follow the first law of thermodynamics.
- The law of thermodynamics states that in a system always increases as the result of the conversion of energy into random molecular motion (heat).
- • Second law of thermodynamics.
  Entropy (disorder) in a system increases as energy is released as heat.
  A room becomes more disorganized over time; it takes energy to keep it in order, while heat is released.
  Cellular processes also increase entropy when chemical energy is converted to kinetic energy, releasing heat.

Chemical reactions can be or , but may require a to speed up the process.

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Chemical reactions involve the breaking or formation of covalent bonds.

In a dehydration synthesis reaction, the reactants are monomers and the product is a macromolecule. Energy is required (`endergonic`).		In a hydrolysis reaction, the reactant is a macromolecule and the products are monomers. Energy is released (`exergonic`).

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The products of endergonic reactions contain more energy than the reactants, and require energy to proceed.	The products of exergonic reactions contain less energy than the reactants (release energy). These reactions can occur spontaneously, but may need an activation energy to get started.	Catalysts (such as proteins called enzymes) promote exergonic reactions by `lowering` the activation energy.

Proteins that act as catalysts are called and act by binding reactants of a reaction to its site, lowering the energy of the reaction.

• A catalytic enzyme (lysozyme).
A groove (active site) in an enzyme fits the shape of its reactant (substrate). When a reactant molecule binds to the site, the protein changes its shape to embrace the substrate more tightly.
The enzyme acts as a catalyst that facilitates breaking covalent bonds within the reactant.

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Enzymes are catalysts that have an active site to which particular reactants (substrates) fit, like a hand in a glove.	An enzyme and its substrate(s) bind tightly, forming an enzyme-substrate complex that `lowers` the activation energy needed to form the product.	A chemical reaction occurs within the active site, forming the product, which is released. The enzyme is free to work again. Review:

An enzyme can be regulated by altering its shape in response to the binding of a molecule.
- • Many enzymes are allosteric and can change their shape in response to a signal (regulator).
  They have 2 binding sites, an active site for the substrate and an allosteric site for a signal molecule.
  When a signal molecule binds to its allosteric site, the shape (conformation) of the enzyme changes.
  This change may repress or activate the active site. Allosteric activation animation:
Factors that affect enzyme function include and , and enzymes need an optimal range of these conditions.
- • The 3-dimensional shape (conformation) of an enzyme is affected by pH and temperature.
  Most human enzymes, such as the intestinal trypsin work best at about 35°C - 40°C and neutral pH.
  Other enzymes such as pepsin are adapted to the acidic environment of the stomach.
triphosphate (ATP) is the main energy currency of the cell.
- • The nucleotide ATP (adenosine triphosphate) is the main energy currency of the cell, and contains the nitrogenous base adenine, a ribose sugar, and 3 phosphates which are linked by high-energy bonds.
  Hydrolysis of ATP releases energy used to drive cellular processes.
- Energy from and aerobic is stored in ATP, which can be hydrolyzed for cellular work.
  - • In mitochondria and chloroplasts, chemical or photosynthetic energy is harnessed to form ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and inorganic phosphate (P_i).
    When energy is needed for cellular work, the ATP is hydrolyzed to ADP and P_i.
- Energy from is used to power many of the cell's activities.
- • How Cells Use ATP.
  1. synthesis of macromolecules
  2. contraction of muscle tissue
  3. activation of proteins
  4. importing macromolecules
  5. active transport
  6. transport in the cytoskeleton
  7. flagella movements
  8. cell crawling
  9. heat production
- • Synthesis of macromolecules. Cells use the energy released from the exergonic hydrolysis of ATP to drive endergonic reactions like those of protein synthesis.
- • Contraction. In muscle cells, filaments made of protein called actin and myosin slide past each other to achieve contraction of the cell. ATP is required for the filaments to reset and slide again. Video:
- • Chemical Activation. Proteins can become activated when a phosphate group from ATP attaches to the protein. Such phosphorylation enables a protein such as the sodium-potassium pump to move molecules against a concentration gradient.
- • Importing Macromolecules.
  A pump first builds up a concentration of ions outside the cell using ATP.
  Macromolecules such as sugars can then be transported into cells against their concentration gradients by coupling their intake to the inward diffusion of ions.
  Video:
- • Active Transport: Na⁺ - Na⁺ Pump.
  Most animal cells maintain a low internal concentration of Na⁺ relative to their surroundings, and a high internal concentration of K⁺.
  This is achieved by the sodium-potassium pump, which actively pumps Na⁺ out of the cell and K⁺ in, using energy from ATP.
- • Cytoplasmic transport.
  Vesicles or organelles can be dragged along microtubule tracks using molecular motor proteins, which use ATP to power their movement.
- • Flagella Movements.
  Microtubules within flagella and cilia slide past each other to produce movements.
  ATP powers the sliding of microtubules.
  Video:
- • Cell Crawling. Actin filaments in a cell's cytoskeleton assemble and disassemble; the resulting changes in shape enable cells to or engulf material. These changes are powered by ATP.
- • Heat Production. The hydrolysis of an ATP molecule releases heat, and often takes place in contracting muscle cells of animals. The heat generated can be used to maintain an organism's temperature by shivering.