Chapter 5 – The Working Cell Energy The capacity to do work What is work? Potential Energy Stored energy, eg. in chemical bonds Kinetic Energy Energy of motion What is the Law of Conservation of Energy?

ATP and Cellular Work The chemical energy of organic molecules is released in cellular respiration to make ATP in the mitochondria Consists of adenosine plus a tail of three phosphate groups Is broken down to ADP, accompanied by the release of energy Energy Adenosine Phosphate transferred to other molecules

ATP functions in what is called energy coupling, or the ATP cycle Cellular respiration: chemical energy harvested from fuel molecules Energy for cellular work

Most enzymes are proteins with diverse structure. Enzymes are chemical catalysts that: Increase the rate of a reaction. Are not permanently altered or used up by the reaction. Do not change the nature of the reaction. enzyme animation..

Reduce the required amount of activation energy required to initiate the reaction. Many enzymes work by orienting molecules so that they can better contact each other. Each type of enzyme has has a highly-ordered, characteristic 3-dimensional shape (conformation). Ridges, grooves, and pockets lined with specific amino acids. Pockets active in catalyzing a reaction are called the active sites of the enzyme..

Activation energy barrier Reactants (a) Without enzyme Products Enzyme (b) With enzyme

The molecule affected by the enzyme is the substrate. Substrates have specific shapes to fit into the active sites (lock-and-key model): Better fit may be induced (induced-fit model): Enzyme undergoes structural change when substrate binds..

Enzyme names end with suffix “-ase.” Enzymes may be named according to their mode of action, e.g. kinase. May specify both the substrate of the enzyme and job category. Enzymes may be named for the substrate, e.g. lipase, maltase. Different organs may make different enzymes (isoenzymes) that have the same activity. Differences in structure do not affect the active sites..

Enzyme activity can be affected by several factors: Changes in pH and/or temperature may denature the enzyme. These changes may break the H bonds stabilizing the molecule.

Small molecules or ions called cofactors alter the shape of the active site and/or is needed for the substrate to bind with the active site, e.g. Ca++. Certain organic molecules called coenzymes are needed to transport small molecules or ions (especially H+) from one enzyme to another, e.g. NAD..

Metabolic pathways are frequently regulated by inhibition Metabolic pathways are frequently regulated by inhibition. End-product inhibition occurs when one product binds with the enzyme and prevents it from binding with the substrate. The product may bind with the enzyme at the allosteric site. This is negative feedback. inhibition animation.. Active site Substrate Inhibitor Enzyme (c) Enzyme inhibition by a molecule that causes the active site to change shape

Competitive inhibition occurs when some other imposter molecule (similar to the substrate) binds with the enzyme. This prevents the substrate from participating in any metabolic pathway. This is typically how poisons work.. Substrate Active site Enzyme (a) Normal enzyme action Inhibitor (b) Enzyme inhibition by a substrate imposter

Membrane Transport Working cells must control the flow of materials This is the primary function of the plasma membrane Transport proteins also help with this task Transport process may be categorized by the energy requirements: Passive transport: Net movement down a concentration gradient. Does not require metabolic energy (ATP). Active transport: Net movement against a concentration gradient. Requires ATP..

Molecules of dye Membrane (a) Passive transport of one type of molecule Equilibrium (b) Passive transport of two types of molecules

Diffusion Molecules/ions are in constant state of random motion due to their thermal energy. Diffusion is the movement of particles from an area of high concentration to and area of low concentration. Net diffusion indicates the final concentration changes. Diffusion Animation Diffusion of some substances through the plasma membrane requires carriers or channels (discussed later)..

Two requirements for osmosis: Osmosis is the special term for the net diffusion of H20 across a selectively permeable membrane. Osmosis is the movement of H20 from a high [H20] to lower [H20] until equilibrium is reached. Two requirements for osmosis: Must be difference in [solute] on the 2 sides of the membrane. Membrane must be impermeable to the solute.

(net movement of water) Hypotonic solution Hypertonic Isotonic solutions Sugar molecule (solute) Selectively permeable membrane Osmosis (net movement of water) Hypertonic solution = has a higher concentration of solute Hypotonic solution = has a lower concentration of solute Isotonic solution = has an equal concentration of solute Osmosis Animation..

Water Balance in Cells The survival of a cell depends on its ability to balance water uptake and loss = Osmoregulation Animal cell Normal Lysing Shriveled Plant Flaccid (wilts) Turgid Plasma membrane (a) Isotonic (b) Hypotonic (c) Hypertonic..

Carrier-Mediated Transport Molecules that are too large and/or polar to pass through the plasma membrane require protein carriers. Facilitated diffusion is passive transport utilizing carriers. Molecules move from high concentration to low concentration. e.g. GLUT carriers transport glucose into human cells..

Active transport required the expenditure of ATP Active transport required the expenditure of ATP. Molecules are moved from low concentration to high concentration. The most common primary active transport mechanism is the Na+/K+ pump. In this case, the protein carrier is an ATPase that converts ATP to ADP + Pi. The carrier transports 3 Na+ out of the cell and takes in 2 K+. Na/K Pump Animation..

Bulk Transport Movement of many large molecules, that cannot be transported by carriers, at the same time. Exocytosis: Fusion of the membrane-bound vesicles that contains cellular products with the plasma membrane. Outside cell Plasma membrane Cytoplasm (a) Exocytosis

Endocytosis: Exocytosis in reverse. Specific molecules can be taken into the cell because of the interaction of the molecule and protein receptor (receptor-mediated endocytosis). Endo/Exocytosis Animation (b) Endocytosis

Phagocytosis Pinocytosis Food being ingested Pseudopod of amoeba