Chapter 5: The Working Cell - Biology PDF

Chapter 5 ========= **The Working Cell** -------------------- Introduction: *Turning on the Lights to Be Invisible* ===================================================== Some organisms use energy-converting reactions to produce light ---------------------------------------------------------------...

Chapter 5 ========= **The Working Cell** -------------------- Introduction: *Turning on the Lights to Be Invisible* ===================================================== Some organisms use energy-converting reactions to produce light --------------------------------------------------------------- - ### Examples are organisms that live in the ocean and use light to hide themselves from predators B. Energy conversion involves not only energy but also membranes and enzymes ------------------------------------------------------------------------- C. So, production of light involves all the topics covered in this chapter ----------------------------------------------------------------------- MEMBRANE STRUCTURE AND FUNCTION ------------------------------- 5.1 Membranes are a fluid mosaic of phospholipids and proteins ============================================================== Membranes are composed of phospholipids and proteins ---------------------------------------------------- - ### Membranes are commonly described as a **fluid mosaic** - ### This means that the surface appears *mosaic* because of the proteins embedded in the phospholipids and *fluid* because the proteins can drift about in the phospholipids Many phospholipids are made from unsaturated fatty acids that have kinks in their tails --------------------------------------------------------------------------------------- - ### This prevents them from packing tightly together, which keeps them liquid - ### This is aided by cholesterol wedged into the bilayer to help keep it liquid at lower temperatures Membranes contain integrins, which give the membrane a stronger framework ------------------------------------------------------------------------- - ### Integrins attach to the extracellular matrix on the outside of the cell as well as span the membrane to attach to the cytoskeleton Some glycoproteins in the membrane serve as identification tags that are specifically recognized by membrane proteins of other cells ------------------------------------------------------------------------------------------------------------------------------------ - ### For example, cell-cell recognition enables cells of the immune system to recognize and reject foreign cells, such as infectious bacteria - ### Carbohydrates that are part of the extracellular matrix are significantly involved in cell-cell recognition Many membranes proteins function as *enzymes*, others in *signal transduction*, while others are important in *transport* ------------------------------------------------------------------------------------------------------------------------- - ### Because membranes allow some substances to cross or be transported more easily than others, they exhibit **selectively permeability** - #### Nonpolar molecules (carbon dioxide and oxygen) cross easily - #### Polar molecules (glucose and other sugars) do not cross easily 5.2 EVOLUTION CONNECTION: Membranes form spontaneously, a critical step in the origin of life ============================================================================================= Phospholipids, the key component of biological membranes, spontaneously assemble into simple membranes ------------------------------------------------------------------------------------------------------ - ### Formation of a membrane that encloses collections of molecules necessary for life was a critical step in evolution 5.3 Passive transport is diffusion across a membrane with no energy investment ============================================================================== **Diffusion** is a process in which particles spread out evenly in an available space ------------------------------------------------------------------------------------- - ### 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 5.4 Osmosis is the diffusion of water across a membrane ======================================================= A.) It is crucial for cells that water moves across their membrane ------------------------------------------------------------------ - ### Water moves across membranes in response to solute concentration inside and outside of the cell by a process called **osmosis** - ### Osmosis will move water across a membrane down its concentration gradient until the concentration of solute is equal on both sides of the membrane 5.5 Water balance between cells and their surroundings 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 is dependent on the concentration of a nonpenetrating solute on both sides of the membrane - #### **Isotonic** indicates that the concentration of a solute is the same on both sides - #### **Hypertonic** indicates that the concentration of solute is higher outside the cell - #### **Hypotonic** indicates a higher concentration of solute inside the cell #### Many organisms can maintain water balance within their cells by a process called **osmoregulation** --------------------------------------------------------------------------------------------------- - ### This process prevents excessive uptake or excessive loss of water - ### Plant, prokaryotic, and fungal cells have different issues with osmoregulation because of their cell walls 5.6 Transport proteins may facilitate diffusion across membranes ================================================================ 5.7 TALKING ABOUT SCIENCE: Peter Agre talks about aquaporins, water-channel proteins found in some cells ======================================================================================================== The cell membrane contains hourglass-shaped proteins that are responsible for entry and exit of water through the membrane -------------------------------------------------------------------------------------------------------------------------- - ### Dr. Peter Agre, a physician at the Johns Hopkins University School of Medicine, discovered these transport proteins and called them aquaporins 5.8 Cells expend energy in the active transport of a solute against its concentration gradient ============================================================================================== Cells have a mechanism for moving a solute against its concentration gradient ----------------------------------------------------------------------------- - ### It requires the expenditure of energy in the form of ATP - ### The mechanism alters the shape of the membrane protein through phosphorylation using ATP 5.9 Exocytosis and endocytosis transport large molecules across membranes ========================================================================= A cell uses two mechanisms for moving large molecules across membranes ---------------------------------------------------------------------- - ### **Exocytosis** is used to export bulky molecules, such as proteins or polysaccharides - ### **Endocytosis** is used to 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 --------------------------------------------------------------------------------------------------- #### There are three kinds of endocytosis ------------------------------------ - ### **Phagocytosis** is engulfment of a particle by wrapping cell membrane around it, forming a vacuole - ### **Pinocytosis** is the same thing except that fluids are taken into small vesicles - ### **Receptor-mediated endocytosis** is where receptors in a receptor-coated pit interact with a specific protein, initiating formation of a vesicle ENERGY AND THE CELL ------------------- 5.10 Cells transform energy as they perform work ================================================ A.) Cells are small units, a chemical factory, housing thousands of chemical reactions -------------------------------------------------------------------------------------- - ### The result of reactions is maintenance of the cell, manufacture of cellular parts, and replication **Energy** is the capacity to do work and cause change ------------------------------------------------------ - ### Work is accomplished when an object is moved against an opposing force, such as friction - ### There are two kinds of energy - #### **Kinetic energy** is the energy of motion - #### **Potential energy** is energy that an object possesses because of its location Kinetic energy performs work by transferring motion to other matter ------------------------------------------------------------------- - ### For example, water moving through a turbine generates electricity - ### Heat, or thermal energy, is kinetic energy associated with the random movement of atoms An example of potential energy is water behind a dam ---------------------------------------------------- - ### **Chemical energy** is potential energy because of its energy available for release in a chemical reaction 5.11 Two laws govern energy transformations =========================================== A.) Energy transformations within matter are studied by individuals in the field of **thermodynamics** ------------------------------------------------------------------------------------------------------ - ### Biologists study thermodynamics because an organism exchanges both energy and matter with its surroundings It is important to understand two laws that govern energy transformations in organisms -------------------------------------------------------------------------------------- - ### The **first law of** **thermodynamics**---energy in the universe is constant - ### The **second law of** **thermodynamics**---energy conversions increase the disorder of the universe - #### **Entropy** is the measure of disorder, or randomness 5.12 Chemical reactions either release or store energy ====================================================== A.) An **exergonic reaction** is a chemical reaction that releases energy ------------------------------------------------------------------------- - ### This reaction releases the energy in covalent bonds of the reactants - ### Burning wood releases the energy in glucose, producing heat, light, carbon dioxide, and water - ### **Cellular respiration** also releases energy and heat and produces products but can use the released energy to perform work B.) An **endergonic reaction** requires an input of energy and yields products rich in potential energy ------------------------------------------------------------------------------------------------------- - ### The reactants contain little energy in the beginning, but energy is absorbed from the surroundings and stored in covalent bonds of the products - ### Photosynthesis makes energy-rich sugar molecules using energy in sunlight A living organism produces thousands of endergonic and exergonic chemical reactions ----------------------------------------------------------------------------------- - ### All of these combined is called **metabolism** - ### A **metabolic pathway** is a series of chemical reactions that either break down a complex molecule or build up a complex molecule A cell does three main types of cellular work --------------------------------------------- - ### Chemical work---driving endergonic reactions - ### Transport work---pumping substances across membranes - ### Mechanical work---beating of cilia To accomplish work, a cell must manage its energy resources, and it does so by **energy** **coupling**---the use of exergonic processes to drive an endergonic one ------------------------------------------------------------------------------------------------------------------------------------------------------------------ 5.13 ATP shuttles chemical energy and drives cellular work ========================================================== A.) ATP, [a]denosine [t]ri[p]hosphate, is the energy currency of cells. ----------------------------------------------------------------------------------------------------------- - ### ATP is the immediate source of energy that powers most forms of cellular work. - ### It is composed of adenine (a nitrogenous base), ribose (a five-carbon sugar), and three phosphate groups. B.) Hydrolysis of ATP releases energy by transferring its third phosphate from ATP to some other molecule --------------------------------------------------------------------------------------------------------- - ### The transfer is called phosphorylation - ### In the process, ATP energizes molecules C.) ATP is a renewable source of energy for the cell ---------------------------------------------------- - ### When energy is released in an exergonic reaction, such as breakdown of glucose, the energy is used in an endergonic reaction to generate ATP ### HOW ENZYMES FUNCTION -------------------- 5.14 Enzymes speed up the cell's chemical reactions by lowering energy barriers =============================================================================== Although there is a lot of potential energy in biological molecules, such as carbohydrates and others, it is not released spontaneously --------------------------------------------------------------------------------------------------------------------------------------- - ### Energy must be available to break bonds and form new ones - ### This energy is called **energy of activation** (**E~A~**) The cell uses catalysis to drive (speed up) biological reactions ---------------------------------------------------------------- - ### Catalysis is accomplished by **enzymes**, which are proteins that function as biological catalysts - ### Enzymes speed up the rate of the reaction by lowering the E~A~ , and they are not used up in the process - ### Each enzyme has a particular target molecule called the substrate ### 5.15 A specific enzyme catalyzes each cellular reaction ======================================================= A.) Enzymes have unique three-dimensional shapes ------------------------------------------------ - ### The shape is critical to their role as biological catalysts - ### As a result of its shape, the enzyme has an **active site** where the enzyme interacts with the enzyme's substrate - ### Consequently, the substrate's chemistry is altered to form the product of the enzyme reaction B.) For optimum activity, enzymes require certain environmental conditions -------------------------------------------------------------------------- - ### Temperature is very important, and optimally, human enzymes function best at 37ºC, or body temperature - #### High temperature will denature human enzymes - ### Enzymes also require a pH around neutrality for best results Some enzymes require nonprotein helpers --------------------------------------- - ### **Cofactors** are inorganic, such as zinc, iron, or copper - ### **Coenzymes** are organic molecules and are often vitamins 5.16 Enzyme inhibitors block enzyme action and can regulate enzyme activity in a cell ===================================================================================== Inhibitors are chemicals that inhibit an enzyme's activity ---------------------------------------------------------- - ### One group inhibits because they compete for the enzyme's active site and thus block substrates from entering the active site - ### These are called competitive inhibitors Other inhibitors do not act directly with the active site --------------------------------------------------------- - ### These bind somewhere else and change the shape of the enzyme so that the substrate will no longer fit the active site - ### These are called **noncompetitive inhibitors** Enzyme inhibitors are important in regulating cell metabolism ------------------------------------------------------------- - ### Often the product of a metabolic pathway can serve as an inhibitor of one enzyme in the pathway, a mechanism called **feedback inhibition** - ### The more product formed, the greater the inhibition, and in this way, regulation of the pathway is accomplished You should now be able to ========================= - Describe the cell membrane within the context of the fluid mosaic model ----------------------------------------------------------------------- - Explain how spontaneous formation of a membrane could have been important in the origin of life ----------------------------------------------------------------------------------------------- - Describe the passage of materials across a membrane with no energy expenditure ------------------------------------------------------------------------------ - Explain how osmosis plays a role in maintenance of a cell --------------------------------------------------------- You should now be able to ========================= - Explain how an imbalance in water between the cell and its environment affects the cell --------------------------------------------------------------------------------------- - Describe membrane proteins that facilitate transport of materials across the cell membrane without expenditure of energy ------------------------------------------------------------------------------------------------------------------------ - Discuss how energy-requiring transport proteins move substances across the cell membrane ---------------------------------------------------------------------------------------- - Distinguish between exocytosis and endocytosis and list similarities between the two ------------------------------------------------------------------------------------ You should now be able to ========================= - Explain how energy is transformed during life processes ------------------------------------------------------- - Define the two laws of thermodynamics and explain how they relate to biological systems --------------------------------------------------------------------------------------- - Explain how a chemical reaction can either release energy or store energy ------------------------------------------------------------------------- - Describe ATP and explain why it is the energy currency of a cell ---------------------------------------------------------------- You should now be able to ========================= - Define enzyme and explain how enzymes cause a chemical reaction to speed up --------------------------------------------------------------------------- - Discuss the specificity of enzymes ---------------------------------- - Distinguish between competitive inhibitors and noncompetitive inhibitors ------------------------------------------------------------------------