ap bio unit 2 review 

Cell Organelles, Membranes, and Transport

Cell Organelles and Their Functions

• two major types of cells:
    * prokaryotic: simpler in structure; found in bacterial organisms
    * eukaryotic: contain membrane-bound organelles; more complex; found in animals, plants, fungi, and protists
    * all cells (prokaryotic and eukaryotic) have the following: genetic material, ribosomes, cytosol, and a plasma membrane
      * the genetic material in prokaryotes is circular and stored in the center of the cell called the nucleoid region
        * plasmids: small circular pieces of genetic material stored outside of the chromosome; often found in some forms of bacteria
      * genetic material in eukaryotes is linear and stored in a membrane-bound nucleus
• ribosomes: functions in protein synthesis; found in prokaryotic and eukaryotic cells; made of proteins and ribosomal RNA (rRNA)
    * sizes of the large and small subunits of ribosomes vary in eukaryotic and prokaryotic cells
    * during translation: ribosomes assemble amino acids into polypeptide chains according to the mRNA sequence
    * there are free ribosomes in the cytosol and organelle-bound ribosomes on the membrane of the rough endoplasmic reticulum
• endoplasmic reticulum: formed of two parts (smooth ER and rough ER)
    * rough ER: covered with ribosomes; functions in proteins synthesis
    * smooth ER: does not contain ribosomes; functions in lipid synthesis and detoxification of harmful substances in the cell
• golgi complex (golgi body/apparatus): a stack of flattened membrane sacs (cisternae); functions in controlling the modification and packaging of proteins for transport
    * lumen: interior of cisternae; contains necessary enzymes for the golgi complex to function
    * proteins made on the free ribosomes of the rough ER are sent to the golgi body to be modified and packed into vesicles for transport throughout the cell
      * vesicles: structure within or outside a cell, consisting of liquid or cytoplasm enclosed by a lipid bilayer
• lysosomes: membrane-bound sacs containing hydrolytic enzymes that are used in various functions including digestion of macromolecules, breaking down of worn-out cellular parts, apoptosis, or destroying bacteria in the cell
    * hydrolytic enzymes: break down protein, lipids, nucleic acids, carbohydrate, and fat molecules into their simplest units
• vacuoles: membrane-bound sac that functions in the storage of food or water for the cell, water regulation, or waste storage (until it can be eliminated)
    * plant vacuole: large central vacuole that helps regulate the water balance of cell
      * well-hydrated plant cells will have proper turgor pressure, which is maintained by the vacuole in the center of the plant cell
        * turgor pressure: provides structural integrity to each cell and to the tissue as a whole; pushes the plasma membrane against the cell wall and causes in-plane mechanical tension within the cell wall
    * animal vacuole: generally small and help sequester waste products
• mitochondria: produces energy (ATP) for the cell; contains a double membrane (smooth outer membrane and folded inner membrane)
    * the folded inner membrane allows for increased surface area, which increases the efficiency of ATP production during cellular respiration
    * the double membrane allows for mitochondria to form proton (H+) gradients which are necessary for ATP production
    * matrix: center of the mitochondria; fluid containing enzymes; the location where the krebs cycle (citric acid cycle) occurs
    * mitochondria also contain their own ribosomes and mitochondrial DNA (mtDNA)\

• chloroplasts: found in plants and algae; carry out photosynthesis; double membrane organelle with smooth outer membrane and structures inside
    * thylakoids: pancake shaped membraneous sacs stacked into structures; functions in light-dependent reaction
    * grana: the structures thylakoids are stacked into
    * stroma: liquid in chloraplast surrounding the grana; enzymes in stroma function in light-independent reactions
    * contain their own dna (cpDNA)

• centrosome: found in animal cells; helps microtubules assemble into spindle fibers (used in cell division)
    * defects in centrosome cause dysregulation of cell cycle (and causes some cancer)
• amyloplasts: starch molecule that store excess glucose produced during photosynthesis; commonly found in starchy root vegetables (ex. potatoes)
• several structures are found in plant and animal cells:
    * peroxisome: helps oxidize molecules and break down toxins in cells
    * nucleolus: not membrane bound organelle; region in the nucleus where ribosomes are assembled
    * cytoskeleton: fibers that help give cells their shape and move items in cell

Endosymbiosis Hypothesis

• endosymbiosis hypothesis: states that membrane-bound organelles (mitochondria and chloroplasts) were once free-living prokaryotes that were absorbed by larger prokaryotes
    * the prokaryotes became interdependent of each other and the larger prokaryotes evolved into membrane bound organelles
• reasons for this theory:
    * mitochondria and chloroplasts have their own DNA (circular like prokaryotic DNA)
    * mitochondria and chloroplasts have their own ribosomes (similar in structure to prokaryotic ribosomes)
    * mitochondria and chloroplasts are produced by binary fission (similar to how bacteria reproduce)

Advantages of Compartmentalization

• membrane-bound organelles form compartments to increase their efficiency
• compartmentalization: allows cells to separate enzymes involved in different metabolic processes
    * this reduces the risk of cross-reacting, which would decrease efficiency of the cellular processes

The Importance of Surface Area to Volume Ratios

• many eukaryotic organelles (ex. mitochondria) have folds in their membranes to increase surface area
    * prokaryotes can fold their single membranes to also increase surface area
• the larger the SA:V ratio, the more efficient the cell is
    * as radius increases, the ratio decreases
• larger cells have a lower SA:V ratio, making them less efficient in certain functions

Structure of Plasma Membranes

• plasma membranes are selectively permeable (some materials can cross and others cannot)
    * selective permeability allows the cell to maintain its internal environment
• plasma membranes are made of a phospholipid bilayer
    * phospholipids have a hydrophilic phosphate head and two hydrophobic tails
    * tails orient themselves away from internal (aqueous) environment
    * phospholipid bilayer contains glycoproteins, glycolipids, and steroids
      * these molecules can move throughout the bilayer and allow the cell to adapt adn respond to changing environmental conditions
        * proteins (in membrane): used to transport materials, participate in cell signaling processes, anchor the cell in place, and catalyze chemical reactions
        * glycoproteins and glycolipids: used in cell recognition
        * steroids: adjust membrane fluidity in response to changing environmental conditions and needs of the cell
      * fluidity of molecules in plasma membrane gives it the term “fluid mosaic model”

Crossing (and Not Crossing) Plasma Membrane

• phospholipid bilayer makes cell membrane selectively permeable
• small hydrophobic molecules (ex. oxygen, carbon dioxide, and nitrogen) can move between phospholipids and in/out of the cell
• larger polar molecules and ions cannot pass through as easily without help
    * large polar and charged molecules must use membrane channels or transport proteins to enter/exit cell
• small polar molecules (ex. H2O) can pass in small quantities; larger amounts also must be assisted
    * aquaporins: special proteins that allow for the movement of (most) water in/out of cells

Passive Transport

• passive transport: movement of molecules in/out of cell without energy required; molecules move from areas of high concentration to areas of low concentration (moving “down” concentration gradient)
    * diffusion: movement of molecules down concentration gradient without energy required
      * osmosis: diffusion of water molecules down a gradient and across a membrane
• facilitated diffusion: process of passive transport with use of membrane protein; used for polar/charged molecules
    * aquaporins are an example of membrane proteins (only used for water)
    * channel proteins: can allow the passive transport of ions (ex. Ca+2 or Cl-1) down the concentration gradient
    * rate of facilitated diffusion is limited by the number of membrane proteins available

Active Transport

• active transport: movement of molecules from areas of low concentration to high concentration; movement of molecules “against” concentration gradient requires the input of energy
• Na+/K+ pump: prime example of active transport
    * membrane protein requires the input of ATP to pump Na+ ions from lower concentration to higher concentration outside the cell
    * membrane protein pumps K+ ions from areas of lower concentration to higher concentration inside the cell
    * for every 3 Na+ ions pumped outside cell, 2 K+ ions are pumped into cell
      * results in higher concentration of positive ions outside of cell and helps cell maintain membrane potential
• endocytosis and exocytosis are also forms of active transport (both require input of energy)
    * endocytosis: used by cell to take in water and macromolecules with vesicles formed from plasma membrane
    * exocytosis: vesicles (with molecules) are merged with cell membrane and molecules in vesicles are expelled from cell

Movement of Water in Cells

Water Potential

• hypotonic: lower concentration of solute outside than inside cell; higher water potential
    * cell swells and bursts
• hypertonic: higher concentration of solute outside than inside cell; lower water potential
    * cell shrinks and shrivels
• isotonic: equal concentration of solute inside and outside cell
    * cell pressure is maintained

• water potential: the potential energy of water in a solution; the ability of water to do work
    * the more water there is in a solution, the higher the water potential
    * water flows down concentration gradients (higher concentration to lower concentration)

Calculating Water Potential

• solute potential (Ψs): water potential due to solute concentration
    * Ψs depends on how many particles in the solute form the solution and the temp. of the solution
    * Ψs = -iCRT
      * i: ionization constant; function of how many particles or ions will form the solution in a given solute
        * covalent compounds: i = 1 (ions don’t separate)
        * ionic compounds: i depends on how many ions form in the solution (ex. NaCl forms 2 ions (Na+ and Cl-) so i = 2)
      * C: concentration of solute in solution; as concentration increases, solute potential decreases
        * solutes with more solute (higher solute concentration) will have lower water potential (if all other variables are equal)
      * R: pressure constant; R = 0.0831 L-bars/mol-K
      * T: temperature of solution; only in Kelvin
• pressure potential (Ψp): water potential due to pressure on system
    * most biological systems are open to equilibrium in their environments which eliminates pressure in the equation and it becomes: Ψ = Ψs
      * when solution is open to atmosphere, Ψp is zero

Osmolarity and Regulation

• osmolarity: total concentration of solutes in solution
• living organisms need to closely regulate internal solute concentration and water potential (to far away from proper conditions could lead to death)
• contractile vacuole: specialized organelle used to store excess water until it is pumped out of the cell; allows cells to maintain internal solute concentration