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