College Physics - Biology 2E Chapter 5

Chapter 5: Plasma Membranes

5.1 Phospholipids

Plasma Membrane Functions

  • Defines the outer border of all cells and organelles.
  • Manages what enters and exits the cell.
  • Receives external signals and initiates cellular responses.
  • Adheres to neighboring cells.

Membrane Components and Structure

  • Fluid Mosaic Model: A mosaic of components (phospholipids, cholesterol, proteins, and carbohydrates) that gives the membrane a fluid character.
  • Proposed in 1972 by S.J. Singer and G.L. Nicolson.

Phospholipids

  • The main fabric of cell membranes is composed of phospholipids, which are amphipathic.
  • Amphipathic nature:
    • Glycerol molecule.
    • Phosphate group (polar).
    • Two fatty acid chains (nonpolar).
  • Each fatty acid can be either saturated or unsaturated.
    • Saturated: Carbons are saturated with H; all single C-C bonds.
    • Unsaturated: At least one double C=C bond occurs.
  • Hydrophobic tails and hydrophilic head.

Phospholipid Bilayer

  • Phospholipids arrange themselves in a bilayer.
    • Polar heads face outward.
    • Hydrophobic tails face inward.

Proteins

  • The second major component of membranes.
  • Functions:
    • Transporters.
    • Receptors.
    • Enzymes.
    • Binding and adhesion.
  • Types:
    • Integral proteins: Integrated completely into the bilayer.
      • Transmembrane proteins are integral proteins that pass completely through the phospholipid bilayer.
      • Integral membrane proteins have one or more regions that are hydrophobic (composed of hydrophobic amino acids) and others that are hydrophilic. The locations and number of regions determine how they arrange within the bilayer.
    • Peripheral proteins: Occur only on the surfaces.

Amino Acids

  • Amino acids can be polar, uncharged, nonpolar aliphatic, negatively charged, positively charged, or nonpolar aromatic.

Carbohydrates

  • The third major component.
  • Located on the exterior surface of the plasma membrane, bound to either proteins (forming glycoproteins) or to lipids (forming glycolipids).
  • Function in cell-cell recognition and attachment.

Membrane Fluidity

  • The membrane needs to be flexible but not so fluid that it cannot maintain its structure.
  • Affected by:
    • Phospholipid type: Phospholipids with saturated fatty acids can pack together more closely than those with unsaturated FA; therefore, more SFA, more rigid.
    • Temperature: Cold temperatures compress molecules, making membranes more rigid.
    • Cholesterol: Located within the fatty acid layer, acts as a fluidity buffer, keeping membranes fluid when cold and preventing them from getting too fluid when hot.

Membrane Asymmetry

  • Plasma membranes are asymmetric; the inner surface differs from the outer surface.
  • Examples:
    • Interior proteins anchor fibers of the cytoskeleton to the membrane.
    • Exterior proteins bind to the extracellular matrix.
    • Glycoproteins bind to substances the cell needs to import.

5.2 Passive Transport

Transport

  • The plasma membrane is selectively permeable, which means it allows some molecules to pass through, but not others.
  • Molecules that can cross the phospholipid bilayer are said to be permeant.
  • This feature allows cytosol solutions to differ from extracellular fluids.
  • Example: All cells maintain an imbalance of sodium and potassium ions between the interior and exterior environments.

Permeability

  • Permeable:
    • Gases: N2, O2, CO_2
    • Hydrophobic/nonpolar molecules (e.g., large hydrocarbons)
    • Small polar molecules (e.g., H_2O, glycerol, urea)
  • Impermeable:
    • Large polar molecules (e.g., glucose and other uncharged mono- and disaccharides)
    • Ions/electrically charged molecules (e.g., AA’s, H^+, HCO_3^-, Na^+, K^+, Ca^{2+}, Cl^-, Mg^{2+})

Moving Substances Across the Membrane

  • Transport across a membrane can be:
    • Passive: Requiring no added energy.
    • Active: Requiring energy (ATP or a coupled electrochemical gradient).

Passive Transport

  • The simplest type of passive transport is diffusion.
  • Diffusion occurs when a substance from an area of high concentration moves down its concentration gradient.
  • In membranes, this occurs through the lipid bilayer.
  • Net movement ceases once equilibrium is achieved.

Factors That Affect Diffusion Rates

  • Concentration gradients: Greater difference, faster diffusion.
  • Mass of the molecules: Smaller molecules diffuse more quickly.
  • Temperature: Molecules move faster when temperatures are higher.
  • Surface area: Increased surface area speeds up diffusion rates.
  • Pressure: In some cells (i.e., kidney cells), blood pressure forces solutions through membranes, speeding up diffusion rates.

Facilitated Passive Transport

  • Facilitated transport, aka facilitated diffusion, moves substances down their concentration gradients through transmembrane, integral membrane proteins.
  • Ions and small polar molecules diffuse this way.
  • Two types of facilitated transport proteins:
    • Channel proteins
    • Carrier proteins

Channel Proteins

  • The top, bottom, and inner core of channel proteins are composed of hydrophilic amino acids, attracting ions and/or polar molecules.
  • Some are open all the time.
  • Others are gated, only opening when a signal is received.

Carrier Proteins

  • All carrier proteins are specific to a single substance.
  • Bind to that substance, change shape, and “carry it” to the other side.
  • Many allow movement in either direction as concentration gradients change.
  • Important example: Glucose transport proteins (GLUTS).

Osmosis

  • Osmosis is the diffusion of water across a membrane.
  • Water always moves from an area of higher water concentration to one of lower water concentration.
  • Differences in water concentration occur when a solute cannot pass through the selectively permeable membrane.

Tonicity

  • Tonicity describes how an extracellular solution can change the volume of a cell by affecting osmosis.
  • When solutions are separated by a membrane permeable to water but not the solute, water moves through the membrane and down its concentration gradient.
  • Hypertonic, isotonic, and hypotonic describe the osmolarity of the cell relative to that of its extracellular fluid.

Tonicity Types

  • Hypertonic: Extracellular fluid has lower osmolarity than the cytosol – water leaves the cell.
  • Isotonic: Extracellular fluid has the same osmolarity as the cytosol – water does not move.
  • Hypotonic: Extracellular fluid has higher osmolarity than the cytosol – water enters the cell.
  • Animal cells function best when extracellular fluids are isotonic.

Osmoregulation

  • Organisms whose cells have cell walls (such as plants, fungi, and bacteria) prefer hypotonic extracellular solutions.
  • The pressure exerted by the plasma membrane against the cell wall (turgor pressure) is critical to organismal growth and function.
  • Hypertonic solutions cause plasmolysis – plasma membrane detaches from the cell wall.

Osmoregulation by Other Organisms

  • Freshwater protists use contractile vacuoles to pump water out of their cells so they do not burst.
  • Marine invertebrates have internal salt concentrations that match their environment.
  • Fishes excrete diluted urine to get rid of excess H_2O or salts.

5.3 Active Transport

Active Transport

  • Active transport is needed any time an ion or molecule is transported through a membrane protein:
    • Against its concentration gradient (from low to high concentration).
    • Against its electrochemical gradient (e.g., H^+ ions to a solution that is more positive).
  • Energy is always required for active transport.
  • Two types of active transport:
    • Primary: Where ATP provides the energy.
    • Secondary: Where an electrochemical gradient provides the energy.

Electrochemical Gradients

  • Electrochemical gradients arise from the combined effects of concentration gradients and electrical gradients.
  • An electrical gradient, where the cytoplasm contains more negatively charged molecules (more negative ions and proteins) than the extracellular fluid, is critical for proper cell functioning.

Carrier Proteins (Pumps)

  • Active transport occurs through transmembrane, integral carrier proteins called pumps.
  • Three types of pumps:
    • Uniporter: Carries one molecule or ion.
    • Symporter: Carries two different molecules or ions in the same direction.
    • Antiporter: Carries two different molecules or ions in different directions.

Primary Active Transport

  • Primary active transport moves an ion or molecule up its concentration gradient using energy from ATP hydrolysis.
  • Example: Na^+-K^+ pump moves 3 Na^+ out and 2 K^+ in using 1 ATP.
  • This pump is an electrogenic pump.

Secondary Active Transport

  • Secondary active transport uses an electrochemical gradient created by primary active transport to move a different substance against its concentration gradient.
  • Many amino acids and glucose enter the cell this way.

5.4 Bulk Transport

Bulk Transport

  • Sometimes cells need to import or export molecules/particles that are too large to pass through a transport protein.
  • Bulk transport is a type of active transport.
  • Energy is required.
  • Importing by bulk transport is called endocytosis, and exporting is called exocytosis.

Endocytosis Types

  • Phagocytosis (cellular eating): The cell membrane surrounds a particle and engulfs it.
  • Pinocytosis (cellular drinking): The cell membrane invaginates, surrounds a small volume of fluid, and pinches off.
  • Receptor-mediated endocytosis: Uptake of a specific substance is targeted by binding to receptors on the external surface of the membrane.

Exocytosis

  • In exocytosis, vesicles containing substances fuse with the plasma membrane, releasing the contents to the exterior of the cell.