Membrane Structure and Function

Cell Review

Various types of cells can all be interactively investigated at this cell comparison website.

The Prokaryotic Cell

• Greek word derivation:
  • Pro: before
  • Karyon: kernel (nucleus)
  • Prokaryote: having no nucleus (i.e. evolved before the nucleus).
• The nucleoid is a region where DNA is found—it is not an organelle with a membrane.
• The prokaryotic cell is ¹∕₁₀ the size of the eukaryotic cell.

The Eukaryotic Cell

• Greek word derivation:
  • Eu: true
  • Karyon: kernel (nucleus)
  • Eukaryote: having a true nucleus (with a membrane)
• Cytoplasm: the region between the nucleus and the cell membrane
• Organelles: membrane-bound compartments in the cytoplasm, each with a specialized function.

Distinguishing Features of Plant Cells

• Cell wall
  • Outside of the cell membrane (plasma membrane)
  • Maintains the shape of the plant cell.
  • Helps protect the plant cell.
• Central vacuole
  • Stores chemicals.
  • Breaks down macromolecules.
  • Takes up a lot of space.
  • Tonoplast: the membrane enclosing the central vacuole
• Chloroplasts: carry out photosynthesis.

The Plasma Membrane

• Phospholipid bilayer
  • Hydrophilic “heads” face the water.
  • Hydrophobic “tails” face each other.
• Proteins “float around” in the membrane.

Figure 8.2(b), page 131, Campbell's Biology, 5th Edition

Membranes Are “Fluid”

• Unsaturated fatty acid tails increase fluidity.
• Saturated fatty acid tails decrease fluidity.
• Cholesterol:
  • Decreases fluidity at moderate temperatures.
  • Hinders solidification at low temperatures.

Figure 8.3, page 133, Campbell's Biology, 5th Edition

The Fluid Mosaic Model

• The cellular membrane is a “fluid mosaic” of structure and function.
• The phospholipid bilayer provides the main structure.
• Proteins provide the function.
  • For example: red blood cells have fifty different membrane proteins.
• Carbohydrates allow for cell-to-cell recognition:
  • Glycolipids
  • Glycoproteins
• Inside Cancer Website (Check out "At the Cell Surface" and "Beneath the Membrane")
• The Inner Life of the Cell Animation (Narrated version- select the "High Speed Version")
• Review CD-ROM Chapter 8, Activity 8.1

Figure 8.5, page 134, Campbell's Biology, 5th Edition

Movement of Molecules Across Membranes

• The plasma membrane is selectively permeable: certain molecules are allowed to pass through the membrane while others are blocked.
• There are six different ways in which molecules move across membranes:
  1. Simple diffusion (diffusion)
  2. Osmosis
  3. Facilitated diffusion
  4. Active transport
  5. Exocytosis
  6. Endocytosis
• A good explanation and animation for many of these forms of membrane transport can be found here.

1. Diffusion (a.k.a. “Simple” Diffusion)

• The movement of molecules from regions of high concentration to low concentration: molecules move down their concentration gradient.
• A natural, or passive, process
  • As long as molecules have kinetic energy, diffusion can occur.
  • Molecules always have kinetic energy except at the (impossible) temperature 0 K.
  • Diffusion Animation
  • See CD-ROM Online Chapter 8, Activity 8.2

2. Osmosis

• The diffusion of water across a selectively permeable membrane: water moves down its concentration gradient.
• The concentration of solute determines the direction of osmosis: water moves from a hypotonic solution (less concentrated) to a hypertonic solution (more concentrated)
• Resources:
  • See CD-ROM Online Chapter 8, Activity 8.3
  • Nice Animation
  • Another animation

3. Facilitated Diffusion

• The transportation of a molecule, that normally cannot move through a lipid bilayer, across the plasma membrane
• Requires no energy.
• Done using a membrane-bound protein.
• Facilitated Diffusion Animation
• See CD-ROM Online Chapter 8, Activity 8.4
  

4. Active Transport

• Molecules are moved across a membrane against their concentration gradient.
• Requires energy (ATP).
• A famous example: the sodium–potassium pump
• See CD-ROM Chapter 8, Activity 8.5
  • Tutorial/animation 1
  • Tutorial/animation 2

5. Exocytosis and 6. Endocytosis

• Ways in which the cell moves larger molecules across the membrane.
• Exocytosis: the secretion of macromolecules out of the cell by fusing vesicles with the plasma membrane
  • Exo: Out of
  • Cyto: Cell
• Endocytosis
  • The absorption of macromolecules by forming new vesicles from the plasma membrane.
    • Endo: Into
    • Cyto: Cell
  • Three types:
    • Phagocytosis: cellular eating
      • Phago: Greek for "to eat"
    • Pinocytosis: cellular drinking
      • Pino: Greek for "drink"
    • Receptor-mediated endocytosis: allows cells to grab specific molecules
• Resources:
  • Receptor-mediated encocytosis animation
  • See CD-ROM Chapter 8, Activity 8.6