Membrane Structure

• Phospholipids

• Proteins

• Cholesterol

Phospholipids

• Have a polar “head”

  • Phosphate

• Have 2 nonpolar “tails”

  • Fatty Acids

• Polar side is attracted to water

• Nonpolar side is repelled to water

• Can have saturated hydrocarbon chains

  • Making the membrane viscous

• Can have unsaturated hydrocarbon chains

  • Making the membrane more fluid

• Move laterally, but rarely flip flop

Proteins

• Used for moving substances in and out of the cell

• Used for signal recognition

• Are embedded in the phospholipid bilayer based on hydrophobic interactions

• Can be integral

  • Through the cell membrane

• Glycoproteins are also common

Cholesterol

• Helps the membrane deal with temperature changes

• Keeps the membrane fluid when cooled

  • Keeps the phospholipids from packing tightly

• Keeps the membrane viscous when heated

  • Restrains the movement of molecules

Movement Across Membranes

Passive Transport

• Diffusion across the membrane

• No energy required

• Spontaneous

• Examples :

  • Diffusion

  • Osmosis

  • Facilitated Diffusion

Active Transport

• Often moves particles against the concentration gradient.

• Occasionally moves with the c.g., but at a faster rate than diffusion.

• Occurs when you need to accumulate particles

• Requires energy to move molecules

• Energy is required

• ATP used

• Examples :

  • The sodium-potassium pump

    • Involved with nerve cells

    • The transport protein has 2 conformations :

      • High affinity for Na+ with binding sites oriented toward the cytoplasm

      • High affinity for K+ with binding sites toward the exterior

    • ATP phosphorylates the transport protein and powers the conformational change from Na+ receptive to K+ receptive

    • 3 Na+ are moved out of the cell leaving room for 2 K+

    • This sets up an electrochemical gradient across the membrane

      • The difference in charge across a membrane is called the membrane potential

      • The combination of the membrane potential and the concentrations gradient is called the electrochemical gradient

    • With the correct stimulus, a gated channel opens

    • The electrochemical gradient is equalized

    • This is a nerve impulse

    • The nerve can’t work again until the gradient is set up

Diffusion

• Due to random movement of molecules

• Particles have a net movement from high concentration to low concentration

• Remember entropy

• Concentration gradient

  • Is the difference in concentration throughout space

  • Particles tend to move “with” or “down” their concentration gradient

  • From high concentration to low concentration

• Equilibrium

  • When the concentration is the same throughout space

Osmosis

• The diffusion of water across a membrane

• Moves down its concentration gradient

  • Toward higher concentration of particles

• Very important in cellular biology

• Water will move from a hypotonic solution to a hypertonic solution

• Hypotonic solution

  • Contains less solute (more water) than a hypertonic solution

• Hypertonic solution

  • Contains more solute (less water) than a hypotonic solution

• Water will move from a hypotonic solution to a hypertonic solution until :

  • Both solutions have equal concentrations (isotonic)

  • The pressure of the cell wall in plants stops the movement of water

Aquaporins

• Due the polarity of water, it has a difficult time moving directly through the membrane

• Water moves through protein channels called aquaporins

Water Control in Cells Without Cell Walls

• In isotonic environment, cells will stay the same (good)

  • There is no net movement of water

  • Cells become limp or flaccid.

  • Plant will wilt

• In hypertonic environment, cell will loose water and shrivel (crenate)

  • Cells will loose water

  • Plasmolysis may occur

    • When membrane pulls away from cell wall

  • Usually fatal to plant cells.

• In hypotonic environment, cell will gain water and swell

  • Water moves into the cell until the internal pressure of the cell wall equals the osmotic pressure

  • At this point, there is equal movement in and out of the cell.

  • Dynamic equilibrium

  • Ideal for most plants.

  • Turgor pressure builds (cells are turgid).

• If water uptake is excessive, the cell could burst (lyse)

• Organelles such as contractile vacuoles keep freshwater protists from bursting

Facilitated Diffusion

• Some molecules can’t diffuse freely across the membrane because they are too big or too charged

• They need the help of proteins.

• Facilitated diffusion

  • Is the diffusion of solute across a membrane with the help of transport proteins

• Does not require energy.

• Moves with the concentration gradient

Transport Proteins

• Solute specific

• Can be saturated

• Use various mechanisms for transport such as :

  • Conformational change

  • Selective channels

• Gated channels (only open with impulse)

Endocytosis

• Import particles into a cell by the formation of a vesicle

Three types are :

• Phagocytosis

  • “Cell eating”

  • Endocytosis of solid (large) particles

  • This is how amoebas eat

• Pinocytosis

  • “Cell Drinking”

  • Endocytosis of fluid droplets (small particles)

• Receptor mediated endocytosis

  • Happens when a specific molecule (called a ligand) binds to a receptor on the cell membrane

Exocytosis

• Exporting particles out of a cell by fusing a vesicle with the cell membrane