Chapter 7  Membranes

 

Outline

•     Composition of cell membranes

–   The roles of lipids, proteins and carbohydrates.

–   The fluid mosaic model

–   Synthesis and sidedness of membranes.

 

•     Traffic across cell membranes.

–   Selective permeability

–   Diffusion

–   Osmosis

–   Active Transport

–   Endocytosis and Exocytosis

 

 

Fig. 7.7 The detailed structure of an animal cell’s plasma membrane, in cross section

 

 

Composition of Cell Membranes

•      Phospholipid bilayer

–   Basic structure of membrane

•      Transmembrane proteins

–   transport, enzyme activity, signal transduction, recognition, joining, attachment

•      Exterior proteins, glycoprotein and glycolipids.

–   Identity and communication

 

 

The hydrophobic interactions of the

phospholipid bilayer provide the underlying structure of cell membranes. Fig 7.2

 

 

 Proteins  Fig 7.7

•     Integral proteins traverse the hydrophobic core of the phospholipid bilayer.

•     Peripheral proteins are loosely bound to the surface of the membrane.

 

 

Transmembrane Protein
(Integral membrane protein)  Fig. 7.8

 

 

 Membrane Protein Functions

•      Transport

•      Enzymatic activity

•      Signal Transduction

•      Intercellular joining

•      Cell-Cell recognition

•      Attachments to the cytoskeleton

 

 

Membrane carbohydrates are important for cell-cell recognition.

•     Membrane carbohydrates are usually branched oligosaccharides.

•     The types of sugars used vary between cell type, species, etc., and provide information.

•     Attached to membrane proteins or lipids (glycoproteins or glycolipids).

•     Found on the external side of the membrane.

 

 

Exterior proteins, glycoprotein and glycolipids  Fig. 7.7

•      Identity and communication

 

 

 The Fluid Mosaic Model

•     A mosaic of proteins float in the fluid phospholipid bilayer.

Membrane Fluidity  (Fig. 7.3)

 

Fig. 7.5  The Fluid Mosaic Model, con’t

•     Rapid lateral movement, about 2 um/sec.

•     Flip-flopping of phospholipids is rare, but can occur.

Membrane Fluidity

•     Unsaturated hydrocarbons increase fluidity.

Membrane Fluidity

•     Cholesterol reduces membrane fluidity.

 

 

Membrane Fluidity  Fig. 7.6

•     Membrane proteins drift and intermingle.

 

 

 

How Cell Membranes are Studied

•     Transmission Electron Microscopy (TEM)

•     Scanning Electron Microscopy (SEM)

•     Freeze-fracturing a membrane

 

 

Fig 7.4 Research Method:  Freeze Fracture

 

 

Synthesis and Sidedness of the Plasma Membrane

•     Membranes have distinct cytoplasmic and extracellular faces.  Fig. 7.7

 

 

Synthesis and Sidedness of the Plasma Membrane  Fig. 7.10

•     Cell membranes have their start in the ER, and have further modifications in the Golgi apparatus.

•     Vesicle bud off of the Golgi and fuse with the plasma membrane.

•     The molecules that start out on the inside of the ER and Golgi, will be on the exterior of the cell.

 

Traffic across cell membranes.

 

Selective Permeability

•     The structure of the plasma membrane results in selective permeability.

•     The phospholipid bilayer is permeable to hydrophobic molecules.

•     Special transport and channel proteins aid in moving hydrophilic molecules across, as they are needed.

 

 

How Materials Enter and Exit Cells

•      Passive Transport

–   Passive Diffusion

–   Facilitated Diffusion

–   Osmosis

•      Active Transport

–   Pumps

–   Bulk Transport

•    Endocytosis and exocytosis

 

 

 Diffusion

•      Passive transport across membranes moves down the concentration gradient.

•      Simple diffusion

–   Small non polar molecules, hydrophobic molecules

 

 

Fig 7.11a Diffusion of one solute across a membrane

•     Random movement causes dye molecules to pass through the pores, down a concentration gradient, until a dynamic equilibrium is reached.

 

What happens if there are two solutes?

 

Fig. 7.11 Diffusion of two solutes across a membrane

Due to random movement, each dye diffuses down its own concentration gradient, until a dynamic equilibrium is reached.

 

Facilitated diffusion
- Uses channel or carrier proteins.

- Specific, passive, and can become saturated.


Osmosis

•     The diffusion of water, but not solutes, across a membrane.

•     Cell membranes are selectively permeable

–   water can flow through aquaporins in the membrane, but solutes cannot

–   the resulting concentration gradient causes water to flow across the membrane toward the side that has the higher concentration of solutes, and the lower relative concentration of water.

 

 

Figure 7.12 Osmosis

-          Two sugar solutions of different concentration are separated by a selectively permeable membrane.

-          Water molecules move randomly and may cross through the pores in either direction.

-          What will happen in this system over time?

 

 

 

 

Figure 7.12 Osmosis

-          Water will move from the solution that is less concentrated to the solution that is more concentrated.

-          Water is moving “down a concentration gradient.

 

 

 

 

Osmosis in cells without walls

•     Tonicity

–  Is the ability of a solution to cause a cell to gain or lose water

–  Has a great impact on cells without walls

 

•     If a solution is isotonic

–  The concentration of solutes is the same as it is inside the cell

–  There will be no net movement of water

 

•     If a solution is hypertonic

–  The concentration of solutes is greater than it is inside the cell

–  The cell will lose water

 

•     If a solution is hypotonic

–  The concentration of solutes is less than it is inside the cell

–  The cell will gain water

 

 

Water balance in cells without walls

•     Animals and other organisms without rigid cell walls living in hypertonic or hypotonic environments

–  Must have special adaptations for osmoregulation

–  An animal cell fares best in an isotonic environment unless it has special adaptations to offset the osmotic uptake or loss of water.

 

 

Water Balance of Cells with Walls

•     Cell walls

–  Help maintain water balance

 

•     If a plant cell is turgid

–  It is in a hypotonic environment

–  It is very firm, a healthy state in most plants

 

•     If a plant cell is flaccid

–  It is in an isotonic or hypertonic environment

 

 

Water balance in cells with walls

 

 

 Maintaining Osmotic Balance

•     Extrusion

–   Excess water collected in contractile vacuole and pumped out. Seen in some protists

•     Isotonic solutions

–   Circulatory system bathes cells in an isotonic solution.

•     Turgor

–   Turgor pressure presses the plasma membrane against the cell wall, making the cell rigid. 

 

 

 Active Transport

•      Allows cells to move substances against a concentration gradient.

•      Uses highly selective protein carriers within the membrane.

•      Requires energy – ATP

 

 

Examples of Active Transport

•     The Sodium Potassium Pump

•     Maintaining Membrane Potential (Electrochemical Gradient)

•     Cotransport

 

 

 The Sodium-Potassium Pump

•      > 1/3 of a cells energy is used in active transport of Na+ and K+ .

•      Maintain low internal [Na+] and high internal [K+].

•      Actively pump Na+ out, and K+ into the cell.

–   Requires ATP

 

 

 

The sodium-potassium pump      Fig 7.16

  1. Cytoplasmic Na+ binds to specific sites in the pump protein.
  2. Na+ binding stimulates the breakdown of an ATP, and the phosphate is added to the pump protein (phosphorylation).
  3. Cytoplasmic Na+ binds to specific sites in the pump protein.
  4. Na+ binding stimulates the breakdown of an ATP, and the phosphate is added to the pump protein (phosphorylation).
  5. Loss of the phosphate restores the protein to its original shape.
  6. K+ is released, and the Na+ sites are now available for Na+ to bind, and the cycle repeats.

 

 

 

Maintenance of Membrane Potential by Ion Pumps

•      Membrane potential is the voltage difference across a membrane – an electrochemical gradient.

•      An electrochemical gradient is caused by the concentration of ions across a membrane.

 

 

An electrogenic pump  (ion pump)

–        Is a transport protein that generates the voltage across a membrane (causes an electrochemical gradient to form).  Fig. 7.18

 

 

Cotransport: Coupled Transport by a Membrane Protein

•      Cotransport

–        Occurs when active transport of a specific solute indirectly drives the active transport of another solute

 

 

Cotransport: active transport driven by a concentration gradient. Figure 7.19

 

Passive and active transport compared, Fig 7.17

 

Passive transport. Substances diffuse spontaneously down their concentration gradients, crossing a membrane with no expenditure of energy by the cell. Includes simple and facilitated diffusion.

 

Active transport. Some transport proteins act as pumps, moving substances across a membrane against their concentration gradients. Energy for this work is usually supplied by ATP.


How do big things cross cell membranes?  

•      Endocytosis

–   The plasma membrane extends outward and envelopes materials.

•    Phagocytosis – takes in particles

•    Pinocytosis – takes in liquid

•    Receptor mediated endocytosis

•      Exocytosis

–   The reverse of endocytosis

 

Both Endocytosis and Exocytosis require energy input by the cell (ATP)

 

 

 

Phagocytosis and Pinocytosis (Fig 7.20)

 

 

Receptor mediated endocytosis  (Fig 7.20) 

•     Used for import of specific molecules

•     Molecules bind to specific receptors

–   Ex. LDL and LDL receptor

 

 

 The End.