Outline

5.1 Membrane structure and function
5.2 Signal transduction

Learning Objectives

1. Illustrate how the structure of a membrane dictates the functions of a membrane.
2. Diagram how signaling pathways act to regulate biological processes.

Section 5.1 Learning Objective

Illustrate how the structure of a membrane dictates the functions of a membrane.

Lipids Defined

Lipids are hydrophobic molecules.

• Polar lipids are found in cell membranes.
  • Contain water soluble functional groups
• Can be amphipathic
  • Can have both polar and nonpolar components

Plasma Membrane

Contains two leaflets of phospholipids

• Leaflets are diverse and asymmetrical.
  This is a fluid (mosaic) structure.

Figure 5.2 Plasma membrane.

The asymmetry helps to maintain the structural integrity of the cell, and it has a role in cell signaling (maintained by flippases).
Permeable to $O_2$, $C{O}_2$, $N_2$, and ions

Types of Lipids

Glycerophospholipids

• Contain glycerol backbone
  Phospholipids
• Contain phosphate group
  Sphingolipids
• Sphingosine backbone
  Cholesterol
• Steroid containing
  If cholesterol increases, the fluidity (typically) decreases.

Diffusion Defined

Diffusion is the movement of particles down a concentration gradient.

• Depends on several factors:
  • Size
  • Shape
  • Solvent viscosity
  • Temperature
• Illustrates the second law of thermodynamics
  Smaller molecules (lipids) move more quickly than larger ones (proteins), and molecules can move more rapidly through a medium with lower viscosity than one with higher viscosity

Active Site Defined

Active site is the location where the enzyme binds to the substrate and catalysis occurs.

• Located on the surface of the enzyme
• Is often in a cleft, pocket, or trench
• Generally formed by residues on turns or coils

Components of Cell Membranes

Figure 5.3 Membrane proteins and the plasma membrane.

Integral Membrane Proteins

Proteins embedded in the plasma membrane
Can be subclassified as:

• monotopic
• ditopic (transmembrane)

Seven-Transmembrane Receptors

An example of membrane-spanning proteins
Contain seven transmembrane α helices that span the membrane
Examples include:

• integrins
• aquaporins

Peripheral Membrane Proteins

Associated with the surface of the membrane
Can be linked through a hydrophobic anchor
Not embedded in the plasma membrane
Can be dissociated from the membrane by:

• changes in pH
• salt
• urea
• guanidinium hydrochloride
  Gα subunit is associated with the plasma membrane through a hydrophobic anchor

Transporters

Methods for molecules to pass in and out of the cell or organelle
Three systems are involved

• Uniporter
• Symporter
• Antiporter

Example of Transporters

Figure 5.4A Transporters, pumps, and channels.

Active Transport

Energy is required.
Molecules move up a concentration gradient.

• Can be primary or secondary
  • Primary—ATP is used directly to drive ion movement
  • Secondary—gradient pumps a second molecule across membrane

Passive Transport

Molecules move down a concentration gradient.
Can occur through channels

Channels

Can be gated or regulated

Figure 5.4B,C Transporters, pumps, and channels.

Osmosis Defined

The movement of solvent through a semipermeable barrier from a region of low to high solute concentration

Lipid Rafts Defined

Lipid rafts are structures composed of cholesterol and sphingolipids that aggregate in the plasma membrane.
They are involved in cell signaling and tethering cell membranes.

Lipid Rafts

Figure 5.5 Other aspects of membrane biology.

Membrane Fusion and Budding

Figure 5.7 Fusion and budding.

Section 5.2 Learning Objective

Diagram how signaling pathways act to regulate biological processes.

Signal Transduction (Cascade)

Chemical signal binds to a receptor

• Conformational change occurs.
  Second messenger is produced
• Signal is amplified
  Activates or inhibits protein kinases
  Chemical signal can be the hormone/neurotransmitter

Signal Transduction Diagram

Figure 5.8 Signal transduction fundamentals.

Protein Kinase A (PKA)

Contains two catalytic and two regulatory subunits
Regulatory subunits contain two cAMP binding sites
Involved in several cellular pathways including:

• lipolysis
• glycogen metabolism
• neurotransmission

PKA Signaling Pathway

Figure 5.9 PKA signaling pathway.

Cholera Toxin: A G Protein Cascade

Figure 5.10 Cholera toxin.

Insulin

Pancreatic hormone
Plays an integral role in glucose metabolism and diabetes
An example of a receptor tyrosine kinase
Also a growth factor

Insulin Signaling Pathway

Figure 5.11 Insulin signaling pathway.

AMP Kinase

Cytosolic kinase
Activated by binding of AMP or phosphorylation by kinases
Acts as an energy sensor
Regulated by PKA, insulin, and calmodulin kinase kinase
Phosphorylates liver kinase B1
ACIAR is a AMPK agonist

AMP Kinase Signaling Pathway

Figure 5.12 AMP kinase signaling.