Vertebrate Physiology - Ch 3

What does cholesterol accomplish in the phospholipid bilayer?

Fills in gaps

Enhances and maintains membrane fluidity

Decreases permeability

Fluid mosaic model of membrane structure

Membrane proteins float freely in sea of lipids

Membrane-skeleton fence model

Membrane protein mobility restricted by cytoskeleton

Some proteins perform specialized functions in specific areas of plasma membrane

Channels

Enable ions to pass

Carriers

Transfer larger substances

Pumps

Transport ions and small molecules

Receptors

Bind to specific molecules

Docking-marking acceptors

Inner surface lock and key with secretory vesicles

Types of passive transport

1) Diffusion down concentration gradient

2) Conduction along electrical gradient

Electrical gradient

Concentration gradient with charged particles

Differences in charges across the membrane drive passive movement of particles

Fick’s Law of Diffusion

Particles tend to travel from high concentration to low concentration

Types of colligative properties

1) Osmotic pressure

2) Elevation of boiling point

3) Depression of freezing point

4) Reduction of vapor pressure

Isotonic solution

Equal

Hypotonic solution

Lower solute concentration in solution than in normal cells

Cell volumes increases → lysis

Hypertonic solution

Higher solute concentration in solution than in normal cells

Cell volume decreases → crenation

What is the phospholipid bilayer impermeable to?

Large molecules; small, poorly lipid-soluble molecules; small, charged molecules

Channel transport

• Transmembrane proteins form selective narrow channels

• Permit passage of ions or water (aquaporins)

• Include gated (open/close) and leak (open) channels

• Faster than carrier-mediated transport

Carrier-mediated transport

Transmembrane proteins actively move small water-soluble substances across the membrane

Includes facilitated diffusion and active transport

Characteristics of carrer-mediated transport systems

1) Specificity: only specific particles it will transport

2) Saturability: limit to number of particles that can be transported at a time

3) Competition: closely related compounds may compete for the same carrier

Facilitated diffusion

Doesn’t require energy (high to low concentration)

1) Molecule attaches to binding site on protein carrier

2) Carrier changes conformation and exposes molecule to other side of membrane

Active transport

Requires energy (moves against concentration gradient)

Primary and secondary active transport

Primary active transport: Na+/K+ pump

Splits ATP to move 2 K+ into cell and 3 Na+ out

Roles:

→ Maintains Na+ and K+ concentration gradients

→ Helps regulate cell volume by controlling solute concentrations inside cell

Secondary active transport

Relies on gradient and energy stored from active transport

Cotransport protein moves substance and ion across membrane at same time

*One thing transported against concentration gradient driven by transport of ion along its concentration gradient

Direct mechanisms of intercellular communication and signal transduction

1) Gap junctions: allow large molecules to pass through

2) Transient direct linkup of surface markers: cell surface proteins communicate

3) Nanotubes: like gap junctions; allows cytoskeletal elements to pass through

Types of intercellular signals

1) Paracrine: local chemical messengers

2) Neurotransmitters: neurons communicate direclty with cells they innervate

3) Hormones: long-range chemical messengers secreted into circulation

4) Neurohormones: hormones released by neurosecretory neurons

5) Pheromones: chemical signals released into environment to reach senses of other organisms

6) Cytokines: regulatory peptides

Signal transduction

Process by which incoming signals conveyed to targer cell’s interior for execution

Lipophilic extracellular messengers

Pass through target cell membrane and bind to intracellular receptors

Produce second messenger

Alter gene transcription

Lipophobic extracellular messengers

Bind to surface membrane receptors as first messenger

→ Some receptors channels

→ Some enzymes phosphorylate target proteins

→ Some transfer signal to intracellular second messenger

Phosphorylating enzyme

Kinase: adds phosphate to target cell protein

→ Changes shape and function of protein

→ Lead to cellular response

*Tyrosine kinase can undergo autophosphorylation

G-Protein-Coupled Membrane Receptors

• Cell surface transmembrane receptor + G protein

• No ligand → inactive

  • Alpha subunit bound to GDP

• Ligand → receptor changes shape → activates G protein

  • Alpha subunit exchanges GDP for GTP

  • G protein breaks into two pieces:

    • Alpha subunit with GTP

    • Beta and gamma subunits

• Subunits interact with other proteins → signaling pathway → response

• Ligand released → G protein reforms and reattaches to receptor

Cyclic AMP second-messenger GCPR pathway

• Hormone attaches to receptor → G protein activates

• Alpha subunit links with adenylyl cyclase in membrane

• Activated adenylyl cyclase converts intracellular ATP to cyclic AMP

• Cyclic AMP activates protein kinase A

• Kinase phosphorylates intracellular proteins → response

Diacylglycerol-inositol triphosphate-Ca2+ second-messenger pathway

• Hormone binds to receptor → activated G protein

• Alpha subunit activates phospholipase C on inner surface of membrane

• Activated phospholipase C coverts PIP2 to DAG and IP3

Second messenger systems

Multiple steps lead to amplification of initial signal

→ Low concentration of chemical messengers can trigger large responses

Receptor regulation

Downregulation or upregulation of receptor number

Antagonists: block step in pathway

Agonists: activate step in pathway

All living cells have membrane potential with excess of _____ charges on inside

Negative

Does a membrane have more K+ or Na+ leak channels?

More K+ leak channels

Are cell membranes more permeable to K+ or Na+?

K+