Membrane Function and Signaling

Membrane Transport and Osmosis

Membrane Function and Signaling

This lecture covers the transport of molecules across cell membranes, local and long-distance signaling, and signal transduction.

Movement of Water Through Membranes - Osmosis

Cell membranes control the movement of substances into and out of cells. This movement can be passive (without energy input) via diffusion, where molecules move from high to low concentration until equilibrium is reached.

Osmosis

Occurs when ions, which cannot pass through membranes, are in solution at different concentrations on either side of a membrane, leading to the movement of water molecules to equalize concentrations.

  • Water moves from an area of low solute concentration to an area of high solute concentration.

Membrane Permeability

  • High permeability: O2, CO2, N2, H2O, glycerol
  • Low permeability: Ions, large uncharged polar molecules

The permeability scale illustrates that small, nonpolar molecules pass through the membrane more easily than ions or large polar molecules.

Movement of Water Alters Cell Structure

The movement of water into and out of cells affects cell structure. The terms hypertonic, isotonic, and hypotonic describe the concentration of the outside solution relative to the inside of the cell.

  • Hypertonic: Net flow of water out of the cell, causing it to shrink.
  • Isotonic: No net change in water flow; cell volume remains constant.
  • Hypotonic: Net flow of water into the cell, potentially causing it to swell or burst.

Water molecules leave the cell to equalize solute concentrations.

Spontaneous Movement of Water

Water moves spontaneously from a region of lower solute concentration to a region of higher solute concentration. For example, net movement of water from 0.3 molar solution of sugar to pure water.

Amphipathic Proteins in Membranes

Proteins can be amphipathic, meaning they have both hydrophilic (polar and charged amino acids) and hydrophobic (nonpolar amino acids) regions. Amphipathic proteins can integrate into lipid bilayers.

Proteins Facilitate and Speed Up Molecular Transport

The transport of most molecules is facilitated or sped up by proteins present in cell membranes.

  • Peripheral membrane proteins: Associated with the membrane surface.
  • Integral membrane proteins (transmembrane proteins): Embedded within the lipid bilayer.

Proteins with the right combination of polar and nonpolar residues over their surface can integrate into the cell membrane. Many integral proteins contain channels or pores that allow the transfer of specific compounds, which is known as facilitated diffusion.

Channels and Pores

The size and shape of a channel is specific for a particular molecule, allowing different proteins to allow different molecules to pass through.

Facilitated Diffusion via Carrier Proteins

Carrier proteins change shape during the transfer of a molecule, aiding in transport and ensuring specificity. Transfer through channels and by carriers is passive, driven by the concentration gradient, not energy input.

  • Example: Facilitated diffusion of glucose through the GLUT-1 carrier protein involves several steps:
    1. Unbound protein.
    2. Glucose binding.
    3. Conformational change.
    4. Release.

Channels vs. Carriers

All of the following are true except: carrier proteins require energy to transport solutes across the membrane; channel proteins do not.

Channel Proteins

Allow many solute molecules to cross the membrane at once.

Carrier Proteins

Allow only a few molecules to cross.

Both channels and carriers facilitate diffusion and change their conformation to allow solutes to cross the membrane.

Regulation of Integral Membrane Protein Channels

Most protein channels are gated; their conformation can change in response to environmental signals, opening or closing the channel.

Active Transport Pumps

Active transport pumps allow molecules to be transported against concentration gradients. For example, Na+ concentration is higher outside the cell, so it wants to diffuse in; K+ concentration is higher inside the cell, so it wants to diffuse out.

Sodium-Potassium Pump (Na+/K+-ATPase)

The Sodium-Potassium pump is a well-understood example of active transport involving the following steps:

  1. Three binding sites within the protein have a high affinity for sodium ions.
  2. Three sodium ions from the inside of the cell bind to these sites.
  3. A phosphate group from ATP binds to the protein. In response, the protein changes shape.
  4. The sodium ions leave the protein and diffuse to the exterior of the cell.
  5. In this conformation, the protein has binding sites with a high affinity for potassium ions.
  6. Two potassium ions bind to the pump.
  7. The phosphate group drops off the protein. In response, the protein changes back to its original shape.
  8. The potassium ions leave the protein and diffuse to the interior of the cell.
    These steps repeat.

Diffusion allows molecules to move only from high to low concentrations, whereas pumps can move molecules from low to high concentrations. This imbalance can drive other cell processes (e.g., ATP generation).

Identifying Membrane Proteins

Yeast cells require a protein to transport glucose from the environment into the cell. You would expect that protein to be an integral membrane protein.

Review: Mechanisms of Molecule Transfer Across a Cell Membrane

  • Diffusion: Passive movement of small, uncharged molecules along an electrochemical gradient.
  • Facilitated Diffusion: Passive movement involving proteins.
  • Active Transport: Active movement involving proteins and energy (ATP).

Cell Communication

Cells need to communicate using chemical messengers (signal molecules such as adrenaline and growth factors) and membrane proteins. Cell signaling helps cells respond to their environment.

Local Signaling

Local signaling involves communication by direct contact between cells:

  • Gap junctions between animal cells
  • Cell junctions
  • Plasmodesmata between plant cells
  • Cell-cell recognition

Tight Junctions

Animal cells lack cell walls, so proteins hold them together. Tight junctions form watertight, dynamic seals between cells. These are important in organism barriers like epithelial cells in intestines and skin. Other protein systems allow specific binding between cells of the same type (cell-cell adhesion).

Cell-Cell Junctions

Cell-cell junctions allow rapid signal transmission between cells.

  • Animal cells communicate directly through gap junctions, enabling shared cytoplasm.
  • Plant cells communicate directly through plasmodesmata, enabling a continuous endomembrane system and shared cytoplasm across cell walls.

Local Signaling — Paracrine and Synaptic Signaling

Local signaling involves signal molecules released to interact only with cells in the local vicinity.

  • Paracrine Signaling: Signaling molecules travel short distances (para=beside).
  • Synaptic Signaling: Occurs between nerve cells where an electrical signal triggers the release of neurotransmitters that diffuse across the synapse.

Long-Distance Signaling - Endocrine Signaling

Specialized cells release hormones that travel through the bloodstream to reach target cells. Some hormones pass into cells; others bind to the surface and their effects are transduced into the cell.

Signal Transduction Across Cell Membranes

For signals that can be transferred across cell membranes (usually signaling molecules are small and/or hydrophobic):

  1. Arrival of signal.
  2. Signal reception.
  3. Direct signal response.

Receptor activated by hormone binding moves to the nucleus and activates genes. Steroid hormones are used as examples of this.

Lipid Insoluble Signals and Transduction

Lipid-insoluble signals require transduction:

  1. Reception: Signaling molecule binds to a receptor.
  2. Transduction: Relay molecules transmit the signal.
  3. Response: Activation of a cellular response.

Signal Transduction Pathway Example

Binding of a signaling molecule to a receptor triggers enzyme activity, such as kinase enzymes adding phosphate groups to other proteins. This initiates a phosphorylation cascade, allowing signal amplification and fine control.

Phosphorylation Cascade

A cascade allows signal amplification and fine control (and requires energy):

  • Signaling molecule binds to a receptor.
  • Activated relay molecule activates protein kinase 1.
  • Active protein kinase 1 activates protein kinase 2.
  • Active protein kinase 2 activates protein kinase 3.
  • Active protein kinase 3 leads to a cellular response.

Cytoplasmic Response to a Signal: Glycogen Breakdown by Epinephrine

The cytoplasmic response to a signal involves the stimulation of glycogen breakdown by epinephrine (adrenaline).

  1. Binding of epinephrine to G protein-coupled receptor (1 molecule).
  2. Inactive G protein becomes active (10^2 molecules).
  3. Inactive adenylyl cyclase becomes active (10^2 molecules).
  4. ATP is converted to Cyclic AMP (10^4).
  5. Inactive protein kinase A becomes active (10^4).
  6. Inactive phosphorylase kinase becomes active (10^5).
  7. Inactive glycogen phosphorylase becomes active (10^6).
  8. Glycogen is broken down into Glucose 1-phosphate (10^8 molecules).

Gene Activation Response to a Signal

The cell’s response to many signals (e.g., growth factor) is to switch on specific genes:

  1. Growth factor binds to a receptor.
  2. Phosphorylation cascade activates a transcription factor.
  3. Transcription factor binds to DNA in the nucleus.
  4. mRNA is produced.

Plasma Membrane-Spanning Receptor Protein Function

Binding of the signal molecule alters the receptor's other binding sites and activities.

Exam 1 Breakdown

  • Average Score: 81%
  • High Score: 94%
  • Low Score: 52%
  • Standard Deviation: 3.61
  • Average Time: 01:54:82

Specific Question Breakdowns:

Some questions with low percentage correct:

  • Template strand used by RNA polymerase (9% correct).
  • Primer binding area(s) with the most hydrogen bonds (12% correct).
  • Amino acids sequences encoded by mRNA sequence (10% correct).
  • Environment in which the operon be expressed at a maximum level (8% correct).

Protein vs. Polypeptide

While studying for the first Bio200 exam, your friend claims that the terms 'protein' and 'polypeptide' have the same meaning. Is your friend correct? Explain. (70% full marks – a lot of people thought the difference was that proteins consisted of multiple polypeptides)