Lecture 8: Biological Membranes and Selective Permeability Gatekeepers of the Cell

Biological membranes are “engineered” to:

Let specific molecules through, and keep other molecules out

Water and Polarity of Cell Membranes

• Shapes chemical Interactions

• Hydrophobic, Hydrophilic, and Amphipathic interactions

Proteins and AAs of Cell Memb.

• Participates in transport of molecules

• Enzymes, receptors, transporters, and hormones

Lipids of Cell Memb.

• Compartmentalize and communicate between cells 

• Structural and signaling roles 

Membranes are gatekeepers of the cell

Cell membranes are composed of:

• Protein

• Lipid 

• Carbs 

Lipids of Membrane provide:

• Structure

• Phospholipids, Glycolipids, Sterols, Sphingolipids 

Proteins of Membranes provide:

• Function

• Enzymes, Receptors, Transporters 

Fluid Mosaic Model

• Phospholipid bilayer: fluid

• Proteins floating in bilayer: mosaic 

Integral proteins in membrane

• Membrane spanning proteins 

• Connected to memb by: 

  • Hydrophobic interactons (lipids) 

  • Covalent bonds (adjacent molecules) 

Peripheral proteins

• Membrane associated proteins 

• Connect to memb. by: 

  • Ionic interactions 

  • Dipole interactions 

Integral protein functions

• Transport

• Enzymatic activity

• Signal transduction 

• Cell-cell recognition 

• Intercellular joining 

• Attachment to the cytoskeleton and extracellular matrix 

For integral proteins to function correctly, bio membranes must have:

• Sidedness

  • The space between phospholipid heads are close together, so there is more space on outside of membrane that integral proteins can fit in

Lipids rafts

• Biological membranes form through chemical interactions

• Less fluid

• Enriched with sterols and sphingolipids

• Less phosphatidylcholine (phospholipids)

• Enriched with protein-protein interactions

  • Protein-substrate interactions

  • Specialized compartments/zones

Transporters

• Allow specific molecules in and out of cell (Selective permeability)

Enzymes

• Catalyze biological chemical reactions

Receptors

• Respond to chemical stimuli 

Selective Permeability 

• Property of cellular membranes that only allows certain molecules to enter or exit the cell 

Proteins facilitate two categories of transport:

• Passive transport 

• Active transport 

Passive transport

• Moves across biological membrane:

  • With the concentration gradient 

  • Without energy 

Simple Diffusion

• Type of passive transport 

• Molecules move accross membrane without assistance 

• Generally hydrophobic molecs. 

Facilitated Diffusion

• Type of Passive Transport

• Osmosis (aquaporins) 

• Channel Proteins (maintains shape)

• Carrier Proteins (changes shape)

Active transport 

• Moves molecules across a bio membrane:

  • Against the concentration gradient

  • Uses energy

Primary Active Transport 

• One transporter completes the entire job 

• Ex. Sodium-Potassium ATPase 

Secondary Active Transport 

• First protein establishes the gradient (uses energy, against gradient) 

• Second protein facilitates diffusion (no energy, with gradient) 

• Ex. Sodium or Proton coupled nutrient absorption 

Bulk transport:

• Gets around the need for transporters

Bulk transporters:

• Phagocytosis 

  • Least specific 

• Pinocytosis 

• Receptor-Mediated Endocytosis 

  • Most specific