2.B.1 Cell Membranes Cell membranes are selectively permeable due to their structure.

Cellular membranes are fluid mosaics of lipids and proteins that are continuously moving in a fluid lipid bilayer.

Cell membranes separate the internal environment of the cell from the external environment.

Phospholipids are a major component of the cell membrane. Amphipathic: have hydrophobic and hydrophillic regions. The hydrophilic phosphate portions of the phospholipids are oriented toward the aqueous external or internal environments. The hydrophobic fatty acid portions face each other within the interior of the membrane itself. Amphipathic: have hydrophobic and hydrophillic regions. The hydrophilic phosphate portions of the phospholipids are oriented toward the aqueous external or internal environments. The hydrophobic fatty acid portions face each other within the interior of the membrane itself.

Phospholipid bilayer

Phospholipids can move laterally and occasionally flip-flop to the other side of the membrane. (a) Movement of phospholipids Lateral movement (  10 7 times per second) Flip-flop (  once per month) Temperature increases phospholipid movement.

Integral membrane proteins are integrated into the phospholipid bilayer.

Transmembrane proteins extend from one side of a membrane to the other. Many function as gateways for the transport of specific substances across the membrane.

Membrane proteins have hydrophilic and hydrophobic regions. Many can be induced to change their conformation, resulting in an activation of their biological activity.

Peripheral proteins are loosely bound to the membrane surface.

Attached to membrane and membrane-bound proteins. Membrane Carbohydrates

Membrane carbohydrates are important in cell-cell recognition. Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane. Glycolipids: important in cell signaling. Glycoproteins: more common, vary among species, individuals, and even cell types in an individual.

Fibers of extracellular matrix (ECM) Glyco- protein Microfilaments of cytoskeleton Cholesterol Peripheral proteins Integral protein CYTOPLASMIC SIDE OF MEMBRANE Glycolipid EXTRACELLULAR SIDE OF MEMBRANE Carbohydrate

Membrane Protein Functions: Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM) Transport Enzymatic activity Signal transduction Cell-cell recognition Intercellular joining Attachment to the cytoskeleton and extracellular matrix (ECM)

Transmembrane glycoproteins Secretory protein Glycolipid 2 Secreted protein Transmembrane glycoprotein Membrane glycolipid Membranes have distinct inside and outside faces.

Membrane structure results in selective permeability. Small molecules, or hydrophobic (nonpolar) molecules, can dissolve in the lipid bilayer and pass through the membrane rapidly. Polar molecules, or large molecules, do not cross the membrane easily. Small molecules, or hydrophobic (nonpolar) molecules, can dissolve in the lipid bilayer and pass through the membrane rapidly. Polar molecules, or large molecules, do not cross the membrane easily.

Transport Proteins Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel Transport proteins allow passage of hydrophilic substances across the membrane Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel

Aquaporins are channel proteins that allow water to diffuse through the membrane.

Cholesterol molecules add firmness and integrity to the plasma membrane and prevent it from becoming overly fluid or overly firm.

Cell walls exist in plants, fungi, and bacteria. They provide a structural boundary as well as a permeability barrier.

Plant cell walls are made of cellulose and are external to the cell membrane.

Prokaryotic cell walls are made of peptidoglycan.

Fungal cell walls are made of chitin, the same substance in arthropod exoskeletons.

Learning Objectives LO 2.10 The student is able to use representations and models to pose scientific questions about the properties of cell membranes and selective permeability based on molecular structure. [See SP 1.4, 3.1] LO 2.11 The student is able to construct models that connect the movement of molecules across membranes with membrane structure and function. [See SP 1.1, 7.1, 7.2]