Membrane Structure and Function

1. Membrane Structure and Function • Cells must contain a cell membrane, cytoplasm and genetic material. • The cell membrane is the EDGE, “boundary of life”, while the cytoplasm is the site of all the reactions of life and the genetic material is the information required for life. • SELECTIVELY PERMEABLE means that the CM allows some substances across more easily than others… some it helps and some it inhibits or rejects all together.

2. Membrane Structure • Development of models as a scientific process (linked to technology) • Models include • Overton – proposed lipid base • Langmuir (phospholipids) – made membrane • Gorter and Grendel (two layers) • Davson and Danielli (sandwich image, protein outside) • Singer and Nicolson = Fluid Mosaic Model • Required electron microscopy • 1972

4. Membrane Structure • Phospholipids – phosphate and fatty acid tails • Hydrophilic phosphates • Hydrophobic fatty acid tails • Two layers • With tails together • Protein • Embedded throughout - integral • Some just surface – peripheral • Motility

6. Idea of Fluidity • Cholesterol molecules prevent membrane from becoming ‘crispy’ • Proteins ‘bob’ along surface • Proteins are anchored by fibers in the cytoskeleton • Phospholipids can even flip flop • Merging of membranes ( pg. 128)

8. Proteins - many purposes within CM • Transport – substance across membrane, specificity to substance, active pumps • Enzymatic activity – sequence reactions • Transmit signals – conformational change when with substrate = message. • Junctions – glue cells into tissues • Recognition – glycoproteins act as targets or ID • Attach to cytoskeleton – change shape of cell

10. Other organelles • The cell membrane is continuous with other membranes in the cell • Endoplasmic reticulum starts/ends at CM • ER makes CM and it is turned inside out as the new CM flows out of the ER….

12. Traffic Across the Membrane • Small particles and ions move in both directions; sugars, amino acids, CO2, NH4 and O2. Also Na+, K+, Ca+2 and Cl- • Hydrophobic molecules ( like CO2) can dissolve in lipid layer • Hydrophilic core of CM impedes movement of ions and polar molecules that are hydophilic. • Proteins built into membrane facilitate the process

13. Transport Proteins • Span the CM • Function as a channel • Some also physically move passengers • Specificity of fit ( form and function ) allows for membrane selectivity. (emergent property and homeostasis )

14. Passive Transport • Molecules have intrinsic kinetic energy • They are in constant motion • Cytoplasm, extracellular fluid and CM vary in concentrations and pressure gradients. • Concentration refers to the overall POPULATION of molecules as well as the RATIO in that location compared to another. • Net Movement • DYNAMIC EQUILIBRIUM

15. Passive transport includes • Diffusion • Tendency of molecules of any substance to spread out into the available space. Substances will diffuse DOWN their concentration gradient. (Entropy) • Osmosis • The diffusion of water molecules across a selectively permeable membrane. • Hypertonic = solution with higher [conc.] of solutes • Hypotonic = solution with lower [conc.] of solutes • Isotonic = solutions are equal in solute concentration • Facilitative Diffusion • Transport proteins are helping molecules to cross membrane, but still diffusion (lowering overall free energy) thus doesn’t require energy from cell.

18. Active Transport • Movement of solutes, UP their concentration gradient, with the help of transport proteins in a CM. • Requires cell to expend energy / do work. • Solute becomes more concentrated or “stock- piled” in a particular area. Example is Sodium-Potassium Pump. (Na-K)

20. Electrogenic Pumps • In addition to moving ions, some active transport pump also generate electric current and/ ATP. • An enzyme called ATP synthase is built into the membrane and ADP is phosphorylated as a result of ions (often H+) passing through the membrane. • Examples are found in the Electron Transport Chains of respiration (inner mitochondrial membrane) and photosynthesis (chloroplasts thylakoid membrane)

21. THE “Answers” for AP Biology  If you don’t know an answer choose either • Hydrogen bond (water, organic compounds, etc) OR • Hydrogen ion (membrane pumps, nerve impulses, etc)