Phospholipids Recall that phospholipids are amphipathic (both hydrophilic and hydrophobic). Artificial membranes showed phospholipids will form a layer in water:

Fluid Mosaic Model Proposed by Singer and Nicolson at UCSD Hydrophobic parts of proteins are embedded within the membrane. Thickness between different membranes is a function of the proteins

Fluid Mosaic Model In what way does this model solve the problems? Hydrophobic parts of proteins are embedded within the membrane. Freeze-fracture technique with electron microscope

How is this fluidity maintained? Kinks in unsaturated fatty acid tails of phospholipids. Cholesterol

How are proteins arranged to contribute to membrane function? Membrane proteins contribute to the mosaic quality of the structure. Different proteins convey different properties to each membrane.

Integral proteins are inserted within the membrane. Peripheral proteins are attached to membrane surface Proteins attach to cytoskeleton or to extracellular fibers to help give animal cells a stronger framework

Membrane Carbohydrates Found only on the outside of the membrane. What is their function? Cell to cell recognition. Sorting cells into tissues. Immune defense.

Usually oligosaccharides (15 or less sugar units) glycolipids or glycoproteins

How do ions and other polar molecules pass into and out of cells? Transport proteins: Provide hydrophilic tunnel for ions. They are specific for the substances they transport.

What is Diffusion?

The movement of molecules from greater concentration to lesser concentration

What causes diffusion? Why is it spontaneous? A concentration gradient Since the direction of movement decreases the free energy of the system it is spontaneous.

Does the diffusion of more than one kind of particle work together or separately? Do the particles stop moving once equilibrium is reached?

If a molecule can move freely through the phospholipid bilayer what always controls the direction of its movement? Concentration gradient. Remember that the concentration gradient represents potential energy.

Osmosis What is osmosis?

The movement of water across a semipermeable membrane from greater concentration to lesser concentration

Since water passes freely across the membrane, how can the cell control the direction of osmosis?

The cell can concentrate solutes that are not permeable to the phospholipid bilayer on one side of the membrane. Which way will water move? Water will follow the solutes! What do the terms hypotonic, hypertonic and isotonic mean?

Water Balance in Cells

Hypertonic = greater solute concentration Hypotonic = lower solute concentration Isotonic = equal solute concentration

Elodea in Salt Water

Elodea in Distilled Water

Which way will the water move?

WHY?

Do Water Molecules Stop Moving in Isotonic Conditions? No. They continue to diffuse, however there is no net movement! In general, which way does water move? From hypotonic to hypertonic!

Figure 8.13 The contractile vacuole of Paramecium: an evolutionary adaptation for osmoregulation

Facilitated Diffusion Diffusion of solutes with the help of transport proteins. Is this a passive or an active process? Passive. Why do these solutes need a protein to facilitate their diffusion?

Because they are too polar or too large to pass through the lipid bilayer. That’s right!

Figure 8.14 Two models for facilitated diffusion Channel Protein Carrier Protein

Active Transport Substances are moved from a lower concentration to a higher concentration Requires ATP Energy!

Figure 8.16 Review: passive and active transport compared

A transport protein that generates voltage across a membrane is called an electrogenic pump. One example is the sodium potassium pump

Figure 8.15 The sodium-potassium pump: a specific case of active transport

What two forces drives the diffusion of ions? Concentration gradient of the ion Effect of membrane potential (charge) on the ion This is called the electrochemical gradient. Ions diffuse down their electrochemical gradient!

Proton pumps are the main electrogenic pumps of bacteria, fungi and plants.

Figure 8.18 Cotransport

Transport of LargeMolecules : Exocytosis – “out of the cell” Endocytosis – “into the cell” –Phagocytosis – endocytosis of large substances –Pinocytosis – endocytosis of fluid and dissolved solutes

Figure 8.19 The three types of endocytosis in animal cells