AP Biology, Chapter 8
Membrane Structure and Function

Summary
Introduction

MEMBRANE STRUCTURE
	Intro
	Membrane models have evolved to fit new data: science as a process
			a. Overton, 1895; lipid soluble substances pass  membranes are lipid
			b. 1915; isolated RBC membranes analyzed
		1. Describe the properties of phospholipids and their arrangement in cellular membranes.
			a. Phospholipids are amphipathic (polar+non-polar, hydrophobic +-philic)
			b. Langmuir, 1917; dried phospholipid in benzene down to a monolayer
			c. Gorter and Grendel, 1925; RBCs have enough phospholipid for a bilayer
			d. Davson and Danielli, 1935; phospholipid coated with protein
				i. Problem: not all membranes look like that in TEM
				ii. Problem: some membrane have much more protein
				iii. Problem: membrane proteins are also amphipathic
			e. Singer and Nicholson, 1972; fluid mosaic model: proteins floating amongst 
			lipids	
	A membrane is a fluid mosaic of lipids, proteins, and carbohydrates
		2. Explain what freeze-fracture techniques reveal about the involvement of proteins in 
		membranes.
			a. Freezing and breaking the membrane allows EM of the inner and outer layers
			b. Some proteins span, some float outward, some inward
		3. Describe the fluid properties of the cell membrane and explain how membrane fluidity 
		is influenced by membrane composition.
			a. Lipids move rapidly and randomly
			b. Some proteins move randomly, others non-randomly, others are anchored
				i. Differently, flourescently labeled mouse and human cells can be fused
				ii. The two colors become randomly distributed after 1 hour
			c. Fluidity varies with temperature and lipid composition
				i. cis-unsaturated fats are fluid at lower temperatures
				ii. Cholesterol solidifies membranes at higher temperatures
		4. Describe how proteins and carbohydrates are spatially arranged in cell membranes 
		and how they contribute to membrane function.
			a. Proteins form patches both as integral and peripheral components
			b. Functions
				i. Transport
				ii. Enzymatic activity
				iii. Signal transduction
				iv. Intercellular joining
				v. Cell-cell recognition of oligosaccharides
				vi. Attachment of cytoskeleton and ECM

TRAFFIC ACROSS MEMBRANES
	Intro
	A membrane's molecular organization results in selective permeability
		5. Describe factors that affect the selective permeability of membranes.
			a. Hydrophobic molecules pass freely
			b. Small uncharged molecules pass easily
			c. Transport proteins are required for all others
		6. Explain the locations and functions of transport proteins.
			a. Transport proteins span the membrane
			b. Allow large and charged molecules to cross
			c. May open a channel or bind and deliver solutes
	Passive transport is diffusion across a membrane
		7. Define diffusion. Explain what causes diffusion and why it is a spontaneous process.
			a. Matter scatters by thermal motion
			b. Substances show a net movement from high to low concentration
			c. More concentrated solutions have less entropy, more free energy
			d. For diffusion; ΔG = ΔH - TΔS
				i. H stays the same, there's no chemical change
				ii. T stays the same, usually
				iii. S, entropy, increases and makes ΔG negative 
		8. Explain what regulates the rate of passive transport.
			a. The greater the concentration gradient, the faster the diffusion
			b. Also depends on permeability and transport proteins
		9. Explain why a concentration gradient across a membrane represents potential energy.
			a. ΔG = ΔH - TΔS
			b. H stays the same, there's no chemical change
			c. T stays the same, usually
			d. For diffusion, S, entropy, increases and makes ΔG negative 
	Osmosis is the passive transport of water
		10. Distinguish hypertonic, hypotonic, and isotonic solutions.
			a. Always used to compare two solutions
				i. Hypertonic has more solutes, less water
				ii. Hypotonic has less solutes, more water
				iii. Iso- has the same
			b. Be careful to specify the comparison
		11. Define osmosis and predict the direction of water movement based on differences in 
		solute concentrations.
			a. Osmosis is the diffusion of water through a selectively permeable membrane
			b. Water moves fromgreater to lesser concentration
	Cell survival depends on balancing water uptake and loss
		12. Describe how living cells with and without walls regulate the balance of water 
		content.
			a. Without walls they must isotone, osmoregulate, or die
				i. Some organisms maintain isotonicity with their surroundings
				ii. Otherwise they have methods for excreting excess water, solutes or 
				both depending on their environment
			b. With cell walls:
				i. In hypotonic solution they swell until pressure potential balances water 
				potential = turgid
				ii. Otherwise, flaccid unto plasmolysis
	Specific proteins facilitate the passive transport of selected solutes
		13. Explain how transport proteins are similar to enzymes.
			a. Do not provide energy
			b. Specific and has a binding site like an active site
			c. Can be saturated
			d. Can suffer competitive inhibition
		14. Explain how transport proteins facilitate diffusion.
			a. Protein changes shape as it binds, just like induced fit
			b. Some provide selective corridors with or without a gate
	Active transport is the pumping of solutes against their gradients
		15. Explain how active transport differs from diffusion.
			a. Going from lower to higher decreases entropy (gathers matter)
			b. ΔS is negative, ΔG is positive, not spontaneous
			c. Cells have to provide energy to work against the concentration gradient
	Some ion pumps generate voltage across membranes
		16. Explain what mechanism can generate a membrane potential or electrochemical 
		gradient. 
			a. Animals cells
				i. Sodium-potassium pump moves 3 Na+ out for every 2 K+ in
				ii. There are chemical concentration gradients for both
				iii. And the inside is negative with a voltage of -0.1 volt
				iv. The Na+ ions are attracted by this electrical gradient
			b. Plant, bacterial, and fungal cells protons are pumped out
	In co-transport, membrane protein couples the transport of one solute to another
		17. Describe the process of co-transport.
			a. Concentration gradient for one chemical can be used to actively transport 
			another
			b. Example: a proton gradient is used to transport sucrose into phloem
	Exocytosis and endocytosis transport large molecules
		18. Explain how large molecules are transported across the cell membrane.
			a. Large molecules can't be passed even by transport proteins
			b. Must be put into or spit out of a vesicle that fuses with the plasma membrane
			c. Examples
				i. Secretion of digestive enzymes
				ii. Phagocytosis 
		19. Compare pinocytosis and receptor-mediated endocytosis.
			a. Pinocytosis is non-specific engulfment of liquid
			b. Receptor-mediated endocytosis is triggered by binding of ligand to receptor
			c. Example: uptake of lipoproteins by the liver