Cell Membranes and Transport

Objectives:

-Describe the structure and function of the cell membrane

-Describe the 4 types of cell transport-Understand how to predict the flow of water in/out of a cell-Understand the concept of water potential

-Pick up your ch. 7 reading guide from the back counter

Homework

• Study

• Quiz over chapter 7/8 (membrane) – Thursday

• Test – Monday

• Post-lab – Tuesday (you get to work on it in class on Thursday)

1 - Review

• 4 organic compounds– Carbohydrates– Lipids– Proteins– Nucleic acids

2 - Amphipathic – contains both hydrophobic (non-polar) and hydrophilic regions (polar)

Phospholipids

3 - Two problems with the sandwich model1.Not all membranes looked the same (some were 3-layered, some not)2.Proteins were amphipathic – hydrophobic regions could not be near water

4 - Fluid mosaic model – proposed by Singer and Nicolson in 1972, proteins imbedded in the

membrane (hydrophobic portions of proteins and lipids could be separated from water

5 - Membranes need to be fluid to work properly

- if a membrane solidifies, its permeability changes and embedded proteins can stop

working

Cell Membrane Animation

• http://www.susanahalpine.com/anim/Life/memb.htm

6 - Membrane Fluidity

• In general, decreasing temperature, slows the phospholipid movement and decreases fluidity

• Unsaturated fatty acid tails will slow this process by keeping the phospholipids from packing together

Cholesterol – temperature buffer

• At lower temps, cholesterol prevents membrane from solidifying

• At higher temps, cholesterol restrains phospholipid movement

7 - Types of proteins• Integral – embedded completely or partially into the membrane

– Transmembrane – span the entire membrane– Unilateral – do not span the entire membrane

• Peripheral – attached to the surface of the membrane

8 - Functions of cell membrane

9 – Cell-to-cell recognition• Examples

– Sorting of tissues/organs– Determining self/non-self

• 10

• Glycolipids – carbohydrates bonded to fat

• Glycoproteins – carbohydrates bonded to protein

11 - Fluid Mosaic Model

Tomorrow

• We will look at diffusion and transport

Today• Observe part A sample – check your

results– Sugar is present inside and outside of the bag

• Complete part B– Solutions are unknown – label carefully

• Prep Part C while part B is running– Get potatoes cut (no skin, similar size, ~ 3cm)– Have part C running before you leave

• Time left over – finish data tables, collaborate data, complete graphs and questions

Clean-up

• Dry part B cups

• Rinse and dry all materials (sugar makes things sticky)

• Save rubber bands please

• Put all materials in tub – leave on table

Dissolvability

• Hydrophobic (nonpolar) molecules (smaller hydrocarbons, CO2, and O2) dissolve easily – can pass through membrane

• Hydrophilic (polar) molecules (ions, water, glucose, sugars) do not dissolve easily – will only pass through slowly or not at all, they need help to get across quickly (transport proteins)

Sidedness of membranes

• Outside of plasma membrane is the same on the surface as the inside of the ER, Golgi, and vesicles

Transport

• Selective permeability

• High concentration to low concentration

• Passive vs active

Diffusion• Molecules move randomly, yet population of molecules

may move directionally• Diffusion creates a system with more entropy (disorder)• Diffusion is spontaneous since it decreases free energy

Fig. 8.10

Types of Transport

• Passive – no ATP required– Diffusion (CO2, O2)– Facilitated diffusion – uses help of transport

proteins (water, polar molecules, ions)• Channel proteins• Aquaporins – H2O

• Active – moving molecules against concentration gradient (requires energy)

Diffusion

• Movement of molecules from an area of high to low concentration

• Does not require energy

• Ex. – Smelling food baking in the kitchen, air freshner

Facilitated Diffusion

• Aquaporins – facilitate water to diffuse• Gated protein channels – can open or close, can

transport molecules in both directions

Facilitated diffusion

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_facilitated_diffusion_works.html

Osmosis

Osmosis

• Hypertonic – higher solute concentration

• Hypotonic – lower solute concentration

• Isotonic – equal concentrations

• Water will move from a hypotonic sol’n to a hypertonic sol’n (Remember – we are thinking in terms of solute)

• Solute will move from hyper to hypo (high to low solute concentration)

Osmotic Potential – think in terms of H2O

• The potential water has to move• Less negative number as higher potential• Distilled water has the highest potential

(zero)• When water has another substance

dissolved in it, the water molecules have less potential to move (osmotic potential is negative)

• Water always move from less negative to more negative (High concentration of water to Low concentration of water)

(High concentration of solute)

(Less concentration of solute)

Active Transport

• Transport protein pumps a molecule against its concentration gradient

• Requires ATP for transport

• ATP can transfer a phosphate to the transport protein inducing a conformational change

• Na-K pump

Na/K Pump

• http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_the_sodium_potassium_pump_works.html

Ion Transport• Anions and cations are unequally distributed

across the plasma membrane• Voltage = electrical potential energy (separation

of opposite charges)• Membrane potential = voltage across

membranes which affect the diffusion of charged substances (favors diffusion of cations into the cell and anions out of the cell)

• Electrochemical gradient = diffusion gradient resulting from combined effects of membrane potential and concentration gradient– Ions always always diffuse down their electrochemical

gradient, but not always down their concentration gradient

Cotransport – couples downhill and uphill diffusion

Endocytosis

• Cell imports macromolecules by forming vesicles derived from the plasma membrane

• Vesicle forms from a region of plasma membrane that sinks inward then pinches off

• Used to incorporate extracellular substances

3 types

• Phagocytosis – cell eating of solid particles (can use pseudopodia), produces food vacuole

• Pinocytosis – cell drinking of fluid droplets, produces small vesicles

• Receptor-mediated – specific molecules (ligands) bind to receptors of coated pits on plasma membrane

Exocytosis

• Cell exports macromolecules by fusion of vesicles with the plasma membrane

• Vesicles usually bud from the ER or golgi and migrates to plasma membrane

• Used by secretory cells to export products (Ex. Insulin)

Endo/Exocytosis

• http://highered.mcgraw-hill.com/olc/dl/120068/bio02.swf