AP Biology

Go to the “Free Response Help” link on my website to check out the Unit 2 FRQs!!!

AP Biology

Pre-Lab Preparation!!! Go to “Labs & Lab Notebook” link on my website. Click on “LabBench” link. Click on “Lab 1: Diffusion & Osmosis.” Read through Concepts #1-5, Design Exercises

#1-2, and Self-Quiz ?s #1-3 of the Pre-Lab. Answer the general questions COMPLETELY that

are listed on your Lab Notebook Guidelines sheet. Remember to ignore the “Water Potential”

section and calculations Due on Friday!!!

AP Biology

The Cell Membrane

AP Biology

Arranged as a Phospholipid bilayer

polarhydrophilic

heads

nonpolarhydrophobic

tails

polarhydrophilic

heads

Serves as a cellular barrier / border

impermeable to polar molecules

AP Biology

Cell membrane defines cell Cell membrane separates living cell from aqueous

environment But…separation CANNOT be complete, or the cell

would die!

Controls traffic in & out of the cell allows some substances to cross more easily than

others hydrophobic (nonpolar) particles can cross more easily

than vs. hydrophilic (polar) particles

AP Biology

Permeability to polar molecules? Membrane becomes “semi-permeable”

via protein channels specific channels allow specific material

across cell membrane

inside cell

outside cell

sugaraaH2O

saltNH3

AP Biology

Cell membrane is more than lipids… Transmembrane proteins embedded in

phospholipid bilayer create semi-permeable channels

lipid bilayermembrane

protein channelsin lipid bilyer membrane

AP Biology

Classes of amino acids

nonpolar & hydrophobicnonpolar & hydrophobic

AP Biology

Classes of amino acids

polar & hydrophilicpolar & hydrophilic

AP Biology

Membrane Proteins

Within membrane nonpolar amino acids

hydrophobic anchors protein

into membrane

On outer surfaces of membrane in fluid polar amino acids

hydrophilic extend into

extracellular fluid & into cytosol

Polar areasof protein

Nonpolar areas of protein

AP Biology

NH2

H+

COOH

Cytoplasm

Retinalchromophore

Nonpolar(hydrophobic)-helices in thecell membrane H+

Porin monomer

-pleated sheets

Bacterialoutermembrane

proton pump channel

aquaporin = water channel

Examples

H2O

H2O

H+

H+

AP Biology

Many Functions of Membrane Proteins

Outside

Plasmamembrane

InsideTransporter Cell surface

receptorEnzymeactivity

Cell surface identity marker

Attachment to thecytoskeleton

Cell adhesion

“Antigen”

“Channel”

AP Biology

Membrane is a collage of proteins & other molecules embedded in the fluid matrix of the lipid bilayer

Extracellular fluid

Cholesterol

Cytoplasm

Glycolipid

Transmembraneproteins

Filaments ofcytoskeleton

Peripheralprotein

Glycoprotein

Phospholipids

Fluid Mosaic Model

AP Biology

Parts of the “mosaic”… Fat composition affects flexibility

membrane must be fluid & flexible % unsaturated fatty acids in phospholipids

keep membrane more fluid Cold weather plants = increased unsaturated fatty

acids (“winter wheat”) cholesterol in membrane

prevents membrane from

“freezing”

AP Biology

Membrane carbohydrates Play a key role in cell-cell recognition

ability of a cell to distinguish one cell from another antigens

basis for rejection of foreign cells by immune system

AP Biology

Movement across the Cell Membrane

AP Biology

Diffusion 2nd Law of Thermodynamics

governs biological systems universe tends towards disorder (entropy)

Diffusion movement from HIGH LOW concentration

Diffusion movement from HIGH LOW concentration

AP Biology

Simple Diffusion Move from HIGH to LOW concentration

“passive transport” no energy needed

diffusion osmosis

movement of water

AP Biology

(a) Diffusion of one solute. The membrane has pores large enough for molecules

of dye to pass through. Random movement of dye molecules will cause some to pass through the pores; this will happen more often on the side

with more molecules. The dye diffuses from where it is more concentrated

to where it is less concentrated (called diffusing down a concentration

gradient). This leads to a dynamic equilibrium: The solute molecules continue to cross the membrane,

but at equal rates in both directions.

Diffusion of two solutes. Solutions of two different dyes are separated by a membrane that is permeable to both.

Each dye diffuses down its own concen-tration gradient. There will be a net

diffusion of the purple dye toward the left, even though the total solute

concentration was initially greater onthe left side.

(b)

Osmosis

Molecules of dye Membrane (cross section)

WATER

Net diffusion Net diffusion Equilibrium

Net diffusion

Net diffusion

Net diffusion

Net diffusion Equilibrium

Equilibrium

AP Biology

Facilitated Diffusion Diffusion through protein channels

channels move specific molecules across cell membrane

no energy neededopen channel = fast transport

facilitated = with help

HIGH

LOW

AP Biology

Active Transport

conformational change

Cells may need to move molecules against concentration gradient conformational shape change transports solute

from one side of membrane to other protein “pump” “costs” energy = ATP

AP Biology

Active transport

ATP

AP Biology

Getting through cell membrane Passive Transport

Simple diffusion diffusion of nonpolar, hydrophobic molecules

lipids HIGH LOW concentration gradient

Facilitated transport diffusion of polar, hydrophilic molecules through a (hydrophilic) protein channel

HIGH LOW concentration gradient

Active transport diffusion against concentration gradient

LOW HIGH uses a transport protein

A.K.A. “protein pump” requires ATP

ATP

AP Biology

Transport summary

simplediffusion

facilitateddiffusion

activetransport

ATP

AP Biology

How about large molecules? Moving large molecules into & out of cell

through vesicles & vacuoles endocytosis

phagocytosis = “cellular eating” pinocytosis = “cellular drinking”

Exocytosis Example:

Pancreas cell

secreting insulin

into bloodstream

exocytosis

AP Biology

Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specific(takes in all molecules in extra-cellular fluid)

triggered bymolecular signal

AP Biology 2007-2008

The Special Case of Water

Movement of water across the cell membrane

AP Biology

Osmosis is just diffusion of water Water is very important to life,

so we talk about water separately Diffusion of water from

HIGH concentration of water to LOW concentration of water across a

semi-permeable membrane

AP Biology

Concentration of water Direction of osmosis is determined by

comparing total solute concentrations Hypertonic - more solute, less water

Hypotonic - less solute, more water

Isotonic - equal solute, equal water

hypotonic hypertonic

water

net movement of water

AP Biology freshwater balanced saltwater

Managing water balance Cell survival depends on balancing

water uptake & loss

AP Biology

Managing water balance Hypotonic

a cell in fresh water high concentration of water around cell

problem: cell gains water, swells & can burst

example: Paramecium ex: water continually enters

Paramecium cell

solution: contractile vacuole pumps water out of cell “osmoregulation”

plant cells turgid = full cell wall protects from bursting

freshwater

AP Biology

Pumping water out Contractile vacuole in Paramecium

ATP

AP Biology

Managing water balance Hypertonic

a cell in salt water low concentration of water

around cell problem: cell loses water &

can die example: salinity increases in

lake plant cells

plasmolysis = wilt can recover if conditions

changesaltwater

AP Biology

Managing water balance Isotonic

animal cell immersed in mild salt solution

no difference in concentration of water between cell & environment problem: none

no net movement of water flows across membrane equally, in

both directions

cell in equilibrium volume of cell is stable

example: blood cells in blood plasma slightly salty IV solution in hospital

balanced

AP Biology

Aquaporinsprotein channels allowing

rapid flow of water across cell membrane

explains efficient nature of osmosis

AP Biology

Cell (compared to beaker) hypertonic or hypotonic

Beaker (compared to cell) hypertonic or hypotonic

Which way does the water flow? in or out of cell

.05 M .02 M

Do you understand Osmosis…