AP Biology2005-2006

Cell Membranes and Movement Across Membranes (Transport)

AP Biology2005-2006

Cell Membrane…more than just a barrier!

AP Biology2005-2006

Cell (plasma) membrane Cells need an inside & an outside…

separate cell from its environment cell membrane is the boundary

INfoodcarbohydratessugars, proteinsamino acidslipidssalts, O2, H2O

OUTwasteammoniasaltsCO2

H2O products

cell needs materials in & products or waste out

IN

OUT

Can it be an impenetrable boundary? NO!

AP Biology

Lipids of cell membrane Membrane is made of phospholipids

phospholipid bilayer

phosphate

lipid

hydrophilic

hydrophobic

inside cell

outside cell

AP Biology

Phospholipid bilayer What molecules can get through directly?

inside cell

outside cell

fats & other lipidscan slip directly through the phospholipid cell membrane, but…

what about other stuff?

lipid

salt

aa H2Osugar

NH3

AP Biology2005-2006

Membrane Proteins Proteins determine most of membrane’s

specific functions cell membrane & organelle membranes each

have unique collections of proteins Membrane proteins:

peripheral proteins = loosely bound to surface of membrane

integral proteins = penetrate into lipid bilayer, often completely spanning the membrane =

transmembrane proteins

AP Biology

Membrane Protein Types Channel proteins – wide open passage Ion channels – gated, only active under

specific conditions Aquaporins – move water only, found

primarily in animal kidneys and plant roots Carrier proteins – conformational change

(change in shape occurs) Transport proteins – require ATP

AP Biology

Membrane Protein Types Recognition proteins – glycoproteins,

carbohydrates are attached (too hydrophilic to be directly on membrane)

Adhesion proteins – anchors to surrounding cells/tissues

Receptor proteins – hormones; receptors that start signal transduction pathways

AP Biology2005-2006

A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer

AP Biology2005-2006

Membrane Carbohydrates Play a key role in cell-cell recognition

ability of a cell to distinguish neighboring cells from another

important in organ & tissue development

basis for rejection of foreign cells by immune system

AP Biology

Cholesterol Provides stability in animal cells Replaced with sterols in plant cells

AP Biology

Getting through cell membrane Passive transport

No energy needed Movement down concentration gradient

high low

Active transport Movement against concentration gradient

low high requires ATP

Think: does it take more energy to move water down a waterfall (from high to low) or up a waterfall (from low to high)?

AP Biology

Diffusion

2nd Law of Thermodynamics - Universe tends towards disorder

Diffusion movement from high low concentration

AP Biology2005-2006

Simple diffusion across membrane

inside cell

outside cell

Which way will lipid move?

low

high

lipid

lipid

lipid

lipid

lipid

lipid lipid

lipidlipid

lipid

lipid

lipid

lipid

lipid

AP Biology

Diffusion of 2 solutes Each substance diffuses down its own

concentration gradient, independent of concentration gradients of other substances

AP Biology2005-2006

Facilitated diffusion Movement from HIGH to LOW

concentration through a protein channel passive transport; no energy needed facilitated = with help

AP Biology2005-2006

Gated channels Some channel proteins open only in the

presence of stimulus (signal) stimulus usually different from

transported molecule ex: ion-gated channels

when neurotransmitters bind to a specific gated channels on a neuron, these channels open = allows Na+ ions to enter nerve cell

ex: voltage-gated channelschange in electrical charge across nerve cell membrane opens Na+ & K+ channels

AP Biology

Active transport Cells may need molecules to move

against concentration situation need to pump against concentration protein pump requires energy

ATP

Na+/K+ pump in nerve cell membranes

AP Biology2005-2006

Transport summary

AP Biology2005-2006

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

requires ATP! through vesicles & vacuoles Endocytosis (taking molecules in)

phagocytosis = “cellular eating” pinocytosis = “cellular drinking” receptor-mediated

endocytosis Exocytosis

Molecules exiting

Exocytosis

AP Biology

Endocytosis

phagocytosis

pinocytosis

receptor-mediated endocytosis

fuse with lysosome for digestion

non-specificprocess

triggered byligand signal

AP Biology2005-2006

The Special Case of Water

Movement of water across the cell membrane

AP Biology2005-2006

Osmosis is diffusion of water Diffusion of water from

high concentration of water to low concentration of water across a

semi-permeable membrane

AP Biology2005-2006

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 Biology2005-2006

Managing water balance Cell survival depends on balancing

water uptake & loss

freshwater balanced saltwater

AP Biology

Hypotonicity animal cell in a hypotonic

solution will gain water, swell & possibly burst (cytolysis)

Paramecium vs. pond water Paramecium is hypertonic H2O continually enters cell specialized organelle, contractile

vacuole, pumps excess H2O out of cell; requires ATP

plant cell in a hypotonic solution is turgid

Turgor pressure Cell wall exerts force

AP Biology2005-2006

Hypertonicity animal cell in hypertonic

solution will lose water, shrivel & probably die

salt water organisms are hypotonic compared to their environment

they have to take up water & pump out salt

plant cell in a hypertonic solution experiences plasmolysis

Vacuole shrinks; cell membrane pulls away from cell wall

Wilting

AP Biology2005-2006

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 .03 M

Osmosis…

AP Biology

Water Potential Water moves from a place of greater

water potential to a place of lesser water potential (net).

As the concentration of a solute increases in a solution, the water potential will decrease accordingly. Which has the greater water potential:

.2M or .8M? Which has the greater water potential:

20% or 80% water?