Image from: http://www.biologie.uni-hamburg.de/b-online/ge22/03.gif

• The size of cells is limited by the surface area to volume ratio of cells
– As cells grow the volume inside of the cell grows faster than the surface area available to diffuse nutrients into the cell and expel wastes out of the cell
• Surface area determines rate molecules enter and leave cell
• Volume determines the demand for resources needed by the cell (increase in volume requires more resources to keep the cell functioning properly)

• Smaller cells have more favorable surface area to volume ratios for exchange with the environment
Root hairs Cells of alveoli:
Lungs
Villi: Small Intestines

• You should be able to solve basic surface area and volume equations given to you on the
AP Biology exam.

• In order for cells to grow, reproduce and maintain dynamic homeostasis, cells must create and maintain an internal environment that is different from the external environment
– Cell membranes are selectively permeable to accomplish this goal
– Cell membranes separate the internal environment from the external environment

Animation from: http://www.sp.uconn.edu/~terry/images/anim/fluidmem.gif
Click here to
See Fluidity
Click here to See
FLUIDITY
1972- S.J. Singer and G. Nicolson propose membrane is a
“mosaic” of proteins and phospholipids that are constantly moving and changing

• The membranes selective permeability is a direct consequence of the membranes structure consisting of phospholipids, embedded proteins, cholesterol, glycoproteins and glycolipids

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

Molecules need to move across membranes in cells
OUT waste ammonia salts
CO
2
H
2
O products
IN food carbohydrates sugars, proteins amino acids lipids salts, O
2
, H
2
O
Image modiified from: http://www.accessexcellence.org/AB/GG/importProt.html

Membranes provide a variety of cell functions

Cell Membranes are made of
PHOSPHOLIPIDS & PROTEINS
Amphipathic –
Molecules with both hydrophilic and hydrophobic regions phosphate hydrophilic lipid hydrophobic

• 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 protein

HYDROPHILIC/HYDROPHOBIC areas determine positions of molecules in cell membranes hydrophobic amino acids
– Those with nonpolar side groups
– stick in the lipid membrane
– anchors the protein in membrane hydrophilic amino acids
– Amino acids with charge and polar side groups
– stick out in the watery fluid in or out of cell

• Attached to proteins (glycoproteins) or lipids (glycolipids)
• 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 http://faculty.southwest.tn.edu/rburkett/GB1-osmosis.htm

• Need to allow passage through the membrane
• But need to control what gets in or out
– membrane needs to be semi-permeable sugar aa lipid H
2
O salt NH
3
So what makes a membrane semi permeable?
See a movie

What molecules can get through directly?
Small non-polar (uncharged) molecules (O
2
, N
2
, &
CO
2
) and hydrophobic molecules (fats & other lipids) can slip directly through the phospholipid cell membrane, but… inside cell lipid O
2
NH
3 salt What about other stuff?
outside cell sugar aa H
2
O

• 2nd Law of Thermodynamics governs biological systems
– Universe tends towards disorder
Animatioin from: http://www.biologycorner.com/resources/diffusion-animated.gif
• Diffusion
– movement from [higher]

[lower] concentration

Diffusion will also happen across a cell membrane as long as there is a difference in concentration and the membrane will let the molecule pass through.

http://facstaff.bloomu.edu/gdavis/links%20100.htm
O
2 automatically moves from
HIGHER concentration (in lungs) to
LOWER concentration (in blood)
CO
2 automatically moves from
HIGHER concentration (in blood) to LOWER concentration (in lungs) http://www.le.ac.uk/pa/teach/va/anatomy/case2/2_2.html

• Each substance diffuses down its own concentration gradient, independent of concentration gradients of other substances

Cell example:
Want to put MORE glucose into mitochondria when there is already glucose in there
Image from: http://www.biologyclass.net/mitochondria.jpg

http://www.d.umn.edu/~sdowning/Membranes/membraneImages/jpegimages/diffusionmedium.jpg

Movement of
Na + & K + ions required to send nerve signals http://www.steve.gb.com/images/science/neuron.png

• Move from HIGH to LOW concentration with aid of membrane transport proteins
– passive transport
– no energy needed
– facilitated = with help
– Examples:
• Way glucose enters cells
• Also how charged and polar molecules enter the cell

Animation from: http://bio.winona.edu/berg/ANIMTNS/facdifan.gif
Carrier Proteins
Grab molecule, undergo conformational change, flip to other side
Channel Proteins
Create passageway for substances to pass through
May be gated or not: open in response to chemical or electrical signals
Animation from: http://www2.uic.edu/~myilma1/ionchannel.gif

• open only in 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 channels change in electrical charge across nerve cell membrane opens Na + & K + channels

inside cell
H
2
O aa sugar
NH
3 outside cell salt
. . . BUT STILL MOVES FROM [HIGHER] to [LOWER]

Uses energy from ATP to move molecules against concentration gradient
– Moves from [lower]
[higher]
– Uses protein pumps
OR vesicles
Animations from: http://academic.brooklyn.cuny.edu/biology/bio4fv/page/cell-movement.html
http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

Moves Protons (H + ) across membrane
EXAMPLES:
- Creates acidic condition inside lysosomes
- Photosynthesis/respiration use H + gradients to generate ATP
See a movie proton pump

SODIUM-POTASSIUM PUMP
Sets up difference in charge across membranes
Pumps 3 Na + out and 2 K + in
Makes cells more + outside more - inside
See a movie about Na+ - K+ pump
Animation from: http://www.cat.cc.md.us/courses/bio141/lecguide/unit1/eustruct/images/sppump.gif

All cells have voltages across their membranes
= membrane potential
• Cytoplasm inside cell is more negative than extracellular fluid outside
• TWO FORCES drive diffusion
• Concentration gradient
• Electrical force
• Ions move DOWN the
electrochemical gradient
Favors the passage of cations (+) into cells

MEMBRANE POTENTIAL created by electrogenic pumps (proteins that generate voltage by pumping ions)
PROTON PUMP
Main electrogenic pump in plants, fungi, and bacteria
Na + - K + PUMP
Main electrogenic pump in animal cells http://fig.cox.miami.edu/~cmallery/150/memb/electrogenic.jpg
Animation from: http://www.lionden.com/cell_animations.htm

Electrogenic pumps can be coupled to actively transport other substances
= COTRANSPORT
Ex: Cells pump H + out of cell then use the diffusion of H + back into cell down its gradient to drive the uptake of sucrose into cells http://faculty.southwest.tn.edu/rburkett/GB1-os29.jpg

+
+
http://hyperphysics.phy-astr.gsu.edu/hbase/biology/nervecell.html#c2

• ACTIVE transport Watch two video clips about endo/exocytosis
• Requires energy (ATP) endo/exocytosis
• Uses vesicles
Animation from: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBooktransp.html

phagocytosis
pinocytosis
“Cell eating” large molecules; whole cells
“Cell drinking”
Fluids;
Small molecules receptor-mediated endocytosis triggered by ligand signal

Example in cells:
WHITE BLOOD CELL ENGULFING
BACTERIA
SEE
PHAGOCYTOSIS
MOVIE http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x28.jpg

• Active transport (requires ATP)
• Uses vesicles
• Releases substances to outside
INSULIN being released by pancreas cells using exocytosis

Video: http://www.southtexascollege.edu/tdehne/BC_ShockwaveAnimations/07SWF-TourOfTheCell/07-16-EndomembraneSystem.swf
Animation from: http://www.franklincollege.edu/bioweb/A&Pfiles/week04.html
See a Golgi movie

http://fig.cox.miami.edu/~cmallery/255/255ion/fig14x26.jpg

The Special Case of Water

• Water is very important, so we talk about water separately
• Diffusion of water from high concentration of water to low concentration of water
– across a semi-permeable membrane
– Passive
– Uses no energy

1991 | 2003
• Transport proteins that move water rapidly into & out of cells
– evidence that there were water channels
Peter Agre
John Hopkins
Roderick MacKinnon
Rockefeller

TONICITYability of a solution to cause a cell to lose or gain water
• Refers to the concentration of SOLUTES
• Is a RELATIVE term, comparing two different solutions
Solute-substance that is dissolved
Solvent- substance solute in dissolved in
Solution = solute + solvent

What if there is a difference in concentration but solute molecules can’t move across a membrane?
WATER will move until concentrations reach equilibrium

Animation: http://www.ouhscphysio.org/humanphys/animations/osmosis1.swf
See an animation
Osmosis1 http://faculty.etsu.edu/currie/images/osmosis1.jpg
Animation

• 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 water hypotonic hypertonic net movement of water

• Pressure potential:
– pressure exerted by the rigid cell wall that limits further water uptake.
• Solute potential
– The effect of solute concentration.
– Pure water at atmospheric pressure has a solute potential of zero.
– As solute is added, the value for solute potential becomes more negative. This causes water potential to decrease also.
– As solute is added, the water potential of a solution drops, and water will tend to move into the solution.

In beaker B, what is the water potential of the distilled water in the beaker, and of the beet core?
– Since water potential = solute potential (-0.4) + pressure potential (0.2),
– water potential = -0.2 (water left the beet core)

Solute concentration
Lower outside than inside
HYPOTONIC
Equal outside and inside
ISOTONIC
Greater outside than inside
HYPERTONIC
What will happen to an animal cell placed in different solutions?

Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis3.swf
See an animation
Osmosis3
Video
HYPOTONIC:
Concentration outside cell is
LESS THAN
Choose Blood
Hypotonic link
More water enters than leaves cell so cell swell and possibly burst

Animation from: http://www.ouhscphysio.org/humanphys/animations/osmosis4.swf
See an animation
OSMOSIS 4
HYPERTONIC: Concentration outside cell is
GREATER THAN
More water leaves cell than enters shrinks
Video Choose Blood
Hypertonic link

Remember:
Cells try to “maintain stable internal conditions = http://bioweb.wku.edu/courses/biol121/Osmosis/Osmosis.asp
So an animal cell in
ISOTONIC conditions stays same size
Water entering = water leaving
Video
Choose Blood Isotonic link

= CYTOLYSIS = CRENATION http://www.stchs.org/science/courses/sbioa/metenergy/bloodcells.gif

http://www.stchs.org/science/courses/sbioa/metenergy/aplantturgor.gif
Cell wall keeps plant cell from bursting made of cellulose rather than phospholipids and is external the cell membrane. Provides structural barrier for some substances to the internal environment.

• Isotonic
– animal cell immersed in isotonic solution
• blood cells in blood
• no net movement of water across plasma membrane
• water flows across membrane, at same rate in both directions
• volume of cell is stable

.05 M .03 M
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

http://www.biology4kids.com/files/cell_vacuole.html
Loss of water from central vacuole= plants “wilt”
Vacuole full of water gives plant support
(turgor pressure)
Turgid = very firm
Flaccid = limp

Sitting in the bathtub makes your fingers plump up and get “pruny”
Grocery stores spray water on their veggies to “plump them up”

If cells can’t maintain
“stable internal conditions” . . .
damage can result and cells can die.
http://www.the-aps.org/education/lot/cell/Quiz.htm

Cell survival depends on balancing water uptake & loss = OSMOREGULATION http://www.microscopy-uk.org.uk/mag/imgjun99/vidjun1.gif
Paramecium vs. pond water
Paramecium is hypertonic
H
2
O continually enters cell to solve problem, specialized organelle, contractile vacuole

Drink salt water
ACTIVELY pump ions OUT;
Urinate less frequently
Gills ACTIVELY pump ions in;
Urinate frequently

Kidspiration by: Riedell