Membranes and TransportUnit 4

What does the Membrane Do?• Support keeps cell

shape• Transport moves

material in and out of the cell

• Recognition receives info on material around the cell

• Communication send info to outside of cell

• Adherence stick cell to other materials; hold it in place

Membrane Structure• What are the parts?1) Phospholipids hydrophilic and hydrophobic ends; 2 layers2) Sterols non-polar rings and polar alcohol groups; cholesterol 3) Embedded proteins Transport proteins and enzymes4) Glycolipids and glycoproteins

protective coat or communication (act like receptors)

Fluid Mosaic Model• Membrane and it proteins

are in a semi-fluid state which allows proteins to move freely in all directions

• How do cells maintain fluidity at low temps?– More unsaturated fatty acid

chains– Raise cholesterol levels

• How do cells maintain stability at high temps?– Cholesterol reduces fluidity

of the membrane; nonpolar region keep nonpolar tails close together

How do we know it is fluid?• Frye and Edidin; 1970• Created antibodies for

human (glow red) and mice (glow green) proteins

• Mixed human and cell membranes and watched the red and green lights over time

• After 40 mins the lights went from half an half to completely diffused around the membrane

• Found membrane fluidity is similar to machine oil

Membrane Proteins• 4 Types of proteins:1) Transport move H2O,

ions, and molecules2) Recognition cell

recognize each other in a system; communication

3) Receptor bind to signal molecules from the environment

4) Adhesion bind cells to each other or other materials

Two Structures1) Integral cross

through both layers of the membrane; all 4 types

2) Peripheral bind to one side of the membrane, mostly the inside, through non-covalent bonds; cytoskeleton parts, glycolipids, and glycoproteins

How do we know these types exist?• Freeze faction

experiments:– Cool cell down and cut

apart membrane layer– Under an electron

microscope, holes and dents in the membranes match like puzzle pieces

Why Are Cells SOOO Small?• Cell Size Limitations:

1) Diffusion limits cell size too big and

it will take too long for things for

move around the cell

2) DNA limits cell size Too big and it

will take too long to make the

proteins needed

3) Surface area-to-volume ratio Too

big and the ratio between the surface

area and cell size takes too long to

take nutrients and release waste

6:1

3:196:641.5:1

Transportation through the Membrane• Transport of material is

the most activity role of the membrane

• Always must be:– Directional in or out– Specific only one type of

cargo• 2 Ways to Go:1) Passive requires no energy by the cell2) Active requires energy for at least one step

Passive Transport• All movement “requires”

energy• Where does the energy come

from for passive transport?– Kinetic energy

• Diffusion the net movement of molecules from a region of high conc. to a region of low conc. down a conc. gradient

• Is it directional?– Yes, though molecules move in

all directions, the over movement is from high to low

• What happens when the gradient levels out?– Dynamic equilibrium

Simple Diffusion• What can diffuse through the

membrane?1) Non-polar molecules can

pass through hydrophobic barrier in the middle• Steroid hormones

2) Inorganic gases O2, N2, and CO2

3) H2O?– If water is polar why does it

diffuse?• It is small enough to pass though

the phospholipids but very slowly

– Water needs a little help

Facilitated Diffusion1) Aquaporins water channels

• Billion/sec

2) Ion channels Na+, Ca2+, and K+

• 2 types:Gated Channels have to be opened by a stimulus that changes proteins shapeCarrier Proteins doors that allow one solute at a time (uniport)What is the rate limiter for facilitated diffusion?

– saturation all the doors are full; can only go so fast

Osmosis• The movement of water molecules

from a region of higher water potential to a region of lower water potential through a partially permeable membrane

• Movement controlled by water potential difference

• 3 solution types:1) Hypotonic high water potential

outside the cell; water flows into the cell (turgor pressure)

2) Hypertonic low water potential outside the cell; water flows out of the cell (plasmolysis)

3) Isotonic equal water potentials; water flows in and out at same rate

Active Transport• The energy-consuming transport of

molecules or ions across a membrane against a conc. gradient made possible by transferring energy from respiration

• 3 main functions:1) Brining important nutrients into

the cell2) Removing waster from the cell3) Maintaining concentrations of ions

across the membraneWhat ions are most important?

– H+, Na+, K+, and Ca2+

– Ions create membrane potential electrical potential difference between sides of membranes

Active Transport• Na+/K+ Pumps:– Essential for all animal

cells and the nerve system of complex organisms

– 1 ATP 3 Na+ out and 2 K+ in

– Generates membrane potentials from -20mV to – 200mV

• Electrochemical gradient concentration gradient produces both a movement of chemicals and an electrical charge

Bulk Transport• Exocytosis:

– Remove waste material and release secretions

– Vesicle fuses with plasma membrane

• What limits exocytosis rates?– Cell size; plasma membrane can

only grow to a certain point• Endocytosis:

– Bulk-phase (pinocytosis)– Cell pulls in part of ECF through

the vesicle; non-specific• What might trigger this?

– Cytosol is too thick; lack of general nutrition; reduce plasma membrane

Bulk Transport• Receptor mediated

Endocytosis– Receptor protein in

membrane binds to specific substrate

– Receptors form coated pit after activation and pull in molecule

– Bind to lysosome to breakdown contents

• Phagocytosis “cell eating”– Cell consumes whole other

bacteria cell for energy or defense (white blood cells)

• Pinocytosis “cell drinking”– Cell consumes fluid from ECF