TRANSPORTING MATERIALS ACROSS CELLS
Permeable ð allows movement across the membrane
Impermeable ð no movement across the membrane
Selectively permeable ð allows certain substances to enter or leave, but not
others
Small molecules pass easily through the cell, medium-sized ones pass less easily, large ones cannot pass through at all. Cells, therefore, have a ..
selective permeability diagram
THE FLUID MOSAIC MEMBRANE MODEL
cell membranes are composed of a phospholipid bilayer that has fluid consistency and is dotted with protein molecules
ï the polar head of the membrane is attracted to water (phosphorus
end)
ï the hydrocarbon chains in the fatty-acid tail (non-polar) are not
attracted to water but are compatible with other lipids
There are many proteins built into the membrane:
§ proteins that carry sugars are glycoproteins and are used to identify different cells
ð this is why transplanted organs are often rejected
§ proteins in phospholipid bilayer act as gatekeepers
§ proteins act as receptor sites for hormones
§ peripheral proteins are suspended in the bilayer (on the outside or inside layers only)
§ integral proteins are deeply embedded (go all the way through)
HOW CELLS MOVE MATERIALS IN AND OUT
ð the substance that is dissolved in solution
ð the substance that does the dissolving of another substance, e.g. H2O
PASSIVE TRANSPORT
§ No energy required to move molecules along a concentration gradient (goes from high concentration [ ] to low).
ð a difference in concentration between two areas
Three types of Passive Transport
1. ð the movement of molecules from an area of concentration to
an area of concentration
§ it is fueled by the constant random motion of particles
§ e.g. air freshener, perfume, the smell of coffee
§ this can also happen across a selectively permeable membrane (it only allows certain substances to pass through them)
Diffusion in Cells
High
Concentration Low Low
O2 O2 High Concentration
CO2 CO2
O2 is in high concentration outside the cell and low concentration inside the cell
ð oxygen diffuses inside the cell!
ð state of balance where particles move at equal rates in all directions
2. ð the movement of water through a semi-permeable membrane
from high concentration to low concentration
Maintaining the correct proportion of water inside and outside the cell is essential for cell survival.
Osmosis occurs in three different solutions:
§ ð the solute concentration is greater outside the cell than
inside
ð water exits the cell
§ ð the solute is equal inside and out
ð the rate of water entering the cell equals the rate it exits the
cell
§ ð the solute is less outside the cell than inside
ð water enters the cell
Note that do best in isotonic solutions while prefer hypotonic ones.
WHY?
Osmosis and Animal Cells
§ animal cells have a cell wall
e.g.
▫ blood serum (liquid part of the blood) is isotonic when compared to red blood cell cytoplasm
▫ this helps RBCs to maintain their shape
ð if the blood serum is diluted (hypotonic solution)
ð the RBCs will swell and may burst. This will be fatal
ð if the blood serum becomes concentrated (hypertonic solution)
ð RBCs lose water and stick to one another
ð this clogs veins and arteries and may be fatal
Osmosis and Plant Cells
§ Plant cells a cell wall made of cellulose
ð in a hypotonic solution, water pressure pushes the
cytoplasm up against the cell wall
ð the rigid cell wall keeps the cell from bursting
ð in a hypertonic solution, the cytoplasm pulls away from
the cell wall and the cell (and plant) wilts
3. ð Diffusion across a membrane with help from
trans-membrane proteins
ð these span the thickness of the phospholipid bilayer
§ movement is still from an area of concentration to an area of
concentration
§ needed for:
▫ large polar molecules such These particles cannot
as glucose diffuse across
▫ large ions such as sodium membranes on their
and chloride ions own
ð trans-membrane protein that assists the diffusion of certain
substances through a membrane
Outside the Cell
Inside the Cell
ACTIVE TRANSPORT
§ energy is used to transport materials from areas of concentration to areas of concentration
§ Just like facilitated diffusion, this requires a trans-membrane carrier protein
§ Adenosine triphosphate (ATP) provides the energy needed for this process
§ As this molecule is broken apart, energy is created
§ 30% to 40% of all energy used while sleeping is for active transport
§ Nutrients must be absorbed quickly in the small intestine or they will be lost as waste.
§ If our bodies relied on simple or facilitated diffusion, about half of the nutrients would be lost
§ Therefore cells pump materials against a concentration gradient to maximize nutrient absorption
SODIUM POTASSIUM PUMP
§ Note that for every three Na+ (sodium ions) pumped out of the cell, two K+ (potassium ions) are pumped in
§ Energy is released when the ATP molecule transfers a phosphate group to the transport protein
ð used to move materials that are not dissolved in solution
Two Types of Bulk Transport
1. ð the process of the cell membrane folding in on itself to form a
vesicle that brings a substance into a cell
ð the cell also often extends its cytoplasm to do this
Pinocytosis ð liquid droplets are engulfed
e.g. cells bring cholesterol into the cytoplasm
Phagocytosis ð solid particles are engulfed
e.g. White blood cells (called macrophages) use this process to engulf
harmful bacteria Movie Clip
2. ð movement of products outside the cell membrane
e.g. used to export materials such as hormones or enzymes