CELL TRANSPORTATION
Cell membranes
are selectively
permeable,
controlling the
entrance and
exit of all
nutrients,
wastes and
other molecules
in order to
maintain
homeostasis.
SOLUTIONS
• Everything in an organism
is in solution.
• A solution is always made
of a solute dissolved in a
solvent.
• In a cell the solvent is
always water.
• The main cell solutes are
sugars, salts, ions like Na+,
Cl-, K+, or the energy
molecule ATP.
CONCENTRATION GRADIENTS
• In a solution in a bottle the solute mixes
evenly in the solvent.
• Often there is a concentration gradient
maintained by the cell.
• This means that the cell must do work to
ensure that there is more solute on one side
of the cell membrane.
• For example, more Na+ outside the cell is
crucial for nerve cell function.
FOUR MAIN FACTORS
There are four main factors influencing how a solute molecule is
moved across a cell membrane:
1. The size of the molecule
- small molecules move more easily
2. The charge of the molecule
- neutral molecules can pass through the phospholipid bilayer
more easily
3. The concentration gradient of the solute molecule across the cell
membrane
- it is always easiest to move a solute molecule from an area of
high concentration to low concentration
4. Distance
- it is harder for a molecule to move to the inside of a large cell
- in a small cell the area > volume of the cell
- in a large cell the area < volume of the cell
Three Main Transport Methods
• There are also three main methods for moving
molecules across a cell membrane:
1. Passive Transport which does not need any
energy
2. Active Transport which requires energy
3. Bulk Membrane Transport
LE 7-17
Passive transport
Active transport
ATP
Diffusion
Facilitated diffusion
1. Passive Transport
A. Diffusion – the movement of
molecules from an area of high
concentration to an area of
low concentration until
equilibrium is reached.
• Ex. Perfume in a room
1. Passive Transport
B. Osmosis – the movement of water molecules
from an area of high water concentration to
an area of low water concentration across a
semi-permeable cell membrane.
• This is a very special type
of diffusion.
1. Passive Transport
C. Facilitated Diffusion – the movement of
molecules across a semi-permeable cell
membrane from an area of high concentration to
low concentration using a protein
• the carrier protein glucose permease is the right
shape to enable a large glucose molecule to move
from an area of high glucose concentration to low
• the glucose can move either in or out of the cell
• a channel protein allows ions to pass the semipermeable cell membrane
Facilitated Diffusion: Carrier Protein
Carrier protein
Solute
A carrier protein, glucose permease, facilitates
diffusion of glucose.
Facilitated Diffusion: Channel Protein
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
A channel protein facilitates diffusion of ions like
H+, Ca+2
2. ACTIVE TRANPORT
• molecules are moved against their
concentration gradient
• this requires the cell to supply energy
• the energy can be supplied by a molecule of
adenosine triphosphate (ATP) which is
hydrolysed to adenosine diphosphate (ADP) and
phosphate, releasing energy
• sometimes the energy is supplied by an
electrical gradient if ions are involved
• for example the sodium-potassium pump
3. BULK MEMBRANE TRANSPORT
• A. Endocytosis
• - the cell membrane folds inwards trapping some extra-cellular
material and forming a vesicle
• - the vesicle breaks from the cell wall and the contents are processed
by the cell
I. Pinocytosis
- also called “cell drinking”
- occurs when a small vesicle is formed
- only fluid and small dissolved particles are ingested
II. Phagocytosis
- also called “cell eating”
- a larger vesicle forms around a small particle of matter like a
bacteria cell
- used by many protists to obtain food
-sometimes specific receptor molecules control what particles are
engulfed. Ex. cholesterol
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis
vesicles forming
(arrows) in a cell
lining a small
blood vessel
(TEM).
Vesicle
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
CYTOPLASM
1 µm
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Food
vacuole
Bacterium
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM)
LE 7-20c
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Coated
pit
Ligand
A coated pit
and a coated
vesicle formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
Bulk Membrane Transport
B. Exocytosis
• this is the reverse of endocytosis and is used
when the cell wants to export material
• a vesicle containing material produced in the cell
fuses with the cell membrane
• the cell contents are expelled into the extracellular fluid
• especially important for hormones that are
transported in the blood like insulin from the
pancreas
Exocytosis