THE TRANSPORT OF SUBSTANCES
ACROSS A CELL MEMBRANE

PASSIVE TRANSPORT
Processes that enable substances to move into and out
of cells without an input of energy from the cell
Powered by a concentration gradient
Three types:
a) Diffusion
b) Osmosis
c) Facilitated Diffusion
PASSIVE TRANSPORT: DIFFUSION
Random movement of substances from regions of
higher concentration to lower concentration
Continues until equilibrium (afterward no net change)
PASSIVE TRANSPORT: DIFFUSION
Factors affecting diffusion
-
Relative concentrations
-
How readily a molecule or ion crosses the membrane
a)
b)
c)
-
Molecule size: faster for smaller molecules
Molecule polarity: faster for non-polar or small polar
molecules
Molecule/Ion charge: faster for uncharged molecules or
ions
Temperature: faster at higher temperatures
PASSIVE TRANSPORT: OSMOSIS
The movement of WATER from higher concentration to
lower concentration across a SEMI-PERMEABLE
MEMBRANE
-
Membrane is impermeable to the solute
-
Water moves until concentrations are equal
(afterward, no net change)
PASSIVE TRANSPORT: OSMOSIS
HYPERTONIC: Solution with higher concentration
HYPOTONIC: Solution with lower concentration
ISOTONIC: Solutions with equal concentrations
PASSIVE TRANSPORT: OSMOSIS
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
Diffusion across a membrane faciliated by a membrane
protein
Two types of proteins exist
-
Channel Proteins
-
Carrier Proteins
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
CHANNEL PROTEINS
-
Tubular shape composed of one or more helixes
-
Can remain open or have gates that respond to a
variety of signals (Ex: hormones, electric charge,
presssure, light, etc)
-
Generally permit ions or small polar molecules to
pass
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
CHANNEL PROTEINS
-
Exterior: amino acids with non-polar side chains
-
Internal: highly specific for the molecules that can
enter (Ex: sodium channels allow Na+ to enter)
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
CHANNEL PROTEINS
Cystic Fibrosis – defective chloride ion channel iwhich
interrupts movement of water into and out of cell
- Results in thick mucus in the breathing passages and
pancreas
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
CARRIER PROTEINS
-
Bind a specific molecule and change shape to
transport them across cell membrane
-
Generally transport larger molecules (Ex: glucose,
amino acids)
-
Slower rate of diffusion than channel proteins
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
CARRIER PROTEINS
-
Exterior: non-polar amino acids
-
Interior: lined with amino acids that are specific to
the molecule being carried
PASSIVE TRANSPORT:
FACILITATED DIFFUSION
CARRIER PROTEINS
Cystinurea – inability for carrier proteins to remove
cystine and other amino acids from urine
- Cystine crystallizes into painful stones that block urine
flow
ACTIVE TRANSPORT
Active transport of substances across a cell membrane
against their concentration gradient
Requires energy usually in the form of ATP
ACTIVE TRANSPORT
ATP – Adenosine triphosphate
-
Derived from an adenosine nucleotide but with a
triple phosphate group
-
The hydrolysis of the end phosphate releases
energy
ACTIVE TRANSPORT
ATP – Adenosine triphosphate
ADP – Adenosine diphosphate
AMP – Adenosine monophosphate
ACTIVE TRANSPORT
Two types of active transport:
a) Primary Active Transport
b) Secondary Active Transport
ACTIVE TRANSPORTPRIMARY ACTIVE TRANSPORT
Cellular process using ATP directly to move molecules
or ions across a cell membrane
ACTIVE TRANSPORTPRIMARY ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
-
3 Na+ transported out of the cell against its gradient
-
2 K+ transported into the cell against its gradient
ACTIVE TRANSPORTPRIMARY ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
-
3 Na+ bind the ion pump on the inside of the cell membrane
-
ATP binds the ion pump and is hydrolysed to ADP + Pi
-
ADP is released and Pi attaches to ion pump
-
Ion pump changes shape
-
Na+ released outside cell and 2 K+ bind
-
Pi is released from the pump and the pump returns to its
original shape releasing 2 K+ inside cell
ACTIVE TRANSPORTPRIMARY ACTIVE TRANSPORT
SODIUM-POTASSIUM PUMP
ACTIVE TRANSPORTSECONDARY ACTIVE TRANSPORT
Ion pumps result in:
-
A difference in charge/electric potential
-
A difference in concentration gradient
*** Electrochemical gradient ***
ACTIVE TRANSPORTSECONDARY ACTIVE TRANSPORT
Use of an electrochemical gradient as a source of
energy to transport molecules or ions across a cell
membrane
ACTIVE TRANSPORTSECONDARY ACTIVE TRANSPORT
HYDROGEN-SUCROSE PUMP
-
H+ pumped out of the cell by a hydrogen ion pump
using ATP
-
Resulting electrochemical gradient powers
movement of sucrose against its gradient from
outside to inside of cell
ACTIVE TRANSPORTSECONDARY ACTIVE TRANSPORT
HYDROGEN-SUCROSE PUMP
MEMBRANE-ASSISTED TRANSPORT
Used to transport large macromolecules
Requires energy from cell
Two types:
a) Endocytosis
b) Exocytosis
MEMBRANE-ASSISTED TRANSPORT ENDOCYTOSIS
Cell engulfs material
Folds cell membrane around material and pinches off
to form a vesicle inside the cell
Three types:
a) Phagocytosis
b) Pinocytosis
c) Receptor-Mediated Endocytosis
MEMBRANE-ASSISTED TRANSPORT ENDOCYTOSIS
PHAGOCYTOSIS – cell eating
- Endocytosis with solid particles
MEMBRANE-ASSISTED TRANSPORT ENDOCYTOSIS
PINOCYTOSIS – cell drinking
- Endocytosis with liquid particles
MEMBRANE-ASSISTED TRANSPORT ENDOCYTOSIS
RECEPTOR-MEDIATED ENDOCYTOSIS
-
Receptor proteins are found on a portion of the cell
membrane (coated pit)
-
Receptor proteins bind molecules and pit folds
inwards to form a vesicle
-
Contents are used by cell or digested by cell
-
Receptor proteins are recycled into the cell
membrane
MEMBRANE-ASSISTED TRANSPORT ENDOCYTOSIS
RECEPTOR-MEDIATED ENDOCYTOSIS
MEMBRANE-ASSISTED TRANSPORT EXOCYTOSIS
Vacuoles fuse with cell membrane to release contents
outside the cell
Vesicle becomes part of cell membrane
MEMBRANE-ASSISTED TRANSPORT EXOCYTOSIS
Ex: Plant cells – used to construct cell wall
Animal cells – releasing hormones, neurotransmitters,
digestive enzymes, etc