Transport Across Membranes
2.4
Importance of Transport
• intake of nutrients
• removal of wastes
• communication with environment &
other cells
• blocking passage of harmful substances
Passive Transport
• the movement of materials across the
cell membrane without the use of
chemical energy (ATP)
• occurs because of diffusion
Diffusion
• the net movement of
a substance from an
area of higher
concentration to an
area of lower
• dynamic equilibrium
Simple Diffusion
• substances move across membrane
unassisted
• small non-polar molecules (O2, CO2,
steroid hormones, some drugs) and
small polar molecules (H2O, glycerol)
• larger molecules and ions can not pass
through membranes unassisted
Facilitated Diffusion
• diffusion across membrane assisted by
integral membrane proteins called
transport proteins
• channel proteins vs. carrier proteins
• ions, water, amino acids, sugars, etc.
Osmosis
• the diffusion of
water across a
membrane
• water follows
concentration
gradient until
equilibrium
Osmosis
• direction of osmosis changes depending
on type of solution surrounding the cell:
• isotonic solution
• hypotonic solution
• hypertonic solution
Cell in Isotonic Solution
10% NaCL
90% H2O
ENVIRONMENT
CELL
10% NaCL
90% H2O
NO NET
MOVEMENT
What is the direction of water movement?
equilibrium
The cell is at _______________.
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10
Cell in Hypotonic Solution
10% NaCL
90% H2O
CELL
20% NaCL
80% H2O
What is the direction of water movement?
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11
Cell in Hypertonic Solution
15% NaCL
85% H2O
ENVIRONMENT
CELL
5% NaCL
95% H2O
What is the direction of water movement?
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12
Cells and Tonicity
Red Blood Cells
hypotonic
hypertonic
isotonic
hypertonic
isotonic
hypotonic
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15
How does this impact…
• …a single-celled organism living in a
freshwater environment?
• …a single-celled organism living in a
salt-water environment?
• …the roots of a plant in the spring after
salt has been sprinkled during the
winter?
More applications…
• Why can’t you water a plant with salt
water?
• Why do vegetables in the grocery store
get sprayed with water periodically?
(What would happen if they were
sprayed with salt water?)
Active Transport
Active Transport
• the movement of materials against the
concentration gradient
• requires cellular energy (ATP)
Primary Active Transport
• pumps move
positively charged
ions (H+, Ca2+, Na+,
K+) across
membranes
• creates
electrochemical
gradient
Secondary Active Transport
• uses gradient established by a primary
active transport pump
Bulk Transport
• movement of larger substances across
a cellular membrane
• requires energy (ATP)
• exocytosis & endocytosis
Endocytosis
Exocytosis
Credits
• Slides #10, 11, 12, and 15 taken from
Powerpoint presentations created by
Cheryl Massengale, educator
extraordinaire & creator of
http://www.biologyjunction.com/