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Cell Transport
Cell Membrane Review
• One of the primary functions of the cell
membrane is to allow the passage of materials
into and out of the cell.
• Many materials pass through the membrane
without any assistance.
• Other materials, however, require proteins to
either passively facilitate their transport or
actively pump them against their concentration
gradient (going from low to high concentration).
Cell Membrane Review
Diffusion
• Diffusion is the movement of molecules from regions of
high concentration to low.
▫ Molecules move with their concentration gradient.
• Molecules will randomly move with the conc gradient
until equilibrium is reached.
▫ Once equilibrium is reached, molecules continue to move;
there’s just no net movement in one direction or another.
• Robert Brown conducted the first studies in random
particle motion – Brownian motion.
▫ From his studies, we first began to understand diffusion and
its role in molecular motion.
• Brownian motion
• Is diffusion a form of active or passive transport?
• Diffusion (4:57)
Osmosis
• Osmosis is the diffusion of H2O across a
selectively permeable membrane (like the cell
membrane)
• Because osmosis is a form of diffusion, does the
H2O move with or against the conc gradient?
• Is osmosis active or passive?
• Osmosis continues until equilibrium is reached.
Iso-, Hypo-, and Hypertonic Solutions
• Isotonic: The conc of H2O inside the cell is equal
to the conc outside the cell
▫ Equilibrium has been reached.
▫ Most cells are in isotonic solutions, like blood, so
they neither shrink nor swell
▫ Osmotic pressure is equal on both sides of the
membrane
Iso-, Hypo-, and Hypertonic Solutions
• Hypotonic: The conc of H2O inside the cell is less
than the conc outside the cell
▫ H2O moves into the cell causing the cell to swell
▫ If the conc difference is great enough, the cell will
cytolyze (burst).
▫ Osmotic pressure is greater outside the cell
▫ Plant and bacterial cells are often in fresh water
(hypotonic solution), but they do not cytolyze.
Why not?
Iso-, Hypo-, and Hypertonic Solutions
• Hypertonic: The conc of H2O inside the cell is
greater than the conc outside the cell
▫ H2O moves out of the cell causing the cell to
shrink
▫ If the conc difference is great enough, the cell will
plasmolyze.
▫ Osmotic pressure is greater inside the cell
• Blood Cells in Solutions (:58)
Iso-, Hypo-, and Hypertonic Solutions
Osmosis and Tonicity (9:27)
Facilitated Diffusion
• Facilitated diffusion is a form of passive transport
that uses a protein (facilitator) to move materials
through the cell membrane.
• Even though a protein is needed, it’s still a form of
diffusion, which means materials move from high to
low conc.
• Why do some molecules need a facilitator protein?
• The proteins are highly specific, so therefore only
one compound/molecule can pass through only one
particular protein.
▫ It’s kinda like enzymes and their substrates.
• Facilitated Diffusion (1:18)
Active Transport
• Active transport is a form of transport that
moves materials against the conc gradient (from
low to high conc)
• In order to move against the gradient, energy is
required. Energy molecule required?
• Protein “pumps” are used to move small or
large, charged or neutral particles through the
membrane.
• Two types of active transport: endocytosis and
exocytosis
Endocytosis
• Endocytosis is the process of taking in materials
via infoldings of the cell membrane
▫ Two types: phagocytosis and pinocytosis
▫ Phagocytosis is taking in solid particles, while
pinocytosis is taking in liquids
▫ In both processes, the cytoplasm extends and
takes in either solids or liquids
Phagocytosis
Pinocytosis
Vesicle
formation
(1:52)
Exocytosis
• Exocytosis is the process by which cells expel
substances.
• A vesicle or vacuole from within the cell fuses
with the membrane and then expels its material
into the extracellular fluid.
Exocytosis
Transport Summary 1
(6:13)
Transport Summary 2
(3:19)