Biology 
 Cell
Membranes and Homeostasis
 Passive Transport
• Diffusion
• Facilitated Diffusion
• Osmosis
 Active Transport
• Molecular Transport
• Bulk Transport (Endocytosis and Exocytosis)
 Function-
regulates what enters and
leaves the cell, and also protects and
supports the cell
 Components-
• Phospholipids
• Proteins
• Carbohydrates
 Phospholipids
• Consist of phosphate “head” water loving
• 2 fatty acid “tails” water fearing
• Forms a bilayer with heads facing out and tails
facing in
 Proteins
• “float” and move along the lipids
• Form channels and pumps that help to move
materials across the membrane
 Carbohydrates
• Usually attach to proteins
• Act like chemical “ID cards”; allow cells to
recognize one another.
 Fluid
• Proteins literally float among the lipids
 Mosaic
• So many different molecules make up the cell
membrane; it looks like a mosaic (a kind of art
that involves bits and pieces of different colors
or materials)
 Semipermeable
membrane:
• Only some things can enter the cell membrane,
other things cannot.
• Things that are too large or too strongly charged
will have difficulty entering, or will not be able
to enter at all
 All
living things want to maintain
homeostasis, relatively constant internal
physical and chemical conditions.
 To stay in balance, molecules in the cell
will move across the cell membrane until
equilibrium is reached.
 What
is passive transport?
• The movement of molecules from high
concentration to low concentration
• Requires no energy (ATP)
• Molecules move with the concentration gradient
• Eventually equilibrium is reached (but
molecules keep moving!)
 Types
of Passive Transport:
• Diffusion
• Facilitated diffusion
• Osmosis
 Diffusion
• The process by which particles move from an
area of high concentration to low concentration.
• How small, nonpolar molecules move across the
cell membrane.
• For example, O2 and CO2
 Facilitated
diffusion
• Molecules that cannot directly diffuse across the
membrane because of size, charge, or polarity
pass through special protein channels.
• For example, molecules like starch are too large,
and require proteins to diffuse into the cell.
 Osmosis
• The facilitated diffusion of water across a
selectively permeable membrane through
proteins called aquaporins.
• Water will move based on its concentration in
solution (solute + solvent):
 Isotonic
 Hypertonic
 Hypotonic
 Osmosis:
• Hypertonic
 Solute concentration is lower inside cell (solvent
concentration is higher inside cell) ;Water goes out
 Cell shrivels
 Causes plasmolysis in plant cells
 Osmosis:
• Isotonic Solution
 Equal concentration of solvent inside and outside of
cell; water goes in and out
 Cell’s volume remains the same; equilibrium
 Osmosis
• Hypotonic
 Solute concentration is greater inside the cell (solvent
concentration is lower inside the cell); water goes in
 Cell swells and may lyse
 Causes cytolysis in animal cells
 What
is active transport?
• The movement of molecules from low to high
concentration.
• Requires energy (ATP)
• Molecules move against the concentration
gradient
 Types
of active transport:
• Molecular transport
• Bulk transport
 Molecular
transport
• For example, sodium/potassium pump
• Small molecules and ions like sodium (Na+) and
potassium (K+) are carried across membrane
proteins
 Bulk
transport
• Moves larger molecules and even solid clumps
of material
• Involves the formation of a vesicle
• Types:
 Endocytosis
 Phagocytosis
 Pinocytosis
 Exocytosis
 Bulk Transport: Endocytosis
• Movement of large molecules or particles into
the cell. The cell membrane folds in, taking in
materials, and forms a vesicle to bring the
contents into the cell.
• Phagocytosis
 “cell eating” solid, can be food particles
• Pinocytosis
 “cell drinking” liquid particles
Endocytosis
 Exocytosis
• The vesicle carrying molecules or particles fuses
with the cell membrane, forcing the contents out
of the cell.