1. The Cellular Basis of Life: Cell
Theory
• A cell is the basic structural & functional unit
Chapter 3: Cells: of living organisms
The Living Units • The activity of an organism depends both the
individual & collective activity of its cells
• Biochemical activities of cells are dictated by
the relative number of their specific
subcellular structures
• Continuity of life has a cellular basis
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Cell Diversity The Composite Cell
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The Plasma Membrane: Structure
• Very thin, selectively
permeable phospholipid
bilayer
• Membrane proteins:
surface receptors,
pumps, channels,
cytoskeletal anchors
• Membrane
carbohydrates- Cell
recognition, “ID tags”
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2. Transport:
The Plasma Membrane: Functions Protein channels and pumps transport specific
substances into the cell
• Defines the cell’s boundary
• Selectively permeable membrane that Enzymatic Activity:
transports substances in and out of the Some enzymes are at the membrane, supplying
cell products where they are needed
• Generates and maintains the resting
membrane potential
Signal Transduction:
• Mediates cell-cell and cell-environment Some membrane proteins are receptors that
interactions receive external signals and initiate cascades of
chemical reactions
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Intercellular Joining:
Cell adhesion molecules (CAMS) provide binding
Plasma Membrane Special Structures
sites for cell-cell interactions
Cell-Cell Recognition:
Some glycoproteins serve as cellular “ID” tags that
allow cells to recognize each other
Attachment to the cytoskeleton and
ECM:
Some membrane proteins act as anchors for the
cell, binding the cell to its environment and
supporting the cytoskeleton
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Special Structures: Tight Junctions Special Structures: Desmosomes
• Interlocking junctional • Plaque & linker proteins
proteins fused together to interdigitate like a zipper
form a relatively • Allow for cell sheets to
impermeable barrier around form
cells • Also prevents cell sheets
• Prevents molecules from subjected to mechanical
passing between adjacent stress from tearing
cells • Examples: Skin, heart
• Example: Digestive tract muscle
lining to prevent “seeping”
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3. Special Structures: Gap Junctions Membrane Transport
• Cells are surrounded by extracellular fluid/
• Communicating junction
between cells interstitial fluid containing nutrients, amino acids,
sugars, fatty acids, vitamins, etc.
• Connexons are
transmembrane proteins that
form channels that span • Cells need to extract essential substances from
across adjacent cells interstitial fluid in order to survive
• Essential for excitable tissue
that require rapid •Traffic across the plasma membrane is constant but
communication in order to is also selective on what substances are introduced
synchronize activities into the cell
• Examples: Cardiac &
smooth muscle
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•Transport is either passive or active 14
Membrane Transport Diffusion
• Passive Processes
– Diffusion
• Simple Diffusion
• Facilitated Diffusion (channel & carrier-mediated)
• Osmosis
– Filtration
• The tendency of molecules or ions to scatter evenly
• Active Processes throughout the environment
– Active Transport • Molecules and ions move down their concentration
– Vesicular Transport gradients until equilibrium is reached
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[High]  [Low] 16
Diffusion Simple Diffusion
Molecules diffuse through the membrane if:
1)Lipid-soluble • Small, non-polar, lipid-soluble
2)Small substances diffuse directly
3)Assisted by a carrier molecule through the lipid bilayer from
regions of high concentration
Types of Diffusion: to low concentration
1)Simple Diffusion: Unassisted diffusion
2)Facilitated Diffusion: Assisted diffusion
a)Channel-mediated • Examples: Oxygen, Carbon
b)Carrier-mediated dioxide, fat-soluble vitamins
3) Osmosis: Diffusion of solvents (water)
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4. Channel-Mediated Facilitated Diffusion Carrier-Mediated Facilitated Diffusion
• Protein channels form • Large, polar, lipid-
“tunnels” that allow small, lipid- insoluble molecules such
insoluble molecules (water, as sugars and amino
ions) into the cell acids bind to protein
carriers and are “shuttled”
across the membrane
• Channels are selective, only
allowing molecules of certain
sizes and charges to pass • Carrier membrane
through proteins are molecule-
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specific 20
Osmosis Diffusion –vs- Osmosis
• Specific to the movement of
water (a solvent) through a
membrane from regions of high
water concentrations to regions
of low water concentrations
• Osmotic diffusion of water may
be directly through the lipid
bilayer or through aquaporins
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Tonicity Filtration
Hypertonic: Osmotic pressure
outside the cell is higher, water • Movement of molecules
leaves cell faster than it enters through membranes
from regions of high
hydrostatic pressure
Isotonic: Osmotic pressure in and to regions of lower
out of cell are equal, water enters hydrostatic pressure
and leaves cell at same rate • Separation of solids
from fluids
Hypotonic: Osmotic pressure • Generally applies only
inside cell the higher, water enters to capillary walls
the cell faster than it leaves 23 24
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