Cell Theory
Cells & Membrane Transport
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Keri Muma
Bio 6
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Cells are the smallest structural and
functional units of life
All living organisms are composed of one or
more cells
Cells arise from other cells
Why do cells stay small?
Plasma Membrane
TO MAXIMIZE SURFACE-TO-VOLUME RATIO!
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Cells must be tiny for
materials to move in and out
of them fast enough to meet
the cell’s metabolic needs
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The plasma membrane separates the living cell
from its nonliving surroundings
General Functions
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Plasma Membrane
Plasma Membrane Structure
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Structure – fluid mosaic model
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Phospholipid bilayer
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Hydrophilic heads orientate
themselves towards the
extracellular and intracellular
fluid
Hydrophobic tails orientate
themselves inward, away
from the fluid
Barrier – separates extracellular fluid from
intracellular fluid
Selective permeability – controls what enters and
exits the cell
Cell markers and receptors – cell recognition,
binds hormones, cell communication
Adhesion – between other cell membranes or
extracellular materials
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Outside cell
Cholesterol – stabilizes the membrane
Glycocalyx – serve as biological markers
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Self vs. non-self
Cytoplasm
(inside cell)
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Plasma Membrane Structure
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Proteins
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Peripheral – attach to inner or outer surface
Integral – embedded in the membrane
Functions of membrane proteins:
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Cell adhesion molecules – play a
role in anchoring cells to each
other and the cytoskeleton,
responsible for stickiness of cells
Receptors– cell recognition, cell
signaling, binding of hormones
Enzymes – facilitate chemical
reactions on inner and outer
membrane surfaces
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Functions of membrane proteins:
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Transport
  Channels – water filled pathways that allow
select ions in/out of the cell
  Carrier proteins- transport select substances
across the membrane
Functions of the Membrane Proteins
c Enzymatic activity
b Cell signaling
a Attachment to
cytoskeleton and
extracellular
matrix
e Intercellular
joining
d Transport
Cytoskeleton
Plasma Membrane Transport
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f Cell-cell
recognition
Cytoplasm
Simple Diffusion
Passive transport – does not require energy to
move solutes across a membrane
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Diffusion – solutes move down their concentration
gradient until evenly distributed throughout the
solution
Due to kinetic energy of molecules
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Diffusion
Diffusion
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Simple diffusion – solutes diffuse across the
membrane unassisted
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Facilitated diffusion – proteins carry or assist
solutes across the membrane
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Small non-polar and lipid-soluble solutes
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Facilitated Diffusion
Step 1
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Carrier proteins
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Transports a specific
substance
Can reach saturation
when all binding sites
are occupied
(Transport maximum)
Other closely related
compounds can
compete for the same
binding sight
ECF
Concentration
gradient
Fick’s law of diffusion
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Factors that affect the rate of diffusion
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ICF
Binding sites exposed to ECF. Molecule to
be transported binds to carrier
Step 3
Direction of
transport
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ICF
Charged ions move through protein channels
Large molecules such as glucose or amino acids are
carried across
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the magnitude of the concentration gradient
the permeability of the plasma membrane to a
substance.
the surface area of the membrane across
which diffusion takes place
the molecular weight of a substance
the distance through which diffusion takes
place
temperature
Binding sites exposed to ICF). Transported
molecule detaches from carrier
Passive Transport
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Osmosis – diffusion of water across a
membrane down its concentration gradient
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Membrane permeable to both water and
solutes
Water moves by osmosis from the area of higher
water concentration to the area of lower water
concentration
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Membrane impermeable to solutes
Osmosis
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Applying these ideas:
Tonicity – ability of a solution to change the shape of a
cell by altering its internal water volume, depends on
concentration of non-penetrating solutes
  Isotonic solution – contains equal concentration solutes
as the cell
  Hypertonic solution – contains more solutes than the
cell
  Hypotonic solution – contains less solutes than the cell
10% Na
Cell
15% Na
Solution
Distribution of Solutes In Fluid Compartments
Passive Transport
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Filtration – water and solutes are pushed
across a membrane from an area of higher
pressure to an area of lower pressure
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Non-selective process, only large molecules cannot
pass
Examples: Blood pressure forces fluids out of
capillaries into tissues and the formation of filtrate in
the kidneys
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Osmotic equilibrium – total amount of solutes per
volume of fluid is equal
Chemical disequilibrium – some solutes are more
concentrated in one fluid compartment than another
(requires input of energy)
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Plasma Membrane Transport
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Active transport – movement of solutes
across the membrane requires energy (ATP)
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Occurs during movement of solutes against their
concentration gradient
Or very, very large molecules
Active Transport
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Solute pumping – proteins use ATP to transport
solutes against concentration gradient
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Primary active transport – energy is provided
directly by the hydrolysis of ATP
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Sodium-Potassium Pump
Sodium-Potassium pump – maintains a higher
concentration of potassium inside the cell and a
higher concentration of sodium outside the cell
Pumps 3 Na+ out and 2 K+ in
Solute Pumping
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Cotransporters – carrier proteins that transport two
or more substrates across a membrane
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Symport: moves two substrates in same direction
Antiport: moves two substrates in opposite
directions
Solute Pumping
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Secondary active transport – primary transport of
one molecule creates an ion gradient used to drive
another molecule against its concentration
gradient
Secondary
Transport in
the kidneys
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Active Transport
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Active Transport
Exocytosis - moves material from cell interior to
the extracellular space
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Vesicles fuse with the plasma membrane expelling the
contents out of the cell
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Phagocytosis
Endocytosis- moves material from extracellular
space into the interior of the cell
Pinocytosis (“cellular drinking”) a cell “gulps”
droplets of fluid
Phagocytosis (“cellular eating”) a cell engulfs a
particle
Endocytosis
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Receptor mediated endocytosis – receptors
bind specific substances and initiates
endocytosis
Receptor Mediated Endocytosis
Receptor Mediated Endocytosis
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Hypercholesterolemia
Normal
HH
Mild disease
Hh
Severe disease
hh
LDL (carries
cholesterol)
LDL receptor
(mops up LDL)
Homozygous
for ability to make
LDL receptors
Heterozygous
Homozygous
for inability to make
LDL receptors
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