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Obtaining Ions, Nutrients and Water
 semipermeable membranes regulate cell
interaction with surroundings
→small openings (pores) allow certain
molecules to penetrate cell but denies
others making it semi
 Small black molecules
allowed through
 Large red molecules
NOT allowed through
WHY?
o
Plasma membrane is composed of a
phospholipid bilayer (two layers of
phospholipids or lipids with phosphates)
2 layers = 1 plasma membrane
♦
Phospholipids composed of:
• lipids (fats): glycerol group and two fatty acid
straight tails
>Nonpolar / hydrophobic (water-fearing),
can’t dissolve in water, found in interior of
plasma membrane
Phosphate head
Lipid tails
• phosphate group: phosphate (PO4)
>Polar / hydrophilic (water-loving) globular
heads, form outside of membrane (can be
exterior or interior side of cell)
>Can dissolve in water, allows plasma
membrane of cell to interact with watery
environment of cell.
Phosphate head
Lipid tail
• properties of phospholipids do NOT allow
water-soluble molecules to move through
membrane easily.
o
Proteins embedded within membrane move
about in a fluid fashion
♦ Fluid mosaic model explains how structural
properties of plasma membrane allow lipids &
proteins to move freely in membrane
Mosaic: something
consisting of variety of
components
o
Plasma membrane also contains:
♦ Transport proteins: allow substances or waste materials
to move through plasma membrane
♦ Proteins & carbohydrates: stick out from cell and help
cells identify each other
–Proteins: give cell flexibility
–Carbohydrates: act as identifier of cell type
 Cell
constantly interacts with environment
& has to respond to external conditions to
maintain homeostasis
Increase in heat makes
temperature rise
99.0ºF
98.8ºF
98.6ºF
Cell will seek to cool
itself down
Homeostasis at work!
Eating a bag of chips will cause cell
to respond to new salty condition
 Cell
membranes maintain homeostasis
by regulating passage of materials into
and out of the cell:
• Passive Transport – no energy required to
move solute or solvent from high to low
concentration (down concentration gradient)
• Active Transport – energy required to move
solute from low to high concentration (against
concentration gradient)
Outside Cell (16)
Inside Cell (4)
 concentration gradient = difference in amount of
substance present inside the cell versus outside
the cell
inside
outside
→Molecules will freely flow from high
concentration to low concentration
→Pathway from high to low concentration
is concentration gradient
→Eventual point where concentration is equal =
equilibrium (particles still randomly move
but equal in/out)
Concentration Gradient
High
Concentration
Low
Concentration
 Three
methods:
a. Simple diffusion – particles (solutes)
b. Osmosis – diffusion of water (solvent)
c. Carrier-facilitated diffusion –
particles (solutes) with protein
assistance

Simple diffusion – free movement of ions &
molecules through protein channels in membrane
Outside cell
Inside cell
 Simple
Diffusion – the homemade animation
o Osmosis – free movement or diffusion of
solvent (water) ONLY
Note how the red sugar particles stay put.
Only the blue water molecules move.
 Osmosis–
the
homemade animation
o three types of solutions that determine
direction of water :
♦ isotonic solution
♦ hypotonic solution
♦ hypertonic solution
♦
Isotonic Solution
> concentration of water on outside of cell is same
as on inside of cell
>Cells do not experience overall osmosis (or diffusion)
>retain their normal shape,
>Still random movement for an overall net gain of 0
90% H2O
10% particles
(inside cell)
90% H2O
10% particles
(outside cell)
♦
Hypotonic Solution
> concentration of water is higher outside cell than
concentration of water inside cell.
>Cells experience osmosis (and diffusion)
>water moves into cell
>cell swells and will burst if animal cell but will not
burst if plant cell due to cell wall
30% H2O
70% particles
(inside cell)
70% H2O
30% particles
(outside cell)
♦
Hypertonic Solution
> concentration of water is higher inside cell than
concentration outside cell.
>Cells experience osmosis (and diffusion)
> water moves out of cell
>Cells shrink
>Plant wilts because of a decrease in pressure on cell
wall
55% H2O
45% particles
(inside cell)
45% H2O
55% particles
(outside cell)
Hypertonic
Isotonic
Higher solute
Equal solute
OUTSIDE
Higher water
Equal water
INSIDE
Water moves OUT No net movement
Cell SHRINKS
Normal
Hypotonic
Higher solute
INSIDE
Higher water
OUTSIDE
Water moves IN
Cell SWELLS
Hypertonic vs. Isotonic vs.
Hypotonic Solutions
Hypertonic vs. Isotonic vs.
Hypotonic Solutions
Hypertonic vs. Isotonic vs.
Hypotonic Solutions
 Often
times, diffusion of large particles
can’t happen or happen fast enough
 Water will diffuse via osmosis to regulate
concentration (PERCENTAGES)
• Outside cell is 50% solute, 50% water
• Inside cell is 10% solute, 90% water
• Solute is too big to move. Will we die? No.
Water will move to get concentration equal
90% H2O
10% particles
(inside cell)
50% H2O
50% particles
(outside cell)
 Hint: look
at water %
only. Where will the
water go?
Question1: Which way will the particles move?
outside
Question 2: Which way will the water move?
inside
20% H2O
80% particles
(inside cell)
80% H2O
20% particles
(outside cell)
 free
movement of larger molecules with
aid (help) of a transport/carrier protein
• Large solutes (sugars, amino acids) are too big
to go through plasma membrane unaided
 Facilitated
animation
Diffusion – the homemade
 Needs
energy (ATP) to move particles
across plasma membrane
• Three types:
1. Active transport – small molecules AGAINST
concentration gradient (from low to high)
2. Endocytosis – large molecules being
engulfed by plasma membrane into vesicles
3. Exocytosis – large molecules being expelled
out by vesicles out of plasma membrane

Requires energy (ATP) to move materials against gradient
(low to high concentration)
♦ carrier protein allows specific molecule or ion to bind
oExample: sodium/potassium (Na/K) pump
♦ ATP changes shape of protein to move molecule or
ion across plasma membrane
ATP
STARRING
train as the CELL
platform as the
EXTRACELLULAR SPACE
people as the
PARTICLES
conductors as the ATP
 cell
surrounds material with
a portion of its plasma
membrane and forms a
vesicle
 Vesicle moves particle inside
cell for digestion
• Bacteria, food
Phagocytosis – large,
solid particles
Pinocytosis – small, liquid
particles
 Vesicle
brings material
to cell membrane &
expels (secretes) it
• Wastes, hormones