Cell Membranes are Gatekeepers
PLASMA MEMBRANE
• Functions:
– Helps cells maintain
balance (homeostasis)
– Holds contents of the cell
in place
– Takes food and nutrients
(glucose, amino acids,
and lipids)
PLASMA MEMBRANE
• Selectively Permeable
– Plasma membrane allows
some molecules to pass in
while keeping others out
– Water: in and out freely
PLASMA MEMBRANE
Phospholipid molecule
Hydrophilic
Head = Glycerol backbone
Hydrophobic
Tail = 2 fatty acid chains
Polar head (includes
phosphate group)
Love water
Nonpolar tails
(fatty acids)
Fear Water
Organization of Plasma Membrame
PLASMA MEMBRANE
• Cholesterol:
– Stabilizes phospholipids by preventing
fatty acid tails from sticking together
Cholesterol
Molecule
Plasma membrane are made up of two layers that are
filled with variety of pores, molecules, and channels
Receptor Proteins:
Trigger a set of cell
responses when specific
molecules bind to cell.
Recognition Proteins
ID tags and cell
surface
attachment sites
“belongs” or
“doesn’t belong”
Enzymatic Proteins
Accelerate
chemical
reactions on
membrane
surface
Transport
Proteins
Allows needed
large substances
or waste materials
to cross
membrane
CELLULAR TRANSPORT:
OSMOSIS
• Concentration gradient, areas of high
and low concentrations
• Salt, sugar, etc. too large to pass
through a membrane
• Diffusion: Movement
From high to low
concentration
CELLULAR TRANSPORT: OSMOSIS
• During osmosis, water diffuses across
selectively permeable membrane.
Notice number of sugar molecules did
not change on each side of membrane
but number of water molecules did.
CELLULAR TRANSPORT:
OSMOSIS
• Hypertonic solution
• water leaves cell by
osmosis
• Cell shrinks
CELLULAR TRANSPORT:
OSMOSIS
• Hypertonic Solution: Concentration of
dissolved substances (Ex: Salt) is higher
outside of cell than inside
• Water flows out of cell to balance out
high concentration of dissolved particles
outside of cell.
CELLULAR TRANSPORT:
OSMOSIS
• Hypotonic Solution: Concentration of
dissolved substances (Ex: Salt) is lower
in the solution outside of cell than inside
• Water flows into cell
CELLULAR TRANSPORT:
OSMOSIS
• In hypotonic solution,
water enters cell by
osmosis causing cell to
swell—could burst.
Cell Membranes: Part 2
CELLULAR TRANSPORT:
OSMOSIS
• Isotonic Solution: Concentration of
dissolved substances in the environment
is same as concentration of dissolved
substances inside cell.
• Water molecules move into and out of cell
at same rate
• Cells maintain normal shape
PASSIVE TRANSPORT
• Simple Diffusion with
concentration gradient
• Requires no energy
PASSIVE TRANSPORT
• FACILITATED DIFFUSION:
– Transport proteins help substances move
through plasma membrane
• Channel proteins
• Diffusion = no energy used
ACTIVE TRANSPORT
• Against concentration gradient
– From lower concentration to higher
• Requires energy
• Sodium-Potassium Pump:
– transports sodium out of the cell and potassium into the
cell against their concentration gradients.
ACTIVE TRANSPORT
• Carrier proteins used in active transport
to pick up ions or molecules near cell
membrane and carry them across
membrane releasing them on other side.
TRANSPORT OF LARGE
PARTICLES
TRANSPORT OF LARGE
PARTICLES
• Endocytosis: Cell surrounds and takes in
material from environment
– Material does not pass directly through
membrane
– Is engulfed and enclosed by portion of cell’s
plasma membrane
– Portion of membrane breaks away, leaving
vesicle with contents which moves into cell
TRANSPORT OF LARGE
PARTICLES
– Endocytosis:
Ingest Food (Take In)
• _phagocytosis________
Ingest liquid
• __pinocytosis__________
– Exocytosis: Release of Wastes, Secretions
TRANSPORT OF LARGE
PARTICLES
TRANSPORT OF LARGE
PARTICLES
• Exocytosis: Expulsion/Release of
__large___ materials from cell
– Used to excrete wastes
– Also used to secrete hormones produced by
cell
Let’s break it down…
Transport through the Cell Membrane
Type of
Transport
Transport
Protein
Used?
Direction of
Movement
Requires
Energy Input
from Cell?
Classificatio
n of
Transport
Simple
Diffusion
No
With
Concentration
Gradient
No
Passive
Facilitated
Diffusion
Yes—channel
proteins or
carrier
proteins
With
concentration
gradient
No
Passive
Active
Transport
Yes—carrier
proteins
Against
concentration
gradient
Yes
Active