Download chapter 9 homeostasis & the plasma membrane

Chapter 5
A Closer Look At Plasma
Membranes
Honors Biology

It Isn’t Easy Being Single? (why not)
• Concentrations of ions & other substances outside the cell may rapidly
become too high or low. A mechanism is needed to selectively permit
substances to enter or leave the cell.

What is homeostasis?
• Homeostasis is the name given to the dynamic processes that enable
optimum conditions to be maintained for constituent cells, in spite of
continual changes taking place both internally and externally.

What mechanism is needed to selectively permit substances to
enter or leave the cell?
• Phospholipid bi-layer with embedded proteins.

How does a cell maintain homeostasis?
• By controlling what enters and leaves the cell.

What are 2 examples of homeostasis with the human body?
Membrane Structure & Function
Phospholipid molecules
form a bilayer.
phospholipid molecule
Note: Do hydrophilic
substances have an easier time
passing thru the membrane? Phosphate/glycerol head is
NO
lipid bilayer
water
polar & hydrophilic, two
fatty acid tails are
nonpolar & hydrophobic.
Cholesterol
(right) is another
lipid molecule
that’s commonly
Question: What is the arrangement found in the
plasma
of the phospholipids?
membrane.
H–T- T-H
• What is the ‘Fluid Mosaic Model’ of the plasma
membrane structure?
•The lipid molecules are free to move and protein
molecules float independently in the lipid bi-layer.
open channel
proteins
gated channel
proteins
transport protein
lipid
bilayer
A wide variety of protein molecules float around in the
lipid bi-layer and perform most of the functions of the
membrane.
oligosaccharide groups
cholesterol
phospholipid
EXTRACELLULAR ENVIRONMENT
cytoskeletal
proteins (beneath
ADHESION
the plasma
PROTEIN
membrane)
open
gated
gated
active
channel
channel
channel
transport
protein
protein
protein
protein
(open)
(closed)
area of
enlargement
TRANSPORT
PROTEINS
RECEPTOR RECOGNITION
PROTEIN
PROTEIN
LIPID
BILAYER
7 Types of Membrane Proteins
(Note: Most of the plasma membrane functions are carried out by these proteins)
 1. Cytoskeletal Proteins - composed of
microtubules. Needed for internal support and
attachment.
 2. Adhesion Proteins - composed of
glycoproteins attached to oligosaccharides. Serve
as the “glue” to hold cells together.
 3. Open-Channel Proteins- needed for
passive transport and facilitated diffusion.
 4. Gated Channel Proteins - help control
the directional flow or transport of ions across
the membrane.
7 types of membrane proteins..cont.
 5. Carrier Proteins - carry on active
transport requiring energy (ATP) to “actively
pump” their cargo across the membrane.
 6. Receptor proteins - these proteins have
binding sites for hormones and enzymes, which
allow them to do their work .
 7.Recognition Proteins -these proteins
are the “fingerprints” of the cells. These selfrecognizing proteins identify their own body
cells or recognize invaders.
•What will happen in the example above?
•What causes the reaction?
•What energy creates the reaction below?
•When will the reaction stop?
Concentration Gradients
Once the molecules have
arranged themselves equally
throughout two adjoining
regions, it’s called “dynamic
equilibrium.”
At this point, a concentration
gradient no longer exists.
 Molecules are in constant
motion, due to their own
kinetic energy.
 If there are more molecules
of a substance in one area
than another, a
concentration gradient
exists.
 Random collisions of
molecules cause them to
move from an area where
they’re highly concentrated
to an area of low
concentration.
Diffusion
The net movement of like molecules down their
concentration gradient is known as diffusion.
The rate at which diffusion occurs can be affected by the
following factors:
(a) Molecular size – smaller molecules move faster than
larger ones.
(b) Temperature – heat energy causes molecules to move
more rapidly & to collide more frequently.
(c) Electrical gradients – the negative side of a membrane
will attract positive ions & repel negative ions.
(d) Pressure gradients – applying pressure can speed up
the rate at which molecules move.
Osmosis
selectively permeable membrane
 The movement of
water across a
selectively permeable
membrane in
response to
concentration
gradients, fluid
pressure, or both is
known as osmosis.
between two compartments
water
protein
molecules
molecules
Osmosis in a Plant Cell
Tonicity
 The relative
concentrations of
solutes in two fluids is
known as tonicity.
 Water tends to move to
areas where solute
concentrations are
greater.
 When solute
concentrations in two
fluids are equal, we say
the two fluids are
isotonic.
 In an isotonic solution,
there is no net
movement of water in
either direction.
Water moves from a hypotonic solution to a
hypertonic solution.
compartment
compartment
1
2
HYPOTONIC
SOLUTION
HYPERTONIC
SOLUTION
membrane permeable to
fluid volume increases
water but not to solutes
In compartment 2
Tonicity
2M sucrose
solution
HYPOTONIC
HYPERTONIC
ISOTONIC
CONDITIONS
CONDITIONS
CONDITIONS
1 liter of
10M sucrose
2M sucrose
distilled water
solution
solution
A hypotonic solution has
fewer solutes dissolved in
it than an adjoining
solution does.
A hypertonic solution has
more solutes dissolved in
it than an adjoining
solution does.
As a result, water
molecules will move from
a hypotonic solution to a
hypertonic solution.
Plasma membranes are selectively permeable.
(This means that some substances are able to enter the cell and others are not.)
O2, CO2, other small
nonpolar molecules,
as well as H2O
C6H12O6, other large, polar
water-soluble molecules,
ions (such as H+, Na+, K+,
Ca++, CI-) along with H2O
X
c
o
n
c
e
n
t
r
a
t
i
o
Passive Transport vs. Active Transport
high
low
DIFFUSION ACROSS
PASSIVE
ACTIVE
LIPID BILAYERS
TRANSPORT
TRANSPORT
lipid-soluble
Water-soluble substances, and water,
Specific solutes are
substances as
diffuse through interior of transport
pumped through interior
well as water
proteins. No energy boost required.
of transport proteins.
diffuse across
Also called facilitated diffusion
Requires energy boost
SPECIALIZED TYPE OF VACUOLE:
CONTRACTILE VACUOLES: PROTISTS HAVE THESE
ORGANELLES - THEY COLLECT EXCESS WATER
AND EXCRETE IT SO THE CELL DOESN’T BURST.
contractile
(cytolysize)
vacuole
(emptied)
contractile
vacuole (filled)
 PLASMOLYSIS: LOSS OF WATER FROM A CELL
RESULTING IN A DROP IN TURGOR PRESSURE.
 Turgor pressure: Water pressure inside the plant cell.
(turgidity)


PASSIVE TRANSPORT:
REQUIRES NO ENERGY TO MOVE SUBSTANCES
ACROSS A MEMBRANE = DIFFUSION , EXAMPLE:
WATER, NON-POLAR MOLECULES, AND LIPIDSOLUBLE SUBSTANCES.
 LIPID BILAYER KEEPS MOST SUBSTANCES OUT
BECAUSE THEY’RE REPELLED OR TOO LARGE.
FACILITATED DIFFUSION:
 PROTEINS PROVIDE CONVENIENT OPENINGS FOR
PARTICLES TO PASS THROUGH - SUGARS AND
AMINO ACIDS ARE MOVED THIS WAY...
 THIS SPEEDS UP THE DIFFUSION RATE!
Facilitated Diffusion
ACTIVE
TRANSPORT
(Requires energy)
 FROM LOW CONC. TO HIGHER CONC.
 In active transport, SOLUTES MOVE AGAINST THE
CONCENTRATION GRADIENT!
 Ex- Sodium-Potassium Pump-helps nerve cells transfer
electrical impulses by maintaining an electrical gradient on
the plasma membrane.
 Ex- Calcium Pump - helps keep the calcium concentration
high in cells.

ENDOCYTOSIS:
a form of active transport in which the cell
uses energy to bring materials into the cell by engulfing.
 A. PHAGOCYTOSIS: (“CELL EATING”) Cell takes in large
molecules by pinching in the plasma membrane.
 B. PINOCYTOSIS: (“CELL DRINKING”) Cell takes in fluid
by pinching in the plasma membrane.
Exocytosis
Active Transport
 EXOCYTOSIS: a form of active transport in which
the cell excretes wastes or secretes substances needed
elsewhere in the organism. These substances do not
move through the plasma
Slide 19membrane itself.
EXOCYTOSIS
Vesicle in cytoplasm moves to plasma membrane,
fuses with it; contents released to the outside