Transcript 3. Cell membranes

Cell Boundaries and
Transport
Objectives
• Explain the role of the cell membrane in
supporting cell functions
• Draw and label the parts of the cell
membrane
• Compare and contrast active and passive
transport
• Describe how tonicity is a measure of the
osmotic pressure gradient (Isotonic,
Hypertonic and Hypotonic)
About Cell Membranes
• All cells have a cell
membrane
• Functions:
a.Controls what enters
and exits the cell to
maintain an internal
balance called
homeostasis
b.Provides protection and
support for the cell
TEM picture of a
real cell membrane.
Why do cells need to control
what enters and exits?
• Plasma membrane – boundary between
the cell and its environment
• Homeostasis – maintaining the cells
environment
• Cells need to bring in nutrients, get rid of
wastes, and keep harmful molecules out
• The plasma membrane also supports the
cytoplasm, recognizes foreign material,
and communicates with other cells.
Structure of cell membranes
• The cell membrane is primarily composed
of a mix of proteins and lipids.
– Lipids help to give membranes their flexibility
– Proteins monitor and maintain the cell's
chemical climate and assist in the transfer of
molecules across the membrane.
About Cell Membranes (continued)
• Structure of cell membrane
Lipid Bilayer -2 layers of
phospholipids
a.Phosphate head is polar
(water loving)
b.Fatty acid tails are nonpolar (water fearing)
c.Proteins embedded in
membrane
Phospholipid
Lipid Bilayer
Polar heads Fluid Mosaic
love water
Model of the
& dissolve. cell membrane
Non-polar
tails hide
from water.
Other Components
• Cholesterol – stabilizes phospholipids, keeps
fatty acid tails from sticking together
• Cell-Surface Markers – chain of sugars
attached to proteins (glycoproteins) that
identifies each cell type
• Receptor Proteins – bind with substances
outside of cell causing changes in the cell
• Enzymes – catalyze chemical reactions
• Transport Proteins – help movement of
substances into and out of the cell
About Cell Membranes (continued)
• 4. Cell membranes have pores (holes) in it
a. Selectively permeable: Allows some
molecules in and keeps other molecules out
b. The structure helps it be selective!
Pores
Structure of the Cell Membrane
Outside of cell
Proteins
Lipid
Bilayer
Transport
Protein
Animations
of membrane
Go to structure
Section:
Phospholipids
Inside of cell
(cytoplasm)
Cell Boundaries: Cell Wall
Plants, algae, fungi and many prokaryotes have this
additional barrier outside their plasma membrane. It
provides additional protection and support for the
cell. The wall is made of carbohydrate and protein
fibers like cellulose. Cellulose is the principle
component of wood and paper!
Which of the four organic compounds
does a book belong to?
Cell Limitations
– A solution is a mixture
of 2 or more
substances
– The liquid doing the
dissolving is the
solvent (usually water)
– The solid being
dissolved is the solute
– The concentration is
the amount of solute in
the solution
Types of Cellular Transport
•Animations of Active
Transport & Passive
Transport
•
Weeee!!
!
Passive Transport
cell doesn’t use energy
1. Diffusion
2. Facilitated Diffusion
3. Osmosis
•
high
low
Active Transport
cell does use energy
1. Protein Pumps
2. Endocytosis
3. Exocytosis
This is
gonna
be hard
work!!
high
low
Passive Transport
•
•
•
cell uses no energy
molecules move randomly
Molecules spread out from an area of
high concentration to an area of low
concentration.
• (HighLow)
Three types
3 Types of Passive Transport
1. Diffusion
2. Facilitative Diffusion – diffusion
with the help of transport
proteins
3. Osmosis – diffusion of water
Passive Transport:
1. Diffusion
Simple Diffusion
Animation
1. Diffusion: random movement
of particles from an area of
high concentration to an
area of low concentration.
(High to Low)
•
Diffusion continues until all
molecules are evenly spaced
(equilibrium is reached)-Note:
molecules will still move around
but stay spread out.
http://bio.winona.edu/berg/Free.htm
Examples of diffusion in your body
• Diffusion occurs throughout the human
body
– Absorption of food in digestive system
(stomach, intestines)
– Absorption of nutrients from food into cells
– Diffusion of oxygen through the cornea in eyes
– Medicine from pills are absorbed into the lining
of the stomach and then into the bloodstream
(both processes of diffusion)
Passive Transport:
2. Facilitated Diffusion A
2. Facilitated diffusion:
diffusion of specific particles
through transport
proteins found in the
membrane
a.Transport Proteins are
specific – they “select”
only certain molecules
to cross the membrane
b.Transports larger or
charged molecules
Facilitated
diffusion
(Channel
Protein)
Carrier Protein
B
Diffusion
(Lipid
Bilayer)
Passive Transport: 2. Facilitated Diffusion
Glucose
molecules
Cellular Transport From aHigh Concentration
High
• Channel Proteins
animations
Cell Membrane
Low Concentration
Through a 
Go to
Section:
Transport
Protein
Protein
channel
Low
Passive Transport:
3. Osmosis
Osmosis
animation
• 3.Osmosis: diffusion of
water through a
selectively permeable
membrane
• Water moves from high
to low concentrations
•Water moves freely
through pores.
•Solute (green) to large
to move across.
The diffusion of solvent molecules,
such as water, is known as
osmosis
water
glucose
Solvent
molecules
e.g. water
Solute
molecules
e.g.glucose
Water is the solvent used in nature
Osmotic concentration relates to
the amount of dissolved solutes in
a solution
high osmotic concentration
(concentrated)
low osmotic concentration
(dilute)
Insoluble molecules do not affect the osmotic concentration
Partially permeable membranes
only let water and very small
molecules through
A partially permeable membrane results in an uneven distribution
across the membrane
Low osmotic
concentration:
A dilute solution
with a high water
potential
High osmotic
concentration:
A concentrated
solution with a
low water
potential
Which way will osmosis occur?
water
glucose
NET osmosis
high osmotic concentration
low osmotic concentration
partially permeable membrane
Active Transport
•cell uses energy (ATP) to actively move
molecules to where they are needed
•Carrier proteins in the cell membrane
•Movement from an area of low concentration
to an area of high concentration . Against the
concentration gradient
•Low  High concentration
Three Types
Types of Active Transport
1. Protein Pumps transport proteins that
require energy to do
work
•Example: Sodium /
Potassium Pumps
are important in nerve
responses.
Sodium
Potassium Pumps
(Active Transport
using proteins)
Protein changes
shape to move
molecules: this
requires energy!
Active Transport
• Shape of carrier protein is complimentary to molecule
they carry
• Binding
• Molecule transported across membrane
• 1 way flow-molecule can only fit into carrier protein
on 1 side of membrane
Other two types of Active Transport:
Bulk Transport-Moving large amounts
• Moving large quantities- in & out:
Endocytosis- & Exocytosis
• Fusing & pinching off of membranes
• Used to form vesicles & to move vesicles around
the cell
•Examples:
• Hormones e.g. insulin released into blood by pancreatic
cells
• Plant cells-vesicles carry materials to make cell wall
• WBC engulfing foreign invaders & fusion with lysosomes
Types of Active Transport
2. Endocytosis: taking bulky
material into a cell
• Uses energy
• Cell membrane in-folds
around food particle
• “cell eating”
• forms food vacuole &
digests food
• This is how white blood
cells eat bacteria!
Types of Active Transport
3. Exocytosis: Forces
material out of cell in bulk
• membrane surrounding the
material fuses with cell
membrane
• Cell changes shape –
requires energy
• EX: Hormones or
wastes released from
cell
Endocytosis &
Exocytosis
animations
Review:
Active vs. Passive Transport
Active
Passive
Energy needed?
 YES, in the form of ATP
Energy needed?
 No
When
 If diffusion cannot meet
needs of cell/is not quick
enough
When
 To transport molecules
across cell membrane
high-low conc.
Tonicity
• Tonicity is a measure of the osmotic pressure
gradient (as defined by the water potential of the
two solutions) of two solutions separated by a
semipermeable membrane.
• Tonicity is influenced only by solutes that cannot
cross the membrane, as only these exert an
osmotic pressure.
– Solutes able to freely cross the membrane do not
affect tonicity because they will always be in equal
concentrations on both sides of the membrane.
http://www.youtube.com/watch?v=6MWl3DCa2uM
Homeostasis
• Defined as one of the fundamental
characteristics of all living systems
• It is the tendency of an organism to
maintain a stable, constant internal
environment
• Maintained by various organ systems in
the body
– An organ system is a group of organs that
works together to perform a common function
Effects of Osmosis on Life
• Osmosis- diffusion of water through a
selectively permeable membrane
• Water is so small and there is so much
of it the cell can’t control it’s movement
through the cell membrane.
Osmosis and Plant Cells
Plant cells placed in solutions with different water concentrations:
Lower water
concentration outside
cell
Water leaves the cell
by osmosis.
Cell membrane pulls
away from cell wall.
The cell is described as
being plasmolysed.
Same water
concentration outside
cell
Water constantly enters
and leaves the cell.
Higher water
concentration outside
cell
Water enters the cell
by osmosis.
Cell membrane pushes
against the cell wall.
The cell is described as
being turgid.
Osmosis and Animal Cells
Animal cells placed in solutions with different water concentrations:
Lower water
concentration outside
cell
Same water
concentration outside
cell
Higher water
concentration outside
cell
Water
Water leaves the cell
by osmosis.
Animal cells shrink.
Water constantly enters
and leaves the cell.
Water enters the cell by
osmosis.
Cell expands and can
burst, this is known as
lysis.
Unlike plant cells, animal cells do not have a strong cell wall to protect them.
•
Hypotonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Hypotonic: The solution has a lower concentration of
solutes and a higher concentration of water than
inside the cell. (Low solute; High water)
Result: Water moves from the solution to inside the
cell): Cell Swells and bursts open (cytolysis)!
Hypotonic solutions have a lower
osmotic concentration than the
cytoplasm of the cell
external solution
blood cell
NET osmosis into the cell
Can cause animal cells to burst
•
Hypertonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Hypertonic: The solution has a higher concentration
of solutes and a lower concentration of water than
inside the cell. (High solute; Low water)
shrinks
Result: Water moves from inside the cell into the
solution: Cell shrinks (Plasmolysis)!
Hypertonic solutions have a higher
osmotic concentration than the
cytoplasm of the cell
external solution
blood cell
NET osmosis out of the cell
Causes animal cells to shrink
•
Isotonic Solution
Osmosis
Animations for
isotonic, hypertonic,
and hypotonic
solutions
Isotonic: The concentration of solutes in the solution
is equal to the concentration of solutes inside the cell.
Result: Water moves equally in both directions and
the cell remains same size! (Dynamic Equilibrium)
Isotonic solutions have the same
osmotic concentration as the
cytoplasm of the cell
external solution
blood cell
No NET osmosis
Review: Diffusion and Osmosis
– Isotonic – concentration of solute is the same
both in and out of cell, water movement is equal
– Hypertonic – concentration of solute is greater
outside the cell, water moves to the outside of
cell to balance out (possibly resulting in cell
shriveling)
– Hypotonic - concentration of solute is lower
outside the cell, water moves into the cell to
balance out (possibly resulting in cell explosion)
Blood cells in animals are isotonic and have roughly the same amount of
dissolved materials inside the cell as surrounding the cell
Plant cells have cell walls that prevent the cell from swelling and
bursting when in contact with water
Respiration and Diffusion
• Respiration depends heavily on
both the circulatory system and
the process of diffusion.
• The passage of air into your
lungs comes in through the
nasal cavity  trachea 
branching system of bronchial
tubes in each lung  alveoli
• The alveoli are lined with
capillaries so that O2 you
breathe in can enter the
bloodstream and CO2 waste can
leave the blood stream.
• Like most animals, your lungs
are lined with moist mucus
providing a wet environment for
gas exchange
Diffusion and Osmosis… a review
• Why does the CO2 leave the blood stream and O2
enter?... Diffusion
• Diffusion is a form of passive transport (no
energy required) in which substances flow from
an area of high concentration to an area of low
concentration. Substances such as O2 and CO2,
are small enough to pass through the walls of the
capillaries.
• A specific type of diffusion is osmosis, or the
movement of water from high to low
concentration due to a change in conditions
Diffusion and osmosis in the kidney
• The main function of the
kidneys is to filter our blood
and remove waste as urine.
Both kidneys do the same
job. Blood is taken to the
kidneys by the renal artery
and when it is cleaned, it is
returned to the heart by the
renal vein. The urine is
taken to the bladder by the
ureters.
Diffusion and Osmosis in the kidney
• Maintaining water balance in the kidney can be inhibited
by a diuretic, a substance that causes the kidneys to
allow excess water loss. A very common diuretic is
caffeine.
– Diuretics block the production of the hormone vasopressin,
an anti-diuretic hormone (ADH). When the blood has a
high concentration of sodium ions, the hypothalamus and
pituitary gland (both in the brain) triggers the release of
ADH which would tell the kidneys to reabsorb more water
to help rehydrate the body. This is an example of
maintaining homeostasis with the endocrine system. The
endocrine system is made up of glands and cells that
release chemical messengers called hormones, directly
into the blood stream.
What type of solution are these cells in?
A
B
C
Hypertonic
Isotonic
Hypotonic
How Organisms Deal
with Osmotic Pressure
•
Paramecium
(protist) removing
excess water
video
•Bacteria and plants have cell walls that prevent them
from over-expanding. In plants the pressure exerted on
the cell wall is called tugor pressure.
•A protist like paramecium has contractile vacuoles that
collect water flowing in and pump it out to prevent them
from over-expanding.
•Salt water fish pump salt out of their specialized gills so
they do not dehydrate.
•Animal cells are bathed in blood. Kidneys keep the
blood isotonic by remove excess salt and water.