Download cell membrane notes

Cell Boundaries
I. Cell membranes essential for
A. helping living organisms
maintain homeostasis by controlling
what substances may enter or leave
cells (narrow range of pH and ionic
concentration for metabolic activities
to take place)
B. Maintaining cell’s integrity –
ensures a distinct identity (marker
proteins embedded in phospholipid
bilayer)
II. Cell membrane structure
A. Thickness – 7-9 nm (bilayer)
B. Selectively permeable – next slide
Small molecules like water, oxygen, &
carbon dioxide can move in and out
freely
Large molecules like proteins &
carbohydrates cannot move easily across
the plasma membrane
C. Chemicals found in membrane
-
Eukaryotic cell = phospholid, cholesterol,
glycolipids, carbohydrate chains
Prokaryotic cells = same except no
cholesterol
D. Fluid mosaic model
1. Structure is fluid – lipids and some proteins
move laterally within the membrane
Phospholipid Bilayer
a. Hydrophobic tails – two fatty acids
inward
b. Hydrophilic heads (phosphates) outward
c. Impermeable to ions and polar molecules
(except water)
d. Intergral proteins embedded – span the
membrane and protrude on either side
3. Cytoplasmic side of membrane
• Peripheral prtns attached to integral prtns
4. On environmental side of
membrane
a. Glycoproteins and glycolipids – short
carbo chains attached to proteins and
lipids
b. fxn of the carbo “coat” –
-
Recognition of molecules that interact w/ cell
(virus, hormones, antibody)
Adhesion process – cell to cell
Functions of proteins
a. Enzymes
b. Transport carriers – allow
movement substances across
membrane
c. Receptor sites – recognition
and binding of hormones
Variation in cell membrane and cell
types
• Vary from membrane to membrane and
place to place on same membrane
• Differences allow for unique properties of
cells
III Functions of cell membrane
• Controls flow of substances in and out of
cell
• Receives info that permits cell to sense
changes in its environment and respond
• Maintains structural and chemical
relationships w/ neighboring cells
• Protects the cell
III. Cell Wall Information
A. Located OUTSIDE of cell membrane
B. Found in:
1. Plants
a. Middle lamella
1) Formed as plant cell divides
2) Thin gluey layer between cells (pectin)
3) Holds cells together
• B. Primary cell wall
– On either side of the lamella
– Contains cellulose bundles
• C. Secondary cell wall (not always
present)
– Cannot expand
– Lignin is present if the cell’s main function is
support- stiffins wall, living material actually
dies
2. Algae – wall like plants
3. Fungi – similar but chitin instead
of cellulose
Prokaryotes – no cellulose,
peptidoglycans (amino acids and
carbs
V. Cell- Cell junctions
• A. Plasmodesmata – plant cells
– 1. cytoplasmic channels traverse cell walls
– 2. 20-60 nm diameter
– 3. pathway for water, ions and nutrients
– 4. desmotubules – tubular extensions of
endoplasmic reticulum
B. Gap Junctions
• Junctions through which cells can
exchange nutrients and molecular
communications (sometimes electrical)
• Appear as clusters of very small channels
(less than 2nm)
• Found in muscle cells of heart
Before doing this lab you should
understand…
• Mechanisms of osmosis and diffusion and
importance to cells
II. Diffusion
A. Diffusion is the movement of
molecules from an area of higher
concentration to an area of lower
concentration
B. Small molecules can pass through the
cell membrane by a process called
diffusion
C. Diffusion across a membrane is a type of
passive transport because it does not
require energy
D. This difference in the concentration of
molecules across a membrane is called a
concentration gradient
E. Kinetic energy keeps molecules in
constant motion causing the molecules to
move randomly away from each other in a
liquid or a gas
• The rate of diffusion depends on
temperature, size of the molecules, &
type of molecules diffusing
• H. Molecules diffuse faster at higher
temperatures than at lower temperatures
• I. Smaller molecules diffuse faster than
larger molecules
• K. Solutions have two parts --- the solute
which is being dissolved in the solvent
• L. Water serves as the main solvent in
living things
M. Diffusion always occurs down a
concentration gradient (water moves
from an area where it is more
concentrated to an area where it is less
concentrated)
N. Diffusion continues until the
concentration of the molecules is the
same on both sides of a membrane
O. When a concentration gradient no longer
exists, equilibrium has been reached but
molecules will continue to move
equally back & forth across a
membrane
Osmosis
• Think back to solutions:
solutes and solvents
• Osmosis is the process by which water
molecules diffuse across a cell membrane
from an area of lower solute concentration
to an area of higher solute concentration.
• Direction of movement depends on
relative concentrations of solutions.
•
http://www.colorado.edu/epob/academics/web_resources/osmosis/
Direction of movement into cells
• Hypotonic to the cytosol
– solution outside the cell is less concentrated than inside the cell
– water moves into the cell
• Hypertonic to the cytosol
– solution outside the cell is more concentrated than inside the cell
– water moves out of the cell
• Isotonic
– concentrations are equal
•
http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.
htm
Water potential
• Definition: measure of the tendency of
water to leave one place in favor of
another
• Important in the study of botany: Why?
Water potential equation
• Solute
potential is
zero or a
negative #
• Pressure
potential is
zero or a
positive #
Movement of water into or out of a cell depends
on the differences in water potential
Ex
Try example 1: Potato cell placed
into distilled water
INITIAL:
FINAL:
Water potential outside of cell:
Water potential outside of cell:
Water potential of potato cell
initially:
Water potential of potato cell
at end of observation:
Pressure in plant cells is called?
• Turgor pressure – keeps veggies crisp
• Plasmolysis – loss of water, wilty
Determining molarity of potato
slices using water potential
Equal water potential
• Water potential outside
of cell when placed in:
Unequal water potential
Water potential outside of
cell:
• Water potential of
potato cell initially:
Water potential of potato
cell final:
Osmosis- %change in mass (class)
Use to Calculate Class Average
1
DW
.2M
.4M
.6M
.8M
1M
2
3
AV.
Water potential - %change in mass
(class average)
1
DW
.2M
.4M
.6M
.8M
1M
2
3
AV.
PASSIVE VS ACTIVE
TRANSPORT
• PASSIVE
TRANSPORT
• No energy (ATP)
required!
• Molecules/ions move
down the
concentration
gradient
– Osmosis
– Facilitated Diffusion
• Protein Channels
• Carrier Proteins
• ACTIVE
TRANSPORT
• Energy required –
ATP NEEDED
• Molecules/Ions move
against the
concentration
gradient
– Protein pumps
• Na+/K+ pump
– Endo/Exocytosis
Active Transport
• Endocytosis
• Active Transport Movies – see
textbook online