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7.3 Cell Boundaries
Boundary
1. Among the most important parts of a cell
are its borders, which separate cell from
surroundings.
2. All cells are surrounded by a thin, flexible
cell membrane. (plasma membrane)
3. Many cells also produce a strong
supporting layer around the membrane
known as the cell wall.
Cell Membrane/
Plasma Membrane
Cytoplasm
Extra cellular Fluid
4. Cell Membrane
Regulates what enters & leaves cell
Provides protection & support
5. The composition of most cell membranes
is a phospholipid bilayer.
* 2 layers of lipids. (Makes cell flexible)
• Most membranes contain proteins embedded
in the lipid bilayer. Carbohydrates may be
attached to many of the proteins.
• Proteins form channels and pumps to help
things cross the membrane.
• Carbohydrates act like chemical identification
cards. Allowing individual cells to identify one
another.
Phosph= phosphate head
lipid=fatty acid tail
Bilayer = 2 layers
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Draw and Label Cell
membrane from pg. 182 for
homework:
Outside
of cell
Proteins
Carbohydrate
chains
Cell
membrane
Inside
of cell
(cytoplasm)
Protein
channel
Lipid bilayer
Known as the “Fluid Mosaic Model”
Find the carbohydrates & proteins…
6. Cell walls are present in
*Plants
*Algae
*Fungi
*Many prokaryotes
 Cells walls are outside the cell membrane.
 Allow H2O, CO2, O2, & other things that pass
through easily
CELL WALL
Algae cells
Main function of the cell wall is to
provide protection and support for cell.
Cell walls are made from fibers of
carbohydrates and protein
Plant cells are mostly cellulose, a
tough carbohydrate fiber
*Middle lamella makes up the outer portion of cell walls and is
shared by neighboring cells
7. Cells of living things are bathed in liquid.
One of the MOST IMPORTANT functions of the
cell membrane is to regulate the movement of
dissolved liquid on one side of the membrane to
the liquid on the other side of the membrane.
8. The cell membrane allows a cell to obtain
certain materials and release certain
materials from the environment.
9. A cell membrane is selectively
permeable. (this means it only allows
certain things to move in or out.)
10. Cell membranes permit certain
molecules to pass in and out, depending
on type and size
H2O, CO2, O2 pass through easily
Sugar and sodium may be stopped or slowed
Acids, chemicals, poisons may be stopped
11. All matter is made up of molecules that
are constantly moving—bumping,
bouncing– they like to move from an
area where it is crowded to an area
where it is less crowded.
The movement of molecules changes the
concentration of molecules inside and outside
of cells
*Occurs in cytoplasm of cells*
12. Diffusion is the process by which
molecules move from an area of high
concentration to an area of lower
concentration
CO2 and O2 in lungs exchange by this process.
Water molecules in cells also exchange in this
way.
*Does not require energy from cells*
13. Most cells live in an aqueous environment,
bathed in fluids that are mostly water.
All life processes take place in water.
14. Remember Veggie lab?...The plant cells of
celery, onion, carrot…etc?
When the cells were in water, there was a higher
concentration of water molecules outside the cells
than inside. Soooo, the water molecules diffused
into the plant cells
When you placed the cells in salt water, the cells
LOST water because the water molecules in the
cells moved from an area of higher water
concentration inside the cells into the surrounding
salt water where the water concentration was less.
The diffusion of WATER is called Osmosis.
15. SOMETIMES, things have to move across
the membrane and they are too big to diffuse
OR the concentration is not right for
diffusion. This is called Active transport.
Facilitated diffusion requires no cellular
energy.
Carrier proteins transport things across the
membrane
Facilitated Diffusion
Glucose
molecules
High
Concentration
Cell
Membrane
Low
Concentration
Protein
channel
Facilitated Diffusion
EXTRACELLULAR
FLUID
Channel protein
Solute
CYTOPLASM
(a) A channel protein (purple) has a channel through which
water molecules or a specific solute can pass.
Figure 7.15
Carrier proteins
Undergo a subtle change in shape that
translocates the solute-binding site across the
membrane
Carrier protein
(b)
A carrier protein alternates between two conformations, moving a
solute across the membrane as the shape of the protein changes.
The protein can transport the solute in either direction, with the net
movement being down the concentration gradient of the solute.
Solute
ENERGY is required for the following types of
Active Transport.
Membrane-Associated Pumps – special
molecules move calcium, sodium, and
potassium across membranes
Endocytosis > Phagocytosis – engulf food
> Pinocytosis – engulf water
Exocytosis > Contractile vacuole
> Vacuole membrane fuses with
cell membrane, forcing out
contents
Active Transport
Molecule to
be carried
Low concentration
High Concentration
Energy
Molecule
being carried