The Cell and its
Environment
Finzer 2013
The Plasma Membrane -
Gateway to the Cell
Structure of the Cell
Membrane
• Homeostasis- “steady state” or balance
•
-cells constantly seek homeostasis
• The Cell Membrane is responsible for
maintaining the steady state within
cells.
• - Most cells live in some kind of fluid
• -Single celled organisms- ponds,
oceans, inside other bodies…
• -Multicellular organisms- cells are
surrounded by body fluid
• - Cells have limits to changes they can
survive
•
- Temperature
•
- Salt concentrations in the H2O
•
- Interstitial fluid- body fluid
between cells
Diffusion of Liquids
• Diffusion- molecules move from an area
of high concentration to an area of low
concentration
• - takes place in both solutions and air
• - Gradual spreading out of molecules
• - They spread out until they reach
equilibrium
• def . equilibrium- molecules evenly
spread out
Simple Diffusion
• Requires NO
energy
• Molecules
move from area
of HIGH to LOW
concentration
DIFFUSION THROUGH
MEMBRANES
• Permeable- when membranes allow all
substances to pass through
• Selectively Permeable- when membranes
allow some substances to pass through,
& others not
DIFFUSION
Diffusion is a
PASSIVE process
which means no
energy is used to
make the
molecules move,
they have a
natural KINETIC
ENERGY
• CELL MEMBRANES ARE SELECTIVELY
PERMEABLE!
• Osmosis- diffusion of water through a
selectively permeable membrane
• (from high concentration to low
concentration)
Q? What decides what moves
into the cell???
• A. Compare the solution outside the cell
to the solution inside the cell.
Diffusion of H2O Across A
Membrane
High H2O potential
Low solute concentration
Low H2O potential
High solute concentration
• Hypotonic- a solution having a lower
concentration of solutes than the cell –
water goes into the cell
Cell in Hypotonic Solution
10% NaCL
90% H2O
CELL
20% NaCL
80% H2O
What is the direction of water movement?
• Hypertonic- a solution having a higher
concentration of solutes than the cell –
water exits the cell
Cell in Hypertonic Solution
15% NaCL
85% H2O
ENVIRONMENT
CELL
5% NaCL
95% H2O
What is the direction of water movement?
• Isotonic- a solution having the same
concentration of solutes as the cell –
equal movement of water into and out of
the cell
Cell in Isotonic Solution
10% NaCL
90% H2O
ENVIRONMENT
CELL
10% NaCL
90% H2O
NO NET
MOVEMENT
What is the direction of water movement?
equilibrium
The cell is at _______________.
Isotonic Solution
NO NET
MOVEMENT OF
H2O (equal amounts
entering & leaving)
Hypotonic
Solution
CYTOLYSIS
Hypertonic
Solution
PLASMOLYSIS
Osmosis in Red Blood Cells
Isotonic
Hypotonic
Hypertonic
Rules for deciding what will
happen to cells…
• 1. look at the solute concentrations to
set up the picture puzzle
• 2. water moves first and fastest….so
• 3. look at the water concentrations to
decide what will happen to the cell
…try some problems
• Turgor Pressure- term used to describe
water pressure within a cell, (usually
plant cells)
• -will continue to build up until
equilibrium is reached
• Plasmolysis- water diffuses out of a cell
and turgor pressure is lost
•
- Plants “wilt”
•
- Cells shrink
• Cytolysis- when cells burst because
water pressure inside the cell is too
great
Cytolysis & Plasmolysis
Cytolysis
Plasmolysis
hypotonic
hypertonic
hypertonic
isotonic
isotonic
hypotonic
What happens to fresh water
organisms?
• Contractile vacuoles- organelles that
pump water out through the cell
membrane
• 2 WAYS SUBSTANCES ARE
TRANSPORTED
THROUGH MEMBRANES
Three Forms of Transport Across the Membrane
• PASSIVE TRANSPORT- the movement
of substances through the cell
membrane when NO ENERGY taken
from the cell
•
Ex- diffusion……osmosis
•
-these both happen “naturally”
• PT uses kinetic energy- the natural movement
of molecules
• Substances using PT:
• -Water
• -Carbon dioxide
• -Oxygen
• -Steroids
• -Alcohol
• -Glucose
• Q ? Do these substances move into or
out of the cell???
Proteins Are Critical to
Membrane Function
Facilitated Diffusion
Molecules will randomly move through
the pores in Channel Proteins.
Facilitated Diffusion
• Some Carrier
proteins do not
extend through
the membrane.
• They bond and
drag molecules
through the lipid
bilayer and
release them on
the opposite side.
• ACTIVE TRANSPORT- transporting
substances across cell membranes
USING ENERGY from the cell
• Ex. Sodium (Na) & potassium (K) (the
cell have high needs of these ions)
• AC moves against natural diffusion----• - Plants have the ability to absorb Na
& K when concentrations are higher
inside the cell than outside the cell
Sodium-Potassium Pump
3 Na+ pumped in for every 2 K+ pumped
out; creates a membrane potential
• Endocytosis- taking into a cell – “cell
eating”
• Exocytosis- putting out of a cell
• *Phagocytosis- taking in large solids
•
*Pinocytosis- taking in or putting
out fluid drops - excrete or secrete –
“cell drinking”
• * Cell reaches around the particle to be
taken in and forms a pouch-à vacuole
Pinocytosis
• Cell forms an
invagination
• Materials dissolve
in water to be
brought into cell
• Called “Cell
Drinking”