Download Cell Membrane and Diffusion

Cell Membranes
&
Movement Across Them
Cell Membranes
Part 1
Cell (plasma) membrane
 Cells need an inside & an outside…

separate cell from its environment

cell membrane is the boundary
IN
OUT
food
- sugars
- proteins
- fats
salts
O2
H2 O
waste
- ammonia
- salts
- CO2
- H2O
products
- proteins
cell needs materials in & products or waste out
Building a membrane
 How do you build a barrier that keeps
the watery contents of the cell separate
from the watery environment?
 FATS 
 LIPIDS 
Remember:
oil & water
don’t mix!!
What substance
do you know
that doesn’t mix
with water?
Fluid Moasiac Model
 We also can call the plasma membrane the
fluid mosaic model.
 Fluid because the double layer moves
around, it never is motionless.
 A mosaic because that is how it appears a
mosaic is a piece of art that is made up of lots
of little pieces put together.
Fluid Mosaic
Model
Human Cheek
Cell
Lipids of Cell Membrane
 Membrane is made of special kind of lipid


phospholipids
“split personality”
“attracted to water”
 Membrane is a double layer

phospholipid bilayer
phosphate
inside cell
lipid
outside cell
“repelled by water”
Semi-permeable membrane
 Cell membrane controls what gets in or out
 Need to allow some materials — but not all
— to pass through the membrane

semi-permeable
 only some material can get in or out
So what needs to get across the membrane?
sugar lipids
aa
O2
H 2O
salt
waste
Crossing the Cell Membrane
 What molecules can get through the cell
membrane directly?

fats and oils can pass directly through
inside cell
waste
outside cell
lipid
salt
sugar aa
H 2O
but…
what about
other stuff?
Cell membrane channels
 Need to make “doors” through membrane

protein channels allow substances in & out
 specific channels allow specific material in & out
 H2O channel, salt channel, sugar channel, etc.
inside cell
waste
salt
H 2O
aa
sugar
outside cell
How do you build a semi-permeable
cell membrane?
 Channels are made of proteins

proteins both “like” water & “like” lipids
bi-lipid
membrane
protein channels
in bi-lipid membrane
Protein Channels
 Proteins act as doors in the membrane

channels to move specific molecules
through cell membrane
HIGH
LOW
Protein Receptor Molecules
 Certain protein molecules in the cell membrane
that can receive chemical messages from other
cells.
Protein Receptor Molecules
 Specific cell receptors receive messages from
specific hormones
Hormones- a chemical produced in the
endocrine glands
Neurotransmitters
 The chemicals which allow the transmission
of signals from one neuron to the next
across synapses. They are also found at the
axon endings of motor neurons, where they
stimulate the muscle fibers.
Hormone # 1
Hormone # 2
Hormone # 3
 What is the target cell for hormone #1?
Cell B
___________________________________
 What is the target cell for hormone #2?
Cell C
___________________________________
 What is the target cell for hormone #3?
Cell A
___________________________________
Once the Messenger Attaches to
the Receptor:
 The message is transported through the


membrane.
The messenger is released back into the blood or
lymph systems.
The cell carries out the appropriate protein
synthesis and action.
Question:
 Which type of cell do you think is a target cell for
estrogen? (bone, ovarian, muscle)
Other Methods of Movement
 Endocytosis – bringing materials into a
cell
Other Methods of Movement
 Exocytosis – taking materials out of a
cell
Absorption
 The process where digested (broken down)
nutrients as well as other dissolved materials enter
the cells of an organism. The key organelle that is
involved with this life function is the PLASMA
(CELL) MEMBRANE
Transport
 The life function that involves the absorption
(taking in) and circulation (moving around) of
materials throughout an organism. We can talk
about both absorption and circulation as separate
processes.
Movement Of
Materials
Part 2
Molecules move from high to low
 Diffusion

move from HIGH to LOW concentration
Diffusion
 Move from HIGH to LOW concentration
passive transport
 no energy needed

diffusion
diffusion of water
osmosis
Simple Diffusion
 Move from HIGH to LOW
fat
inside cell
LOW
fat
fat
fat
fat
fat
Which way
will fat move?
HIGH
outside cell
fat
fat
fat
fat
fat
fat
fat
fat
Facilitated Diffusion
 Move from HIGH to LOW through a channel
sugar sugar
sugar
inside cell sugar
sugar
sugar
LOW
Which way will
sugar move?
HIGH
outside cell
sugar
sugar sugar
sugar
sugar
sugar
sugar
Diffusion
 Move from HIGH to LOW concentration

directly through membrane
 simple diffusion
 no energy needed

help through a protein channel
 facilitated diffusion (with help)
 no energy needed
HIGH
LOW
Simple vs. facilitated diffusion
simple diffusion
inside cell
lipid
facilitated diffusion
inside cell
H 2O
protein channel
outside cell
outside cell
H 2O
Active transport
 Cells may need molecules to move
against concentration “hill”

need to pump “uphill”
 from LOW to HIGH using energy
protein pump
 requires energy
 ATP

ATP
Transport summary
simple
diffusion
facilitated
diffusion
active
transport
ATP
Osmosis
Movement of Water Across
The Cell Membrane
Osmosis
 Water is very important, so we talk about
water separately
 Osmosis

diffusion of water from HIGH concentration
of water to LOW concentration of water
 across a semi-permeable membrane
Keeping water balance
 Cell survival depends on balancing
water uptake & water loss
freshwater
balanced
saltwater
 Tonicity - relative concentration of solute on either


side of a membrane
Solute - cannot pass through cell (Salt, sugar,
proteins)
Solvent - can pass through cell (water universal
solvent)
 HYPERtonic -solution has more solute, less water.
 Therefore, the cell will lose water and may shrivel
up and die.
 HYPOtonic - solution has more water, less solute.
 Therefore, the cell may burst
 ISOtonic - solution has balance of solute and
water.
Solution
Description
Movement
Hypotonic
Solution-
Solute concentration
outside the cell is
higher (less water)
Water diffuses out of the
cell until equilibrium is
reached
Hypertonic
Solution-
Solute concentration Water moves into the cell
greater inside the cell
until equilibrium is
(less water)
reached
Isotonic
Solution
Concentration of
solutes same
inside & outside
the cell
Water moves into & out of
cell at an equal rate so
there is no net movement
of water
1
Keeping right amount of water in cell
 Freshwater
KABOOM!
a cell in fresh water
 high concentration of water around
cell

 cell gains water
 example: Paramecium
 problem: cells gain water,
swell & can burst
 water continually enters
Paramecium cell
 solution: contractile vacuole
 pumps water out of cell
freshwater
Controlling water
 Contractile vacuole in Paramecium
2
Keeping right amount of water in cell
 Saltwater
I’m shrinking,
I’m shrinking!
a cell in salt water
 low concentration of water
around cell

 cell loses water
example: shellfish
 problem: cell loses water

 in plants: plasmolysis
 in animals: shrinking cell

solution: take up water
saltwater
3
Keeping right amount of water in cell
 Balanced conditions

no difference in
concentration of water
between cell & environment
 cell in equilibrium
 example: blood
 problem: none
 water flows across
membrane equally,
in both directions
 volume of cell doesn’t
change
That’s
better!
balanced