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Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
- Phospholipid bilayer
- Selectively permeable
- Fluid Mosaic Model
WATER
Hydrophilic
head
Hydrophobic
tail
WATER
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
- Phospholipid bilayer
- Selectively permeable
- Fluid Mosaic Model
2. Why fluid/what affects fluidity?
Lateral movement
Flip-flop
- FA - Sat vs unsaturated
7
(~10 times per second)
(~ once per month)
- Temperature
(a) Movement of phospholipids
- Cholesterol
Viscous
Fluid
Unsaturated hydrocarbon
tails with kinks
Saturated hydroCarbon tails
(b) Membrane fluidity
Cholesterol
(c) Cholesterol within the animal cell membrane
Rarely!!
Students
- Learning Log mid-point check – Thursday 11/19
- Does anyone suffer from triskaidekaphobia?
- Fear of the number 13
- Phones in bin….off or muted….please & thank you
Chapter 7 Essential Questions
LO 2.10 The student is able to use representations and models to
pose scientific questions about the properties of cell membranes
and selective permeability based on molecular structure.
LO 2.11 The student is able to construct models that connect the
movement of molecules across membranes with membrane structure
and function.
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
–
–
–
–
–
Fats
Phospholipids
Steroids
Oils
Waxes
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
4. How are fats made?
Figure 5.11 The synthesis and structure of a fat, or
triacylglycerol
H
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OH
HO
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Fatty acid
(palmitic acid)
OH
H
Glycerol
(a) Dehydration reaction in the synthesis of a fat
Ester linkage
O
H
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O
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(b) Fat molecule (triacylglycerol)
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Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
4. How are fats made?
5. What is the difference between a saturated &
unsaturated fat?
Figure 5.12 Examples of saturated and unsaturated fats
and fatty acids
Stearic acid
(a) Saturated fat and fatty acid
Oleic acid
(b) Unsaturated fat and fatty acid
cis double bond
causes bending
Saturated vs Unsaturated Fats
-
No double bonds (C-C)
Carbons are saturated
Solid at RT
Animal fats
Butter
Bacon grease
What are trans fats?
- Formed by hydrogenation
- C=C without the “kink”
- Double bonds (C=C)
- Carbons not saturated
- Oil at RT
- Plant or fish fats
- Vegetable oil
- Olive oil
Saturated vs Unsaturated Fats
-
No double bonds (C-C)
Carbons are saturated
Solid at RT
Animal fats
Butter
Bacon grease
What are the functions of fats?
- Energy storage (2X carbs)
- Cushion
- Insulation
- Double bonds (C=C)
- Carbons not saturated
- Oil at RT
- Plant or fish fats
- Vegetable oil
- Olive oil
Figure 5.13 The structure of a phospholipid
+N(CH )
CH2
3 3
Choline
CH2
O
O
P
–
O
Phosphate
Amphipathic – molecules
both polar & non-polar
O
CH2
CH
O
O
C
O C
CH2
Glycerol
O
Fatty acids
Hydrophilic
head
Hydrophobic
tails
(a) Structural formula
(b) Space-filling model
(c) Phospholipid
symbol
Figure 5.15 Cholesterol, a steroid
H3C
CH3
CH3
HO
CH3
CH3
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
4. How are fats made?
5. What is the difference between a saturated & unsaturated fat?
6. What makes a membrane mosaic?
Figure 7.7 The detailed structure of an animal cell’s plasma membrane
Fibers of
extracellular
matrix (ECM)
Glycoprotein
Carbohydrate
Glycolipid
EXTRACELLULAR
SIDE OF
MEMBRANE
Microfilaments
of cytoskeleton
Cholesterol
Peripheral
protein
Integral
protein
CYTOPLASMIC SIDE
OF MEMBRANE
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
4. How are fats made?
5. What is the difference between a saturated & unsaturated fat?
6. What makes a membrane mosaic?
7. How are integral proteins held in the membrane?
- α-helix
EXTRACELLULAR
SIDE
N-terminus
C-terminus
a Helix
CYTOPLASMIC
SIDE
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
4. How are fats made?
5. What is the difference between a saturated & unsaturated fat?
6. What makes a membrane mosaic?
7. How are integral proteins held in the membrane?
8. What are the functions of proteins?
Figure 7.9 Some functions of membrane proteins
(a) Transport. (left) A protein that spans the membrane
may provide a hydrophilic channel across the
membrane that is selective for a particular solute.
(right) Other transport proteins shuttle a substance
from one side to the other by changing shape. Some
of these proteins hydrolyze ATP as an energy source
to actively pump substances across the membrane.
ATP
(b) Enzymatic activity. A protein built into the membrane
may be an enzyme with its active site exposed to
substances in the adjacent solution. In some cases,
several enzymes in a membrane are organized as
a team that carries out sequential steps of a
metabolic pathway.
(c) Signal transduction. A membrane protein may have
a binding site with a specific shape that fits the shape
of a chemical messenger, such as a hormone. The
external messenger (signal) may cause a
conformational change in the protein (receptor) that
relays the message to the inside of the cell.
Enzymes
Signal
Receptor
(d) Cell-cell recognition. Some glyco-proteins serve as
identification tags that are specifically recognized
by other cells.
Glycoprotein
(e) Intercellular joining. Membrane proteins of adjacent cells
may hook together in various kinds of junctions, such as
gap junctions or tight junctions (see Figure 6.31).
(f) Attachment to the cytoskeleton and extracellular matrix
(ECM). Microfilaments or other elements of the
cytoskeleton may be bonded to membrane proteins,
a function that helps maintain cell shape and stabilizes
the location of certain membrane proteins. Proteins that
adhere to the ECM can coordinate extracellular and
intracellular changes (see Figure 6.29).
Students
- Did you park appropriately today? 
- Learning Log mid-point check is Thursday
- Test corrections – I need your Exam report (21/35)
- I think I found a “new Friday outfit”  Panthers 9-0
- Phones in bin…off or muted…please & thank you!!
Chapter 7: Membrane Structure & Function
1. What do you know about membranes?
2. Why fluid/what affects fluidity?
3. What are some common lipids?
4. How are fats made?
5. What is the difference between a saturated & unsaturated fat?
6. What makes a membrane mosaic?
7. How are integral proteins held in the membrane?
8. What are the functions of proteins?
9. Word association……
Membrane = Selectively permeable!!!!
10. What things are selected for?
- Non-polar pass easily - steroids
11. What things are selected against?
- large, polar, charged molecules – glucose, amino acids, ions
12. What is the difference between hypertonic & hypotonic solutions?
- Hypertonic – more total solute than another sol’n = less water
- Hypotonic – less total solute than another sol’n = more water
Figure 7.11 The diffusion of solutes across a membrane
(a) Diffusion of one solute. The membrane Molecules of dye
Membrane (cross section)
has pores large enough for molecules
of dye to pass through. Random
movement of dye molecules will cause
some to pass through the pores; this
will happen more often on the side
WATER
with more molecules. The dye diffuses
from where it is more concentrated
to where it is less concentrated
(called diffusing down a concentration
gradient). This leads to a dynamic
Net diffusion
Net diffusion
equilibrium: The solute molecules
continue to cross the membrane,
but at equal rates in both directions.
Equilibrium
(b) Diffusion of two solutes. Solutions of
two different dyes are separated by a
membrane that is permeable to both.
Each dye diffuses down its own concentration gradient. There will be a net
diffusion of the purple dye toward the
left, even though the total solute
concentration was initially greater on
the left side.
Net Osmosis
diffusion
Net diffusion
What is osmosis?
- Diffusion of water
Net diffusion
Net diffusion
Equilibrium
Equilibrium
Figure 7.12 Osmosis
Lower
concentration
of solute (sugar)
Higher
concentration
of sugar
Same concentration
of sugar
Selectively
permeable membrane: sugar molecules cannot pass
through pores, but
water molecules can
Water molecules
cluster around
sugar molecules
More free water
molecules (higher
concentration)
Fewer free water
molecules (lower
concentration)
Osmosis

Water moves from an area of higher
free water concentration to an area
of lower free water concentration
What about transport in actual cells?
Figure 7.13 The water balance of living cells
Placed into
Hypotonic solution
(a) Animal cell. An
animal cell fares best
in an isotonic environment unless it has
special adaptations to
offset the osmotic
uptake or loss of
water.
H2O
Isotonic solution
(b) Plant cell. Plant cells
are turgid (firm) and
generally healthiest in
a hypotonic environment, where the
uptake of water is
eventually balanced
by the elastic wall
pushing back on the
cell.
H2O
Turgid (normal)
H2O
H2O
Normal
Lysed
Hypertonic solution
H2O
Shriveled
H2O
H2O
Flaccid
H2O
Plasmolyzed
Chapter 7: Membrane Structure & Function
10. What things are selected for?
- Non-polar pass easily - steroids
11. What things are selected against?
- large, polar, charged molecules – glucose, amino acids, ions
12. What is the difference between hypertonic & hypotonic solutions?
13. How are substances transported across membranes?
- Passive transport – no energy expended & things flow down
concentration gradient from high to low
- Simple diffusion
- Facilitated diffusion – uses transport proteins
- Active transport – energy required & things flow AGAINST a
concentration gradient from low to high
- uses transport proteins
Figure 7.17 Review: passive and active transport compared
Passive transport. Substances diffuse spontaneously
down their concentration gradients, crossing a
membrane with no expenditure of energy by the cell.
The rate of diffusion can be greatly increased by transport
proteins in the membrane.
Active transport. Some transport proteins act as pumps,
moving substances across a membrane against their
concentration gradients. Energy for this work is usually
supplied by ATP.
ATP
Diffusion. Hydrophobic
molecules and (at a slow
rate) very small uncharged
polar molecules can diffuse through
the lipid bilayer.
Facilitated diffusion. Many hydrophilic
substances diffuse through membranes
with the assistance of transport proteins,
either channel or carrier proteins.
Chapter 7: Membrane Structure & Function
10. What things are selected for?
- Non-polar pass easily - steroids
11. What things are selected against?
- large, polar, charged molecules – glucose, amino acids, ions
12. What is the difference between hypertonic & hypotonic solutions?
13. How are substances transported across membranes?
- Passive transport – no energy expended & things flow down
concentration gradient
- Simple diffusion
- Facilitated diffusion
- Active transport – energy required & things flow AGAINST a
concentration gradient
14. What are some other mechanisms of transport across membranes?
- Endocytosis – moving things into a cell
- Exocytosis – moving things out of a cell
Figure 7.20 Exploring Endocytosis in Animal Cells
In phagocytosis, a cell
engulfs a particle by
wrapping pseudopodia
around it and packaging
it within a membraneenclosed sac large
enough to be classified
as a vacuole. The
particle is digested after
the vacuole fuses with a
lysosome containing
hydrolytic enzymes.
PHAGOCYTOSIS
EXTRACELLULAR
FLUID
1 µm
CYTOPLASM
Pseudopodium
Pseudopodium
of amoeba
“Food” or
other particle
Bacterium
Food
vacuole
Food vacuole
An amoeba engulfing a bacterium via
phagocytosis (TEM).
In pinocytosis, the cell
“gulps” droplets of
extracellular fluid into tiny
vesicles. It is not the fluid
itself that is needed by the
cell, but the molecules
dissolved in the droplet.
Because any and all
included solutes are taken
into the cell, pinocytosis is
nonspecific in the substances
it transports.
PINOCYTOSIS
0.5 µm
Plasma
membrane
Pinocytosis vesicles
forming (arrows) in
a cell lining a small
blood vessel (TEM).
Vesicle
Receptor-mediated endocytosis enables the
cell to acquire bulk quantities of specific
substances, even though those substances
may not be very concentrated in the
extracellular fluid. Embedded in the
membrane are proteins with
specific receptor sites exposed to
the extracellular fluid. The receptor
proteins are usually already clustered
in regions of the membrane called coated
pits, which are lined on their cytoplasmic
side by a fuzzy layer of coat proteins.
Extracellular substances (ligands) bind
to these receptors. When binding occurs,
the coated pit forms a vesicle containing the
ligand molecules. Notice that there are
relatively more bound molecules (purple)
inside the vesicle, but other molecules
(green) are also present. After this ingested
material is liberated from the vesicle, the
receptors are recycled to the plasma
membrane by the same vesicle.
RECEPTOR-MEDIATED ENDOCYTOSIS
Coat protein
Receptor
Coated
vesicle
Ligand
Coated
pit
A coated pit
and a coated
vesicle
formed
during
receptormediated
endocytosis
(TEMs).
Coat
protein
Plasma
membrane
0.25 µm
Students
- Get
- Lab handout – Diffusion & Osmosis
- Test folders
- Piefest
- First Presbyterian Church
- Saturday, November 21: 10AM – 2PM
- $5 – Students
- Proceeds support Habitat for Humanity
- 2nd Annual Creative Media Open House
- Thursday 5:30 – 7:30
- Anyone missing notes….folded up?
- Phones in bin…muted or off…please & thank you
Students
- Progress reports – return them signed by PARENTS
- Recall your Honor Code
- Creative Media Open House – Tomorrow 5:30 – 7:30
- Piefest – Saturday 10 – 2
- Learning logs – due TOMORROW
- Mercer’s Room tomorrow
- Pull out experimental design
- Phones in bin…muted or off….please & thank you!
Experimental design items
- Unknown sucrose solutions (A – E)
- Water
- Dialysis tubing – tie knots as close to end as possible!!
- Pipets
- Beakers – plastic cups
- Scales – blot bags BEFORE weighing
- Weigh boats
- Paper towels
- Clock
- 5 mL
- 20 minutes
- % change = Final – Initial x 100%
Initial