1. CELL MEMBRANE Structure and Function
Chapter 5
CELL MEMBRANE
Structure and Function

2. Chapter 5 At a Glance 5.1 How Is the Structure of the Cell
Membrane Related to Its Function?
5.2 How Do Substances Move Across
Membranes?
5.3 How Do Specialized Junctions Allow
Cells to Connect and Communicate?
Case Study Investigation:
Y:\Biology\D Cell Membrane and Transport\Transport CSI Reading.pdf

3. 5.1 How is the Structure of the Cell Membrane Related to its Function?
All cell membranes have a similar basic structure
Proteins suspended in a double layer of phospholipids
Responsible for:
selectively exchanging substances
Communicating with environment
Controlling biochemical reactions
Forming connections btwn cells
Responsible for:
Isolating cells contents

4. 5.1 How is the Structure of the Cell Membrane Related to its Function?
Crucial functions of the plasma cell membrane:
isolates cell’s contents from external environment
regulates exchange of essential substances
allows communication between cells
creates attachments within and btwn cells
regulates biochemical reactions
extremely thin–10,000 membranes still thinner than txtbk page
membranes change in response to their surroundings

5. Cell Membrane = Fluid Mosaic Model
named in 1972 by S.J. Singer and G.L. Nicolson
proteins and other structures move within the membrane
carbohydrate
interstitial fluid (outside)
protein
extra-
cellular
matrix
glycoprotein
receptor
protein
phospholipid
bilayer
binding site
phospholipid
transport
protein
connection
protein
pore
cholesterol
recognition
protein
enzyme
cytoskeleton
cytosol (fluid inside cell) 5

6. Phospholipids are basis of membrane structure
phospholipid bilayer is specifically arranged between an interior (cytosol) and exterior (interstitial fluid) watery layer
interstitial fluid–weakly salty liquid (blood w/o its cells & proteins) surrounds outer surfaces of animal cell membranes
cytosol – fluid portion of cytoplasm (mostly water)
head
(hydrophilic)
tails
(hydrophobic)

7. Phospholipids are basis of membrane structure
phospholipid bilayer’s flexibility allows cellular shape changes
individual phospholipid molecules not bonded to each other
unsaturated fat acids are kinked b/c of their double bond(s)
all reasons why membrane is fluid

8. Phospholipids structure = selectively permeable
membranes  more fluid at high temps (more movement)
and less fluid at low temps (less movement)
cell membranes of organisms living in low temps tend to be
more unsaturated to help maintain fluidity
cholesterol stabilizes membranes – makes it less fluid at high temps and less solid at lower temps; becomes more selective

9. Phospholipids structure = selectively permeable
water soluble (polar) substances cannot easily cross phospholipid bilayers (i.e. salts, amino acids, sugars)
small molecules (i.e. H2O, O2, CO2) can slip in
larger lipid soluble (nonpolar) substances (i.e. estrogen, testosterone) can pass through

10. Variety of Proteins Form a Mosaic
variety of proteins are embedded within or attached to
phospholipid bilayer
many have carbohydrates attached to their outer surface (glycoproteins)
5 major categories of membrane proteins
enzymes
receptor proteins
recognition proteins
connection proteins
transport proteins

11. Membrane Proteins
1. Enzymes - proteins that promote chemical reactions that synthesize or break apart molecules (i.e. enzymes that line the small intestine that digest carbs & proteins)
2. Receptor proteins –
trigger cellular responses
(i.e. hormones that bind,
immune response)
3. Recognition proteins –
glycoproteins that serve as
identification tags on cell
surface (i.e. self vs non-self)

12. Membrane Proteins 4. Connection proteins: anchor cell membranes
maintain shape by linking membrane to cytoskeleton
anchor cell in place within a tissue by linking cell’s cytoskeleton to extracellular matrix outside
form connections between adjacent cells
5. Transport proteins:
regulate movement
of hydrophilic molecules
through membrane
Channel proteins - form pores (can open and close)
Carrier proteins – bind to and carry substances through membrane

13. 5.2 How Do Substances Move Across Membrane
some substance can move across the membrane by diffusing through phospholipid bilayer through specialized proteins
solute: substance that can be dissolved in a solvent (i.e. sugar, salt, etc)
solvent: fluid capable of dissolving a solute (i.e. H2O, gasoline, alcohol)
Water is a universal solvent

14. concentration: defines the amount of solute in a given amount of solvent
gradient: physical difference in temperature, pressure, charge or concentration of a solute in a fluid - btwn 2 adjoining regions of space
concentration gradient: differences in solute concentrations across a membrane

15. Molecules in Fluids Diffuse in Response to Gradients
atoms, molecules, & ions are in constant random motion
diffusion: net movement of solutes from regions of high concentration to regions of low concentration (down a gradient)
gradients cause
molecule movement
the greater the gradient
the faster the rate of
diffusion
the higher the temperature
movement continues until
molecules are evenly
dispersed (unless disrupted)
Hot Water Cold Water

16. Dye molecules
diffuse into the water;
water molecules diffuse
into the dye
Both dye molecules
and water molecules are
evenly dispersed
A drop of dye is
placed in water
dye
molecules
water
molecule

17. Movement Through Membranes Occurs by Passive or Active Transport
cells w/o gradients are dead so…..
membrane proteins must use energy to create and maintain gradients in order to carry out crucial biochemical process
selective permeable membrane creates a barrier that helps maintain gradients
PASSIVE TRANSPORT
diffusion of substances
across cell membranes DOWN
concentration gradients
simple diffusion
facilidated diffusion
osmosis
ACTIVE TRANSPORT
requires energy to move
substances usually AGAINST
concentration gradients
endocytosis
exocytosis

18. Active Transport
Passive Transport

19. Passive Transport: Simple & Facilitated Diffusion
Simple Diffusion: some
molecules diffuse directly
through phospholipid bilayers
small molecules with no
net charge (H2O, O2, CO2)
lipid soluble molecules
regardless of size (ethyl
alcohol, vit A, D, and E,
steroid hormones)
(interstitial
fluid) O2
phospho-
lipid
bilayer
(cytosol)
Simple diffusion through
the phospholipid bilayer

20. channel proteins: form pores through cell membranes
Facilitated Diffusion: transport proteins help larger polar molecules & ions (sugars, K+, Na+, Cl-, Ca 2+) to cross membranes
carrier proteins: loosely bind to specific ions/molecules (i.e. sugars & small proteins), change shape & transfer the bound particles across membrane
channel proteins: form pores through cell membranes
Ion channels
very selective
(charge/size)
channel
protein
carrier
protein
Facilitated diffusion
through carrier proteins
Facilitated diffusion through
channel proteins

21. small size and positive charge attracts negative side of H2O
aquaporins: specialized water channel proteins for faster diffusion of water (osmosis)
small size and positive
charge attracts negative
side of H2O
billions of water
molecules can move through
an aquaporin in single file
every second
repels larger molecules
and smaller positive ions
aquaporin
channel 
(cytosol)
water

22. Osmosis is the Diffusion of Water
Osmosis: diffusion of water across a selectively permeable membrane in response to concentration gradient, pressure, or temp.
H2O moves from
regions of high
concentration to regions
of low concentration
across a membrane
dissolved substances
reduces concentration
of free H2O molecules
(& purity) in a solution
osmotic strength:
tendency to attract H2O
across a membrane; greater
the solute conc., the greater
the o.s.
Low
water
concentration
High
water
concentration

23. Isotonic, Hypertonic, Hypotonic Solutions
No net flow
of water
Water flows out;
the sac shrinks
Water flows in;
the sac expands
A sac in an
isotonic solution
A sac in a
hypertonic solution
A sac in a
hypotonic solution
iso = same
equal
concentrations of solute and solvent
hyper = more
more solute
higher osmotic strength
H2O moves out of less
concentrated sln
hypo = less
less solute
lower osmotic strength
H2O moves into more concentrated sln
Egg Experiment

24. Osmosis Plays an Important Role in Lives of Cells
water uptake by roots of plants
absorption of dietary water from intestine
reabsorption of water in kidneys
organisms that live in fresh water must use energy to counteract osmosis b/c cells are hypertonic to surrounding water
Turgor Plasmolysis
pressure: when water
when water leaves cell
flows into a causing cell
cell and to shrink
inflates the away from
cell; pressure cell wall
on the cell wall (droopy
plants)

26. When water is plentiful, it fills the central
central vacuole
cytoplasm
When water is plentiful, it fills the central
vacuole, pushes the cytoplasm against the
cell wall, and helps maintain the cell’s shape
Water pressure supports
the leaves of this
impatiens plant
Turgor pressure provides support
cell wall
plasma membrane
When water is scarce, the central vacuole
shrinks and the cell wall is unsupported
Deprived of the support
from water, the plant wilts
Loss of turgor pressure causes the plant to wilt

27. Active Transport, Endocytosis, Exocytosis
energy-expending cellular activities are crucial to sustaining life
maintaining concentration gradients
acquiring food
excreting wastes
cell to cell communication
active transport: membrane proteins use energy to move molecules/ions across a membrane AGAINST their concentration gradient (low concentration to a high concentration)

28. Active Transport ATP & specific molecule/ion
ATP & specific molecule/ion
Energy comes from breaking high energy bond of 3rd phosphate
active transport proteins = pumps
(interstitial fluid)
Energy from ATP
changes the shape
of the transport protein
and moves the ion
across the membrane
The transport
protein binds both
ATP and Ca2
The protein
releases the ion
and the remnants
of ATP (ADP and P)
and closes
ATP
binding
site
ADP
binding
site
ATP P
ATP
Ca2
(cytosol)

29. Endocytosis Allows Cells to Engulf Particles or Fluids
(interstitial fluid)
When cells need materials that
are too large to pass through
membrane they use…
Endocytosis: form of active
transport that allows cells to
engulf particles or fluids
1. Pinocytosis:“cell drinking”
moves liquids into cells
vesicle containing
interstitial
fluid
(cytosol)
A dimple forms in the plasma membrane, which
deepens and surrounds the interstitial fluid The
membrane encloses the interstitial fluid, forming a vesicle.
Pinocytosis
interstitial fluid
cytosol
TEM of pinocytosis

30. 2. Receptor-mediated endocytosis: moves specific molecules into cells (packets of protein and cholesterol)
Receptor proteins for specific
molecules or complexes of
molecules are localized at coated
pit sites.
nutrient molecule
(interstitial fluid)
receptor
The receptor bind the
molecules and the membrane
dimples inward.
coated pit
The coated pit region of the
membrane encloses the receptor-
bound molecules.
coated vesicle
A vesicle (“coated vesicle”)
containing the bound molecules
is released into the cytosol.
(cytosol)
Receptor-mediated endocytosis
extracellular particles
bound to receptors
coated vesicle
(interstitial fluid)
(cytosol)
protein
coating
coated pit
plasma membrane
TEM of receptor-mediated endocytosis

31. Phagocytosis: “cell eating” moves large particles (or whole
organisms) into
cells
(interstitial fluid)
food particle
pseudopods
food
vacuole
(cytosol)
The plasma membrane extends pseudopods toward an extracellular
particle (for example, food) The ends of the pseudopods fuse,
encircling the particle A vesicle called a food vacuole is formed
containing the engulfed particle.
Phagocytosis
An Amoeba engulfs a
Paramecium
A white blood cell engulfs a
disease-causing fungal cell

32. Exocytosis Moves Material Out of the Cell
energy is used to dispose of undigested particles of waste or secrete substances (i.e. hormones) into interstitial fluid
(interstitial fluid)
secreted
material
plasma
membrane
plasma membrane
vesicle
Material is enclosed in a
vesicle that fuses with the
plasma membrane, allowing
its contents to diffuse out
(cytosol)

34. Exchange of Materials Across Membranes Influences Cell Size and Shape
most cells too small to be seen with naked eye (1-100mm in diameter) WHY?
as a spherical cell enlarges, its innermost parts get farther away from the plasma membrane
diffusion (already slow) takes too long to supply important processes deep within cell
volume increases more rapidly than surface area
larger cells (require more nutrients & create more waste) would have a smaller area of membrane for acquiring nutrients and eliminating wastes  not enough to exchange/survive

35. Case Study
many rattlesnake & spider venoms contain phospholipases - break down membrane phospholipids causing cells to rupture and die
phospholipases attack membranes of capillary cells causing blood vessels to rupture & release blood into surrounding tissue
causes anemia (inadequate # of oxygen-carrying RBC)
attack muscle cell membranes
antivenin – contains specialized proteins that bind and neutralize snake venom proteins
no antivenin for brown recluse bites (treatments only)
How does the role of phospholipases in snake venom differ from its role in the snake’s digestive tract?

36. Study….
Reread each section; reread your notes
Reorganize your notes; re-write; make charts, tables, lists
Ch. 5 Vocab
Read Summary of Key Concepts (pg. 91)
Complete Thinking Through the Concepts 1 – 6 (pg 91-92)
Be able to answer Review Questions 1-8 (pg. 92)
Concept check questions
Use Mastering Biology to help study