1. Cell Membrane/Plasma Membrane
plasma membrane is the boundary that separates the living cell from its surroundings
functions:
1. integrity of the cell – size and shape
2. controls transport = “selectively permeable”
3. excludes unwanted materials from entering the cell
forms a physical barrier with the external environment
allows the cell to create different environments outside and inside
4. maintains the ionic concentration of the cell & osmotic pressure of the cytosol
allows for the creation of gradients – electrical and chemical
5. forms contacts with neighboring cells = tissue
6. sensitivity - first part of cell that is affected by changes in the
extracellular environment

2. Glyco- protein
Carbohydrate
Glycolipid
Microfilaments of cytoskeleton
EXTRACELLULAR SIDE OF MEMBRANE
CYTOPLASMIC SIDE OF MEMBRANE
Integral protein
Peripheral proteins
Cholesterol
Fibers of extra- cellular matrix (ECM)

3. Cell Membrane/Plasma Membrane
comprised of a phospholipid bilayer
phospholipids are the most abundant lipid in the plasma membrane – about 75% of lipids
phospholipid:
glycerol + 2 fatty acids
addition of a phosphate group
phospholipids are amphipathic molecules
containing hydrophobic and hydrophilic regions
hydrophobic fatty acid “tails”
hydrophilic phosphate “head group”
makes the phospholipid both non-polar and polar
Hydrophilic head
Hydrophobic tail
WATER

4. Surface area increases while total volume remains constant
Plasma Membrane Function – Cell Integrity & Size
the amount of plasma membrane determines the size of the cell
metabolic requirements of the cell set upper limits on its size
the surface area to volume ratio of a cell is critical
as the surface area increases by a factor of n2, the volume increases by a factor of n3
small cells have a greater surface area relative to volume
i.e. 6 vs. 1.2
Total surface area [sum of the surface areas of all box sides  number of boxes]
SA = height x width
Total volume [height  width  length  number of boxes]
Surface-to-volume (S-to-V) ratio [surface area  volume]
6 cm2
150 cm2
25x bigger
750 cm2
125x bigger
125 cm3
125x larger
1 cm3
1.2 6
Tissues
Surface area increases while total volume remains constant 5 1 1

5. Plasma Membrane Models: Scientific Inquiry
scientists began building models of plasma membranes before they were seen with electron microscopes
RBC membranes were chemically analyzed in 1915
found to be made of proteins and lipids
10 yrs later – hypothesis that membranes were composed of phospholipid bilayers
with the hydrophobic tails facing inward away from the aqueous environment
BUT – where were the proteins located in the membrane?

6. Plasma Membrane Models: Scientific Inquiry
HYPOTHESIS: membrane is a phospholipid bilayer “sandwiched” between two layers of proteins
PROBLEM: membrane proteins are not very water soluble because they are amphipathic (just like phospholipids)
made up of hydrophilic and hydrophobic regions
1972 – Singer and Nicolson proposed the model seen below
Phospholipid bilayer
Hydrophobic regions of protein
Hydrophilic regions of protein

7. microtome splits the frozen cell along the hydrophobic interior
freeze-fracturing
microtome splits the frozen cell along the hydrophobic interior
SEM used to image the cell layers
Knife
Plasma membrane
Cytoplasmic layer
Proteins
Extracellular layer
Inside of extracellular layer
Inside of cytoplasmic layer
TECHNIQUE
RESULTS

8. Plasma Membrane Fluidity
plasma membrane also contains embedded proteins
many lipids and proteins are also modified through the addition of carbohydrates
glycoproteins
glycolipids
these proteins must be able to change shape to function
some of them even laterally diffuse through the PM
so the membrane must be fluid

9. Plasma Membrane Fluidity
fluid mosaic model states that a membrane is a fluid structure with a “mosaic” of various proteins embedded in it
membrane fluidity is due to several factors
temperature - lipids move around more with increased temp
lipid packing – lipids with shorter fatty acid tails are less stiff
saturation of fatty acids – more C=C bonds (more unsaturated) increase fluidity
cholesterol – decreases fluidity at warmer temps; increases fluidity at lower temps
Lateral movement occurs 107 times per second.
Flip-flopping across the membrane is rare ( once per month).

10. Scientific Inquiry: Do membrane proteins move?
Frye and Eddin – fluorescently labelled membrane proteins of mouse and human cells and fused the cells
production of “hybrid” cells with a homogenous mixture of proteins proved that proteins do move within the plasma membrane
Membrane proteins
Mouse cell
Human cell
Hybrid cell
Mixed proteins after 1 hour
RESULTS
Figure 7.7 Inquiry: Do membrane proteins move? 10

11. Evolution of Differences in Membrane Lipid Composition
Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions
Ability to change the lipid compositions in response to temperature changes has evolved in organisms that live where temperatures vary
© 2011 Pearson Education, Inc.

12. Membrane Proteins and Their Functions
membrane proteins determine most of the membrane’s specific functions
proteins link on the extracellular side to an extracellular matrix of proteins – support the cells within a tissue
proteins link on the cytoplasmic side to the cytoskeleton
via adaptor proteins
Glyco- protein
Carbohydrate
Glycolipid
Microfilaments of cytoskeleton
EXTRACELLULAR SIDE OF MEMBRANE
CYTOPLASMIC SIDE OF MEMBRANE
Integral protein
Peripheral proteins
Cholesterol
Fibers of extra- cellular matrix (ECM)

13. Membrane Proteins and Their Functions
N-terminus
 helix
C-terminus
EXTRACELLULAR SIDE
CYTOPLASMIC SIDE
two kinds of membrane proteins:
1. Extrinsic or Peripheral: bound to the surface of the membrane
function mainly as enzymes
2. Intrinsic or Integral: penetrate the hydrophobic core
those that span the membrane are called transmembrane proteins

14. 6 major functions of integral membrane proteins:
1. transport – channel proteins & transporters
2. enzymatic activity
3. signal transduction – receptor proteins
4. cell-cell recognition – cell identity markers
5. intercellular joining - linkers
6. attachment- to the cytoskeleton and extracellular matrix (ECM)
Enzymes
Enzymatic activity
Glyco- protein
Cell-cell recognition
Intercellular joining
Attachment to the cytoskeleton and extracellular matrix (ECM)

15. The Role of Membrane Carbohydrates in Cell-Cell Recognition
cell surface carbohydrates – found on the extracellular surface of the plasma membrane
these carbohydrates are covalently bonded to lipids (glycolipids) or to proteins (glycoproteins)
on the external side of the PM - carbohydrates vary among species, individuals, and even cell types in an individual
often used to identify a type of cell
can be exploited by viruses – gain entry to a host cell
Receptor (CD4)
Co-receptor (CCR5)
HIV
Receptor (CD4) but no CCR5
Plasma membrane
HIV can infect a cell that has CCR5 on its surface, as in most people.
HIV cannot infect a cell lacking CCR5 on its surface, as in resistant individuals.

16. Synthesis and Sidedness of Membranes
membranes have distinct inside and outside faces
asymmetrical distribution of proteins, lipids, and associated carbohydrates in the plasma membrane is determined when the membrane is built by the ER and Golgi apparatus
Transmembrane glycoproteins ER
ER lumen
Glycolipid
Plasma membrane:
Cytoplasmic face
Extracellular face
Secretory protein
Golgi apparatus
Vesicle
Transmembrane glycoprotein
Secreted protein
Membrane glycolipid

17. Membrane structure results in selective permeability
a cell must exchange materials with its surroundings
controlled by the plasma membrane
plasma membranes are selectively permeable
permeability = property that determines the effectiveness of the PM as a barrier
permeability varies depending on the organization and characterization of the membrane lipids and proteins
hydrophobic (nonpolar) molecules dissolve in the lipid bilayer and pass through the membrane rapidly
polar molecules do not cross the membrane easily
require transport mechanisms
provided by transport proteins

18. Transport Proteins
transport proteins allow passage of hydrophilic substances across the membrane
specific for the substance it moves
numerous types:
1. channel proteins – have a hydrophilic channel that certain molecules or ions can use as a tunnel to move across the membrane
2. carrier proteins - bind to molecules and change shape to shuttle them across the membrane
3. pumps – carrier proteins that require the hydrolysis of ATP (or GTP) to move substances

19. What determines the direction of transport??
two basic things
1. concentration of what is being moved
2. available energy

20. Membrane Permeability and Transport
transport across the membrane may be: passive or active
passive transport
diffusion
osmosis
facilitated
active transport
primary AT
secondary AT
endocytosis
exocytosis

21. A. Diffusion = movement of materials from [high] to [low]
-random movement, no energy needs to be synthesized
-the movement is driven by the inherent kinetic energy of the particles moving down their concentration gradient
-three ways to diffuse:
1. through the lipid bilayer: lipid soluble (non-polar), alcohol, gases, ammonia, fat-soluble vitamins
2. through a channel: charged ions or small polar molecules
-some channels are “gated” – open and close
3. facilitated diffusion: larger, polar molecules too big for channels
-uses a transporter protein

22. B. Osmosis = diffusion of water from [high] to [low]
OR movement of water from [low solute] to [high solute]
Lower concentration of solute (sugar)
Higher concentration of solute
Sugar molecule
H2O
Same concentration of solute
Selectively permeable membrane
Osmosis
-in osmosis – the membrane is permeable to water and NOT to the solutes
-BUT it is the concentration of solutes that causes the water to move
-the solutes are surround by a hydration shell of water molecules
-this decreases the amount of free water molecules available to move
-so increased solute concentration decreases the concentration of free water molecules
-water movement is affected by this drop in free water molecule concentration

23. B. Osmosis = diffusion of water from [high] to [low]
OR movement of water from [low solute] to [high solute]
-EXPERIMENT: U shaped tube divided by a membrane permeable to water only
-increase the solute concentration in the right half of the tube
-decrease free water concentration on the right side also
-water rises on the side of the solutes – due to osmosis
-counteract this rise in water by applying pressure = osmotic pressure (OP)
-therefore increasing solute concentration increases osmotic pressure
-water will move toward that area to decrease this OP

24. Water Balance of Cells Without Walls
How do cells behave in solutions?
must consider solute concentration and membrane permeability
the two things combine to create = TONICITY
Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water
degree to which a the concentration of a specific solute surrounding a cell causes water to enter or leave the cell
Isotonic solution: Solute concentration is the same as that inside the cell = no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell = cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell = cell gains water

25. Water Balance of Cells Without Walls
Hypotonic solution
Osmosis
Isotonic solution
Hypertonic solution
(a) Animal cell
(b) Plant cell
H2O
Cell wall
Lysed
Normal
Shriveled
Turgid (normal)
Flaccid
Plasmolyzed
Isotonic solution: Solute concentration is the same as that inside the cell = no net water movement across the plasma membrane
Hypertonic solution: Solute concentration is greater than that inside the cell = cell loses water
Hypotonic solution: Solute concentration is less than that inside the cell = cell gains water

26. Osmoregulation = control of solute concentrations and water balance
hypertonic or hypotonic environments create osmotic problems for organisms
Osmoregulation = control of solute concentrations and water balance
the protist Paramecium- hypertonic to its pond water environment
has a contractile vacuole that gets rid of the excess water it takes on due to osmosis
in medicine – hypertonic IVs are given to induce the movement of water out of a tissue
make the blood plasma hypertonic – increased OP results
osmosis will drive water from the tissue into the plasma – low solute to high solute
used as a treatment for edema

27. Water Balance of Cells with Walls: Plants
cell walls also help maintain water balance
plant cell in a hypotonic solution swells until the wall opposes uptake
the cell is said to be turgid (firm)
if a plant cell and its surroundings are isotonic, there is no net movement of water into the cell
the cell becomes flaccid (limp) - the plant may wilt
Hypotonic solution
Osmosis
Isotonic solution
Hypertonic solution
(a) Animal cell
(b) Plant cell
H2O
Cell wall
Lysed
Normal
Shriveled
Turgid (normal)
Flaccid
Plasmolyzed
Video: Turgid Elodea

28. in plant cells in a hypertonic environment – cells lose water
the membrane will pull away from the wall
usually lethal effect called plasmolysis
Video: Plasmolysis 28

29. Facilitated transport = movement of a molecule aided by a protein
- movement of a solute from [high] to [low] either:
1. through a channel protein
e.g aquaporins- for facilitated diffusion of water
e.g. Ion channels that open or close in response to a stimulus (gated channels)
2. through a carrier protein
EXTRACELLULAR FLUID
CYTOPLASM
Channel protein
Solute
(a) A channel protein

30. Carrier proteins and Facilitated Diffusion
-solute binds to the carrier protein located on the plasma membrane
-transported across the membrane by the carrier protein’s change in shape
-no energy required – still moving high to low concentration
-but there is a limit to the amount of facilitated diffusion the cells can undergo - has to do with the # of carrier proteins on the PM
EXTRACELLULAR FLUID
CYTOPLASM
Solute
Carrier protein
(b) A carrier protein

31. Active transport uses energy to move solutes against their gradients
some transport proteins can move solutes against their concentration gradients
Active transport moves substances against their concentration gradients
requires energy, usually in the form of ATP
is performed by specific proteins embedded in the membranes called pumps
two kinds of active transport:
Primary
Secondary

32. A. Primary Active transport
requires a protein carrier called a pump – a pump that hydrolyzes ATP (ATPase)
e.g. sodium/potassium ATPase pump : maintains a specific concentration of Na+ and K+ in and out of the cell
three Na+ are pumped out of a cell and 2 K+ are pumped into the cell
3 Na+ ions bind to the pump
ATP binds to the pump and is hydrolyzed
a phosphate group remains bound to the pump = phosphorylation
phosphorylation changes the activity of the pump by altering its shape
Na+ is expelled out of the cell – against its concentration gradient
2 K+ ions then bind the pump and causes the release of the P group
another shape change by the pump - releases the K+ into the cell

33. Selective Permeability: Membrane Gradients
selective permeability of the plasma membrane PLUS the action of pumps like the Na/K ATPase results in a distinct distribution of positive and negative ions inside and outside the cell
typically the inside of the cell is more negatively charged
this difference in electrical charge and chemical composition between the inside and outside produces a potential energy = electrochemical gradient
because it occurs across the PM – we call this potential energy = membrane potential
MEMBRANE GRADIENTS ARE THE UNDERLYING REASON DRIVING SECONDARY ACTIVE TRANSPORT

34. -the pump that creates this gradient = electrogenic pump
B. Secondary Active transport = ATPase pumps are used to establish a concentration gradient of one ion - which then drives the movement of another ion or ions
-i.e. the energy stored in a concentration gradient is used to drive the transport of other materials
-the pump that creates this gradient = electrogenic pump
-S.A.T is usually is driven by the establishment of either a Na+ or proton gradient
-movement of the additional ions occurs through a membrane protein called a co-transporter
-two kinds: symporters and anti-porters
diffusion
Na pump

35. Sucrose-H cotransporter
e.g. Na/Ca antiporter – opposite direction for Na and Ca movement
– the Na+ concentration gradient creates potential energy which is used by an antiporter protein
- as Na+ leaks back in through this antiporter – the potential energy is converted
into kinetic energy which drives the movement of a Ca ion against its gradient
e.g. Na/glucose symporter
-Na+ diffusion coupled to movement of glucose in the same direction
ATP H
Proton pump
Sucrose-H cotransporter
Sucrose
Diffusion of H  

36. Bulk transport across the plasma membrane occurs by exocytosis and endocytosis
small molecules and water enter or leave the cell through the lipid bilayer or via transport proteins
large molecules (e.g. polysaccharides and proteins) cross the membrane in bulk via vesicles
this bulk transport requires energy
two kinds of bulk transport:
1. Exocytosis – known as secretion
2. Endocytosis – known as internalization

37. Exocytosis
exocytosis= migration of transport vesicles to the plasma membrane – followed by fusion and release of contents
transport vesicles bud from the Golgi apparatus
many secretory cells use exocytosis to export their products

38. Endocytosis
endocytosis = internalization following the formation of a vesicle
vesicles form by invaginating from the plasma membrane
reversal of exocytosis - involving different proteins
three types of endocytosis:
Phagocytosis (“cellular eating”)
Pinocytosis (“cellular drinking”)
Receptor-mediated endocytosis

39. 1. pinocytosis = “cell drinking”
2. phagocytosis = “cell eating”
molecules are taken up when extracellular fluid is “gulped” into tiny pinocytic vesicles
fusion with a lysosome and digestion into component
cell engulfs a particle into a vacuole called a phagosome
fusion with a lysosome to digest the particle

40. -binding of a ligand with its receptor  internalization into the cell
3. receptor-mediated endocytosis = internalization of specific substances at specific locations in the plasma membrane
-binding of a ligand with its receptor  internalization into the cell
-internalization requires a protein called clathrin
-receptor-ligand complexes internalized into clathrin- coated pits clathrin- coated vesicle
-CC vesicle fuses with endosomes – eventually fuse to the lysosome for processing

41. Extracellular components and connections between cells help coordinate cellular activities
most cells synthesize and secrete materials that are external to the plasma membrane
these extracellular materials include:
the extracellular matrix (ECM) of animal cells
intercellular junctions
cell walls of plants
For the Cell Biology Video Ciliary Motion, go to Animation and Video Files.

42. Cell Junctions – Joining Plasma membranes
neighboring cells can interact, and communicate through direct physical contact
one kind of contact is through the formation of intercellular junctions
several types of intercellular junctions
Tight junctions - animals cells
Desmosomes – animal cells
Gap junctions – animal cells
Plasmodesmata – plants cells
Tight junctions prevent fluid from moving across a layer of cells
Tight junction
TEM
0.5 m
1 m
0.1 m
Extracellular matrix
Plasma membranes of adjacent cells
Space between cells
Ions or small molecules
Desmosome
Intermediate filaments
Gap junction

43. Tight Junctions, Desmosomes, and Gap Junctions – Intercellular Junctions in Animal Cells
tight junctions: anchoring junction
PMs are bound together by interlocking membrane proteins
forms a very tight union – can prevent passage of water and solutes between the two cells and between the two cells
also gives the tissue strength
-complex of numerous proteins:
-these protein complexes interact with the actin of the cells cytoskeleton

44. Tight Junctions, Desmosomes, and Gap Junctions – Intercellular Junctions in Animal Cells
2. Desmosomes: anchoring junction
fasten cells together into strong sheets
comprised of cellular adhesion molecules/CAMs and proteoglycans
adhesion plaque links to components of the cytoskeleton (keratin)
desmosomes that link the cell to the underlying basement membrane = hemidesmosome

45. Tight Junctions, Desmosomes, and Gap Junctions – Intercellular Junctions in Animal Cells
Gap junctions = communicating junction
provides cytoplasmic channels between adjacent cells
made of protein complexes called connexons
capable of opening and closing and connecting the two cell interiors

46. Cell Walls of Plants
cell wall = extracellular structure that distinguishes plant cells from animal cells
prokaryotes, fungi, and some protists have cell walls
functions:
1. protects the plant cell
2. maintains its shape
3. prevents excessive uptake of water
made of cellulose fibers embedded in a mixture of polysaccharides and protein

47. Cell Walls of Plants may have multiple layers:
primary cell wall: thin and flexible portion of the wall
made of cellulose and a sugar called pectin
middle lamella: thin layer between primary walls of adjacent cells
secondary cell wall –found in some cells
found between the plasma membrane and the primary cell wall
impregnated with lignin for strength
Secondary cell wall
Primary cell wall
Middle lamella
Central vacuole
Cytosol
Plasma membrane
Plant cell walls
Plasmodesmata
1 m

48. Plasmodesmata in Plant Cells
Plasmodesmata are channels that perforate plant cell walls
like gap junctions between two plant cells
plasmodesma (singular)
clusters of plasmodesmata found in pit fields
through plasmodesmata, water and small solutes (and sometimes proteins and RNA) can pass from cell to cell
Interior of cell
0.5 m
Plasmodesmata
Plasma membranes
Cell walls