1. Chapter 7
Membrane Structure & Function

2. 7.1 Life at the Edge
• Plasma Membrane is a thick film
– About 8 nm thick
– Controls traffic in and out of the cell
• Exhibits selective permeability
– Allows some substances into the cell, and others
cannot enter
– Ability to discriminate between certain materials
can be life or death

3. Fluid Mosaic Model
• Lipids and proteins are staple materials in
the membrane
– Most abundant lipids are phospholipids
• Amphipathic molecule – both hydrophilic region and
hydrophobic region

4. Fluid Mosaic Model
• Fluid Mosaic Model
– Membrane is a fluid structure with a “mosaic” of
various proteins

5. The Fluidity of Membranes
• Membranes are held together by
hydrophobic interactions
– It is possible for the lipids and proteins to shift
around within the membrane itself
– Adjacent phospholipids switch places about 107
times per second
– Proteins are larger and move a lot slower

6. The Fluidity of Membranes
• A membrane remains fluid as temperature
decreases
– Eventually, the lipids get packed together and the
membrane solidifies
– Similar to bacon grease turning into lard
– If the membrane is rich in unsaturated
hydrocarbons, the kinks in their tails make it
harder to pack together
• So, the membrane will remain fluid at lower
temperatures

7. The Fluidity of Membranes
• Cholesterol
– Steroid lodged between phospholipid molecules
in the membrane
– At higher temperatures, it makes the membrane
less fluid by restraining phospholipid movement
– Lowers temperature for membrane to solidify

8. The Fluidity of Membranes
• Membranes must be fluid in order to function
properly
– When it solidifies, the permeability changes
– Enzymatic proteins may become inactive if they
cannot move within the membrane
– If too fluid, however, it cannot support protein
function

9. Evolution of Differences in
Membrane Lipid Composition
• Variations in the cell membrane lipid
composition of many species are speculated
to be caused by evolutionary adaptation
– Fishes that live in extreme cold have a higher
amount of unsaturated hydrocarbon tails which
allows their membrane to stay fluid at these
temperatures
– At high temperature extremes, some bacteria and
archaea have unusual lipids that may prevent
excess fluidity at these temperatures

10. Evolution of Differences in
Membrane Lipid Composition
• The ability to change the lipid composition
based off the current temperature has also
been shown as an adaptation
– Some plants that live in cold, such as winter
wheat, adjust the amount off unsaturated
phospholipids in autumn to keep the membranes
from solidifying in the winter

11. Membrane Proteins and their
Functions
• A membrane is a collage of different proteins
that are often grouped and embedded into
the fluid matrix of the lipid bilayer
– More than 50 unique proteins have been
discovered in the plasma membrane of the red
blood cell
– Phospholipids make the “material” or structure of
the membrane, and proteins determine the
function

12. Membrane Proteins and their
Functions
• Two major types of proteins
– Integral
– Peripheral
• Integral Proteins
– Penetrate the hydrophobic interior of the bilayer
– They are “integrated” into the membrane
– Mostly transmembrane proteins
• Peripheral Proteins
– Not embedded; on outside of the membrane
– Often exposed to parts of integral proteins

13. Membrane Proteins and their
Functions
• On the cytoplasmic side of the cell, some
proteins are held in place by being attached
to the cytoskeleton
• On the extracellular side, certain membrane
proteins are attached to fibers of the
extracellular matrix
– These attachments give a stronger structure than
the membrane could provide by itself

14. Membrane Proteins and their
Functions
• Proteins on the surface of the cell are
important in the medical field
– Some can help outside agents invade the cell
• HIV (human immunodeficiency virus) infects the cells
through the help of a surface protein
– Also important in medicine being transferred to
the cells, etc.

15. The Role of Membrane Carbohydrates
in Cell-Cell Recognition
• Cell-cell recognition
– Ability to distinguish one type of neighboring cell
from another
– Important in defense and making sure everything
is working properly
• Glycolipids
– Short, branched chains of sugars that bond to
lipids
• Glycoproteins
– Short, branched chains of sugars that bond to
proteins

16. Selective Permeability
• The fluid mosaic model helps explain S.P.
– A steady traffic of small molecules and ions
moves across the plasma membrane in both
directions
– Cell regulates inorganic ions too
• Na+, K+, Ca2+, Cl-
– Nonpolar molecules such as hydrocarbons,
carbon dioxide, and oxygen, are hydrophobic
• They can dissolve easily in the lipid bilayer and cross
easily
– Passive transport

17. Transport Proteins
• Transport Proteins
– Proteins that allow hydrophilic substances to pass
through without making contact with lipid bilayer
– Channel Proteins
• Hydrophilic channel that specific ions and materials use
to pass into/out of the cell
• Aquaporins
– Water can pass through these at 3 billion H2O
molecules/second

18. Permeability
• Diffusion
– The movement of molecules of any substance so
that they spread out evenly into the available
space
• A higher concentration of material X will diffuse into
whatever solution has a lower concentration of material
X
• Concentration Gradient
– The region along which the concentration of a
chemical substance increases or decreases
• No work must be done to make this happen

19. Passive Transport
• Passive transport requires no energy
– Like following the flow of a river

20. Tonicity
• Isotonic
– The cell and its surrounding solution are the same
concentration
• Hypertonic
– The cell is hypotonic, so the surrounding solution
is hypertonic and the contents of the cell will
diffuse out of the cell and into the solution
• Hypotonic
– The cell is hypertonic, so the surrounding solution
is hypotonic and the solution around the cell will
diffuse into the cell

21. Water Balance Without Walls
• Osmoregulation
– Control of solute concentrations and water
balance
– Paramecium caudatum
• Live in hypotonic pond water
• To prevent water from rushing in, it has adapted to the
water by developing a membrane that is much less
permeable to water than other organisms

22. Water Balance of Cells with Walls
• Cells of plants, prokaryotes, fungi, and some
protists are surrounded by walls
– The walls help to maintain water balance
– After being pushed so far, the walls’ inelasticity
will cause the wall to rebound, demonstrating
turgor pressure
• Opposes further water uptake

23. Water Balance of Cells with Walls
• The cell will become Turgid
– Very firm, and healthy state for a lot of these cells
– This is what allows non-woody plants to stand
upright
• Flaccid
– If the cell is placed in an isotonic solution, there is
no pushback of water; so, the cells become limp,
or flaccid
• Plasmolysis
– If placed into a hypertonic solution, the membrane
will pull from wall, causing wilting in plants &
eventual death

24. Facilitated Diffusion
• Facilitated Diffusion
– Passive transport aided by proteins
– Polar molecules can diffuse through proteins in
the membrane and get into/out of the cell
• Ion Channels
– Channel proteins that transport ions
• Gated Channels
– Ion channels that open/close in response to
stimuli

25. Active Transport
• Active transport
– To pump a solute across a membrane, against its
concentration gradient
– Proteins that help do this are called carrier
proteins

26. How Ion Pumps Maintain Membrane
Potential
• Membrane Potential
– The voltage across a membrane
– Acts like a battery, an energy source that affects
the traffic of all charged substances across the
membrane
• Electrochemical Gradient
– Two forces (electrical, chemical) driving ions
across the membrane

27. How Ion Pumps Maintain
Membrane Potential
• Electrogenic Pump
– A transport protein that generates electrical
voltage across a membrane
• Proton Pump
– The main electrogenic pump of plants, fungi, and
bacteria

28. Cotransport
• Cotransport
– When H+ may enter through into the membrane
and “carry” another material with it
• Amino acids, sugars, etc

29. Bulk Transportation
• Exocytosis
– The transport of a bulk amount of materials
through vesicles containing the substance fusing
with the membrane
• Endocytosis
– The transport of a bulk amount of materials by
pushing into the membrane, and as it deepens, it
eventually pinches off into a vesicle that travels
into the cell