1. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
All the membranes of a cell have a similar basic structure
Proteins suspended in a double layer of phospholipids
Phospholipids are responsible for the isolating function of membranes
Proteins are responsible for selectively exchanging substances and communicating with the environment, controlling biochemical reactions, and forming attachments

2. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
Functions of the plasma membrane
It isolates the cell’s contents from the external environment
It regulates the exchange of essential substances
It allows communication between cells
It creates attachments within and between cells
It regulates biochemical reactions

3. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
Membranes are “fluid mosaics” in which proteins move within layers of lipids
The “fluid mosaic” model of a membrane was proposed in 1972 by S.J. Singer and G.L. Nicolson
This model indicates that each membrane consists of a mosaic, or “patchwork,” of different proteins that constantly shift and flow within a viscous fluid formed by a double layer of phospholipids
A fluid is any substance whose molecules can flow past one another and includes gases, liquids, and cell membranes

4. Figure 5-1 The plasma membrane
interstitial fluid (outside)
carbohydrate
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) 4

5. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
The fluid phospholipid bilayer helps to isolate the cell’s contents
Phospholipids are the basis of membrane structure and consist of two very different parts
A polar, hydrophilic head
Two nonpolar, hydrophobic tails
The outer surfaces of animal plasma membranes are bathed in watery interstitial fluid, a weakly salty liquid resembling blood without its cells or proteins

6. Figure 5-2 A phospholipid
tails
(hydrophobic)
head
(hydrophilic) 6

7. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
The fluid phospholipid bilayer helps to isolate the cell’s contents (continued)
The phospholipid bilayer is the fluid portion of the membrane
Plasma membranes face both exterior and interior watery environments

8. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
The fluid phospholipid bilayer helps to isolate the cell’s contents (continued)
Hydrophobic and hydrophilic interactions drive phospholipids into bilayers
Hydrogen bonds between water and the hydrophilic heads cause the heads of the outer layer to orient outward toward the watery exterior, while the heads of the inner layer face the watery interior
The nonpolar tails, being hydrophobic, face the inside of the membrane, away from the watery environment around the bilayer

9. Animation: Plasma Membrane Structure

10. Figure 5-3 The phospholipid bilayer of the cell membrane
interstitial fluid
(watery environment)
phospholipid
hydrophilic
head
hydrophobic
tails
bilayer
hydrophilic
head
cytosol
(watery environment) 10

11. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
The fluid phospholipid bilayer helps to isolate the cell’s contents (continued)
The phospholipid bilayer’s flexible, fluid membrane allows for cellular shape changes
Individual phospholipid molecules are not bonded to one another
Some of the phospholipids have unsaturated fatty acids, whose double bonds introduce “kinks” into their “tails”
The above features make the membrane fluid

12. Figure E5-1 Tail kinks in phospholipids increase membrane fluidity
more unsaturated
greater fluidity
more saturated
less fluidity 12

13. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
The fluid phospholipid bilayer helps to isolate the cell’s contents (continued)
Water-soluble substances such as salts, amino acids, and sugars cannot easily cross phospholipid bilayers
However, the isolation provided by the plasma membrane is not complete, and very small molecules such as water, oxygen, and carbon dioxide as well as larger, lipid-soluble molecules can pass through this selective barrier

14. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
The fluid phospholipid bilayer helps to isolate the cell’s contents (continued)
Membranes become more fluid at high temperatures (more movement) and less fluid at low temperatures (less movement)
Cell membranes of organisms living in low temperatures tend to be more unsaturated (more kinks help them maintain fluidity)
Cholesterol stabilizes membranes, affecting fluidity and reducing permeability

15. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane
Proteins are embedded within, or attached to, the phospholipid bilayer
Many proteins have attached carbohydrates (glycoproteins) on their outer membrane surface

16. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Membrane proteins may be grouped into five major categories
Enzymes
Recognition proteins
Receptor proteins
Connection proteins
Transport proteins

17. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Enzymes are proteins that promote chemical reactions that synthesize or break apart biological molecules
Some plasma membrane enzymes are used to synthesize the extracellular matrix, a web of protein and glycoprotein fibers that fills spaces between animal cells

18. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Receptor proteins trigger cellular responses upon binding of specific molecules, such as hormones, sent by other cells
Recognition proteins are glycoproteins that serve as identification tags on the surface of a cell

19. Figure 5-4 Receptor protein activation
(interstitial fluid)
A messenger molecule
binds to the receptor protein
messenger
molecule
receptor
protein
Messenger molecule
binding activates the
receptor protein,
changing its shape
The activated receptor
protein stimulates a
response in the cell
(cytosol) 19

20. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Connection proteins anchor cell membranes in various ways
Maintain cell shape by linking the plasma membrane to the cell’s cytoskeleton
Link the cytoskeleton inside the cell with the extracellular matrix outside, anchoring the cell in place within a tissue
Form connections between adjacent cells

21. 5.1 How Is the Structure of the Cell Membrane Related to Its Function?
A variety of proteins form a mosaic within the membrane (continued)
Transport proteins regulate the movement of hydrophilic molecules through the plasma membrane
Channel proteins are pores that can be opened or closed to allow specific substances to pass across the membrane
Carrier proteins bind substances and carry them through the membrane, sometimes using cellular energy