Plasma membrane Structure and Function
See the fluid-mosaic model of the plasma membrane on p 81 fig 3.15
The plasma membrane separates the internal and external environment of the cell and regulates the entrance and exit of molecules from the cell. This allows the cell and the organism to maintain a steady internal environment, a process called
homeostasis.
There are numerous proteins embedded in the membrane
1)peripheral that are held in place by cytoskeleton and
2) integral, which can move back and forth).
It forms a fluid mosaic model because of the fluid nature of the membrane and the steroids and the proteins that are in it.
Carbohydrate (sugar) chains are attached to the phospholipids (glycolipids) and to the proteins (glycoproteins) and play a role in cellular identification.
Draw a picture here later p 81

Draw and describe the different kinds of proteins in the chart below (see p 82)
Type of Protein function drawing
Channel Allows a particular molecule or ion to move freely (eg. Cl )
Cystic fibrosis if its not working properly
Carrier Selectively interacts with a specific molecule or ion so that it can cross.
(eg. GLUT carries glucose across)
Cell recognition MHC glycoproteins are different for each person.
Cells that come from another person get attacked by white blood cells b/c their MHC gp are different
Receptor
Enzymatic
Shaped in a way that a specific molecule can bind to it.
(eg. If growth hormone receptors are faulty and cannot interact with growth hormone, dwarfism happens)
Catalyzes a specific reaction
(Cholera bacteria release toxins that interfere with adenalate cyclase, involved in ATP metabolism. This leads to severe diarrhea)

The permeability of the Plasma membrane
Selectively permeable: certain substances can move across the membrane an others cannot
The plasma membrane is selectively permeable and helps to maintain “normal” conditions of the cell.
1. Some molecule can pass easily through the membrane with NO additional energy.
They cross the membrane through PASSIVE TRANSPORT
Here are three forms of passive transport. a) Osmosis: the movement of water from an area of high concentration of water to low concentration (down the concentration gradient) b) Diffusion: the movement of molecules (not water) from an area of high concentration to low concentration (lipid soluble molecules and gases, like CO
2
and
O
2
) c) Facilitated transport: molecules move down their concentration gradient but through a channel or carrier (Some sugars and amino acids).
(fig 3.22 on p 87): draw

Other molecules can go against their concentration gradient from an area of low concentration to an area of high concentration but this requires energy
Here are three forms of transport that require energy: a) Active Transport: a carrier is required plus energy (ATP) and the cells involved often have lots of mitochondria (which make ATP). The cells are often called pumps; eg. Iodine collects in thyroid gland; cells lining digestive tract absorb glucose
Draw the steps in the sodium-potassium pump below (Fig 3.23, p88)
The final two other forms that require energy are types of bulk transport b) Exocytosis: a vesicle fuses with the plasma membrane as secretion occurs; hormones (like the insulin produced by the pancreatic cells), neurotransmitters and digestive enzymes,.
The Golgi apparatus often supplies the vesicles. The vesicle becomes part of the plasma membrane c) Endocytosis: cells take in substances by vesicle formation. A part of the plasma membrane folds in on itself to envelop the substance and then the membrane pinches off to form an intracellular vesicle.
There are three forms of endocytosis;

phagocytosis (for large molecules), pinocytosis (for small molecules) and receptor mediated endocytosis (a special form of pinocytosis)
(see Fig 3.25 on page 91)
Draw and explain the three forms of endocytosis below:
Names of group members(circle your name): ____________________________________________
In groups of three make analogies for each of the forms of transport
Type of transport Function and how it works?
Does it require energy?
Analogy
What is it like? Think borders, clubs, schools…
Facilitated transport The passage of glucose (not lipid soluble) and amino acids from high to low concentration is sped up by bonding with carrier proteins that are specific to one molecule.
No energy expenditure is required
Active transport
Bulk transport
Molecules or ions move through the plasma membrane, accumulating inside or outside and may go in the opposite direction of the concentration gradient. (Iodine taken out from urine by cells in kidneys)
Carrier proteins (pumps) AND energy expenditure are required.
There are LOTS of mitochondria in these cells! A great example is the sodium potassium pump.
Macromolecules (polypeptides, polysaccharides, polynucleotides) enter and exit a cell by vesicle formation (or membrane assisted transport because part of the cell membrane is needed to make vesicle)
This requires energy .
Exocytosis (exit cell)/Endocytosis (enter cell)

a) Exocytosis
P 90, Fig 3.24
A vesicle fuses with plasma membrane as secretion occurs
(hormones, neurotransmitters and digestive enzymes). b) Endocytosis p 91 Fig 3.25 a) phagocytosis b) pinocytosis c) Receptor mediated
Endocytosis (a special form of pinocytosis)
Vesicles take in substances by vesicle formation. Plasma membrane folds in on itself to envelop substance, then membrane pinches off to form an intracellular vesicle a) Phagocytosis: the material taken in by endocytosis is large; can be seen with light microscope
(amoebas and white blood cells in humans engulf debris) b) Pinocytosis vesicles form around a liquid or very small particles
(blood cells in kidneys or intestines and plant root cells)
A receptor protein shaped so a specific molecule like a vitamin, peptide hormone or lipoprotein can bind to it
Usually a coated pit with layer of protein on cytoplasm side of pit.
The vesicle becomes uncoated and may fuse with lysosome
It is involved in uptake and transfer between cells; maternal to fetal blood
Hypercholesterolemia: LDL is not binding in the pit as it should, cells can’t take up cholesterol and collects in walls of arteries