Download Packet 3- Cells and tissues

Lecture 3: Cells and Tissues
Reading: Silverthorn Chapters 3, 5
The cell: A summary
We begin our survey of human physiology by looking at the most significant parts of the cell. Our treatment of the cell assumes a
solid understanding of cell biology gained in a general biology course, like Bio 1. You are welcome (and encouraged) to review Bio
1 lectures on the cell, if you want to brush up on your basics. In this lecture, we will focus on the cell membrane, which mediates
critical interactions between the ECF and ICF.
1. A CELL: the fundamental unit requiring a homeostatic environment (Review Ch 3)
2. There are 100 trillion cells in human body (Guyton, Med Phys 11e pg3) and 25 trillion are RBC. (Perspective: it would take you
3.2 million yrs to count to 100 trillion if you count 1 #/sec!)
3. 200 different cell types in human body (based on selective gene activation)
A. Interesting view of many types- (Wikipedia claims 300 types of cells...it would be interesting to look at why there is a
discrepancy here!) http://en.wikipedia.org/wiki/List_of_distinct_cell_types_in_the_adult_human_body
Structure of the cell membrane
1. The cell membrane is a semipermeable separation between ICF and ECF.
2. Fluid compartments are separated by cell membranes It mediates interactions between the environment and the cell.
3. It is composed of a phospholipid bilayer that is studded with many different types of molecules, including:
A. Glycoproteins and glycolipids (both involved in immunity and cell recognition)
B. Cholesterol chills within the lipid layer, providing structure and impermeability to water soluble molecules. The more
cholesterol found in the cell membrane, the more “water tight” the cell becomes.
C. Many embedded proteins (The more proteins there are, the more metabolically active the cell)
Proteins found in the cell membrane
1. Structural proteins (connect cells together, keep shape, attach cell to extracellular matrix…)
A. Gap junctions (connexins):
i. Simple tunnel between 2 cells, like a rivet; holds them together AND allows stuff through.
B. Tight junctions (claudins, occludins):
i. Connects two cells, but does not allow passage between cytoplasm, though they can be leaky…
2. Enzymes
A. Catalyze chemical reactions
B. Can be attached to outside surface…
C. Can be attached to inside surface
3. Receptors
A. A cellular protein that binds to a ligand (a molecule that binds to a protein…hormone, NT, ion…)
B. Can be located on cell membrane…also on DNA…also can float!
C. Hormones bind to receptors, triggering a RESPONSE…
4. Transporters
A. Channel proteins
i. Facilitated diffusion
B. Carrier proteins
i. Facilitated diffusion
ii. Primary active transport
iii. Secondary active transport
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Transport: Moving “stuff” through the cell membrane
1. DIFFUSION
A. SIMPLE DIFFUSION: Random movement of molecules from areas of high concentration to areas of lower concentration.
i. Requires no input of energy (passive)
ii. Relies simply on the kinetic energy of the molecules in question.
iii. Stops when equilibrium is attained.
iv. Direct DIFFUSION through cell membrane,
v. Restricted to molecules that are:
a. Lipophyllic (hydrophobic)
b. Small… (Ex: Oxygen, CO2, Nitrogen and alcohol)
c. (Note: water CAN diffuse through the membrane, if there is not much cholesterol in the membrane, even though it
is lipophobic, BUT this is pretty slow…)
2. FACILITATED DIFFUSION requires a protein helper to make it through the cell membrane. There are a couple of types of
proteins that enable movement of molecules in this way.
A. CHANNELS are water-filled tunnels that directly link ECF and ICF.
i. Usually the channel is SPECIFIC for a molecule
ii. Diffusion can be very fast
a. Example: Aquaporins (Water channels)
b. In 1 second, the amount of water that diffuses (in each direction) through the cell membrane of a RBC is about
100x the volume of the cell!
iii. Example: Ion channels
a. Very specific to the substance that can go through…
b. Can be gated (closed or opened based on stimulus, including voltage, a chemical signal/enzyme, or even pressure/
temp stimuli)
c. Gated channels are CONDITIONAL…
d. Example: Gated Na+ channel
I. 0.3x0.5 nanometers in diameter
II. Inner surface VERY NEGATIVELY CHARGED!
III. This negative charge pulls Na+ away from their hydrating water molecules and into the channel.
IV. Ions diffuse down conc gradient.
e. Compare to: Gated K+ channel
I. 0.3x0.3 nanometers in diameter
II. Inner surface NOT NEGATIVELY CHARGED!
III. This lack of negative charge keeps Na+ out b/c the hydrated Na+ is too big to go through
IV. Hydrated K+ is much smaller than hydrated Na+…
V. K+ diffuses down conc gradient.
B. TRANSPORTERS can engage in facilitated diffusion if they move substances DOWN a concentration gradient…
i. Protein carrier provides a way through the cell membrane
ii. Does NOT require direct energy
iii. Is open to one side or other, never form a direct link between ECF and ICF
iv. Is SLOWER than a channel, b/c it must open and close
v. Example: GLUT
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3. ACTIVE TRANSPORT
A. Primary active transport:
i. Molecules are moved against conc gradient
ii. Requires a direct energy source, usually in the form of ATP
iii. Example: Na+/K+ pump! (a membrane protein)
a. Pump is OPEN to the intracellular fluid
b. Pump binds 3 Na+ ions
c. Causes shape change that enables ATP to bind and leave a Ph ion attached to the pump ATP provides the energy for
this
d. Ph ion binding causes pump to change its shape and open to the OUTSIDE
e. Na+ ions fall off, and K+ can bind
f. K+ binds, causing Ph ion to fall off
g. PO4- ion falling off causes shape to change back, and K+ released inside
B. Secondary Active transport
i. Molecules are moved against conc gradient
ii. Does not require a direct energy source, though DOES require another molecule’s concentration gradient
iii. The energy comes from another molecule going DOWN its concentration gradient
iv. Example: (SGLT)
C. Phagocytosis/ endocytosis and exocytosis
i. Does require energy…
ii. Does NOT require a transport protein
iii. Will discuss more when talking abt the immune system
Epithelial transport
Sometimes substances must be transported from outside the body to inside the body, or vice versa. The previous types of transport
moved substances from the ECF to the ICF, or vice versa. If a substance is to move from the outside to the inside of the body, it
MUST CROSS A LAYER OF EPITHELIAL TISSUE.
mm Anatomy link!
Epithelial tissue (ET) lines spaces. Any time there is a space in the human body, it is lined by epithelium.
ETs are classified by the shape of the cells and the number of layers they have. Simple ETs consist of a
single layer of cells, while stratified ETs have multiple layers of cells. Transport happens more easily across
simple ET.
ET is polar. This means you can label the sides of an epithelial cell, because one side makes contact with
the space and the other side makes contact with underlying connective tissue. The apical, or luminal side of
the cell borders the space. The basolateral side makes contact with the underlying connective tissue (at the
basement membrane).
If a substance is going to pass from the outside to the inside, it must pass through TWO layers of cell membrane, because it must first
enter the ET cell from the external environment, and then it must pass through the ET cell and into the ECF. This means there must
usually be transporters on BOTH the luminal and basolateral membranes.
Your Task
Draw a picture of a substance being transported across an epithelial cell. Include
the transporters that would be necessary to make this happen. Defend your answer.
(Hint: Usually one side of the cell actively transports materials into/out of the cell
and the other side makes use of passive (or facilitated) transport.
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External Brain 3: Cells and Tissues
Complete all the following tasks on a separate sheet of paper and include it in your External Brain binder. Remember the rules for the
External Brain. All work must be your OWN, although you may use UNLABELED images if you want. Make sure you cite all your
sources!
Study Guide Questions
1. Describe the structure and function of the cell membrane. Your answer should be specific and detailed.
2. Describe several mechanisms by which molecules move through the cell membrane. Clearly connect the general type of
molecules with the mechanism of transport.
3. Given several cells with different numbers of proteins embedded in the membrane, be able to determine the relative rates of
metabolic activity for each cell. (In other words, label the cell as more or less metabolically active...)
4. Be able to describe different ways cells are connected.
5. Compare and contrast the following transporters:
a. SLGT
b. GLUT
c. Na+/K+ Pump
d. Ion channels
6. List the functions of proteins and other molecules embedded in the cell membrane.
7. Compare and contrast diffusion, active transport, and facilitated diffusion.
8. Compare and contrast primary and secondary active transport.
9. Compare and contrast endocytosis and exocytosis. What kind of transport is this?
10.What is epithelial transport?
11. Draw a single epithelial cell. Label the side that faces the external environment and the side that attaches to the body’s
connective tissue.
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