Download Unit IV Teacher Notes

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UNIT IV – CELL TRANSPORT
I. Cell Membrane (pgs. 182-183)
Every cell is surrounded by a cell membrane. The cell membrane is
_selectively permeable_ which means it only allows certain substances in
and certain substances out. This characteristic is critical in helping the cell maintain
_homeostasis__.
A. The Cell Membrane is composed of three biomolecules:
1. Lipids – There are two lipids that make up the cell membrane:
__phospholipids_ and _cholesterol (steroid)__.
a. phospholipid bilayer - _Non-polar_ glycerol backbone with two fatty acid
tails (hydrophobic) and a _polar_ (hydrophilic) phosphate group on the
other end. The bilayer consists of 2 layers of phospholipid molecules.
b. cholesterol – found in the cell membranes of _animal_ cells to help
stabilize them. Cholesterol is wedged between the non-polar fatty acid tails
of the phospholipid bilayer to maintain stability and fluidity.
2. Carbohydrates – found on the outside of the cell membrane and Serve as “ID
tags” to _identify the cell__.
3. Proteins – Proteins are embedded in the phospholipids bilayer. Serve as
__channels__ and _pumps_ for transport of specific molecules across the
cell membrane.
II. Traffic Across the Cell Membrane
A cell is a working unit and thus, needs to move materials in and out of the cell.
A. Passive Transport (pgs. 183 – 187)
Movement of molecules across a membrane without using energy. There is a
movement of substances from a HIGH concentration to a LOW concentration.
There are 3 types of passive transport:
1. Diffusion – random movement of molecules in a liquid or gas from high
to low. Ex. perfume, food coloring
Many substances move across the cell membrane (phospholipid bilayer)
by diffusion. Ex. CO2, O2
DIFFUSION
2. Facilitated Diffusion – diffusion of specific substances from high to low
through a protein channel. Protein channels provide for larger openings for
molecules like glucose, fructose, galactose to get through.
3. Osmosis – diffusion of water from a high water concentration to a low
water concentration through a selectively permeable membrane. Cells must
have a mechanism for counteracting the pressure osmosis can
create, otherwise a cell could swell & burst or explode when it comes in
contact with a dilute water solution.
Cells control this osmotic pressure in one of three ways:
a. cell wall – physically prevents the cell from expanding. Ex. bacteria,
plants, fungi
b. contractile vacuole – actively pumps water out of the cell. Ex.
protists
c. blood – cells are bathed in blood which is mostly water, but yet also
has other components to equalize the pressure on both sides of the cell
membrane. Ex: Human cells
**Comparative Terms: These terms describe the solution outside the cell compared
to inside the cell.
Hypertonic – increased concentration of solute (sugar, salt), and decreased
concentration of water.
Hypotonic – decreased concentration of solute (sugar, salt), and increased
concentration of water.
Isotonic – equal concentration of water & solute inside the cell compared to
outside the cell.
(crenated)
B. Active Transport – requires energy (ATP) (pgs. 188-189)
1. Protein Pumps – energy requiring process during which membrane
proteins pump molecules across a membrane against a concentration
gradient from a LOW concentration to a HIGH concentration. Ex.
Pumping of ions such as which are important in nerve transmission and
muscle contraction.
2. Endocytosis – “ sucking IN” Import of macromolecules INTO the cell
from the exterior by forming a vesicle around the molecule from the
existing cell membrane.
Specific types of endocytosis are:
a. phagocytosis – “cellular eating” Large, solid particles. Ex. Amoeba
eating; WBC’s in our immune system
b. pinocytosis – “cellular drinking” Small, fluid droplets
3. Exocytosis – “spitting OUT” Export of macromolecules FROM the inside of
the cell to the exterior of the cell by a vesicle fusing with the cell
membrane. Ex. Insulin, neurotransmitters