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5-3 Notes
Active Transport = requires the cell to expend energy (ATP) to move substances across the
membrane

Goes against the gradient from [Lo]  [Hi]

Helps the cell maintain steep ionic gradients
o Example: The Sodium-Potassium Pump (See handout)

May be used to generate a membrane potential = voltage across a membrane

Cell’s interior is usually negatively charged, which favors the diffusion of cations (+) into
the cell and anions (-) out of the cell
o Creates potential energy

Electrochemical Gradient = a concentration gradient and an electrical gradient
o Due to separation of ions (charges) across a membrane
o Ex. Proton Pump = transports H+ out of the cell and generates a membrane
potential in plants, fungi, and bacteria
BULK TRANSPORT
Exocytosis 
 Endocytosis
Phagocystosis (“cell-eating”)= endocytosis of solids
Pinocytosis (“cell-drinking”) = endocytosis of liquids or solutes
Receptor-Mediated Endocytosis = Specific substances are brought into the cell

Ligands = specific molecules that bind to the receptors

Receptors = proteins embedded in the membrane’s surface in clusters in coated pits,
which are coated with clathrin, that dip in to form vesicles around the ligands
o Allow the cell to bring in large quantities of specific molecules

Ex. Cholesterol and LDL receptors
CELL SURFACES AND JUNCTIONS (that is, things OUTSIDE the plasma membrane)
Extracellular Matrix (ECM)

In animal cells

Meshwork of macromolecules outside the plasma membrane of animal cells

Made mostly of glycoproteins, the most common of which is collagen

Helps cells adhere to each others

Provides support and anchorage for cells

Helps control gene activity in the cell’s nucleus
Intercellular Junctions in Animals
Adhesion Junctions

Desmosomes = junctions that rivet cells together into strong sheets, but still permit
substances to pass between through intracellular spaces.


Like “Plinko” (from The Price is Right)
Tight Junctions = Intercellular junctions that hold cells together tightly enough to block
transport of substances through the intercellular space.

Occur as belts all the way around each cell that block intercellular transport.
Communication Junctions

Gap Junctions = connect the cytoplasm of adjacent cells.

Formed by two connecting protein rings (connexon), each embedded in the
plasma membrane of adjacent cells.

Have pores large enough to allow cells to share small molecules, but not
macromolecules
Cell Walls

Most cells produce them, but NOT animal cells

Stronger than the plasma membrane

Protect plant cells, maintain their shape, and prevent excess water uptake.

Plant cells develop as follows:

Young plant cell secretes a thin, flexible primary cell wall


Made mostly of cellulose
Between primary cell walls of adjacent cells is a middle lamella made of pectin, a
sticky polysaccharide that cements cells together.


Allows cells to stretch when growing
Some cells add lignin to produce a secondary cell wall (which is harder)
Neighboring cells often adhere and interact through special patches of direct physical contract
Intercellular junctions in plants:

Plasmodesmata = Channels that perforate plant cell walls, allowing cytosol to flow freely
between cells

Plasma membranes of adjacent cells are continuous through a plasmodesma.

Allows free passage of water and small solutes.

Similar to gap junctions in animal cells