Cell Junctions

“serve to anchor cells to one another or to provide a passageway for cellular exchange.”

Three major types: Desmosomes Tight junctions Gap junctions

Cell Junctions

Desmosomes Hold two cells together Intermediate filaments (made of protein

keratin) Provide mechanical stability and strength Like rivets fastening cells into strong

sheets Tissues: skin, uterus, heart

Desmosomes

Tight Junctions Create impermeable seals around cells Example: digestive tract tube does not

allow enzymes from digestive tract to pass through bloodstream

Animal cells only Prevents leakage of molecules

Tight Junctions

Communication junctions “passageways between cells that allow

transfer of chemical or electrical signals” Two kinds:

plasmodesmata gap junctions

Gap Junctions Narrow tunnels made of protein connexins Prevents cytoplasm of each cell from

mixing Allows passage of ions/small molecules Tunnels-1.5 nm diameter Gap between cells- 2-3 nm Pattern of channel formation determines

permeability

Endocytosis (endo = inside, cyto = cell)

Capture of substances outside the cell when the plasma membrane merges to engulf it.

***There are three types of endocytosis1. phagocytosis2. pinocytosis3. receptor-mediated

Pinocytosis (pino = to drink, cyto = cell)

Pinocytosis occurs when dissolved materials enter a

cell. The plasma membrane folds inward to form a channel allowing the liquid to enter. The

plasma membrane closes off the channel, encircling the liquid inside a vesicle.

Oils enter cells through pinocytosis

Phagocytosis (phago = to eat, cyto = cell)

Phagocytosis occurs when undissolved solids enter a

cell. The plasma membrane wraps around

the solid material and engulfs it, forming a

vesicle. Phagocytic cells, such as white blood cells, attack and engulf bacteria

in the manner.

Receptor-Mediated Endocytosis

Occurs when specific molecules bind to specialized “receptors”

(proteins) in the plasma membrane. The membrane, the

receptors, and the specific molecules, called ligands, fold

inward forming vesicles. Hormones target special “target

cells” by receptor-mediated endocytosis.

Ligand

Receptor Protein

Molecule being ingested

Exocytosis (exo = outside, Cyto = cell)moving substances outside the cell

Process of vesicles fusing with the plasma membrane and releasing their content to the outside

of the cell.

Passive Transport No energy expended by the

cell Goes “downhill” with the

concentration gradient Substances move from [HI] [LOW] Examples: diffusion, osmosis,

facilitated diffusion

Diffusion

Solution = solute + solvent Solute- substance that is dissolved Solvent- substance that does the dissolving

(fluid) Solute particles dissolved in a solvent are

constantly in motion. Temperature motion

This constant motion causes DIFFUSION Diffusion- movement of particles from areas of

high concentration to areas of low concentration

Osmosis and Tonicity Hypertonic- the solution that has [solute]

Water flows toward the hypertonic environment

Hypotonic- the solution that has [solute]

Water flows away from a hypotonic environment

Isotonic- the solutions have equal concentrations of solute (no net osmosis)

Osmosis and Tonicity

Facilitated Diffusion

• Down gradient• No ATP used

Carrier – Mediated Transport

Carrier – Mediated Transport

Active Transport (Sodium-Potassium Pump)

• Needed because Na+ and K+ constantly leak through membrane

• One ATP utilized to exchange three Na+ pushed out for two K+ brought in to cell

Carrier – Mediated Transport

Secondary Active Transport

Sodium-Potassium Pump

CYTOPLASM

Glucosemolecule

Sodiumion

Na+–K+

pump

+

+

• Regulation of cell volume• Heat production • Maintenance of a membrane potential

• Secondary active transport (No ATP used)

Voltage-Gated Sodium Channels

Voltage-Gated Potassium Channel