Cell Junctions + Transport

Objectives: Define tissues Describe the membrane junctions that structure tissues and allow cells to communicate Describe types of diffusion and cellular transport between cells Compare and contrast solute movement across the cell membrane Name different transporters according to their role Compare/contrast hormones and neurotransmitters

what are tissues

cells that act and are associated together 

• Dependent on intercellular junctions and communication 

what are junctions

contact points between cells that hold cells together to create functional unit

tight junctions

impermeable and binds cells together so It's leak-proof 

• Apical (top) 

adherens junctions

binds cells together using actin (the actins are part of the cytoskeleton and so you’re joining the cytoskeletons of neighboring cells)

• Right underneath tight junctions 

desmosomes

anchoring junctions that prevent cells from being pulled apart, distributes stress through intermediate filaments 

• Below adherens junctions 

hemidesmosomes

• Hemidesmosome: ½ of a desmosome that anchors cell to basement membranes 

gap junctions

allows direct chemical communication between cells 

• Has connexons: intercellular channels that allow small molecules to flow between cells 

what is membrane transport and what are the 2 types

movement of substances in/out the cell 

• Passive transport 

• Active transport 

passive transport: definition + 2 types

down a concentration gradient 

• No energy needed 

• 2 types: simple diffusion and facilitated diffusion

simple diffusion

movement of particles down a gradient without a transporter 

• Small, uncharged molecules like O2 and Co2 

facilitated diffusion

requires transporter protein 

• For bigger molecules  

• Ex: aquaporins because simple diffusion of water is really slow because it's polar and the plasma membrane is hydrophobic inside, so aquaporins speed up the movement of water, ion/leakage channels that move ions/charged molecules 

active transport: definition + 2 types

against a concentration gradient 

• Needs energy (either ATP or membrane potential) 

• 2 types: primary and secondary active transport

Primary active transport example

prime example is Na-K pump: breaks ATP to force NA out and K inside cell

secondary active transport examples

the flow of Na back in can pick up other molecules along with it when it goes back in  

• Na and amino acids flow back into cell 

• Na flows in, H+ flows out 

• Na flows in, Ca flows out 

• Cl- flows in, HCO3 flows out 

uniporter

carries only 1 molecules

• for active transporter

symporter

2 molecules flow down in the same direction

• for active transport

antiporter

one molecule flows in, one molecule flows out

• for active transport

what is cell-cell communication + why is cell-cell communication important

Cells sense and respond to the environment via molecules at their surface (the cell membrane). Basis for processes like: 

• Cell differentiation 

• Endocrine signaling 

• Neural signaling 

Ligand-gated channel

opens up and allows ions to pass through when a ligand (chemical messenger molecule) binds to it  

voltage-gated channel

opens up when there is a change in voltage/membrane potential 

• Na+ channel 

• K+ channel 

Na+/K+ ATPase Pump:

uses energy to move 3 Na+ out of the cell and 2 K+ into cell

leakage channel

passive diffusion, allows continuous flow of ions down their concentration gradient (lets K out and Na in)

mechanically gated channel

opens/closes in response to physical touch, pressure, vibration, or stretch

temperature-gated channel

opens/closes in response to thermal energy

what does endo and exocytosis do

imports and exports nutrients, waste, neurotransmitters, hormones, etc 

endocytosis and its 3 types

brings in substances. There are 3 types: 

• Phagocytosis: plasma membrane senses substance and binds it to the plasma membrane, enclosing around the substance into a vacuole to bring it in; big substances 

• Pinocytosis: used for very small particles 

• Receptor-mediated cytosis: ligand binds to receptor on plasma membrane, enclosing the substance into a coated vesicle to bring it in 

exocytosis

pushes out substances 

• Vesicle fuses with plasma membrane, releasing substances into extracellular matrix 

hormones vs neurotransmitters

hormones:

• Released by cells for long distance cell to cell communication 

• long term effect, slow to act

• hormone is only expressed when there’s a receptor

neurotransmitters:

• Released by neurons for neuron to cell communication, short-distance  

• immediate and short-term effect

some signaling molecules can be both a hormone and neurotransmitter