KPE260 Week 2: Cell Signaling PART 1

membrane dynamics

constant movement & restructuring of lipids & proteins with biological membranes

why are membranes important for cells?

• Survive

• Communicate

• Maintain homeostasis

• compartmentalize

• Control what enters and leaves

major functions of the cell membrane

• Physical/electrical barrier → separates inside from outside

• Mechanical properties → helps with cell shape and movement

• Biochemical hub → site for some cellular reactions

• Communication → allows cells to receive signals

• Compartmentalization → separates different environments

cell signalling

communication between cells using signals.

types of cell signals

1) mechanical = touch/physical force

2) chemical = hormones, growth factors, neurotransmitters

two main types of physiological signals

• Electrical signals → changes in membrane potential

• Chemical signals → signalling molecules such as hormones and neurotransmitters

Where are electrical signals especially important?

In neurons and muscle cells, where changes in membrane potential help produce cellular responses.

The two major categories of membrane transport

1) passive transport

2) active transport

passive transport

Transport that does not require metabolic energy.

Substances move down their gradient.

active transport

Transport that requires energy and can move substances against their gradient.

simple diffusion

he movement of molecules from higher concentration → lower concentration due to random thermal motion.

Does simple diffusion require ATP?

No. Simple diffusion is passive transport.

Where does simple diffusion occur?

Molecules move directly through the lipid bilayer of the membrane.

What types of molecules can easily cross the lipid bilayer by simple diffusion?

small, nonpolar/lipid-soluble molecules.

Examples:

• O₂

• CO₂

• Fatty acids

• Steroid hormones

Why nonpolar molecules are able to cross the membrane easily

The inside of the phospholipid bilayer is lipid-based, so nonpolar molecules can dissolve in it and move through it.

diffusion equilibrium

The point when the concentration of a substance is equal on both sides of the membrane.

What happens to molecules when diffusion equilibrium is reached?

Molecules continue moving randomly, but there is no net movement in one direction.

net flux

The difference between the two one-way movements of a solute across a membrane.

three factors that affect the magnitude of net flux across a membrane:

• Concentration difference

• Surface area

• Membrane permeability

According to Fick's First Law, what happens to diffusion when the concentration difference increases?

Net flux increases.

Greater concentration difference → greater diffusion.

What happens to diffusion when membrane surface area increases?

Net flux increases.

More surface area → more opportunity for molecules to cross.

What happens to diffusion when membrane permeability increases?

Net flux increases.

basic idea of Fick's First Law

concentration difference + surface area + permeability = the greater the net flux.

Why can't ions easily cross the lipid bilayer?

Ions are charged/hydrophilic, so they do not dissolve well in the hydrophobic interior of the lipid bilayer.

examples of ions that use ion channels.

• Na⁺

• K⁺

• Cl⁻

• Ca²⁺

How do ions cross the plasma membrane?

They can move through ion channels down their electrochemical gradient.

electrochemical gradient

The combined effect of: Chemical gradient + electrical gradient on the movement of an ion.

chemical gradient

The difference in the concentration of an ion across the membrane.

three major types of gated ion channels

• Ligand-gated

• Voltage-gated

• Mechanically gated

ligand-gated channel

• shortest duration

• A channel that opens or closes when a chemical messenger/ligand binds to it.

voltage-gated channel

• long duration

• A channel that opens or closes in response to a change in membrane voltage.

mechanically gated channel

• longest duration

• channel that opens or closes in response to physical or mechanical forces

facilitated diffusion

A passive transport process where molecules move from high → low concentration using a membrane protein.

Does facilitated diffusion require ATP?

No.

It is passive transport.

main difference between simple and facilitated diffusion

Simple diffusion:
→ directly through lipid bilayer

Facilitated diffusion:
→ requires a membrane protein/channel

Both:
→ move down their gradient
→ do NOT require ATP

If a transport protein is involved, does that automatically mean the transport is active?

No.

Facilitated diffusion uses a protein but is still passive because the substance moves down its gradient without ATP.

What makes active transport different from passive transport?

Active transport requires energy and can move substances against their gradient.

Where does the energy for many active transport processes come from?

ADP + energy

Why would a cell need to transport something against its gradient?

the cell sometimes needs to maintain specific concentrations of ions or molecules rather than simply allowing them to move down their gradients.

What is the Na⁺/K⁺-ATPase?

An active transport pump that uses ATP to move Na⁺ and K⁺ across the membrane.

What does the Na⁺/K⁺ pump move?

3 Na⁺ OUT 2 K⁺ IN

using ATP.

osmosis

The movement of water across a selectively permeable membrane

direction that water moves during osmosis

higher to lower water concentration (water follows solute)

tonicity

the surrounding solution affecting the volume or size of a cell

3 types of tonicity

1. isotonic

2. hypotonic

3. hypertonic

isotonic solution

a solution that causes no major change in cell volume

• no net water movement

• cell stays the same size

hypotonic solution

a solution with a lower concentration of nonpenetrating solutes outside the cell than inside.

• water enters cell, cell swells

hypertonic solution

a solution with a higher concentration of nonpenetrating solutes outside the cell than inside

• water leaves cell, cell shrinks

What happens to a cell in a hypotonic solution?

Water moves IN → cell swells.

What happens to a cell in a hypertonic solution?

Water moves OUT → cell shrinks.

What happens to a cell in an isotonic solution?

There is no net water movement, so the cell stays approximately the same size.

"nonpenetrating solute"

a solute that cannot freely cross the lipid bilayer (sodium, chloride)

What is the approximate osmolarity of intracellular and extracellular fluid?

300 mOsm/L.

major nonpenetrating solutes in extracellular fluid

sodium, chloride

Hypotonic

water in, cell swells

isotonic

cell remains the same

hypertonic

water out, cell shrinks

endocytosis

A process where the plasma membrane folds inward and pinches off, forming a vesicle that brings material into the cell.

three major types of endocytosis

1. Pinocytosis

2. Phagocytosis

3. Receptor-mediated endocytosis

pinocytosis

"Cell drinking."

The cell takes in:

• Extracellular fluid

• Dissolved substances

phagocytosis

"Cell eating."

The cell engulfs large particles, such as bacteria.

receptor-mediated endocytosis

A specific form of endocytosis where a ligand binds to a receptor, causing the membrane to form a vesicle containing the receptor-ligand complexes.

clathrin-coated pits

Specialized areas of the membrane involved in receptor-mediated endocytosis that help the membrane pinch inward to form a vesicle.

What usually happens to endocytotic vesicles after they enter the cell?

Many fuse with endosomes, which can transfer their contents to lysosomes for digestion.

exocytosis

A process where an intracellular vesicle fuses with the plasma membrane and releases its contents outside the cell.

two functions of exocytosis

1. Releases cellular products outside the cell

2. Adds components to the plasma membrane

easiest way to remember endocytosis vs. exocytosis

endo = in

exo = out