Unit 3: Cell Membranes

osmosis

• what affects osmosis?

• why does osmosis occur?

movement of water across a SEMIpermeable membrane (H to L)

• CONCENTRATION GRADIENT 

• to balance out concentration of solution across the membrane 

what makes up a solution?

SOLVENT (dissolved IN) & SOLUTE (BEING dissolved)

Tonicity (3 kinds)

Comparative description of a solution ; isotonic, hypertonic, hypotonic

Isotonic

Concentration is SAME

Hypertonic 

Concentration of SOLUTES (BEING dissolved) is GREATER

• Means there’s LESS water 

hypotonic 

concentration of solutes (BEING dissolved) is LOWER 

• Means there's MORE water 

Is pure water hypo or hyper-tonic?

HYPO

Hypotonic animal cell

Lysed (burst)

• KILLS cell

Isotonic animal cell

Normal

• Cell is happy

Hypertonic animal cell

Shriveled 

• Cell is mad

Hypotonic plant cell

Turgid (normal)

• Helps the plant stand up b/c central vacuole is so full its putting pressure on cell wall

  • Cell is happy

Isotonic plant cell

Flaccid

• Cell is meh b/c central vacuole isn't consistently full

Hypertonic plant cell

Plasmolyze

• Cell is sad

• Membrane pulls away from cell wall

What does the plasma membrane do?

"selects“ what goes in/out 

• Allows SOME substances to pass more easily than others 

Fluid Mosaic Model

Model showing the membrane composed of LIPIDS & PROTEINS

(Called MOSAIC because its composed of diff. parts)

Phospholipids in plasma membrane

• amphipatic

most abundant lipid in plasma membrane ; can move w/n membrane

• containing both hydrophobic & hydrophilic regions

how do phospholipids move in the : 

COLD 

Warm

Cold: UNsaturated phosph. to allow gaps for diffusion since they move SLOWER 

Warm: SATURATED phosph. to prevent membrane gaps getting too big 

What does cholesterol do in membrane ; where?

• What does it do in COOL/WARM temp?

acts a temp. buffer ; located b/w phospholipids

• Cool: Increases fluidity (stops them from getting too slow)

• Warm: Decreases fluidity (stops them from moving too much)

Integral proteins

• transmembrane proteins

Penetrates hydrophobic core of lipid bilayer

• pushes molecules through membrane

• span the membrane

Peripheral proteins 

LOOSELY bound to surface of membrane 

Protein functions (6)

1) transport

2) enzymatic activity

3) signal transduction

4) cell-cell recognition

5) intercellular joining

6) attachments to ECM & cytoskeleton

Cell-cell recognition

Ability to distinguish self from non-self

• interacts w/ surface molecules of OTHER CELLS

Passive transport ; 2 kinds?

Diffusion of a substance (NO ATP INVESTMENT)

• Simple diffusion

• Facilitated diffusion

Simple diffusion

• What decides what can pass through the plasma membrane?

W/O PROTEIN

• Molecules move DIRECTLY through plasma membrane

• HydroPHOBIC tails (ONLY NONPOLAR)

Facilitated diffusion

Uses protein assistance (POLAR OR IONS)

What do transport proteins do

• 2 kinds

Allow passage of hydrophilic substances across membrane

• Channel / Carrier

Channel proteins

(Aquaporins)

Has a tube (channel) through it

• Allows H2O to pass through plasma membrane

Carrier proteins 

Binds specific substances ; UNDERGO CONFORMATIONAL SHAPE CHANGE to displace molecule

• Passive (w/o ATP) or active (w/ ATP)

Cystinuria

Absence of carrier proteins for amino acid CYSTEINE in kidney cells

• Result : accumulation of amino acids that crystalize into painful stones

Active transport

• What does it create?

Moves substance AGAINST THEIR CONCENTRATION GRADIENT

• REQUIRES ATP

• Creates artificial H & L to create gradients

Electrogenic Pumps 

• Electrochemical gradients

Transport proteins that generates the voltage across a membrane 

• An UMBRELLA TERM for proteins that actively move IONS across cell membrane 

• EG: Positive-Negative attraction

Protein pumps

• Membrane Potential

Use ATP to move POSITIVE IONS (proton gradient)

• Makes the cell NEGATIVE across membrane

• Generates membrane potential (one positive side on negative side of membrane)

  • AKA voltage difference across membrane

Sodium Potassium pump

Kind of electrogenic pump

• Found most commonly in nerve cells

  • 3 sodiums OUT : 2 potassiums In

  • Inside of cell is therefore negative
    

Cotransport 

Occurs when passive transport of a specific solute INDIRECTLY drives active transport of another solute 

• DOES NOT USE ATP DIRECTLY 

• Active transport driven by a protein gradient

• 2 molecules move TOGETHER

  • One by NATURAL DIFFUSION which PULLS the other

Bulk transport

• 2 Types

For LARGE PROTEINS

• EXOcytosis

• ENDOcytosis

Exocytosis

Transport vesicles migrate to the plasma membrane, fuse w/ it, & release their contents

Endocytosis

• 3 types (just names)

Cell takes in macromolecules by forming new vesicles from plasma membrane

• Phagocytosis, Pinocytosis, Receptor Mediated Endocytosis

Phagocytosis 

Cell "eating“ / engulfing 

• FOR SOLIDS 

• Generalized strategy

Pinocytosis

Cell "drinking“ (l solutes) (NOT WATER)

• Generalized strategy

Receptor Mediated Endocytosis

Used for 1 specific solute at a time

• Receptor protein usages