Lecture 7 - Movement Across Cellular Membranes

Diffusion

• Net movement of molecules from HIGH TO LOW CONCENTRATION

Simple Diffusion

• SMALL, NON POLAR molecules sometimes move across membrane like its not there

• Molecules move across membrane DOWN concentration gradient (high to low)

• Characteristics

  • NO energy required

  • Not saturable

  • Like membrane is not there

Electrochemical Gradient

• Movement of molecules across membrane DOWN GRADIENT

  • Facilitated Diffusion

  • Net “+” Charge → Net “-” Charge

• Requires a pore (protein channel/opening)

Pores and Carriers

• Pore → channels

• Carriers → shuttle system

Facilitated Diffusion and Pores

• Polar solutes cannot cross lipid bilayer alone

• Aided by TRANSPORT PROTEINS forming:

  • CHANNELS (pores)

  • SHUTTLE SYSTEM (channels)

• Transport proteins are SPECIFIC for cargo

Diffusion of Water Across Membrane

• H2O moves across membrane via AQUAPORINS (H2O channel proteins)

• Hypotonic or hypertonic

Hypotonic

• Solution with LESS SOLUTES than surrounding media

Facilitated Diffusion

• Movement of molecules across membrane WITH AID OF TRANSPORT PROTEINS

• DOWN CONCENTRATION GRADIENT

• Characteristics

  • NO energy required

  • Is saturable

Hypertonic

• Solution with MORE solutes than surrounding media

Aquaporins

• H2O channel proteins

Active Transport

• Movement of molecules across membrane AGAINST CONCENTRATION GRADIENT

• REQUIRES AID OF TRANSPORT PROTEINS/FACILITATOR

• REQUIRES ENERGY

  • WORK to move solutes UP (AGAINST) CONCENTRATION GRADIENT

  • ATP (energy currency of cell)

• Is saturable

Saturable v. Non Saturable Transport

• Saturable → requires membrane proteins to move molecules

• Non-Saturable → occurs via simple diffusion

Types of Active Transport

• Uniporter (Primary active transport)

• Symporter (Secondary active transport)

• Antiporter

Uniporter

• Primary active transport

• Molecules move across membrane in SINGLE DIRECTION

  • LOWER TO HIGHER CONCENTRATIOn

Symporter

• Secondary active transport

• Moves TWO molecules in SAME DIRECTION

• Example: Sucrose and H+

  • Uniporter moves H+ to outside of cell

    • Primary active transport

    • Low to high concentration

  • High concentration of H+ ions on outside of cell COUPLES with sucrose

    • Both will enter cell together

Antiporter

• TWO different substances move in OPPOSITE DIRECTION

• Example: Sodium-Potassium Pump

  • Unbound protein

    • K+ ON OUTSIDE

    • NA+ ON INSIDE

  • Sodium binds to protein

  • ATP releases phosphate from phosphate group

    • Attaches to newly binded protein

    • Changes shape

    • ATP changes to ADP

  • Sodium released to outside

  • Unbounded protein with phosphate attached allows K+ to enter

    • K+ binds to phosphate

  • Shape of protein changes

  • Releases K+ to inside

Properties Cells MUST DO

• Store and replicate genetic material (DNA)

• Make membranes, proteins, and other macromolecules

  • Business for the cell

• Metabolism

• Transport materials

  • One part of cell to another

• DEGRADE MATERIALS that are damaged or have come into cell and might be harmful

Cell Similarities

• PROKARYOTES AND EUKARYOTES

• DNA AS THE GENETIC INFORMATION (Makes RNA and proteins)

• RELATED ENZYMES (includes enzymes making DNA and RNA)

• RIBOSOMES (molecule machines making protein)

• LIPD BILAYERS (Separates inside from outside)

  • CYTOPLASM → area INSIDE all cells

Plasma/Cytoplasmic Membranes

• Selective permeable barriers

  • Boundary OR filter

• Lipid bilayer

Nucleus

• Location of cell’s chromosome

• Large

• Highly organized

• Structure → Surrounded by DOUBLE-MEMBRANE NUCLEAR ENVELOPE

• Function → INFORMATION STORAGE AND PROCESSING

  • DNA is converted to RNA (transcription)

• Molecules MUST BE TRANSPORTED into and out of nucleus

Nucleous

• Region of nucleus

• Associated with high rates of TRANSCRIPTION (makes RNA)

Endomembrane System

• Integrated network of molecular trafficking

Endoplasmic Reticulum

• Organelle

• Complex out-foldings of other nuclear membrane

  • MEMBRANE ENCLOSED sacs and tubes

• ER Types

  • Smooth

  • Rough

Smooth Endoplasmic Reticulum

• Lacks ribosomes attached

• Involved in:

  • Lipid and carbohydrate synthesis

  • Steroid hormone production

  • Detoxification

Rough Endoplasmic Reticulum

• Ribosomes attached

• Modifies proteins

• Directs protein transport to cell surface or other parts of cell

Ribosomes

• Machines functioning to make proteins

• “Read” RNA molecules to make proteins

• ALL cells have ribosomes

• Not considered organelles

Locations of Ribosomes in Eukaryotic Cell

• Free in CYTOPLASM → organelles AND cytosol (fluid portion of cytoplasm)

• BOUND TO ROUGH ER

  • Make:

    • Proteins staying in endomembrane system

    • Proteins exported/secreted from cell

• MITOCHONDRIA and CHLOROPLAST

Locations of Ribosomes in Prokaryotic Cell

• Free in cytoplasm

Movement of Proteins to Different Parts of Cell

• Golgi organelle

  • Moves protein to cytoplasmic membrane OR into an organelle

Golgi Apparatus

• Organelle

• Protein and Vesicle SORTING CENTER

• Contains STACKS OF MEMBRANES

• Where proteins get TAGGED AND SORTED for delivery

  • Tagged with carbohydrates

• VESICLES BLEB OFF (pinch/bud off) Golgi → Filled with proteins destined for different places around cell

Secretory Pathway

• Ribosome deposit protein in ER

• Protein exits ER

• Protein enters Golgi for processing

• Protein exits Golgi

• Protein exits cell