BIOL 151- Molecules & Membranes

MOLECULES & MEMBRANES

CELL MEMBRANES

• Composed of a phospholipid bilayer

  • Double layer of phospholipids

• Proteins “float” in the sea of lipids (Fluid Mosaic model)

• Carbohydrates are attached to some proteins and lipids

THE MEMBRANE HAS A HYDROPHOBIC INTERIOR

• The polar phospholipid “heads” stick out on either side

  • Interact with water inside & outside of the cell

• The nonpolar “tails” are inside, away from water

• Membrane proteins must have both hydrophilic and hydrophobic regions

MEMBRANE COMPONENTS

• Lipids

  • Provide a semi-permeable barrier

• Proteins

  • Transport proteins aid in moving substances across the membrane

  • Others receive signals from outside the cell

• Carbohydrates

  • Attached to proteins or lipids (glycolipids and glycoproteins)

  • Important for cell-cell recognition and adhesion

THE CELL MEMBRANE IS A SEMI-PERMEABLE BARRIER

• Due to the very non-polar interior, it is permeable to only some molecules

  • Non-polar molecules can cross easily (e.g., steroids, lipid hormones)

  • Polar molecules (e.g., glucose, amino acids) cannot diffuse across

  • Small exceptions (e.g., water)

  • Ions can never cross on their own (e.g., Na+)

SUPPORT FOR THE FLUID MOSAIC MODEL

• Method

  • A mouse cell membrane protein is labeled with a green dye

  • A human cell membrane protein is labeled with a red dye

  • Cells are fused to create a heterokaryon

• Results

  • Initially, membrane proteins are separate

  • After 40 minutes, proteins intermixed, supporting the fluid mosaic model

MEMBRANE COMPOSITION VARIES

• Different cells or organelles have varied lipid compositions

  • Includes saturated vs unsaturated fatty acids,

  • Types of phospholipids, amount of cholesterol

• Importance for membrane stability and fluidity

• Affected by:

  • Lipid composition (more saturated = less fluid)

  • Temperature (colder = less fluid)

  • Cold-blooded animals may increase unsaturated fatty acids in colder months

TWO TYPES OF MEMBRANE PROTEINS

• Peripheral membrane proteins

  • No hydrophobic region, not embedded in membrane

  • Interact with integral membrane proteins or phospholipid head groups

• Integral membrane proteins

  • Partially embedded in the membrane

  • Contains both hydrophilic and hydrophobic parts

  • If it spans the membrane and protrudes on both sides, it is a transmembrane protein

FREEZE-FRACTURE METHOD TO VISUALIZE MEMBRANE PROTEINS

• Frozen tissue fractured with diamond/glass knife

• Fracturing separates one half of the membrane along weak hydrophobic interfaces

• Proteins sticking out of the fractured membrane suggest they were embedded in the bilayer

CELL-CELL RECOGNITION AND ADHESION

• Involves plasma membrane and is essential for tissue formation and maintenance

  • Skin and muscle cells recognize and stick to similar cells

• Demonstrated in sponges, multicellular animals that can re-form tissues after separation

CELL JUNCTION TYPES

• Tight junctions

  • Prevent substances from moving through spaces between cells

  • Seal intercellular spaces to prevent leaks

  • Found in bladder and intestinal cells

• Desmosomes

  • Hold neighboring cells firmly together, reinforcing attachments

  • Important for tissues under physical stress (e.g., skin, heart)

• Gap junctions

  • Channels between membrane pores in adjacent cells

  • Allow rapid communication (e.g., electric current in heart muscle)

EXTRACELLULAR MATRIX (ECM)

• Secreted mix of proteins and glycoproteins surrounding cells

• Functions:

  • Holds cells together in tissues

  • Facilitates communication and cell movement during development/repair

  • Forms barriers between different tissues

CELL MEMBRANES ADHERE TO THE ECM

• Integrin

  • Supports attachment in animal cells

  • Binds ECM outside the cell and cytoskeleton inside

INTEGRIN AND CELL MOVEMENT

• Detaches from ECM on one side

• Extends the other side in direction of movement, forming new attachment

• Rearrangement facilitates cellular locomotion

TRANSPORT ACROSS CELL MEMBRANE

• Passive Transport

  • Requires no energy; substances move down concentration gradient

  • Diffusion from high to low concentration

PASSIVE TRANSPORT TYPES

• Simple diffusion

  • Nonpolar molecules diffuse across the membrane independently

• Facilitated diffusion

  • Larger polar molecules/ions require membrane proteins

• Regulation by cell or extracellular stimuli can control channel opening

CARRIER PROTEINS

• Used instead of channels for transport

• Limited speed for molecule diffusion across the membrane

SUMMARY - PASSIVE TRANSPORT

• Nonpolar substances diffuse across membrane without energy

  • Examples: O2, CO2, steroids

• Ions require protein channels; larger polar molecules (e.g., glucose) utilize protein carriers

• Transport limited to movement down concentration gradient

ACTIVE TRANSPORT

• Movement against concentration gradient

• Requires energy from the cell (ATP)

• Three types of transporters exist

EXAMPLE: SODIUM-POTASSIUM PUMP

• Antiporter moving 2 K+ into and 3 Na+ out against concentration gradients

• Uses 1 ATP per cycle

PRIMARY vs. SECONDARY ACTIVE TRANSPORT

• Primary: Uses ATP directly (e.g., Na/K pump)

• Secondary: Utilizes energy from established concentration gradient for additional transport

BULK TRANSPORT

• Endocytosis and exocytosis

  • Methods to move groups of molecules into or out of the cell

  • Both require energy (ATP)

• Endocytosis: Plasma membrane folds inward to form a vesicle around materials

TYPES OF ENDOCYTOSIS

• Phagocytosis (“cell eating”)

  • Engulfs large particles or whole cells

  • Vesicle fuses with lysosome for digestion

• Pinocytosis (“cell drinking”)

  • Forms vesicles to bring in fluids

  • Common in endothelial cells for nutrient acquisition

• Receptor-mediated endocytosis

  • Captures specific extracellular substances using receptor proteins

  • Forms vesicles upon binding

RECEPTOR-MEDIATED ENDOCYTOSIS

• Involves receptor proteins and clathrin

• Formation of "coated pits" helps in vesicle creation

SUMMARY OF ENDOCYTOSIS

• Visual representation of three types:

1. Phagocytosis: Engulfing large particles

2. Pinocytosis: Bringing in fluids

3. Receptor-mediated endocytosis: Targeted substance uptake

EXOCYTOSIS MOVES THINGS OUT OF CELLS

• Reverse process of endocytosis

• Transports substances out in bulk

A SIDE NOTE ON OSMOSIS

• Defined as "the diffusion of water across a semipermeable barrier"

• Just like all substances, water moves down its concentration gradient

TONICITY

• Used to compare solute concentrations between two solutions

• Hypertonic: Higher solute concentration

• Hypotonic: Lower solute concentration

• Water always moves toward hypertonic solution

THIS APPLIES TO CELLS TOO

• Hypertonic Solutions: Water loss leading to cell shrinkage

• Isotonic Solutions: Equal water movement in and out, stable cell shape

• Hypotonic Solutions: Water uptake, causing swelling or bursting if no cell wall is present.