Chapter 7- Membrane Structure and Function

Phospholipids

Amphipathic molecules and are the major component of biological membranes

Biological membrane

Phospholipid bilayer with other embedded molecules (Proteins and cholesterol)

Biological membrane is referred to as

Cell membranes or plasma membrane

Fluid Mosaic Model

Biological membranes are fluid and a mosaic of membrane-embedded proteins

Two types of membrane associated proteins

Integral membrane proteins and peripheral membrane proteins

Integral Membrane Proteins

Integrated within the cell membrane usually spanning the entire bilayer

Peripheral Membrane Proteins

Proteins on the periphery (or perimeter) of a cell membrane

1) Membrane-associated proteins functions

Recognition- Marks cell for identification

2) Membrane-associated proteins functions

Anchorage- Cell cytoskeleton and ECM anchoring

3) Membrane-associated proteins functions

Transduction- signal molecule receptors

4)Membrane-associated proteins functions

Transport- molecular transport across membrane

5) Membrane-associated proteins functions

Linkage- connects two cells via protein linkage

6) Membrane-associated proteins functions

Enzymes- many times of enzymatic processes

Concentration Gradient

Difference in the concentration of a substance between 2 areas

A molecules moves with (or down)

gradient when going from an area of high to low concentration

A molecule moves against (or up)

its gradient when going from an area of low to high concentration

Diffusion

The movement of a substance from an area of high concentration to an area of low concentration

Molecules have natural tendency

to diffuse with (down) their concentration gradients (from high to low)

Biological membranes

are semi-permeable and can act as barriers to prevent diffusion of molecules

Selectively permeable (or semi-permeable)

Chooses what crosses the membrane

Some molecules can freely diffuse across a membrane

without facilitation from a protein

CAN Freely diffuse

Small, uncharged, nonpolar/hydrophobic

CANT freely diffuse

Large, Charged (+/-), Polar/Hydrophilic

What are the two general types of molecular transport across biological membranes

Passive transport (no energy), Active transport (Requires energy)

Passive Transport

No energy, transports molecules from a high to low concentration

Active Transport

Requires energy, transports molecules from a low to high concentration

What are the three types of transport proteins

Uniportors, Symporters, Antiporters

Uniporters

Transport one molecule at a time in just 1 direction

Symporters

Cotransport more than 2 molecules at a time in the same direction

Antiporters

Cotransport more than 2 molecules at a time in opposite directions

Osmosis

Passive diffusion of a solvent (Usually water) across a semi-permeable membrane

Direction of water flow depends on toncity

Relative concentration of solutes dissolved in the solutions

Hypotonic solutions

have lower solute concentration

Isotonic solutions

have equal solute concentrations

Hypertonic solutions

have a higher solute concentrations

Directions of osmosis

Water will move from hypotonic to hypertonic solutions if the solutes cannot diffuse across the membrane

Direction of osmosis

Water moves towards the more concentrated solution of solute to dilute it until it becomes isotonic

Hypotonic Environments

H2O enters cells causing them to swell and potentially lyse (Burst)

Isotonic environments

H2O enters and exists the cell at equal rates (preferred by animal cells)

Hypertonic Environments

H2O exits cells causing them to dehydrate

Simple diffusion

Simple and direct diffusion of small uncharged molecules through a cell membrane

Facilitated diffusion

Non-energetic diffusion of charged molecules facilitated by a transport protein

2 types of transport proteins involved in facilitated diffusions

Porins/channels and transporters/carriers

Porins/Channels

Form an obvious membrane-spanning-tunnel

Aquaporins

Used to transport water molecules through a cell membrane (facilitating osmosis)

Transporters/Carriers

Undergoes conformational changes to move molecules across a membrane

What are the two types of active transport that require enegy since molecules are transported against their gradient

Primary active transport and secondary active transport

Primary Active transport

Directly driven by energy source (Such as ATP hydrolysis)

Secondary active transport

Directly Driven by another molecules concentration gradient

Primary active transport

An ATP-driven process transporting molecules against their concentration gradient

Directly driven by energy derived from ATP hydrolysis

Used to generate and maintain important concentration gradients for cell survival

Primary active transport: Na+/K+ pump

An example of primary active transport that moves Na+ and K+ ions in opposite directions (antiporter)

3 Na+ ions are exported while 2 K+ ions are imported

Secondary active transport

Directly driven by a concentration gradient instead of ATP hydrolysis

however, its indirectly driven by Primary Active Transport

4 steps to Na+-Glucose Secondary Active Transport: 1:

1.Na+ is transported against its concentration gradient using primary

2. Higher concentration of Na+ is generated on the outside of the cell

3. Glucose has a higher concentration inside the cell than outside

4. Na+ transportation down its gradient “powers” Glucose transport against its gradient

Endocytosis and exocytosis

Large biomolecules (Proteins, Carbohydrates, DNA) are too large to diffuse through membranes or channels

-Instead macromolecules are transported across cell membranes via endocytosis and/or exocytosis

Endocytosis

Macromolecule engulfment by the cell membrane, allowing entry into the cell via a lipid vesicle

What are the main 3 types of endocytosis

Phagocytosis, pinocytosis, receptor-mediated-endocytosis

Phagocytosis

A large solid material is taken in by endocytosis (Cell “eating)

Pinocytosis

Small, liquid material taken in by endocytosis (Cell-”drinking”)

Receptor-Mediated-Endocytosis

Specific form of pinocytosis using receptor proteins

Exocytosis

Vesicles fusion with a cell membrane, allowing its contents to exit the cell to the extracellular space