UNIT 4 - MEMBRANES

THE PLASMA MEMBRANE

STRUCTURE A phospholipid bi-layer (lipids 50% by weight) Contains embedded proteins (50% by weight)

Fluid consistency (movement) Mosaic pattern of proteins

Main FUNCTION Separation of the internal cell environment from its surroundings

MEMBRANE STRUCTURE EXPLAINED

Phospholipids Hydrophilic Head – Polar head containing a phosphate group Hydrophobic Tails – two fatty acid chains with one or more double bonds

Proteins Peripheral – protein to protein associations. Are not embedded into the membrane Integrated – proteins that are embedded into the membrane

MEMBRANE STRUCTURE EXPLAINED

Cholesterol (animal) and Sterols (Plants) Major membrane constituent Stops membranes from freezing at low temp. Interferes with fluidity at high temp.

Carbohydrates Component of Glycoproteins and Glycolipids Only found on cell exterior Forms the glycocalyx Cell to cell interations and cell surface protection

MEMBRANE STRENGTH

There are 4 components that aid in the strengthening of the plasma membrane. 1. Polar and Non-polar interactions 2. Embedded Proteins 3. Cholesterol 4. Cytoskeleton

PLASMA PROTEINS - FUNCTIONS

Transport – Some proteins act to move substances between the internal and external cell environment

Reception – Some proteins receive specific substances, such as hormones, from the external environment.

Enzymes – Some proteins associated with the plasma membrane will act as enzymes in various ways.

CHANNEL PROTEIN

•FUNCTION: Allow molecules or ions of appropriate size to cross freely.

•Disease: Cystic fibrosis (CF) gene mutation on chromosome 7; produces faulty chloride channel protein.

•Results in thick mucus forming in airways and ducts.

CARRIER PROTEIN

•FUNCTION:

•Selectively interacts with specific molecules.

•Attaches to and assists the molecule across the membrane.

•Disease: Diabetes mellitus-faulty carrier for glucose.

GLYCOPROTEINS

In animal cells, the carbohydrate chains of cell recognition proteins are collectively called the glycocalyx. “sugar-coated”

The glycocalyx can function in cell-to-cell recognition, adhesion between cells, and reception of signal molecules.

The diversity of carbohydrate chains is enormous, providing each individual with a unique cellular “fingerprint”.

CELL RECOGNITION PROTEIN

•Major Histocompatibility Complex (MHC) glycoproteins are specific for each and every person.

•Makes transplants difficult as tissue is considered a foreign body and attacked by blood cells.

RECEPTOR PROTEIN

•FUNCTION: The receptor is shaped in such a way that it is specific to only one molecule.

•Disease: Pygmies are short because their GH receptor proteins are faulty and do not interact with growth hormone.

ENZYMATIC PROTEIN

•FUNCTION: Carry out specific metabolic reactions.

•Disease: Cholera bacteria release a toxin that interferes with an enzyme that regulates Na+ ions in the cell. Severe diarrhea results.

MEMBRANE PERMEABILITY

The plasma membrane is differentially permeable

Small, uncharged molecules may pass freely through membranes (CO2, O2) via their concentration gradient.

Larger, uncharged molecules may not pass freely(glucose).

Charged molecules (H+, Na+) pass via transmembrane proteins (channel proteins).

MEMBRANE TRANSPORT

Passive Transport Movement in the direction of the positive electrochemical gradient No energy is required (diffusion, facilitated transport)

MEMBRANE TRANSPORT

Active Transport Movement in an energy unfavourable direction (requires ATP)

MEMBRANE TRANSPORT

Vesicle Transport Movement of macromolecules Endocytosis – Extracellular material enters the cell Exocytosis – Intracellular material exits the cell

DIFFUSION AND OSMOSIS

Diffusion Passive movement of molecules past the plasma membrane Moves from high to low concentration Ceases after equilibrium has been met

Osmosis Passive movement of water past the plasma membrane from high to low concentration Concentration can be altered by the cell with active transport of certain ions

OSMOSIS IN CELLS

A solution contains a solute (solid) and a solvent (liquid).

Cells are normally isotonic to their surroundings, and the solute concentration is the same inside and out of the cell.

“Iso” means the same as, and “tonicity” refers to the strength of the solution.

OSMOSIS IN PLANT AND ANIMAL CELLS

OSMOSIS IN CELLS

Hypotonic solutions cause cells to swell and possibly burst.

“Hypo” means less than.

Animal cells undergo lysis in hypotonic solution.

Increased turgor pressure occurs in plant cells in hypotonic solutions.

Plant cells do not burst because they have a cell wall.

OSMOSIS IN CELLS

Hypertonic solutions cause cells to lose water.

“Hyper” means more than; hypertonic solutions contain more solute.

Animal cells undergo crenation (shrivel) in hypertonic solutions.

Plant cells undergo plasmolysis, the shrinking of the cytoplasm.

HYPO/HYPER/ISO TONIC

10% Salt

15% Salt

HYPO/HYPER/ISO TONIC

0.9% Fructose

0.9% Fructose

HYPO/HYPER/ISO TONIC

20% Salt

15% Salt

HYPO/HYPER/ISO TONIC

Pure Water 2% Salt

HYPO/HYPER/ISO TONIC

3% Salt 95%

water

TRANSPORT BY CARRIER PROTEINS

Used to move biologically significant molecules past the plasma membrane.

Each carrier protein interacts with a specific molecule

Carrier proteins move molecules with both Facilitated Transport and Active Transport methods.

FACILITATED TRANSPORT

Does not require energy

Movement in the direction of the concentration gradient takes place.

Molecules bind with the carrier protein and the protein undergoes a conformational change

Allows for polar and charged molecules to pass through the plasma membrane

GLUCOSE – FACILITATED DIFFUSION

ACTIVE TRANSPORT

Requires energy (ATP) to be carried out.

Movement against the concentration gradient takes place. (low to high concentration)

Binding of the target molecule to the protein initiates ATP hydrolysis and a conformational change of the protein.

ACTIVE TRANSPORT

NA+/K+ PUMP

Carrier proteins involved in active transport are called pumps.

Step 1 – Na+ binds to the protein inside the cell

Step 2 – ATP hydrolyzes and Na+ is released externally

Step 3 – K+ binds to the protein outside the cell

Step 4 – Hydrolysis of phosphate bound to the pump releases K+ into the cytoplasm

Each ATP hydrolysis of ATP pumps 3 Na+ ions outside the cell and 2 K+ ions into the cell.

NA+/K+ PUMP

Draw Steps on Board

During exocytosis, vesicles fuse with the plasma membrane for secretion.

Some cells are specialized to produce and release specific molecules.

Exocytosis

EXOCYTOSIS

EXAMPLES OF EXOCYTOSIS

The release of digestive enzymes from cells of the pancreas.

Secretion of the hormone insulin in response to rising blood glucose levels.

Called regulated secretion since it only happens when insulin is needed to reduce blood glucose.

ENDOCYTOSIS

During endocytosis, cells take in substances by invaginating a portion of the plasma membrane, and forming a vesicle around the substance.

Endocytosis occurs as: Phagocytosis – large particles Pinocytosis – small particles Receptor-mediated endocytosis – specific particles

PHAGOCYTOSIS

• Phagocytosis occurs when the substance to be transported into the cell is large.

• Amoebas ingest food by phagocytosis.

PHAGOCYTOSIS

PHAGOCYTOSIS

White blood cells are amoeboid and engulf worn-out cellular debris or bacteria using phagocytosis.

PHAGOCYTOSIS

PINOCYTOSIS

Pinocytosis occurs when a macromolecule, such as a polypeptide, is to be transported into the cell.

The macromolecule or polypeptide is still considered small when compared to things brought in by phagocytosis--the resulting vesicle or vacuole is also small.

PINOCYTOSIS

PINOCYTOSIS

Pinocytosis occurs continuously and this uses up the plasma membrane.

What keeps the relative size of the plasma membrane constant?

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor-mediated endocytosis is a form of pinocytosis.

The substance to be taken in binds with a specific receptor protein, which migrates to a pit or is already in a coated pit.

RECEPTOR-MEDIATED ENDOCYTOSIS

RECEPTOR-MEDIATED ENDOCYTOSIS

Receptor-mediated endocytosis is responsible for cells taking up low-density lipoprotein (LDL) when LDL receptors gather in a coated pit.

Disease - Hypercholesterolemia

LDL receptors are unable to properly bind to the coated pit. Cholesterol builds up in arteries.

DIFFUSION – FACTORS AFFECTING RATE

Concentration

Temperature

Ionic/Molecular Size

Shape of Ion/Molecule

Viscosity

Movement of Medium

DIFFUSION – FACTORS AFFECTING RATE

CHAPTER SUMMARY

The structure of the plasma membrane allows it to be differentially permeable.

The fluid phospholipid bilayer, its mosaic of proteins, and its glycocalyx make possible many unique functions of the plasma membrane.

SUMMARY CONTINUED…

Passive and active methods of transport regulate materials entering and exiting cells.

Osmosis describes the movement of water (solvent) as opposed to the solute.

Hypotonic solutions cause increased turgor pressure or even cause lysis.

SUMMARY CONTINUED…

Hypertonic solutions cause crenation (animals) or plasmolysis (plants) in cells.

SUMMARY CONTINUED…

Hypertonic solutions cause crenation (animals) or plasmolysis (plants) in cells.

Passive transport includes simple diffusion or facilitated transport with carrier proteins--no energy. Active transport involves ATP with carrier proteins.

Exocytosis and endocytosis round out the final mechanisms by which materials pass through the plasma membrane. Can you name and explain the different variations of endocytosis?