Chapter 5 membranes

Active transport uses energy to move materials against a concentration gradient

- against means up a concentration gradient - requires energy from the cell - atp is used directly or indirectly to fuel active transport - move substances , molecules from low to high concentration Like facilitated diffusion , actively transport required the use of a protein carrier ex : the sodium potassium pump which is a transmembrane protein that is embedded in the cell membrane

The sodium potassium pump runs directly on atp

- an ative transport mechanism ; requires the cell to spend energy - it moves sodium ions and potassium ions across the membrane in opposite directions - in a single sodium potassium pump cycle ATPis use with the result that three sodium ions leave and two potassium ions enter - breakdown hydrolysis of ATP to ADP & phosphate is the energy source used to power the pump

Transport can occur by simple diffusion

- molecules and ions dissolved in water are in constant random motion due to their kinetic energy ex. Lump of sugar in water - through this motion dissolved molecules are distributed throughout the solution

The rate of diffusion is affected by

- tempature - size of molecules - steepness of the concentration gradient

In an aqueous solution

- water is the solvent - dissolved substances are the solutes

Couple transport uses ATP indirectly

-During coupled transport carrier proteins create a concentration gradient using energy from atp -coupled transport ( cotransport ) captures energy from the movement of one of the transported substances down its concentration gradient

The phospholipid bilayer is fluid

-Hydrogen bonding of water holds the two layers together -Individual phospholipids & unanchored proteins can move through the membrane -Saturated fatty acids make the membrane less fluid than unsaturated fatty acids - warm tempatures make the membrane more fluid than cold tempatures

Cellular membranes consist of four component groups

1)PHOSPHOLIPID BILAYER : forms a semipermeable barrier ; and can allow diffusion of gases ex. O2 & co2 2) transmembrane proteins : span both layers of a membrane form channels to allow small molecules to cross 3) interior protein network: helps determine the overall shape of the cell 4 ) cell surface identity markers : often carbohydrates attached to lipid or protein "self recognition " tissue recognition

Osmotic pressure

A cell placed in a hypotonic solution would take in water And well A red blood cell placed in a hypertonic solution would loose water shrink and shrivel

Phospholipids

Are arranged in a bilayer

Transmembrane (globular) proteins

Are inserted into the lipid bilayer of the plasma membrane

Atp

As sodium moves back into the cell it's concentration gradient it releases the energy needed to move glucose up its concentration gradient - the transporter is a symporter

Facilitated diffusion by carrier proteins

At high enough concentration all carriers will be occupied the rate of transport will be constant & the carrier exhibits saturation.

Bulk transport of substances is accomplished by

Endocytosis - the movement of substances into the cell Exocytosis - movement of material out the cell

Pores

Extensive non polar regions within a transmembrane protein can create a pore through the membrane B barrel

Plants use TURGOR PRESSURE to push the cell membrane against the cell wall ( keep the cell rigid ) maintain osmotic balance and maintain their shape

H

The partially hydrophilic partially hydrophobic phospholipid spontaneously forms a bilayer

Hydrophobic fatty acid tails are repelled by water and end up on the inside Phosphate groups ( heads ) are on bothe surfaces of the bilayer

Facilitated diffusion of ions through channels

ION CHANNELS : which the span membrane ; allow the passage of ions (charged atoms or molecules ) which are associated with water

Osmotic pressure

If a cell is placed in an isotonic solution there will be no net movement to water water diffused into and out of the cell at the same rate the cell will maintain shape no shrinking or swelling The osmotic concentrations of two solutions are equal

Simple diffusion

Is a process in which materials , small molecules such as oxygen and carbon dioxide will pass through a cell membrane most easily ; they are exchanged between a cell and its environment .

Osmosis

Is the movement of water across membranes

simple diffusion

Is the net movement of molecules across a membrane from areas of higher concentration to areas of lower concentration

Proteins and protein complexes perform key functions

Membrane proteins function, act as : transporters, enzymes , cell surface receptors , cell surface identity markers, cell to cell adhesion proteins & attachments to the cytoskeleton.

Osmosis

Most polar molecules such as sugars and proteins cannot freely cross the lipid membrane Water molecules are polar and small enough to pass through the membrane freely

Antiporters

Move two molecule steps in opposite directions

Symporters

Move two molecules at a time

Material can leave the cell by exocytosis

Occurs when material is discharged from a cell - vesicles in the cytoplasm fuse with the cell membrane & release their contents to the exterior of the cell - used in plants to export cell wall material - used in animals to secrete hormones neurotransmitters , digestive enzymes

Endocytosis

Occurs when the plasma membrane envelops food particles and liquids

Transmembrane protein domain

Often forms an A helical structure - can span both layers of a membrane , crosses the plasma membrane - composed of hydrophobic , non polar amino acids , which anchor ( hold ) the transmembrane protein in the plasma membrane - can form channels to allow small polar molecules to cross membranes

Phospholipid structure

One molecule of glycerol, a three carbon polyalchol acting as a backbone for the phospholipid Two nonpolar hydrophobic fatty acid tails attached to the glycerol One phosphate group made up of a polar hydrophilic head , which is attached to the glycerol ; the phosphate group can have a choline molecule attached to itself

Isosmotic regulation

Organisms in seawater are isoosmotic with respect to their environment ( keep their body cells isotonic to their environment ) also many terrestrial animals ( live on land ) maintain osmotic balance by surrounding their cells with an issomtic ( isotonic fluid)

PAssive transport across membranes

PAssive transport - is a movement of the molecules through the membrane in which - no energy is required - molecules move in response to a concentration gradient

Examples of endocytosis are phagocytosis and pinocytosis

Phagocytosis - the cell takes in particulate matter Pinocytosis - the cell takes in only fluid , liquid material

Permeability also depends on

Polarity - nonpolar by molecules cross easily Electrical charge - molecule is rejected without help of a transport protein ( carrier protein)

Proteins allow membrane diffusion to be selective

SELECTIVELY PERMEABLE : integral membrane (transmembrane) proteins , which are embedded into the cell membrane , allow the cell to be selective about what passed through the plasma membrane

Maintaining osmotic balance

Some single cell end eukaryotes use extrusion in which excess water is constantly pumped out through contractile vacuoles in order to maintain osmotic balance

Another example of endocytosis is receptor - mediated endocytosis

Specific molecules are taken in after they bind to a receptor

Structural features of membrane proteins relate to function

The anchoring of proteins in the bilayer PERIPHERAL MEMBRANE PROTEINS - anchored to a phospholipid in one layer of the membrane INTEGRAL MEMBRANE PROTEINS - span the entire lipid bilayer membrane ( transmembrane proteins )

Phospholipids spontaneously for bilayers

The fatty acid tails are non polar chains- phospholipid tails of carbon and hydrogen -their nonpolar nature makes them hydrophobic ( water fearing ) - the phosphate group is polar and hydrophilic ( water loving )

Osmosis

The movement of water molecules across a membrane from an area of high water concentration to an area of lower concentration (however higher in solute) - is a type of diffusion it is the diffusion of water across a membrane in response to the concentration of one or more of the solutes

The fluid mosaic model shows proteins embedded in a fluid bilayer

The plasma membrane consists of a PHOSPHOLIPID BILAYER - a sheet of lipids two molecules thick The correct model the FLUID MOSAIC MODEL of a membrane structure argues contends that cellular membranes consists of two major components & had transmembrane proteins inserted into the plasma membrane

Aquaporins : water channels

To cross the cell membrane water molecules must pass through aquaporins which are specialized protein channels for

Carrier ( transport) proteins used in active transport include

UNIPORTERS - move one molecule at a time

The sodium potassium pump runs directly on atp

Uses energy from atp to alter the conformation of the carrier protein

Osmosis

When two solutions have different osmotic concentrations -the HYPERTONIC solution has a higher solute concentration - HYPOTONIC HAS A LOWER SOLUTE CONCENTRATION ISOTONIC SOLUTION : the solute concentration in the two solutions is the same

Carrier proteins

Which are embedded in the cell membrane are specific the bind to the specific molecule that they transport across the membrane, they are PAssive

Carrier proteins

Which are embedded into the cell membrane bind to a specific molecule to facilitate its passage

gATED CHANNEL PROTEINS

Which span the cell membrane , are opened or closed in response to stimulus - the stimulus may be mechanical or electrical

Channel proteins

Which span the membrane , have a polar interior allowing polar molecules to pass through these proteins are involved in facilitated diffusion which use PAssive transport

Proteins : multifunctional components

the specific function of a membrane within a cell is determined by the Type and number of membrane proteins