Membrane pt.1

MembranesStructure & Function ~Transport &

Signaling

The Cell Membrane

• Sometimes called the plasma membrane• Outer boundary of lipids and proteins

– Formed of phospholipids– Proteins are embedded in the lipids

• The membrane is approximately 5 nm thick

• Holds the cell together - gives it shape• Regulates what enters and leaves the cell

The Phospholipid Bilayer• Biological membranes are a bilayer

formed from phospholipids. • Phospholipids create a spherical, three

dimensional shell around the cell.• They are often represented 2-

dimensionally as:

Orientation of Phospholipids

• Each phospholipid has a negatively charged phosphate head– These are hydrophilic

• The heads point out: toward the environment and the interior of the cell

• Each phospholipid has two tails that are highly hydrophobic hydrocarbon chains

• The tails orient in, towards each other and away from the watery exterior of the cell– Create a hydrophobic interior part of the

membrane

Membrane Polarity

The Fluid Mosaic

• Lipid bilayers are fluid– Individual phospholipids diffuse

throughout the 2-dimensional surface

• Mosaic property of the membrane– proteins, cholesterol and other

molecules are embedded in the phospholipids

• Membrane proteins diffuse throughout the membrane

Membrane Fluidity• Several factors influence membrane fluidity:• Cholesterol

– A necessary component of biological membranes– Breaks up Van der Waals forces and the close

packing of phospholipid tails– Makes membrane more fluid– Regulating cholesterol regulates membrane fluidity

• Ratio of saturated to unsaturated hydrocarbon chains in phospholipids – Impacts fluidity– Phospholipids with saturated hydrocarbon chains

pack close together & form numerous Van der Waals bonds

The Membrane is Semi-permeable

• The arrangement of phopholipids and embedded proteins makes the membrane semipermeable

• Some molecules are allowed to pass freely (diffuse) through the membrane.

• Other molecules cannot enter the cell– Virtually impermeable to large molecules– Molecules as small as charged ions need a

special process to enter the cell– Quite permeable to lipid soluble low

molecular weight molecules (like CO2 & O2).

Homeostasis• The process of maintaining a stable

internal environment despite changing external conditions

• The cells' environment is surrounded by fluids

• The cytoplasm is mostly water• Most transport involves molecules in

liquid state, in solution– solvent - water in most cell processes– solute

Diffusion• Molecules move from areas of higher

concentration to areas of lower concentration • This process continues until the material is

evenly distributed throughout the substance• A major means of molecular transport in the

cell• Diffusion through membrane doesn't require

energy • Increased temperature increases diffusion rate• Increased pressure increases diffusion rate• Limited by the diffusion rate of the molecule• Effective for some substances: e.g. water

The Concentration Gradient

• Diffusion occurs down a concentration gradient

• Concentration gradient – Difference in concentration of molecules of a

substance from the highest to the lowest concentration

• Moving from an area of high concentration to an area of low concentration is moving with the concentration gradient

• The steeper the concentration gradient (greater difference between concentrations) the more rapid the diffusion.

Osmosis• Diffusion of water molecules through a selectively

permeable membrane from an area of greater concentration to an area of lesser concentration

• Water flows back and forth across the cell membrane until the concentration of water molecules is = on each side

• When concentration is = on both sides: equilibrium

• Concentration of water on each side of membrane determined by concentration of solutes in water

• Requires no energy

Solute Concentration• The concentration of water on each side of

membrane determined by concentration of solutes in water

• Isotonic Solution – concentration of solutes inside = outside– rate of osmosis equal both ways

• Hypotonic – concentration of solutes outside is less than inside– water moves from solution into cell

• Hypertonic – concentration outside is greater than

concentration inside– water flows out of cells

Water Balance

Cells in Hypotonic Solution• Hypotonic

– concentration of solutes outside is less than inside– water moves from solution into cell

• Osmotic Pressure– Pressure caused inside a cell or any sack by the

passage of water in through osmosis

• Freshwater plants• Turgor = pressure in cells from water flowing in

– causes cell to be rigid– cell wall prevents bursting

• Animal cells can't reach equilibrium in hypotonic solution because they lack a cell wall– Some cells have developed mechanisms to remove

excess water before they burst

Contractile Vacuole

Cells in Hypertonic Solution

• In hypertonic solution cells shrivel because water flows out of the cell

• Drinking sea water • Road salt bad for plants• Some animals are specially

adapted – Salmon 

Carrier Transport• Most molecules cannot move across

the membrane freely– Require carriers

• Carrier molecules are proteins in the cell membrane– transport large molecules or molecules

that cannot dissolve in the lipids that make up the cell membrane

•  2 types:– Facilitated Diffusion– Active Transport

Facilitated Diffusion• Like simple diffusion • Substances move with the concentration

gradient• Carrier molecules speed up the movement of

the diffusing substances• Protein channels• Allows charged molecules that couldn’t cross the

membrane to diffuse freely in & out of the cell• Greatest use is small ions: Na+, K+, Cl-

• Speed is limited by the number of protein channels, not the concentration gradient

Picturing Facilitated Diffusion

Active Transport• Also uses carrier molecules• Involves movement of materials against

the concentration gradient– e.g. liver cells store glucose - have higher

concentration of glucose than surrounding tissue, so active transport needed to move glucose in.

• Requires energy• Two categories:

– Primary & secondary

Primary Active Transport

• Uses energy at the membrane protein itself to cause a conformational change– Energy from ATP hydrolysis

• Shape change results in transport of molecule through the protein

• Best known is Na+/K+ pump• Antiport

– Transports K+ in and Na+ out at the same time

The Na+/K+ Pump – An Antiport

Secondary Active Transport

• Uses energy to establish a gradient across the cell membrane

• Utilizes the gradient to transport the desired molecule up its concentration gradient

Establishing a Gradient

The E.coli Lactose Symport

• Symport – coupled transport in the same direction across a

cell membrane

• E. coli establishes a proton (H+) gradient across the cell membrane

• Uses energy to pump protons out of the cell

• Those protons are coupled to lactose at the lactose permease transmembrane protein

• Lactose permease uses the energy of the protons moving down their concentration gradient to transport lactose into the cell

A Model Symport

The Lactose Symport

The Na+-Glucose Symport

• Another secondary active transport system

• Uses the Na+-K+ pump as its first step

• Establishes a strong Na+ gradient across the cell membrane

• Glucose-Na+ symport protein uses the Na+ gradient to transport glucose into the cell

The Na+-Glucose Symport

Na+-Glucose Symport in Gut Epithelium

• Used in human gut epithelial cells• Cells take in glucose and Na+ from the

intestines• Transport them through the blood stream

using Na+-glucose symports, glucose permease (a glucose facilitated diffusion protein) & the Na+/K+ pump

• Epithelial cells are joined together by tight junctions– Prevent anything from leaking through from

the intestines to the blood stream without being filtered by the epithelial cells

Na+-Glucose Symport in Gut Epithelium

Bulk Transport• Allows movement across the cell boundary

without passing through the membrane• Materials that cannot pass through the membrane

need to be transported into or out of the cell– e.g. droplets of fluid, particles of food, etc.

•  Endocytosis – Bulk transport into the cell – Cell membrane encloses particle forming a pouch – Phagocytosis = feeding this way (amoebas, packman)– Pinocytosis = movement of liquids with solutes this way

•  Exocytosis – the reverse - bulk transport out of cell

• Require energy

Endocytosis