The Cell Membrane

Membrane structureMembranes form compartments around and

within cells. For example:

Plasma membraneNuclear membraneEndoplasmic

reticulumGolgi apparatusLysosomesMitochondriaChloroplastsVacuoles

Membrane structure

Basic characteristics of cell membranesThey are 8 nm thick. They are fluid mosaics with proteins embedded or attached to a double layer of phospholipids.They separate a living cell from its nonliving

surroundings.They control traffic into and out of the cell.They are selectively permeable: some sub-

stances pass more easily than others.

Membrane structure

Membrane structureA fluid mosaic with proteins embedded or

attached to a double layer of phospholipids.Everything is free to move; not locked in

place.

Membrane structureFluidity: lipids and proteins are not stuck in

one place; they can drift side-to-side.

Membrane structureProteins and phospholipids have hydrophobic

(water-hating ) and hydrophilic (water-loving ) regions.

PhospholipidPhospholipids & proteins in a membrane

Membrane structureThe hydrophobic and hydrophilic regions of

phospholipids cause them to align as a bilayer when water is present.

Proteins fall into the proper region depending upon their sequence of hydrophobic and hydrophilic

amino acids. ⇒

Membrane structureMembranes also contain cholesterol, which

keeps the hydrophobic tails of the phospho-lipids from bunching up in cold temperatures.

Animals that live in cold climates have more cholesterol in their cell membranes.

Membrane structurePlacement of membrane proteins

Integrated proteins are within the membrane.Peripheral (external) proteins are on surface.

Membrane structureFunctions of membrane proteins

Hormone-binding sitesImmobilized enzymesCell adhesionCell-to-cell communicationChannels for passive transportPumps for active transport

Functions of membrane proteinsHormone-binding sites

The receptor protein scoops up a hormone, such as estrogen, traveling in the blood.

The receptor protein ⇒

Functions of membrane proteinsImmobilized enzymes

Enzymes catalyze chemical reactions.Groups of enzymes create a metabolic pathway.

Ex: enzymes for respiration in mitochondria.

Functions of membrane proteinsCell adhesion

Tight junction proteins bind cells together. Adhesion belts and desmosomes glue cells together.

Functions of membrane proteinsCell-to-cell communication

Receptor proteins also aid cell recognitionEx: Blood group glycoproteins (have sugar chains)Ex: receptors for HIV let virus attach to cells

Functions of membrane proteinsChannels for passive transport

Passive – no energy required.Some channel proteins let large molecules (like sugars) pass

through the membrane by dif- fusion. Carriers help ions pass.More real design above

Channel conceptualized designs Carrier

Passive transportSmall uncharged molecules like O2, CO2, and lipids can

pass between the phospholipids of the mem-brane by simple diffusion.

Non-polar fatty acids are barriers

to charged molecules

Diffusion of one thing does not affect dif-fusion of another.

Passive transport

Click box for video.

Functions of membrane proteinsPumps for active transport

Cells can accum-ulate large quantities of a material if they use ATP energy.

Examples:Proton pump in mitochondriaNa+/K+ pump in nerve cells

Active transportIn active transport, ATP energy must be used to do

work.Work involves moving a substance against a concentration gradient – from a low to a high concentration.

Diffusion goes from high to low concentration.

Membrane fluidityMembrane fluidity allows it to break, change

shape, and reform.The membrane is a mosaic as a result.

Membrane fluidityThe loose packing of phospholipids and proteins

makes the membrane fluid.It’s like a bunch of marbles rolling around together.

New ones come up from below and push the old ones aside.

Diffusionand Osmosis

Diffusion and osmosisDiffusion – the passive movement of particles from

a region of high concentration to a region of low concentration.

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_diffusion_works.html

Examples: perfume in air, smoke in air, sugar in water, oxygen through cell membranes

Diffusion and osmosisDiffusion – the passive movement of particles from

a region of high concentration to a region of low concentration.

Example: Bromine gas in a beaker separated from pure air by a glass plate.

Diffusion and osmosisOsmosis – the passive movement of water molecules,

across a partially permeable membrane, from a region of lower solute concentration to a region of higher solute concentration.

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_osmosis_works.html

Hypotonic = low concentration ofsolute (high H2O).

Water is “pulled”to the hypertonicside.

Concentrationseven out (same g/ml of the soluteand of water).

Diffusion and osmosisOsmosis

Ex: Two solutions with different concentrations of sugar are separated by a membrane that only lets water pass.

The hypertonic solution (right) has a lower water concentration than the hypotonic solution (left) since some water is displaced by sugar molecules.

Diffusion and osmosisOsmosis

Two sugar solutionsWater molecules will move from the hypotonic solution where they are abundant to the hypertonic solution where they are rarer.

Diffusion and osmosisOsmosis

Osmosis requires a selectively permeable membrane.Osmosis continues until the solutions are isotonic (=same concentration).The direction of osmosis is determined only by a

difference in total solute concentration.

Diffusion and osmosisWater moves across cell membranes by osmosis.

top – animal cell; bottom – plant cell

Sea water pulls water out of cell –causes dehydration.

The cell is swollen by distilled water.

Passive transportSmall uncharged molecules like O2, CO2, and lipids can

pass between the phospholipids of the mem-brane by simple diffusion.

Non-polar fatty acids are a bar-

rier to polar moleculesDiffusion of one thing does not affect dif-fusion of another.

Passive transportLarger molecules, with or without a charge (polar),

must pass through channel proteins.This is called facilitated diffusion.

The diffusion is aided by membrane tunnels.No energy is used (movement is passive).The rate of movement is determined by the number of channel or carrier proteins.Carrier proteins are very specific.

Carrierprotein

Channelprotein

Passive transport

Active transportIn active transport, ATP energy must be used to do

work.Work involves moving a substance against a concentration gradient – from a low to a high concentration.

Diffusion goes from high to low concentration.

Active transportUsing ATP, cells can maintain a much higher

concentration of a substance than would be allowed by diffusion.

ATP is used with a carrier protein to pump materials into or out of the cell.

Ex: a Na+/K+

pump in nerve cells maintains a high level ofintracellular K+.

Active transportConcentrating ions on one side of the

membrane

Transport within the cellWithin the cell, materials may be moved from the

endoplasmic reticulum to the Golgi apparatus and past the cell membrane within vesicles.

Vesicles are membrane-bound bubbles.This is how hor-

mones, etc., are secreted.

Bubble becomes part of the cell membrane. Note the sequence above!

ExocytosisMoving large molecules from inside to

outside the cell – large proteins.

EndocytosisMoving large molecules from outside to

inside the cell – food (bacteria into white blood cells).