1665: Robert Hooke 1895: Charles Overton - composed of lipids 1900-1920’s: must be a phospholipid 1925: E. Gorter and G. Grendel - phospholipid

bilayer 1935: J.R. Danielli and H. Davson – proteins also

part, proposed the Sandwich Model 1950’s: J.D. Robertson – proposed the Unit

Membrane Model 1972: S.J. Singer and G.L. Nicolson – proposed

Fluid Mosaic Model

Gorter + Grendel Red Blood Cells analyzedEnough for Phospholipid bilayerPolar heads face out and

Nonpolar tails face inDoes not explain why some

nonlipids are permeable

Sandwich Model(Danielli + Davson)2 layers of globular proteins with phospholipid inside to make a layer and then join 2 layers together to make a channel for molecules to pass

Unit Membrane Model (Robertson)Outer layer of protein with phospholipid bilayer inside, believed all cells same composition, does not explain how some molecules pass through or the use of proteins with nonpolar parts, used transmission electron microscopy

Fluid Mosaic Model (Singer + Nicolson)Phospholipid bilayer with proteins partially or fully imbedded, electron micrographs of freeze-fractured membrane

1) Rapidly freeze specimen

2) Use special knife to cut membrane in half

3) Apply a carbon + platinum coating to the surface

4) Use scanning electron microscope to see the surface

According to the electron micrograph which membrane model is correct?

Why?

Fluid-Mosaic Model

Fluid – the plasma membrane is the consistency of olive oil at body temperature, due to unsaturated phospholipids. (cells differ in the amount of unsaturated to saturated fatty acid tails)

Most of the lipids and some proteins drift laterally on either side. Phospholipids do not switch from one layer to the next.

Cholesterol affects fluidity: at body temperature it lessens fluidity by restraining the movement of phospholipids, at colder temperatures it adds fluidity by not allowing phospholipids to pack close together.

Mosaic – membrane proteins form a collage that differs on either side of the membrane and from cell to cell (greater than 50 types of proteins), proteins span the membrane with hydrophilic portions facing out and hydrophobic portions facing in. Provides the functions of the membrane

Phospholipid bilayer

Phospholipid Hydrophilic head Hydrophobic tails

Cholesterol

Proteins Transmembrane/

Intrinsic/Integral

Peripheral/Extrinsic

Cytoskeletal filaments

Carbohydrate chain

Glycoproteins

Glycolipids

1) Transport Proteins2) Receptor Proteins3) Enzymatic Proteins4) Cell Recognition Proteins5) Attachment Proteins 6) Intercellular Junction Proteins

Channel Proteins – channel for lipid insoluble molecules and ions to pass freely through

Carrier Proteins – bind to a substance and carry it across membrane, change shape in process

– Bind to chemical messengers (Ex. hormones) which sends a message into the cell causing cellular reaction

– Carry out enzymatic reactions right at the membrane when a substrate binds to the active site

– Glycoproteins (and glycolipids) on extracellular surface serve as ID tags (which species, type of cell, individual). Carbohydrates are short branched chains of less than 15 sugars

- Attach to cytoskeleton (to maintain cell shape and stabilize proteins) and/or the extracellular matrix (integrins connect to both).

- Extracellular Matrix – protein fibers and carbohydrates secreted by cells and fills the spaces between cells and supports cells in a tissue.

- Extracellular matrix can influence activity inside the cell and coordinate the behavior of all the cells in a tissue.

– Bind cells togetherTight junctionsGap junctions

Tight Junctions

Desmosomes

Gap Junctions

Transmembrane Proteins of opposite cells attach in a tight zipper-like fashion

No leakage Ex. Intestine, Kidneys, Epithelium of skin

Cytoplasmic plaques of two cells bind with the aid of intermediate filaments of keratin

Allows for stretching Ex. Stomach, Bladder, Heart

Channel proteins of opposite cells join together providing channels for ions, sugars, amino acids, and other small molecules to pass.

Allows communication between cells. Ex. Heart muscle, animal embryos

• Materials must move in and out of the cell through the plasma membrane.

• Some materials move between the phospholipids.

• Some materials move through the proteins.

• Molecules move across the plasma membrane by:

1) Diffusion2) Facilitated Diffusion3) Osmosis

ATP energy is ATP energy is notnot needed to move the needed to move the molecules through.molecules through.

• Molecules can move directly through the phospholipids of the plasma membrane

This is called …

•Diffusion is the net movement of molecules from a high concentration to a low concentration until equally distributed.

•Diffusion rate is related to temperature, pressure, state of matter, size of concentration gradient, and surface area of membrane.

http://www.biologycorner.com/resources/diffusion-animated.gif

•Gases (oxygen, carbon dioxide)

•Water molecules (rate slow due to polarity)

•Lipids (steroid hormones)

•Lipid soluble molecules (hydrocarbons, alcohols, some vitamins)

•Small noncharged molecules (NH3)

• Cell respiration• Alveoli of lungs• Capillaries• Red Blood Cells• Medications: time-

release capsules

• Molecules can move through the plasma membrane with the aid of transport proteins

This is called …

• Facilitated diffusion is the net movement of molecules from a high concentration to a low concentration with the aid of channel or carrier proteins.

• Ions (Na+, K+, Cl-)

• Sugars (Glucose)

• Amino Acids

• Small water soluble molecules

• Water (faster rate)

• Channel and Carrier proteins are specific:• Channel Proteins allow ions, small solutes, and

water to pass• Carrier Proteins move glucose and amino acids• Facilitated diffusion is rate limited, by the number

of proteins channels/carriers present in the membrane.

• Cells obtain food for cell respiration

• Neurons communicate

• Small intestine cells transport food to bloodstream

• Muscle cells contract

• Water Molecules can move directly through the phospholipids of the plasma membrane

This is called …

• Osmosis is the diffusion of water through a semipermeable membrane. Water molecules bound to solutes cannot pass due to size, only unbound molecules. Free water molecules collide, bump into the membrane, and pass through.

• What will happen in the U-tube if water freely moves through the membrane but glucose can not pass?

• Water moves from side with high concentration of water to side with lower concentration of water. Movement stops when osmotic pressure equals hydrostatic pressure.

• Cells remove water produced by cell respiration.

• Large intestine cells transport water to bloodstream

• Kidney cells form urine

Tonicity refers to the total solute concentration of the solution outside the cell.

What are the three types of tonicity?1) Isotonic2) Hypotonic3) Hypertonic

Solutions that have the same concentration of solutes as the suspended cell.

What will happen to a cell placed in an Isotonic solution?

The cell will have no net movement of water and will stay the same size.

Ex. Blood plasma has high concentration of albumin molecules to make it isotonic to tissues.

Solutions that have a lower solute concentration than the suspended cell.

What will happen to a cell placed in a Hypotonic solution?

The cell will gain water and swell. If the cell bursts, then we call this lysis. (Red

blood cells = hemolysis) In plant cells with rigid cell walls, this creates

turgor pressure.

Solutions that have a higher solute concentration than a suspended cell.

What will happen to a cell placed in a Hypertonic solution?

The cell will lose water and shrink. (Red blood cells = crenation)

In plant cells, the central vacuole will shrink and the plasma membrane will pull away from the cell wall causing the cytoplasm to shrink called plasmolysis.

• Diffusion – O2 moves in and CO2 moves out during cell respiration

• Facilitated Diffusion – glucose and amino acids enter cell for cell respiration

• Osmosis – cell removal or addition of water

What will happen to a red blood cell in a hypertonic solution?

What will happen to a red blood cell in an isotonic solution?

What will happen to a red blood cell in a hypotonic solution?

1) Active Transport1) Primary2) Secondary (no ATP)

2) Bulk Transport 1) Exocytosis2) Endocytosis

1) Phagocytosis2) Pinocytosis3) Receptor-Mediated

endocytosisATP energy is ATP energy is requiredrequired to move the to move the molecules through.molecules through.

Molecules move from areas of low concentration to areas of high concentration with the aid of ATP energy.

Requires protein carriers called Pumps.

Bring in essential molecules: ions, amino acids, glucose, nucleotides

Rid cell of unwanted molecules (Ex. sodium from urine in kidneys)

Maintain internal conditions different from the environment

Regulate the volume of cells by controlling osmotic potential

Control cellular pH Re-establish concentration

gradients to run facilitated diffusion. (Ex. Sodium-Potassium pump and Proton pumps)

3 Sodium ions move out of the cell and then 2 Potassium ions move into the cell.

Driven by the splitting of ATP to provide energy and conformational change to proteins by adding and then taking away a phosphate group.

Used to establish an electrochemical gradient across neuron cell membranes. http://www.biologie.uni-hamburg.de/b-online/library/biology107/bi107vc/fa99/terry/images/ATPpumA.gif

Counter Transport – the transport of two substances at the same time in opposite directions, without ATP. Protein carriers are called Antiports.

Co-transport – the transport of two substances at the same time in the same direction, without ATP. Protein carriers are called Symports.

Gated Channels – receptors combined with channel proteins. When a chemical messenger binds to a receptor, a gate opens to allow ions to flow through the channel.

Movement of large molecules bound in vesicles out of the cell with the aid of ATP energy. Vesicle fuses with the plasma membrane to eject macromolecules.

Ex. Proteins, polysaccharides, polynucleotides, whole cells, hormones, mucus, neurotransmitters, waste

Movement of large molecules into the cell by engulfing them in vesicles, using ATP energy.

Three types of Endocytosis:PhagocytosisPinocytosisReceptor-mediated endocytosis

“Cellular Eating” – engulfing large molecules, whole cells, bacteria

Ex. Macrophages ingesting bacteria or worn out red blood cells.

Ex. Unicellular organisms engulfing food particles.

“Cellular Drinking” – engulfing liquids and small molecules dissolved in liquids; unspecific what enters.

Ex. Intestinal cells, Kidney cells, Plant root cells

Movement of very specific molecules into the cell with the use of vesicles coated with the protein clathrin.

Coated pits are specific locations coated with clathrin and receptors. When specific molecules (ligands) bind to the receptors, then this stimulates the molecules to be engulfed into a coated vesicle.

Ex. Uptake of cholesterol (LDL) by animal cells

What is phagocytosis?

What is pinocytosis?

What is receptor-mediated endocytosis?