fluid mosaic model

• describes membrane as fluid, with proteins embedded in or associated with the phospholipid bilayer

plasma membrane

• selectively permeable
• it allows some substances to cross more easily than it does others

membranes are predominantly made of...

• phospholipids and proteins held together by weak interactions
• cause the membrane to be fluid

phospholipids

• provide a hydrophobic barrier that separates the cell from its liquid environment

hydrophilic molecules cannot easily enter the cell, but hydrophobic molecules can

cholesterol

• hydrophobic steroid
• found embedded in animal membranes
• helps membranes resist changes in fluidity when the temperature changes
• at high temp - makes membrane less fluid
• at low temp - helps membrane retain fluidity

proteins

• embedded in the membrane
• can serve as transport channels to move materials across hydrophobic interior of the phospholipid bilayer
• can act as molecular receptors to bind to signaling molecules (ligands)

peripheral proteins

• loosely bound to membrane's surface
• not embedded in the lipid bilayer

membrane carbohydrates

• crucial in cell-cell recognition
• important in the sorting of cells into tissues in an animal embryo
• basis for rejection of foreign cells by the immune system
• short, branched chains
• fewer than 15 sugar units

intergral proteins

• penetrate the hydrophobic interior of the lipid bilayer

glycolipids

• membrane carbohydrate chains covalently bonded to lipids
• short, branched

glycoproteins

• membrane carbohydrate chains covalently bonded to proteins

integrins

• cell-surface receptor proteins

non-polar molecules (hydrophobic)...

• can dissolve in the hydrophobic interior of the phospholipid bilayer
• cross the membrane easily
• examples: hydrocarbons, oxygen, carbon dioxide

hydrophobic core of the membrane...

• impedes the passage of ions and polar molecules (hydrophilic)

transport proteins

• enable hydrophilic substances to avoid the lipid bilayer and pass through
• span the membrane
• are specific, like enzymes, for the substances they transport

aquaporins

• transport (channel) proteins
• accelerate the speed at which water can cross membranes (three billion water molecules per second)

passive diffusion

• substance travels from where it is more concentrated to where it is less concentrated
• diffuses down its concentration gradient
• does not require energy
• relies only on the thermal motion energy intrinsic to the molecule in question

concentration gradient

• process in which particles move from an area of high concentration to low concentration
• region along which the density of a chemical substance increases or decreases

osmosis

• the diffusion of water across a selectively permeable membrane
• water diffuses from the solution with the less concentrated solute to that of the more concentrated solute

isotonic solution

• no net movement of water across the plasma membrane
• water crosses the membrane at the same rate in both directions

hypertonic solution

• cell loses water to surroundings (shrivels, may die)
• more solutes in the water around the cell

hypotonic solution

• water will enter the cell faster than it leaves (will swell and may burst)
• fewer solutes around the cell

water moves from...

Hypo-> Hyper

ions and polar molecules...

• cannot move easily across the membrane

facilitated diffusion

• process by which ions and polar molecules diffuse across the membrane with the help of transport proteins

how do transport proteins work?

• provide a hydrophilic channel through which the molecules in question can pass
• bind loosely to molecules in question and carry them through the membrane

active transport

• substances are moved against their concentration gradient
• low concentration -> high concentration
• requires energy, usually in the form of ATP

sodium-potassium pump

• good example of active transport
• transmembrane protein
• pumps sodium out of the cell and potassium ions into the cell
• necessary for proper nerve transmission and is a major energy consumer in the body

membrane potential

• difference in electrical charge across a membrane
• expressed in voltage
• inside of the cell is negatively charged compared with outside the cell

why are positively charged ions on the outside of the cell attracted to the inside of the cell?

• inside of cell is negatively charged

what two forces drive the diffusion of ions across a membrane?

• chemical force - ion's concentration gradient
• voltage gradient across the membrane - attracts positively charged ions and repels negatively charged ions

chemical force

• ion's concentration gradient

voltage gradient

• across the membrane
• attracts positively charged ions and repels negatively charged ions

combination of forces acting on ion forms...

• electrochemical gradient

cotransport

• ATP pump that transports a specific solute indirectly drives the transport of other substances
• substance that was initially pumped across the membrane can do work as it moves back across the membrane by diffusion
• brings with it a second compound against gradient

carrier protein

• change shape in a way that shuttles their "passengers" across the membrane

channel proteins

• function by having a hydrophilic channel that certain molecules or ions use as a tunnel through the membrane

large molecules are moved across the cell membrane through...

• exocytosis and endocytosis

both processes require energy

exocytosis

• vesicles from the cell's interior fuse with the cell membrane
• expelling contents

endocytosis

• cell forms new vesicles from the plasma membrane
• allows the cell to take in macromolecules
• examples: engulfing of foreign particles by white blood cells or amoebas
• reverse of exocytosis

examples of molecules that pass through phospholipid bilayer using simple diffusion

• CO2
• O2

examples of molecules that pass through phospholipid bilayer using carrier proteins

• glucose

examples of molecules that pass through phospholipid bilayer using channel proteins

• H+ ions (protein pump)

bulk transport

• moves large molecules
• exocytosis and endocytosis
• requires energy

what is meant by membrane fluidity?

• membranes are not static sheets of molecules locked rigidly in place
• they move and shift sideways, hence the "fluid" classification

how can decrease in temperature affect membrane fluidity?

• membrane becomes more solid
• phospholipids settle into closely packed arrangement

how do phospholipids with unsaturated hydrocarbon chains affect membrane fluidity?

• with temperature decrease, membrane still remains fluid
• kinks in tails don't allow the phospholipids to pack tightly together

how does cholesterol affect membrane fluidity?

• acts as a "fluidity buffer"
• resists changes in membrane fluidity that can be caused by changes in temperature
• at low temperatures - hinders solidification by disrupting the regular packing of phospholipids
• at moderate temperatures - reduces phospholipid movement, reducing membrane fluidity

how do phospholipids with saturated hydrocarbon chains affect membrane fluidity?

• saturated carbon tails pack together, increasing membrane viscosity

major functions of membrane proteins

• transport
• enzymatic activity
• signal transduction
• cell-cell recognition
• intercellular joining
• attachment to cytoskeleton and ECM

transport

• proteins guide and pump substances across the membrane using energy

enzymatic activity

• enzymes in membrane organized into teams to carry out sequential steps of metabolic pathway

signal transduction

• signaling molecule binds to receptor
• may cause receptor to change shape
• allows message to be relayed

cell-cell recognition

• glycoproteins act as identification tags
• recognized by membrane proteins of other cells

intercellular joining

• membrane proteins hook together to those of the adjacent cell

attachment to cytoskeleton and ECM

• cytoskeleton elements bind to membrane proteins
• maintains cell shape
• stabilizes protein location
• proteins bind to ECM
• coordinate cellular changes outside or inside cell

integrins

• cell surface receptor proteins

cytoskeleton microfilaments

• thin, solid rods
• form structural networks when certain proteins bind along the side of other filaments

ECM fibers

• made up of glycoproteins
• embedded in a network woven out of proteoglycans

example of transport proteins being specific

• doesn't allow fructose to pass, a structural isomer of glucose

diffusion

• the movement of particles of any substance so that they spread out into the available space

turgid

• healthy state for most plant cells
• very firm

flaccid

• no net tendency for water to enter
• plant wilts
• limp

plasmolysis

• plasma membrane pulls away from the cell wall at multiple places as plant cell shrivels
• causes plant to wilt and can lead to plant death

why does the plant cell not burst like the red blood cell when placed in a hypotonic solution?

• the plant cell has a cell wall
• uptake of water is eventually balanced by the wall pushing back on the cell

summary: sodium-potassium pump

1. cytoplasmic Na+ binds to sodium-potassium pump
2. binding of 3 Na+ stimulates phosphorylation by ATP
3. phosphorylation leads to change in protein shape - Na+ is released
4. new shape attracts K+ - K+ binds to extracellular side, triggers release of the phosphate group
5. phosphate group restores protein's OG shape
6. 2 K+ released, cycle repeats with affinity for Na+ once again

receptor-mediated endocytosis

• enables the cell to acquire bulk quantities of specific substances
• specialized type of pinocytosis
• key feature: receptor sites that bind with specific solutes

phagocytosis

• a cell engulfs a particle by extending a pseudopodia fluid around it, and packaging it within food vesicle

pinocytosis

• cell continually gulps droplets of extracellular fluid into tiny vesicles formed by infolding of plasma membrane