Lecture 16 - Membranes

composition of the plasma membrane

• The plasma membrane separates a living cell from its surroundings and regulates exchange with the environment.
• It is mainly a phospholipid bilayer with embedded and associated proteins, cholesterol, and carbohydrates.
• The membrane is selectively permeable: some substances cross more readily than others. Transport proteins help regulate this molecular traffic.

dynamic structure of the membrane through the fluid mosaic model

• In the fluid mosaic model, proteins are embedded in a fluid phospholipid bilayer. The membrane is held together mainly by weak hydrophobic interactions.


• Most lipids and some proteins can move laterally within the membrane; lipid “flip-flop” between layers is rare.


• Heterokaryon experiments, in which mouse and human cells were fused, demonstrated the intermixing of membrane proteins.


• Membrane components are distributed asymmetrically. The endoplasmic reticulum and Golgi apparatus help establish this sidedness as membranes are built.


• “The plasma membrane is the boundary that separates the living cell from its surroundings.”

describe phospholipids

• Phospholipids are the most abundant lipids in the plasma membrane and are amphipathic: each has a hydrophilic region and a hydrophobic region.

• A phospholipid consists of glycerol, two hydrophobic fatty-acid tails, and a phosphate-containing hydrophilic head.


• In water, phospholipids self-assemble into bilayers:
Hydrophilic heads face the watery environments on both sides of the membrane.
 + Hydrophobic tails point inward, sheltered from water.


• The bilayer forms the boundary between the cell and its external environment.

integral proteins

• Integral proteins penetrate the hydrophobic core; those spanning the entire membrane are transmembrane proteins.


• Their membrane-spanning regions commonly contain nonpolar amino acids, often arranged in alpha helices.

peripheral proteins

bound to the membrane surface

essential functions of membrane proteins

1. Transport


2. Enzymatic activity


3. Signal transduction through receptors


4. Cell-cell recognition


5. Intercellular joining


6. Attachment to the cytoskeleton and extracellular matrix (ECM)

note that proteins are not randomly distributed and they determine many of the membrane’s functions

describe membrane carbohydrates and cell recognition

• Membrane carbohydrates are generally on the extracellular surface and may be attached to:
 Proteins, forming glycoproteins
 + Lipids, forming glycolipids


• Carbohydrate patterns vary among species, individuals, and cell types. They help cells recognize one another.


• The ER and Golgi apparatus contribute to the distribution of membrane proteins, lipids, and associated carbohydrates.

general properties of lipids

• Lipids mix poorly with water, consist largely of hydrocarbon regions, and are not true polymers.
• Important biological lipids include fats, phospholipids, and steroids.
• Glycerol is a three-carbon alcohol with a hydroxyl group attached to each carbon.
• A fatty acid has a carboxyl group attached to a long carbon skeleton.
• A fat, or triacylglycerol (triglyceride), consists of three fatty acids joined to glycerol by ester linkages. Its fatty acids may be identical or different.

fats ad fatty acids

• Fatty acids vary in length and in the number and positions of their double bonds.
• Saturated fatty acids have no double bonds and the maximum possible number of hydrogen atoms.
• Unsaturated fatty acids have one or more double bonds.
• Fats made from saturated fatty acids are generally solid at room temperature; fats rich in unsaturated fatty acids are generally liquid oils. Most animal fats are saturated, while plant and fish fats are usually unsaturated.
• Fats separate from water because water molecules hydrogen-bond to one another and exclude the fats.

steroids and cholesterol

• Steroids have a carbon skeleton of four fused rings.
• Cholesterol is a steroid found in animal cell membranes and is a precursor for the synthesis of other steroids.
• High blood cholesterol may contribute to cardiovascular disease.
• Cholesterol buffers membrane fluidity:
• At warm temperatures, such as 37°C, it restrains phospholipid movement and reduces fluidity.
• At cool temperatures, it hinders tight packing and helps prevent solidification.
• Plants contain cholesterol but use related steroid lipids to help buffer membrane fluidity.

describe membrane fluidity in cells

• Membranes become less fluid as temperatures cool and can solidify; the solidification temperature depends on lipid composition.
• Membranes rich in unsaturated fatty acids are generally more fluid than those rich in saturated fatty acids.
• Saturated tails pack closely together.
• Double bonds in unsaturated tails create kinks that impede close packing.
• Membranes need to remain fluid to function properly; the source compares their usual consistency to salad oil.
• Organisms in environments with changing temperatures may alter membrane lipid composition in response to temperature.

passive transport

• Passive transport moves substances down their concentration gradients without energy expenditure by the cell.


diffusion

Diffusion is the tendency of molecules to spread out through available space. Individual molecules move randomly, though the population may show net movement.


concentration gradient

A concentration gradient is a region along which the concentration of a substance changes.


dynamic equilibrium

Hydrophobic, nonpolar molecules such as hydrocarbons, oxygen, and carbon dioxide can dissolve in the lipid bilayer and cross rapidly.
• Ions and other hydrophilic or polar substances do not cross the bilayer easily.
“No work must be done to move substances down a concentration gradient.”

passive transport and diffusion across membrane

Hydrophobic, nonpolar molecules such as hydrocarbons, oxygen, and carbon dioxide can dissolve in the lipid bilayer and cross rapidly.


Ions and other hydrophilic or polar substances do not cross the bilayer easily.
“No work must be done to move substances down a concentration gradient.”

osmosis

• Osmosis is the diffusion of water across a selectively permeable membrane.
•

Water moves from a region of lower solute concentration (more free water) toward a region of higher solute concentration (less free water), until solute concentrations are equal on both sides, as described in the source.


tonicity

Tonicity is the ability of a surrounding solution to cause a cell to gain or lose water. It depends on the relative concentration of solutes that cannot cross the membrane.
• Isotonic: Solute concentrations are equal inside and outside the cell; there is no net water movement.
• Hypertonic: The surrounding solution has a higher solute concentration; the cell loses water.
• Hypotonic: The surrounding solution has a lower solute concentration; the cell gains water.

describe cells without cell walls

• Cells without rigid walls may shrivel in hypertonic solutions and lyse (burst) in hypotonic solutions.
• Hypertonic and hypotonic environments create osmotic challenges for organisms whose cells lack rigid walls.


describe cells with cell walls

Cell walls help plant cells maintain water balance.
• In a hypotonic solution, a plant cell takes up water and swells until the wall resists further expansion; the cell becomes turgid (firm).
• In an isotonic environment, there is no net water movement into the cell; it becomes flaccid (limp).
• In a hypertonic environment, a plant cell loses water. Its membrane pulls away from the cell wall, causing plasmolysis, which can make the plant wilt and may be lethal.

facilitated diffusion

passive movement down a concentration gradient aided by transport proteins; it requires no energy.

transport proteins + types

Transport proteins are specific for the substances they move and include:
• Channel proteins, which provide corridors for particular molecules or ions.
• Carrier proteins, which bind a solute and change shape to move it across the membrane.

aquaporins

Aquaporins are membrane-spanning channel proteins that greatly facilitate water movement.


what are some ion channels

Some ion channels are gated channels that open or close in response to a stimulus. In nerve cells, some open in response to electrical stimulation.

what is active transport

Active transport uses energy, usually from ATP hydrolysis, to move substances against their concentration gradients.

Active transport proteins are carrier proteins.


Active transport enables cells to maintain concentrations that differ from those in their surroundings.

what is the sodium potassium pump?

• Animal cells typically maintain a higher potassium-ion concentration and a lower sodium-ion concentration than their surroundings.


• The sodium-potassium pump uses energy from ATP, transferred through phosphorylation, to maintain these gradients.

describe the sodium potassium pump cycle

1. Cytoplasmic Na⁺ binds to the pump, which has high affinity for Na⁺ in its initial shape.


2. Na⁺ binding stimulates phosphorylation by ATP.


3. Phosphorylation changes the pump’s shape and lowers its affinity for Na⁺, which is released outside.


4. The altered shape has high affinity for extracellular K⁺; K⁺ binds and triggers release of the phosphate group.


5. Loss of the phosphate restores the pump’s original shape, which has lower affinity for K⁺.


6. K⁺ is released into the cytoplasm, and the pump can bind Na⁺ again.


7. In the standard pump cycle, three Na⁺ ions are transported out of the cell and two K⁺ ions are transported in for each ATP hydrolyzed.

membrane potential

Membrane potential is the voltage across a membrane, created by unequal distributions of positive and negative ions.


The cytoplasmic side of the membrane is negative relative to the extracellular side.

what does the electrochemical gradient combine

The chemical force from an ion’s concentration gradient + The electrical force exerted by the membrane potential.

what is an electrogenic pump

• An electrogenic pump is a transport protein that generates voltage across a membrane.

• The sodium-potassium pump is the major electrogenic pump in animal cells.


• Proton pumps are the main electrogenic pumps in plants, fungi, and bacteria; they actively transport H⁺ out of the cell.

what is co transport (coupled transport)

• Cotransport occurs when active transport of one solute indirectly drives the transport of another substance.


• The movement of an actively transported solute down its concentration gradient can be coupled to the movement of a second substance against its own gradient.


• In the source example, a proton pump establishes an H⁺ gradient, and an H⁺/sucrose cotransporter uses H⁺ diffusion to drive sucrose transport.

exocytosis

• Small molecules and water can cross through the bilayer or via transport proteins.


• Large molecules, including polysaccharides and proteins, cross in bulk using vesicles; bulk transport requires energy.


• In exocytosis, transport vesicles move to and fuse with the plasma membrane, releasing their contents outside the cell.


• Many secretory cells, including cells of the digestive tract, use exocytosis to export products.

endocytosis

the cell takes in macromolecules by forming vesicles from the plasma membrane. It is described as a reversal of exocytosis and involves different proteins

the 3 types of endocytosis

1. Phagocytosis (“cellular eating”): The cell engulfs a particle into a vacuole; the vacuole can fuse with a lysosome for digestion.


2. Pinocytosis (“cellular drinking”): Extracellular fluid and its dissolved molecules are taken up in small vesicles.


3. Receptor-mediated endocytosis: Specific solutes bind to receptors, triggering vesicle formation. Receptors and other molecules may enter the vesicle, and emptied receptors are recycled to the plasma membrane.

examples of receptor-mediated endocytosis

• Human cells take up cholesterol through receptor-mediated endocytosis. Cholesterol is carried in particles called low-density lipoproteins (LDLs).
• Familial hypercholesterolemia is associated with missing or defective LDL receptor proteins.
• HIV must bind to the immune-cell surface protein CD4 and the co-receptor CCR5 to infect a cell.
• Cells with CCR5 on their surface can be infected.
• Cells lacking CCR5 can resist infection.
• The source notes that drugs were being developed to mask CCR5.