BIO 329 Plasma Membrane Structure

Estimate the thickness of the plasma membrane and list its main constituents and functions

• Thickness: The plasma membrane is typically estimated to be about 5 nm thick.

• Main Constituents: Its primary components are lipids (primarily phospholipids and cholesterol), proteins, and carbohydrates (attached to lipids and proteins on the non-cytosolic side).

• Functions: Its main functions include: acting as a barrier to define the cell and organelles;
  regulating transport (selectively importing and exporting molecules); receiving external signals (via receptors); and allowing cell movement and expansion.

Assess the solubility of the lipids that constitute cell membranes.

Membrane lipids are generally insoluble in water. They are amphipathic, meaning they have a hydrophobic (water-hating) part and a hydrophilic (water-loving) part. This structure prevents them from dissolving fully in the aqueous environment.

Explain why fats such as triacylglycerol coalesce into fat droplets in water.

Triacylglycerols (fats) are almost entirely
hydrophobic (non-amphipathic). They lack a significant hydrophilic head. Therefore, when placed in water, they coalesce into large
fat droplets to minimize the surface area exposed to the aqueous environment

1. Articulate how the amphipathic nature of membrane lipids drives the assembly of a lipid bilayer, as well as the formation of closed vesicles, organelles, and even whole cells.

The amphipathic nature of phospholipids drives them to spontaneously assemble in water. The hydrophobic tails cluster together to avoid water, while the hydrophilic heads face the aqueous environment. This energetically favorable arrangement forms a  lipid bilayer. Because any break in the sheet would expose hydrophobic tails to water, the bilayer spontaneously seals, forming a closed compartment (vesicles, organelles, or a cell).

what is coalesce 

it is an act of joining into a single whole 

Phospholipids

contain two hydrophobic fatty acid tails and a hydrophilic head group (often with a phosphate and a small polar molecule) attached to a glycerol backbone. They are amphipathic

Glycolipids

Similar to phospholipids, but the hydrophilic head is a sugar (carbohydrate) group instead of a phosphate-containing group. They are also amphipathic.

Triacylglycerol (Fats)

Contain three hydrophobic fatty acid tails linked to a glycerol molecule. They are hydrophobic.

Cholesterol

Has a rigid, characteristic four-ring steroid structure. It is amphipathic, having a small hydrophilic hydroxyl (
-OH) group and a short, hydrophobic hydrocarbon tail.

how do lipids move within the lipid bilayer.

• Lipids move in three main ways:

  • Lateral Diffusion: Rapid movement sideways within the same monolayer (frequent).

  • Flexion/Rotation: Rapid movement of the hydrocarbon tails (frequent).

  • Flip-flop (Transverse Diffusion): Rare spontaneous movement to the opposite monolayer (rare, requires enzyme assistance).

summarize how the length and saturation of lipids' hydrophobic tails affect the fluidity of a cell membrane.

tail Length: Shorter hydrophobic tails increase fluidity because they reduce the tendency of the tails to interact with one another.

• Saturation: Unsaturated tails (which have double bonds creating kinks) increase fluidity because they pack less closely together than saturated (straight) tails.

Explain how the inclusion of cholesterol affects the fluidity of animal cell membranes

• Cholesterol has a dual effect:

  • At warm temperatures (37∘C): It makes the membrane less fluid by restricting the movement of phospholipid tails.

At low temperatures: It makes the membrane more fluid by preventing the hydrocarbon tails from packing too tightly and solidifying.

Propose how single-celled organisms such as yeast and bacteria can maintain the fluidity of their membranes as temperatures vary.

• Single-celled organisms change the
  lipid composition of their membranes.

• When temperatures decrease (colder), they increase the proportion of short-tailed and unsaturated lipids to maintain fluidity.

• When temperatures increase (warmer), they increase the proportion of long-tailed and saturated lipids to make the membrane less fluid.

List the reasons why cells carefully regulate their membrane fluidity.

• Fluidity is crucial for membrane function:

  • It allows proteins to diffuse laterally and cluster in specific regions.

  • It ensures proteins and lipids are distributed evenly during cell division.

  • It allows membranes to fuse with one another (e.g., during endocytosis, vesicle transfer).

  • It is necessary for certain functions, such as cell signaling and enzyme activity.

State where new phospholipids are produced and explain how membranes are able to grow evenly.

• New phospholipids are produced at the
  cytosolic face of the smooth endoplasmic reticulum (ER).

• Membranes grow evenly because
  scramblase enzymes catalyze the random transfer of phospholipids from the cytosolic monolayer to the non-cytosolic monolayer, distributing the new lipids equally.

outline how the asymmetric distribution of phospholipids, characteristic of different membrane types, is established and maintained.

• The initial random distribution is established by scramblase in the ER.

The final asymmetric distribution is established and maintained in the Golgi apparatus and plasma membrane by flippase enzymes. Flippases selectively move specific phospholipids (like those with choline) from the non-cytosolic monolayer to the cytosolic monolayer

Describe how membranes retain their orientation during transfer between cell compartments.

• The two faces of a membrane (the cytosolic face and the non-cytosolic face) always retain their original orientation throughout the process of membrane budding and fusion.

The side of the membrane that faces the
cytosol always stays facing the cytosol, and the side that faces the lumen of an organelle or the cell exterior always stays facing that compartment or exterior.

Contrast the actions of flippases and scramblases

• scramblase: Found primarily in the ER. It is a
  non-selective enzyme that equilibrates the random distribution of phospholipids by transferring them from one monolayer to the other.

• Flippase: Found primarily in the plasma membrane and Golgi. It is a
  selective enzyme that transfers specific phospholipids (often those containing amine groups) from the non-cytosolic monolayer to the cytosolic monolayer to establish and maintain asymmetry.

List some functions of plasma membrane proteins.

• Plasma membrane proteins serve many functions, including:

  • Transporters (move specific molecules across the membrane)

  • Anchors (link the membrane to the cytoskeleton or ECM)

  • Receptors (detect chemical signals in the external environment)

  • Enzymes (catalyze specific reactions)

Categorize membrane proteins based on the way they interact with the lipid bilayer.

• Membrane proteins are broadly categorized into
  Integral and Peripheral proteins.

• Integral proteins (e.g., Transmembrane, Monolayer-associated, Lipid-linked) are permanently attached to the membrane.

Peripheral proteins are indirectly or temporarily associated with the membrane.

Distinguish between integral membrane proteins and peripheral membrane proteins.

• Integral Membrane Proteins: Directly interact with the hydrophobic interior of the lipid bilayer. They can only be removed by disrupting the bilayer with detergents. Examples include transmembrane proteins.

• Peripheral Membrane Proteins: Are non-covalently bound to the surface of the membrane, often via interactions with integral proteins or the hydrophilic head groups of lipids. They can be easily removed by mild treatments that don't disrupt the membrane (e.g., high salt concentration or change in
  pH).

Explain how the polypeptide chain of a transmembrane protein, with its hydrophilic backbone, is able to span the hydrophobic interior of the lipid bilayer.

• The polypeptide chain usually crosses the membrane as an
  α-helix. In this structure, the
  hydrophilic polypeptide backbone (the N−H and C=O groups) is shielded from the hydrophobic lipid tails by forming hydrogen bonds with itself. The
  hydrophobic amino acid side chains project outward from the helix, interacting directly with the hydrophobic fatty acid tails of the lipids.

Contrast the structure and potential function of single-pass and multipass transmembrane proteins

• Single-pass: The polypeptide chain crosses the lipid bilayer only once. Its primary function is often as a
  receptor (binding an external signal and transmitting it to the cell interior).

• Multipass: The polypeptide chain crosses the lipid bilayer multiple times (as a series of α-helices). These often function as
  channels, pores, or transporters, with the multiple helices arranged to create a hydrophilic pathway for molecules to cross the membrane.

Describe the structure of a membrane-spanning β-barrel transmembrane protein.

• The β-barrel is formed when multiple beta sheets are rolled into a large, barrel-like structure. The hydrophilic side chains of the β-sheets line the central channel of the barrel, while the hydrophobic side chains face outward to interact with the lipid bilayer.

Explain how hybrid cells allowed investigators to monitor the movement of membrane proteins within the plane of the lipid bilayer.

• In the classic experiment, two different cells (e.g., human and mouse) were
  fused to create a single hybrid cell. Each cell's plasma membrane proteins were initially labeled with a different colored antibody (e.g., mouse proteins with red, human proteins with green). Over time, the proteins of the two species were observed to mix evenly across the entire surface of the hybrid cell, demonstrating that membrane proteins are mobile and can diffuse laterally within the plane of the membrane.

Outline the ways that cells restrict the lateral movement of their membrane proteins.

• Cells use four main strategies to restrict protein movement:

  • Tethering to the Cell Cortex: Proteins are tethered to molecules inside the cell (e.g., the cytoskeleton).

  • Tethering to the Extracellular Matrix: Proteins are linked to molecules outside the cell.

  • Binding to Proteins on Other Cells: Proteins on one cell bind to proteins on another cell (e.g., at cell-cell junctions).

Diffusion Barriers: Tight junctions or other specialized barriers (like occluding junctions) restrict proteins to a specific, localized membrane domain (e.g., apical vs. basal surface of an epithelial cell).