Lecture 5: Single Cell Dynamics; Membrane Structure Bio Sci 93

Prokaryote

cell with no nucleus (the name means 'before nucleus'). DNA sits in a region called the nucleoid. Domains Bacteria and Archaea are prokaryotes. Have no membrane-bound organelles. Has ribosomes.

Eukaryote

cell with a true nucleus: DNA is in a nucleus bounded by a double membrane, and the cell has membrane-bound organelles. Includes animals, plants, protists, and fungi.

3 domains of life

Carl Woese's Bacteria, Archaea, and Eukarya. Bacteria and Archaea are prokaryotes; Eukarya are eukaryotes.

Basic features of all cells

plasma membrane, cytosol (semifluid substance), chromosomes (carry genes), and ribosomes (make proteins).

Prokaryotic cell structures

nucleoid with the bacterial chromosome, ribosomes, plasma membrane, cell wall, glycocalyx, fimbriae, and flagella. No nucleus.

Eukaryotes beyond animals and plants

protists (e.g., Amoeba, Paramecium, Volvox, Chlamydomonas) and fungi (e.g., mushrooms and baker's yeast).

Organelles

specialized structures inside a cell that perform specific jobs to keep the cell alive. membrane-bound compartment in eukaroytes

Plasma membrane functions

protects the cell from the extracellular environment, allows a fixed environment inside the cell, and controls the traffic of substances in and out.

Phospholipid structure

one glycerol backbone, two fatty acid chains (usually one saturated and one unsaturated), and a polar hydrophilic phosphate group with a variable polar group attached. Head is hydrophilic; tails are hydrophobic.

Amiphipathic (amphiphile)

a molecule with both a hydrophilic region and a hydrophobic region, such as a phospholipid.

Phospholipid bilayer

in aqueous solution they self-assemble into a bilayer 5-10 nm thick: hydrophilic heads face the water on both sides and hydrophobic tails face each other in the middle. held together by weak hydrophobic interactions

Fluid mosaic model

membrane is fluid (lipids and proteins drift laterally) and a mosaic (proteins embedded in and bound to a phospholipid bilayer, plus carbohydrates on the outer surface).

Lateral movement

phospholipids drift laterally about 10^7 times per second

Flip-flop movement

phospholipids flip between laters only about once per month.

Saturated tails

tails pack tightly, making the membrane more viscous

Unsaturated tails

have kinks (cis double bonds) that prevent tight packing, so the membrane is more fluid, especially at low temperatures.

Cholesterol in the membrane

stabilizes fluidity: reduces fluidity at moderate temperatures but hinders solidification at low temperatures. Plants don't really have this, but insert other sterols for stability.

What affects membrane fluidity?

temperature, the ratio of saturated to unsaturated phospholipid tails, and cholesterol content. Some organisms can change membrane lipid composition on demand.

Membrane asymmetry

phospholipids are asymmetrically distributed between the inner and outer layers of the bilayer. (You do not need individual phospholipid names for exams.)

Integral membrane protein

inserted into the lipid bilayer and sometimes spans both sides (transmembrane).

Peripheral membrane protein

not embedded in the bilayer; associates loosely with the membrane surface, mostly through interactions with other proteins, including integral proteins.

How does a protein sit in the bilayer?

transmembrane alpha helices are amphipathic: hydrophobic (nonpolar) residues face the lipids, while polar or charged residues face inward and cluster into a hydrophilic core. Example: 7-transmembrane domain proteins that transduce signals into the cell.

Glycoprotein

protein with an attached carbohydrate

Glycolipid

lipid with an attached carbohydrate (carbohydrates are found on the extracellular surface and function in cell-cell recognition).

Six functions of membrane proteins

transport, enzymatic activity, signal transduction, cell-cell recognition, intercellular joining, and attachment to the cytoskeleton and extracellular matrix (ECM).

Where the membrane is built

most phospholipids are assembled in the ER. Proteins bound for the plasma membrane and secreted proteins begin assembly in the ER, then travel in vesicles through the Golgi to the plasma membrane.

How membrane orientation arises

cargo facing the interior (lumen) of a vesicle becomes the extracellular face of the plasma membrane. Cargo facing the exterior of the vesicle becomes the cytoplasmic face.

FRAP

Fluorescence Recovery After Photobleaching: tag a molecule with a fluorescent label, bleach a small area with a laser, and monitor how fast fluorescence returns to measure the rate of diffusion in the membrane.

Frye and Edidin experiment

mouse and human cells were fused into a hybrid cell. Labeled membrane proteins from each intermixed over about 1 hour, showing that membrane proteins move laterally. Lowering the temperature would reduce fluidity and slow the mixing.

ABO blood groups

the ABO gene encodes a glycosyltransferase that adds different sugars to the H antigen base structure, forming different glycan chains on glycolipids and glycoproteins of red blood cells.

HIV entry

HIV binds the CD4 receptor and the CCR5 co-receptor on host cells. Viral membrane proteins mimic natural ligands. Cells lacking CCR5 can't be infected, as in resistant individuals.

SARS-CoV-2 entry

spike protein binds the ACE-2 receptor. The protease TMPRSS2 cleaves ACE-2 and the spike protein, activating the spike and promoting viral entry. ACE-2 is found on cells throughout the body.

CAR T-cell therapy

a patient's T cells are collected and modified to carry a chimeric antigen receptor that recognizes a surface antigen on cancer cells. Reinfused T cells bind the antigen, triggering a signaling cascade that kills the cancer cell.

Organelles

Animal cell: nucleus (envelope, nucleolus, chromatin), rough and smooth ER, Golgi, mitochondria, lysosomes, peroxisomes. Also ribosomes and the cytoskeleton (microfilaments, intermediate filaments, microtubules). Plant cells also have chloroplasts, a central vacuole, a cell wall, and plasmodesmata.