Chapter 5 - Biomembranes And Cell Architecture

Smallest and simplest cells
Surrounded by a plasma membrane
No internal membrane-limited sub-compartments
DNA is concentrated in the center
Enzymes and metabolites thought to diffuse freely within the single internal aqueous compartment

Partitioned into smaller sub-compartments called organelles.
Each organelle is surrounded by a biomembrane.
Each organelle contains a unique complement of proteins (some embedded in the membrane(s), other in the aqueous interior called the lumen) which allow the organelle to carry out a cellular function.
Cytoplasm is the part outside the largest organelle, the nucleus.
Cytosol is the aqueous part of the cytoplasm outside all of the organelles.

Closed structures
Simple lipid bilayer structure that is responsible for the shape of the cell, cellular function, anchoring proteins to the membrane, modifying membrane protein activities and transducing signals to the cytoplasm.
Responsible for movement of molecules between the in/outside of the cell and in/out of the organelles (in case of eukaryotic cells).

Each cell type exhibits a distinctive design defined by the shape of the cell and the location of its organelles.
The cytoskeleton provides the structural basis for the unique design of each cell type.
It is a dense network of three different classes of protein filaments that permeate the cytosol and mechanically support cellular membranes.

Concept Check

The fundamental structure and function of a cell membrane depends on lipids (e.g. phospholipids, steroid derivatives, etc).

The specific function of each membrane depends on the membrane proteins that are present in a specific membrane.

Phospholipids (PPL) of the composition present in cells will spontaneously form these sheetlike bilayers.
Two molecules thick
Hydrocarbon chains of the PPL in each layer (leaflet) form a hydrophobic core.
Two main properties:

Diffusion Prevention ⇒ Hydrophobic core is an impermeable barrier that prevents the diffusion of water-soluble solutes.
Stability ⇒ The bilayer structure is maintained by hydrophobic and van der Waals interactions between the lipid chains.

All cellular membranes enclose an entire cell or compartment. The membranes therefor have a cytosolic (internal) face that is oriented towards the interior and an exoplasmic (external) face that faces towards the environment.
Topologically, the internal aqueous space of vesicles and some organelles is equivalent to the outside of the cell. This results in their cytosolic face being on the “outside” rather than the inside.
Three organelles (nucleus, mitochondrion and chloroplasts) are enclosed by two membranes separated by a small intermembrane space. The exoplasmic faces of the inner and outer membranes around these organelles border the intermembrane space between them.
The two faces are asymmetric in terms of lipid and protein composition.

Typical biomembrane is assembled from phosphoglycerides, sphingolipids and steroids, all of which are amphipathic molecules.
Hydrophobic effect and van der Waals interactions cause the tail groups to self-associate into a bilayer with the polar head groups oriented towards water.
Phosphoglycerides

Made of a glycerol backbone, two fatty acid tails (hydrophobic), and a phosphate group with an additional polar head (hydrophilic).
Primary components of cell membranes, forming bilayers that provide structural integrity and regulate substance passage.

Consists of a sphingosine backbone instead of glycerol, often with one fatty acid and a polar head group.
Found in nerve cell membranes, they play roles in signal transmission and cell recognition.

Lipids composed of a four-ringed hydrocarbon structure.
Examples include cholesterol (a membrane component and precursor for steroid hormones) and hormones like cortisol, estrogen, and testosterone, which regulate various biological processes.