Cell Membranes and Cell Walls

Cell Membranes

Overview of Cell Boundaries:

Cells require boundaries with their environments (i.e. plasma membranes). Eukaryotes also have internal boundaries for compartmentalisation. Boundaries allow:

The formation of a suitable semi-fluid matrix (protoplasm) for cell functioning.
The formation of a specialised microenvironment where the cell can concentrate solutes and have dedicated cell processes.
Segregation of harmful activities.

Cell Membranes:

Membranes are sheet-like structures composed of lipids, proteins and carbohydrates in varying ratios. They are typically 5-8nm thick.

Membranes are amphipathic, meaning that they contain both hydrophilic and hydrophobic regions.
→ They are all different types, but all contain a hydrophilic head and hydrophobic tail.

The most common phospholipid in eukaryotes is phosphatidyl choline.

Fluid-Mosaic Model:

The phospholipid bilayer is able to move (i.e. is fluid).

It contains integral proteins embedded, and peripheral protein outside linked by non-covalent bonds. Glycolipids can also be present on the surface and are often receptor molecules.
In eukaryotes, cholesterol is embedded, whereas in fungi, ergosterol is embedded.

Structural Features:

Lipid bilayers are self-assembling.
Lipid layers are fluid (can move up to 2um a second), however some are restricted by cytoskeletons, rafts and domains.
Asymmetry - inner and outer layers of the bilayer have different components and compositions. Lipids rarely cross monolayers unless under action of scramblase and flippase enzymes.
Integral proteins pass through the lipid bilayer.

Structure Linked to Function:

Provides a semi-permeable barrier for the movement of solutes and gases.
→ Small non-polar molecules such as O₂ and CO₂, and lipid-soluble molecules can pass through easily.
Transportation of Solutes - integral proteins provide a means of accumulating sugars, amino acids and ions. They also act as channels for active processes.
Flexibility for cell movement, division, and membrane fission.
Receptors in the membrane allow for response to external stimuli.
Inter-cellular interaction - glycolipids are used in cell recognition.

Cell Walls

Cell walls provide cells with structural support, but also function as selective barriers.

Bacterial cells have a rigid peptidoglycan cell wall.
Fungal cells have a chitin cell wall.
Plant cells have a cellulose cell wall.
Animal cells do not have a cell wall.

Cell Cortex:

The cell cortex is a specialised layer of cytoplasmic proteins beneath the plasma membrane, and is attached to the rest by transmembrane proteins.

For example, erythrocytes contain spectrin proteins, which form a meshwork responsible for providing support and maintenance of cell shape.
→ This spectrin meshwork is connected to the plasma membrane by actin and anchor proteins, which allows it to provide elasticity.

Extracellular Matrix:

Some cell types have an exterior cell ‘coat’ based on sugar molecules (‘glycocalyx’), which are attached to membrane proteins and/or glycolipids.
The extracellular matrix is a collagen ‘coat’ which forms strong fibres outside cells.
→ It protects the cell surface from mechanical and chemical damage, whilst also playing a key role in cell recognition.