23. Cell wall-structure and function

Unlike animal cells plant cells have an outer cell wall composed of polysaccharides, in particular cellulose.

The cell wall of a plant cell is made up of primary and secondary cell walls. When one cell divides into two, a primary cell wall forms around each two new cells. The primary cell wall is laid down just outside the cell membrane, and is flexible, allowing the new cells to grow. This new cell wall is mostly made of cellulose molecules, arranged into thin hair-like strands called microfibrils. The microfibrils are arranged in a meshwork pattern along with other components such as hemicellulose, glycans and pectins, which link them together and help strengthen the cell wall.  The secondary cell wall is constructed between the plasma membrane and the primary cell wall after the cell has finished growing. It is made by laying down successive layers of cellulose microfibrils and lignins. Mature xylem cells are heavily lignified - they make up the 'wood' of woody plants 

Functions of cell walls:

• Provide tensile strength and limited plasticity which are important for:

  • keeping cells from rupturing from turgor pressure

  • turgor pressure provides support for non-woody tissues

• Thick walled cells provide mechanical support

• Tubes for long-distance transport

• Cutinized walls prevent water loss

• Provide mechanical protection from insects & pathogens

• Physiological & biochemical activities in the wall contribute to cell-cell communication

During growth and development

• Cell division involves synthesis of new cell wall

• Cell enlargement involves changes in cell wall composition

• Cell differentiation involves changes in cell wall composition

29. Mechanisms of phosporilation

While phosphorylation is a prevalent post-translational modification (PTM) for regulating protein function, it only occurs at the side chains of three amino acids, serine, threonine and tyrosine, in eukaryotic cells. These amino acids have a nucleophilic (–OH) group that attacks the terminal phosphate group (γ-PO₃^2-) on the universal phosphoryl donor adenosine triphosphate (ATP), resulting in the transfer of the phosphate group to the amino acid side chain. This transfer is facilitated by magnesium (Mg²⁺), which chelates the γ- and β-phosphate groups to lower the threshold for phosphoryl transfer to the nucleophilic (–OH) group. This reaction is unidirectional because of the large amount of free energy that is released when the phosphate-phosphate bond in ATP is broken to form adenosine diphosphate (ADP).

For a large subset of proteins, phosphorylation is tightly associated with protein activity and is a key point of protein function regulation. Phosphorylation regulates protein function and cell signaling by causing conformational changes in the phosphorylated protein. These changes can affect the protein in two ways. First, conformational changes regulate the catalytic activity of the protein. Thus, a protein can be either activated or inactivated by phosphorylation. Second, phosphorylated proteins recruit neighboring proteins that have structurally conserved domains that recognize and bind to phosphomotifs. These domains show specificity for distinct amino acids. For example, Src homology 2 (SH2) and phosphotyrosine binding (PTB) domains show specificity for phosphotyrosine (pY), although distinctions in these two structures give each domain specificity for distinct phosphotyrosine motifs (2). Phosphoserine (pS) recognition domains include MH2 and the WW domain, while phosphothreonine (pT) is recognized by forkhead-associated (FHA) domains. The ability of phosphoproteins to recruit other proteins is critical for signal transduction, in which downstream effector proteins are recruited to phosphorylated signaling proteins.

Protein phosphorylation is a reversible PTM that is mediated by kinases and phosphatases, which phosphorylate and dephosphorylate substrates, respectively. These two families of enzymes facilitate the dynamic nature of phosphorylated proteins in a cell. Indeed, the size of the phosphoproteome in a given cell is dependent upon the temporal and spatial balance of kinase and phosphatase concentrations in the cell and the catalytic efficiency of a particular phosphorylation site.