Plant Physiology

PLANT PHYSIOLOGY

Plant Cells

  1. Plastids are double membrane bound organelles that exist within some plants and algae. They are found in almost all plant cells, although the specific types of plastids may reside in particular cells more than others. Plastids are primarily involved in the manufacture and storage of food, and include cell organelles such as proteinoplasts, elaioplasts, amyloplasts, leucoplasts, chloroplasts, chromoplasts, proplastids, and etioplasts. Proplastids are colorless and undeveloped plastid, and depending on the particular path of maturation that the parent cell will take, the proplastid may either develop in colored plastids or colored leucoplasts. Colored plastids include the more commonly known chloroplast, and also chromoplasts. Chloroplasts are widely known due to their vital role in photosynthesis. Chromoplasts are pigments synthesized and stored in the plant, and are typically found in aging leaves, fruits, and flowering plants. Leucoplasts are further subdivided into amyloplasts, elaioplasts, and proteinoplasts, which are concerned with storage of starch, oil, and protein respectively. Endosymbiosis is cited as the origin of plastids, whereby an captured or engulfed cell was reduced to a functional organelle.

  2. Lipids, as well as other molecules contribute to cell structure. However, lipids have no single common structure, although the most commonly occurring types are phospholipids and triglycerides. The former, phospholipids are the main lipids found in membranes, in addition to carbohydrates, proteins, and cholesterol. The latter is important for maintaining the consistency of the membrane. Proteins serve a number of function and can transport materials across the membrane, act as receptors for certain molecules, or act as enzymes for chemical reactions.

  3. Lipid-bound proteins are entirely located within the lipid bilayer. Peripheral proteins are easily detached from the lipid bilayer and are found of the exterior of this layer. They are however less mobile within the layer. Integral proteins are found within the lipid bilayer and cannot easily b removed without the destruction of the bilayer.

  4. Plant cells have mitochondria are these are the main sites of cellular respiration. The primary role of mitochondria is to produce energy, a function they share with chloroplasts.

  5. Vacuoles are formed by the fusion of multiple membrane vesicles, which fuse to form a single tonoplast, an incredible and unique membrane within the cell due to its ability to change size. Vacuoles can be as large as to occupy 90% of the cell, although most plant cells typically have vacuoles that occupy approximately 30% of the cell size.

  6. Although being double-membrane organelles, plastids essentially have three membranes due to the existence of a third internal membrane referred to as the thylakoid membrane. This membrane forms the thylakoids, a series of flattened discs, which are typically arranged in stacks called grana. The cytosol of plastids is referred to as the stroma.

  7. In order for proplastids to develop into chloroplasts, they have to occur in cells that have exposure to light. This prerequisite is benficial to plants as it ensures that chloroplast are only developed in areas where they will be useful, and that proplastids in other parts of the plant can develop into other plastids.

  8. Plasmodesmata only occur in plant cells and interconnect most plant cells, leading to the development of an extended or continuous cytoplasm referred to as the symplast. The size exclusion limit of plasmodesmata was found to be between 1.5 to 2 nanometers. Plant cells often undergo cytokinesis whereby Golgi-derived vesicles containing precursors to plant cell walls leading to formation of primary plasmodesmata.

Plasmodesmata are tubularextensions of the plasma membrane which traverse the cell wall andconnect to the cytoplasms of other cells.

Cell Wall

  1. Middle lamella are the connective substance between neighboring plant cell walls. It is a product of cytokinesis and is the first formed layer deposited at this time. Its function is to enable stability between the cells so as to enable formation of plasmodesmata.

  2. Phragmoplasts are plant cell structures created during the final stages of mitosis and acts as a scaffold for the assembly of cell plates as well as the subsequent formation of new cell walls separating daughter cells. Dissociated spindle sub-units are the main components used in the formation of phragmoplasts. The phragmosplast, during cytokinesis, gives rise to the cell plate in a series of steps that entails vehicle fusion.

  3. Primarily, cell wall provide structural support to the plant enabling it to form the necessary tissues and organs. Furthermore, cells walls also have an important communication role on the intercellular level.

  4. Primary cell wall are characteristic of growing and young plants and are typically covered by a polysaccharide-rich primary wall. They are primarily composed of polysaccharides, together with phenolic esters, structural glucoproteins, covalently and ionically bound minerals, and enzymes.

Microfibrils are highly orderedcrystalline aggregates of cellulose, and are arranged in layers alsoreferred to as lamellae.

The matrix consists of proteins,pectins and hemicelluloses, the latter two being types ofpolysaccharides. The function of the individual components of thematrix is to increase structural support to the cell wall.Non-polysaccharide components of cell walls include proteins.

  1. Secondary walls are created inside the primary cell walls. The major component difference between primary and secondary cell walls is the inclusion of lignin, which increases the strength and water-resistance of the wall. Therefore, the major function of secondary cell walls is to provide additional protection to cells and also

  2. Plant cells either grow through the process of cell expansion or cell differentiation. The latter refers to the specific growth path taken by plant cells within different regions of the plant, for example, the roots and the leaves. Cell expansion simply refers to the individual increase in plant cell size.

In tip growth, the growth islocally focused at the cell’s leading edge, and includes cells suchas root airs and pollen tubes. Diffuse growth is however more commonand involves membrane growth across the entire cell surface, or atleast a large part of it. Microfibrils in elongating cells typicallylie perpendicular to the axis of elongation.

  1. The process that provides the force for plant cell expansion is osmosis. In order for growth of the cell to take place, the cell wall has to be loosened and stretched, after which it is thickened. The acid growth hypothesis involves the acidification of the plant cell wall by the substance auxin, which either optimizes the activity of cell wall-localized proteins which loosen the wall or lowers the yield threshold of the wall. Either way, the hypothesis describes how low pH causes loosening and stretching of cell walls, hence facilitating elongation and expansion.