SIMPLAST: PARTS AND CHARACTERISTICS - BIOLOGY - 2025

The syplast is the continuous system formed by the cytoplasm of all the cells of a plant united by the plasmodesmata. The term is contrasted with that of apoplast, which is the system formed by all cell walls and intercellular spaces forming a continuous structure.

Both cell walls and cytoplasms are involved in the transport of water and nutrients inside plants. Transport via the cell wall is called apoplastic transport, while the transport that takes place through the cell cytoplasm is called simplstic transport.

Apoplast and syplast. Taken and edited from: Jackacon, vectorised by Smartse.

Although the simplistic transport was first observed in 1879 by E. Tangl, the term sympathetic was coined a year later by J. Hanstein. For his part, the German physiologist E. Munch used this term and that of apoplast to expose his theory of flow pressure that tries to explain the transport of solutes in the phloem of plants.

Parts

Cytoplasm

It is made up of all the parts of the cell that are contained within the plasma membrane, with the exception of the nucleus.

Plasmodesmus

Plasmodesmus is a microscopic channel that passes through the cell walls of plant cells. The plural of the term is plasmodesmata, although plasmodesmata is also used.

Plasmodesmata are formed during cell division by entrapment of endosplasmic reticulum fractions in the middle lamina during cell wall synthesis. The holes formed are normally aligned with those of neighboring cells to allow communication between the cytoplasms.

Apoplast

The apoplast is formed by the cell walls of contiguous cells and by the extracellular spaces forming a continuous structure that allows the transport of water and nutrients in plants.

The flow of substance through the apoplast is called apoplastic transport and is interrupted by the air spaces inside the plant as well as by the cuticle. The apoplastic pathway is also interrupted at the root by Caspary Bands.

Caspary bands

Caspary bands are structures present in the endodermis of plant roots. They are formed by suberin and to a lesser extent by lignin and surround the endodermis cells on four of their six faces, except those facing the outside and inside of the plant.

The waterproof barrier formed by the Caspary bands forces water and minerals to pass through cell membranes and cytoplasms instead of traveling only through cell walls.

In this way, the endodermis cell membranes can control both the type of nutrients that circulate between the cortex and the vascular tissue and their quantity.

Transport

Plants acquire water and inorganic nutrients from the soil through the roots and produce organic nutrients mainly in the leaves. Both water and organic and inorganic nutrients must be transported to all cells of the body.

To facilitate this transport, the nutrients are dissolved in the water that circulates inside the plant, forming a substance known as sap. Transport takes place through vascular tissue.

The xylem carries water and inorganic nutrients (eg nitrogen, potassium and phosphorus) from the root to the rest of the body (raw sap). The phloem for its part transports the nutrients produced during photosynthesis from the leaves to the rest of the plant (elaborated sap).

In both xylem and phloem, transport can be both apoplastic and syplastic. Apoplastic transport occurs within cell walls and may be faster than simplstic transport because the transported material is not filtered by cell membranes or cytoplasm.

Simplistic transport

The plasma membrane is a semipermeable barrier that surrounds the cytoplasm of each cell. Due to its semi-permeable condition, it can control the entry of molecules into the cytoplasm, allowing or promoting the passage of some molecules and preventing or restricting the passage of others.

At the root

In the roots, water and minerals reach the cells of the plant's endodermis via apoplastic transport. Once in endodermal cells, these substances cannot continue their movement through the apoplastic pathway because the Caspary Bands constitute a barrier for said transport.

In this way, the raw sap must pass through the cell membranes and the cytoplasm of the cells of the endodermis. The cell membrane is selectively permeable and can control the flow of nutrients between the cortex and the vascular tissue.

After filtration, the solutes reach the cells of the pericycle, with the help of the plasmodesmata, from where they can pass into the xylem for long-distance transport.

Simplistic and apoplastic water uptake diagram of a plant root. Taken and edited from: Dylan W. Schwilk.

On the sheets

Most of the photosynthesis of plants takes place in the leaves, and in this area is where the synthesis of carbohydrates and other organic molecules takes place. The carbohydrates must then be transported to the sugar sinks (places in the plant where sugar is consumed or stored).

The sugar molecules must be transported from the foliar mesophyll to the phloem cells in dissolved form in the sap and for this the presence of the semi-permeable membranes of the cells is required. This transport can be carried out both by the apoplastic route and by the simplistic route.

In simplstic transport, the sugar molecules of the foliar mesophyll cells remain inside the cells and move between them through the connecting plasmodesmata until they reach the phloem cells.

In apoplastic transport, the movement of sugar molecules occurs via the route external to the plasma membrane. Frequently in these cases, the plant stores the sugar molecules in the cell walls of the cells near the phloem.

When this occurs, the cells can then absorb the stored molecules and pass it to the phloem cells through the plasmodesmata (simplastic pathway).

The simplistic pathway of sugar transport to the phloem is more frequent in plants from warm climates, while plants from temperate and cold climates more frequently use apoplastic transport.

References

1. MW Nabors (2004). Introduction to Botany. Pearson Education, Inc.
2. Symplast. On Wikipedia. Recovered from en.wikipedia.org.
3. Apoplast. On Wikipedia. Recovered from en.wikipedia.org.
4. Plasmodesma. On Wikipedia. Recovered from en.wikipedia.org.
5. FB Lopez & GF Barclay (2017). Plant anatomy and physiology. Pharmacognosy.
6. I. Taiz & E. Zeiger (2002). Plant Physiology. Sinauer Associates.
7. H. Arjona (1996). Taking, transport and metabolism of water and nutrients in the plant. Colombian Agronomy.