Cell Plate

Definition

The cell plate is a structure that forms in the cells of land plants while they are undergoing cell division.

The cells of land plants, unlike animal cells, have a cell wall made of stiff sugars which surround their cell membranes. In addition to protecting the cell from damage, the cell walls help to maintain the plant’s rigid upright structures, such as leaves and stems.

These rigid support structures allow plants to grow tall and spread their leaves wide, obtaining more sunlight. In most plants, the cell wall is made of cellulose – an arrangement of glucose molecules that forms hard, rigid surfaces.

Interestingly, the cellulose that makes up cell walls is not digestible to humans or animals – but it can be broken down into sugar by some methane-producing archaebacteria. This is one reason for the symbiotic relationship between many animals and the archaebacteria in our gut.

During cell division, plant cells must form a new cell wall to separate their daughter cells. This new fragment of cell wall must form in the middle of the parent cell, to ensure that half of the parent cells’ chloroplasts, gene copies, etc. end up on each side of the cell wall.

The “plate” of hard sugars that forms in the middle of the parent cell, which will become the cell wall of the future daughter cells, is called the cell plate.

It is formed when vesicles from the Golgi apparatus carrying phospholipids needed to make the cell membrane, and sugars needed to form the cell wall, are delivered and assembled along a network of cytoskeleton spindle fibers that forms in the middle of the cell as the cell prepares to divide.

Function of Cell Plate

Cell walls serve the double purpose of protecting the precious contents of plant cells, such as their nuclei, and allowing a plant to have free-standing structure.

Since plants do not have skeletons like animals, and are constantly growing and changing in competition to get more sunlight, it is important that the individual parts of plants, such as stems and leaves, be able to stand straight against the force of gravity on their own.

This is why land plants have cell walls, but animals who have skeletons, and ocean plants that live in the weightless environment under water may not have cell walls.

Having cell walls makes cell division a bit tricky for plants. To split in two and produce daughter cells – a process called “cytokinesis” – cells without cell walls simply pinch their cell membrane in two around the middle. The cell membrane is like a flexible bag, which can be pinched and re-formed as needed when a cell needs to change shape.

The rigid cell wall, however, cannot be bent or pinched in the same way. It constrains the shape of a cell during reproduction, and cannot be simply pinched around the middle.

Instead, to perform cytokinesis, plants must assemble a new section of cell wall to ensure that their daughter cells will have the structural integrity that the plant needs to maintain its form. They do this through a series of steps that we’ll discuss more below.

Cell Plate Formation

The “cell cycle” describes the process that cells go through, from their “birth” as new daughter cells, until they themselves are ready to split and become “parent cells” to two new daughter cells.

The formation of the cell plate takes place during the mitotic phase. In this description we will briefly describe all phases of the cell cycle to paint a complete picture, but feel free to skip to the section labeled “mitotic phase” to get a play-by-play on how the cell plate forms.

The stages of the cell cycle are divided into:

• Interphase – where the cell grows and matures
• The mitotic phase – where the cell begins working towards dividing itself

Interphase

These stages are further developed into specific steps in which the cell takes all the actions needed to produce two healthy daughter cells. The steps of interphase are:

• G1 Phase – Immediately after a daughter cell becomes independent, it spends some time growing and creating more organelles, such as chloroplasts.
• S Phase – In S Phase – shirt for “Synthesis” phase – the cell synthesizes a new copy of its DNA. All of its chromosomes are copied, as is its centrosome, which will help ensure that one copy of each chromosome goes to each daughter cell during cell division.
• G2 Phase – The cell continues to make more proteins and materials for its daughter cells, and cytoskeleton structures begin to form which will help the cell perform cytokinesis – and, in the case of land plant cells, form their cell plate.

Mitotic Phase

After G2 comes the mitotic phase, where the cell begins taking the actions it needs to split.

For the cell plate to form, space must be cleared of any vacuoles or any other obstructions that could get in the way; the chromosomes must be assorted so that each daughter cell receives a copy of each; and then the cell plate can form, separating the cytoplasm of the two daughter cells.

All that, and more, occurs during the mitotic phase.

The stages of the mitotic phase are:

1. Preprophase. Some plant cells have extremely large vacuoles, or vesicles that store materials such as fuel.In plant cells like these, the vacuoles first need to be moved or split in half to create a clear path through the cell’s cytoplasm where the cell plate can form. The nucleus also needs to be moved into the center of this path, so that its DNA can be split between the two daughter cells.This clear band of cytoplasm is called the phragmosome.
2. Prophase, in which the cell’s DNA curls up tightly into compact chromosomes. These compact chromosomes are easier to sort and move, ensuring that no DNA is left out of either daughter cell.The mitotic spindle, which will help move chromosomes to opposite sides of the cell which will then become separate daughter cells, also begins to form. In land plants, the mitotic spindle also lays the groundwork for the cell plate to form along the phragmosome.
3. In metaphase, motor proteins line the chromosomes up along a network of microtubules in the middle of the cell. This network is called the “metaphase plate.” In plant that have a Phragmosome, the metaphase plate usually runs through the Phragmosome, and occupies the same location where the cell plate will later form.
4. In anaphase, chromosomes lined up along the metaphase plate, chromosomes are separated, with one copy of each being dragged away from the center and toward each end of the cell. In this way, one copy of each chromosome ends up in the cytoplasm of what will become the daughter cells.
5. In telophase, a new nucleus forms in each daughter cell. This happens in the daughter cells’ cytoplasm, away from the cell plate.
6. The final phase of cell division, following telophase, is cytokinesis. In this stage the cell’s cytoplasm splits in two to produce two daughter cells. This is when the cell plate is formed.First, a network of microtubules, microfilaments, and parts of the parent cell’s endoplasmic reticulum form a structure called the “phragmoplast.”With the phragmoplast serving as its scaffold, the cell plate begins to assemble. Vesicles from the Golgi apparatus bring phospholipids to form the daughter cells’ new cell membranes, and sugars to form their rigid cell walls.The growth of the cell plate along the phragmoplast is pictured below:

The process of cell division for plants is truly complete when the cell plate has fused with the cell walls, forming a fully functioning divider between the two daughter cells.

Now, each daughter cell will begin to grow, and prepare to split into two daughter cells of its own!

Related Concepts

• Cell wall – A wall made of hard sugars, which protects plant cells and allows them to keep their rigid structure.
• Cellulose – The rigid sugar that makes up the cell walls of most plants. Although it is made of glucose, it cannot be digested by animals, but only by their symbiotic bacteria.
• Microtubules – Long, rigid structures that form within cells. Microtubules can act as a “railroad” track for materials to be delivered to a specific place in the cell, or can help to change the cells’ shape.

Quiz

1. Why don’t animal cells form a cell plate?
A. Animal cells do not need rigid cell walls, since animals have skeletons.
B. Animals cells can simply “pinch” their cell membranes until they form two separate compartments.
C. Animal cells need to be flexible, so that animals can move their limbs freely.
D. All of the above.

2. During which phase of the cell cycle does the cell plate form?
A. G1
B. S Phase
C. G2
D. Mitotic phase

3. Which of the following is NOT a correct pairing of cell division phases with cell plate formation steps?
A. Preprophase – Phragmosome forms as vacuoles are cleared away to form a clear band of cytoplasm where the cell plate can later form.
B. Anaphase – Cell plate is assembled along the metaphase plate.
C. Cytokinesis – Cell plate is assembled from phospholipids and sugars, along the scaffold of the phragmoplast.
D. None of the above.