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Plant Cell Wall: Function, Structure & Composition

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Taught by

Kelly Robson

Kelly has taught High School Science and Applied Communications. She holds an Education Specialist Degree in Ed. Leadership.

All cells have a cell membrane on the outside of them that acts like a skin. Plant cells go one step further and have a cell wall - a protective outside that provides support and other functions.

What is a Cell Wall?

All living things are composed of cells. They are the building blocks of all life. Cells come in many different shapes and have different functions. Plant and animal cells are different too. The main difference between plant and animal cells is that plant cells have a cell wall on the outer layer, whereas animal cells only have a cell membrane.

Function

A plant cell wall has many important jobs. The main two jobs it has are to provide protection and support for the plant. In fact, it is the cell wall that gives the plant its actual shape. It acts as a gate keeper, determining what can come in and out of the cell, in order to keep the cell protected. It is kind of like the outside bricks of a castle, only, as you will learn as you read on, there are holes throughout this castle. Those holes do make the cell vulnerable, but they are important to the function of the cell.

A plant cell has a cell wall, whereas an animal cell only has a cell membrane.
Plant Cell

A redwood tree and a dandelion both have cell walls on the outside of all of their cells. The cell walls are there to give the plants their shape and support; however, the cell walls act and are constructed a little different to meet the needs of the particular plant. For instance, a 100-foot redwood tree needs a very strong and rigid plant cell wall so that it can grow to it's great height and not fall over in the wind. On the other hand, a little yellow dandelion out in the field needs to have more plasticity so that it can bend, not break, as the wind blows through the field.

A dandelion needs to have plasticity to be able to bend in the wind.
Dandelion

Have you ever forgotten to water the flowers? They may not be able to talk, but they will let you know they're thirsty as they begin to droop over. Their shape is still being maintained by the cell wall so that, as soon as you water the plant, it can pick itself back up again. On the other hand, if you water too much, the cell wall also makes sure that the cell does not burst. It protects the cell from overexpansion.

The cell wall protects the plant and cells from the many insects and pathogens that could harm the plant, but the cell wall still has its vulnerable areas. There are holes all over the cell wall called plasmodesmata. These holes allow for nutrients to enter the cell as well as waste to exit the cell. These small holes can cause the cell to lose water, and this is when the plant will start to droop. But as soon as the plant can get a drink, it will bounce right back up to its proper shape.

Here is a list of the basic plant cell wall functions:

  • Provide support and limited plasticity
  • Prevent loss of water
  • Protection from insects and pathogens
  • Filter
  • Prevent overexpansion caused by too much water
  • Keep the shape of the plant
  • Allow plants to grow to great heights

Structure and Composition

The plant cell wall is composed of cellulose. Cellulose is a structural carbohydrate and is considered a complex sugar because it is used in both protection and structure. The plant cell wall consists of three layers. Each layer has its own unique structure and function. The layers may vary depending on the type of plant and its needs.

Cell walls are composed of three layers.
Cell Wall Layers

The top layer is the strangely-named middle lamella. This outer layer is shared by neighboring cells, and it connects the cells together to form a strong structure. It is also very flexible. The middle lamella is rich in pectins, which help to strengthen the plant and give it the ability to resist compression. They also contain enzymes that help to breakdown the cell wall, allowing the plant to change its structure. This process is important when fruit is ripening.

The primary wall is the next layer. It is composed of cellulose in the form of microfibrils. These cellulose microfibrils weave together with glycans, increasing the strength of the cellulose. Pectins can also be found in the primary cell wall. With all of this strength, you would think nothing could squish a dandelion.

The third and final layer is the secondary wall. This layer is extremely rigid and provides compression strength. It helps stop the plant from getting squished. The secondary wall has a very similar composition as the primary wall, only it has more stuff in it - it contains lignin, which is very hard and has considerable strength. The secondary wall also protects the plant from invading bacteria or fungi.

Lesson Summary

A plant cell wall is constructed around the outside of the cell to give plants a strong and protective building block. The cell wall can be divided into three layers, each of which is made for strength and protection. These layers and their compositions not only protect the cell but allow for the cell to function. There are passage ways for nutrients to enter and waste to leave. The plant cell wall is unique to plants and plays many vital roles to making plants what they are.

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