Tonoplast: Definition & Function

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  • 0:00 What are Tonoplasts?
  • 1:30 Tonoplast Structure
  • 2:31 Tonoplast Function
  • 3:45 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista

Jeremy has a master of science degree in education.

In the word of plants, we often hear the suffix, 'plast.' From chloroplasts to leucoplasts to amyloplasts, we often associate the suffix plast with plant. Such is the case here, with tonoplasts.

Tonoplasts: What Are They?

Tonoplasts are nothing like what their name would suggest (unless you know Greek). A tonoplast is actually the membrane that surrounds the large vacuole in a mature plant cell. We get the name 'tonoplast' from Greek, which translated looks something like 'tone, tension, stretching, and molded.' This would coincide well with where the tonoplast exists. It is also known as the 'vacuolar membrane.'

A vacuole is basically an organelle that is filled with food and/or waste particles. They are used to take in certain food substrates and then dissolve them. A large vacuole can make up around 95% (in some plants) of the plant cell and is rather unique to plants. Also called the central vacuole, its main job is to maintain turgor pressure inside of the plant. Turgor pressure helps the plant keep its shape by pressing the plasma membrane against the cell wall. This maintains a nice, rigid structure for the plant. Think about nice, crisp lettuce versus old, wilt-y looking lettuce. This is the best example of turgor pressure.

The membrane that surrounds the vacuole is the tonoplast. The name makes sense now, as we see the name means 'tension' and the tonoplast keeps tension on the vacuole. This helps to give the vacuole some form of structure. It also helps to maintain proper balance of nutrients and ions inside and out of the vacuole, thus keeping proper turgor pressure in the plant cell.

Tonoplast Structure

The tonoplast is unique to plant cells as animal cells do not have a central vacuole. The tonoplast is a lipid bilayer, typical of other cellular membranes. Recall that a lipid is part of the fats, oils, waxes, etc. group. A bilayer indicates that there are two layers, each with the hydrophillic (water loving) head facing out into the cellular cytosol and the vacuolar cytosol. Cytosol is a liquid component of a cell that surrounds organelles like the vacuole. The hydrophobic (water fearing) tails face inwards.

Here you can see the hydrophobic tails and hydrophillic tails. The tails keep away from the water where the heads turn towards them.
Lipid Bilayer drawing

The tonoplast is semi-permeable as well. This means that certain molecules and substrates can pass through it but others cannot. This allows the tonoplast to maintain a balance of ions inside and outside of the central vacuole. Again, this maintains proper turgor pressure inside the cell.

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