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Plants and animals may have split off to form different branches of the evolutionary tree eons ago, but we still share alot of similarities. For example both our cells and that of our green cousins are eukaryotic (refer to previous article).
But this article will highlight the differences, not just for convenience, but due to their different needs and functions. |
"It is not so much that the cells make the plant; it is rather that the plant makes the cells."
Heinrich Anton de Bary
In this article, we'll talk about some of the things that make plant cells so different from our cells. In addition to being photosynthesizing machines, plant cells have cell walls and central vacuoles to make them unique.
Plant Cells Have Specialized Components
Imagine this: your friend asked you to watch her plants while she went away for a vacation. You know plants need plenty of sunshine and water, so you put them on your windowsill and give them a daily drink. But maybe one day you get home late from work and you just completely forget to water the plants. Today turns into tomorrow, which turns into the next day. Before you know it, you walk by the plants and notice they've started to wilt!
Well, even though the plants are wilting, you notice they still look like the same plant with roughly the same shape. There's still hope! With a few extra days before your friend comes back, you still have some time to get back on track and water them daily. Thankfully, the plants perk back up in no time and you're still rewarded for your plant-sitting with a souvenir.
If you ever stopped to wonder why plants don't completely collapse when you forget to water them, there's a good reason for that. Plant cells share a lot of components with the cells in your body, such as DNA in a nucleus, an endoplasmic reticulum and ribosomes. However, plants have some original cell components that are different than what's in your cells. For one, plant cells have chloroplasts that uses sunlight to turn nutrients and carbon dioxide into food. But before that, we'll talk about two structures important to plant cells.
Plant Cells Have Specialized Components
Imagine this: your friend asked you to watch her plants while she went away for a vacation. You know plants need plenty of sunshine and water, so you put them on your windowsill and give them a daily drink. But maybe one day you get home late from work and you just completely forget to water the plants. Today turns into tomorrow, which turns into the next day. Before you know it, you walk by the plants and notice they've started to wilt!
Well, even though the plants are wilting, you notice they still look like the same plant with roughly the same shape. There's still hope! With a few extra days before your friend comes back, you still have some time to get back on track and water them daily. Thankfully, the plants perk back up in no time and you're still rewarded for your plant-sitting with a souvenir.
If you ever stopped to wonder why plants don't completely collapse when you forget to water them, there's a good reason for that. Plant cells share a lot of components with the cells in your body, such as DNA in a nucleus, an endoplasmic reticulum and ribosomes. However, plants have some original cell components that are different than what's in your cells. For one, plant cells have chloroplasts that uses sunlight to turn nutrients and carbon dioxide into food. But before that, we'll talk about two structures important to plant cells.
Cell Walls
All plant cells have a plasma membrane just like an animal cell, which provides the same barrier and regulates transport. However, plant cells also have a specialized structure called the cell wall. The cell wall is a protective layer surrounding the cell on the outside of the plasma membrane. A cell wall can be up to 800 times thicker than the plasma membrane. It's composed largely of cellulose, a polysaccharide sugar that provides strength to the cell wall. If you've ever noticed how strong the bark of a tree is, that's because this bark is composed of dead cells with really tough cell walls.
The cell wall also grows with the cell, getting bigger as the cell gets bigger. Although plants aren't the only organisms with a cell wall, this structure is a characteristic of all plants.
The cell wall serves several purposes. Its toughness provides great protection, strength and shape to the cell, helping a plant cell to be both flexible and rigid. Think about creeping vines. The stems are strong enough that you need a tool to cut them, but flexible enough that they can wrap around trees and sway with it without breaking.
The plasma membrane is flexible enough to move closer to or away from the cell wall - according to changes in the water content of the cytoplasm within the cell. These are done using Plasmodesmata, which are tiny strands of cytoplasm that pass through pores in plant cell walls, forming "connections" or "pathways" between adjacent cells. Specifically, plasmodesmata form the symplast pathway for the movement of water and solutes through plant structures (see diagram, above-left). These cell-cell connections are especially important for the survival of plant cells during conditions of drought.
All plant cells have a plasma membrane just like an animal cell, which provides the same barrier and regulates transport. However, plant cells also have a specialized structure called the cell wall. The cell wall is a protective layer surrounding the cell on the outside of the plasma membrane. A cell wall can be up to 800 times thicker than the plasma membrane. It's composed largely of cellulose, a polysaccharide sugar that provides strength to the cell wall. If you've ever noticed how strong the bark of a tree is, that's because this bark is composed of dead cells with really tough cell walls.
The cell wall also grows with the cell, getting bigger as the cell gets bigger. Although plants aren't the only organisms with a cell wall, this structure is a characteristic of all plants.
The cell wall serves several purposes. Its toughness provides great protection, strength and shape to the cell, helping a plant cell to be both flexible and rigid. Think about creeping vines. The stems are strong enough that you need a tool to cut them, but flexible enough that they can wrap around trees and sway with it without breaking.
The plasma membrane is flexible enough to move closer to or away from the cell wall - according to changes in the water content of the cytoplasm within the cell. These are done using Plasmodesmata, which are tiny strands of cytoplasm that pass through pores in plant cell walls, forming "connections" or "pathways" between adjacent cells. Specifically, plasmodesmata form the symplast pathway for the movement of water and solutes through plant structures (see diagram, above-left). These cell-cell connections are especially important for the survival of plant cells during conditions of drought.
A macrostructure of plant tissue
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Details of a cell wall
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Central Vacuoles
In addition to a cell wall, plant cells also have this very large structure that can take up as much as 80% of its volume. This is called a central vacuole, a large storage compartment in plant cells. What can this vacuole hold? Well, although other non-plant cells can also have vacuoles, plant cells have these characteristically gigantic vacuoles that are largely a storage place for water. While animal cells are around 70% water, plant cells can be up to 90% water - and they need a place to put it. However, in addition to water, a vacuole can also contain food in the form of sugars and other nutrients, as well as waste products.
The central vacuole can also contain digestive enzymes like those in animal cell lysosomes. A central vacuole also can help maintain a neutral pH in the cell by pumping hydrogen atoms, or protons, from the cytoplasm into the vacuole. What happens if you pump a lot of protons into the vacuole? This makes this plant cell component acidic, which also is similar to a lysosome.
In addition to a cell wall, plant cells also have this very large structure that can take up as much as 80% of its volume. This is called a central vacuole, a large storage compartment in plant cells. What can this vacuole hold? Well, although other non-plant cells can also have vacuoles, plant cells have these characteristically gigantic vacuoles that are largely a storage place for water. While animal cells are around 70% water, plant cells can be up to 90% water - and they need a place to put it. However, in addition to water, a vacuole can also contain food in the form of sugars and other nutrients, as well as waste products.
The central vacuole can also contain digestive enzymes like those in animal cell lysosomes. A central vacuole also can help maintain a neutral pH in the cell by pumping hydrogen atoms, or protons, from the cytoplasm into the vacuole. What happens if you pump a lot of protons into the vacuole? This makes this plant cell component acidic, which also is similar to a lysosome.
Chloroplasts
Now back to something we’re familiar with, no not photosynthesis, we'll get into the details of how photosynthesis works in the future. This time we'll discuss an important structure essential to this process. Each plant is made of many plant cells performing photosynthesis. Inside of each of these tiny plant cells are one to hundreds of small structures called chloroplasts. Chloroplast comes from the Greek word 'chloros' for 'green' and 'plastis' for 'the one that forms.' Chloroplasts are the structural sites of photosynthesis, where light energy is used to convert nutrients into food. A cell that resides in a plant leaf, for example, might have hundreds of chloroplasts that capture light from its tanning session.
Chloroplasts exist in the cytoplasm of plant cells. They are flat, Frisbee-shaped structures filled with thylakoids. Thylakoids are small disk-like compartments composed of membranes that are the sites of sunlight-dependent photosynthesis. The thylakoids are surrounded by the stroma, or the inner liquid portion of the chloroplast. Both the stroma and the thylakoids contain important molecules for photosynthesis. Thylakoids are often stacked on top of each other - they look like a stack of flapjacks. Grana, or singular 'granum,' are stacks of thylakoids within chloroplasts. They contain their own DNA like mitochondria do and hence must have evolved via endosymbiosis, as mentioned in our previous article.
Now back to something we’re familiar with, no not photosynthesis, we'll get into the details of how photosynthesis works in the future. This time we'll discuss an important structure essential to this process. Each plant is made of many plant cells performing photosynthesis. Inside of each of these tiny plant cells are one to hundreds of small structures called chloroplasts. Chloroplast comes from the Greek word 'chloros' for 'green' and 'plastis' for 'the one that forms.' Chloroplasts are the structural sites of photosynthesis, where light energy is used to convert nutrients into food. A cell that resides in a plant leaf, for example, might have hundreds of chloroplasts that capture light from its tanning session.
Chloroplasts exist in the cytoplasm of plant cells. They are flat, Frisbee-shaped structures filled with thylakoids. Thylakoids are small disk-like compartments composed of membranes that are the sites of sunlight-dependent photosynthesis. The thylakoids are surrounded by the stroma, or the inner liquid portion of the chloroplast. Both the stroma and the thylakoids contain important molecules for photosynthesis. Thylakoids are often stacked on top of each other - they look like a stack of flapjacks. Grana, or singular 'granum,' are stacks of thylakoids within chloroplasts. They contain their own DNA like mitochondria do and hence must have evolved via endosymbiosis, as mentioned in our previous article.
Inside the chloroplast
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How chloroplasts photosynthesise
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In Summary
The plant body is composed of cells and their products. All plant cells are surrounded by a rigid cell wall that is produced by the cell. Inside the wall is the cell membrane, which selectively regulates the movement of materials into and out of the cells. Plant cells are compartmentalized into organelles that are structurally organized to support their function. The nucleus, chloroplast, ribosomes, and vacuole are examples of plant cell organelles.
Molecules flow into and out of the cells by simple diffusion, facilitated diffusion via passive transport, and active transport. Osmosis is the diffusion of water across a membrane. Osmosis occurs in response to a water potential gradient produced by differences in solute concentration on each side of a membrane.
The plant body is composed of cells and their products. All plant cells are surrounded by a rigid cell wall that is produced by the cell. Inside the wall is the cell membrane, which selectively regulates the movement of materials into and out of the cells. Plant cells are compartmentalized into organelles that are structurally organized to support their function. The nucleus, chloroplast, ribosomes, and vacuole are examples of plant cell organelles.
Molecules flow into and out of the cells by simple diffusion, facilitated diffusion via passive transport, and active transport. Osmosis is the diffusion of water across a membrane. Osmosis occurs in response to a water potential gradient produced by differences in solute concentration on each side of a membrane.
Ponder this
Why do plants need cell walls? Animal cells also have the ability to retain water, but why do plant need this thick tough wall?
Chloroplasts are the key to photosynthesis, and yet they exhibit in all plant cells. The trunk and branches of a tree are not the main factories for photosynthesis, and yet there too are chloroplasts. Why?
Discuss
Plant and animal (eukaryotic) cells are more similar to each other than they are to bacterial (prokaryotic) cells. Both have mitochondria and ribosomes, and unlike bactrial cells, have a distinctive nucleus. What is the evolutionary story behind these two? Where do they branch off, and why? Why don't animal cells also adopt chloroplasts the same way they took in mitochondria?
Further readings
Plant cells, chloroplasts, and cell walls, at SciTable by Nature
"How the first plant came to be", an article on the theorised evolution of plant cells
"How endosymbiosis changed life on Earth", a very good resource on cellular evolution, courtesy of UC Berkeley.
