Topics in Depth
The Theme of Structures Only in Plant (and Other) Cells in Cells
Structures Only Found in Plant (and Some Algae and Fungus) Cells
Now we move on to the fun stuff that you only wish you had in your cells. It's OK to be jealous.
Like the cell walls in bacteria, fungi, algae, and some Archaea (single-celled organisms), plant cell walls are found just outside the plasma membrane and provide structure and protection to the cells.
The Cell Wall: Not Yours.
Transmission electron micrograph source: Paungfoo-Lonhienne et al. PLoS ONE. 2010; 5(7).
The cell wall prevents plant cells from bursting (lysing…) when too much water moves into the cell across the membrane. As water pushes against the cell wall from the inside, plant cells become large and firm because pressure, known as turgor pressure, builds up against the inside of the cell wall.
You have experienced the presence of turgor pressure when you have broken a piece of crisp celery. In the same way, if you have every tried to break a piece of wilted celery, you also know what effect the absence of turgor pressure can have on a plant. Plant cell walls also have small openings, called plasmodesmata, that allow cells to communicate with adjacent cells. Plant cell walls are composed primarily of a protein called cellulose, while fungal cell walls are made of a protein called chitin.
Large Central Vacuole
Most plant cells have a large membrane-bound sac called a vacuole. In many types of plant cells, this vacuole can occupy between 30% and 80% of the total volume of the cell, making it the largest single cellular structure. The main function of the large central vacuole is to help the cell maintain water pressure, aka turgor pressure, on the cell wall. Water molecules flow into the central vacuole and, like a big balloon inside a cardboard box, fills up and pushes outward on the cell wall.
Helpful tip: Vacuole sounds a lot like vacant, or vacuum, and this is a good way to think about this organelle. It mostly contains a water-based solution with some organic compounds and enzymes.
Many plant and algae cells contain small, green organelles called chloroplasts.
Here is what our green friend looks like:
Transmission electron micrograph source: Wikimedia Commons
These organelles are responsible for converting the energy from the sun into chemical energy, usually in the form of glucose, through the elaborate process of photosynthesis. Chloroplasts are green because they contain large amounts of the green pigment chlorophyll bound to proteins embedded in internal stacks of membranes called thylakoids. Sunlight is captured by chlorophyll molecules and transferred throughout the thylakoid membranes to give off energy. This energy is used to strip carbon from carbon dioxide in the air to make sugar. Generally, in any given plant, only some of the cells will contain chloroplasts.
The chloroplast was also a free living bacterial cell at one point, and its genome is much larger than that of the mitochondrion.
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