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An illustration shows the location of each organelle within a plant cell. At the center of the cell is the nucleus, which appears as a large green sphere. Its surface is lined with flattened ovals; one oval is labeled nuclear pore. A portion of the nucleus has been cut away, revealing a collection of many smaller spheres that are labeled nucleolus. Immediately above the nucleus is the rough endoplasmic reticulum, depicted as a matrix of flattened compartments with many small dots appearing along the exterior. The smooth endoplasmic reticulum is to the right of the rough endoplasmic reticulum, appearing smaller in size and lacking the dots along its exterior surface. Between the smooth endoplasmic reticulum and the nucleus is a mitochondria. It's a bean shaped organelle with a squiggly line that runs lengthwise down its center. To the right of the mitochondria is a collection of 6 rod shaped organelles labeled Cytoskeleton. Right of the cytoskeleton is a lysosome, which is relatively small and resembles a quarter of a circle. Below and slightly right of the nucleus is a chloroplast; it's shown as an oval shaped structure containing two squiggly lines that run lengthwise down its center. To the right of the chloroplast is a plastid, depicted as a moderately sized quarter of a circle. The golgi body is below and left of the plastid, and is represented by a collection of flattened circles and ovals. Left of the golgi body is a vacuole, which resembles a very large, fluid fillled sac. Two more chloroplasts are shown above the vacuole and left of the nucleus. The space between each organelle is filled with the cytoplasm, and the cytoplasm is encased by the cell membrane. The exterior of the cell, which encapsulates the cell membrane, is the cell wall.

Figure 2.08: Generalized Plant Cell 

Structurally, plant and animal cells are very similar because they are both eukaryotic cells. They both contain membrane-bound organelles such as the nucleus, mitochondria, endoplasmic reticulum, Golgi apparatus, lysosomes, and peroxisomes. Both also contain similar membranes, cytosol, and cytoskeletal elements. The functions of these organelles are extremely similar between the two classes of cells (peroxisomes perform additional complex functions in plant cells having to do with cellular respiration). However, the few differences that exist between plant and animals are very significant and reflect a difference in the functions of each cell. 

Plant cells can be larger than animal cells. The normal range for an animal cell varies from 10 to 30 micrometers while that for a plant cell stretches from 10 to 100 micrometers. Beyond size, the main structural differences between plant and animal cells lie in a few additional structures found in plant cells. These structures include: chloroplasts, the cell wall, and the central vacuoles. 

A graphic compares an animal cell to a plant cell. On the left is an animal cell containing mitochondria, lysosomes, an endoplasmic reticulum, a cell membrane, a golgi apparatus, peroxisomes, a nucleus, cytosol, and cytoskeleton. On the right, a plant cell contains all of the organelles within an animal cell, as well as a cell wall, chloroplasts, and vacuoles.

Figure 2.09: Plant Cell vs. Animal Cell 

Chloroplasts 

In animal cells, mitochondria produce the majority of the cells energy from food. It does not have the same function in plant cells. Plant cells use sunlight as their energy source; the sunlight must be converted into energy inside the cell in a process called photosynthesis. Chloroplasts are the structures that perform this function. They are rather large, double membrane-bound structures (about 5 micrometers across) that contain the pigment chlorophyll, which absorbs sunlight.  

Chloroplasts carry out energy conversion through a complex set of reactions similar to those performed by mitochondria in animals. The double membrane structure of chloroplasts is also reminiscent of mitochondria. The inner membrane encloses an area called the stroma, which is analogous to the matrix in mitochondria and houses DNA, RNA, ribosomes, and different enzymes and is where the Calvin Cycle occurs. Contained withing the stroma, are thylakoids, membrane bound sacs that are stacked to form grana. This membrane contains chlorophyll and is where the light-dependent reactions of photosynthesis occur which will be discussed in Unit 3. Inside the thylakoid membrane, is the lumen. By having this additional internal membrane, the surface area for the light-dependent reactions of photosynthesis is increased and the cell can more efficiently convert light energy into ATP. 

The Cell Wall 

Another structural difference between in plant cells is the presence of a rigid cell wall surrounding the cell membrane. This wall can range from 0.1 to 10 micrometers thick and is composed of fats and cellulose. The tough wall gives added stability and protection to the plant cell. Cell walls also exist in other kinds of organisms such as fungi and some prokaryotes. 

Central Vacuoles 

Central vacuoles are large, liquid-filled organelles found primarily in plant cells. Vacuoles can occupy up to 90% of a cell's volume and have a single membrane. Their main function is to manage the tonicity of the cell and maintain turgor pressure to keep the cell rigid, but they can also fill digestive functions similar to lysosomes. Vacuoles can contain a number of enzymes that perform diverse functions, and their interiors can be used as storage for nutrients or, as mentioned, provide a place to degrade unwanted substances. 

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