Category: 1. Cell Structure

The following structures are found exclusively in plant cells and are absent in animal cells.

Cell wall

Though a cell wall is commonly found in prokaryotes and fungi as well as plants, their diversity is due to convergent evolution, not common ancestry, when it comes to these three groups of organisms. Plant cell walls are composed of cellulose are an excretion found outside the plasma membrane. They serve as a covering that provides structural support and gives shape to the cell.

Central Vacuole

The central vacuole is a large, membrane-bound structure that fills much of the plant cell. The membrane surrounding the central vacuole is called the tonoplast. The central vacuole plays a key role in regulating the cell’s concentration of water in changing environmental conditions. Have you ever noticed that if you forget to water a plant for a few days, it wilts? That’s because as the water concentration in the soil becomes lower than the water concentration in the plant, water moves out of the central vacuoles and cytoplasm (Figure 9.2.89.2.8). As the central vacuole shrinks, it leaves the cell wall unsupported. This loss of support to the plant’s cell walls results in the wilted appearance. The central vacuole also supports the cell’s expansion. When the central vacuole holds more water, the cell becomes larger without having to invest considerable energy in synthesizing new cytoplasm. Lastly, central vacuoles store nutrients, accumulate ions, or become a place to store wastes.

Plastids

Plastids are a group of storage organelle found in plants and algae. Chloroplasts are a type of plastid that store chlorophyll and other pigments for photosynthesis. Chromoplasts are plastids that store orange or yellow pigments, found in plants and fruit such as bell peppers. They are rich in carotenes and xanthophyls. Amyloplasts store starch and can be found in plants such as potato tubers, carrot roots, sweet potato roots, and grass seeds.

Chloroplasts store their pigments in interconnected sacs called thylakoids (Figure 9.2.99.2.9). These sacs are often found in stacks called grana (singular granum). The fluid portion of the double membraned chloroplast is called the stroma. Because the thylakoid stores chlorophyll a, b, and accessory pigments, it is the main region for the first reaction of photosynthesis, where sunlight is used to create molecular energy. In the stroma, the products of the first reaction are used to produce organic molecules such as glucose. The combination of these reactions allow these autotrophic organisms to produce their own organic food.

The chloroplast, like the mitochondria, contains its own DNA, ribosomes, and is double membraned.

Eukaryotic cells frequently have smaller vesicles including peroxisomes which, among other functions, help in photosynthesis in plant cells. In addition, many plant cells accumulate lipids as oil drops located directly in cytoplasm. Peroxisomes are small, round organelles enclosed by single membranes.

They carry out oxidation reactions that break down fatty acids and amino acids. They also detoxify many poisons that may enter the body. (Many of these oxidation reactions release hydrogen peroxide, H₂O₂, which would be damaging to cells; however, when these reactions are confined to peroxisomes, enzymes safely break down the H₂O₂ into oxygen and water.) For example, alcohol is detoxified by peroxisomes in liver cells. Glyoxysomes, which are specialized peroxisomes in plants, are responsible for converting stored fats into sugars.

Mitochondria (singular = mitochondrion) are often called the “powerhouses” or “energy factories” of a cell because they are responsible for making a nucleic acid called adenosine triphosphate (ATP), the cell’s main energy-carrying molecule. ATP represents the short-term stored energy of the cell. Cellular respiration is the process of making ATP using the chemical energy found in glucose and other nutrients. In mitochondria, this process uses oxygen and produces carbon dioxide as a waste product. In fact, the carbon dioxide that you exhale with every breath comes from the cellular reactions that produce carbon dioxide as a byproduct.

In keeping with our theme of form following function, it is important to point out that muscle cells have a very high concentration of mitochondria that produce ATP. Your muscle cells need a lot of energy to keep your body moving. When your cells don’t get enough oxygen, they do not make a lot of ATP. Instead, the small amount of ATP they make in the absence of oxygen is accompanied by the production of lactic acid.

Mitochondria are oval-shaped, double membrane organelles that have their own ribosomes and DNA. Each membrane is a phospholipid bilayer embedded with proteins. The inner layer has folds called cristae. The area surrounded by the folds is called the mitochondrial matrix. The cristae and the matrix have different roles in cellular respiration.

The cellular skeleton is a collection of protein filaments within the cytoplasm. Microtubules are key organelles in cell division, they form the basis for cilia and flagella. Plant cells do not have cilia, which are short projections from the cell that function in movement, but the sperm cells of early diverging plants, like bryophytes and seedless vascular plants, have flagella.

These are long projections that function in movement. Microtubules are also are guides for the construction of the cell wall, and cellulose fibers are parallel due to the microtubules. The movement in microtubules is based on tubulin-kinesin interactions. In contrast, the movement of microfilaments is based on actin-myosin interactions. Microfilaments guide the movement of organelles within the cell.

The lipids or proteins within the transport vesicles still need sorting, packaging, and tagging so that they end up in the right place. Sorting, tagging, packaging, and distributing lipids and proteins takes place in the Golgi apparatus (also called the Golgi body), a series of flattened membranes.

We call the side of the Golgi apparatus that is closer to the ER the cis face. The opposite side. closer to the plasma membrane, is the trans face. The transport vesicles that formed from the ER travel to the cis face of the Golgi, fuse with it, and empty their contents into the Golgi apparatus’ lumen. As the proteins and lipids travel through the Golgi, they undergo further modifications that allow them to be sorted. The most frequent modification is adding short sugar molecule chains. These newly modified proteins and lipids then tag with phosphate groups or other small molecules in order to travel to their proper destinations.

Finally, the modified and tagged proteins are packaged into secretory vesicles that bud from the Golgi’s trans face. While some of these vesicles deposit their contents into other cell parts where they will be used, other secretory vesicles fuse with the plasma membrane and release their contents outside the cell.

In plant cells, the Golgi apparatus has the additional role of synthesizing polysaccharides, some of which are incorporated into the cell wall and some of which other cell parts use.

Transport vesicles, composed of endomembrane system material, bud off the from the RER, carrying material into the Golgi Apparatus, the next component of the endomembrane system.

The endoplasmic reticulum (ER)  is a series of interconnected membranous sacs and tubules that collectively modifies proteins and synthesizes lipids. They are formed as an extension of the nuclear membrane and fold out towards the cytoplasm. The two functions of the ER take place in separate areas: the rough ER and the smooth ER, respectively.

The rough endoplasmic reticulum (RER) can be found has ribosomes along its surface, and the proteins they create are either secreted or incorporated into membranes in the cell. The smooth endoplasmic reticulum (SER) is continuous with the RER but has few or no ribosomes on its cytoplasmic surface. SER functions include synthesis of carbohydrates, lipids, and steroid hormones; detoxification of medications and poisons; and storing calcium ions.

The endomembrane system (endo = “within”) is a group of membranes and organelles in eukaryotic cells that works together to modify, package, and transport lipids and proteins. It includes the nuclear envelope, lysosomes, and vesicles, the tonoplast (see below), and the endoplasmic reticulum and Golgi apparatus. Although not technically within the cell, the plasma membrane is included in the endomembrane system because, as you will see, it interacts with the other endomembranous organelles. The endomembrane system does not include either mitochondria or chloroplast membranes.

If a peripheral membrane protein were synthesized in the lumen (inside) of the ER, would it end up on the inside or outside of the plasma membrane?

All cells contain DNA, as described above. However, plant cells, which are eukaryotic, contain organelles and a nucleus while prokaryotic cells do not possess organelles or a membrane bound nucleus. We will start by going over the structures that are unique to all eukaryotic. Next, we will go over structures unique to plant cells.

Ribosomes are the cellular structures responsible for protein synthesis. They are not organelles. They can be small dot-like structures that float freely in the cytoplasm (known as free ribosomes) or they may be attached to the plasma membrane’s cytoplasmic side or the endoplasmic reticulum’s cytoplasmic side and the nuclear envelope’s outer membrane, and called attached ribosomes.

Ribosomes are large protein and RNA complexes consisting of two subunits, a large and a small. Ribosomes receive their “orders” for protein synthesis from the nucleus where the DNA transcribes into messenger RNA (mRNA). The mRNA travels to the ribosomes, which translate the code provided by the sequence of the nitrogenous bases in the mRNA into a specific order of amino acids in a protein. Amino acids are the building blocks of proteins.

Because protein synthesis is an essential function of all cells (including enzymes, hormones, antibodies, pigments, structural components, and surface receptors), there are ribosomes in practically every cell. Ribosomes are particularly abundant in cells that synthesize large amounts of protein. For example, the pancreas is responsible for creating several digestive enzymes and the cells that produce these enzymes contain many ribosomes. Thus, we see another example of form following function.