Plant

The cells of plants, fungi, and bacteria are surrounded by rigid cell walls. Plant cell walls are typically one to five micrometers thick, and their primary constituent is cellulose, a molecule consisting of many glucose units connected end-to-end. In plant cell walls, many cellulose molecules are bundled together into microfibrils (small fibers), like the fibers of a string. These microfibrils have great tensile strength, because the component strands of cellulose are interconnected by hydrogen bonds. The cellulose microfibrils are embedded in a dense, cell-wall matrix consisting of other complex molecules such as hemicellulose, pectic substances, and enzymes and other proteins. Some plant cells become specialized for transport of water or physical support, and these cells develop a secondary wall that is thick and impregnated with lignin, another complex carbohydrate.

All living cells are surrounded by a plasma membrane, a viscous lipid-and-protein matrix which is about 10 nm thick. The plasma membrane of plant cells lies just inside the cell wall, and encloses the rest of the cell, the cytoplasm and nucleus. The plasma membrane regulates transport of various molecules into and out of the cell, and also serves as a sort of two-dimensional scaffolding, upon which many biochemical reactions occur.

The nucleus is often considered to be the control center of a cell. It is typically about 10 micrometers in diameter, and is surrounded by a special double-membrane with numerous pores. The most important molecules in the nucleus are DNA (deoxyribonucleic acid), RNA (ribonucleic acid), and proteins. DNA is a very long molecule, and is physically associated with numerous proteins in plants and other eukaryotes. Specific segments of DNA make up genes, the functional units of heredity which encode specific characteristics of an organism. Genes are connected together into chromosomes, thread-like structures that occur in a characteristic number in each species. Special enzymes within the nucleus use DNA as a template to synthesize RNA. Then, the RNA moves out of the nucleus where it is used as a template for the synthesis of enzymes and other proteins.

Plastids are organelles only present in plants and algae. They have a double membrane on their outside, and are specilized for the storage of starch (amyloplasts), storage of lipids (elaioplasts), photosynthesis (chloroplasts), or other functions. Chloroplasts are the most important type of plastid, and are typically about 10 micrometers in diameter. Chloroplasts are specialized for photosynthesis, the biological conversion of light energy absorbed by chlorophylls, the green leaf pigments, into potential chemical energy such as carbohydrates. Some of the component reactions of photosynthesis occur on special, inner membranes of the chloroplasts, referred to as thylakoids; other reactions occur in the aqueous interior of the chloroplast, referred to as the stroma. Interestingly, plastids are about the size of bacteria and, like bacteria, they also contain a circular loop of DNA. These and many other similarities suggest that cells with chloroplasts originated several billion years ago by symbiogenesis, the union of formerly separate, prokaryotic cells.

Mitochondria are organelles which are present in nearly all living, eukaryotic cells. A mitochondrion has a double membrane on its outside, is typically ovoid or oblong in shape, and is about 0.5 micrometers wide and several micrometers long. Mitochondria are mainly responsible for the controlled oxidation (metabolic breakdown) of high-energy food molecules, such as fats and carbohydrates, and the consequent synthesis of ATP (adenosine triphosphate), the energy source for cells. Many of the mitochondrial enzymes that oxidize food molecules are embedded in special internal membranes of the mitochondria. Like plastids, mitochondria contain a circular loop of DNA, and are believed to have originated by symbiogenesis.

Golgi bodies are organelles present in most eukaryotic cells, and function as biochemical processing centers for many cellular molecules. They appear as a cluster of flattened vesicles, termed cisternae, and associated spherical vesicles. The Golgi bodies process carbohydrates, which are used to synthesize the cell wall, and lipids, which are used to make up the plasma membrane. They also modify many proteins by adding sugar molecules to them, a process referred to as glycosylation.

Vacuoles are fluid-filled vesicles which are separated from the cytoplasm by a special membrane, referred to as a tonoplast. Vacuoles are present in many eukaryotic cells. The vacuoles of many plant cells are very large, and can constitute 90% or more of the total cell volume. The main constituent of vacuoles is water. Depending on the type of cell, vacuoles are specialized for storage of foods, ions, or water-soluble plant pigments.

The endoplasmic reticulum is a complex system of interconnected double membranes, which is distributed throughout most eukaryotic cells. The membranes of the endoplasmic reticulum are often continuous with the plasma membrane, the outer nuclear membrane, the tonoplast, and Golgi bodies. Thus, the endoplasmic reticulum functions as a conduit for chemical communication between different parts of the cell. The endoplasmic reticulum is also a region where many proteins, lipids, and carbohydrates are biochemically modified. Many regions of the endoplasmic reticulum have ribosomes associated with them. Ribosomes are subcellular particles made up of proteins and RNA, and are responsible for synthesis of proteins from information encoded in RNA.