We will begin our study with the cell wall, which surrounds the outside of plant cells. This structure protects and supports the plant cell and renders it somewhat rigid. The polysaccharide cellulose is the main component of the plant cell wall.
All cells are enclosed by a cell membrane (or plasma membrane) which is composed of phospholipids and proteins. Various biochemical mechanisms permit small nutrients to pass through the membrane to the cell’s interior.
Within the cell membrane is the cytoplasm (or, cytosol). This fluid-gelatinous portion of the cell suspends organelles, and enzymes and other proteins that are produced in the cytoplasm (but not by the cytoplasm).
The cytoplasm contains an internal protein framework called the cytoskeleton. Microfilaments within the cytoskeleton provide the mechanism for contraction in muscle cells, and other cytoskeleton fibers called microtubules participate in reproduction.
Extending out from the cell membrane of animal cells are projections called microvilli. These fingerlike projections are found in cells of the digestive tract of animals and humans, where absorption takes place. Longer hairlike extensions called cilia are found on cells of the respiratory tract, where they trap dust particles in mucus in order to prevent them from entering the lungs.
The first internal cell structure we will study is the centrosome. The centrosome contains two bodies called centrioles. Centrioles are situated at right angles to one another and are composed of microtubules; they are involved in mitosis.
Ribosomes are seen at numerous locations within the cell. These ultramicroscopic bodies are the “workbenches” of the cells; they are the sites of protein synthesis from amino acid subunits (monomers). Ribosomes are especially numerous in cells that synthesize proteins, such as pancreatic cells, muscle cells, and epidermal (skin) cells. Ribosomes are commonly located along the infoldings of the cell membrane called the ER, or endoplasmic reticulum, which forms an internal network within the cytoplasm (discussed below).
Plant cells have a few structures that animal cells do not. For example, the larget organelle in the plant cells is located near the center of the plant cell and is known as the vacuole, which contains water, sugars, ions, pigments, and other substances. It also applies pressure to the cell membrane (turgor pressure), causing it to expand and stick close to the cell wall.
A key organelle in a plant cell are chloroplasts. Chloroplasts are the site of photosynthesis, in which the sun’s energy is converted into chemical energy in the form of glucose and other carbohydrates. Photosynthesis is an essential metabolic process that will be discussed in the future.
An important organelle of the cytoplasm is the mitochondrion (mitochondria). The mitochondrion is a double-membrane enclosed organelle that produces ATP, which is the energy currency of the cell. Cells that require large amounts of energy, such as muscle cells and sperm cells, contain many mitochondria, while fewer of these organelles exist in less active cells.
The center of genetic activity in the cell is the nucleus. With the exception of red blood cells and gametes (sex cells), all human cells have forty-six chromosomes (DNA + specialized proteins) in their nucleus. A large body of RNA (dark spot in the nucleus) is called the nucleolus and is suspended in the fluid-like nucleoplasm (similar to the cytoplasm) in the nucleus. The genes within the nucleus are small sections of DNA that are specific nucleotide sequences that contain the instructions for the synthesis of particular proteins (one gene - one protein). The nucleus is surrounded by a nuclear membrane, which is characterized by shallow depressions called nuclear pores (animal cells have these, as well). RNA passes out of the nucleus through these pores and travels to ribosomes for translation (to make proteins). Proteins and nucleotides also pass into the nucleus through these pores.
The endoplasmic reticulum, also called the ER, is a system of interconnected membrane channels in the cytoplasm. These membranes may or may not have ribosomes associated with them. If ribosomes are associated with the ER, it is referred to as a rough ER. Rough ER predominates in cells that are actively synthesizing proteins for export (sent to other cells). Where the endoplasmic reticulum has few or no ribosomes, it is known as smooth ER.
After proteins have been manufactured, they are generally stored in a series of flattened membranes called the Golgi body. The Golgi body, which is made up of a series of about ten to twenty flattened membranes. This organelle modifies, packages, and secretes proteins after they are synthesized at the endoplasmic reticulum.
The cell stores digestive enzymes in organelles known as lysosomes. Enzymes in lysosomes help break down organic molecules into components that are useful to the cell in protein synthesis and energy metabolism.
Enzymes are also stored in peroxisomes. This is the site in which toxic compounds are neutralized. For this reason, peroxisomes are abundant in liver cells where they participate in the breakdown of alcohol, among other toxins.