Cells – Pieces and Parts

Membranes and Organelles

Despite their microscopic size, cells contain numerous internal structures called organelles. The structure and function of organelles is often highly complex, and nearly all organelles are involved in intricate biochemical reactions that maintain the living cell. The major organelles in cells are outlined below.

1. The cell membrane, or plasma membrane, is the thin (0.005 micrometer wide) membrane that encloses all components within the cell and controls the passage of materials into and out of the cell. Cell membranes are composed of phospholipids (both hydrophilic and hydrophobic) and sterols (mainly cholesterol). The hydrophobic “tails” attached to these molecules are clustered on the inner part of the membrane. The exterior of the membrane may contain chains of carbohydrates as well as transport proteins, or permeases, which regulate the transport of materials across the cell membrane. Some proteins also occur on the inner membrane.
2. Internal membranes, which are present only in eukaryotic cells, are membranes that surround organelles. They have the same basic structure as the plasma membrane, often with modifications, and also regulate transport into and out of the organelle. Because the various functions that organelles perform in the cell may require different conditions (pH for example), internal membranes maintain these distinct conditions by isolating organelles from each other.
3. The cell nucleus contains its genetic material, or DNA (deoxyribonucleic acid). The nucleus is therefore the control center of the cell and is separated from the rest of the cell by the nuclear envelope (whose presence distinguishes eukaryotes from prokaryotes, the latter which lack a nuclear envelope). The minute pores in the nuclear envelope permit substances to pass from the nucleus to the cytoplasm. As cellular conditions warrant, subsets of DNA in the nucleus are created that program for the building of specific proteins required by the cell. RNA acts as the intermediary between the DNA in the nucleus and the protein-building apparatus outside the nucleus in the cell, called cytoplasm. The cell nucleus contains a suborganelle called the nucleolus, which is the site of ribosome production.
4. Mitochondria are the metabolic engines of the cell, occurring in both plant and animal cells. Mitochondria are surrounded by a permeable bilipid membrane similar to the plasma membrane. The mitochondrion’s inner membrane is shaped to contain a huge number of folds, called cristae. The inner part of the organelle, called the matrix, is a space filled with enzymes and other metabolic substances.

Organelles

Mitochondria oxidize the products of metabolism from the cytoplasm and convert this energy into ATP (adenosine triphosphate), which fuels cell function. The essence of ATP production, which takes place in the matrix, lies in the transfer of electrons to and from various compounds, with ATP as the final product. A mitochondrion converts fats, amino acids, and sugars into acetyl coenzyme A. The acetyl part of this compound is then oxidized to yield carbon dioxide and hydrogen atoms, which are transferred to the hydrogen acceptors, coenzymes NAD (nicotinamide adenine dinucleotide) and FAD (flavin adenine dinucleotide). Adding hydrogen produces NADH and FADH 2. Oxidation of these compounds occurs gradually, controlled by proteins in the cristae, and eventually yields ATP.

1. Lysosomes have a single phospholipid bilayer membrane called vesicles. Vesicles are large, liquid-filled sacs containing enzymes, including hydrolases, which break down and process some cell products, such as proteins and lipids. Their low pH (~ 5) assists in the degradation of cell materials. Once broken down, or lysed, some of these products are transported across the lysosome membrane and used to make organic compounds required by the cell. The remaining waste usually remains inside the lysosome and is termed a residual body.
2. Peroxisomes are also vesicles that contain lysing enzymes. Peroxisomes specialize in degrading purines—nitrogenous compounds in cells. The degradation of purines produces hydrogen peroxide, a substance toxic to cells. So peroxisomes use other enzymes, like catalase, to break down hydrogen peroxide into harmless water and oxygen.
3. The Golgi apparatus, or Golgi bodies, is made up of a stack of five to eight flat, dish-shaped, membrane-bound sacs. The stack of sacs is surrounded by tubules and vesicles. Golgi bodies are like factories and redistribution centers. They receive the proteins and carbohydrates that occur on the surfaces of cell membranes, even their own, and then reassemble them. The Golgi apparatus gets vesicles from the endoplasmic reticulum, then alters the vesicle membranes, and finally processes and distributes their contents to other cell components, including the plasma membrane.

The endoplasmic reticulum (ER), which occurs only in eukaryotic cells, is a membranous network of connected flattened sacs, tubes, and channels that may make up as much as half the total membrane in the cell. There are two types of ER , rough and smooth, which are continuous with each other; although each has a distinct function.

Smooth endoplasmic reticulum (SER) is a mesh of membranous tubules that synthesizes phospholipids and cholesterol. The SER accomplishes this with enzymes bound to its membrane. Some of these products remain in the ER; others are transported to other organelles. SER is called “smooth” because it lacks ribosomes, cell particles made of RNA and proteins that synthesize other proteins from amino acids. The membrane of rough endoplasmic reticulum (RER) is made up of flattened sacs and vesicles and is covered with bump-like ribosomes. Therefore RER is key in making and releasing important cell proteins and glycoproteins. The RER synthesizes and distributes the enzymes necessary to lysosomes and proteins required by the Golgi apparatus, the ER, and the plasma membrane. A freshly synthesized protein moves from the RER through the SER and then to the Golgi apparatus. The protein is processed and slightly altered at each step, sometimes by splitting its amino acids, sometimes by adding carbohydrates.

Cross-section of a typical eukaryotic cell sowing the many different organelles

Staying in Shape

The cytoplasm is the living contents inside a cell, excluding the nucleus and the large vesicles. The cytoplasm contains a clear, colorless liquid termed the hydroplasm in which the organelles exist. Cytoplasm is about 90 percent water, but that water is a rich solution of ions, especially potassium, sodium, and chloride. Of course, the cytoplasm is bound and contained within the plasma membrane.

A cell’s vesicles help support the cell and keep it in shape. A cell’s cytoskeleton consists of interconnected protein filaments in the cytoplasm. The cytoskeleton not only maintains the cell’s three-dimensional shape; it also anchors its organelles and helps the cell move.

There are three basic types of structures that make up the cytoskeleton:

1. Microtubules, often only 20 nanometers in diameter, are long, hollow strands of the protein tubulin. Microtubules extend from the center of the cell to its plasma membrane. They are a “scaffold” for the building of other cell structures; they also play a key role in cell division. Many microtubules are constantly being built or taken apart, as required by the cell. In unicellular organisms, they are a permanent part of the flagella and cilia and thus are key to locomotion (as in the tail of a sperm cell). Microtubules are the main organizers of the cytoskeleton and may control the position and function of both actin filaments and intermediate fibers, as well as the movement of certain organelles inside the cell.
2. Actin filaments are far smaller than microtubules and are made from the cell protein actin. In non-muscle cells, actin filaments link the plasma membrane to the cytoplasm and are involved in some types of cellular movement. In muscle cells, actin filaments have a broader role. Arranged in an orderly fashion, they interact with filaments made from another protein called myosin, which with actin is crucial for muscle contraction.
3. The size of intermediate fibers falls between that of the microtubules and actin filaments. Intermediate fibers are rope-like and made of fibrous proteins that are not easily disassembled. They are found primarily in cells undergoing mechanical stress.