The Secret Life of Cells
Most cells in the human body consist of several organelles, units within the cell that have a specific function. Organelles include the membrane, cytoplasm, nucleus, endomembrane, and mitochondria.
Membranes
The cell membrane or plasma membrane forms the boundary between the inside and outside of a cell. It consists of both protein and lipid molecules in varying ratios depending on the cell type. Typically, there are 50 lipid molecules for each protein in the membrane. However, since proteins are much bigger than lipids, proteins make up 50 percent of the mass of the membrane. These molecules are arranged in two opposing sheets, creating a bilayer of lipid and protein. One layer faces the outside of the cell, or the extracellular surface, and the other faces the interior, the cytosolic surface.
The main lipid type in the membrane is a phospholipid, a molecule made of a charged phosphate group attached to a base molecule (either a glycerol or sphingosine). Because of the negative charge of the phosphate, one end of the phospholipid layer can interact with water molecules, which are also charged, or polar. At the other end are fatty acid hydrocarbon chains that create a nonpolar (hydrophobic) area. Just as oil and water don’t mix, this part repels water or charged molecules and makes an effective filtration barrier, called a semipermeable membrane.
One role of cholesterol in cell structure
Cholesterol is abundant in the plasma membrane and serves to regulate membrane fluidity. Like phospholipids, a cholesterol molecule has a hydrophilic head and a hydrophobic tail, meaning the cholesterol molecule can align with the phospholipid and create a more rigid structure.
Proteins embedded in the phospholipid bilayer may be associated with either or both of the surfaces of the membrane. This membrane-spanning arrangement enables the proteins to serve many cellular functions. They transport material into or out of the cell and provide membrane attachment for stationary cells or adhesion for migratory cells. Many proteins in the membrane are receptors that recognize chemical signals and relay those signals to the inside of the cell to alter cellular activity.
Free-floating proteins
Proteins in the membrane are not locked in place. They may float throughout the membrane, spin, or flip horizontally.
Cytoplasm
Effectively separated from the extracellular environment by the plasma membrane, the inside of the cell is the location of most metabolic activity. The cytoplasm houses the workshops of the cell. Here, incorporated material is broken down, new proteins are generated, and new phospholipids are produced.
Nucleus
The nucleus, which contains the DNA (genetic code for the cell), is positioned in the center of the cell. The nucleus turns copies of the DNA code into RNA, which is used in the cytoplasm to make proteins.
The nuclear membrane consists of the same materials as the plasma membrane. But the nuclear membrane is composed of 4 phospholipid layers (2 bilayers) with a perinuclear space between.
The most prominent structure within the nucleus is called the nucleolus and is made up of proteins and nucleic acids. Here, rRNA (ribosomal RNA), required for protein production, is synthesized and prepared for transport outside the nucleus.
Anatomy of a Word
ribosome
Ribosomes are structures found in all living cells, responsible for producing most of the proteins that are created in an organism.
Protein complexes within the nuclear membrane regulate transport of materials into and out of the nucleus. While small water-soluble molecules may pass unimpeded, larger molecules must be assisted to get from place to place. The “helper” is a cotransport molecule, which must bind to the “cargo” molecule to allow the transport. The helper that moves molecules into the nucleus is called importin. Exportin moves molecules out of the nucleus.
Endomembrane System
Many of the membrane-bound organelles of a cell are either physically or functionally connected or both, and are thus grouped together as part of the endomembrane system. These include:
· nuclear envelope (the membrane surrounding the nucleus)
· endoplasmic reticulum
· Golgi apparatus
· vesicles
· plasma membrane
Endoplasmic Reticulum
The endoplasmic reticulum (ER), which plays a vital role in protein and lipid production, is made up of large folded sheets of membranes that occupy vast expanses of the cytoplasmic compartment. ER comes in two types:
· Rough ER (rER) is covered with ribosomes, the organelles for protein synthesis, which give the ER a rough appearance.
· Smooth ER (sER) does not contain ribosomes and is the site of lipid synthesis.
Material of the ER is transported in membrane-bound spheres called vesicles that move toward and fuse with the membranes of the Golgi apparatus.
Golgi Apparatus
The Golgi apparatus, a cell structure made up of flat sacklike layers, is essential for sorting proteins and packaging them for specific targets. As the vesicles fuse together on the incoming side, they form a new layer termed the cis face of the Golgi. Like an assembly line, these layers are moved higher and higher in the stacks of the Golgi as new cis layers are added. At the opposite side of the stacks, the trans face, or last layer, breaks up into transport vesicles and shuttles material to its target.
Some materials will be shipped to the plasma membrane, while others will be placed into vesicles going to other membrane-bound organelles, such as the mitochondria.
Vesicles
In addition to the transport vesicles of the endomembrane system, other vesicles are essential for proper cellular function. Lysosomes are spheres of enzymes that break down proteins, carbohydrates, or fats. Peroxisomes contain hydrogen peroxide and are prominent in liver and kidney cells where the hydrogen peroxide detoxifies ethanol and breaks down fatty acids.
Mitochondria
The mitochondria is a structure that creates the energy used by the cell. It consists of a double membrane system, much like the nuclear membrane. Like the nucleus, mitochondria also possess DNA. The mitochondrial genome encodes for over 30 genes whose products play essential roles in metabolism and energy production.
Shaped like a capsule, the outer mitochondria membrane is flat over the surface of the organelle while the inner membrane is folded (to increase surface area) into sheets, which are called cristae. Proteins in the inner membrane create an electron transport system where protons or hydrogen ions (H+) are transported from the interior of the mitochondria, called the matrix, to the intermembrane space between the inner and outer membranes. The energy of the flowing H+ is used to produce ATP (adenosine triphosphate), the molecule that all cells of the human body use for energy.