Cells are the smallest units of life. They are a closed system, can self-replicate, and are the building blocks of our bodies. In order to understand how these tiny organisms work, we will look at a cell’s internal structures.
We will focus on eukaryotic cells, cells that contain a nucleus. Prokaryotic cells, cells that lack a nucleus, are structured differently.
A cell consists of two major regions, the cytoplasm and the nucleus. The nucleus is surrounded by a nuclear envelope and contains DNA in the form of chromosomes. The cytoplasm is a fluid matrix that usually surrounds the nucleus and is bound by the outer membrane of the cell. Organelles are small structures within the cytoplasm that carry out functions necessary to maintain homeostasis in the cell. They are involved in many processes, for example energy production, building proteins and secretions, destroying toxins, and responding to external signals.
Organelles are considered either membranous or non-membranous. Membranous organelles possess their own plasma membrane to create a lumen separate from the cytoplasm. This may be the location of hormone synthesis or degradation of macromolecules. Non-membranous organelles are not surrounded by a plasma membrane. Most non-membranous organelles are part of the cytoskeleton, the major support structure of the cell. These include: filaments, microtubules, and centrioles.
The plasma membrane surrounds the cell to create a barrier between the cytosol and the extracellular matrix. Plasma membranes also enclose lumens of some cellular organelles. The structure of the membrane resembles a fluid mosaic made up of phospholipids, cholesterol, and membrane proteins. Phospholipid molecules, the main structural components of the membrane, form an amphipathic bilayer. An amphipathic structure is both hydrophilic and hydrophobic; part of the structure has a high affinity for water and the other part is repelled by water. The inner surface of each layer is made up of lipid chains and thus is hydrophobic. The outer surface of each layer is made up of the polar heads of the phospholipids and is hydrophilic.
Proteins associated with the plasma membrane are either peripheral membrane proteins or integral membrane proteins. Peripheral membrane proteins interact closely with the membrane through ionic interactions. Integral membrane proteins are embedded within or pass through the lipid bilayer. There are six broad categories of integral membrane proteins: pumps, channels, receptors, linkers, enzymes, and structural proteins.
- Pumps - transport ions, sugars, and amino acids across membranes
- Channels - allow small ions and molecules to pass freely in and out of the cell
- Receptors - recognise and bind with ligands
- Linkers - anchor the cytoskeleton to the extracellular matrix
- Enzymes - have many roles, for example ATPases participate in ion pumping
- Structural proteins - form junctions with neighboring cells
The endoplasmic reticulum (ER) is a large network of membranes responsible for the production of proteins, metabolism and transportation of lipids, and detoxification of poisons. There are two types of endoplasmic reticulum with separate functions: smooth endoplasmic reticulum and rough endoplasmic reticulum. The presence or absence of ribosomes in the ER’s plasma membrane determines whether it is classified as smooth or rough ER.
The outer plasma membrane of rough endoplasmic reticulum (rER) is carpeted with ribosomes, causing it to appear dotted under a microscope. Protein production occurs in the ribosomes of rER. The ribosomes synthesize a peptide strand which enters the lumen of the rER and folds into its functional shape. From there it will be transported to the Golgi apparatus in a membrane bound vesicle formed from budding of the rER membrane.
Smooth endoplasmic reticulum, abbreviated sER, lacks ribosomes and thus appears smooth under a microscope. Its functions vary among cell types. For example, sER in cells of the liver have detoxifying functions while sER in cells of the endocrine system mainly produce steroid hormones. Detoxification occurs through enzymes associated with the sER membrane and usually involves adding hydroxyl groups to molecules. The presence of hydroxyl groups makes the molecules more water soluble and therefore able to be flushed from the body through the urinary tract. Steroid hormone synthesis occurs through reactions that modify the structure of cholesterol.
The Golgi apparatus appears as a series of flattened, membranous sacs, or cisternae, that resemble a stack of pancakes just off the rough endoplasmic reticulum. It receives vesicles containing proteins recently produced by the rER. The Golgi apparatus can be compared to a warehouse or post office for newly formed proteins. Here the proteins are further modified, packaged, and sent off to their final destinations in the cell or body.
Because the Golgi apparatus receives and sends off vesicles from opposite sides of its stack of cisternae, it is considered polar, meaning it has a directional structure. The cis-face is located near the rER and receives vesicles. The trans-face is on the opposite side of the organelle and releases vesicles through budding of the plasma membrane. The quantity of stacks depends on the function of the cell.
The mitochondrion, mitochondria denotes plural, is a double membrane bound organelle. Its inner membrane contains many infoldings called cristae. The space between the outer and inner membranes is referred to as the intermembrane space and the matrix is the space inside the inner membrane. Free ribosomes and mitochondrial DNA can be found in the matrix. Mitochondrial DNA is unique in that it is entirely maternally inherited.
Mitochondria are the powerhouses of the cell. Cellular respiration, the generation of energy from sugars and fats, occurs in these organelles. Some of the enzymes that catalyze respiration are found within the matrix. Other proteins involved in these reactions are built into the wall of the inner membrane. The cristae of the inner membrane are highly convoluted to increase surface area. This allows for more proteins lining the membrane and thus greater productivity.
Peroxisomes are single membrane compartments that contain enzymes used to remove hydrogen atoms from substrates. The free hydrogen atoms then bind to oxygen and create hydrogen peroxide.
Peroxisomes are especially important in the liver because transferring hydrogen from poisons or alcohol to oxygen atoms detoxifies harmful compounds.
Lysosomes are membranous sacs that hydrolyze macromolecules to carry out intracellular digestion. This may occur for a variety of reasons. Single-celled organisms, such as amoebas, use lysosomes to digest food products. This process is referred to as phagocytosis. Phagocytosis occurs in human cells as well, however in humans this process is used in defense to destroy invaders and bacteria.
Lysosomes are also used to recycle the cell’s own materials. This processes is referred to as autophagy. Damaged organelles that are broken down in the lysosome and its organic monomers are returned to the cell cytosol for reuse. In this way the cell is constantly renewing itself.
Transport vesicles are membrane bound sacs used to transport materials through the cytoplasm. They are formed from budding of the plasma membrane of other organelles and release their contents through exocytosis. Transport vesicles are used to move proteins around the cell and to release neurotransmitters into the synaptic space.
Ribosomes, either free in the cytosol or associated with rER, synthesize proteins as polypeptide chains. This occurs through the translation of RNA. Specifically, ribosomes bind to messenger RNA, abbreviated mRNA. The ribosome reads a series of nucleotide bases in groups of three called codons. The first codon read is the start codon. Each codon following the start codon represents a specific amino acid that is then brought to the ribosome by transfer RNA, abbreviated tRNA. The tRNA carrying the amino acid is bound into the A site of the ribosome. Here the amino acid is linked to the amino acid that precedes it, in the P site. The bond between two amino acids in a polypeptide chain is referred to as a peptide bond. After the peptide bond is created the ribosome translocates to the next three nucleotide bases on the mRNA strand and repeats the process until a stop codon is reached.
Microtubules are involved in the movement of organelles and other structures, for example lysosomes and mitochondria. They are elongated, nonbranching polymers made up of dimers of α-tubulin and β-tubulin. Microtubules contain approximately 13 circular dimeric tubulin molecules. Dimers can be added or removed to change the length of the microtubule. This process is termed dynamic instability and requires GTP hydrolysis. All of the tubulin dimers are arranged in a specific pattern so that they have the same orientation. Because of this orientation microtubules are considered polar, with a plus and minus end. Growth occurs at the plus end. The minus end of the microtubule does not grow.
Actin filaments are nearly ubiquitous among all cell types. Their structure is similar to that of microtubules in that they are formed by a helical arrangement of smaller molecules. However, actin filaments are thinner and more flexible than microtubules. Multiple cell functions require participation of actin. Actin filaments are, for instance, used as anchors in movement of membrane proteins and they form the core of microvilli.
The size of intermediate filaments, as their name implies, is between that of microtubules and actin filaments. Intermediate filaments consist of a rod domain with globular domains on either end. The rod domain is made up of a pair of helical monomers twisted around each other to form coiled-coil dimers. Although the subunits of intermediate filaments are diverse and tissue specific, the filaments generally perform a structural role in the cell. They primarily form a linked continuum of filaments in the nucleus, cytosol, and extracellular environment. They are especially involved in the formation of cell-to-cell and cell-to-extracellular matrix junctions.
Centrioles are structural organelles consisting of nine microtubule triplets organized into cylinders. The two main functions of centrioles are the formation of basal bodies and mitotic spindles. Basal bodies are used as building blocks for flagella and cilia. Mitotic spindles are involved in the separation of chromosomes during cell division. Centrioles determine the location of mitotic spindles during anaphase.
Cells are the smallest units of life. They are a closed system, can self-replicate, and are the building blocks of our bodies. A cell consists of two major regions, the cytoplasm and the nucleus. The nucleus is surrounded by a nuclear envelope and contains DNA in the form of chromosomes. The cytoplasm is a fluid matrix that usually surrounds the nucleus and is bound by the outer membrane of the cell. Organelles are small structures within the cytoplasm that carry out functions necessary to maintain homeostasis in the cell. A list of cellular organelles are given below:
- Endoplasmic reticulum
- Golgi apparatus
- Transport vesicles
- Actin filaments
- Intermediate filaments