A Cellular Adventure

Intercellular junctions are the means through which animal cells adhere, interact and communicate. Junctions are especially common in epithelial tissue which lines the external and internal surfaces of the body. There are three types.
Tight junctions form when proteins hold adjacent cell membranes tightly together creating an impermeable seal across a layer of cells. This prevents leakage of extracellular fluid.
Desmosomes or anchoring junctions function like rivets, fastening cells together into strong sheets. Intermediate filaments anchor desmosomes in the cytoplasm, and these types of junctions attach muscles cells to each other.
Gap junctions are cytoplasmic channels that allow for the exchange of ions and small molecules like sugars and amino acids between cells through protein-lined pores. These are necessary for communication between cells.

Intercellular junctions are the means through which animal cells adhere, interact and communicate. Junctions are especially common in epithelial tissue which lines the external and internal surfaces of the body. There are three types.

Tight junctions form when proteins hold adjacent cell membranes tightly together creating an impermeable seal across a layer of cells. This prevents leakage of extracellular fluid.

Desmosomes or anchoring junctions function like rivets, fastening cells together into strong sheets. Intermediate filaments anchor desmosomes in the cytoplasm, and these types of junctions attach muscles cells to each other.

Gap junctions are cytoplasmic channels that allow for the exchange of ions and small molecules like sugars and amino acids between cells through protein-lined pores. These are necessary for communication between cells.

The extracellular matrix regulates cell behavior because signals from the matrix influence the activity of genes in the nucleus. It is composed primarily of glycoproteins. Collagen is the most abundant glycoprotein in the matrix, and it forms very strong fibers outside the cell. These fibers are embedded in a network woven from proteoglycans which consists of a small core protein with many carbohydrate chains. Fibronectin is another type of glycoprotein in the matrix that  binds to cell surface receptor proteins called integrins which are built into the plasma membrane.

The extracellular matrix regulates cell behavior because signals from the matrix influence the activity of genes in the nucleus. It is composed primarily of glycoproteins. Collagen is the most abundant glycoprotein in the matrix, and it forms very strong fibers outside the cell. These fibers are embedded in a network woven from proteoglycans which consists of a small core protein with many carbohydrate chains. Fibronectin is another type of glycoprotein in the matrix that  binds to cell surface receptor proteins called integrins which are built into the plasma membrane.

The cell wall protects plant cells, maintains their shape, and prevents excessive uptake of water. This support structure is made of microfibrils of cellulose embedded in a matrix of polysaccharides and proteins. The primary cell wall is secreted by a young plant cell, is relatively thin and flexible, and determines the direction of cell expansion. The middle lamella is a layer of sticky polysaccharides called pectin that glue adjacent plant cells together. The secondary cell wall is secreted by plant cells when they stop growing. It is thicker and stronger and is located between the primary cell wall and the cell membrane.

Plasmodesmata are channels in plant cell walls through which the plasma membranes of bordering cells connect. Water, small solutes, proteins and RNA can move through them. These channels unify the plant into one living continuum because the cytosol passes through the cells connecting the chemical environment of adjacent cells.

Motor proteins work together with the cytoskeleton and the cell membrane to allow whole cells to move along fibers outside the cell. They facilitate the bending of cilia and flagella as well as help microtubules function as “monorails” for vesicles inside the cell.

Microtubules form cilia (numerous and short) and flagella (1 or 2 per cell, long). These two types of appendages on the cell can be used to move a cell through aqueous media or move fluid past a stationary cell. Cilia in particular have the ability to transmit environmental signals to the cell’s nucleus.

The cytoskeleton is a network of three types of protein fibers. It provides mechanical support and allows for cell motility. The cytoskeleton can also transmit mechanical signals from the cell’s surface to the interior, and it can interact with motor proteins to produce cellular movement

Microtubules are the largest fiber, and their structures are like hollow rods which are made of columns of globular proteins called tubulins. These fibers can change length by the addition or subtraction of tubulin dimmers. The microtubules grow out from a region near the nucleus called the centrosome, a microtubule organizing center which duplicates during mitosis. Two centrioles are located in the centrosome each composed of nine sets of triplet microtubules arranged in a ring (they replicate before cell division). Microtubules provide supporting framework of the cell and serve as monorails for organelles to move along.

Microfilaments are solid rods made of actin (a globular protein) chains, and two of these chains form a helix. These fibers form a network just inside the plasma membrane. The sliding of these actin filaments and thicker myosin filaments past each other causes the contraction of muscles. Actin and myosin filaments also interact in local contractions like cleavage furrows in animal cell division and in amoeboid movement through pseudopodia.

Intermediate filaments are intermediate in size and diverse in composition. They serve as a framework especially to maintain cell shape (hold nucleus securely in place). Intermediate filaments make up the nuclear lamina which lines the nuclear envelope.

            The cytosol is the semi-fluid substance that supports organelles and other cellular components in a cell. The cytosol, along with all of its organelles, compose the cytoplasm. The cytoplasm is the region between the nucleus and the plasma membrane of a eukaryotic cell. Within the cytoplasm of a eukaryotic cell, suspended in the cytosol, are a variety of organelles of specialized form and function. The interior of a prokaryotic cell is also called the cytoplasm.

            The cytosol is the semi-fluid substance that supports organelles and other cellular components in a cell. The cytosol, along with all of its organelles, compose the cytoplasm. The cytoplasm is the region between the nucleus and the plasma membrane of a eukaryotic cell. Within the cytoplasm of a eukaryotic cell, suspended in the cytosol, are a variety of organelles of specialized form and function. The interior of a prokaryotic cell is also called the cytoplasm.

            The plasma membrane functions as the selective barrier that allows sufficient passage of oxygen, nutrients, and wastes to service the entire cell. Since a limited amount of a particular substance can cross per second, the ratio of surface area to volume is crucial. A cell’s volume grows proportionally more than its surface area as a cell’s size increases. A high ratio of surface area to volume is important in cells that exchange a lot of material with their surroundings (e.g. intestinal cells). The plasma membrane consists of both lipids and proteins, with its fundamental structure being the phospholipid bilayer; the bilayer creates a barrier between two aqueous compartments (separates the outside of the cell from the inside). Proteins embedded in the phospholipid bilayer carry out specific functions within the plasma membrane, such as selective transport of molecules and cell-cell recognition. 
*The plasma membrane is also described in the Golgi apparatus post, as well*

            The plasma membrane functions as the selective barrier that allows sufficient passage of oxygen, nutrients, and wastes to service the entire cell. Since a limited amount of a particular substance can cross per second, the ratio of surface area to volume is crucial. A cell’s volume grows proportionally more than its surface area as a cell’s size increases. A high ratio of surface area to volume is important in cells that exchange a lot of material with their surroundings (e.g. intestinal cells). The plasma membrane consists of both lipids and proteins, with its fundamental structure being the phospholipid bilayer; the bilayer creates a barrier between two aqueous compartments (separates the outside of the cell from the inside). Proteins embedded in the phospholipid bilayer carry out specific functions within the plasma membrane, such as selective transport of molecules and cell-cell recognition. 

*The plasma membrane is also described in the Golgi apparatus post, as well*

A peroxisome is a metabolic compartment that is bound by one membrane. It contains enzymes that transfer hydrogen from various substrates to oxygen, hydrogen peroxide is a by-product. This organelle has several functions including: using oxygen to break fatty acids down into smaller molecules that can be transported to mitochondria as fuel for cellular respiration, detoxifying alcohol and other harmful compounds in the liver, and converting hydrogen peroxide (the poisonous by-product of peroxisomes) to water. Glyoxysomes are specialized peroxisomes in fat-storing tissues of the plant seed which convert fatty acids to sugar which the seedling uses for energy until it can produce its own sugar.

A peroxisome is a metabolic compartment that is bound by one membrane. It contains enzymes that transfer hydrogen from various substrates to oxygen, hydrogen peroxide is a by-product. This organelle has several functions including: using oxygen to break fatty acids down into smaller molecules that can be transported to mitochondria as fuel for cellular respiration, detoxifying alcohol and other harmful compounds in the liver, and converting hydrogen peroxide (the poisonous by-product of peroxisomes) to water. Glyoxysomes are specialized peroxisomes in fat-storing tissues of the plant seed which convert fatty acids to sugar which the seedling uses for energy until it can produce its own sugar.

A chloroplast is of the type of plant organelle known as plastids. They contain the green pigment chlorophyll and sites for photosynthesis in plant cells only. The organelle has two membranes separated by a thin intermembrane space. Inside the chloroplast is a fluid called stroma (DNA, ribosomes, and enzymes are within it) which surrounds a membranous system of flattened sacs called thylakoids. Photosynthetic enzymes are embedded in these thylakoids and the stacks of thylakoids are named grana.

A chloroplast is of the type of plant organelle known as plastids. They contain the green pigment chlorophyll and sites for photosynthesis in plant cells only. The organelle has two membranes separated by a thin intermembrane space. Inside the chloroplast is a fluid called stroma (DNA, ribosomes, and enzymes are within it) which surrounds a membranous system of flattened sacs called thylakoids. Photosynthetic enzymes are embedded in these thylakoids and the stacks of thylakoids are named grana.

The mitochondria carry out cellular respiration by the production of ATP. The number of these organelles in the cell correlates with the cell’s level of metabolic activity. Mitochondria are unique because they have the ability to move, change shape, and divide in two because they contain their own DNA. Mitochondria are enclosed by two phospholipid membranes with embedded proteins. The outer membrane is smooth and the inner membrane is convoluted with foldings called cristae. Cristae allow a larger surface area in the membrane which makes cellular respiration more productive. There are two inner compartments: the intermembrane space between the inner and outer membrane, and the matrix which is enclosed by the inner membrane. The matrix contains enzymes, mitochondrial DNA and ribosomes.

The mitochondria carry out cellular respiration by the production of ATP. The number of these organelles in the cell correlates with the cell’s level of metabolic activity. Mitochondria are unique because they have the ability to move, change shape, and divide in two because they contain their own DNA. Mitochondria are enclosed by two phospholipid membranes with embedded proteins. The outer membrane is smooth and the inner membrane is convoluted with foldings called cristae. Cristae allow a larger surface area in the membrane which makes cellular respiration more productive. There are two inner compartments: the intermembrane space between the inner and outer membrane, and the matrix which is enclosed by the inner membrane. The matrix contains enzymes, mitochondrial DNA and ribosomes.