Getting substances into and out of cells

The cell (plasma) membrane is a barrier specifically designed to prevent random objects from crossing at will. Yet cells must constantly exchange materials with their outside environment if they are to survive. How do some substances get across the cell membrane barriers when other do not? As we are about to see, there are four principle ways in which materials are moved into and out of cells.

Diffusion

The process of diffusion is the random movement of substances from a region of high concentration to a region of low concentration until no differences in concentration remain. For example, consider what happens when you make a cup of tea. The tannin molecules, which give the tea its golden brown color, are very concentrated in a tea bag but absent from the hot water. As they start to dissolve, the molecules inside the bag begin to move faster and faster. One molecule bumps into anotherabd then into a second and a third and so on until the molecules gradually spread out and leave the vicinity of the tea bag, and turn the water a marvelous golden color. Slowly, the tannin molecules have moved away from a region of high concentration near the tea bag and into a region of low concentration in the surrounding water until eventually the tannin molecules are at equal concentrations throughout the liquid.

As the tannin molecules randomly disperse themselves throughout the liquid, they are obeying the second law of thermodynamics, and the amount of disorder in the universe is increased.

Osmosis

Like other substances, water diffuses from regions where it is in high concentration to regions where it is in low concentrarion. One important region of high water concentrarion is just outside the cell where the fluid is mainly water that contains few dissolved substances. On the other hand, water is at a lower concentration inside the cell because of the presence of sugars, amino acids, proteins and organelles as well as other substances. Water therefore, diffuses from its high concentration outiside the cell to the low concentration inside the cell, passing through pores in the hydrophobic cell membrane.

The cell membrane, however, is a very selective barrier not permeable to many other substances found at high concentrations inside the cell. It is said to be semipermeable; that is, water can move freely across the cell membrane, but other materials cannot. As a result, there is a net flow of molecules into the cell as water diffuses in and other materials are prevented from moving out. Passage of water from a dilute solution to a more concentrated solution across a semipermeable membrane is called osmosis. If a cell does not compensate for the effects of osmosis, it will gradually swell with water and burst.

How do cells compensate for the effects of osmosis? Cell walls surrounding plants, fungi and most prokaryotes are strong and rigid. Such cells can expand only until the swollen cell presses up against this immovable barrier. Single-celled, freshwater animals contain pumps. These organelles, called contractile vacuoles, specialize in collecting excess water and expelling it from the cell. In multicellular organisms like ourselves, each cell is surrounding by fluid that contains salts, sugars and so forth at the same concentration as those found inside the cell. Osmosis is not a problem, therefore, unless the balance of materials outside the cells is disturbed, asi it is in some medical conditions.

Transport

Diffusion plays an important role in moving small molecules short distances within and around cells. However for large molecules and many others diffusion is too slow and nonselective. If they are to obtain vital ingredients in a timely manner, cells must have a way of speeding up the movement of molecules into and out of their cytoplasm.

Also, the cell membrane is impermeable to many of the larger substances needed by cells. Sugars, amino acids, etc. Cannot simply diffuse from one side of the membrane to the other. Cells, therefore, transport these needed molecules across the membrane using special carrier proteins. These proteins, located in the membrane, are very specific and attach only to certain types of molecules. In this way, a cell can carefully regulate the amount and types of molecules that pass into and out of it.

Passive transport

Carrier proteins can move molecules only in the same direction they woul normally move by diffusion. This process, called facilitated diffusion, allows the cell to control what materials are transported across the membrane and in what quantity. No energy is required.

As its name suggests, diffusion is still important in this transport mechanism. Molecules, like sugars, reach carrier proteins in the membrane by diffusion and are then moved across the membrane from a region of high concentration to one of low concentration. For example, if a cell is placed in a sugar solution, sugar molecules will diffuse and make contact with carrier proteins within the membrane. Carrier proteins will then transport these sugars across the membrane and into the cell, but only as long as the concentration of sugar is greater outside the cell than inside.

Active transport

There are many instances, however, in which the cell must move substances from regions of low concentration to regions that are already highly concentrated. For example, a particular sugar may be in dilute solution outside the cell as well as in high concentration inside the cell, but it would be beneficial for the cell to accumulate more of this valuable resource. Built into the membrane are carrier proteins that attach to sugar molecules outside the cell and then use energy to pump these molecules against the normal direction of diffusion and into the cell. This system is known as active transport. Energy is required for active transport because work is being done against a natural force of diffusion.

Endocytosis and Exocytosis

For larger particles, cell must use other processes. Endocytosis is a general term for the process whereby very large particles of material are wrapped with plasma membrane and moved into the cell in form of vesicles of vacuoles. None of the trapped material actually moves through the membrane, but it remains on the other side of the original membrane, even while the vacuole is inside the cell.

Exocytosis is the reverse of endocytosis. Quantities of material are expelled from the cell without ever passing through the membrane as individual molecules. By using the processes fo endocytosis and exocytosis, some specialized types of cells move large amounts of bulk material into and out of themselves.

Solid particles are engulfed by phagocytosis (cell eating) a process that begins when solids make contact with the outer cell surface, triggering the movement of the membrane. The desired particles are then enclosed within a small piece of plasma membrane, which forms a sac called vacuole (vesicle), with the food particles inside. This vacuole is the moved to the interior of the cell. Strictly speaking, the food particles are not yet part of the cell because they are still surrounded by membrane.

Before food can be used, it must be broken down into smaller pieces and those pieces moved into the cytoplasm. Digestion occurs when the food vacuole is fused with a second vacuole called a lysosome that contains power digestive enzymes. Food is degraded, its nutrients are absorbed by the cell, and its waste products are left in the digestive vacuole, which may then leave the cell by exocytosis.

Phagocytosis occurs in the scavenging white blood cells of our body. They prowl around looking for invading bacteria and viruses, which they engulf and destroy. Pinocytosis (cell drinking) is almost the same process as phagocytosis, except it involves liquids instead of solids.

During exocytosis, a vacuole containing material to be excreted from the cell moves to the plasma membrane and fuses with it. The vacuole membrane becomes part of the plasma membrane and the contents are released to the outside. Cells use this method to eliminate wastes left from digestion and metabolism and also to release a whole variety of materials that have been synthesized inside the cell but are needed outside the cell. Release of hormones and digestive enzymes, found in multicellular animals, are two examples of this process.

To view video animations as well as more information on cell transport. Click the numbers 1 2 3.

The cell membrane

The cell membrane (sometimes called plasma membrane or plasmalema) forms the thin molecular surface of every cell. It is made up of lipids, proteins and some carbohydrates in a flexible, dynamic, and ever changing array. The plasma membrane acts as a selective barrier between the inside and the outside of the cell and controls the exchange of materials between the cytoplasm and the surrounding liquid. It is almost impermeable to certain ions and molecules, and the cell uses specialized transport mechanisms to move molecules from one side to another.

Phospholipids are the major srtuctural components of most membranes. These molecules form a bileyer (a double layer of material) on the surface of the cell with their long hydrophobic hydrocarbon chains pointing inward to the center of the bilayer and their hydrophilic phosphate groups facing outward. Within this bilayer of lipids float various kinds of proteins, rather like ships in a lipid sea. Some proteins remain on the surface and are called extrinsic proteins, whereas intrinsic proteins are partially submerged or extend through the phospholipid bilayer.

Protein and lipid constituents of membranes are not fixed in any other location, but they can move and locate themselves at different points on the cell surface as required. Some, having carbohydrates or lipids attached to them, are complex glycoprotein and glycolipid macromolecules that play roles in recognition between cells and acts as receptor for molecules such as hormones.

The physical state of membranes is dynamic. For example, when a cell adds extra cholesterol to a membrane, this changes the fluidity and converts the cell membrane from a liquid like state to a more viscous gel like state. Components may be added or taken away as the cell charges, grows or becomes a specialist. Our modern picture of the cell membrane is a dynamic one of constant change, movement, modification and adaptation. To view a neat video about cell membrane structure and the fluid mosaic model click here. What is the mosaic model of the cell membrane?

Unit II : The Cell

Introducción

Los organismos vivientes están formados por unidades básicas llamadas células. Las características ascociadas con la vida dependen de las actividades que ocurren dentro de las células. Algunos organismos pequeños se componen se componen de una célula. Los organimos de una célula se llaman organismos unicelulares. Dentro de esta célula se llevan a cabo todas las actividades de vida del organismo unicelular. Los organismos más grandes están formados por muchas células y son llamados organismos multicelulares. Las actividades de los organismos multicelulares se dividen entre sus muchas células.

La mayoría de las células son tan pequeñas que el ojo humano no puede verlas a simple vista. No fue hasta la invención del microscópio que se descubrieron y estudiaron las células. Este instrumento de magnificación demostró ser uno de los inventos más importantes de la historia de la ciencia. El desarrollo de los microscópios permitió a los científicos estudiar las células en detalle.

Los primeros microscopios se hicieron alrededor de 1600. Galileo, un científico italiano, hizo un microscópio con el que observó insectos. El microscópio de Galileo era un microscopio compuesto, es decir, tiene dos lentes. Cada una de esas lentes está montada en cada extremo de un tubo hueco. Dos fabricantes holandeses de espejuelos, Jans y Zaccharias Jansen, también desarrollaron los primeros microscopios compuestos.

Robert Hook, un científico inglés, mejoró en algo el diseño del microscopio compuesto. Con su microscopio, Hooke observó muchos objetos, incluyendo cortes finos de corcho. Lo que él vio le recordó unas pequeñas celdas, como las de un monasterio. En 1665, en su libro Micrographia, Hooke usó la palabra células (celdas pequeñas) para describir las celdas que había observado en el corcho. Hooke no había observado células v ivientes, pero sí había visto las paredes de células que habían estado vivas. Sin embargo, se le recconoce a Hooke el haber sido la primera persona que observó e identificó las células.

Unos años después de las observaciones de Hooke, Anton van Leeuwenhoek, un comerciante holandés, vió también las células. El microscopio de Hooke aumentaba unas 30 veces los objetos. Leeuwenhoek construyó microscopios simples con sólo una lente que aumentaban los objetos unas 200 veces. Con ellos observó células sanguíneas, baceterias y organismos simples que nadaban en una gota de agua.

La teoría celular

Para el siglo XIX, los microscopios se habían mejorado mucho. Los científicos habían podido estudiar estructuras nunca antes vistas en las células. En 1883, Robert Brown, un botánico escocés, descubrió que las células de las hojas de las orquídeas tenían una estructura central. A esta estructura le llamamos ahora núcleo. Pocos años mas tarde, se usó la palabra protoplasma para referirse al material viviente del interior de las células. En 1838, Mathew Schleiden, un botánico alemán, propuso, como resultado de sus observaciones en tejidos vegetales, la hipótesis de que todas las plantas están formadas por células. Al año siguiente, Theodor Schwann, un zoólogo alemán, amplió la hipótesis luego de sus observaciones en tejidos animales, y propuso que los animales también están formados por células. Schwann propuso también que los procesos de la vida de los organismos ocurren en las células. En 1858, Rudolf Virchow presentó evidencia de que las células se reproducen para formar nuevas células.

Como resultado de muchas investigaciones, incluyendo las de Schleiden, Schwann y Virchow, se desarrolló la teoría celular. La teoría celular se puede resumir en estos postulados.

• Todos los organismos están formados por una o más céluilas.
• La célula es la unidad básica de estructura y función de los organismos.
• Las células nuevas provienen, por reproducción celular, de células que ya existen.

Las hipótesis y las investigaciones de una persona pueden ser la base principal de algunas teorías. Sin embargo, la teoría celular fue el resultado de lso descubrimientos de muchos biólogos. Hoy se reconoce la Teoría Celular como una de las principales de la biología. Ha servido de base para los biólogos que buscan nuevos conocimientos acerca de la célula y de sus propiedades.

Assignment: Read the following article on a historical perspective on cell theory

Tomado de: Alexander et al (1987) Biología. Prentice Hall. p 21-22.

Proteins, Nucleic Acids.

Proteins

Amino acids

Proteins are among the most abundant and versatile macromolecules. They have different functions as enzymes, hormones, storage, transport, structural proteins, membrane proteins, and antibodies. Eventhough their strurctural and functional diversity is overwhelming, their properties depend as much on their final shape as on their constituent parts.

Proteins are polymers of amino acids. All amino acids have the same fundamental structure.

1.- An amino group (-NH2).- It is a weak base

2.- A carboxyl group (-COOH).- It is a weak acid

3.- A hydrogen (-H)

4.- A functional group that varies from one amino acid to the next and is generally called and -R group.

The only difference between the 20 life relevant aminoacids is that of ther lateral groups (-R). In 8 of the molecules, -R is formed by short chains or rings of C, H. These amino acids are non polar thus, hydrophobic. 7 of the amino acids have –R formed by weak acids or weak bases. Depending on the solution’s pH, they can be possitively or negatively charged.

Proteins

Amino acids link to each other in a condensation reaction. The “head” of an amino acid bonds to the “tail” of another one by the elimination of a molecule of water. This bond is called peptide bond and the molecule that forms by the union of several amino acids is called a polypeptide. The sequence of aminoacids in a polypeptide gives the molecule its particular biological function, even the slightest variation in the sequence can result in an alterated or nule protein function.

Because there are about 20 different amino acids commonly found in proteins and each protein can contain anywhere from 200 to 300 individual amino acids, the diversity of possible amino acid sequences for any given polypeptide is extremely large. For example, 400 different dipeptides are possible and 8,000 tripeptides. For peptides of 300 amino acids, the possible number of sequences is so large that there are not enough atoms in the entire universe to build one example of each possible sequence!

The final properties of a protein depend largely on its three dimensional shape. Before a polypetide can function within a cell, therefore, it must twist and fold into a unique and special properties and functions as a protein emerge.

The final shape of a protein molecule is determined by three set of conditions.

1.-The sequence of amino acids in the polypeptide.

2.-The interaction between the amino acids in the polypeptide.

3.-The interactions of the amino acids with the surrounding water.

The sequence of amino acids from one end of the molecule to the other is called primary structure. Under orders from the genetic and synthetic apparatus of the cell, aminoacids are joined together in a precise and predetermined fashion. The primary structure is only the start.

In water a polypeptide twists and turns until it takes up the most stable configuration, a shape that requires the least energy to maintain and the most to disrupt. It is termed the secondary structure of the molecule. For example, amino acids close to each other interact and form hydrogen bonds. This forces the polypeptide to into and alpha helix or a beta sheet.

Next comes the interaction with water. Many parts of the molecule are either polar and hydrophillic or nonpolar and hydrophobic. As the folded and coiled polypeptide chain interacts with the surrounding water molecules, hydrophobic –R groups are forced , as much as possible, into the interior of the emerging structure, away from the surrounding water. Hydrophilic –R in the other hand, are stabilized by the interaction with water; thus, they are found close to the surface of the protein and in contact with water. The shape taken on by the macromolecule as a result of these forces is called a tertiary structure.

For many proteins, this is the highest level of structure they achieve. At the tertiary level, the protein suddenly takes on its major property and begins to function as a vital macromolecular component of the cell. But for some, there is one more step., the highest order os structure called the quaternary structure. At this level, the functioning protein unit is a complex built by the combination of more than one polypeptide chain and /or by the addition of other substances such as metallic ions, carbohydrates, lipids, nucleic acids, or other carbon based structures.

Hemoglobin, the oxygen-carrying protein found in red blood cells is an example of a protein showing the all four levels of structure. This macromolecule, consists of four polypeptide chains. Each chain has its own amino acid sequence (primary structure), and parts of chain twists into regions of alpha helix (secondary structure). All the chains fold into unique shapes in water (tertiary structure) and the come together in a fourfold complex with four additional iron-containing structures called heme groups (quaternary structure).

Function of Proteins

Protein functions are as varied as their structures. Almost every vital chemical reaction in every living cell is mediated by its own highly specialized protein known as enzyme. These enzymes are critically important in the reactions that break down food molecules, build new cellular structures, or repair existing components. Very few, if any, chemical reactions within a living organism can take place at any appreciable rate without these proteins.

The hard shell of a turtle, the skin of a new born baby, the claws of a lion and the feathers of a hummingbird all contain a type of protein known as keratin. This protein helps build and maintain physical structures, as does collagen, an essencial component of ligaments and tendons that connect bones and muscles.

Other proteins are involved in movement. Actin and myosin are fibrous contractile proteins found in muscle cells and involved in changing the shape of individual cells.

When attacked by invading organisms, our bodies respond by producing antibodies, a type of protein designed to recognize and neutralize and invader. Messenger molecules called hormones help regulate complex processes such as immune responses, growth and metabolism. Some hormones are steroids but other are proteins.

Within the human body, there is a probable minimum of 10,000 different proteins, each with its own unique structure shape and function. Information needed to construct each of these proteins, with all amino acids in the correct sequence, is stored in the genetic and hereditary material of cells: The nucleic acids.

Nucleic Acids

The information that orders the myriad of proteins that are found in the organisms is coded by two macromolecules known as nucleic acids and is translated by them. Nucleic acids are polymers formed by long chains of monomers called nucleotides. A nucleotide is a molecule formed by three subunits: A phosphate group, a five-carbon sugar and a nitrogenous base.

The sugar subunit can be either a ribose or a deoxyribose, the latter having one oxygen atom less than the ribose. Ribose is the sugar that is present in ribonucleic acid (RNA) whereas, deoxyribose is present in deoxyribonucleic acid (DNA). There are five nitrogenous bases which are the primary skeleton of the nucleic acids. Two of them adenine (A) and guanine (G) have a two ring structure and are known as purines . The other three, cytosine (C), thymidine (T) and uracil (U) have only one ring and are known as pyrimidines.

Both nucleic acids present A,C,G but RNA presents U instead of T which is present in DNA.

Most RNA molecules found in cells consist of a single polynucleotide chain, whereas DNA molecules are almost invariably double stranded, consisting of two polynucleotide chains twisted around one another in a double helix, which resembles a spiral stair case. DNA is usually synthesized only at one specific time in the cell cycle whereas RNA is more or less produced constantly throught the cell cycle.

The sugar-phosphate backbones of both DNA strands are on the outside of the molecule, forming the handrails of the staircase, the nitrogenous bases face inward, forming the stairs. Each step in the staircase is a pair of bases, one from each strand. These bases are paired together in specific combinations. Adenine is always paired with thymine and guanine is always paired with cytosine. Using the letter abreviation A=T, G=C. These are the base pairing rules. Thus if the sequence of bases is known for one DNA strand, it is possible to deduce the sequence of the partner strand, known as the complementary strand. For example, if one of the strands has the sequence

ATGTTCAAT

The sequence for the complementary strand is

TACAAGTTA

The two strands would go together like this

ATGTTCAAT

TACAAGTTA

DNA stores information for the sequence of amino acids in polypeptides and also acts as the hereditary substance, passing along this stored information from one generation to the next. Cells utilize RNA in several ways: As a messenger molecule carrying information away from DNA molecules, as structural molecules, or as transfer molecules in protein synthesis.

For a nice review on macromolecules click here.

Session 2 (August, 21st, 2009) Lipids

Lipids

They are a general group of non water soluble organic molecules that can be dissolved in non polar organic solvents, such as chloroform, ether and benzene. Tipically, lipids are energy storage molecules in the form of fatty acids and fat and when structural, as phospholipids, glucolipids and wax.

They are constructed from the simplest possible monomer. A carbon atom is covalently bonded to two hydrogen atoms forming a -CH₂ – unit. When several of these monomers are linked together, a polymer called hydrocarbon is created. Hydrocarbons have two very important properties: 1) A lot of energy is stored in the C-C and C-H covalent bonds of the molecule and 2) They are very hydrophobic and will not dissolve in water to any great extent.

Fatty Acids.- They are not usually found in the cells as free forms. They are composed of a long chain of an even number of C atoms, between 14 and 22. They differ from each other on the number of carbons and in the fact that some of them could have double bonds in different positions in the chain. A fatty acid such as stearic acid, which has no double bonds it is said to be saturated. This is, all the posibilities of more bonds are already used. In other words, all the carbons in the chain present four bonds. In the other hand oleic acid which presents double bonds between carbon atoms in its chain, is said to be unsaturated, because its carbon atoms have the potential of bonding with more atoms.

Fat.- A fat molecule is formed by three molecules of a fatty acid linked to a glycerol molecule. Glycerol is an alcohol with three carbons with three hydroxyl groups (-OH). A fatty acid is a long hydrocarbon chain that finishes with an carboxyl group (-COOH). This chain is non polar and hydrophobic while the carboxyl group gives the molecule and acid property. As with polysaccharides, each bond between glycerol and the fatty acid is formed by the elimination of water (condensation, dehydration). These molecules are said to be neutral lipids because they do not have net positive or negative charges.

Phospholipids and Glucolipids.- As in fats, phospholipids and glucolipids are formed of fatty acid chains linked to a gylcerol molecule.

Phospholipids (phosphate lipids).- In phospholipids the third carbon atom of the glycerol is linked to a phosphate group as opposed to a fatty acid. Phosphate groups are highly hydrophilic since phospate is negatively charged while the fatty acid portion is highly hydrophobic. This kind of molecules are called amphipatic molecules. The relevance of this is that phospholipids are the main component of cell membranes.

Glucolipids (sugar lipids).- In glucolipids the third carbon atom of the glycerol is linked to a short carbohydrate chain. Depending on the particular glucolipid, this chain may contain in any position between one to fifteen monomers. The carbohydrate “head” is highly hydrophilic and the fatty acid “tails” are hydrophobic. In a water solution, glucolipids behave in the same way as phospholipids so they are also important components of cell membranes.

Waxes.- They are formed by joining together a single fatty acid molecule to another hydrocarbon molecule that has as part of its structure a hydroxyl (-OH) group at one end. The wax macromolecule, therefore, has two hydrophobic ends and is insoluble in water, which makes it an excellent waterproof material and constitutes its main role in living systems as coat that cover surfaces such as insect exoskeleton, or wax that covers feathers, skin etc.

Cholesterol and steroids.- Although not strictly polymers, steroids are usually classified along with lipids because of their strong hydrophobic character. All steroids are composed of the same basic four rings of carbon atoms. One steroid, cholesterol, acts in ways not fully undestood to change the properties of cell membranes and to regulate membrane fluidity. Other steroids, called hormones, act as messengers. They are released in tiny amounts by specialized glands and tissues and circulate in the fluids of multicellular organisms. Eventually, specific target cells recognize hormones, which become attached to or taken up by these cells and trigger changes in their metabolim or developmental fate. Sex hormones, for example, control the function of both male and female reproductive organs and ensure the correct formation of sex cells.