Saturday, February 26, 2011

Cell Parts

The Cell Theory:
The Cell Theory states that all living things are made up of cells and that all cells come from pre-existing cells. It also states that the cell is the basic unit of life - structures and functions.
The cell is a remarkable phenomenon that is the foundation of all life forms. This blog will examine the cell with particular focus on its internal structure.

Let's examine the basic structures that are common amongst both plant and animal cells:

Nucleus - Eukaryotic cells all contain a membrane enclosed structure called a nucleus. This organelle's membrane is perforated by nuclear pores and is continuous with the endoplasmic reticulum.
In terms of function, the nucleus serves as the control center for the cell; it is where the genetic information is stored. In the form of chromosomes, DNA strands are condensed around proteins. Further,the nucleus also contains nucleoli which are responsible for producing ribosomes, which will be discussed later in this blog.

Endoplasmic Reticulum - this organelle is an extensive network of membrane bound tubules and sacs. This membrane separates the inner contents of the endoplasmic reticulum (lumen) from the cytosol. There are two main types of endoplasmic reticulum which differ slightly interms of function and structure: rough and smooth endoplasmic reticulum.
Rough endoplasmic reticulum is covered with ribosomes. It aids in synthesizing proteins, adds carbohydrates to proteins to create glycoproteins and produces new membranes.
Smooth endoplasmic reticulum is responsible for synthesizing lipids, metabolizing carbohydrates, detoxification and calcium ion storage.

Ribosome - ribosomes are composed of two subunits made of ribosomal RNA and protein. They are found either bond to the surface of rough endoplasmic reticulum, or floating in the cytosol of the cell. These ribosomes are called 'free ribosomes'.
In terms of function, the ribosomes role is to synthesize proteins.
Golgi Apparatus - These are stacks of flattened membranous sacs which are polar (have cis and trans regions)
A cell's golgi apparatus is responsible for the modification of proteins, the carbohydrates added to proteins and phospholipids. It is also involved in the synthesis of phospholipids as well as sorting and releasing products in vesicles.

Vacuole - this organelle is a large, membrane bound vesicle.
It is involved in digestion, storage, waste disposal, water balance, cell growth and protection.
Peroxisome - This organelle is defined as a specialized metabolic compartment within the cell which is bounded by a single membrane.
It contains enzymes that can transfer hydrogen atoms to oxygen producing hydrogen peroxide which is then converted to water by yet another enzyme.
Mitochondrion - Known as the cell's powerhouse, the mitochondria is bonded by a double membrane. The inner membrane has a large surface area with many folds called cristae.
This organelle plays a key role in cellular respiration, which we will learn about in a future blog post.
In plant cells ONLY:
Cell wall - This outer layer of plant cells enable them to maintain their shape and serves as a method of protection. Further, it is made mostly of cellulose.
Chloroplast - this photosynthetic organelle can convert solar energy into potential chemical energy which plants and animals rely on to thrive. We will explore this process in a future blog.

In animal cells ONLY:
Lysosome - Lysosomes are known as the cell's 'clean up crew'. They are responsible for digestion. Macromolecules are hydrolyzed within them.
Centrosome - This is a region within the cell where the cells microtubules are initiated. It contains a pair of centrioles and plays a key role in the division of animal cells.
Flagella - This structure is only found in some animal cells. It allows for motility of certain cells. Sperm cells would be an example.



Saturday, February 19, 2011

Macromolecule 4: Nucleic Acids

Nucleic acids are polymers of nucleotides.Nucleotides are composed of a phosphate group, a sugar and a nitrogen base. There are 5 types of nitrogenous bases: adenine, cytosine, guanine, thymine, and uracil. Uracil is only found in RNA (a very important nucleic acid that allows genetic information to leave the nucleus and be used by other cellular organs-specifically ribosomes-to code for protein synthesis).
Deoxyribonucleic acid (DNA) is the nucleic acid found in the nucleus of all eukaryotic cells. It contains the code of life. In this nucleic acid, two anti-parallel strands of amino acids are connected to one another by hydrogen bonds. Adenine bonds to thymine with two hydrogen bonds while guanine and cytosine are connected by three hydrogen bonds. The structure is similar to that of a twisted ladder.

Macromolecule 3: Proteins


Proteins are essential in making each individual unique. Although all humans have the same number of chromosomes, and the same types of genes, it is small variations in protein shape/structure than make each of us look different. Enzymes, immunoglobuins, hemoglobin, keratin, and fibrin are all examples of proteins which play important roles in our bodily functions. Classified by function, there are seven classes of proteins: structural, contractile, storage, defense, transport, enzymes and some hormones.

Proteins are made of several polypeptide chains linked together. These polypeptide chains are composed of nucleic acids linked together by peptide bonds as can be seen in the image to the right.
There are 20 amino acids in humans, of these, 9 are essential amino acids that can not be made in the body, but rather must be obtained through one's diet. There are aromatic, polar and non polar nucleic acids. These characteristics are determined by the R-group of the nucleic acid.

There are 4 levels of structure of proteins:

Primary: The sequence of the amino acids.
Secondary: The shape of the polypeptide chain. (Either alpha helix or beta pleated sheets)
Tertiary: The folds within the chain (caused by interactions between the R-groups of the various amino acids) They can be either fibrous or globular.
Quaternary: The overall shape of the numerous polypeptide chains linked together to form the protein.

Macromolecule 2: Lipids

In contrast to carbohydrates which provide the body with the short term energy needed, lipids are often used as a long term energy source. All lipids are hydrophobic, and are composed of oxygen, carbon and hydrogen atoms. They contain fewer oxygen atoms relative to carbohydrates. There are four main types of lipids: fats, phospholipids, steroids and waxes. In this blog, I will focus primarily on the first three.

Fats
Glycerol is a type of fat containing a three carbon glycerol linked to three fatty acid chains. This linkage is done through the formation of ester bonds. These fatty acid chains can be either saturated or unsaturated based on the types of bonds present within the carbon chains: saturated fatty acids contain only single bonds within their carbon chains whereas unsaturated fatty acids contain double or even triple bonds within the structure of their chains. We find these types of fat in many of our foods. Unsaturated fats (such as those found in oils) are considered healthier as they are less likely to form clots in our blood vessels.

Phospholipids
Phospholipids are a key component of our cell membranes. Composed of a polar choline and phosphate group attached to a non polar group of glycerol and two fatty acid chains, phospholipids are ideal for this purpose. When dissolved in water, they form two layers in which the polar heads are exposed to the water, and the non polar tales mix in the middle. They can also take on a spherical formation. These spheres are called micelles. This is what creates the cell membrane. The polar heads allow the cell exist in water while the highly non polar layer prevents polar substances (such as water or ions from penetrating. There are, however hydrophilic pores which allow for these polar substances to pass through as required.

Steroids
Steroids are a type of lipid which play significant roles in the human body. They are composed of four fused hydrocarbon rings and several functional groups. In the cell membrane, cholesterol is an important steroid which keeps the membrane intact. In the body, it is also converted to vitamin D, bile salts and hormones.


Saturday, February 12, 2011

Macromolecule 1: Carbohydrates


Carbohydrates are one of the four major groups of macromolecules which are essential to life as we know it. Composed primarily of carbon, hydrogen and oxygen in a fixed 1-2-1 ratio, these molecular compounds are produced and consumed by living organisms in order to attain energy. In addition, they are also used as building materials and for means of identification. Their importance is therefore undeniable.

Sugars are the subunit of carbohydrates. They can be categorized based on the number of carbon atoms present within their structure: for example, a sugar with 6-carbons would be called a "hexose". However, there are also different types of hexose sugars which are isomers of each other. Two examples would include glucose and galactose. These are examples of simple sugars are called monosaccharides and can be identified by the carbonyl group they possess.

These simple sugars can be assembled (through condensation reactions) or disassembled (through hydrolysis). The covalent bonds that hold these monomer units together are called glycosidic linkages. When linked together, two monosaccharides can form disaccharides. Likewise, many monomer units of this kind form polysaccharides.

For example:

Glucose + Glucose = Maltose
Glucose + Fructose = Sucrose
Glucose + Galactose = Lactose

It is also important to consider the position of the hydroxyl group in these polymer chains. A chain of alpha glucoses form starch (a carbohydrate that can be digested by humans), whereas a chain of beta glucoses for cellulose (a carbohydrate found in plants that cannot digested by humans).