Cellular Biology

Basic Cellular Anatomy and Physiology



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The Cell

The cell is a basic building block for all living things, without it, life would not exist. Bacteria and the parasite that causes malaria consist of single cells, while plants and animals are made up of trillions of cells. Most cells are spherical or cube in shape but some are a range of different shapes. When you look in a light microscope you'll see only a few simple structures, that being the plasma membrane, cytoplasm, and the nucleus. However, when looking in an election microscope it's easy to see many defined shapes and structures.

The Plasma Membrane

The thin plasma membrane surrounds the cell, separating its contents from the surroundings and controlling what enters and leaves the cell. The plasma membrane is composed of two main molecules, fats (in fact phospholipids) and proteins. The fats are arranged in a double later with the large protein molecules dotted about the membrane, as seen in figure 1-1. Some of the protein molecules form tiny channels in the membrane while others help transport substances from one side of the membrane to the other.

The Cell Wall

The cell wall is a fairly rigid layer surrounding a cell composed of cellulose (fiber), located external to the cell membrane, which provides structural support, protection and filtering mechanism. The cell wall also prevents over-expansion when water enters the cell. They are only found in prokaryotes, bacteria, archaea, fungi, and algae. Eukaryotes and most protests do not have cell walls.

The Cytoplasm

Within the plasma membrane is the cytoplasm. It consists of a clear jelly-like fluid called the a) cytosol or intracellular fluid in which b) cell inclusions, c) organelles and d) microfilaments and microtubules are found.

a) Cytosol The cytosol consists mainly of water in while various molecules are dissolved or suspended. These molecules include proteins, fats and carbohydrates as well as sodium, potassium, calcium, and chloride ions. Many of the reactions that take place in the cell occur in the cytosol.

b) Cell Inclusions These are large particles of pat, glycogen and melanin that have been produced by the cell. They are often large enough to be seen with a light microscope. For example the cells of adipose tissue (as in the insulating fat later under the skin) contain fat that takes up most of the cell.

c) Organelles Organelles are the "little organs" of the cell like the heart, kidney, and liver are the organs of the body. They are structures with characteristic appearances and specific "jobs" in the cell. Most can not be seen with a light microscope and so it was only when the electron microscope was developed that they were discovered. The main organelles in the cell are robosomes, the endoplasmic reticulua, mitochondrion, Golgi complex (apparatus) and lysosomes.

Mitochondrion

Mitochondrion, (plural is mitochondria) are specialized organelles of all eukaryotic cells that use oxygen. They are small, spherical to rod-shaped, often called the powerhouse of the cell; mitochondria are responsible for energy generation by the process of oxidative phosphorylation. In this process, electrons produced during the oxidation of simple organic compounds are passed along a chain of four membrane-bound enzymes, finally reacting with and reducing molecular oxygen in the water. The movement of the electrons releases energy that is used to build a gradient of protons across the membrane which the electrons transport chain is situated. These protons flow back through as adenosine triphosphate (APT), a membrane bound enzyme, which is a high energy molecule.

Endoplasmic Reticulua

The endoplasmic reticulua (singular is endoplasmic reticulum, also referred to as ER), is a membrane system within the cytoplasm of a eukaryotic cell, and is important in the development of proteins and lipids. The ER usually makes up more then half of the membrane of the cell and is continuous with the outer membrane of the nuclear envelope. There are two distinct regions of the ER, which include the rough ER (RER) and the smooth ER (SER). The RER is studded with protein-synthesizing ribosomes, while the SER is not associated with ribosomes and is involved in the development of lipids and the detoxification of some toxic chemicals.





Golgi Apparatus

Also called the Golgi body, or Golgi complex, this organelle is a stack of flattened membrane-bound sacs and tubes. The Golgi apparatus is mainly devoted to processing the proteins and lipids synthesized in the endoplasmic reticulum. When it finishes processing the proteins and lipids it repackages them into vesicles for the transport to the plasma membrane, and within the cell as lysosome.

Lysosomes

A membrane-enclosed organelle, or vesicle found in all eukaryotic cells that is responsible for the cell's digestion of macromolecules, old cell parts, and microorganisms. Lysosomes contain a wide variety of enzymes that break down macromolecules such as nucleic acids, proteins, and polysaccharides. Many of the products of lysosomal digestion, including amino acids and nucleotides, are recycled back to the cell for use in synthesizing new cellular components.

Chloroplast

A complex, discrete green organelle contained in the cytoplasm of plant cells and eukaryotic algae that conduct photosynthesis. Chloroplasts aboard sunlight and use it in conjunction with water and carbon dioxide to produce sugar and oxygen.
Chloroplasts are responsible for the green color of almost all plants and are lacking only in plants that do not make their only food, such as fungi and non-green parasitic or saprophytic higher plants. The chloroplast is usually flattened and lens-shaped and consists of a body, in which are embedded from a few to as many as 50 submicroscopic bodies. The chloroplast contains chlorophyll pigments, as well as yellow and orange carotenoid.

Other plastids include chromoplast (for colored pigment storage), amyloplasts (for starch storage), elaioplasts (for fat storage) and proteinoplasts (for storing and modifying protein).

Vacuole

A membrane-bound compartment within some eukaryotic cells that can serve a variety of secretory, excretory, and storage functions. Vacuoles and their contents are considered to be distinct from the cytoplasm. Vacuoles are especially large in most plant cells. Vacuoles generally do the following:

a) Remove unwanted structural debris
b) Isolate materials that might be harmful or a threat to the cell
c) Contain waste products
d) Maintaining internal hydrostatic pressure or turgor within the cell (allow a plant to stand up and not wilt)
e) Maintaining an acidic internal pH
f) Containing small molecules
g) Exporting unwanted substances from the cell to another place
h) Enable the cell to change shape
i) Aid in the destruction of invading bacteria or misfolded proteins that have begun to build up

Ribosome

A small, dense, functional structure found in all known cells that assemble proteins, and are not membrane-bound. They contain approximately 60 percent RNA and 40% protein. Ribosomes are mainly found in attached to the endoplasmic reticulum and the nuclear envelope, as well as scattered throughout the cytoplasm depending on cell type.

Microtubule

Straight, hallow cylinders found throughout the cytoplasm of all eukaryotic cells (not found in prokaryotic cells). Microtubules form part of the cytoskeleton and give structure and shape to a cell and also work as a conveyer to move other organelles throughout the cytoplasm. In addition, microtubules are a major component in cilia and flagella, and participate in the formation of spindle fibers during cell division (mitosis).

Microfilament

Microfilaments are the thinnest filaments of the cytoskeleton found in the cytoplasm of all eukaryotic cells. These linear polymers of actin subunits are flexible and strong, and therefore help give the cell shape and support. They also propel cell crawling and ameboid movement.

Cilia

Cilia (singular cilium) are an organelle found in eukaryotic cells. Cilia are thin and tail-like, extending outwards from the cell body, off the plasma membrane. There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile cilia, which typically serve as sensory organelles.

Flagella

A flagellum (plural flagella) is a long, slender projection from the cell body, composed of microtubules and surrounded by the plasma membrane. In small, single-cell organisms they may function to propel the cell by beating in a whip-like motion; in larger animals they often serve to move fluids along mucous membranes such as the lining of the trachea.



Centriole

A barrel shaped microtubule structure that is found in most eukaryotic cells, though absent in higher plants and fungi. The walls of each centriole are usually composed of nine triplets of microtubules. Centrioles appear to help in cell division by producing spindle fibers which help organize chromosomes.



The Nucleus and its Compartments

The nucleus is a membrane enclosed organelle found in most eukaryotic cells, which contains the cells genetic material organized as multiple long linear DNA molecules.

Cytoskeleton

The cytoskeleton is made up of microtubules and other filaments. It provides the cell's shape, mechanical shape, locomotion, and helps with intercellular transport of organelles.

Nuclear Envelope and Pores

The nuclear envelope consists of two cellular membranes, an inner and outer membrane (inner membrane is attached to the RER and has ribosomes attached), arranged parallel to one another. The nuclear envelop completely encloses the nucleus and separates the cell's genetic material from the surrounding cytoplasm, serving as a barrier to prevent macromolecules from freely diffusing between the nucleoplasm and the cytoplasm.

Nuclear pores, which provide aqueous channels through the envelope, are composed of multiple proteins, collectively referred to as nucleporins. The size of the pores allow free passage of small water-soluble molecules while preventing larger molecules, such as nucleic acids and proteins, from inappropriately entering or exiting the nucleus.

Chromatin

Chromatin is a complex of DNA and protein that makes up chromosomes, which in eukaryotes is found inside the nuclei. In prokaryotes, the chromatin is held within the nucleoid. Chromatin is the template for the cell's transportation, replication and overall characteristics. During cell division the chromatin compact into chromosomes.

Nucleoplasm

Similar to the cytoplasm of a cell, the nucleus contains nucleoplasm or nuclear sac. The nucleoplasm is a highly viscous liquid that surrounds the chromosomes and nucleoli. Many substances such as nucleotides (necessary for purposes such as replication of DNA) and enzymes (which direct activities that take place in the nucleus) are dissolved in the nucleoplasm.

Nucleolus

The nucleolus is a sub-organelle of the cell nucleus which contains RNA and proteins, which has a main function of producing and assembling ribosome components such as ribosomal RNA.

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