Introduction to Cell Biology

Biology hopes to describe the structure, function, and origins of living things. It is such a wide area that covers many subject area and disciplines and sometimes crosses over to other disciplines. The biology on the MCAT is mainly focused on life sciences.

The biology section can be divided into cell and molecular biology and systems (human biology) biology. We are hoping to build a bottom up approach, by describing simple things first and then adding denser layers as we progress. Lastly, we hope to connect how simple things we talked about earlier influence both our structure and our function.

We begin our review of biology by talking about the simplest unit of life: the cell.

Although each cell might provide different functionality they all have one thing in common:

1. Each cell is small so that it has a large surface area to volume ratio which allows it perform very complex tasks in structured environments.

Cells are typically divided into two types: Prokaryotic and Eukaryotic Cells

We will now further describe the structure and organelles that are found in a typical eukaryotic animal cell. Different cell types might not contain all the below organelles. There are roughly two types types of eukaryotic cells: Plant Cells and Animal cells.

The following three structures are only found in plants along with the other organelles listed above:

Animal cells contain two additional structures that aid in movement: cilia and flagella. They are both made form microtublules. Cilia are hairlike structures found in the respiratory tract. Sperm contain a structure called a flagellum which helps to aid movement.

Certain cells need to be able to contact other nearby cells. Cellular junctions adhere membranes of adjacent cells.

There are three different types of cellular junctions:

1. Gap junctions- Connect the cytoplasm of adjacent cells, which allows for the movement of molecules between the cells. Gap junctions are found in the heart muscle. Gap junctions allow for rapid spread of information between cells.
2. Tight Junctions- Provide waterproof seals between adjacent cells to prevent leakage of fluid. These are found in inner stomach lining.
3. Adhering Junctions- Are the types of junctions found in epithelial cells which allow for stretching.

The outer boundary of cell is called a plasma membrane. It functions to protect the contents of the cell by only allowing select molecules contact with the outside environment.

The plasma membrane is unique in its structure which gives it his dual properties. The plasma membrane is composed of phospholipids.

Phospolipids belong to a class of molecules that are amphiphilic. They contain distinct hydrophillic and hydrophobic regions. The hydrophilic region is referred to as the polar head group while the hydrophobic region is referred to as phospholipid tail. The tails are made from fatty acids which may or may not be saturated. Phospholipids in solution self-assemble into a bilayer. This bilyar structure is unique such that all the hydrophobic lipid tails point in to each and hence are shielded away from water. This is sometimes referred to as the sandwich model.

From this picture you can see that are some additional molecules found in the lipid bilayer. Some of these additional molecules are: cholesterol, glycolipids, proteins (integral and peripheral), various filaments.

The presence of these structures led to what is known as the fluid-mosaic model. The fluid-mosaic model conveys the idea that lipid bilayer is ever changing, and these additional molecules are present so that to influence the structural characteristics of the ever-changing bilayer. Cholesterol serves to regulate the fluidity of the membrane. Most animals are able to change the amount of cholesterol to varying outside conditions. More cholesterol make the membrane more rigid, which might be needed in very cold environments where you want to prevent the membrane from freezing.

Integral proteins are proteins that are found only within the hydrophobic bilayer. Peripheral proteins are proteins that are found on the outside, which allows it to contact the outside the world. Integral proteins usually do not provide any complex tasks other than providing structural support. Glycolipids and proteins are use to identify the cell. Each cell can be differentiated from other cell by the presence of these glyclolipids and proteins. They serve as 'locks' which will only let something in when it find the appropriate 'key' in this case a certain signal molecule.

Although the lipid bilayer prevents many molecules from entering the cell. Nature has designed certain mechanisms which allow for the entry of molecules. That are three unique mechanism:

1. Passive Transport- Do not require energy in the form of ATP.
2. Active Transport- Requires energy in the form of ATP
3. Bulk Transport- Transport of very large molecules (endocytosis, exocytosis, etc)

Passive Transport describes spontaneous processes. All mechanism that involve the movement of molecules in a spontaneous process involves the movement of molecules from area of high concentration to areas of low concentration. So all molecules involved in passive transport are less concentrated within in the cell and hence are able to move freely without requiring energy. Two such important spontaneous processes are diffusion and osmosis.

Diffusion involves the movement of molecules down a concentration gradient. Simple diffusion describes the movement of hydrophobic molecules such as oxygen or carbon dioxide which can simply transverse the bilayer. Facilitated diffusion describes movement of mostly polar molecules that require a carrier protein to aid in its movement. Due to the presence of the carrier protein, the rate at which the molecule enters the cell is carefully controlled by the saturation rate of the carrier protein.

Osmosis- describes the movement of water. Movement of water can be described in three cases:

1. isotonic solution- The concentration of solute inside and outside the cell is the same and hence there is no movement of water.
2. hypotonic solution- The concentration of solute in the outside enviroment is smaller than the concentration within the cell, as a result water moves into the cell and the cell potentially burst
3. hypertonic solution- The concentration of solute in the outside environment is more than the concentration within the cell. As result, water leaves the cell and thus the cell shrivels.

Active Transport- Active transport uses energy to move molecules against their concentration gradient. An example of active transport is the Na-K pump, which uses molecules of ATP to move sodium outside of the cell and potassium into the cell (against their respective concentration gradients).

Endocytosis- Is a type of bulk transport that brings material into the cell It uses a mechanism where the membrane come together to form a vesicle which then enters the cell. Pinocytosis describes ingestion of liquids by cells, while phagocytosis describes ingestion of anything that is not a liquid.

Exoctyosis- Refers to the forming of vesicle, which pinch of from the membrane and leave the cell. Exocytosis occurs all the time especially in the release of neurotransmitters at axon terminal which allow for propagation of information.