Chapter 4: Cell Structure and Function

Cells are the basic organizational component of living things. Nothing that is simpler than a cell can be considered to be alive.

I. Cell Theory:

1. Every living thing is composed of one or more cells.

2. The cell is the smallest/simplest unit of life.

3. Under current conditions, all cells come from preexisting cells.

Life on earth can be broken into two major divisions, based on cell structure. Eukaryotic cells are characterized by the presence of membrane-bound organelles and a defined nucleus. In contrast, prokaryotic cells are less organized, with an outer membrane but no membrane-bound organelles. Prokaryotic cells do contain DNA and ribosomes, but no membrane-bound organelles.

II. Eukaryotic Cells:

When we look at eukaryotic cells, we study the organelles. Analogous to organs in a human body, each organelle has a defined job within a eukaryotic cell.

Plasma Membrane: The membrane is made of two layers, known as a lipid bilayer. Each component making up the bilayer looks like a balloon with two tails. The tails are fatty acid chains (hydrophobic) and the heads contain phosphate groups (hydrophilic). This unique combination in phospholipids makes the molecule orient itself with the phosphates out towards the aqueous cytoplasm. The plasma membrane also has proteins embedded within it that allow charged and/or large molecules into and out of the cell. The basic function of the plasma membrane is to keep the contents within the membrane separate from the contents outside of the membrane.

Nucleus: The nucleus is surrounded by two lipid bilayers, forming the nuclear envelope. The nuclear envelope has numerous pores, allowing certain items to leave the nucleus. The pores are not large enough to allow DNA to leave the nucleus. Also, the nucleus holds the nucleolus, which is responsible for making the components of the ribosomes.

Endoplasmic Reticulum: Rough endoplasmic reticulum is so-named due to the presence of ribosomes on the surface of the membrane. The ribosome is responsible for assembling amino acids into proteins. When a protein is complete, it is often released into the ER for processing and/or ‘shipping’ to another location. Smooth ER is involved with the modification of lipids rather than proteins.

Similar to ER, the Golgi Bodies(GB) are hollow tubes that are involved with processing of lipids and proteins. Golgi bodies produce vesicles for shipment to specific locations within a cell. GB release vesicles, which are membrane-bound sacs. The membrane surrounding the materials within a vesicle protects the material from being digested or otherwise reacting with the cytoplasm of the cell. Two of these vesicles are of special importance: Lysosomes are loaded with enzymes and act to digest cell items, even entire cells. Peroxisomes are vesicles that break down fats and proteins.

Mitochondrion: The cell generally uses energy in the form of ATP. The mitochondrion is an organelle with a double membrane that functions to convert stored energy such as sugar into useable energy, ATP. Interestingly, the mitochondrion contains its own distinct DNA.

Chloroplast: Chloroplasts act as solar panels, collecting sunlight (energy) and converting it into stored organic molecules, such as glucose. They are often large and easy to see within a cell due to the fact they have a lot of surface area (useful in collecting light). The outer membrane hold pigments (such as chlorophyll) that collect light and the conversion to stored energy occurs within the inner membrane. Always remember: energy is neither created nor destroyed so the chloroplast doesn’t make energy! It converts light energy into stored, chemical energy!

Cytoskeleton: Know the two main components of the cytoskeleton, the microtubules (MT) and microfilaments (MF). The MTs are hollow tubes make out of proteins. The monomer of the tubes is a protein called tubulin. MTs can grow or contract by adding/subtracting tubulin monomers (amino acids). MTs are very important in cell division and also play a role in providing for cellular shape and movement. Microfilaments are thin ‘ropes’ made by twisting two polypeptide chains of actin together. MFs are very important in managing cell shape and cell movement.

These two components of the cytoskeleton are the functional components of muscle. Muscles contract when these fibers slide past one another. When we mention muscles being made largely of protein, we are speaking primarily about the cytoskeleton.

III. Cell Movement

There are three general means by which cells can move:

1) Cytoskeleton components can elongate/shorten due to addition/subtraction of monomers.

2) The coordinated beating/movement of cytoskeletal components.

3) Cytoskeletal components moving organelles around the cell.

The coordinated movement of flagellum and cilium are important cases of cell movement. Both are arranged in a "9+2" arrangement, with 9 microtubules arranged around 2 central MTs. The arrangement is anchored at a centriole, which is a protein body responsible for the production of MTs. Generally, if there are few bundles, it is a flagellum (e.g. sperm). If there are many moving in unison, it is a case of cilium.

 

IV. Cell Junctions

In the living world, it is very common for cells to surround themselves with a cell wall. In plants, this wall included cellulose (along with other macromolecules). Cellulose allows water to pass freely and it provides the cell with a solid, rigid structure. The cell will also export waxes and other materials to make the surface more water repellent.

In animal cells, a matrix is deposited around cells. In bone cells, calcium salts and the protein collagen are commonly deposited around the living bone cells. The disease Osteogenesis imperfecta (Mr. Glass in Unbreakable) is a result of cells not properly assembling the polypeptide chain in the bone matrix.

Often times, it is necessary to cells to communicate or share resources with neighboring cells. For example, the vascular tissue of plants needs to be porous enough to allow water and nutrients to flow along its length, so all cells are interconnected. In animals, we see three types of cell junctions:

a) Tight junction – the lining of the stomach and skin cells (epithelial cells) are examples of tight junctions, where cell connections don’t allow materials to flow past a surface. Proteins extend from the cell membranes of the neighboring cells, acting to bring the two cells close together.

b) Adhering junctions – muscles cells are connected but also must be able to flex and stretch. They are connected with adhering junctions.

c) Gap junctions – the nerve cells of your brain/nervous system have small gaps between connecting cells. Neurotransmitters jump this gap when an impulse flows down a neuron.

V. Prokaryotic Cells

One of the primary divisions of life forms on earth is between prokaryotic (the oldest forms of life on the planet) and eukaryotic cells. Prokaryotes lack a true nucleus and other membrane-bound organelles. This does not mean that prokaryotes have no DNA or lack the means to carry on basic cell functions. Bacteria (prokaryotes) have a single strand of DNA; it just isn’t contained and protected within a single nucleus. The DNA molecule is circular and called a nucleoid. Also, the cells are very small and are not able to simultaneously conduct many cellular functions, which eukaryotes are able to do because the functions occur within specialized organelles.