Cell Study Guide - Biology 101 Free Biology Study Guides

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The Cell

The cell is possibly the most important concept in biology since it is the basic unit of life. Every living organism is made up of cells (or just one in the case of bacteria). Your body is composed of microsopic cells that are only visible if viewed under a microsocope. All the animals, trees and plants are made up of cells that share many similar characteristics. They all have cell membranes separating them from the outside environment, DNA to store information, RNA to pass this information to the ribosome- and ultimately protein that is translated from the RNA (mRNA to be specific). These common features make it possible to learn clues about how our own cells function by studying the cells of other model organisms like mice, fruit flies or nematodes. Most organisms that we are used to seeing are multicellular and have different types of cells that are suited for a particular function. Bacteria are composed of a single cell but also share the same characteristics just described. There are a few important differences though that will be explained below.

Organelles of the Cell

Eukaryotic cells (all organisms except bacteria and archaea (prokaryotes)) have complex organelles which are surrouned by their own membrane (similar to the cell membrane).

Nucleus - the nucleus is where DNA is stored and where RNA transcription occurs
Mitochondria - often refered to as the "powerhouse" of the cell, this is the organelle that generates ATP (the energy currency of the cell)
Endoplasmic reticulum - the system of membranes used for the folding and transport of proteins. Rough ER (endoplasmic reticulum) is covered by ribosomes while Smooth ER is not. Ribosomes are structures made of protein and rRNA (ribosomal RNA) where proteins are made (translated).
Golgi apparatus - used for modifying and packaging of proteins
Chloroplast - in plants this organelle is responsible for the reactions of photosynthesis

Cell Parts

There are other important components of the cell that are not considered organelles since they are not surrounded by their own lipid bilayer.
Ribosomes - the ribosome is a large complex made of RNA and protein that translates mRNA into protein.
Lysosomes - where the breakdown of nutrients can occur using enzymes
Cell Membrane - this is the structure composed of a lipid bilayer that separates the cell from the outside environment
Cell Wall - found only in plant and bacteria this structure is found outside the cell membrane and serves as a more rigid protective barrier

Differences Between Eukaryotes and Prokaryotes

Bacteria (and archaea which are seldomly mentioned) are prokaryotes. The term prokaryotes is derived from pro (before) and karya (nucleus): before-nucleus = pro-karya = prokaryote. This is because it is thought that bacteria are still very similar to their primitive ancestors which did not have a nucleus. So- bacteria (prokaryotes) do not have a nucleus, while all eukaryotic cells do have a nucleus (this is a popular question for exams- and a common mistake). Bacteria also lack all other membrane bound organelles.

Bacteria do not have:
organelles
nucleus
mitochondria
endoplasmic reticulum
golgi appartus
chloroplast
lysosomes

Bacteria do have:
ribosomes
cell membrane
cell wall (which eukaryotes don't have)
Its also important to remember that while bacteria don't have a nucleus they definitely still have DNA organized in a chromosome that is transcribed to mRNA and then translated to protein on a ribosome.

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