The Cell

The cell is possibly the most important concept in biology since it is the basic unit of life.

A model with labelled organelles of an animal cell

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.

The cell membrane, or plasma membrane separates the cell from exterior environment and is composed of a phospholipid bilayer. It is composed of phospholipids which each have a polar (hydrophillic) head and a polar (hydrophobic tail). The polarity of the phospholipids helps them self assemble into a structure where the hydrophobic tails all face inward away from the aqueous interior and exterior of the cell.

How phospholipids assemble to make the <a href=cell membrane" title="The Phospholipids of the Cell Membrane">

The cell membrane is embeded with proteins. Transmembrane proteins pass all the way through the membrane, while peripheral proteins only pass through one side of the bilayer. Transmembrane proteins are often are involved in the transport of compounds and nutritents across the lipid bilayer since only small hydrophobic molecules, water and gas can diffuse freely through the hydrophobic interior.

The Cell Membrane With Embeded Proteins

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).

Drawing of an animal cell with organelles labelled

Nucleus - the nucleus is where DNA is stored and where RNA transcription occurs. Each cell has one nucleus. Within the nucleus is a structure called the nucleolus which is the site of ribosome assembly.

The image below is an image of a cell with the nucleus stained blue and the multiple mitochondria stained red

Microscope image of a cell with the mitochondria, nucleus and microfilaments stained

Mitochondria - often refered to as the "powerhouse" of the cell, this is the organelle that generates ATP (the energy currency of the cell). Mitochondria have a highly folded inner membrane that provides surface area for the enzymatic reactions that produce ATP. The interior of the two membranes is called the matrix, the space in between the two membranes is called the intermembrane space and the folds created by the inner membrane are called cristae. Mitochondria also contain their own DNA which encodes some of the enzymes that are used inside the mitochondria.

The Mitochondria With Parts Labelled

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:
endoplasmic reticulum
golgi appartus

Bacteria do have:
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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