Organisms may be divided into two main forms :prokaryotic and eukaryotic This is a typical prokaryotic bacterial cell and shows the following features all of which are mentioned in the monograph P2 Prokaryotic Cell Structure Plasmids

Nucleoid – This is highly condensed DNA in the form of a single circular molecule. It is often referred to as a bacterial chromosome Plasmid – these are smaller rings of DNA which contain genes Cytosol – This is the watery gel making up the majority of the cell content and is the site of bacterial metabolism Ribosomes – Bacteria have many ribosomes suspended in the cytosol. These as smaller and less complex than the ones found in Eukaryotic cells Cell wall – Tough outer coat made of peptidoglycan (linked dissacharide and peptide) and is used as the basis of the gram stain Capsule – The bacterial has a mucilaginous coating known as the capsule. This protects the cell against chemicals such as acids Pili – These can be used to help bacteria attach to surfaces or to help them transfer their plasmids. Flagella – Both aid bacterial movement

Nucleus and DNA Eukaryotic cells have a true nucleus, bound by a double membrane. Prokaryotic cells have no nucleus. Prokaryotic cells do have DNA and DNA functions. Biologists describe the central region of the cell as its "nucleoid" (-oid=similar or imitatingbut the nucleoid is essentially an imaginary "structure." There is no physical boundary enclosing the nucleoid. Eukaryotic DNA is linear; prokaryotic DNA is circular (it has no ends). Eukaryotic DNA is complexed with proteins called "histones," and is organized into chromosomes; prokaryotic DNA is "naked," meaning that it has no histones associated with it, and it is not formed into chromosomes. A eukaryotic cell contains a number of chromosomes; a prokaryotic cell contains only one circular DNA molecule and a varied assortment of much smaller circlets of DNA called "plasmids." The smaller, simpler prokaryotic cell requires far fewer genes to operate than the eukaryotic cell.

Ribosomes Both cell types have many, many ribosomes, but the ribosomes of the eukaryotic cells are larger and more complex than those of the prokaryotic cell. Ribosomes are made out of a special class of RNA molecules (ribosomal RNA, or rRNA) and a specific collection of different proteins. A eukaryotic ribosome is composed of five kinds of rRNA and about eighty kinds of proteins. Prokaryotic ribosomes are composed of only three kinds of rRNA and about fifty kinds of protein. Membrane bound organelles The cytoplasm of eukaryotic cells is filled with a large, complex collection of organelles, many of them enclosed in their own membranes; the prokaryotic cell contains no membrane-bound organelles which are independent of the plasma membrane. This is a very significant difference, and the source of the vast majority of the greater complexity of the eukaryotic cell. There is much more space within a eukaryotic cell than within a prokaryotic cell, and many of these structures, like the nucleus, increase the efficiency of functions by confining them within smaller spaces within the huge cell, or with communication and movement within the cell.

Structure of a typical Eukaryotic animal cell

Plant and animal cells eukoryotic cells have a nucleus and other membrane bound organelles. Common to both are :- a)Cell membrane – controls entry and exit of molecules b) Cystol – Fluid part of cytoplasm in which many reactions take place c) Ribosomes – site of protein synthesis. These are bound to rough ER d) Endomembrane system – various membrane bound organelles such as: Nucleus – Contains genetic information e.g. DNA Rough ER – Transport of proteins Golgi – packages protein for secretion Mitochondria – the sites of oxidative phosphorylation Lysosomes – sub cellular digestion using powerful protease enzymes Peroxisomes – breakdown of hydrogen peroxide Cytoskeleton – System of fibres inside the cell which add strength, allow cell movement and allow cell to change shape

Certain features are only found in animal cells e.g. a)Intermediate filaments – these are part of the cytoskeleton which is discussed in more detail in the next section. These filaments are found under the cell membrane and add mechanical strength. Plant cell don’t need them as they have a cell wall. b) Centrioles – These orginise the spindle fibres during cell division c) Microvili – the lack of a cell wall allows the animal cell to form fold called micro- villi which further increase the surface area in specialised cells e.g. epithelial cells of gut.

Plasmodesmata These are cytoplasmic connections between plants which all the movement of substances. Middle Lamellae - This is the layer between two adjacent cells Chloroplasts - Synthesis of carbohydrates using light energy Large permanent vacuole - Contains cell sap Cellulose Cell Wall – Flexible, strong and elastic wall which adds mechanical strength and prevent cell from bursting

Differences Between Prokaryote and Eukaryote Cells FeatureProkaryoteEukaryote Size small, mean diameter  m Up to 40  m common Genetic material Contained in nucleoid and plasmids DNA associated with proteins to form a chromosome, found within a nucleus Organelles Few present and none surrounded by a plasma membrane Many organelles; Some with double membranes e.g. nucleus, mitochondria and chloroplasts. Many with a single membrane e.g. golgi apparatus, endoplasmic reticulum and lysosomes Cell Walls Rigid formed from glycoproteins (mainly murein) Fungi: Rigid, formed from polysaccharide chitin Plants: Rigid, formed from polysaccharide cellulose