Cell Molecular Biology Comparison of Features Ultrastructure of

Cell & Molecular Biology Comparison of Features & Ultrastructure of Prokaryotic and Eukaryotic Cells

Learning Intentions • • Microscopy Classification of Cells Ultrastructure of eukaryotes and prokaryotes. Comparison of features & ultrastructure

Classification • Living organisms are grouped into 3 domains; – Bacteria – Archaea (thermophiles, chemophiles) – Eukaryotes • Both Bacteria and Archaea are structurally defined as Prokaryotes. • Eukaryotes are split into 4 kingdoms; – Plant – Animal – Fungi – Protists (eg. algae and paramecium)

Structure of a typical Eukaryotic animal cell

Structure of a typical Eukaryotic plant cell

Common features of all Eukaryotic cells a) Cell membrane – controls entry and exit of molecules b) Cytosol – Fluid part of cytoplasm in which many reactions take place c) Ribosomes – site of protein synthesis. 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 Microbodies (peroxisomes )– breakdown of hydrogen peroxide Cytoskeleton – System of fibres inside the cell which add strength, allow cell movement and change of shape

Features Unique to Animal Eukaryotes Certain features are only found in animal cells e. g. a) Intermediate filaments – part of the cytoskeleton, are found under the cell membrane and add mechanical strength. b) Centrioles – organise the spindle fibres during cell division c) Microvilli – the lack of a cell wall allows the animal cell to form folds called micro-villi which further increase the surface area in specialised cells e. g. epithelial cells of gut.

Features Unique to plant eukaryotes Middle Lamellae - This is the layer between two adjacent cells Plasmodesmata These are cytoplasmic connections between plants allowing movement of substances. Cellulose Cell Wall – Flexible, strong adds mechanical strength, prevents cell from bursting Chloroplasts Synthesis of carbohydrates using light energy Large permanent vacuole - Contains cell sap

Additional features common to all Eukaryotes • Microbodies (peroxisomes )– breakdown of hydrogen peroxide • Cytoskeleton – System of fibres inside the cell which add strength, allow cell movement and change of shape

Starter questions 1. In eukaryotes which organelles have a double membrane? 2. How does the structure of the internal membranes allow for increased metabolic rate? 3. What role do microbodies and lysosomes play? 4. Convert 0. 2 mm into µm.

Prokaryotic Cell Structure Plasmids

Nucleoid – This is a region of highly condensed DNA in the form of a single circular molecule. (bacterial chromosome) Plasmid – smaller rings of DNA with limited genetic information Cytosol (cytoplasm) gel substance majority of the cell content and is the site of bacterial metabolism Ribosomes – suspended in the cytosol, are smaller and less complex than Eukaryotic ribosomes Cell wall – Tough outer coat made of peptidoglycan (linked disaccharide and peptide) and is used as the basis of the gram stain Capsule – some have a mucilaginous coating known as the capsule. This protects the cell against chemicals such as acids (slime layer) Pili (fimbriae)– These can be used to attach to surfaces or in transferring plasmids. Flagella – longer structure than pili, involved in motility.

Prokaryote Cell Wall and the Gram Stain

Comparison of Prokaryotic and Eukaryotic Cells 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.

Comparison of Prokaryotic and Eukaryotic Cells Ribosomes Both cell types have many ribosomes. 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 r. RNA) and a specific collection of different proteins. A eukaryotic ribosome is composed of five kinds of r. RNA and about eighty kinds of proteins. Prokaryotic ribosomes are composed of only three kinds of r. RNA 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.

Feature Size Genetic material Prokaryote small, mean diameter Up to 40 m common 0. 5 - 5 m Circular DNA found in Linear DNA associated with proteins region of nucleoid, (histones) to form a chromosome, found may contain within a nucleus additional plasmids Few present and Organelles none surrounded by a plasma membrane Cell Walls Eukaryote Rigid structure composed of peptidoglycan 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, microbodies Fungi: Rigid, formed from polysaccharide chitin Plants: Rigid, formed from polysaccharide cellulose

Essay due Friday 23/08 • Compare prokaryotic and eukaryotic cells under the following headings. i. ii. Organisation of genetic material Ultrastructure and other features 5 10