Unit 1 Cell and Molecular Biology Section 1 Cell Structure

Cells Organisms can be divided into two major groups, depending on the structure of their cells. 1. Prokaryotes – bacteria and archaea 2. Eukaryotes – Protists, fungi, animals and plants.

Prokaryotic Cell Structure 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 Plasmids

StructureDefinition NucleoidThis is highly condensed DNA in the form of a single circular molecule. It is often referred to as a bacterial chromosome PlasmidThese are smaller rings of DNA which contain genes CytosolThis is the watery gel making up the majority of the cell content and is the site of bacterial metabolism RibosomesBacteria have many ribosomes suspended in the cytosol Cell wallTough 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 Pili and Flagella – Both aid bacterial movement

Eukaryotic animal cell structure

Features common to all Eukaryotic cells 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

Features unique to animal Eukaryotes 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 organise 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.

Features unique to plant Eukaryotes 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