(a) Membrane system - site of light reactions (photosynthesis) - chlorpophyll pigments - enzymes - electron carriers - flattened, fluid-filled sacs (called thylakoids which are stacked to form grana) - lamellae between grana (b) Stroma - site of dark reactions in photosynthesis - photosynthetic gel containing enzymes associated with calvin cycle and sugars and organic acids (c) Starch grains - excess sugars Golgi apparatus (a.k.a. Golgi body) - series of complex tubules - flattened cisternae - present in cells which actively produce enzymes - modify and package proteins produced by ribosomes - e.g. carbohydrates are added to proteins in the golgi body to become glycoproteins, which are released into the cytoplasm and are fused with the surface membrane to be released outside. - the golgi body is also involved with the transportation of lipids within the cells and the formation of lysosomes (which contain digestive enzymes) - lysosomes fuse with and digest unwanted substances in the cells or old worn-out organelles released from golgi vesicle cisternae cytasol (between cysternae) starts from - protein molecules are transferred between cisterneae - secretory vesicles pass through the cytosol and fuse with adjacent cisterna Endoplasmic Reticulum - membrane-bound flattened sacs or tubules cisternae - form a network continuous with outer membrane of nuclear envelope - same structure as other membranes in cell - increases the surface area within the cell o sites of metabolic activity o channels for movement of substances o separate different activities of the cell proceeding simultaneously Page 6 packages from the endoplasmic reticulum
2 types 1. Rough endoplasmic reticulum - the bulk of the endoplasmic reticulum - encrusted with ribosomes - isolates and transports proteins synthesised by the ribosomes - many of these proteins are not required by the cell, but are secreted by the cell (e.g. digestive enzymes, hormones etc.) - rough endoplasmic reticulum acts as a transport system for these products. 2. Smooth endoplasmic reticulum - not encrusted with ribosomes - involved in synthesis and transport of lipids and steroids ribosomal Ribosomes - composed of RNA (rrna) - each consists of 3 sub-units one slightly larger than the other ( cottage loaf shape) - assembles amino acids into proteins - operates in conjunction with mrna - bound to endoplasmic reticulum - often several ribosomes collectively (polysome) - N.B. not all ribosomes are associated with the endoplasmic reticulum. In growing cells or cells that are making proteins for internal use, large numbers of ribosomes are found free in the cytoplasm. Lysosomes - formed by golgi body - they are vesicles containing enzymes - carry out intracellular digestion - 0.2µ 0.5µ wide - enzymes include nuclease, protease, lipase, acid phosphotase (i.e. hydrolases) o Function: to release enzymes outside the cell (exocytosis) to fuse with a particle taken in by exocytosis and digest it internally (residual bits may be expelled by exocytosis) Autophagy: digest old worn-out parts of cell / organelle Centrioles - not seen in plant cells - act as anchors for spindle fibres when they pull chromosomes apart Microtubules - fine tubular organelles making up the cytoskeleton in the cytoplasm (i.e. maintain shape) - straight and hollow of various lengths - made of protein sub-units called tubulin - used to make centrioles and spindles for cell division Page 7
Studying Cells (a) Naked eye - only see life size - Examples: egg (e.g. chicken), amoeba. Not much else. 1 - humans have a resolving power of mm 10 (this means that humans are only able to distinguish between 1 points that are further than mm apart.) 10 (b) Light (optical) microscope - The quality of the picture depends on: 1. The quality of the eyepiece lens. 2. The quality of the objective lens. - As the magnification is increased : the field of view diminishes the depth of field increases the resolution decreases The maximum magnification that the eye can cope with is 1500 Resolution Magnification Resolving power the ability of an optical system to distinguish between two adjacent objects increases the apparent size of an object the degree of detail that can be seen with a microscope Maximum resolutions: Light microscope: 0.2 µ Electron microscope: 0.2 nm The resolving power is inversely proportional to the wavelength of the radiation used (i.e. the shorter the wavelength, the greater the resolution). (a) Binocular microscope - 3D external structures - reduced magnification ( 3) - greater depth of field - less detail Page 8
(b) Light microscope - preparation of specimens needed 1. Whole mounts* - only if small and thin 2. Sections - thin slices cut * A mount is a single celled organism (e.g. amoeba, bacteria, daphnia) As an organism gets bigger, sections are needed. If these are too thick, the light will not be able to pass through. B C BC: longitudinal section 3. Smears - tissues may be smeared across a slide (e.g. blood) slide 2 A D AD: transverse section (normally done from top) drop of blood slide 1 Place drop of blood on slide 1 Add leishman s stain This makes white blood cells turn blue Smear the drop of blood with slide 2 This is to ensure thin layers are present so it can be viewed. Summary 4. Squash - cells are broken up or macerated by physical or chemical means - cells are then squashed to separate them - examples which can be used are: root tip squash and locust testes Objects must be: (a) Transparent - light must be able to penetrate (b) Thin - a microtone may be used to cut thin slices Individual sections: with longitudinal ad transverse sections, a 3D picture can be built. Page 9
Stains: - most biological structures are transparent - the stain gives a contrast between different structures - the stain combines with certain chemicals in the structure - Iodine solution: Starch blue-black - Eosin solution: cytoplasm pink - Feulgens agent - Aceto-orcein agent DNA dark red / purple Advantages of a light microscope (a) (b) (c) (d) (e) (f) relatively cheap to buy and operate small and portable preparation of material relatively quick and easy little experience needed unaffected by magnetic fields materials are rarely distorted by preparation living material may be viewed. Disadvantages of light microscope (a) limited magnification (maximum 1500) (b) limited resolution (200nm / 0.2 µ) Unit 1.3 Cellular Organisation Prokaryotic cells SYLLABUS CHECKLIST - describe the structure of a bacterial cell and its inclusions as illustrated by Escherichia coli; - understand the roles of the cell wall, cell surface (plasma) membrane and its invaginations, flagella, bacterial chromosomes, - plasmids, glycogen granules and lipid droplets; recognise and identify structures in electron micrographs of bacterial cells. Eukaryotic cells - understand the organisation of eukaryotic cells as illustrated by a leaf palisade cell and a liver cell; recognise and identify the structure of these cells as revealed by light and electron microscopy; - understand the magnification and resolution that can be achieved - using light and electron microscopy; interpret electronmicrographs and identify the organelles; - describe the structure and understand the roles of the nucleus, nucleolus, rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, chloroplasts, mitochondria, ribosomes, centrioles and microtubules, the cellulose cell wall; - describe the structure and understand the properties and roles of the cell surface (plasma) membrane. Page 10