cell and membrane structure Flashcards
what does the cell theory state?
the cell theory states that the cell is a fundamental unit of structure, function and organisation in all living organisms, and that new cells are formed from existing cells
what are the main principles of cells?
- all living organisms are composed of one or more cells
- the cell is the most basic unit of structure in all organisms
- all cells arise only from pre-existing cells
what are the differences between a prokaryotic and a eukaryotic cell?
- prokaryotic cells are typically 0.2-2 nanometers in diameter, while eukaryotic cells are typically 10-100 nanometers in diameter
- prokaryotic cells have no nuclear membrane or nucleoli, while eukaryotic cells have a true nucleus, consisting of nuclear membrane and nucleoli
- prokaryotic cells do not contain membrane-enclosed organelles, while eukaryotic cells contain membrane-enclosed organelles
- prokaryotic cells contain a glycocalyx which is present as a capsule or slime layer, while eukaryotic cells contain a glycocalyx is present in some cells that lack a cell wall
- prokaryotic cells have a flagella that consist of two protein building blocks, while eukaryotic cells have a flagella that is complex and consists of multiple microtubules
- in prokaryotic cells, the plasma membrane contain no carbohydrates and generally lack sterols, while in eukaryotic cells, sterols and carbohydrates that serve as receptors are present
- prokaryotic cells have no cytoskeleton or cytoplasmic streaming, while eukaryotic cells have a cytoskeleton and have cytoplasmic streaming
- prokaryotic cells have a single circular chromosome, and lack histones, while eukaryotic cells have multiple linear chromosomes with histone arrangement
what are the five general functions that a cell is capable of performing?
- intake of raw materials
- extract useful energy, and synthesise its own molecules
- grow in an organised manner
- reproduce after its own kind
- respond and adapt to the external environment
what is the lower limit of cell size determined by?
it is determined by the minimum amount of space needed to contain the essential elements of its function (eg. DNA and enzyme molecules)
what is the upper limit of cell size determined by?
the upper limit of cell size is determined by the surface area of volume ratio needed for exchange of materials between the cell and its environment
why is cell size kept as small as possible?
as the size of the cell increases, the surface area to volume ratio decreases, and the number of chemical exchanges that can be performed with the extracellular environment would be inadequate to maintain the cell, because most of its cytoplasm is relatively far from the outer membrane. exchange with the extracellular environment is vital as substances like oxygen and nutrients can only enter the cell, and waste products can only leave in this fashion
how is a generalised eukaryotic cell organised?
- eukaryotic cells have a plasma membrane on their outer surface, and extensive and elaborately arranged internal membranes compartmentalise the cell
- the nucleus contains the genetic material of the organism, and the contents external to the nucleus are known as the cytoplasm
- the cytoplasm consists of organelles, which are compartments that carry out the various functions in the cell, as well as the cytosol, which is an aqueous matrix in which the organelles and nucleus are suspended
what are the three forms of cell representation?
- 2D or 3D diagrams
- 2D photomicrographs (taken using a LM or TEM)
- 3D photomicrographs (taken using a SEM)
what constitutes the cytoplasm of the cell?
the cytoplasm refers to all the organelles and cytosol within the cell surface membrane, with the exception of the nucleus.
what are the properties and constituents of the cytosol
the cytosol is an aqueous solute rich matrix that appears transparent and lacking structure under the LM. it contains about 90% water, and dissolved in it are
1. various essential ions and soluble organic molecules
2. soluble proteins
3. the cytoskeleton which is a network of fine strands of globular and fibrous proteins which provide infrastructure and support for the cell
what is the definition of a membranous organelle?
a membranous organelle is a compartmentalised space within the cytoplasm and are surrounded by membranes which are structurally and biochemically similar to the plasma membrane.
what are the advantages of having membranous organelles?
- the presence of membranes surrounding the organelles allows for the maintenance of characteristic differences between the contents of each organelle and the cytosol
- the compartmentalisation of specific reactions provide different local environments for which incompatible processes can occur simultaneously
- internal membranes allow for the embedding of enzymes and proteins that mediate many cellular reactions. the greater the membrane surface area, the greater the number of enzyme complexes that can be embedded, and thus increasing efficiency of many reactions by providing optimal enzyme concentration for reactions to occur.
what are the general characteristics of the nucleus?
the nucleus is the largest organelle in the animal cell, and is easily seen with the light microscope. it is usually spherical or oval, with a size of between 5-20 micrometers
what is the general function of the nucleus?
the nucleus encloses genetic material and protects DNA from the metabolically active cytoplasm. the double membrane is perforated with pores to enable exchange of substances between the nucleus and cytoplasm
what is the structure of the nuclear membrane?
- each of the two membranes is a lipid bilayer, with the outer membrane of the nucleus being continuous with the membrane of the endoplasmic reticulum
- the inner and outer membrane and continuous with each other and the region between them is known as the perinuclear space, which is continuous with the ER lumen
- the nuclear envelope is perforated by nuclear pores, made up of a large protein complex, which allows macromolecules to exit and enter the nucleus
what is the structure of the nucleoplasm
- nucleoplasm is an aqueous matrix within the nucleus containing proteins, metabolites, ions and chromatin
- chromatin is composed of coils of DNA wound around basic proteins known as histones
- chromatin can exist as loosely coiled euchromatin, or tightly coiled heterochromatin
what is the function of the nucleolus?
the nucleolus appears as a dense mass in the nucleus when viewe under the EM, and is composed of DNA carrying rRNA genes and proteins, which function to synthesise a specific type of RNA known as ribosomal RNA< which forms a component of ribosomes
what is the definition of the endomembrane system?
the endomembrane system is composed of a number of inter-related membrane sacs within the cytoplasm of the cell. these membranes are related either by direct physical continuity or by the transfer of membrane segments known as vesicles
what are the components of the endomembrane system?
- rough ER and smooth ER
- golgi apparatus
- lysosomes
- vacuoles
what is the general structure of the ER?
the ER consists of an extensive network of hollow, membranous tubules, sacs or sheets called cisternae. the internal space of the ER is known as the lumen, which is continuous with the perinuclear space. compared to the golgi apparatus, it has a flatter, more compact packing that is sheet-like
what are the specialised structures in ER that contribute to its function?
- the extensive network of cisternae increases membrane surface area for synthesis. this allows rER to be embedded with more ribosomes for synthesis of polypeptides, and for sER to be embedded with more enzymes so that steroids and phospholipids can be synthesised
- the hollow cisternae accommodate newly synthesised substances and allow for the packaging of contents into vesicles for transport to the golgi apparatus
what is the structure of the rER?
the rER has a sheet-like appearance which appear rough due to the presence of ribosomes that stud the cytosolic face of the rough ER
what is the function of the rER?
- the rER-bound ribosomes are sites of protein synthesis where a polypeptide chain is synthesised at the bound ribosome
- the polypeptide chain then enter the ER lumen, which is the site of protein folding, though a protein channel in the rER membrane where the polypeptide chain folds into its specific conformation
- these proteins are either destined for export, or are targeting to various cellular organelles
what is the structure of the sER?
the sER is a network of tubules which lack ribosomes, resulting in its smooth appearance
what are the functions of the sER?
- synthesise of lipids, including oils, phospholipids and steroids. as such, cells that are active in hormone secretion usually have abundant sER
- metabolism of carbohydrates
- detoxification of drugs and poisons
- storage of calcium ions for use in muscle contraction, as well as cell signalling
what is the structure of the golgi apparatus?
the golgi apparatus consists of a stack of flattened, membrane bound sacs called cisternae, which separate its internal space from the cytosol. each sack differs in thickness and molecular compositon
explain the distinct polarity that exists in the stack of golgi apparatus
- new cisternae are constantly being formed at the cis face by receiving transport vesicles from the ER. the membranes of transport vesicles from the ER fuses with the cis face membrane and deposit their contents into golgi cisternal space
- at the trans face, membranes bud off to form either secretory vesicles, containing materials to be transported to the extracellular matrix, or lysosomes
- between the golgi sacs, golgi vesicles are responsible for transferring materials between the parts of the golgi, and some vesicles also bud off from the trans face to transport substances to other organelles in the cell
what are the functions of the golgi apparatus?
- the golgi apparatus is the site of modification of ER products, including glycosylation which is the addition of sugar groups, and trimming which is the removal of excess monomers.
- different golgi cisternae contain different enzymes for modification, and hence ER products are progressively modified as they move through the stacks of the golgi complex
- the modified products are packaged into vesicles and then passed onto other components of the cell where they bud off the GA complex
what are the specialised structures in GA that contribute to its function?
- the abundant flattened cisternae provide increased surface area for vesicle reception and budding
- multiple cisternae also allow for different modification processes to occur simulataneously
what is the structure of lysosomes?
- the lysosome is a membranous organelle that appears homogenously electron-dense under the EM.
- it contains hydrolytic enzymes that can digest most biological macromolecules
why are lysosomes membranous organelles?
- due to the acidic nature of lysosome contents and hydrolytic activity of enclosed enzymes, lysosomal contents must be prevented from spilling into the cytoplasm under normal cell conditions
- thus, segregation of contents within the membrane provides optimal pH for hydrolytic reactions and protects cellular contents from hydrolysis
what are the three major function of the lysosome?
- digestion of materials taken into cells. food particles are engulfed by endocytosis to form food vacuoles with fuse with lysosomes to form endosomes. the enzymes then digest the endosome’s contents which later end up in the cytosol to be used as food for the cell. in the case of defence again bacteria, the result of fusion is a phagocytic vacuole
- autophagy of worn-out organelles occur when unwanted structures within the cell are enclosed by a membrane of unknown origin, forming a vesicle, which then fuses with the lysosome to form an autophagic vacuole
- lysosomes also undergo autolysis, where cells self-destruct in a controlled manner when programmed to through apoptosis when it senses that it has become a threat to its environment. for autolysis to occur, there must be a mass release of lysosomal contents in the whole cell.
where in the cell do vacuole membranes originate?
- ER
- GA
structure and function of vacuoles in animal cells?
in animal cells, vacuoles are small, mobile organelles that serve to house and transport substance. eg. food vacuoles and phagocytic vacuoles
what is the structure of the vacuole in plant cells?
in mature plant cells, there generally is a large central vacuole surrounded by a single membrane called the tonoplast. the solution within the tonoplast is known as the cell sap, and differs in composition from the cytoplasm
what are the functions of the vacuole in plant cells?
- storage of organic compounds like proteins and inorganic ions
- disposal site for toxic metabolic by-products
- contains pigments
- accumulating compounds that are toxic or unpalatable to consumers as a protective mechanism
- cell growth and elongation as water accumulates in the vacuole. plant cells can therefore increase in size with minimal investment in cytoplasm synthesis and without sacrificing surface area to volume ratio, as cytoplasmic contents are pushed to the periphery of the cell
what is the function of the mitochondria?
mitochondria are the sites of cellular respiration, which is the catabolic process that generates ATP by extracting energy from sugars, fats and other metabolic fuels in the presence of oxygen
what is the structure of mitochondria?
- mitochondria are usually elongated or spherical, ranging from between 1-10 micrometers
- they are enclosed in an envelope of two membranes, each with a unique collection of proteins and enzymes. the outer membrane is smooth, but the inner membrane is highly convoluted, with foldings known as cristae to increase the surface area for the attachment of various enzyme systems
- the narrow, fluid-filled space between the two membranes is known as the inter-membrane space
- the compartment enclosed by the inner membrane is known as the mitochondrial matrix, which houses enzymes, circular DNA, RNA and ribosomes so the mitochondria can synthesise its own proteins.
why is it necessary for the compartmentalisaion of the mitochondrial matrix to occur?
it is necessary for a proton gradient across the inner mitochondrial membrane to be set up. the generation of the proton gradient is due to the inner mitochondrial membrane being impermeable to protons
what is the function of chloroplasts?
chloroplasts are the sites of photosynthesis and are only present in photosynthetic cells. they convert solar energy to chemical energy by absorbing sunlight, and use it to drive the synthesis of organic compounds from carbon dioxide and water. they also synthesise ATP and NADPH which are used in the calvin cycle
what is the structure of chloroplasts?
- chloroplasts are lens-shaped and are about 5-10 micrometers in length
- each chloroplast is surrounded by a double membrane called the chloroplast envelope. the inner membrane encloses a semi-fluid compartment known as the stroma
- the chloroplasts have a third set of membranes within the stroma known as the thylakoids, which enclose an area known as the thylakoid lumen, which together form a flattened sac called the thylakoid disc, which are stacked up to form granum. connecting stacks of granum are sheet-like thylakoids known as intergranal lamellae
what is the function of the stroma?
- the stroma contains circular DNA that enables the synthesis of chloroplast proteins
- sugars synthesised by the chloroplasts are stored as starch grains in the stroma
- stroma also contains enzymes required for the calvin cycle
what is the function of thylakoids?
- thylakoids are the site for light-dependent reactions of photosynthesis
- thylakoids allow for increased surface area for attachment of chlorophyll and other photosynthetic pigments
why is it necessary for the compartmentalisaion of the thylakoid lumen to occur?
it is necessary for a proton gradient across the thylakoid membranes to be set up, as the thylakoid membrane is impermeable to protons
what is the function of ribosomes?
ribosomes serve as the site of protein synthesis in both eukaryotes and prokaryotes. they synthesise proteins that are either destined for export, inserted into the plasma membrane, or targeting to various membrane bound organelles
what is the structure of ribsomes?
- eukaryotic (80s) ribsomes are about 30 nanometers in diameter, while prokaryotic (70s) ribosomes are slightly smaller
- each ribosome consists of two subunits, the large ribosomal subunit and the small ribosomal subunit, which are made up of proteins and ribosomal RNA
- the ribosome can be found either attached to the rER, free in the cytosol, in the mitochondrial matrix, or in the chloroplast stroma
what is the definition of the cytoskeleton?
the cytoskeleton is an intricate 3D array of interconnected filaments and tubules. the cytoskeleton consists of microtubules, microfilaments, and intermediate filaments
what are the functions of the cytoskeleton?
- giving mechanical support to the cell and maintaining cell shape
- allowing for anchorage and directs the movements of organelles and molecules within the cell
- providing cell motility
what are the functions of microtubules?
specialised arrangements of microtubules arise from microtubule organising centers, which give anchorage and orientation to the microtubule assembly. they are also involved in transportation of molecules from one site in the cell to another
what are the functions of microfilaments?
microfilaments are involved in the movement of cells and changes in cell shape
what are the functions of intermediate filaments?
intermediate filaments may stabilise organelles, like the nucleus, or they may be involved in specialised cell junctions
where are centrioles located?
centrioles are found in animal cells, and are located near the nucleus, in a region known as the centrosome that contains specialised proteins required for microtubule assembly. they are found in pairs at right angles to each other, each member of the pair consisting of nine triplets of microtubules arrange in a ring
what are the functions of centrioles?
before cell division, each centriole replicates itself and moves to opposite poles of the cell. the centrosome acts as the MTOC for the formation of spindle fibres that play a role in nuclear division
what is the structure of the plant cell wall?
the plant cell wall is a relatively rigid and inflexible structure consisting mainly of the structural polysaccharide cellulose. it is secreted by the plant cell from within. the plant cell wall is freely permeable to all but very large molecules. the region between the cell walls of adjacent cells is separated by the middle lamella, which is rich in pectin that helps the cells adhere together
what are the functions of the plant cell wall?
- protecting the cells from mechanical injury and invasion
- enables the cell to withstand the hydrostatic pressure exerted by uptake of water by the cell, and enables plant cells to prevent excessive uptake of water
what are the features of tight junction between animal cells?
cell membranes are joined along ridges, and there is no intermembrane space
what are the features of anchoring junction between animal cells?
there are localised points of attachment between intermediate filaments, and there is an intermembrane space of 25-35 nanometers
what are the features of gap junction between animal cells?
transmembrane proteins with 1.5 nm pores called connexons in one membrane align with those in another to form channels between cells. there is an intermembrane space of 2-3 nm
what are the functions of tight junction between animal cells?
to allow spaces between membranes to seal
what are the functions of anchoring junction between animal cells?
to allow for cell adhesion
what are the functions of gap junction between animal cells?
to allow for exchange of ions and molecules between cells
how are junctions in plant cells formed?
the presence of a cell wall renders it impossible for close association of membranes of plant cells to occur. however, plant cells are perforated by numerous plasmodesmata, where water and small solutes can pass though these pores.
what are the differences between a light microscope and an electron microscope?
- working principles - LM involves passing visible light through specimens, whereas EM involves the use of an electron beam instead.
- types of specimen that can be viewed - LM are generally used to view sections of tissues and entire cells, and can be used on living or dead specimens. EM is generally used for sub-cellular studies and specimens are non-living
- magnification - EM have much greater magnifying power at X250000 in comparison to LM at X1500
- resolution - EM has much greater resolving power at 0.5nm than a LM at 200nm, as electron beams have a much shorter wavelength than light rays. resolution is inversely proportion to wavelength of radiation being used
- contrast - EM has much higher contrast, but only produces images in black and white, while LM can produce coloured images
- stains - EM uses stains with heavy metals to reflect electrons, while LM uses coloured dyes
how do you prepare a sample for light microscopy?
- fixation - preserve material in a life-like condition with minimum distortion
- dehydration - remove traces of water from the fixed material
- clearing - remove dehydrating alcohol so that material is made transparent
- embedding - support the material so that it is firm enough for sectioning
- sectioning - prepare slices of material which are thin enough to allow light to pass through
- staining - improve contrast between different structures, as most biological material is transparent
- mounting - embed and protect material so that is it is suitable for viewing over a long period
what colours do different organelles in a cell present themselves in when stained in methylene blue?
nuclei stain blue
what colours do different organelles in a cell present themselves in when stained in safranin?
- cytoplasm and cellulose stain green
- nuclei and lignin stain red
- chloroplasts stain pink
what colours do different organelles in a cell present themselves in when stained in haematoxylin?
- nuclei stain blue
- cytoplasm stains pink
why can’t live specimens be viewed under the EM?vv
thin sections of the specimen are destroyed after electron bombardment, and live specimens are unable to survive staining with toxic heavy metal stains and the high vacuum required for viewing
what is the function of cell fractionation in cellular studies?
cell fractionation is used to separate out a homogenous population of subcellular organelles from the cytoplasm as a whole, to allow for the studies of isolated organelles
what are the 2 main steps of cell fractionation?
- disruption of cells and release of the individual cellular components, known as homogenisation
- purification and separation of cell components, known as differential centrifugation
what is the purpose behind homogenisation?
to break tissues into small fragments and to release organelles
what is the methodology behind homogenisation?
- to mechanically rupture the cell membrane, employ either: mortar-pestle and abrasive material, homogenisation, pressure cell, ultrasound, or osmotic lysis using a hyperosmotic homogenisation medium to rupture protoplasm
- chemically dissolve the membrane phospholipids, using various detergents
what are some precautions when performing homogenisation?
in order to maintain the integrity of organelles, homogenisation is done in
1. an isotonic medium containing sucrose, mannitol or sorbitol
2. a buffer solution to maintain suitable pH
3. a temperature of 4 degrees celcius to inhibit protease activity
what is the purpose of differential centrifugation?
to separate organelles using centrifugation
what is the methodology behind differential centrifugation?
- the homogenate is placed on a motor and is subjected to progressively increasing speeds and duration of centrifugation to separate particles in descending order of size and densit
- the faster the rotation, the greater the gravitational force generated and the smaller the particles which will be sedimented
- after each speed, the liquid above the pellet can be drawn off and re-centrifuged at a higher speed for a longer duration
- a series of pellets containing cell organelles of smaller sizes can therefore be obtained.
what is the purpose behind autoradiography
- to identify the sites of synthesis of cellular distribution of metabolic products by tagging specimen molecule with radioisotopes
- used with LM or EM to trace secretion products and their intermediates from their synthesis sites, through cellular organelles, to their eventual destinations
what is the methodology behind autoradiography?
- cells or tissue sections are incubated with radioactive substances, depending on the compartment of the cell for which visualisation is required
- after the radioactive substance has been taken up, the cell or tissue section is mounted on a glass slide which is dipped into photographic emulsion. the decay of the radioactive isotopes results in a dark spot on the photographic emulsion, allowing for the identification of the location of the radioactive substance
