Cell: the unit of life Flashcards
Who discovered the cell?
Robert Hooke - discovered cell in 1665
Anton van Leeuwenhoek - saw live cell
Who discovered the nucleus?
Robert Brown
Contributors to cell theory
Matthias Schleiden, Theodore Schwann, Rudolf Virchow
How did Schleiden contribute to cell theory?
Discovered all plants are made of cells
How did Schwann contribute to cell theory?
- reported that animal cells have a thin outer membrane
- found out that plants cells have a cell wall
- hypothesised that bodies of plants and animals are composed of cells and products of cells
What are the postulates of cell theory?
- all living organisms are composed of cells and products of cells
- all cells arise from pre-existing cells
How did Virchow contribute to cell theory?
As it was not explained before, he explained that new cells are formed from pre-existing cells
Give the overview of a cell.
- has an outer membrane as the delimiting structure
- plants cells have a cell wall
- each nucleus contains a dense membrane bound structure called nucleus
- nucleus contains genetic material (DNA) in chromosomes
- semi fluid matrix called cytoplasm occupies the volume of the cell
- cytoplasm is main area for cellular activities
- eukaryotes have other membrane bound organelles like ER, Golgi apparatus, lysosomes, micro-bodies, vacuoles, etc
- ribosomes are non-membrane bound and formed in all cells in cytoplasm and on RER
- centrosome in animal cell helps in cell division
Differentiate between eukaryotic and prokaryotic cells
eukaryotic cell:
- has nuclear membrane
- has 80s ribosomes
- has membrane bound organelles
- comparatively larger
- many chromosomes
- dna present in nucleus, not nucleoid
What is the smallest cell?
Mycoplasma - bacteria
What is the largest isolated cell?
Ostrich egg
What is the longest cell?
Nerve cell
Organisms under prokaryotic cells
Bacteria, cyanobacteria, mycopasma, PPLO (pleuro pneumonia like organisms)
Shapes of bacteria
bacillus, coccus, vibrio, spirillum
Overview of prokaryotic cells
- cell wall is there except in mycoplasma
- no well defined nucleus
- has smaller circular dna called plasmid in addition to genomic dna
- plasmid confers phenotypic characteristics to bacteria
- only have rbosomes
- have mesosomes
Parts of cell envelope
- three tightly bound parts - glycocalyx, cell wall, cell membrane
- act together as a single protective unit
- glycocalyx can be loose sheath called slime later or think and tough layer called capsule
- cell wall determines the shape of the cell and provides structural support to prevent collapsing and bursting
- plasma membrane is selectively permeable and interacts with surroundings
- mesosomes are the extensions of plasma membrane into the cell in the form of vesicles, tubules, and lamellae
What is gram positive and gram negative
- based on the response of bacterial cell wall to staining procedures, bacteria are classified as:
gram positive - take up the stain (presence of thick wall of peptidoglycan)
gram negative - don’t take up stain (presence of thick wall of peptidoglycan whose stain can be washed away with ethanol)
What are chromatophores?
- membranous extensions into cytoplasm that contains pigments
- present in cyanobacteria
Functions of mesosomes
cell wall formation, dna replication, respiration, secretion, increase surface area
Structures of bacterial cell wall
motile structures: flagella
non motile: pili and fimbriae
What is flagella and what are its parts?
- thin filamentous extension from cell wall
- basal body, filament, hook
What are pili and fimbriae?
- elongated tubular structures made of special protein - pili
- fimbriae - small bristle like fibres
- help in adhesion to surfaces and to host tissues and in conjugation
Ribosomes in prokaryotes
- non membrane bound
- 70 S made of two subunits of 50S and 30S
Polysome or polyribosome
- chains of ribosomes attached to a single mRNA
- translate mRNA into proteins
Inclusion bodies
- non membrane bound
- store reserve material in cytoplasm
- phosphate granules, glycogen granules
Composition of cell membrane
- made of bilayer of phospholipids
- polar head faces outer side
- hydrophobic tail faces inside
- this ensures non polar tail of saturated hydrocarbons is protected from aqueous environment
- presence of cholesterol, proteins and carbohydrates
- proteins can be peripheral or integral based on ease of extraction
- peripheral lie on the surface while integral are partially or totally buried in the membrane
Describe fluid-mosaic model
- made by Singer and Nicolson
- quasi fluid nature of lipid enables lateral movement of proteins within the overall bilayer
- this ability to move within the membrane is measured as its fluidity.
What is the importance of fluid nature of cell membrane?
The fluid nature of the membrane is also important from the point of
view of functions like cell growth, formation of intercellular junctions,
secretion, endocytosis, cell division etc.
Passive transport
- movement of molecules across membrane without requiring energy
- neutral solutes and water can move along the concentration gradient by simple diffusion/osmosis
Active transport
- movement of ions or molecules which requires the utilisation of ATP
- happens against concentration gradient with the help of carrier protein
- polar molecules need active transport to move across non-polar lipid bilayer
eg- Na+/K+ pump
Function of cell wall
- gives shape to the cell
- protects the cell from mechanical
damage and infection
-helps in cell-to-cell interaction
-provides barrier to undesirable macromolecules.
Composition of cell wall in algae
- cellulose, galactans, mannans, minerals
Composition of cell wall in plants
- cellulose, hemicellulose, pectins, proteins
What is primary cell wall and secondary cell wall?
- primary cell wall in young plant cell capable of growth, diminishes with maturity
- secondary cell wall forms on the inner side of the cell
Middle lamella
The middle lamella is a layer mainly of calcium pectate which holds or glues the different neighbouring cells together.
plasmodesmata
connect the cytoplasm of neighbouring cells.
Endomembrane system
- organelles are considered together as an endomembrane system because their functions are coordinated
- ER, golgi apparatus, vacuoles, lysosomes
Endoplasmic reticulum
- network of tiny tubular structures scattered in the cytoplasm
- divides intracellular space into luminal (inside ER) and extra luminal(cytoplasm) compartments
- continuous with the outer membrane of nucleus
SER
- major site for lipid synthesis
- in animal cells lipid-like steroidal hormones are synthesised in SER
RER
- has ribosomes on surface
- observed in the cells actively involved in protein synthesis and secretion.
Golgi apparatus
- densely stained reticular structures near the nucleus
- consist of many flat, disc-shaped sacs or cisternae
Function of GA
- principally performs the function of packaging
materials to be delivered either to the intra-cellular targets or secreted outside the cell - A number of proteins synthesised by ribosomes on the endoplasmic reticulum are modified in the cisternae of the golgi apparatus before they are released from its
trans face.
Golgi apparatus is the important site of formation of glycoproteins and glycolipids.
Explain cis face and trans face
- The Golgi cisternae are concentrically arranged near the nucleus with distinct convex cis or the forming face and concave trans or the maturing face.
Lysosomes
- membrane bound vesicular structures formed by GA
- rich in hydrolytic enzymes optimally active as the acidic pH
- capable of digesting proteins, carbs, fats, and nucleic acids
Vacuoles
- membrane bound space in cytoplasm
- contains water, sap, excretory product
- bound by membrane called tonoplast
- tonoplast facilitates the transport of a number of ions and other materials against concentration gradients into the vacuole
Contractile vacuole
- found in amoeba for osmoregulation and excretion
Food vacuole
formed by engulfing food in protists
Mitochondria
- membrane-bound structure with the outer membrane and the inner membrane dividing its lumen into two aqueous compartments
- inner compartment filled with a dense homogeneous substance called matrix
- matrix also possesses single circular DNA molecule, a few RNA molecules, ribosomes (70S) and the components required for the synthesis of proteins.
What are cristae?
The inner membrane forms a number of infoldings called the cristae (sing.: crista) towards the matrix. The cristae increase the surface area in mitochondria.
Functions of mitochondria
Mitochondria are the sites of aerobic respiration. They produce cellular energy in the form of ATP, hence they are called ‘power houses’ of the
cell.
Classification of plastids
Based on the type of pigments plastids can be classified into chloroplasts, chromoplasts and leucoplasts.
- chloroplast - has chlorophyll and carotenoid pigments that trap light energy for photosynthesis
- chromoplasts - fat soluble carotenoid pigments which give red, yellow or red colour
- leucoplasts - colourless plastics with stored nutrients
Types of leucoplasts
- Amyloplasts store carbohydrates (starch), e.g., potato
- elaioplasts store oils and fats
- aleuroplasts store proteins
Structure of chloroplasts
-double membrane bound
-the space limited by the inner membrane of the chloroplast is called the stroma.
-flattened membranous sacs called the thylakoids are present in the stroma
-thylakoids are arranged in stacks called grana (singular: granum) or the intergranal thylakoids
-flat membranous tubules called the stroma lamellae connect thylakoids of different grana
-the membrane of the thylakoids enclose a space called a lumen
-the stroma of the chloroplast contains enzymes required for the synthesis of carbohydrates and proteins
-it also contains small, double- stranded circular DNA molecules and ribosomes
chlorophyll pigments are present in the thylakoids.
-the ribosomes of the -chloroplasts are smaller (70S) than the cytoplasmic ribosomes (80S)
Structure of ribosomes
- granular structures
- has larger and smaller subunit
- eukaryotes have 80S (60S, 40S)
- prokaryotes have 70S (50S, 30S)
- composed of RNA and protein
What is S in 70S?
S - Svedberg’s Unit
- sedimentation coefficient
- indirect measure of density and size
Cytoskeleton
- elaborate network of filamentous proteinaceous structures consisting of
microtubules, microfilaments and intermediate filaments present in the cytoplasm - provide mechanical support, and maintain shape of the cell
Cilia
small structures which work like oars, causing the movement of either the cell or the surrounding fluid
Flagella
- long hair like outgrowth responsible for cell movement
Structure of cilia or flagella
- Core - axoneme
- Contains 9 doublets of microtubules radially arranged running parallel to long axis and a pair of central microtubules
- Central ones connected by bridge and enclosed by central sheath
- Each doublet connected to sheath by radial spoke
- Each doublet connected to each other by linker
Centrosome
- contains two centrioles perpendicular to each other
Centrioles
- made of 9 triplets of peripheral fibrils
- adjacent triplets are linked (A-C linker)
- central part is proteinaceous and called hub
- hub connected to triplets by protein radial spokes
Structure of nucleus
- nuclear membrane - two parallel membranes that separate nucleoplasm and cytoplasm
- space between two membranes is perinuclear space
- nuclear pores are formed by fusion of membrane and allow the transport of RNA and protein
- outer membrane in continuous with ER and bears ribosomes
- nucleoplasm is nuclear matrix and contains nucleolus and chromatin
What is nucleolus?
- non membrane bound spherical structure in nucleus
- site for active ribosomal RNA synthesis
What is chromatin?
a loose and indistinct network of nucleoprotein fibres
Chromosome
- contains DNA, histones, RNA and non-histone proteins
- primary constriction called centromere
- disc shaped structures called kinetochores on either side of centromere
Types of chromosome based on position of centromere
- metacentric: middle centromere with equal arms
- sub metacentric: slightly away from middle and slightly unequal arms
- acrocentric: centromere extremely close to one end and one very long arm
- telocentric - terminal centromere
Satellite chromosome
- chromosomes with non-staining secondary constrictions at a constant location
- this gives the appearance of a small fragment called the satellite
MIcrobodies
- membrane bound minute vesicles called microbodies that contain various enzymes, are present in both plant and animal cells
