Cell Structure & Function

Question 1

what are the three tenets of cell theory?

Answer 1

1. all living organisms are composed of one or more cells
2. the cell is the most basic unit of structure in all organisms
3. all cells arise only from pre-existing cells

Question 2

what are the differences between prokaryotic and eukaryotic cells?

Answer 2

prokaryotes have neither a true nucleus nor membrane bound organelles, whereas eukaryotes have both, and are generally much larger

for more, refer to table 1 on page 7 of notes

Question 3

what are the five functions a cell (as a fundamental unit of life) are able to perform?

Answer 3

1. intake of raw materials, and from these:
2. extraction of useful energy and synthesising its own molecules
3. growing in an organised manner
4. reproducing after its own kind
5. responding and adapting to external environment

Question 4

what are the lower and upper limits of cell size?

Answer 4

lower limit: minimum amount of space needed to contain the essential elements of its function (eg. DNA and enzyme molecules)
upper limit: surface area to volume ratio needed for exchange of materials between the cell and its environment (as cell size increases, SA:V ratio decreases

Question 5

why must cells be kept small, and how do increases in organism size thus have to arise?

Answer 5

to ensure the number of chemical exchanges (only transported through cell membrane) that can be performed with the extracellular environment would be adequate to maintain the cell, so most of the cytoplasm cannot be relatively far from the outer membrane

cell size kept small, hence increase in organism size is due to a greater number of cells

Question 6

what are the units of measurement for cells and cell parts, and what is the approximate diameter of prokaryotic and eukaryotic cells?

Answer 6

cells / larger organelles: micrometer (um), 10^(-6)
smaller organelles (eg. ribosomes) / thickness of membranes: nanometer (nm), 10^(-9)
prokaryotic is 0.2-2.0um, eukaryotic is 10-100um

Question 7

what does the cytoplasm consist of?
hint: cytosol, membranous / non-membranous organelles

Answer 7

cytosol is an aqueous matrix in which organelles and the nucleus are suspended
membranous: chloroplast, mitochondria, rough and smooth endoplasmic reticulum, Golgi apparatus, lysosomes, peroxisomes, chloroplasts (for plants)
non-membranous: cytoskeleton, centrioles, centrosomes (for animal), cilia, flagella, ribosomes

Question 8

what is the cytoplasm?

Answer 8

it refers to all organelles and cytosol within the cell membrane, except the nucleus
cytoplasm = cytosol (aqueous solute rich matrix) + organelles
cytosol is 90% water, dissolved in it are essential ions and soluble organic molecules like sugars and aa, soluble proteins including enzymes, cytoskeleton (network of fine globular and fibrous protein strands, providing infrastructure and support to the cell

Question 9

what are membranous organelles, and their advantages?

Answer 9

compartmentalised spaces within cytoplasm, surrounded by membranes (structurally and biochemically similar to plasma membrane)
- allows maintenance of characteristic differences between contents of each organelle and cytosol, compartmentalisation of specific reactions provides diff local environments, incompatible processes can occur simultaneously
- increases membrane SA, internal membranes have embedded enzymes and proteins, so more enzyme complexes can be embedded and increases efficiency of many reactions by providing optimal enzyme concentration

Question 10

what is the nucleus and nuclear envelope? (microscope, size, function, structure)

Answer 10

largest organelle, easily seen with light microscope
5-20um
encloses genetic material, protects DNA from metabolically active cytoplasm, double membrane perforated with nuclear pores for substance exchange

nuclear envelope: double membrane (each a lipid bilayer), separates cytoplasm and nucleus’ contents, inner and outer membranes are continuous and region between is perinuclear space

Question 11

describe the function and structure of the nucleoplasm and nucleolous

Answer 11

nucleoplasm: aqueous matrix within nucleus containing proteins, metabolites, ions, RNA, and chromatin (genetic material of the cell, coils of DNA wound around histone proteins)
- chromatin’s two forms: loosely coiled is euchromatin (light-coloured patches), tightly coiled is heterochromatin (dark-coloured patches)

nucleolus: dense mass in nucleus seen under electron microscope, composed of DNA carrying rRNA genes, RNA and protein, functioning to synthesise rRNA that forms a component of ribosomes

Question 12

how are ribosomes synthesised in eukaryotic cells?

Answer 12

their two large and small ribosomal subunits are first synthesised at the rough endoplasmic reticulum’s ribosomes, then they are folded in the RER lumen and reenter the nucleus through nuclear pores, and meet up with the rRNA synthesised at the nucleolus

Question 13

what is the endomembrane system?

Answer 13

it is composed of inter-related membrane sacs, related either by direct physical continuity or by transfer of membrane segments known as vesicles
components are: rough and smooth endoplasmic reticulum, Golgi apparatus / body, lysosomes, vacuoles

*the plasma membrane is not part of the endomembrane system, but continuously interacts with it

Question 14

what is the endoplasmic reticulum (its structure and adaptations)?

Answer 14

extensive network of hollow, membranous tubules, sacs or sheets called cisternae (singular: cisterna)
internal space of the ER is known as the lumen, continuous with perinuclear space

extensive network of cisternae increases membrane surface area for synthesis
hollow cisternae accommodate newly synthesised substances and allow for packaging of contents into transport vesicles to the GA

Question 15

what is the structure, function and significance of rough endoplasmic reticulum?

Answer 15

structure: sheet-like, rough due to presence of ribosomes that stud the cytosolic face of RER
function: RER-bound ribosomes are sites of protein synthesis, pp chain then enters ER lumen to be folded into its native conformation, and sent for export or targeted to various cellular organelles
significance: cells active in protein secretion are abundant in RER, some proteins synthesised in RER can also enter RER membrane to form ER membrane proteins

Question 16

what is the structure and function of the smooth endoplasmic reticulum?

Answer 16

network of tubules which lack ribosomes, smooth appearance
functions in diverse metabolic processes

synthesis of lipids (cells active in hormone secretion usually have abundant SER)
metabolism of carbohydrates
detoxification of drugs and poisons
storage of calcium ions (for muscle contraction and cell signalling)

Question 17

what is the structure, function and significance of the Golgi body (cis and trans faces)?

Answer 17

structure: stack of flattened, membrane-bound sacs called cisternae (singular: cisterna, each stack differs in thickness and molecular composition)
- distinct polarity: cis (forming) face, and a trans (maturing) face. new cisternae made of transport vesicle membranes formed at cis face by receiving transport vesicles from the ER, and secretory vesicles bud off at trans face to go to extracellular matrix or to form lysosomes
*Golgi vesicles transfer materials between cisternae, or transport substances to other organelles off the trans face

function: site of modification and packaging of ER products
- glycosylation: addition of sugar groups
- trimming: removal of excess monomers
*different cisternae have different enzymes, progressively modified at each cisterna between cis and trans faces

significance: any cell active in any form of secretion have abundant GA, increased SA for vesicle reception and budding. multiple cisternae: different modification processes can occur simultaneously

Question 18

what is the structure and three major functions of lysosomes?

Answer 18

membranous organelle that is homogeneously electron-dense, contains hydrolytic enzymes (proteases, nucleases, lipases, acid phosphates) that digest most macromolecules, with an optimal acidic pH of 5, segregation of contents provides optimal pH for hydrolytic reactions and protects cellular contents from hydrolysis

1. digestion: food particles or bacteria engulfed by endocytosis to fuse w lysosomes to form endosomes (with food) or phagocytic vacuole (with bacteria)
2. autophagy of worn-out organelles: enclosing unwanted structures with membrane of unknown origin, forming autophagic vacuoles
3. autolysis: self-destruction of cells is apoptosis, mass release of lysosomal contents in whole cell

Question 19

what is the role and structure of vacuoles (in animal and plant cells)?

Answer 19

animal: small, mobile organelles that store and transport substances (eg. food vacuoles and phagocytic vacuoles)

plant: large central vacuole (single-membraned) known as a tonoplast that contains cell sap
functions include storage of organic compounds like proteins and inorganic ions like K+ and Cl-, disposal site for toxic metabolic by-products, contains pigments, plant protection by accumulating toxic or unpalatable compounds to consumers, cell growth and elongation by accumulating water and pushing cytoplasmic contents to periphery of cell so minimal investment in cytoplasm synthesis needed, and SA:V ratio not sacrificed when making plant cells bigger

Question 20

what are both non-endomembrane system membranous organelles and energy transducers?

Answer 20

mitochondria and chloroplasts are energy transducers: not involved in endomembrane system, but they convert energy from one form into another

Question 21

what is the structure, size, function, and significance of mitochondria?

Answer 21

elongated or spherical, ranging from between 1.0-10um
double membraned, smooth outer but highly convoluted inner with infoldings known as cristae to increase the SA

• synthesises ATP during cellular respiration!

space between membranes is inter-membrane space / perimitochondrial space
inner membrane encloses the mitochondrial matrix, storing enzymes, its own circular DNA, and own protein synthesis machinery (RNA and ribosomes), for mitochondria to synthesise its own proteins (site of the Krebs cycle)
compartmentalisation is necessary for proton gradient across inner mitochondrial matrix which is impermeable to protons

Question 22

what is the structure, size, function, significance of chloroplasts?

Answer 22

sites of photosynthesis, convert solar energy to chemical energy, using sunlight to drive synthesis of organic compounds from CO2 and H2O
lens-shaped, 5-10um in length, only organelles other than nucleus visible under light microscope, double-membraned chloroplast envelope

inner membrane encloses semi-fluid compartment known as the stroma, contains DNA and protein synthesis machinery
sugars synthesised also stored as starch grains in stroma
thylakoids are the third set of membranes that enclose thylakoid lumen, forming thylakoid discs that stack up to form granum that are connected by intergranal lamellae, to increase SA for attachment of photosynthetic pigments
compartmentalisation is necessary for proton gradient

Question 23

what is the structure, function, and location of ribosomes?

Answer 23

site of protein synthesis, consisting of two subunits (large and small ribosomal subunits made up of proteins and rRNA)
location: attached to RER, free in cytosol (self-sufficient: mitochondrial matrix and chloroplast stroma)
bound ribosomes synthesise proteins destined for export, insertion into membranes, or targeted to various membrane-bound organelles
free cytosolic ribosomes synthesise proteins that remain in the cell and function within the cytosol

Question 24

what is the structure and function of the cytoskeleton (microtubules, microfilaments, intermediate filaments)

Answer 24

3D array of interconnected filaments and tubules: giving mechanical support and maintaining cell shape, allowing for anchorage and directing movement of organelles and molecules, providing cell motility (cilia and flagella)

microtubules: specialised arrangements arise from microtubule organising centers (MTOCs), involved in transport of substances
microfilaments: movement of cells
intermediate filaments: stabilise organelles or specialised cell junctions

Question 25

what is the structure and purpose of centrioles?

Answer 25

found in animal cells, absent in plant cells, located near nucleus in a region known as centrosome
found in pairs at right angles to each other, with each member of the pair consisting of nine triplets of microtubules (cytoskeleton) arranged in a ring
before cell division, each centriole replicates itself and moves to opposite poles of the cell, acting as MTOCs for the formation of spindle fibres

Question 26

what is the structure and function of cell walls (and what is the extracellular matrix)?

Answer 26

rigid and inflexible structure mainly made of cellulose, protecting cells and high tensile strength enables cell to withstand hydrostatic pressure
it is freely permeable to all but very large molecules, region between cell walls of adjacent cells separated by middle lamella, rich in polysaccharides such as pectin that adheres cells

extracellular matrix in animal cells supports and binds cells

Question 27

compare the light and electron microscopes (working principles, type of specimens and specimen preparation, magnification, resolution, contrast)

Answer 27

light passes visible light, electron uses electron beam
light uses tissues or entire cells (living or dead), electron uses sub-cellular studies (dead)
magnification = image size / actual size, electron’s is much greater
resolution is the shortest distance found between two points that can be distinguished by the observer, inversely proportional to wavelength of radiation used, electron’s is much greater than light
contrast can be improved by staining (coloured dyes for light, heavy metals for electron)

Question 28

explain the purpose, methodology, precautions of homogenisation

Answer 28

to break tissues into small fragments, releasing organelles
motar, pestle, abrasive material / homogeniser / pressure cell / ultrasound / osmotic lysis OR chemical digestion (non-mechanical)
precautions: done in isotonic medium, buffer solution, temperature of 4 degrees Celsius to inhibit protease activity

Question 29

explain the purpose, methodology of differential centrifugation

Answer 29

separate organelles using centrifugation
subject homogenate to progressively increasing speeds and duration, separating particles in descending order of size and density (size, shape, density all affect sedimentation coefficient)
faster rotation = greater gravitational force = smaller sedimented particles
after each speed, supernatant (liquid above pellet) can be re-centrifuged at higher speed for longer duration, to obtain series of pellets
- possible bc organelles are discreet particles with unique sizes and densities

Question 30

explain the purpose, methodology, precautions for autoradiography

Answer 30

using X-ray film to visualise molecules or fragments that have been radioactively labelled

to identify sites of synthesis and cellular distribution of metabolic products by tagging with radioisotopes
incubate cells or tissue sections with radioactive substances, then mount on glass slide dipped into photographic emulsion, decaying the radioactive isotope that result in a dark spot