Organelles And Cellular Structures

Question 1

Outline the three principles of the cell theory

Answer 1

1. All living organisms are composed of one or more cells
2. The cell is the smallest and most basic unit of life
3. Cells arise from pre-existing cells

Question 2

Average cell size

Answer 2

Prokaryotic (smaller): 0.5-10um

Eukaryotic (bigger): 10-100um -> contain organelles and structural adaptations

Smaller= increased SA:V = increased rate of diffusion

Question 3

Tools and tech use for studying cells : microscopy

Answer 3

Light microscope
Magnification: ratio of object’s image size to real size
Resolution: minimum distance which allow two points to be distinguished as separate points
Contrast: visible differences between various part of sample

Electron microscope (organelles within eukaryotic cells)
- scanning electron microscope (surface of cell)
- transmission electron microscope (internal structure)

Question 4

Tools and techniques for studying cells : cell fractionation

Answer 4

Used to separate and isolate specific organelles

1. Homogenisation
   - breaking open cells to release cellular contents
   - using chemicals, enzymes, sound waves, pressure
2. Centrifugation
   - separation and isolation of different size cell organelles using centrifugal force
   - differential: using speed to spin down organelles
   - density gradient: sucrose gradient set up, organelles migrate to form band where its density = sucrose solution density

Question 5

Types of cells

Answer 5

1. Prokaryotic cells (bacteria)
   - circular DNA found in nucleoid, not enclosed by membrane
   - plasmids (small circular DNA molecule) found throughout cytoplasm
   - no membrane bound organelles, fewer organelles in total (eg ribosomes)
   - have cell surface membrane
   - mostly no internal membrane
2. Eukaryotic cell (animal/plant)
   - linear DNA in nucleus, enclosed by nuclear envelope
   - mitochondria and chloroplasts contain circular DNA
   - various membrane-bound and non-membrane bound organelles
   - have cell surface membrane
   - complex internal membrane system called endomembrane system : synthesis, transport and modification of proteins and lipids

Question 6

Eukaryotic cells consist of

Answer 6

1. Cell surface membrane
2. Nucleus
3. Cytoplasm
   - cytosol : fluid portion of cytoplasm, contains ions and organic compounds
   - organelles
   - cytoskeleton : framework of protein filaments eg micro tubules, microfilaments, intermediate filaments give cells shape + serve other cellular functions (eg movement of vesicles within cell)

Question 7

Organelles of eukaryotic cells : nucleus (structure)

Answer 7

Largest organelle (10-20um)

Enclosed by double membrane (made up of phospholipid bilayer) called nuclear envelope
- outer membrane is continuous with endoplasmic reticulum
- nuclear pores on nuclear envelope = channels for regulated movement between nucleus and cytoplasm

Filled with semi-fluid matrix called nucleoplasm

DNA located in nucleus, coiled around histones (proteins) to form highly elongated chromatin threads
- during nuclear division chromatin condenses into chromosomes
- in non-dividing cell chromatin exists as heterochromatin (more condensed) and euchromatin (less condensed)

Nucleolus in nucleus (clear and distinct non-membrane bound structure)
- one or more nucleoli may be found within nucleoplasm
- contains genes coding for rRNA
- functions as site of transcription of rRNA and assembly of ribosomal subunits

Question 8

Organelles of eukaryotic cells : nucleus (function)

Answer 8

Stores hereditary material (DNA) = continuity of genetic info to next generation of cells

DNA contains genes = templates for protein synthesis

Controls and directs cell activities by regulating gene expression and protein synthesis

Question 9

Organelles of eukaryotic cells : mitochondrion (structure)

Answer 9

Cylindrical/rod-shaped, width ranges 0.5-1.5um, length ranges 3-10um

Double membrane bounded, inner and outer membranes separated by inter-membrane space
- outer forms smooth continuous boundary
- inner is extensively folded into cristae which project into matrix

Semi-fluid matrix contains circular DNA (codes for some proteins and enzymes in organelle) and many 70S ribosomes (protein synthesis)

Presence of both DNA and ribosomes in mitochondria = originated from prokaryotic organism that invaded eukaryotic cell = endosymbiotic theory

Question 10

Organelles of eukaryotic cells : mitochondria (function)

Answer 10

Site of cellular respiration (produce ATP)

Question 11

Organelles of eukaryotic cells : chloroplast (structure)

Answer 11

Large organelle (diameter 0.5-2um, length 5-10um), larger and more complex than mitochondria

Double membrane bounded (chloroplast envelope)

Third membrane (thylakoid membrane) forms network of thylakoids (flattened coin-like sacs enclosing fluid-filled thylakoid space)
- thylakoids are stacked to form grana (singular: granum)
- stacks of grana joined by intergranal lamellae
- both grana and integranal lamellae contain photosynthetic pigments (eg chlorophyll)

3 membranes divide chloroplasts into 3 distinct internal compartments
1. Inter-membrane space
2. Stroma, lies inside envelope but outside thylakoid membrane
3. Thylakoid lumen

Interior of chloroplast filled with gel-like matrix called stroma
- contains circular DNA and many 70S ribosomes like mitochondria; starch grains and oil droplets found here
- light-independent reactions occur here (Calvin cycle)

Question 12

Organelles of eukaryotic cells : chloroplast (function)

Answer 12

Site of photosynthesis
Light-dependent reactions occur on thylakoid membrane
Light-independent reactions (Calvin Cycle) occur in stoma

Question 13

The endosymbiotic theory

Answer 13

Ancestor of eukaryotic cell engulfed prokaryotic cell, formed relationship = endosymbiont
Evolution = merged into single organism, eukaryotic cell with mitochondrion
Evidence : mitochondria and chloroplast
- contains own circular DNA
- similar size to prokaryotic cells
- double membrane bound

Question 14

Organelles of eukaryotic cells : endoplasmic reticulum (structure)

Answer 14

Series of interconnected membranous sacs (r) and tubules (s) called cisternae (singular: cisterna)

ER membrane separates internal compartment of ER (ER lumen) from cytosol (cytoplasm)

1. sER = series of interconnected tubules with no ribosomes attached
2. rER = continuous with outer membrane of nuclear envelope, consists of network of flattened membrane-bound sacs called cisternae (increase SA:V) with attachment of ribosomes on cytoplasmic site = studded appearance

Membranes of 2 types of ER are connected, internal spaces (ER lumen) are continuous

Question 15

Organelles of eukaryotic cells : endoplasmic reticulum (function - rER)

Answer 15

rER: site of protein synthesis and transport

Proteins synthesised by ribosomes attached are either
- inserted into cell’s membranes
- secreted out of cell, or
- packaged into lysosomes

Proteins synthesised are folded into specific shape in ER lumen

Most secretory proteins = glycoproteins
Addition of carbohydrates can also take place in rER but limited to few types of carbs

These proteins are then packaged into transport vesicles which bud off the rER and move to other parts of cell (eg Golgi apparatus)

rER is engaged in modifying proteins (eg enzymes) that will be secreted from cell = abundant in cells that secrete proteins (eg cells of liver)

Question 16

Organelles of eukaryotic cells : endoplasmic reticulum (function - sER)

Answer 16

Site of lipid synthesis (including sex hormones)
Involved in detoxification of drugs and poisons as well as storage of calcium ions required for signal transduction during cell signalling or muscle contraction

Question 17

Organelles of eukaryotic cells : Golgi apparatus (structure)

Answer 17

Consists of a stack of flattened membrane-bound sacs (cisternae) that are not physically connected (unlike ER cisternae)

Distinct structural polarity, membranes of cisternae on opposite sides have differing molecular composition

Forming/cis face: where transport vesicles from ER fuse to (faces nucleus/ER, convex shape)

Maturing/trans face: budding of secretory vesicles occurs (concave shape)

Question 18

Organelles of eukaryotic cells : Golgi apparatus (function)

Answer 18

Chemically modifies, sorts, and packages proteins or lipids made in rER and sER respectively

Modification: addition of short chains of sugar molecules (glycosylation) and/or phosphate groups (phosphorylation) /other small molecules = proteins and lipids can be routed to proper destinations

Modified proteins are then sorted and packaged into secretory vesicles which bud off from maturing/trans face

Depending on the protein’s (in the vesicle) function, proteins are either
- inserted into cell surface membrane
- released out of cell via exocytosis
- vesicles can fuse with each other to form lysosomes

In plants, involved in formation of new cell wall
- Golgi vesicles membrane form new cell membrane, contents form new cell wall

Question 19

Organelles of eukaryotic cell: lysosome (structure)

Answer 19

Small spherical vesicles (0.2-0.5um) formed by vesicles from GA

Contains hydrolytic enzymes (lipases, proteases, nucleases) surrounded by single membrane

Contents are acidic, enzymes have low optimum pH

Question 20

Organelles of eukaryotic cells: lysosome (function)

Answer 20

Animal cell: digestive component and organelle-recycling facility

Fuse with vesicles formed by endocytosis (food vacuole) and release hydrolytic enzymes into vesicles to digest materials within
- materials taken in as food (amoeba) or for defence of body (wbc)
- products of digestion are absorbed and assimilated into cytoplasm of cell

Autophagy: engulf and digest worn-out organelles

Autolysis: trigger self-digestion of cell by releasing lysosomal contents within cell after cell damage/death

Question 21

Organelles within eukaryotic cells: ribosomes (structure)

Answer 21

Complexes composed of proteins and rRNA
In eukaryote: diameter of about 20nm
- 80S
- small subunit 40S, large subunit 60S

Can be attached to cytoplasmic side of rER or free in the cytosol (cytoplasm)

Occur in clusters called polyribosome or polysomes - simultaneous synthesis of many polypeptide chains from single mRNA

Question 22

Organelles of eukaryotic cells: ribosome (function)

Answer 22

Site of protein synthesis (AAs are joined together to form polypeptide chain via formation of peptide bonds)

Proteins made on free ribosomes: function within cytosol of cell

Proteins made by rER ribosomes: insertion into membrane/ packaging within lysosomes/ secretion from cell

Both ribosomes are structurally identical, can switch between 2 roles

Question 23

Organelles of eukaryotic cells : centrioles (structure)

Answer 23

Exist as a pair of rod-like structures positioned at right angles next to each other + situated next to nucleus at region called centrosome

Transverse section of each rod-like structure = 9 triplets of microtubules, joined by fibrils
- microtubules are long hollow tubes made up of tubulins

Absent in plant cells

Question 24

Organelles of eukaryotic cells: centrioles (function)

Answer 24

Nuclear division in animal cells (replicate before cell divides)

Function as microtubules organising centres (MTOC) for assembly of spindle fibres (separation of homologous chromosomes and sister chromatids during nuclear division)

Plants do not have centrioles but still have organised microtubules = other mtoc

Question 25

The endomembrane system

Answer 25

System of intracellular membranes Membranes within cell compartmentalise various organelles involved in synthesis, transport and modification of proteins and lipids -> intracellular membranes compose system - nuclear envelope, endoplasmic reticulum, Golgi apparatus, lysosomes, vesicles - cell surface membrane is part of system as it interacts with organelles in system DOES NOT INCLUDE MEMBRANES LIKE MITOCHONDRIAL OR CHLOROPLAST ENVELOPE

Question 26

What are prokaryotes

Answer 26

All bacteria = prokaryotes Simple unicellular microorganisms which lack nuclei or membrane-bound organelles Small size (length 0.1-10um, diameter 1um) Three common shapes 1. Spheres (cocci) 2. Rods (bacilli) 3. Spirals Possess Peptidoglycan cell wall, cell surface membrane, circular DNA, cytoplasm containing ribosomes

Question 27

Prokaryotes : Peptidoglycan cell wall

Answer 27

Strong rigid cell wall surrounding bacteria due to Peptidoglycan (aka Murein) - Consists of parallel polysaccharide chains made up of NAG and NAM - Parallel polysaccharide chains are cross-linked at regular intervals by short chains of AA Cell wall prevents bacteria from bursting when it absorbs water, tiny pores allow passage of water, ions, small molecules Bacteria - gram Positive : more Peptidoglycan, gram staining= Purple - gram negative : cell wall covered by outer membrane, gram staining = red Some bacteria’s outer surface of cell wall may be coated with glycocalyx -> not essential but advantageous (helps bacteria adhere to surfaces, protects from viral attack/toxic substances/phagocytosis, conserve water, aids mobility)

Question 28

Prokaryotes : cell surface membrane

Answer 28

Consist of phospholipid bilayer and proteins (same as eukaryotes) Higher proportion of Proteins as compared to eukaryotic membrane -> need to perform various functions that are performed by organelles in eukaryotes Do not have Cholestrol (unlike eukaryotes) Form **in-folding** to carry out specialised metabolic functions (respiration) - eg **mesosome** for attachment of bacterial chromosome in process of binary fission

Question 29

Prokaryotes : circular DNA

Answer 29

1 double-stranded circular DNA (bacterial chromosome) Not associated with histones (unlike eukaryotes) Found in non-membrane bound region of cytoplasm called **nucleoid** -> appears less darkly stained in micrograph Presence of plasmids: smaller, double-stranded, circular, extra-chromosomal DNA in cytoplasm (can have multiple) - contain genes that are not essential but are beneficial under stressful conditions

Question 30

Prokaryotes : cytoplasm containing ribosomes

Answer 30

Cytoplasm in bacteria lacks membrane-bound organelles = no complex compartmentalisation 70S ribosomes - small subunit 30S - large subunit 50S Also storage granules of glycogen and lipid droplets on cytoplasm

Question 31

What do ribosomal subunits consist of

Answer 31

Ribosomal RNA - Coded for by genes in nucleolus Ribosomal proteins - coded for by genes in nucleus