Organelles And Cellular Structures Flashcards
Outline the three principles of the cell theory
- All living organisms are composed of one or more cells
- The cell is the smallest and most basic unit of life
- Cells arise from pre-existing cells
Average cell size
Prokaryotic (smaller): 0.5-10um
Eukaryotic (bigger): 10-100um -> contain organelles and structural adaptations
Smaller= increased SA:V = increased rate of diffusion
Tools and tech use for studying cells : microscopy
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)
Tools and techniques for studying cells : cell fractionation
Used to separate and isolate specific organelles
- Homogenisation
- breaking open cells to release cellular contents
- using chemicals, enzymes, sound waves, pressure - 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
Types of cells
- 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 - 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
Eukaryotic cells consist of
- Cell surface membrane
- Nucleus
- 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)
Organelles of eukaryotic cells : nucleus (structure)
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
Organelles of eukaryotic cells : nucleus (function)
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
Organelles of eukaryotic cells : mitochondrion (structure)
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
Organelles of eukaryotic cells : mitochondria (function)
Site of cellular respiration (produce ATP)
Organelles of eukaryotic cells : chloroplast (structure)
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)
Organelles of eukaryotic cells : chloroplast (function)
Site of photosynthesis
Light-dependent reactions occur on thylakoid membrane
Light-independent reactions (Calvin Cycle) occur in stoma
The endosymbiotic theory
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
Organelles of eukaryotic cells : endoplasmic reticulum (structure)
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)
- sER = series of interconnected tubules with no ribosomes attached
- 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
Organelles of eukaryotic cells : endoplasmic reticulum (function - rER)
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)
Organelles of eukaryotic cells : endoplasmic reticulum (function - sER)
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
Organelles of eukaryotic cells : Golgi apparatus (structure)
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)
Organelles of eukaryotic cells : Golgi apparatus (function)
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
Organelles of eukaryotic cell: lysosome (structure)
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
Organelles of eukaryotic cells: lysosome (function)
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
Organelles within eukaryotic cells: ribosomes (structure)
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
Organelles of eukaryotic cells: ribosome (function)
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
Organelles of eukaryotic cells : centrioles (structure)
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
Organelles of eukaryotic cells: centrioles (function)
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
The endomembrane system
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
What are prokaryotes
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
Prokaryotes : Peptidoglycan cell wall
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)
Prokaryotes : cell surface membrane
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
Prokaryotes : circular DNA
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
Prokaryotes : cytoplasm containing ribosomes
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
What do ribosomal subunits consist of
Ribosomal RNA - Coded for by genes in nucleolus
Ribosomal proteins - coded for by genes in nucleus
