C5 — Eukaryotic Cells Flashcards
The cell theory definition
The cell is a fundamental unit of structure, function and organization in all living organisms, and new cells are formed from other existing cells.
States that…
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.
Cytoplasm definition
Cytoplasm refers to all the organelles and cytosol within the cell surface membrane, with the exception of the nucleus.
Mitochondria definition
Mitochondria (singular: mitochondrion) are the sites of cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats and other metabolic fuels in the presence of oxygen.
Chloroplasts definition
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.
Resolution in microscopy definition
Resolution is defined as the shortest distance that is found between two points that can be distinguished by the observer.
Magnification definition
Magnification is defined as the number of times larger an image is to its specimen, i.e., the ratio of an object’s image size to its real size.
A eukaryotic cell has compartmentalised cell structures known as membrane-bound organelles. In contrast, a prokaryotic cell’s cytosol is one undivided space without compartments.
Can you think of any advantage(s) that the membrane-bound organelles can offer to eukaryotic cells?
- Enzymes and substrates for a particular process can be more concentrated when confined within an organelle, as opposed to if enzymes and substrates spreading throughout the cytosol. Hence, the chemical process can be more efficient.
- Optimum conditions e.g. pH required for a particular process can be maintained and confined to the organelle. Other cellular processes may need a different level of condition.
- Any harmful substances that can potentially cause damage to the cell can be confined in the organelle. E.g. lysosome contains enzymes that hydrolyse proteins, lipids or nucleic acids. The membrane of lysosome keeps these hydrolytic enzymes stored away from the rest of the cell to prevent killing of the cell itself.
State 2 features of a mitochondrion that can be seen in figure 1.1. [3]
- Elongated/spherical
- Double membrane
- Highly folded inner membrane/cristae
State 2 features of a rough endoplasmic reticulum that can be seen in figure 1.1. [3]
- Extensive network of tubules/cisternae
- Rough appearance due to ribosomes
Describe 2 functions of the Golgi body. [2]
- Site of modification of ER products, via glycosylation/removal of excess monomers
- Packaging of proteins and budding off as vesicles
Suggest 2 advantages to eukaryotic cells of having membrane-bound organelles. [2]
- Allows compartmentalisation of specific reactions/functions (eg hydrologic enzymes within lysosomes) to provide optimal/specific conditions/environment for enzymatic reaction to occur
- regulation of cell’s contents by the protein and other components of the membrane eg generation of proton gradients necessary for photosynthesis and respiration
- increases membrane surface area allowing for embedding enzymes and proteins
- increases efficiency of enzymatic reactions as reactants and enzymes of a metabolic pathway could be kept together
- maintaining characteristic differences in specific environments allow for incompatible processes to occur simultaneously/maintain characteristic differences
Explain the role of glycogen in animal cells. [2]
- Major form of storage polysaccharide in animals/found in liver and skeletal muscle
- used as a source of glucose to maintain sugar levels
- serves as a fuel source to generate ATP for muscle contraction
5 functions the cell can perform
- Intake of raw materials, and from these,
- 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
Cytosol
aqueous solute rich matrix (the soluble part of the cytoplasm) that appears transparent and lacking structure under the Light Microscope (LM). It contains about 90% water, and dissolved in it are:
- various essential ions and soluble organic molecules such as sugars and amino acids
- soluble proteins including enzymes
- the cytoskeleton – a network of fine strands of globular and fibrous proteins, which provides infrastructure and support to the cell.
Non-membranous organelles
Cytoskeleton
Centrioles
Centrosomes
Cilia
Flagella
Ribosomes
Membranous organelles + endomembrane system
Endomembrane system:
Endoplasmic reticulum (rough and smooth)
Golgi apparatus
Lysosomes
Vacuoles
Nuclear envelope (not nucleus)
cell surface membrane
Membranous organelles:
- whole of endomembrane system
- peroxisomes
- chloroplast (contains double membrane)
- mitochondria (contains double membrane)
- nucleus (contains double membrane)
Advantages of compartmentalisation:
-> The compartmentalisation of specific reactions provides different local environments for which incompatible processes can occur simultaneously, allowing the maintenance of characteristic differences between the contents of each organelle and the cytosol.
-> Increase membrane surface area.
The greater the membrane surface area, the larger the number of enzyme complexes that can be embedded, and thus increasing efficiency of many cellular reactions by providing optimal enzyme concentration for reactions to occur.
Chloroplast structure + function:
- Involved in energy transduction
- lens-shaped and are about 5 μm – 10 μm in length.
- Visible under the light microscope (other than nucleus)
- Surrounded by a double membrane called the chloroplast envelope
Function: Sites of photosynthesis + 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
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Stroma:
-> inner membrane encloses a semi-fluid compartment known as the stroma
-> Sugars synthesised by the chloroplasts are stored as
starch grains in the stroma.
Stroma contains circular DNA that enables the synthesis of chloroplast proteins and enzymes required for the light-independent reactions (Calvin cycle) of photosynthesis
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Thylakoids:
-> chloroplasts have a third set of membranes within the stroma known as thylakoids, which enclose an area known as the thylakoid lumen.
The thylakoid and the thylakoid lumen together form
a flattened sac called thylakoid disc.
-> Some thylakoid discs are stacked up like a stack of coins to form granum (plural: grana). Connecting stacks of grana are sheet-like thylakoids known as intergranal lamellae.
Function:
- Thylakoids are the site for light-dependent reactions of photosynthesis. The thylakoids allow for increased surface area for attachment of chlorophyll and other photosynthetic pigments.
- Compartmentalisation of thylakoid lumen is also necessary for the setting up of proton gradient across the thylakoid membranes. The generation of the proton gradient is due to the thylakoid membrane being impermeable to protons.
- Chloroplasts carry out photosynthesis to synthesise organic compounds (e.g. sugars such as glucose).
- Chloroplast absorb light to synthesise ATP and NADPH (during light dependent reaction)
which are used in Calvin Cycle.
Mitochondria (plural)
Mitochondrion (singular)
- Involved in energy transduction
- Number of mitochondria per cell corresponds to the level of metabolic activity.
- Usually elongated or spherical, ranging
from between 1.0μm - 10μm in length. - Contain double membranes
Function: Sites of cellular respiration, the catabolic process that generates ATP by extracting energy from sugars, fats and other metabolic fuels in the presence of oxygen.
Inner membrane is highly convoluted, with infoldings known as cristae (singular: crista) to increase the surface area for the attachment of various enzyme systems and involved in cellular respiration.
The narrow, fluid-filled space between the two membranes is known as the inter-membrane space (or peri-mitochondrial space).
The compartment enclosed by the inner membrane is known as the mitochondrial matrix, which houses enzymes, circular DNA, RNA and ribosomes To allow for the mitochondria to synthesise its own proteins. This is also the site of Krebs cycle.
The compartmentalisation of the mitochondrial matrix is Necessary for a proton gradient across the inner mitochondrial membrane to be set up due to the inner mitochondrial membrane being impermeable to protons.
Mitochondria synthesises ATP during cellular respiration.
Endoplasmic reticulum
Consists of an extensive network of hollow, membranous tubules, sacs or sheets called cisternae (singular: cisterna).
Hollow cisternae accommodate newly synthesised substances and allow for packaging of contents into vesicles for transport to the GA i.e. budding of vesicles.
The extensive network of cisternae increases membrane surface area for synthesis e.g. for RER it is embedded with ribosomes for synthesis of polypeptides, and for SER, the membrane allows for enzymes to be embedded, so that steroid and phospholipid can be synthesised.
The internal space of the ER is known as the cisternal space, or lumen, which is continuous with the perinuclear space.
Rough endoplasmic reticulum
RER-bound ribosomes are sites of protein synthesis where a polypeptide chain is synthesised..
The polypeptide chain then enters the ER lumen, which is the site of protein folding, through a protein channel in the RER membrane where the polypeptide chain folds into its native conformation.
These proteins are either exported or are targeted to various cellular organelles
(Proteins synthesized by free ribosomes in the cytosol remain in the cytosol)
Cells that are active in protein secretion usually have abundant rough ER.
Some proteins synthesised in the rough ER can also directly enter the membrane of the ER to form ER membrane proteins.
Proteins that leave the RER are enclosed in vesicles known as transport vesicles.
Smooth endoplasmic reticulum
Functions in diverse metabolic processes which require ATP:
-> Synthesis of lipids (including oils, phospholipids and steroids like sex hormones); 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
Golgi apparatus
Consists of a stack of flattened, membrane-bound sacs called cisternae (singular: cisterna), which separates its internal space from the cytosol. Each stack differs in thickness and molecular composition. Consists of a cis face (forming face) and a trans face (maturing face).
-> 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 fuse with the cis face membrane and deposit their contents into Golgi cisternal space.
-> At the trans face, membranes bud off to form secretory vesicles, which contain materials to
be transported to the extracellular matrix or to form lysosomes.
-> Between the Golgi sacs, Golgi vesicles are responsible for transferring materials between the parts of the Golgi.
-> Some Golgi vesicles also bud off from the trans face to transport substances to other organelles in the cell.
Functions:
- site of modification and packaging of ER products
- Some modifications include glycosylation (addition of sugar groups) and trimming (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 from the cis face to the trans face. The processed and packaged contents are then passed on to other components of the cell by vesicles that bud off the GA complex.
- Therefore, cells that are active in any form of secretion usually have abundant GA, as the abundant flattened cisternae provide increased surface area for vesicle reception and budding.
- Multiple cisternae also allow for different modification processes to occur simultaneously.
Lysosomes
Structure: Contains hydrolytic enzymes (proteases, nucleases, lipases and acid phosphatases) that can digest most biological macromolecules. Enzyme contents are synthesised
on rER and transported to GA for further processing.
Function: Digestion of materials taken into cells
-> Food particles are engulfed by endocytosis to form food vacuoles, which 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.
-> It can also be a defence mechanism against bacteria in certain cell types; the result of fusion is known as a phagocytic vacuole.
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Structure: Due to the acidic (pH 5) nature of lysosome contents and hydrolytic activity of enclosed enzymes, lysosomal contents must be prevented from spilling into the cytoplasm under normal cell conditions.
Function: Therefore, segregation of contents within the membrane provides optimal pH for hydrolytic reactions and protects cellular contents from hydrolysis.
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Function:
Autophagy of worn-out organelles:
-> Unwanted structures within the cell are enclosed by a membrane of unknown origin, forming a vesicle. This vesicle then fuses with the lysosome to form an autophagic vacuole.
Autolysis:
-> Cells can self-destruct in a controlled manner when programmed to, in
apoptosis. It is a normal event in development and differentiation.
-> Can also occur when a cell 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.
Vacuole
Animal cells:
-> small, mobile organelles that serve to house and transport substances
Plant cells:
-> Storage of organic compounds (e.g. proteins) and inorganic ions (e.g. K+ and Cl-)
-> Disposal site for toxic metabolic by-products
-> Contains pigments (e.g. red and blue pigments that colour the petals)
-> Plant protection by accumulating compounds that are toxic or unpalatable to consumers
-> 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.
> large central vacuole surrounded by a single membrane called tonoplast
Nucleus
- size of between 5 - 20 μm.
Function:
encloses genetic material and protects DNA from metabolically active cytoplasm.
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Nuclear envelope:
- double membrane
- Outer membrane of the nucleus continuous with endoplasmic reticulum membrane
- inner and outer membranes are continuous with each other and the region between them is known as the perinuclear space, which is continuous with the ER lumen.
Function:
-> separates the contents of the nucleus from the cytoplasm
-> perforated by nuclear pores, made up of a large protein complex, which allow macromolecules such as mRNA and rRNA to exit the nucleus, and proteins e.g. enzymes to enter and exit the nucleus. (Exchange of substances between the nucleus and cytoplasm)
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Nucleoplasm:
- An aqueous matrix within the nucleus containing proteins, metabolites, ions, RNA and chromatin
- Chromatin is composed of coils of DNA wound around basic protein known as histones and exist in 2 forms:
- Loosely coiled chromatin known as euchromatin, which appears as light-coloured patches in the electron micrograph of the nucleus.
- Tightly coiled chromatin known as heterochromatin, which appears as dark-coloured patches in the electron micrograph of the nucleus
Nucleolus:
- composed of DNA carrying rRNA genes, RNA and protein, which functions to synthesize ribosomal RNA (rRNA) that forms a component of ribosomes
Cell centrifugation sequence:
Cell centrifugation: nuclei > chloroplasts > mitochondria > lysosomes > endoplasmic reticulum > ribosomes
ribosomes
Function: site of protein synthesis in both eukaryotes and prokaryotes.
Eukaryotic ribosomes:
-> about 30 nm in diameter, 80S (S stands for Svedberg units)
Prokaryotic ribosomes:
-> slightly smaller than 30 nm, 70S ribosomes
Consists of two subunits, the
large ribosomal subunit and the small ribosomal subunit, which are made up of proteins and ribosomal RNA (rRNA)
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Found in four cellular locations:
● attached to the rough endoplasmic reticulum (80S)
-> Called bound ribosomes. Bound ribosomes synthesise proteins that are destined for export, insertion into membrane or targeted to various membrane-bound organelles
● free in the cytosol (80S)
-> called free ribosomes. Free ribosomes synthesise proteins which remain in the cell, functions within the cytosol or within the nucleus
● in the mitochondrial matrix (70S)
● in the chloroplast stroma (70S)
Cytoskeleton
An intricate 3 dimensional array of interconnected filaments and tubules that permeates the cytosol of eukaryotes
Functions:
- Organise structures and activities of the cell
- 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 (cilia and flagella)
Microtubules are used in structural support. Specialised arrangements of microtubules are used for varying functions. These arrangements of microtubules arise from microtubule organizing centres (MTOCs), which give anchorage and orientation to the microtubule assembly. Microtubules are also involved in transportation of molecules from one site in the cell to another.
Microfilaments are involved in the movement of cells, e.g. amoeboid crawling, and changes in cell shape.
Intermediate filaments may stabilise organelles, like the nucleus, or they may be involved in specialised cell junctions.
Centrioles
Present in animal cells, absent in plant cells.
Located near the nucleus, in a region known as the centrosome that contains specialised proteins required for microtubule assembly.
Found in pairs at right angles to each other, each member of the pair consisting of nine triplets of microtubules arranged in a ring.
Before cell division, each centriole replicates itself and moves to opposite poles of the cell. The
centrosome act as MTOC (microtubule organising centre) for the formation of spindle fibres that play a role in nuclear division.
Cell wall
- relatively rigid and inflexible structure consisting mainly of the structural polysaccharide cellulose. It is secreted by the plant cell from within, and supports and defines the shape of plant tissues.
Functions:
- Protecting the cells from both mechanical injury and invasion
- Cellulose’s high tensile strength 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.
- 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 polysaccharides such as pectin that helps the cells adhere together.
Cell fractionation
- Homogenisation:
- Disruption of cells and release of the individual cellular components
- To break tissues into small fragments and to release organelles
Methods:
- To mechanically rupture the cell membrane, the following methods may be employed:
-> Mortar, pestle and abrasive material (e.g. sand, silica)
-> Homogeniser / blender
-> Pressure cell
-> Ultrasound
-> Osmotic lysis using a hyperosmotic homogenisation medium to rupture protoplasts
- To chemically dissolve the membrane phospholipids, various detergents may be used. - Differential centrifugation: Purification / separation of cell components
- To separate organelles using centrifugation
- The homogenate is subjected to progressively increasing speeds and duration of centrifugation to separate particles in descending order of size and density.
- The size, shape and density of the organelle all affect its sedimentation coefficient (which is measured in Svedberg units, S)
- The faster the rotation, the greater the gravitational force generated and the smaller the particles which will be sedimented.
- After each speed, the supernatant (liquid above the pellet) can be drawn off and re-centrifuged at a higher speed and for a longer duration.
- A series of pellets containing cell organelles of smaller and smaller size can therefore be obtained.
Precautions:
- To maintain the integrity of organelles, homogenisation is done in:
● an isotonic medium containing sucrose, mannitol or sorbitol
● a buffer solution to maintain suitable pH
● a temperature of 4dgC to inhibit protease activity
