6 A Tour of the Cell Flashcards
Define Magnification.
The ratio of an object’s image size and its real size.
Define Resolution.
The smallest distance two points can be separated yet still discernable.
What are the forms of electron microscopy?
SEM (scanning electron microscope) and TEM (transmission electron microscope).
How do SEM microscopes operate?
In SEM the sample is coated in gold and electrons are fired at it. This stimulates the gold atoms to release secondary electrons which are detected.
How do TEM microscope operate?
Electrons are passed through the sample which has been stained with heavy metals so that certain regions have a higher electron density. The electrons from the TEM that pass near these regions are scattered more, allowing an image to be formed.
How can the various parts i.e. organelles of a cell be separated?
They can be ‘homogenised’ in a blender to disconnect all the parts and then placed in a centrifuge to separate them based on density.
What may be on the surface of a bacterium?
Cillia, Flagella and Fimbriae.
What are Cillia?
Fine projections on the surface of the bacterium that can be moved, like oars, for propulsion.
What are flagella?
A long projection that can be moved to propel the bacterium.
What are Fimbriae?
Fine projections that can be used to attach to things like limpets on a rock.
What structures of a prokaryote are used for protein synthesis?
- The nucleiod is where the DNA is located but is not membrane bound.
- Like eukaryotes, prokaryotes have ribosomes for protein synthesis.
Do prokaryotes have a cytoplasm?
Yes.
What surrounds the cytoplasm of a prokaryote? (in order from inside to out)
- A plasma membrane which surrounds the cytoplasm.
- Cell wall - a tough rigid structure
- Capsule - a jellylike outer coating found on some bacteria.
Which cells are typically large: prokaryote or eukaryote?
Eukaryote.
How large are bacteria typically?
1-5 micrometers in diameter.
How large is a typical eukaryotic cell?
10-100 micrometers in diameter.
What is the function of the plasma membrane?
To surround and protect the cytoplasm and its components and to regulate the intake and release of substances such as oxygen, nutrients and wastes.
Why are cells so small?
To provide them with a greater surface area to volume ration and thus allow rapid diffusion and thus exchange with their surroundings.
How can the SA/V ratio be maximised?
- Small cells
- Highly folded structures i.e. the cerebrum.
- Fine projections on the surface known as microvilli (not necessarily intestinal)
How does a eukaryote fundamentally differ from a prokaryote?
It has membrane bound organelles.
What are most biological membrane composed of?
A phospholipid bilayer i.e. two layers of phospholipids.
Where are flagella seen in eukaryotes?
In the sperm of plants and animals etc.
What are the basic organelles/structures which are present in both plant and animal cells?
Nucleus, Endoplasmic Reticulum, Cytoskeleton, Peroxisome, Mitochondria, Golgi apparatus and Ribosomes.
What is the nucleus composed of?
The nuclear envelope, the nucleolus and chromatin.
What is the nuclear envelope?
The double membrane that surrounds the nucleus which has pores. It is continuous (joined to) the ER.
What is the nucleolus?
A region (not membrane bound) inside the nucleus which is involved in the production of ribosomes.
How many nucleoli (plural of nucleolus) are there in a nucleus?
One or more.
What is chromatin?
A material found in the nucleus which consists of DNA and Proteins and is visible when the cells divides as individual chromosomes.
What is the endoplasmic reticulum?
A network fo membranous sacs and tubes which are involved in membrane synthesis etc. and as smooth and rough regions (smooth ER and rough ER)
What is the cytoskeleton?
A network of protein filaments and tubules that reinforce the cell’s shape and are involved in its movement.
What is the cytoskeleton composed of?
Microfilaments, Intermediate filaments and Microtubules.
What is the peroxisome?
An organelle with various functions such as breaking down cellular products.
What are the mitochondria?
Organelles where cellular respiration occurs to allow the regeneration of ATP, the energy currency of the cell.
Which organelles are found in animal but no plant cells?
Lysosomes and Centrosomes (with centrioles)
What is the lysosome?
An organelle found only in animal cells which contains enzymes that hydrolyse i.e. break down macromolecules.
What are Centrosomes?
An organelle where the cell’s microtubules are initiated. It contains a pair of centrioles which guide cell division.
What is the golgi apparatus?
An organelle which synthesises, modifies, sorts and secretes cell products.
What are ribosomes?
Structures (not organelles) which make proteins.
Where are ribosomes found?
They can either be found freely floating in the cytosol or bound to either the rough ER of the nuclear envelope.
What is cytosol?
The aqueous component of the cytoplasm in which organelles etc. are found.
How do yeast cells reproduce?
By budding (asexually)
What organelles/structures are found in plant but not animal cells?
Chloroplasts, a central vacuole, plasmodesmata and a cell wall.
What is the cell wall made of?
Cellulose.
What is the function of the central vacuole?
It is filled with sap to store water and nutrients etc. This sap may be under pressure, known as turgor pressure, which supports the plant’s structures.
Why do plants wilt when dehydrated?
The vacuole of their cells lose water and thus turgor pressure so no longer support the plant’s structures.
How does the shape of animal and plant cells differ? Why?
Plant cells are rectangular due to their rigid cell walls whereas animal cells are more oval and flexible.
What is special about the pores of the nuclear envelope?
Each pore has a protein structure called a ‘pore complex’ which regulates the entry and exit of proteins and RNA.
Besides the nuclear envelope, nucleoli and chromatin, what other main structures are there of the nucleus?
The nuclear lamina and the nuclear matrix.
What is the nuclear lamina?
A layer on the inside edge of the nuclear envelope which consists of a netlike array of protein filaments which maintain the shape of the nucleus.
What is the nuclear matrix?
A network fo protein fibres which extend through the nucleus, presumably to help organise the genetic information.
What does chromatin form?
Chromosomes.
How do chromosomes appear during and before division?
During division they are tightly coiled discrete structures. When not under division they are a tangled mess.
How does the nucleolus appear under an electron microscope?
A mass of densely stained fibres and granules.
What is the function of the nucleolus?
rRNA (ribosomal RNA) is synthesised from instruction in DNA.
Also, proteins that entered through the nuclear pores from the cytoplasm are assembled into the subunits of ribosomes?
How are ribosomes formed?
Proteins from the cytoplasm reach the nucleolus through the nuclear pores. Here they are assembled, along with rRNA, into large (60-s) and small subunits (40-s) of ribosomes.
This subunits then leave the nucleus through the pores into the cytoplasm. Here a small and a large subunit combine to form a ribosome.
What is the function of ribosomes? What cell will have most?
They are ‘protein factories’ therefore cells which secrete a large number of proteins i.e. pancreatic cell will have more of them.
Where can bound/free ribosomes be found?
Bound ribosomes can be found attached to the endoplasmic reticulum and nuclear envelope.
Free ribosomes are typically in the cytosol of the cell.
How do bound and free ribosomes differ structurally?
They have no difference and in fact can switch between roles.
How do bound and free ribosomes differ in terms of function?
Both synthesise proteins.
However most of the proteins produced by free ribosomes are used within the cytosol such as enzymes to hydrolyse any sugar taken in.
Bound ribosomes typically make proteins that are destined for use in an organelle i.e lysosome, for insertion into a membrane i.e. plasma membrane, or that are to be secreted out of the cell.
What are the components of the endomembrane system?
The nuclear envelope, ER, golgi apparatus, lysosomes and various vesicles and vacuoles.
What functions does endomembrane system perform?
Synthesis of proteins and the implantation of them into membranes and organelles. Also metabolism, the movement of lipids, and the detoxification of poisons.
How do products move through the endomembrane system?
Either through segments that are connected or in vesicles (membrane-bound sacs)
What is the ER composed of?
A network of membranous sac and tubules named cisternae.
This membrane surrounds the interior of the ER, named the ER lumen/cisternal space.
What is an analogy for the Golgi apparatus?
It acts as a warehouse to receive, sort and ship the products of the ER i.e. proteins.
What is the Golgi composed of?
Flattened membranous discs named cistern. Which are not connected.
How do products move to and through the Golgi?
Products arrive at in vesicles at the cis face of the Golgi stack.
These products move through the layers of cisternae and thus are transformed.
Eventually they reach the end of the golgi stack, known as the trans face, where they pinch of into new vesicles.
What are some specific products modified in the Golgi?
Glycoproteins formed in the ER have their carbohydrates modified i.e. some sugars monomers are removed and substituted with others.
Membrane phospholipids are also modified.
Besides modifying products what function does the Golgi have?
It manufactures some new non-protein products such as pectin which like modified products exit through the trans face.
What is the function of the Golgi?
To modify mainly protein products of the ER and to manufacture a few non-proteins.
How is the Golgi a non-static structure?
It undergoes ‘cisternal maturation’ in which the actual cisternae move from cis to trans face.
This is because the trans face is gradually degraded as vesicles pinch off and remove some membrane. Mean while vesicles which fuse with the cis face add new membrane.
Note: enzymes inside are recycled.
How does the Golgi ensure its products go to the right place?
On some products it adds markers such as phosphate groups to make them recognisable.
The vesicles which form may have extra molecules added to their surface which bind to receptors and thus identifying the contents.
How do the smooth ER and rough ER differ in basic function?
The broad function of the rough ER is to secrete the proteins made by ribosomes whereas the smooth ER undertakes many tasks.
What are the main functions of the smooth ER?
The synthesis of lipids, the metabolism of carbohydrates, detoxification of drugs and poisons, and the storage of calcium ions.
How does the smooth ER detoxify poisons?
Smooth ER’s i.e. in the liver’s cells, add hydroxyl groups to toxic molecules which makes them more soluble and thus easier to flush from the body.
How does the rough ER take the products the ribosomes that are attached to them?
As the ribosomes form the protein the polypeptide cain is threaded into the ER through pores. It entered the lumen of the ER where it folds into its native shape.
What form are most secretory proteins?
Glycoproteins which are proteins with a carbohydrate covalently bonded to them.
Besides protein formation, what function does the rough ER have?
The creation of membrane i.e. for vesicles.
What are the functions of the rough ER?
To production of proteins (by the attached ribosomes) and the formation of membranes.
What is a lysosome?
A membranous sac that contains hydrolytic enzymes that breakdown macromolecules.
What are the conditions inside the lysosome?
Acidic to allow the functioning of the enzymes inside.
Why would any enzymes that leak from a burst lysosome not be particularly dangerous to a cell?
Since the enzymes are used to the acidic lysosome if they are released they will not have a large effect as the cytosol is of a neutral pH.
When might a lysosome be used?
The vesicle that takes in the products of phagocytosis may bind to a lysosome allowing its enzymes to break the products down. The simpler molecules are then released into the cytoplasm etc. to be used by the cell.
Besides phagocytosis, when might lysosomes be used?
In autophagy, which is the recycling of damaged organelles etc. by breaking down into products that can be re-used i.e. amino acids.
What causes Tay-Sachs disease?
The lysosomes lack an enzyme to break down certain lipids which then accumulate, causing the cells of the brain to cease normal functioning.
What disease is caused by the improper functioning of lysosomes?
Tay-sachs as they lack the enzymes to break down a certain lipid, causing it to accumulate in the brain.
What are vacuoles?
Membrane bound sacs that can store products and through their selectively permeable membranes regulate the intake and release of these substances.
What are the forms of vacuoles?
Food vacuoles, contractile vacuoles and central vacuoles.
What form of vacuole can be found in animals?
Food vacuoles.
What are food vacuoles?
Vacuoles that are formed through phagocytosis i.e. when the cell engulfs macro molecules.
What are contractile vacuoles?
A type of vacuole found in many freshwater protests which contract to pump excess water out of the cell and thus maintain a correct ion/molecule concentration.
What purpose do typical vacuoles have?
They acts are stores of nutrients such as sugars. In plants and fungi, which have no lysosomes, they may contain hydrolytic enzymes.
What are central vacuoles?
Large vacuoles which in plants store sap that contains water and sugars etc. to act as a store.
Why are central vacuoles important?
They enable growth by expansion as they take on water and swell which increases the size of the individual cells and thus the stem etc. they form.
This swelling also creates a pressure, known as turgor pressure, which supports leafs etc. to stop them from wilting.
Why are chloroplasts and mitochondria evolutionarily significant?
Their integration is explained through the Endosymbiotic Theory.
What is the endosymbiotic theory?
The theory that mitochondria and chloroplasts were originally distinct prokaryotic organisms which were engulfed by a cell and through a symbiotic relationship lived together. Since cell theory dictates that all cells are formed from other cells all descendant cells have them.
What evidence is there for the endosymbiotic theory?
Chloroplasts and mitochondria both have their own circular DNA, their own ribosomes and also grow independently.
Note:
Some cells have one mitochondria, other i.e. those most active may have 100s or 1000s.
What is the structure of a mitochondrion?
It has an inner membrane that is folded to form cristae. The outer membrane is smooth and encloses the inner membrane.
What are the regions of the mitochondria called?
The area between the membranes is known as the inter membrane space whereas the are enclosed by the inner membrane is known as the matrix.
Where are mitochondrial ribosomes found?
Floating freely in the matrix.
What is the structure of a chloroplast?
A double membrane encloses a fluid named stroma.
Within the stroma are interconnected disk-shaped sacs names thylakoids. Some of the thylakoids are in stacks named grana (sing.: granum)
What are chloroplasts a type of?
Plastids
What are plastids?
A group of closely related organelles found only in plants.
What are the main types of plastids?
Chloroplasts, Amyloplasts and Chromoplast
What are amyloplasts?
Colourless plastid organelles found mostly in roots which store starch in the form of amylose.
What are chromoplasts?
Plastids which contain the pigments that make fruits and flowers orange or yellow.
How do peroxisomes break down substances?
They have enzymes which take hydrogen atoms from a substrate and thus denature it. These H atoms are then combine with oxygen to form Hydrogen Peroxide as a BYPRODUCT (it is the removal of H atoms, not the H2O2 which detoxifies)
What do peroxisomes typically break down?
Alcohol is detoxified in the liver by removing hydrogen atoms. Similarly fatty acids can be broken down into smaller molecules that can be used for energy.
How do peroxisomes stop the hydrogen peroxide from damaging them?
They have enzymes to break the hydrogen peroxide down into water.
What is a specialised form of peroxisome? What is their function?
Glyoxysomes that are found in the fat-storing tissues of plant seeds. They have enzymes that convert fatty acids to sugar. This provides a source of energy for the emerging seedling before it can perform photosynthesis.
Why might peroxisomes be endosymbiotic?
They grow larger by incorporating lipids and proteins made by the ER etc. and can then divide into two.
What are the primary functions of the cell cytoskeleton?
Provide structural support for the cell AND to provide ‘tracks’ along which substances can be transported.
Why is the cytoskeleton essential in animal cells?
They lack cell walls and turgor pressure so depend on them to remain in shape.
How does the cytoskeleton differ functionally from an animal skeleton?
It can be taken apart and reassembled repeatedly allowing it to dynamically change.
Besides structure, what does the cytoskeleton allow?
‘Cell motility’ which is the movement of the entire cell or its constituent parts i.e. organelles and substances.
How is cell motility achieved?
Vesicles, Organelles or the entire plasma membrane (uses filaments outside cell) attach to motor proteins which intern attach to MICROTUBULES or microfilaments. These motor proteins can, using ATP, ‘walk’ along these structures, causing movement.
Where is the movement of substances by motor proteins seen?
In axons it is used to move neurotransmitters to the axon terminal. In all cells it is used to move vesicles containing ER products to the Golgi.
What are the components of the cytoskeleton?
Microtubules, microfilaments and intermediate filaments.
What are microfilaments also known as?
Actin filaments
What are the components of the cytoskeleton in order of diameter?
Biggest: Microtubules, intermediate filaments and microfilaments (thinnest.)
In what cells are microtubules found?
All eukaryotic cells.
What is the structure of microtubules?
They are a hollow rod formed from a spiral of the globular protein tubulin.
Tubulin is in turn a dimer made of two subunit polypeptides: α-tubulin and β-tubulin
What is a dimer?
A molecule made up of two subunits.
How do microtubules adjust?
Their dimers can be taken off and then added again to adjust their length.
How is the motion of microtubules intrinsically regulated?
Due to the way the dimers are arranged one end of the microtubule is more active at gaining and losing dimers.
This end, known as the ‘plus end’ is therefore the one which undergoes the most change which ensures the lengthening is in the correct direction.
What are the centrioles?
Fibres composed of nine sets of triplet microtubules which are regulated by the centrosomes/
What functions do cilia have?
In many single celled eukaryotes they are used for locomotion. They are also used in multicellular organism to move external substances such as mucus by trachea cells.
Most cells in vertebrates each have a single cilium, known as the ‘primary cilium’ which has membrane proteins which acts as receptors for chemical signals.
In what functions are microtubules used?
- Maintenance of cell shape
- Mobility i.e. cilia and flagella
- Chromosome movement during cell division
- Organelle movements
With what cytoplasmic component are cilia and flagella operated?
Microtubules.
How do flagella and cilia differ in terms of their motion?
Cillia are like oars in that they have a power stroke, are cocked back, and then released again in a repeating fashion. This generates movement that is PERPENDICULAR to where they face.
Flagella, like the tail of a fish, beat rhythmically to generate motion in the axis they point.
What is the arrangement of microtubules in cilia and flagella?
9 pairs of microtubules are arranged in a ring around two central non-paired microtubules. This is known as a 9+2 arrangement.
What are the microtubules of the cilia/flagellum attached to?
The basal body which is similar to a centriole and thus prevent the cilia/flagellum from detaching.
How are the flagella/cillia moved?
The outer ring microtubule doublets of these structures are joined by ‘cross-linking proteins’ Motor proteins named dyenins are also attached to both tubules.
Using its two ‘feet’ the dyenins ‘walk’ along the tubules, causing them to slide past each other. However because they are joined by ‘cross-linking proteins’ the cilia/flagellum bend.
What is the structure of microfilaments?
They are a solid rod built from a double twisted chain (like a double helix) of actin.
What type of protein is actin?
Globular.
How do microtubules and microfilaments differ in the way they support the cell?
Microtubules support the cell by resisting compression like a rigid steel rod.
Microfilaments support the cell by bearing tension like the guy wires of an antenna.
In what cells are microfilaments found?
All eukaryotic.
Where are microfilaments found within the cytoplasm?
At the edges, called the cortex, near the membrane ‘cortical microfilaments’ give the fringes a more rigid structure.
In some cells these microfilaments form microvilli.
How do microfilaments initiate motion?
In muscle fibres Myosin filaments attach to the Actin-microfilaments. These myosin filaments have ‘heads’ which can ‘walk’ along the actin filament and thus cause contraction.
What is cytoplasmic streaming?
The fact that the cytosol is circulate through the cell like a current through the interaction of actin. This is important to evenly distribute nutrients amongst the cell.
What is the phenomena that ensures the equal distribution of solutes in the cytosol?
Cytoplasmic streaming
In what forms of motion are microfilaments used?
- Muscles
- The pseudopodia of amoebas
- Actin forms a contractile ring which pinches the cell into two during division.
In what functions do microfilaments participate?
- Motion (muscles,pseudopodia,contractile rings @ division)
- Structural support (under tension)
- Cytoplasmic streaming
What are intermediate filaments structurally?
A chain of various subunits such as keratin.
How do intermediate filaments move?
They generally do not and thus are more permanent, often existing even after cell death.
How do intermediate filaments act as structural support?
By bearing tension like guy wires on a tower.
Where do intermediate filaments provide structural support?
They hold organelles like the nucleus in place by anchoring them to the cytoplasm. They also strengthen axons and maintain the shape of all cells.
Besides structural support, what is the function of intermediate filaments?
The nuclear lamina is composed of them.
Beside support, what function does the cell wall have?
It prevents excessive uptake of water as it prevents the cell from swelling too much.
Besides plants, which organism have cell walls?
Prokaryotes (including bacteria), fungi and some protists.
How is the cell wall formed?
Microfibrils made of cellulose are formed by the enzyme cellulose synthase. These microfibrils are then excreted out the cytoplasm where they tangle to form the wall.
How are cell walls formed into a specific rectangular shape?
As the microfibrils are formed they are guided by microtubules so that the specific orientation they are place guides the growth of the cell and thus its shape.
How are plant cells kept together?
Between the cell walls of adjacent cells (the middle lamella) sticky polysaccharides named pectins are secreted to ‘glue’ the cells together.
At a cellular level how is wood formed?
Cells sometime produce additional harder layer of cell wall between the first (primary) cell wall and the plasma membrane.
What do animal cells have that is analogous to a cel wall?
An extracellular matrix (ECM)
What is the ECM composed of?
Mainly glycoproteins like collagen and other carbohydrate-containing molecules.
What is the primary component of ECM?
Collagen.
How are the collagen fibres of the ECM held together?
They are woven together with proteoglycan.
What are proteoglycans?
A molecule consisting of a small protein core with many carbohydrate chains.
How are cells attached to the ECM?
The cell membrane contains integrins which are proteins on the surface that span from outside the cell to the inside cytoskeleton.
These integrins are linked to the ECM by ‘ECM glycoproteins’ such as fibronectin.
What are the basic types of cell junction?
Plants: Plasmodesmata
Animals: Tight junctions. Desmosomes and Gap junctions.
Which cell junctions are found in plant cells?
Only plasmodesmata.
What is the function of plasmodesmata?
To act as channels through which nutrients and hormones etc. can pass between cells to coordinate actions etc. (bypasses the plasma membrane)
What is the function of tight junctions?
To tightly join cells to form a watertight seal.
Where are tight junctions seen in the body?
In intestines to prevent leakage etc.
What are desmosomes?
A form of cell junction that anchors cells together using intermediate fibres made of keratin/
What are gap junctions?
The form of animal cell junction that forms a channel between two cells to allow small substances to pass through for coordination etc.
Where are gap junctions seen?
In the heart to ensure coordinated contractions.
Where are membranes formed?
The rough ER
