a2.2 cell structure

Question 1

what does cell theory state?

Answer 1

1. all living organisms are composed of cells
2. cells are the smallest units of life
3. all cells originate from pre-existing cells

Question 2

what are the equations that relate to microscopy?

Answer 2

total magnification = eyepiece lens x objective lens
image size = actual size x magnification

Question 3

what are stains?

Answer 3

to help visualise certain structures, we use stains. stains bind preferentially to particular organelles or areas on a cell, making that structure easier to see

Question 4

what are some examples of stains?

Answer 4

1. iodine: binds to starch in plant cells
2. methylene blue: binds to the nuclei in animal cells
3. gram staining: bacteria
4. digital micrographs: add colour digitally to help differentiate between structures

Question 5

what is a graticule?

Answer 5

a graticule is a scale that is put in the eyepiece. the widths of the divisions can vary, so we need to first decide what they represent. it is calibrated with a graduated slide (stage micrometer) which is divided into 100 10 micrometre sections

Question 6

what is the maximum magnification of a light microscope?

Answer 6

the resolution of a light microscope is limited by the wavelength of visible light. if we try to resolve smaller objects by increasing the magnification, we just get a blurry image. so the maximum magnification is usually x400 and are used to see cell structure

Question 7

what is the maximum magnification of an electron microscope?

Answer 7

beams of electrons have a much shorter wavelength than light so electron microscopes have a much higher resolution. some electron microscopes can magnify up to x1000000 and are used to see cell ultrastructure

Question 8

what are fluorescent dyes?

Answer 8

fluorescent dyes can be used in light microscopy. when the dye is added to the sample it will preferentially attach to certain structures (be absorbed). as in immunofluorescence, the labelled areas will appear as brightly coloured spots, allowing visualisation of the target molecule throughout the specimen

Question 9

what is immunofluorescence?

Answer 9

immunofluorescence is a technique used in light microscopy to better visualise certain structures. a fluorescent tag, called a fluorophore, is attached to antibodies specific for antigens on a structure or cell being viewed. when the antibody binds to the antigen, the structure is then ‘tagged’ with immunofluorescence. when a certain wavelength of light is shone onto the fluorescence tag, the tag will emit light of a different wavelength that can then appear as brightly coloured spots, allowing the visualisation of the location of these target molecules

Question 10

what is freeze fracture?

Answer 10

freeze fracture involves freezing a sample and then using a specialised tool to break the sample into small pieces. these small pieces are then observed using an electron microscope to see the internal structure. this is a particularly useful technique for being able to visualise structures that are not normally visible, such as the internal plasma membrane

Question 11

what is cryogenic electron microscopy?

Answer 11

cryogenic electron microscopy involves freezing a sample to cryogenic temperatures to fix the molecules, making them more firm or stable. the specimen is then viewed using electron microscopy. freezing the sample improves the resolution of the image formed and reduces damage that may occur from the electron beam. used to observe protein structure

Question 12

what is the plasma membrane?

Answer 12

-> an outer boundary that encloses all contents of the cell
-> controls entry & exit of substances
-> can pump in substances even if external concentration is very low
-> very effective at preventing entry of unwanted or even toxic substances
-> allows cell to maintain concentrations of substances which are very different from those of the surroundings
-> permeability relies on a lipid based structure
-> can lyse (due to pressure, viruses, autolysis, ect.) which can lead to a cells death

Question 13

what is the cytoplasm?

Answer 13

-> main component of the cytoplasm is water - many substances are dissolved or suspended in it
-> enzymes catalyse thousands of different chemical reactions which is the cells metabolism
-> metabolism provides the cell with energy and produces all proteins that make up it’s structure
-> proteins are easily damaged, so even when the cell is not growing, the cytoplasm must continually break down and replace them

Question 14

what are ribosomes?

Answer 14

in both prokaryotes and eukaryotes, ribosomes:
-> catalyse the synthesis of proteins during translation
-> are composed of two subunits that come together to form a functioning structure
prokaryotes have 70s ribosomes, but eukaryotes have 80s ribosomes

Question 15

what is the cell structure of prokaryotes?

Answer 15

-> plasma membrane
-> cytoplasm
-> no cytoplasmic organelles apart from 70s ribosomes
-> cell wall (provides shape, prevents lysis, made of peptidoglycan)
-> pili (enable to cell to attach to surfaces and swap DNA with other cells)
-> capsule (polysaccharide layer used to adhere to other surfaces)
-> flagellum (long extensions used in cell locomotion)
-> no nucleus, instead has one circular DNA molecule found in the nucleoid (lacks proteins & ribosomes)
-> DNA is naked and not associated with proteins

Question 16

how do prokaryotic cells reproduce?

Answer 16

prokaryotic cells divide by binary fission. this creates two equal sized and genetically identical cells
1. replication of DNA
2. movement of copies to two areas of the cytoplasm
3. the cytoplasm splits and the membrane reforms around the two new cells

Question 17

what is the nucleus?

Answer 17

-> contains the DNA which stores information for making proteins via transcription and translation
-> contains the nucleolus, which is where ribosome subunits are made
-> has a double membrane with pores through it which allows eukaryotic cells to separate the activities of gene transcription and translation

Question 18

what are ribosomes like in eukaryotes?

Answer 18

ribosomes are either:
-> free: floating in the cytoplasm synthesising polypeptides used within the cell
-> bound: attached to the rough ER, synthesising polypeptides that are secreted from the cell or become integral proteins in the cell membrane

Question 19

what is the rough endoplasmic reticulum?

Answer 19

-> the rER is a series of connected, flattened membrane sacs that play a central role in the synthesis and transport of polypeptides
-> has bound ribosomes with synthesise the polypeptide and release it to the inside of the rER
-> the rER membrane is continuous with the nuclear envelope, which surrounds the cell nucleus

Question 20

what is the smooth endoplasmic reticulum?

Answer 20

-> the sER is a series of connected, flattened membrane sacs that are continuous with the rER
-> in contrast to the rER, sER lacks ribosomes and is not involved in protein synthesis
-> the main functions of sER are the synthesis of phospholipids and cholesterol for the formation and repair of membranes

Question 21

what is the golgi apparatus?

Answer 21

-> the GA modifies polypeptides into their functional state
-> the GA sorts, concentrates and packs proteins into vesicles
-> depending on the contents, the vesicles are dispatched to one of three destinations: within the cell to lysosomes, the plasma membrane, or outside the cell via exocytosis

Question 22

what are vesicles?

Answer 22

-> vesicles are membrane bound sacs that contain and transport materials within cells
-> transport vesicles move molecules between locations inside the cell by budding off one organelle compartment and fusing with another
-> secretory vesicles secrete molecules from the cell via exocytosis. they are also how new phospholipids are added to the cell membrane

Question 23

what is a lysosome?

Answer 23

-> lysosomes are small spherical organelles, enclosed by a single membrane. they contain enzymes that work in oxygen poor areas and lower pH
-> the enzymes digest large molecules to degrade and recycle the components of the cells own organelles when they are old or damaged, or if the cell is ‘starving’ in the absence of nutrients
-> it also has an immune defense function, by digesting pathogens that have been engulfed by phagocytes

Question 24

what is the mitochondria?

Answer 24

-> mitochondria are adapted for production of ATP by cellular respiration
-> the mitochondrion is surrounded by a double membrane
-> mitochondria evolved by endosymbiosis

Question 25

what are chloroplasts?

Answer 25

-> chloroplasts are adapted for photosynthesis, which captures light energy and uses it with water and carbon dioxide to produce glucose -> within the chloroplasts are light-absorbing pigments such as chlorophyll, which give the chloroplast its characteristic green colour -> chloroplasts evolved by endosymbiosis -> type of plastid: only in plants

Question 26

what are vacuoles?

Answer 26

-> mature plant cells have a central vacuole that occupies 30% - 90% of the volume of the cell -> in addition to water storage, the main role of the vacuole is to maintain turgor pressure against the cell wall. the turgor pressure is the mechanism that plants use to remain upright -> in animals, plants and fungi

Question 27

what is the cytoskeleton?

Answer 27

-> present in both prokaryotic and eukaryotic cells, the cytoskeleton is not considered to be an organelle -> the cytoskeleton helps cells maintain their shape, organises cell parts and enables cells to move and divide

Question 28

what are microtubules?

Answer 28

-> microtubules are polymers of a protein called tubulin and form part of the cytoskeleton -> microtubules are used for the intracellular transport of organelles and the separation of chromosomes during mitosis

Question 29

what are centrioles?

Answer 29

-> centrioles are paired, cylindrical shaped organelles composed of nine groups of three microtubules organised with radial symmetry -> functions of centrioles include: arrangement of the mitotic spindle during cell division and serving as anchor points for microtubules in the cytoplasm -> in animals and plants, but not fungi

Question 30

what are cilia and flagella?

Answer 30

-> cilia and flagella are extensions from the cell surface which aid in cell movement. they are formed from modified centrioles called a basal body -> in animals and plants (absent in conifers and flowering plants), but not fungi

Question 31

what are the definitions of the common processes carried out by all life?

Answer 31

1. homeostasis: the maintenance of a constant internal environment 2. metabolism: all biochemical reactions occurring in an organism 3. nutrition: supplying nutrients needed for energy, growth and repair 4. excretion: removal of metabolic waste products 5. growth: increase in size or number of cells 6. response to stimuli: perception of stimuli and carrying out appropriate actions in response 7. reproduction: sexual or asexual production of offspring

Question 32

what are the characteristics of a paramecium that enable it to perform the functions of life?

Answer 32

1. homeostasis: excess water is collected into contractile vacuoles and then expelled through the cell membrane 2. metabolism: contains dissolved enzymes 3. nutrition: heterotrophs, eat smaller unicellular organisms 4. excretion: waste products removed via an anal pore 5. growth: grows until it reaches a maximum SA:V 6. response: beats cilia to move towards environmental changes 7. reproduction: divides via mitosis (asexual) or two will fuse and then divide (sexual)

Question 33

what are the characteristics of a chlamydomonas that enable it to perform the functions of life?

Answer 33

1. homeostasis: excess water is collected into contractile vacuoles and then expelled through the cell membrane 2. metabolism: contains dissolved enzymes 3. nutrition: autotroph, photosynthesis 4. excretion: waste products of photosynthesis diffuses out the cell membrane 5. growth: grows until it reaches a maximum SA:V 6. response: light sensitive eyespot senses light and moves towards it 7. reproduction: divides via mitosis (asexual) or two will fuse and then divide (sexual)

Question 34

why are red blood cells atypical cells?

Answer 34

-> eukaryotic cells without a nucleus or mitochondria -> during their maturation, they discard their nucleus and mitochondria. this makes their cells very small, increasing their SA:V for efficient gas exchange and the ability to move through the narrow capillary vessels

Question 35

why are aseptate fungal hyphae atypical cells?

Answer 35

-> hyphae are the tubular projections of multicellular fungi that form an underground network. fungal hyphae are sometimes not divided up into individual cells (called aseptate hyphae), resulting in a continuous cytoplasm along the length of the hyphae. -> aseptate hyphae are not made of clearly defined individual cells, rather continuous structures with multiple nuclei

Question 36

why are skeletal muscles fibers atypical cells?

Answer 36

-> skeletal muscle fiber cells result from the fusion of multiple cells. this results in a single large cell with multiple nuclei

Question 37

why are phloem sieve tubes atypical cells?

Answer 37

-> sieve tube elements are specialised cells that are part of the phloem, the tissue that transports organic compounds made during photosynthesis throughout a plant -> sieve tube elements lose their nucleus and other organelles during their development. this allows the cells to have more space for transport of phloem sap

Question 38

what is endosymbiosis?

Answer 38

symbiosis is an interaction between two different organisms living in close physical association, typically to the advantage of both. in endosymbiosis, one cell lived within the other and became increasingly interdependent until the unit could only exist as a whole

Question 39

how has endosymbiosis contributed to the evolution of eukaryotic cells?

Answer 39

the theory states that mitochondria were once free-living prokaryotes that developed aerobic respiration. larger prokaryotes that could only respire anaerobically took them in by endocytosis. instead of killing and digesting them, they allowed them to live in their cytoplasm as endosymbionts. the endosymbionts aerobic respiration supplied energy to the host far more efficiently than its own anaerobic one and the host supplied the endosymbiont with food. they eventually evolved into mitochondria. (same with chloroplasts!)

Question 40

what evidence is there for endosymbiosis?

Answer 40

mitchondria & chloroplasts: -> have a double membrane (expected if a cell with a single membrane is ingested by endocytosis) -> have their own genes, on circular DNA molecules like that of prokaryotes -> transcribe their own DNA and use mRNA to synthesise some of their own proteins -> have their own 70s ribosomes, typical of some prokaryotes -> are only produced by the division of pre-existing mitochondria and chloroplasts

Question 41

what is cell differentiation?

Answer 41

multicellular organisms have an advantage as cells can develop differently to perform different functions. specialised cells only develop the features they need to carry out their functions, making them more efficient. in human cells, about 4000 genes are active in all cell types - these are 'housekeeping' genes and are not associated with specific roles. other genes vary in the expression and are only ever active in one type of cell. this is the process of cell differentiation. in differentiated cells, different genes are 'switched on' and expressed, so the cell makes particular proteins and other gene products.

Question 42

how did multicellularity evolve?

Answer 42

all plants and animals are multicellular. multicellularity has evolved independently more than once in the origins of plants and at least once in animals. many fungi and eukaryotic algae are multicellular. most cells in a multicellular organism have lost the ability to live independently or to divide. there are several advantages to multicellularity: 1. organisms tend to have longer life spans, as the death of one cells does not prevent their continued survival 2. organisms are generally larger, so they can exploit niches that unicellular organisms cannot 3. it allows for complexity as there can be differentiation of cell types