2.1.1 - cell structure Flashcards
describe the structure of the nucleus
- split into 3 sections: nuclear envelope, nucleoplasm, nucleolus
- nuclear envelope -> semipermeable double membrane
- small gaps between nuclear envelope known as nuclear pores -> allow substances to enter/exit
- dense nucleolus -> made of RNA
- contains chromosomes which form a tangle known as chromatin (stains)
describe the function of the nucleus
- contains DNA that carries code that instructs the cell about making proteins
- controls cellular processes (e.g mRNA transcription)
describe the structure and function of the endoplasmic reticulum (ER)
- cisternae: network of tubules, membranes and flattened sacs
- extends from cell surface membrane and connects to nuclear envelope
- rER (rough): many ribosomes attached for protein synthesis and transport
- sER (smooth): different roles in different cells, lipid synthesis
describe the structure of the Golgi apparatus
- stack of curved membranes enclosing a series of flattened sacs
- constantly changing structure
- cis face -> aligns with rER, vesicles fuse forming a new layer
- trans face -> sacs break down and vesicles bud off via exocytosis
describe the function of the Golgi apparatus
- modifies and packages proteins into required product for export
- synthesises glycoproteins
describe the structure and function of ribosomes
- formed of proteins and rRNA
- not membrane bound, often bind to rER
- 2 subunits -> large which joins amino acids and small that has mRNA binding site
describe the structure and function of mitochondrion
- surrounded by double membrane
- folded inner membrane forms cristae
- inside is the fluid ‘matrix’ - (Krebs cycle)
- site of aerobic respiration, make ATP (energy currency)
describe the structure of a chloroplast
- double membrane isolates reactions from the rest of the cell
- contains membranes called ‘grana’ (contain chlorophyll) which form stacks called ‘thylakoids’
- stroma -> fluid filled matrix
- intergranal lamellae: tubes attack thylakoids in adjacent grana
state the function of chloroplasts
site of photosynthesis to convert solar energy to chemical energy
describe the structure of a lysosome
- specialised secretory vesicles
- sac surrounded by
single membrane - contains proteins that function as digestive enzymes
- glycoprotein coat protects interior
describe the function of a lysosome
- fuse with other vesicles that contain something that needs to be digested
- help destroy worn out/unwanted organelles
- enzymes break down large molecules, producing soluble substances
describe the structure and function of a plant cell wall
- made of cellulose microfibrils for mechanical support
- composite structure -> great resistance to stretching forces
- are permeable and allow substances to pass through
- made of cellulose
- provides strength and maintains cell shape
describe the structure and function of centrioles
- located in centrosomes
- make and organise microtubules
- during cell division these form the spindle and are responsible for moving chromosomes and pulling them to opposite ends of the cell
describe the structure and function of the plasma/cell surface membrane
- allow selective movement of substances into/out cells
- phospholipid bilayer with intrinsic and extrinsic proteins embedded to allow more movement of substances
- isolates cytoplasm from extracellular environment
describe the structure and function of flagella
- rotates to propel (usually unicellular) organism
- movement is produced by microtubules sliding against each other
- hollow helical tube made of the protein flagellin
describe the structure and function of cilia
- hairlike protrusions on eukaryotic cells
- move back and forth rhythmically to sweep foreign substances away and enable the cell to move
- movement, like in flagella, is produced by microtubules sliding against each other
why is the cytoskeleton important?
- it provides mechanical strength
- it aids transport within cells
- it enables cell movement
what is the cytoskeleton made up of and how do these help?
- microfilaments -> allow cell to move + contract
- microtubules -> ‘scaffolding’ for the cell, tracks movement of organelles
- intermediary fibres -> provide mechanical strength
how do organelles work together for protein synthesis?
- the DNA from the nucleus is copied/converted into mRNA by the process of transcription
- this mRNA strand leaves the nucleus through a nuclear pore
- it then attaches to a ribosome on rER
- the ribosome ‘reads’ the genetic instructions contained in the mRNA and uses this code to synthesise an amino acid sequence by the process of translation
- the amino acid sequence is transported to the Golgi apparatus where it is modified into a protein and packaged into a secretory vesicle
- vesicle moves to the cell surface membrane with the help of the cytoskeleton
- vesicle fuses with cell surface membrane and releases protein by exocytosis
define ‘organelle’
a membrane bound subcellular structure that has one or more specific function to perform
define ‘eukaryotic’
cells with a nucleus and other membrane bound organelles; can be multicellular or unicellular
define ‘prokaryotic’
single celled organism with no nucleus or membrane bound organelles
define ‘endocytosis’
a cellular process in which substances are brought into the cell
define ‘exocytosis’
the process by which materials or substances are removed from, or transported out of cells
what is ATP?
- adenosine triphosphate
- energy carrying molecule found in cells of all living things
what is RNA?
- ribonucleic acid
- present in all living cells
- acts as a messenger carrying instructions from DNA for protein synthesis
compare eukaryotic and prokaryotic cells
- prokaryotic are smaller
- prokaryotic are always unicellular whereas eukaryotic is often muticellular
- prokaryotic has no nucleus or membrane bound organelles
- eukaryotic has mitochondria
- both can have a cell wall, but this is made from different things
- prokaryotics genetic information is in circular DNA without proteins whereas eukaryotic has linear DNA with proteins
- both have ribosomes but prokaryotics are 70S whereas eukaryotics are 80S
- prokaryotic reproduces by binary fission (asexual) whereas eukaryotic reproduces by mitosis and meiosis (either)
what is a ‘plasmid’?
- in prokaryotic cells only
- small, circular, short strands which carry DNA with few genes
what are ‘mesosomes’?
invaginations formed by cell surface membrane for special functions, such as the site of respiration
define ‘peptidoglycans’
- polymers of modified sugars, cross linked by short chains of polypeptides
- found in bacteria cell walls
describe the DNA in a prokaryotic cell
- fundamentally the same as in eukaryotes but packaged differently
- supercoiled so it is compact
- circular DNA and sometimes plasmids
- no nucleus
what is the difference in flagella in eukaryotes and prokaryotes?
- thinner in prokaryotes
- 9 +/- 2 arrangement in eukaryotes
- energy in eukaryotes comes from ATP, instead comes from chemiosmosis in prokaryotes
define ‘magnification’
the number of times larger an image appears compared to the real object/structure size
define ‘resolution’
the ability to distinguish between two separate points
name the 4 types of microscopes
- light/optical
- transmission electron
- scanning electron
- laser scanning (confocal)
describe how light microscopes work
- lenses focus rays of light and magnify the view of a thin slice of specimen
- different structures absorb different amounts of wavelength of light
- reflected light is transmitted to the observer via the objective lens and eyepiece
describe how a transmission electron microscope (TEM) works
- pass a high energy beam of electrons through a thin slice of specimen
- more dense structures appear darker since they absorb more electrons
- focus image onto fluorescent screen or photographic plate using magnetic lenses
describe how a scanning electron microscope (SEM) works
- focus a beam of electrons onto a specimens surface using electromagnetic lenses
- reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate
describe how a laser scanning confocal microscope works
- focus a laser beam onto a small area on a samples surface using objective lenses
- fluorophores in the sample emit photons
- photomultiplier tube amplifies the signal onto a detector; an image is produced pixel by pixel in the correct order
compare use of a TEM and SEM microscope
- both use a beam of electrons focused using magnets
- both require dehydrated specimens
- electrons pass through in TEM whereas they reflect off in SEM
- both are greyscale
- TEM is 2D whereas SEM is 3D
- TEM has a bigger magnification
- SEM has a bigger resolution
which microscope can be used on living specimens?
laser scanning confocal
what is a feature of the laser scanning confocal microscope?
it has deph selectivity
what is a disadvantage of a light microscope?
it only has 2 magnifications
order from smallest to largest the different types of microscopes according to their magnifications
light, SEM, TEM
order from smallest to largest the different types of microscopes according to their resolution
TEM, SEM, light
define ‘photomicrograph’
a photograph of a specimen taken at magnification using a microscope
define ‘differential staining’
using specific stains to distinguish different subcellular structures
outline how a student could prepare a temporary mount of tissue for a light microscope
- obtain thin section of tissue
- place tissue in a drop of water
- stain tissue on a slide to make structures visible
- add coverslip (using mounted needle at 45 degrees) to avoid trapping air bubbles
how should the field of view in microscopy be recorded?
- draw a diagram with a sharp pencil
- do not sketch rough lines or use shading
- ensure proportions are correct
- annotate visible structures
- include a scale or magnification
give the equation for magnification
magnification = image size ÷ actual size
why do samples need to be stained for light microscopes?
- coloured dye binds to structure
- contrast between darker and lighter stained parts distinguishes separate structures
what are the different ways of preparing slides?
- dry mount -> solid specimen and thinly cut, e.g pollen, insects etc
- wet mount -> aquatic or stain specimens, cover slips
- smear slides -> cover slip is slid over the sample, e.g blood
explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure
- place micrometre on stage to calibrate eyepiece graticule
- line up scales on graticule and micrometer
- count how many graticule divisions are in 100µm on the micrometer
- length of one eyepiece division = 100µm / number of divisions
- use calibrated values to calculate actual length of structures
