Cells Flashcards
Define the term eukaryotic cell
DNA is contained in a nucleus, contains membrane-bound organelles.
Define the term prokaryotic cell
DNA is “free” in cytoplasm, no organelles
Describe the structure and function of the cell-surface membrane
‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded.
Explain the role of cholesterol in the cell-surface membrane
Cholesterol: steroid molecule connects phospholipids & reduces fluidity
Explain the role of glycoproteins in the cell-surface membrane
Cell signalling, cell recognition & binding cells together
Explain the role of glycolipids in the cell-surface membrane
Cell signalling & cell recognition
Describe the structure of the nucleus
Surrounded by nuclear envelope.
Nuclear pores allow substances to enter/exit
Dense nucleolus made of RNA & proteins assembles ribosomes.
Describe the function of the nucleus
Contains DNA coiled around chromatin into chromosomes
Controls cellular processes: gene expression determines specialisation & site of mRNA transcription, mitosis, semi-conservative replication.
Describe the structure of a mitochondrion
Surrounded by double membrane folded inner membrane forms cristae: site of electron transport chain
Fluid matrix: contains mitochondrial DNA, respiratory enzymes
Describe the structure of a chloroplast
Vesicular plastid with double membrane.
Thylakoids: flattened discs stack to form grana
Intergranal lamellae: tubes attach thylakoids in adjacent grana.
Stroma: fluid-filled matrix
State the function of mitochondria
Site of aerobic respiration to produce ATP.
State the function of chloroplasts
site of photosynthesis to convert solar energy to chemical energy
Describe the structure and function of the Golgi apparatus
Planar stack of membrane-bound, flattened sacs cis face aligns with RER.
Molecules are processed in cisternae vesicles bud off trans face via exocytosis:
● modifies & packages proteins for export
● synthesises glycoproteins
Describe the structure and function of a lysosome
Sac surrounded by single membrane embedded H+ pump maintains acidic conditions contains digestive hydrolase enzymes glycoprotein coat protects cell interior:
● digests contents of phagosome
● exocytosis of digestive enzymes
Describe the structure and function of a ribosome.
Formed of protein & rRNA free in cytoplasm or attached to ER.
Site of protein synthesis via translation:
large subunit: joins amino acids
small subunit: contains mRNA binding site
Describe the structure and function of the endoplasmic reticulum (ER).
Cisternae: network of tubules & flattened sacs extends from cell membrane through cytoplasm & connects to nuclear envelope:
● Rough ER: many ribosomes attached for protein synthesis & transport.
● Smooth ER: lipid synthesis.
Describe the structure of the cell wall
Bacteria:
Made of the polysaccharide murein.
Plants:
Made of cellulose microfibrils
plasmodesmata allow molecules to pass between cells, middle lamella acts as boundary between adjacent cell walls.
State the functions of the cell wall.
● Mechanical strength and support.
● Physical barrier against pathogens.
● Part of apoplast pathway (plants) to enable easy diffusion of water.
Describe the structure and function of the cell vacuole in plants.
Surrounded by single membrane: tonoplast contains cell sap: mineral ions, water, enzymes, soluble pigments.
● Controls turgor pressure.
● Absorbs and hydrolyses potentially harmful substances to detoxify cytoplasm.
State the role of plasmids in prokaryotes.
Small ring of DNA that carries non-essential genes.
Can be exchanged between bacterial
cells via conjugation.
Why are viruses referred to as ‘particles’ instead of cells?
Acellular & non-living: no cytoplasm, cannot self-reproduce, no metabolism.
Describe the structure of a viral particle.
Linear genetic material (DNA or RNA) & viral enzymes e.g. reverse transcriptase.
Surrounded by capsid (protein coat made of capsomeres).
No cytoplasm.
Describe how optical microscopes work.
- Lenses focus rays of light and magnify the view of a thin slice of specimen.
- Different structures absorb different amounts and wavelengths of light.
- Reflected light is transmitted to the observer via the objective lens and eyepiece
Outline how a student could prepare a temporary mount of tissue for an optical microscope.
- Obtain thin section of tissue e.g. using ultratome or by maceration.
- Place plant tissue in a drop of water.
- Stain tissue on a slide to make structures visible.
- Add coverslip using mounted needle at 45° to avoid trapping air bubbles.
Suggest the advantages and limitations of using an optical microscope.
colour image
+ can show living structures
+ affordable apparatus
- 2D image
- lower resolution than electron microscopes = cannot see ultrastructure
Describe how a transmission electron microscope (TEM) works.
- Pass a high energy beam of electrons through 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.
Suggest the advantages and limitations of using a TEM.
+ electrons have shorter wavelength than light =
high resolution, so ultrastructure visible
+ high magnification (x 500000)
- 2D image
- requires a vacuum = cannot show living structures
- extensive preparation may introduce artefacts
- no colour image
Describe how a scanning electron microscope (SEM) works.
- Focus a beam of electrons onto a specimen’s surface using electromagnetic lenses.
- Reflected electrons hit a collecting device and are amplified to produce an image on a photographic plate.
Suggest the advantages and limitations of using an SEM.
+ 3D image
+ electrons have shorter wavelength than light =
high resolution
- requires a vacuum = cannot show living structures
- no colour image
- only shows outer surface
Define magnification
factor by which the image is larger than the actual specimen
Define resolution.
smallest separation distance at which 2 separate structures can be distinguished from one another
Explain how to use an eyepiece graticule and stage micrometer to measure the size of a structure.
- Place micrometer 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 1 eyepiece division = 100μm / number of divisions
- Use calibrated values to calculate actual length of structures.
Outline what happens during cell fractionation and ultracentrifugation.
- Mince and homogenize tissue to break open cells & release organelles.
- Filter homogenate to remove debris.
- Perform differential centrifugation:
a) Spin homogenate in centrifuge.
b) The most dense organelles in the mixture form a pellet.
c) Filter off the supernatant and spin again at a higher speed.
State the order of sedimentation of organelles during differential centrifugation.
most dense → least dense
nucleus → mitochondria → lysosomes → RER → plasma membrane → SER → ribosomes
Explain why fractionated cells are kept in a cold, buffered, isotonic solution.
cold: slow action of hydrolase enzymes.
buffered: maintain constant pH.
isotonic: prevent osmotic lysis/ shrinking of organelles.
State what the cell cycle is and outline its stages.
cycle of division with intermediate growth periods
- interphase
- mitosis or meiosis (nuclear division)
- cytokinesis (cytoplasmic division)
What is the difference between the cell cycle and mitosis?
Cell cycle includes growth period between divisions; mitosis is only 10% of the cycle & refers only to nuclear division.
Outline what happens during interphase.
G1: cell synthesises proteins for replication e.g. tubulin for spindle fibres & cell size doubles
S: DNA replicates = chromosomes consist of 2 sister chromatids joined at a centromere
G2: organelles divide