Cell Structure Flashcards
Define the terms eukaryotic and prokaryotic cells.
- Eukaryotic: DNA is contained in a nucleus, contains membrane-bound specialised organalles.
- Prokaryotic: DNA is ‘free’ into cytoplasm, no organelles e.g. bacteria and archaea.
State the relationship between a system and specialised cells.
Specialised cells –> tissues that perform a specific function –> organs made of several tissue types –> organ systems.
Describe the structure and function of the cell-surface membrane.
‘Fluid mosaic’ phospholipid bilayer with extrinsic & intrinsic proteins embedded.
- isolates cytoplasm from extracellular environment.
- selectively permeable to regulate transport of substances.
- involved in cell signalling / cell recognition.
Explain the role of cholesterol, glycoproteins & glycolipids in the cell-surface membrane.
- Cholesterol; steroid molecule connects phospholipids & reduces fluidity.
- Glycoproteins; cell signalling, cell recognition (antigens) & binding cells together.
- Glycoproteins; cell signalling and cell recognition.
Describe the structure of the nucleus.
- Surrounded by nuclear envelope, a semi-permeable double membrane.
- 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 and 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, lipids and proteins.
Describe the structure of a chloroplast.
- Vesicular plastid with double membrane.
- Thylakoids: flattened discs stack to form Grana: contain photosystems with chlorophyll.
- Intergranal lamellae: tubes attach thylakoids in adjacent grana.
- Stroma: fluid-filled matrix.
State the function of mitochondria and chloroplasts.
- Mitochondria: site of aerobic respiration to produce ATP.
- 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 the cytoplasm or attached to the ER.
- site of protein synthesis via translation: large subunit joints amino acids, small subunit contains mRNA binding site.
Describe the structure and function of the endoplasmic reticulum (ER).
Cisternae: network of tubules & flattened sacs extend from cell membrane through cytoplasm & connect 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 polysaccharide murein.
- Plants: made off cellulose microfibrils. Plasmodesmata allow molecules to pass between cells, middle lamella acts as a 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
Explain some common cell adaptations.
- Folded membrane or microvilli increase surface area e.g., for diffusion.
- Many mitochondria = large amounts of ATP for active transport.
- Walls one cell thick to reduce distance of diffusion pathway.
State the role of plasmids in prokaryotes.
- Small ring of DNA that carries non-essential genes.
- Can be exchanged between bacterial cells via conjugation.
State the role of flagella in prokaryotes.
Rotating tail propels (usually unicellular).
State the role of the capsule in prokaryotes.
Polysaccharide layer:
- prevent desiccation
- acts as food reserve
- provides mechanical protection against phagocytosis & external chemicals
- sticks cells together
Compare eukaryotic and prokaryotic cells.
Both have:
- cell membrane
- cytoplasm
- ribosomes
Contrast eukaryotic and prokaryotic cells.
Prokaryotic:
- small cells & always unicellular
- no membrane-bound organelle & no nucleus
- circular DNA not associated with proteins
- 70s small ribosomes
- asexual reproduction (binary fission)
- cellulose cell wall / chitin
- capsule, sometimes plasmids & cytoskeleton
Eukaryotic:
- larger cells & often multicellular
- always have organelles & nucleus
- linear chromosomes associated with histones
- 80s larger ribosomes
- mitosis & meiosis (sexual and asexual)
- murein cell walls
- no capsule, plasmids & always cytoskeleton
Why are viruses referred to as ‘particles’ instead of cells?
Acellular & non-living: no cytoplasm, cannot self-produce, no metabolism.
Describe the structure of a viral particle.
- Linear genetic material (DNA / RNA) & viral enzymes e.g., reverse transcriptase.
- Surrounded by capsid (protein coat made of capsomeres).
- No cytoplasm.
Describe the structure of an enveloped virus.
- Simple virus surrounded by matrix protein.
- Matrix protein surrounded by envelop derived from cell membrane of host cell.
- Attachment proteins on surface.
State the role of the capsid on viral particles.
- Protect nucleic acid from degeneration by restriction endonucleases.
- Surface sites enable viral particle to bind & enter host cells or inject their genetic material.
State the role of attachment proteins on viral particles.
Enable viral particle to bind to complementary sites of host cell: entry via endosymbiosis
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 degrees to avoid trapping air bubbles.
Suggest the advantages and limitations of using an optical microscope.
+ colour change
+ can show living structures
+ affordable apparatus
- 2D image
- lower resolution than electron microscopes = cannot see ultrastructure
Describe how a transmission electron microscope 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 lens.
Suggest the advantages and limitations of using a TEM.
+ electrons have shorter wavelength than light = high resolution, so ultrastructure visible
+ high magnification (x500000)
- 2D imagine
- requires a vacuum = cannot show living structures
- extensive preparation may introduce artefacts
- no colour image
Describe how a scanning electron microscope 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 a 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
Describe magnification and resolution.
- Magnification: factor by which the image is larger than the actual specimen.
- Resolution: smallest separation distance at which 2 separate structures can be distinguished from one another.
Explain how to use an eyepiece graticule and stage micrometre to measure the size of a structure.
- Place micrometre on stage to calibrate eyepiece graticule.
- Line up scales on graticule and micrometre. Count how many graticule divisions are in 100um on the micrometre.
- Length of 1 eyepiece division - 100um / number of divisions.
- Use calibrated values to calculate actual length of structures.
State the equation to calculate the actual size of a structure from microscopy.
Actual size = image size / magnification
I = A x M
M = I / A
A = I / M
Outline what happens during a cell fractionation and ultracentrifugation.
- Mince and homogenize tissue to break open cells & release organelle.
- Filter homogenate to remove debris.
- Perform differential centrifugation:
a) spin homogenate in centrifuge
b) the densest 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 organelle 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 organelle.
