Topic 3 - Cell structure Flashcards
Principles of optical microscopes
Simple convex glass lenses used in pairs in a compound light microscope to focus an object at a short distance by 1st lens, then magnified by 2nd lens
Pros and cons of optical microscope
Pros:
- Cheap
- Images in colour
- No training required
- Live specimens
Cons:
- Low magnification
- Low resolution
- 2D images
Principles of transmission electron microscope
- Electron gin produces e- beam, focused onto the specimen by a contender electromagnet
- Beam passes through a thin section of the specimen from below. Parts absorb e- and appear dark; others let e- pass through and appear bright - produces image on screen - photomicrograph
Pros and cons of transmission electron microscope
Pros:
- High resolution images
- High magnification
- Visible internal structures
Cons:
- Expensive
- Training is required
- No colour images
- 2D images
- Only thin specimens
Principles of scanning electron microscope
- Beam of e- directed onto surface of specimen - passed back and forth across specimen
- e- scattered by specimen - scattering pattern analysis allows us to get a 3D image
Pros and cons of scanning electron microscope
Pros:
- 3D images
- High magnification
- High resolution
- Thick specimens
Cons:
- Expensive
- Training is required
- No colour images
How do you prepare a slide for an optical microscope
- Pipette a drop of water onto a slide
- Use tweezers to place a thin section of your specimen on top of the droplet
- Add a drop of a stain
- Add a cover slip - remove all air bubbles
What is the difference between magnification and resolution?
Magnification: Increasing the size of an image. Up until the limit of resolution, an increase in magnification = an increase in detail
Resolution: minimum mistaken apart that two objects can be for them to appear separate items
What is the formula to calculate magnification
Can you describe the principles of cell fractionation and ultracentrifugation in separating cell components?
- Homogenisation
- Tissue is broken up in a cold, isotonic buffer solution to release the organelles into a solution - Filtration
- The homogenised cell solution is filtered through a gauze
- This separates any large cell debris - Ultracentrifugation
- The cell fragments are poured into a test tube and placed in a centrifuge and spun at a low speed
- a thick sediment - the pellet - is at the bottom of the tube and the fluid above is the supernatant
- The supernatant is drained into a new tube and spun again at a higher speed
- a new pellet forms and again, the supernatant is drained off and spun again at an even higher speed
- this process is repeated at higher speeds each time until all the organelles are separated out
Why is a cold, isotonic buffer needed
Cold - to reduce enzyme activity that could break down organelles
Isotonic - same water potential as tissue sample - to prevent water moving in or out of the cells by osmosis, causing lysis
Buffered - to prevent changes in pH which could affect/denature enzymes
How are organelles separated out during centrifugation?
They are separated in order of mass and the order is usually
Nuclei
Mitochondria
Lysosomes
Endoplasmic reticulum
Ribosomes
Distinguishing features of eukaryotic cells
- Cytoplasm contains membrane bound organelles
- DNA is enclosed in a nucleus
general structure of animal cell
- Mitochondrion
- nucleus
- Nucleolus
- RER
- SER
- Golgi apparatus
- Golgi vesicle
- Cytoplasm
- Cell surface membrane
- Ribosomes
- Lysosome
General plant structure
Same as animal
- Chloroplast
- Cell vacuole
- Cell wall
Cell surface membrane
- Selectively permeable barrier between the cell and its environment –> enables control of passage
- Also contains molecules / receptors / antigens on surface allowing cell recognition / signalling
What does the nucleus consist of
Nuclear envelope:
- Double membrane
- Has nuclear pores which allow substances e.g. mRNA to move between nucleus and cytoplasm
Nucleolus:
- Makes ribosomes
Nucleoplasm: granular jelly like material that makes up the bulk of the nucleus
Protein bound, linear DNA:
- Chromatin = condensed
- Chromosome = highly condensed
Ribosomes
Two subunits = large subunit and small subunit, each of which contains ribosomal RNA and protein
- Not surrounded by a membrane but can be attached to RER
- Site of protein synthesis - translation
smooth endoplasmic reticulum
- No ribosomes
- Synthesises and processes lipids
- Synthesises and processes carbohydrates
Rough endoplasmic reticulum
- Ribosomes on surface synthesise proteins
- Proteins processed / folded and transport inside RER
- Proteins packaged into vesicles for transport e.g. to Golgi apparatus
Golgi apparatus and vesicles
Golgi apparatus = flattened membrane sacs
- Modifies/processes protein from RER
- e.g. protein + carbohydrate –> gylcoproteins
- Packages them into Golgi vesicles
- Produces lysosomes (a type of Golgi vesicle)
Golgi vesicle = small membrane sac
- Transports proteins / lipids to their required destination
- e.g. to the cell-surface membrane
Lysosomes
- Contains/releases lysozymes (hydrolytic enzymes)
- To break down / hydrolyse pathogens
- Or worn out cell components
Mitochondria
Components of the mitochondria:
- Outer membrane
- Cristae - inner membrane fold
- Matrix, containing: small / 70s ribosomes, circular DNA
- Site of aerobic respiration
- To produce ATP for energy release
Chloroplasts
Components of the chloroplast:
- Double membrane
- Stroma, containing: Thylakoid membrane, Small 70s ribosomes, Circular DNA, Starch granules / lipid droplets
- Lamella - thylakoid linking grana
- Grana - stacks of thylakoid
- Absorbs light energy for photosynthesis
- To produce organic substances e.g. carbohydrates / lipids
Cell wall
- Formed outside of the cell membrane
- Composed mainly of cellulose (a polysaccharide) in plants and algae
- Composed of chitin (a nitrogen containing polysaccharide) in fungi
- Provides mechanical strength to the cell
- So prevents cell changing shape or bursting under pressure due to osmosis
- Note = it is permeable to most substances (unlike the cell-surface membrane)
Cell vacuole
- Maintains turgor pressure in the cell, supporting the plant
- Contains cell sap - s store of sugars, amino acids, pigments and any waste chemicals
Organisation in complex multicellular organisms
- In complex multicellular organisms, eukaryotic cells become specialised for specific functions
- Specialised cells are organised into tissues, tissues into organs and organs into systems
Tissue
Group of specialised cells with a similar structure working together to perform a specific function, often with the same origin
Organ
Aggregations of tissues performing specific functions
Organ system
Group of organs working together to perform specific functions
Explaining adaptations of specialised eukaryotic cells
- Many cells need a Hugh rate of protein production - e.g. antibodies, enzymes, hormones:
Many ribosomes and rough endoplasmic reticulum
For a high rate of protein synthesis - Many cells need a high rate of ATP production - e.g. for active transport or muscle contraction:
Many mitochondria
For a high rate of aerobic respiration / ATP production
Distinguishing features of prokaryotic cells
- Cytoplasm lacks membrane-bound organelles
- So genetic material not enclosed in a nucleus
Structure of a general prokaryotic cell
Examples of prokaryotic organisms:
- Bacteria
- Archaea
Contrasting eukaryotic and prokaryotic cells
Viruses: acellular and non living
- Acellular - not made of or able to be divided into cells
- Non-living - unable to reproduce without a host cell, no metabolism
General structure of a virus particle
- Nucleic acids surrounded by a capsid (protein coat)
- Attachment proteins allow attachment to specific host cells
- No cytoplasm, ribosomes, cell wall, cell surface membrane etc.
- Some also surrounded by a lipid envelope
Interphase
- S phase - DNA replicates semi-conservatively leading to two sister chromatids
- G1 and G2 - number of organelles and volume of cytoplasm increases; protein synthesis; ATP content increased
Mitosis
- Parent cell divides = two genetically identical daughter cells, containing identical/exact copies of DNA of the parent cell
- Stages - ‘PMAT’
Prophase
- Chromosomes condense, becoming shorter and thicker = appear as two sister chromatids joined by a centromere
- Nuclear envelope breaks down and centrioles move to opposite poles forming spindle network
Metaphase
- Chromosomes align along equator
- Spindle fibres attach to chromosomes by centromeres
Telophase
- Chromosomes uncoil, becoming longer and thinner
- Nuclear envelope reforms = two nuclei
- Spindle fibres and centrioles break down
Anaphase
- Spindle fibres contract, pulling sister chromatids to opposite poles of the cell
- Centromere divides
Cytokinesis
- The division of the cytoplasm, usually occurs, producing two new cells
The importance of mitosis in the life of an organism
Parent cell divides to produces 2 genetically similar identical daughter cells for:
- Growth of multicellular organisms by increasing cell number
- Repairing damaged tissues / replacing cells
- Asexual reproduction
Recognising different stages of the cell cycle
- Interphase - C –> no chromosomes visible (visible nucleus)
- Prophase - B –> Chromosomes visible but randomly arranged
- Metaphase - D –> chromosomes lined up of the equator
- Anaphase - E –> chromatids (in two sets) being separated to opposite poles by spindles, V shape shows sister chromatids have been pulled apart at their centromeres
- Telophase - A –> chromosomes in two sets, one at each pole
Many cancer treatments are directed at controlling the rate of cell division
Disrupt the cell cycle - cell division / mitosis slows - tumour growth slows
- Prevent DNA replication –> prevent / slows down mitosis
- Disrupts spindle activity / formation –> chromosomes cant attach to spindle by their centromere –> sister chromatids cant be pulled to opposite poles of the cells –> prevents / slows mitosis
Con = Disrupt cell cycle of normal cells too, especially rapidly dividing ones e.g. cells in hair follicles
Pro = Drugs more effective against cancer cells because dividing uncontrollably / rapidly
Prokaryotic cells replicate by binary fission
- Circular DNA and plasmids replicate ( circular DNA replicates once, plasmids can be replicated many times)
- Cytoplasm expands (cells get bigger) as each DNA molecule moves to opposite poles of the cell
- Cytoplasm divides
- = two daughter cells, each with a single copy of DNA and a variable number of plasmids
Viral replication
Viruses don’t undergo cell division because they are non-living
1. Attachment protein binds to complementary receptor protein on surface of host cell
2. Inject nuclei acid (DNA/RNA) into host cell
3. Infected host cell replicates the virus particles
