Biology - B1 OCR Gateway

CELLS AND LIGHT MICROSCOPY NOTES:

• Eukaryotic cells - nucleus, complex, animal and plant cells, bigger than 10um (10,000mm)

• Prokaryotic cell - no nucleus, simpler, smaller than 10um, bacteria cells

Animals cells :

• Nucleus - has Chromosomes (DNA), control cell activity

• Cytosplasm - chemical reactions occure

• Mitochondria - cell respiratiion + has enzymes for this

• Ribosomes - protiens made here

• Cell membrane - secures + controls what leaves and enters cell (permeable membrane) + receptor molecules for cell communication

Plant cells (contain all animal structures) :

• Cell wall - holds cell together + made of cellulose

• Chloroplasts - inside is green chlorphyll → for photosynthesis

• Vacoule - contains cell sap → stores minerals

Bacteria cells :

• Chromosonal DNA - control cell activity + replication (no nucleus)

• Plasmids - extra DNA in small loops + genes for drug resistence

• Cell membrane - already covered

• Pili - little hairs allowing bacteria to stick to surface

• Slime Capsule - allows sticking to surfaces

Microscopes :

• Magnify objects

• Increase resolution (distinguishes space between 2 points)

• Light microscopes:

  Advantages: Cheaper, Able to use on living cells, Portable

  Disadvantages: lower magnification + resolution,

• Electron microscopes:

• Advantages: Highest magnification and reoslution, can see IN subcellular structures

• Disdvantages: Expensive, cannot be transported, cannot use on living cells

• TEMs microscope (transmission electron microscope) - images in flurecent black and white images on a computer by - firing electrons at the speciment, and when they pass through they’re detected, producing an image

• SEMs (scanning electron microscope) - 3D black and white images produced - electrons are fired at the sample and are reflected back from the specimen, producing the image

What are the advantages and disadvantages of light microscopes?

• Cheaper

• Easier to use

• Can use on living tissue

• Low magnification + resolution

What are the advantages and disadvantages of electron microscopes?

Answer: Advantages:

1. High magnification and resolution.

2. Ability to visualize subcellular structures.

Disadvantages:

1. Expensive

2. Complex to use

How do TEMs produce images?

• Electrons fired at the sample

• Pass through

• When detected = create image on computer

• Flourecent black and white image

What is the function of the cell membrane?

• Controls what enters and leaves the cell -

• Receptor molecules for cell communication

What is a key difference between prokaryotic and eukaryotic cells?

• Prokaryotic cells lack a nucleus and are simple cells

• Eukaryotic cells have a nucleus and are complex cells

What is the function of a plasmid?

• Small loops of extra DNA

• Drug resistence genes (can be passed on during replication

Which bacterial subcellular structure allows for replication?

• Chromosonal DNA

NOTES FOR LIGHT MICROSCOPY PRACTICAL:

• MEMORY TRICK FOR THIS = FEO

• Focusing knobs (Fine + Course adjustment knobs) - move the stage up or down to focus image

• Eye piece lense - look through + magnifys image

• Objective lense - magnifies image (change the magnification power)

Specimen preperation :

1. Prepare thin piece (light wont let light pass through if its thick - wont see the image)

2. Use a clean slide (with a drop of water to stick the specimen)

3. Tweezers to place specimen on the slide

4. Stain (if needed) → use if specimen is colourless → stains make it easier to see the specimen + highlight different structures/tissues

   Iodine → Plant Cells

   Methylyn blue → Animal cells

   Crystal Violot → Bacteria cells

5. Place cover slip on top

6. Lower cover slip onto slide (which has the specimen)

Viewing the speciment once prepared :

1. Lowest powered objective lense

2. Course adjustment knob to move stage up or down to focus the image

3. Adjust focus with fine adjustment knob

4. Use a greater magnification if the image isnt clear and repeat steps 1-4 again before increasing magnification

What is the use of the fine adjustment knob?

• Very slowly - stage up or down - focus image

Scientists use a light microscope to view the courlerless samples of a cell wall. Explain how they should prepare this practical.

• Prepare a thin sample of the cell wall

• Get a clean slide and use a pipette to place a drop of water to keep the sample steady

• With tweezers, place the sample on the slip

• Use a drop of IODINE to highlight the sample and make it easier to see

• Place a coverslip on top of the sample

• Place the slip containing the sample on the stage ready to observe

NOTES ON MORE LIGHT MICROSCOPY:

1. If you know the magnification of the objective and eyepiece lens, you can work out total magnification

   → Total magnification = Eyepiece lens x objective lens

   → (the magnification of the eyepiece and objective lens used)

2. If you dont know the magnification of the lens used, you can use the (magnified) image size, and also the ACTUAL image size

   → Magnification = magnified image size / real image size

3. Sometimes the question will ask you to leave the answer in mm or micrometres (um)

   → to go from mm to um, you multiple by 1000

   → to go from um to mm, you divide by 1000

How do you work out the total magnification of an image viewed if you’re given the eyepiece and objective lense?

• Multiply both lenses magnification together

How do you work out the magnification of an image if you’e not given the magnification of the lenses?

• Magnified image size / real image size

What is 1mm converted to um measurement?

• 1000um

Calculate the magnification of an image viewed with an eyepiece lens magnification of x8 and an objective lens magnification of x15?

• 8 × 15 = x120 total magnification

NOTES FOR DNA:

• DNA = all the genetic material in an organism

• Double Helix shape - made of 2 long strands (polymer)

• So DNA is a polymer

• Polymers are made up of many many monomers - DNA strands (the polymer) are made up of thousends of nucleotides (monomers)

• Nucelotides are made up of a Sugar, Phosphate and Base

• There are 4 dfferent bases:

  • Adenine

  • Thymine

  • Cytosine

  • Guanine

• So there are 4 different Nucleotides (the sugar and phospate never change)

• Each base forms a cross link between the 2 DNA strands

• Adenine and Thymine bind together and Cytosine and Gaunine bind together (this is called complementary base pairing)

• A gene is a particuler sequence of bases that code for a protien - for every 3 bases — theres an amino acid (the type of amino acid depends on the sequence of bases - A T G for example codes for one type of amino acid)

• A chain of amino acids makes up one protein, and this protien carries out a specific function (depending on the sequence of amino acids)

Why is DNA describe as a polymer?

• DNA is made up of monomer nucleotides

Describe the structure of DNA

• Double Helix

• 2 Polymer strands

• Each strand is made of monomer nucelotides

• A nucelotide is made up of a Sugar, Phosephate and a Base

• There are 4 different nucelotides only the base changes

• A T and C G pair together which allows a cross lonk between the 2 strands

• When they pair, this is called complimentery base pairing