2.1- Structure Of Cells Flashcards
1) List three ways eukaryotic cells differ from prokaryotic cells.
- membrane bound organelles
- DNA is enclosed in a nucleus
- Have larger ribosomes (80S) than prokaryotic cells
2) Prokaryotic cells include bacterial cells. List the differences between Prokaryotic and Eukaryotic cells.
- No membrane bound organelles (e.g. mitochondria, endoplasmic reticulum, golgi)
- No nucleus - instead they have a single circular DNA molecule that is free in the cytoplasm and is not associated with proteins
- Smaller ribosomes
- Has a murein cell wall, a capsule and plasmids
Nucleus
ccontaining chromosomes, consisting of protein-bound, linear DNA within the nucleoplasm. Contains one (nucleolus) or more nucleoli
EA, EP
Nuclear Envelope
Double membrane surrounding the nucleus with nuclear pores to let molecules in and out
EA, EP
Plasma Membrane
Description of structure: A phospholipid bilayer with hydrophllic phosphate heads facing outwards and hydrophobic fatty acid tails facing inwards.
Function: Involved in the transport of substances via diffusion or facilitated diffusion, active transport (with the aid of proteins) EA, EP, P
Cytoplasm
Where chemical reactions take place EA, EP, P
Ribosome
Where amino acids are joined together to make proteins (i.e. protein synthesis)
They have no membrane • EA, EP = have 80s ribosomes
• P = have 70s ribosomes
Mitochondria
Site of AEROBIC respiration Produces ATP (releases energy) Contains short, circular DNA (not associated with proteins) Has a double membrane. The inner membrane is folded forming cristae. Contains smaller (70S) ribosomes (thought to have evolved from prokaryotic cells.) EA, EP
Rough Endoplasmic Reticulum (RER)
Has ribosomes on their surface which are involved in protein synthesis
Proteins are also folded up inside the RER
EA, EP
Smooth endoplasmic reticulum
Synthesis and storage of molecules such as steroids and sterols EA, EP
Golgi Apparatus and golgi vesicles
Modifies proteins (e.g. by adding carbohydrate groups to form a glycoprotein or lipid groups to make a lipoprotein)
Stores proteins
Packages proteins into vesicles
Transport vesicles to cell surface EA, EP
Lysosomes
A vesicle that contains hydrolytic enzymes (lysozymes) which are used to digest molecules EA, EP
Chloroplast
Chlorophyll absorb light for photosynthesis to produce carbohydrates
Has a double membrane. Inside there are thylakoid membranes which can form a stack called a granum (pl. grana). The grana are linked by lamellae.
Contain starch grains EP and ALGAE
Cell Wall
For strength and support EP (cellulose) and P (Murein) Algae and fungi Vacuole
Vacuole
Tonoplast – a membrane surrounding the vacuole
When the vacuole is full of fluid the cell becomes turgid.
Temporary food store containing sugars and amino acids
Contains pigments (colour petals)
Flagella
For movement
Plasmids
Circular rings of DNA
(can reproduce independently and may give resistance to harmful chemicals such as antibiotics) P
Capsule
A ‘slime’ layer on the outside of the cell, made of polysaccharides
It provides protection for the cell and might help cells stick together P
Circular DNA
Genetic material in a prokaryote (not enclosed in a nucleus) P
- Define magnification
Magnification is how much bigger the image is than the specimen
- Define resolution
Resolution is the minimum distance apart that the two objects can be in order for them to appear as separate items.
- Describe the procedure to prepare a slide
- Add a drop of water to the slide
- Remove a thin section of tissue and place it onto the slide (flat as possible)
- Add 1 drop of iodine dissolved in potassium iodide to stain the sample (This is only correct if it is plant tissue)
- Lower a coverslip on top using a mounting needle
- Explain why it was important that the sections of tissue were thin
- A thin section allows more light through;
* allows a single layer of cells to be viewed.
- Explain why you should push down hard on the cover slip, but should not push the cover slip sideways.
- Push hard to squash the tissue to create a single layer of cells.
- Do not push sideways as this will cause the cells to roll together.
- A plant cell was observed with an optical microscope. Describe how the length of the cell could be estimated.
- Use a stage micrometer to help us calibrate the size of the eyepiece graticule.
- Measure the length of the plant cell with an eyepiece graticule.
Calibrating a stage micrometer
1) Find out the length of the stage micrometer? e.g. 10mm long = 10000m
2) Find out how many divisions there are? e.g 100 divisions
3) Work out how big each division is in m? e.g 10000/100 = 100m = 1 division
4) Place the stage micrometer on the stage
5) Line up the divisions on the eyepiece graticule with those of the micrometer
6) Lets say 100 units on the eyepiece graticule fit 30 units of the stage micrometer
7) Now work out the length of one eyepiece graticule unit in m
If the question is asking what is the actual length of the cell
8) Place your slide on the stage
9) Measure the length of the cell using eyepiece graticule units
10) Multiply the number of eyepiece graticule units by the length of one eyepiece graticule unit in m (part 7 – see above)
- Fill in the table below comparing light (optical) and electron microscopes
Scanning electron microscope Transmission electron microscope Light microscope
Images are black and white
Can use living specimens
Specimens are always dead
Highest resolution
Lowest resolution
Allows you to see the structures and details within cells
Have to cut cross sections of samples
Do not need thin sections to be cut
Shows the surface of the specimen
Electrons pass through the object to create an image on the screen below (the denser the sample, the darker it appears)
Electrons bounce off the surface of the object and create an image
There has to be a vacuum maintained inside the microscope (so that air particles do not interfere with the electrons)
Resolution of 20nm
Resolution of 0.1nm
Resolution of 0.2m
Can see the natural colours of the object
The microscopes are relatively small and mobile
These microscopes are very big and cannot be moved
2-D image
3-D image produced
False colour can be added
Use magnets to focus the beam of electrons
Use lenses to focus the light
- Why are electron microscopes used to view cells?
• They have a HIGH resolution
• because electrons have a shorter wavelength than light.
• This allows you to view internal structures/organelles of a cell.
(Remember using this – Light microscope = Low resolution = Longer wavelength)
- Name two structures in a eukaryotic cell that cannot be identified using an optical microscope.
• Mitochondrion / ribosome / endoplasmic reticulum / lysosome / cell-surface membrane
- Describe the principles and the limitations of using a transmission electron microscope to investigate cell structure.
. Electrons pass through / enter (thin) specimen;
2. Denser parts absorb more electrons;
3. (So) denser parts appear darker;
4. Electrons have short wavelength so give high resolution;
Limitations:
5. Cannot look at living material / Must be in a vacuum;
6. Specimen must be (very) thin;
7. Artefacts present;
8. Complex staining method / complex / long preparation time;
9. Image not in 3D / only 2D images produced.
- What is the advantage of TEM (transmission) compared to SEM (scanning) is
- Higher resolution
* Allows internal structures within cells to be seen
- What is the advantages of SEM (scanning) compared to TEM is
- Thin sections do not need to be prepared
- shows surface of specimen
- can have 3-D images
- Write an equation to calculate magnification and draw a formula triangle
Magnification = size of image
size of object
- How many micrometers in a millimeter?
1mm = 1000m
- What is meant by ‘cell fractionation’ and why would scientists want to do it?
- Separating out the contents of a cell into the different ‘fractions’ (i.e. different parts).
- This usually means separating out the different organelles.
- This is useful for scientists because it allows them to study individual organelles
- What is homogenisation? What is the point of it?
• Using a blender (or a homogenizer) to break open the cell membrane to release all of the organelles inside
- Why is the solution filtered?
• To remove any whole cells or large cell debris before carrying out centrifugation
- The cells are kept in a specific type of solution: (you must memorise the whole sentence!)
A. Why is it cold? To slow down enzyme activity to prevent digestion of organelles
B. Why is it isotonic? To prevent osmosis, so that the ORGANELLES do not shrivel or burst
C. Why is it buffered? So that the pH is kept constant, so that proteins are not denatured
- What does ultracentrifugation mean? Describe what happens:
- The sample is put into a tube
- It is spun at a low speed (low Revolutions Per Minute = RPM)
- The most dense organelle forms a sediment/pellet at the bottom of the test tube
- The other organelles remain suspended in the supernatant
- The supernatant is removed and put into a clean test tube and spun at higher RPM
- The second most dense organelle forms a sediment at the bottom etc
