3.2 Cells Flashcards
Explain why organisms with smaller sizes can be seen with a TEM but not with an optical microscope.
TEM has higher resolution as it uses a shorter wavelength of electrons, rather than a long wavelength of light
Why is a TEM microscope used to produce an image of a plant cell, including its mitochondria?
- TEM has a higher resolution
- Electron beam passes through the cell so that the internal structures of the cell can be seen
Describe the principles and limitations of TEM to investigate cell structure.
- Short wavelength of electrons pass through sample, high resolution
- Sample must be extremely thin to allow electrons to pass through
- Sample cannot be living as it is carried out in a vacuum
- Image produced is not in colour or 3D
Describe the structure and function of the nucleus.
- Has nucleolus
- Has nuclear envelope with pores (important for mRNA and ribsome movement)
- Contains chromatin (DNA and histones)
- Site of trancription
What organelle in a eukaryotic cell modifies proteins?
The golgi body
What is the function of the smooth ER?
Synthesises, stores and transports carbohydrates and lipids
What are the main structures of chloroplasts?
Stroma, thylakoid membrane, grana, lipid droplets, starch grain
What are the main structures of mitochondria?
Matrix, inner membrane, outer membrane, cristae, inter membrane space
Why do you use an ice-cold, isotonic buffer solution when preparing a solution for centrifuging?
Ice-cold = slows down the activity of hydrolytic enzymes
Isotonic = stops osmosis, cells are not damaged (burst, shrivel)
Buffer = control pH, slows down enzyme activity which may destroy organelles
Describe how you would use cell fractionation to isolate chloroplasts from leaf tissue.
- Homogenate leaf tissue in cold, isotonic buffer solution.
- Filter off any debris
- Collect 2nd pellet after centrifuging
What organelles require the lowest speed for centrifuge and why?
- Nuclei, mitochondria
- They are the larger and heavier organelles
What organelles require the highest speed for centrifuge and why?
- Lysosomes, ribosomes, endoplasmic reticulum.
- Smallest organelles, have a low density
What are the two components once the mixture is centrifuged?
Pellet and supernatant
What are the cell walls of the following organisms primarily made up of:
a) bacteria
b) fungi
a) Murein
b) Chitin
What is the theory that relates prokaryotic cells to mitochondria and chloroplasts?
Endosymbiotic theory
What are the similarities between prokaryotes and mitochondria/chloroplasts?
Both:
- Contain 70s ribosomes
- Replicate by binary fission
- Have similar sizes
- Have circular DNA
- Have a double membrane
Describe the structure of a bacterial cell.
All:
- No membrane-bound organelles
- 70s ribosomes
- No nucleus, circular DNA instead that is not associated with histones
- Cell wall containing murein
Some have:
- Plasmids
- Capsules
- Flagellum
Describe the structure of a virus.
- Nucleic acid core (RNA/DNA)
- Protein coat/capsid
- Attachment proteins (glycoproteins)
- Phospholipid envelope
Why are viruses classed as acellular and non-living?
- Invade host cell in order to reproduce
- They contain no cellular structures like cytoplasm or cell membranes
What are the phases of the cell cycle?
Interphase, mitosis, cytokinesis
What are the phases of interphase? Describe what happens in each one.
G1 - organelles replicate, protein synthesis, cell grows
S - DNA replicates, sister chromatids created
G2 - DNA checked for errors, energy stores grow
What are the phases of mitosis? Describe what happens in each one.
Prophase:
- chromosomes condense
- centrioles move to form spindle fibres
Metaphase:
- nucleus disappears
- chromosomes line up along spindle fibres
Anaphase:
- centromere splits
- sister chromatids pulled apart to opposite poles
Telophase:
- chromosomes decondense
- nuclear envelope reforms
Where in plants does mitosis occur?
Meristems
Explain how the events in interphase and mitosis leads to the production of genetically identical daughter cells.
- Semi-conservative replication during interphase produces two identical sister chromatids
- Each chromatid seperated during anaphase and pulled to opposite poles
How many cells does mitosis produce and are they haploid or diploid?
Produces 2 genetically identical diploid cells
What two pieces of evidence show that a cell is in the anaphase stage of mitosis?
- V-shape of chromosomes show that they have been pulled apart at the centromere
- Chromosomes are at opposite poles of the spindle fibres
What occurs during cytokinesis in an animal plant? How does this differ in plant cells?
- In animal cells, a cleavage furrow forms and separates the two cells, splitting the cell membrane in two.
- In plant cells, a cell plate forms and new cell walls form once the cell plate reaches the old cell walls
What happens during binary fission?
- Circular DNA and plasmids replicated
- Parent cell divides into two new cells, cytoplasm split in two
- Produces two daughter cells, each with one copy of the circular DNA and with a variable number of plasmids
How does a virus replicate?
- Virus attachment proteins bind to complimentary receptor proteins on the surface of a host cell
- Virus injects DNA/RNA into the host cell
- Host cell DNA changed and it begins to code for new virus particles
- Virus particles then released by the cell (budding- leave one by one through membrane)
What is cancer?
When cells break away from a primary tumour and form a secondary tumour elsewhere. (metastasis)
What two types of tumours are there? Describe their differences.
Malignant and benign
- Malignant is usually cancerous and benign non-cancerous
- Malignant tumours grow and spread more quickly (rapidly dividing)
- Malignant tumours do more harm, however, benign can still put pressure on adjacent organs and tissues
- Symptoms of malignant tumours are systemic but localised with benign tumours
What 4 ways can you treat cancer?
- Radiotherapy
- Chemotherapy
- Surgery
- Immunotherapy
Why would a potato lose mass when placed in a sucrose solution for a period of time?
- Sucrose solution had a lower water potential (hypertonic)
- Water moves by osmosis from area of high w.p in potato to low w.p in solution
- Loss of water in potato so smaller mass
Describe how you would carry out an experiment to find the water potential of a potato tissue?
- Evenly cut 5 potato cylinders, measure the mass of each one
- Add each cylinder to a different conc. of sucrose solution for a set time
- Remeasure the mass after patting each potato dry with paper towels
- Calculate % change in mass
- Plot graph:
x-axis = conc. sucrose solution
y-axis = % change in mass - Draw line of best fit, find where the line crosses 0% change in mass
- Use an external resource to find the water potential of the potato at this conc. of sucrose solution
What happens to a plant cell when it is placed in a hypertonic solution?
- Cells shrink (flaccid) and become plasmolysed, cell membrane pulled away from cell wall
- Plant wilts and possibly dies
Predict what would happen to a sample of cheek cells that are placed into a test tube containing pure water. How does this differ with onion cells?
- Cheek cells would burst, water moves into cell by osmosis from high water potential in test tube to low water potential in cell.
- Onion cell will become turgid, but not burst as it has a cell wall
State two functions of membranes in living cells.
- Compartmentalisation, separate organelles from one another and the reactions that occur in each
- Able to control what molecules leave and enter a cell
What lipids are found in the phospholipid bilayer?
- Cholesterol
- Glycolipid
- Phospholipid
What proteins are found in the phospholipid bilayer?
- Glycoprotein
- Intrinsic proteins (channel, carrier)
- Extrinsic proteins
What is the purpose of cholesterol in the phospholipid bilayer?
- Regulates fluidity of the membrane
What is the purpose of glycoproteins and glycolipids in the phospholipid bilayer?
Act as receptor molecules:
- Signalling receptors for hormones/neurotransmitters
- Receptors involved in endocytosis
Describe the phospholipid bilayer and how it forms?
- Bilayer of phospholipids
- Hydrophilic phosphate heads face outwards and hydrophobic fatty acid tails face inwards
What is the impact of temperature on the phospholipid bilayer?
As temperature increases:
- Cell membrane permeability increases
- Increased movement of phospholipids
- Channel/carrier proteins denature
At very low temperatures:
- Membranes become damaged as ice crystals pierce the cell membrane
- Once the cells thaw, the membranes are highly permeable
What can pass through the cell membrane by simple diffusion?
Small, non-polar, lipid-soluble molecules
Describe how a molecule moves by facilitated diffusion across the cell membrane.
- Molecule diffuses from area of high conc. to low conc. down the conc. gradient
- Through a carrier/channel protein
How does a protein enter a cell by facilitated diffusion?
- Protein binds to receptor on the cell membrane surface
- Protein travels through carrier/channel protein down conc. gradient
What factors affect the rate of diffusion of a molecule across the cell membrane?
- Temperature
- Surface area
- Steepness of conc. gradient
- Property of molecule (size, charge etc)
What is active transport and how does it work?
- The movement of molecules against the conc. gradient from an area of low conc. to high conc.
- Requires energy (provided by ATP) which allows the carrier protein to change shape
Why is active transport important in cells? Give examples.
- Absorbing glucose and amino acids from digestion
- Loading sucrose into phloem sieve cells from companion cells in plants
- Taking up inorganic ions from soil in root hair cells
Describe the co-transport involved in the absorption of glucose.
- Na+ actively transported into blood
- Lower conc. of Na+ in cells
- Na+ diffuses into cells by facilitated diffusion, taking with them glucose molecules
- Glucose enters blood by facilitated diffusion
Describe how cell-mediated immunity works.
- Phagocyte/own cell presents antigens (antigen presenting cells)
- T cell has specific receptors which bind to the antigen
- This causes the T cell to divide by mitosis, producing T helper cells and cytotoxic T cells
Describe how humoral immunity works.
- Antigen taken up, processed and presented by B cell
- T-helper cell binds to presented antigen and activates B cell, causing it to divide by mitosis.
- Plasma cells produced which produce and secrete antibodies.
- Antibodies bind to antigens, pathogen destroyed
- Memory cells also produced (secondary response)
What are the differences between B and T lymphocytes?
B:
- Humoral immunity
- Produced in bone marrow
- Respond to pathogens suspended in blood
T:
- Cell-mediated immunity
- Produced in thymus
- Respond to pathogens in own cells
Describe the process of phagocytosis.
- Phagocyte attracted to pathogen by chemicals released
- Phagocyte engulfs pathogen, forming phagosome
- Lysosome and phagosome fuse,hydrolytic enzymes from lysosome hydrolyse the pathogen
- Products used or removed, antigens presented on outer membrane
What is an antibody?
Protein produced by plasma cells which binds to specific foreign antigens
What is an antigen?
Non- self/foreign protein which triggers an immune response
Name the key structures of an antibody?
2 binding sites, variable/constant region, disulphide bridges, hinge region
What is the benefit of antibodies having 2 binding sites?
Agglutination
- Antibody can bind to two pathogens/bacteria at once
- Groups of same pathogen clump together
- Acts like a marker to attract phagocytes to engulf and destroy
What are monoclonal antibodies?
Antibodies produced by the same genetically identical plasma cell
How would you produce monoclonal antibodies?
- Mouse B cells fused with tumour cells using detergent (breaks down cell membrane)
- Fused cells are tested and cloned
- Antibodies are collected on large scale and modified for humans (humanisation)
What are the ethical issues surrounding monoclonal antibodies?
Mice are given tumours (cancer) in order to produce fast dividing cells
How are monoclonal antibodies used in an ELISA test?
- Antigens fixed to surface
- Add specific monoclonal antibodies. Linked to enzyme
- Surface is washed to remove antibodies not attached to the antigen.
- Enzyme substrate is added, detectable signal produced
What is active and passive immunity?
Active = produce the antibodies yourself
Passive = antibodies injected
What is herd immunity?
- When a large enough proportion of the population is immune/vaccinated that the chance of spread of the pathogen is very small
How does a vaccine work?
- Given dead/inactive pathogen
- Antigens cause immune response, T cells are activated and T helper cells stimulate B cells to divide by mitosis
- Plasma cells (antibodies) and memory cells produced
- Memory cells act as the vaccine and produce many more antibodies in a shorter period of time, this reduces the symptoms
For what 3 reasons do vaccines not always work?
- Antigen drift (mutations)
- Immunosuppressed individuals (malnourished, drugs)
- Antigenic concealment (hide)
What is HIV and AIDS?
- Human Immunodeficiency Virus
- Acquired immune Deficiency Syndrome
How does HIV replicate and how can this lead to AIDS?
- Virus attachment proteins bind to complimentary receptor proteins on the surface of a host cell
- Virus uses reverse transcriptase enzyme to make DNA from its RNA, which it can then insert into the T-helper cell (host cell).
- T-helper cell produces HIV particles and releases these, when this happens the host cell is destroyed
- When HIV particles start infecting more host cells, more T-helper cells are destroyed.
- Weaker immune system as there are fewer T-helper cells to activate B cells and produce antibodies
