3.2 Cells Flashcards
3.2.1 Cell structure
What is a eukaryotic cell? (1 point)
A cell that has a true nucleus with a nuclear envelope surronding the chromosomes and membrane-bound organelles
Describe the nucleus of a eukaryotic cell (4 points)
A large organelle surrounded by a nuclear envelope (double membrane), which contains many pores. Nucleus contains chromosomes and nucleolus
The nucleus controls the cell’s activities by controlling the transcription of DNA.
DNA contains instructions to make proteins.
The pores allow substances to move between the nucleus and cytoplasm.
The nucleolus makes ribosomes
Describe the cell surface plasma membrane of a eukaryotic cell (4 points)
The membrane found on surface of animal cells and inside cell wall of other cells.
Mainly made of lipids and protein
Regulates the movement of substances into and out of the cell.
Also has receptor molecules on it which allow it to respond to chemicals like hormones
Describe the mitochondrion of eukaryotic cells (6 points)
Usually oval-shaped.
Have double membrane.
Inner membrane folded to form structures called cristae.
Inside is matrix, which contains enzymes involved in respiration
The site of aerobic respiration, where ATP is produced.
Found in large numbers in cells that require a lot of energy as they are very active
Describe the chloroplasts in eukaryotic cells (5 points)
A small, flattened structure found in plant and algal cells.
Its surrounded by a double membrane and also has membranes inside called thylakoid membranes.
These membranes are stacked up in some parts of the chloroplast to form grana.
Grana linked together by lamellae thin, flat pieces of thylakoid membrane
The site of photosynthesis
Describe the Golgi Apparatus in eukaryotic cells (4 points)
A group of fluid-filled, membrane bound flattened sacs.
Vesicles are often seen at the edges of the sacs
It processes and packages new lipids and proteins.
Also makes lysosomes
Describe the Golgi Vesicles in eukaryotic cells (2 points)
A small fluid-filled sac in the cytoplasm, surrounded by a membrane and produced by the golgi apparatus
Stores lipids and proteins made by the golgi apparatus and transports the out of the cell via the cell-surface membrane
Describe the lysosome of a eukaryotic cell (4 points)
A round organelle surounded by a membrane, with no clear internal structure.
Its a type of golgi vesicle
Contains digestive enzymes called lysozymes. These are kept separate from the cytoplasm by the surrounding membrane, and can be used to digest invading cells or to break down worn out components of the cell.
Describe the ribosomes in eukaryotic cells (4 points)
A very small organelle that either floats free in the cytoplasm or is attached to the rough endoplasmic reticulum.
Its made up of proteins and RNA.
Its not surrounded by a membrane
The site where proteins are made
Describe the Rough Endoplasmic Reticulum in eukaryotic cells (3 points)
A system of membranes enclosing a fluid-filled space.
The surface is covered with ribosomes
Folds and processes proteins that have been made at the ribosomes
Describe the Smooth Endoplasmic Reticulum in eukaryotic cells (2 points)
Similar to RER but has no ribosomes
Synthesises and processes lipids
Describe the Cell Wall in eukaryotic cells (4 points)
A rigid structure that surrounds cells in plants, algae and fungi.
In plants and algae its made mainly of the carbohydrate cellulose.
In fungi its made of chitin
Supports cells and prevents them from changing shape
Describe the Cell Vacuole in eukaryotic cells (6 points)
A membrane-bound organelle found in the cytoplasm of plant cells.
It contains cell sap which is a weak solution of sugar and salts.
The surrounding membrane is called the tonoplast
Helps to maintain pressure inside the cell and keep the cell rigid.
This stops plants wilting.
Also involved in the isolation of unwanted chemicals inside the cell
What are specialised cells? (2 points and example in small intestine)
Eukaryotic cells can become specialised to carry out specific functions
E.g. Epithelial cells in small intestine adapted to absorb food efficiently because:
Walls of small intestine lined with villi which increase surface area for absorption
Epithelial cells have folds in cell-surface membranes called microvilli to increase surface area even more
Have lots of mitochondria to provide energy for transport of digested food molecules into cell
Specialised cells grouped together to form tissues which work together to form organs which make up an organ system
What is a prokaryotic cell? (1 point and example)
A cell that lacks a membrane-bound nucleus or any other membrane bound organelle
E.g. Bacteria
Describe the cytoplasm in prokaryotic cells (1 point)
Contains no membrane bound organelles whereas eukaryotes do (mitochondria)
Describe the ribosomes in prokaryotic cells and how they differ to eukaryotic cells (1 point)
Same function and structure as in eukaryotic cells but are smaller (70s) whereas eukaryotes have 80s
Describe DNA in prokaryotic cells (2 points)
Prokaryotic cell doesn’t have a membrane bound nucleus unlike eukaryotic cells instead has circular DNA present as one long coiled up strand and floats free in cytoplasm.
DNA is not attached to any histone proteins
Describe prokaryotic cell Plasmids (4 points)
Small loops of DNA that aren’t part of main circular DNA molecule.
Plasmids contain genes for antibiotic resistance and can be passed between prokaryotes.
Not always present but some can have several.
Eukaryotes don’t have a plasmid
Describe prokaryotic cell Capsule (2 points)
Some have capsule made up of slime
Helps to protect bacteria from attack by cells of the immune system
Describe prokaryotic cell wall and how it differs from eukaryotic cell wall (3 points)
Supports the cell and prevents it from changing shape
Made of a polymer called murein which is a glycoprotein.
In eukaryotes however cell wall is made from cellulose
Describe prokaryotic cell plasma membrane (2 points)
Mainly made of lipids and proteins
Controls the movement of substances into and out of cell
How does prokaryotic cell size differ from eukaryotic cell? (1 point)
Prokaryotic cell is 1 - 10 micrometres whereas eukaryotic cell is 10 – 100 micrometres
What is a virus? (1 point)
A virus is a microscopic, intracellular, parasitic organism that infects other organisms
Features of a virus (4 points)
Require a host in order to take over the nucleus and release their DNA
Viruses use host enzymes, energy and ribosomes
Have no ribosomes, RER, SER, Golgi apparatus or mitochondria unlike eukaryotic cells
Have no cytoplasm, no ribosomes or plasma membrane unlike bacteria
Structure of a virus (6 points)
Lipid envelope = Phospholipid Genetic material = viral RNA Enzyme = viral enzyme (Reverse transcription Attachment protein = CD4 proteins Matrix Capsid
All viruses have nucleic acid (RNA or DNA)
All viruses have a capsid which protects the genetic material
All viruses have attachment proteins which attach to host
Some have an outer phospholipid membrane surrounding the capsid
Life cycle of a virus (5 points)
Attachment proteins bind to complimentary receptors on the cell surface membrane
Capsid fuses with membrane
Genetic material incorporates with cell genetic material
Viral structure is replicated
Virus particle leaves the cell
Explain HIV’s replication cycle (10 points)
Following infection HIV enters the bloodstream and circulates around the body
A protein on HIV binds to a protein called CD4 mostly on T- helper cells
The protein capsid fuses with the cell surface membrane.
The RNA and enzymes of HIV enter the T-helper cell
The HIV reverse transcriptase converts the virus’s RNA to DNA
The newly made DNA moves into the T-helper cell’s nucleus where it is inserted into the cell’s DNA
The HIV DNA in the nucleus creates mRNA using the cells enzymes. This mRNA contains instructions for making new viral proteins
The mRNA passes out of the nucleus through a nuclear pore and uses the cells protein synthesis mechanisms to make new HIV proteins
The HIV particles break away from the T-helper cell with a piece of its cell membrane surrounding it.
The new virus matures and moves on to infect other cells
Why are microscopes useful? (4 points)
Increased understanding at subcellular level
Can identify specialised cells to see how organisms’ function
Diagnostic tools e.g. magnify blood samples
Enlarge tiny viruses to develop vaccines and cures
Can see things you can’t with naked eye
What is magnification? (1 point)
The act/process of enlarging the physical appearance or image of something
What is resolution? (1 point)
The minimum distance apart 2 objects can be in order to appear as separate items
What is the calculation for magnification?
Magnification = size of image/ Size of actual object
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What are Optical Light Microscopes? (definition, 3 disadvantages, 1 advantage and preparation)
Use light radiation to form an image
- Have poor maximum resolution of 0.2 micrometres because electrons have short wavelength
- Have maximum magnification of x1500
- Less detailed images and cannot see small organelles
+Cheaper
Preparation: Sample washed and stained with iodine solution then covered with cover slip
What are Transmission Electron Microscopes? (definition, 7 disadvantages, 2 advantages and preparation)
Electrons pass through the specimen and only the electrons that pass through are seen and produce an image. Denser parts of specimen absorb more electrons, which makes them look darker on the image. Use electromagnets to focus the beam of electrons, which is then transmitted through the specimen
- Expensive
- Specimen must be dead
- Vacuum required
- Only produces a 2D image
- No colour
- Stained with heavy metal
- Can only pass through a thin slice of specimen
+ High Resolution of 0.1 nanometres so chloroplasts
can be seen
+ High magnification of x 1,500,000
Preparation: Slice and stain specimen with heavy metal and must be in a vacuum
What are Scanning Electron Microscopes? (definition, 5 disadvantages, 4 advantages and preparation)
Scan a beam of electrons across the surface of the specimen. This reflects electrons off the surface of the specimen, which are gathered in a cathode ray tube to form an image. Reflected beam is observed
- Expensive
- Specimen must be dead
- Vacuum required
- Lower resolution than TEMs
- Stained with gold
+ 3D images produced
+ Colour images produced
+ High resolution of 20 nanometres apart
+ High magnification of x 1,500,000
Preparation: Coat surface of specimen with thin layer of gold, must be in a vacuum
What are artefacts and how are they a problem? (Definition, 2 problems and an example)
They are something we see that is not visually present
Gives appearance of structures which are not really there
Create uncertainty about structures apparently seen in electron microscopes
May include wrinkles in cells
What are the common scales? (5 points)
1km = 1000m 1m = 100cm 1cm = 10mm 1mm = 1000µm micrometres 1µm = 1000nm nanometres
What is the stage micrometre and how do you use it? (definition and 7 steps)
Stage micrometre is a slide with a small etched scale on it
1. Select magnification
2. Remove specimen slide
3. Place stage micrometre onto microscope stage
4. Focus at selected magnification
5. Measure diameter of field of view
6. Remove the slide and place specimen back onto stage
7. Using the size you have calculated for field of view you can then estimate the size of whole cell, a particular organelle or an interesting section
LIMITATION = Could make an error as it is only an estimation
What is the eyepiece graticule? (2 points)
Gives more accurate way to measure
An eyepiece graticule is fitted into the eyepiece lens and is visible all the time whilst looking down microscope
Calibrating the graticule (6 points)
- Choose which magnification to use. You want one that takes up most of field of view
- Replace the specimen with a stage micrometre
- When focused, line up the stage micrometre with the eyepiece graticule so one main division of each is in line
- Work out the length of one eyepiece graticule unit in micrometres
E.g. 3 eyepiece graticule units = 4 stage micrometre units. Since 1 division on stage micrometre is 1000µm, you do 4 x 1000 = 4000µm. So 1 eyepiece graticule unit = 4000/3 = 1333µm - Remove the stage micrometre and put the specimen back on the stage
- Calculate how many eyepiece graticule units the cell measures
What is cell fractionation? (2 points)
The process where cells are broken up and different organelles they contain are separated out
Useful for studying individual specific organelles
Describe the process of cell lysis (3 points)
Breaking down of the membrane of a cell
A buffer solution is added
The buffer solution is:
1. Buffered to maintain same pH and prevent proteins being denatured
2. Ice cold to slow down enzyme activities that could damage the cell
3. Isotonic so the water potential in the cytoplasm is the same as outside the cell to prevent water entering or leaving the organelles by osmosis to prevent damage (osmotic lysis)
Describe the process of cell fractionation (5 stages)
- Cell lysis
- Homogenisation to disrupt the tissue and cells and break open the cell. This can be done by high frequency sound, mild detergent to make holes in plasma membrane, High pressure by forcing cells through small holes or shear cells between close-fitting rotating plunger and thick wall of glass vessel
- Filter the homogenate through a gauze to remove debris, unopened cells and damaged cells
- Place solution in centrifuge
- Ultracentrifugation where you crush and filter cell using pestle. Then spin at a low speed where the heaviest organelles like nuclei form a thick sediment at the bottom (the pellet). The rest of organelles are suspended in a fluid above pellet (the supernatant). Drain off supernatant into another tube and spin supernatant at a higher speed to release mitochondria and then repeat at a higher speed. The order of sedimentation is heaviest density to lightest
3.2.2 All cells arise from other cells
What is the cell cycle? (1 point)
A regular cycle of cell division separated by periods of cell growth
Why does the cell cycle need to happen? (2 points and 1 however
The cell needs to grow and synthesise proteins such as enzymes to make sure the cell and nucleus are prepared to divide for replication
DNA has to be replicated because the resulting cells all need to contain 23 pairs of chromosomes
HOWEVER, not all cells go through the cell cycle as some go into cell arrest
What is cell arrest? (2 points and 2 examples)
The G₀ stage of the cell cycle
Some cells don’t divide at all and some cells stop dividing such as when we reach adulthood due to overcrowding and differentiation
E.g. 1. Liver cells are in G₀ but can be called back into cell cycle by external causes for example if the cell is damaged
2. Nerve and muscle cells are highly specialized so are arrested in G₀ and nerve cells can never divide but some muscle cells can
What are the stages of the cell cycle? (5 points)
G₁ Phase = Growth of the cell and synthesis of DNA polymerase which is needed to form long chains of repeated nucleotides to create DNA. It is important to ensure the cell has correct organelles to replicate and produce 2 functional cells
S Phase = DNA Replication to ensure each of 46 chromosomes are duplicated to produce 2 cells with 23 pairs
G₂ Phase = Replication of other organelles and prepare cell for division. This is important as it acts as a check point to ensure there are no errors
Stage 5 = Mitosis (nuclear division) to form new nuclei
Stage 6 = Cytokinesis where the cytoplasm divides and new membranes form
What happens during uncontrolled cell division? (3 points)
Uncontrolled cell division happens as a result of mutated genes if they are not picked up at the checkpoint
Most mutated cells die or are destroyed but some may continue to divide and form tumours either benign or malignant
Cancer is a result of damage to the genes that control mitosis
What is the mitotic index? (3 points)
The ratio between the number of cells in a population undergoing mitosis to the number of cells
Number of actively dividing cells in field of view / Total number of cells in field of view
X100 if expressed as percentage
How do chromosomes change before, during and after mitosis? (3 points)
Chromosome before mitosis: 1 chromatid per chromosome
Chromosome after S-phase (replication): 2 chromatids per chromosome (sister chromatids) 2 copies remain attached at centromere but still regarded as 1 chromosome
Chromosome after mitosis: Each cell receives 1 chromatid per chromosome
What is the centromere? (1 point)
The site where spindle fibres attach
What are the stages of mitosis? (5 stages)
Stage 1 Prophase:
Chromosomes condense and become visible
Nuclear membrane breaks down
Stage 2 Metaphase:
A microtubule spindle forms within the cell
All chromosomes line up along the equator (middle) of the cell attached to the spindle
Stage 3 Anaphase:
Microtubules shorten pulling apart the sister chromatids
Chromosomes are pulled to opposite poles of the cell
Stage 4 Telophase:
Nuclear membranes form
Chromosomes decondense become longer, thinner and less visible
After Mitosis – Cytokinesis:
Cell membrane divides
2 daughter cells each has own nucleus containing full set of chromosomes
Cell cycle continues into G₁
How do cancer treatments work? (1 point and 1 problem of treatment)
Disrupt cell cycle by preventing DNA from replicating and inhibit metaphase by inhibiting formation of spindle fibres
PROBLEM: Rapidly dividing cells e.g. hair cells are also killed in the process
What is binary fission? (3 points)
A-sexual form of reproduction
Cell division used by all prokaryotes
A-sexual reproduction has one parent, no mixture of genetic material, identical offspring, produces clones, quick process
What are the stages of binary fission? (6 stages)
- DNA Replication:
DNA uncoils
Plasmids and DNA replicate
Copies of DNA stay attached to cell membrane - Segregation:
Replicated DNA and plasmids move to opposite ends of cell - Elongation:
DNA and plasmids moving to opposite ends causes cell to lengthen - Formation of the membrane:
The equatorial plate separates the plasma membrane - Formation of the cell wall:
New cell wall forms to separate the new cells - 2 cells are formed:
Two genetically identical cells are formed (clones)
Compare and contrast the process of nuclear division in prokaryote and eukaryote cells (4 similarities and 6 differences)
Similarities:
Both binary fission and mitosis are a-sexual
Both create two new identical daughter cells
Both processes involve replicated DNA moving to opposite poles
Both involve DNA replication
Differences:
Prokaryotes divide by binary fission whereas eukaryotes use mitosis
Mitosis requires spindle fibres to pull cell apart whereas binary fission does not
Mitosis requires formation of a nuclear envelope whereas binary fission does not
Only Binary fission requires replication of plasmids
Only mitosis has sister chromatids joined at centromere
Binary fission is quicker than mitosis
Why can bacteria multiply in less than 20 minutes? (1 main points and 4 sub-points)
Binary fission is much simpler and very quick because there is no nuclear envelope/nucleus, there are fewer/no membrane bound organelles to replicate, free floating DNA, simpler DNA
How does prokaryote DNA differ from eukaryote DNA? (5 points)
- In prokaryote it is single, circular DNA whereas in eukaryote it is multiple, linear chromosomes
- In prokaryote DNA condenses in nucleoid whereas in eukaryote DNA condenses in nucleus
- Majority of prokaryotes contain only 1 copy of each gene (haploid) whereas eukaryote cells contain 2 copies of each gene (diploid)
- In prokaryotes plasmids are present whereas in eukaryotes plasmids are not commonly present
- In prokaryote cells there is no non-coding and repetitive DNA as the genomes are efficient and compact whereas in eukaryotes there is large amounts of non-coding and repetitive DNA
3.2.3 Transport across cell membranes
What is the Fluid Mosaic Model and why is it called Fluid Mosaic? (Definition and 2 points)
The structure of a cell membrane
Called fluid because the phospholipids are constantly moving
Called mosaic because proteins are scattered through the bilateral like a mosaic
What is the membrane? (4 points)
A bilayer (two)
Made of phospholipids (phospholipid bilayer)
Mosaic of different components
Selectively permeable
What molecules can pass through the phospholipid bilayer? (2 points)
- Lipid-soluble substances such as Oxygen and Carbon Dioxide as they can dissolve in lipids so pass
- Fatty acids as they have the same properties/are lipid based so can pass
What molecules cannot pass through? (3 points)
- Large molecule such as glucose as are too large to pass between phospholipids
- Water soluble ions as cannot pass through hydrophobic tails (repel water)
- Polar (electrically charged) molecules as charged molecules prefer to interact with polar heads of the membrane (outside). The inside is non-polar/hydrophobic
What are the components of the membrane? (5 points)
- Phospholipid bilayer
- Glycolipids (extrinsic protein)
- Glycoproteins (extrinsic protein)
- Intrinsic proteins
- Cholesterol
What do the glycolipids do in the membrane? (3 points)
Are lipids with a carbohydrate chain
On the surface of the membrane so help with cell recognition and cell adhesion
Also form hydrogen bonds with water which helps to increase stability of the membrane
What does the phospholipid bilayer do in the membrane? (2 points)
Makes up the overall structure of the membrane
Is main barrier preventing passage of specific molecules such as large molecules, water soluble ions and polar molecules
