Cells Flashcards
Define the terms eukaryotic and prokaryotic cell.
Eukaryotic: DNA is contained in a nucleus, contains membrane bound specialised organelles
Prokaryotic: DNA is free in the cytoplasm, no organelles
State the relationship between a system and specialised cells.
specialised cells -> tissues that perform a specific function -> organs made of several tissue types -> organ system
Describe the structure and function of the cell-surface membrane
‘fluid mosaic’ phospholipid bilayer.
Isolates cytoplasm from extracellular environment.
Selectively permeable to control substances transported.
Involved in cell recognition/cell signaling
Explain the role of cholesterol in the cell-surface membrane
connects phospholipid and reduces fluidity.
Explain the role of glycoproteins in the cell-surface membrane.
Cell signalling, cell recognition (antigens) and cell binding
Explain the role of glycolipids in the cell-surface membrane.
Cell signalling and cell recognition
Describe the structure of the nucleus
•Surrounded by nucleur envelope (a semi permeable double membrane)
•Nuclear pores allow substances to exit/enter
•Dense nucleolus made of RNA and proteins assembles ribosomes
Describe the function of the nucleus
•Contains DNA which is coiled around the chromatin into chromosomes
•Controls cellular processes: gene expression (determines specialisation), site of mRNA transcription, mitosis, semi conservative replication.
Describe the structure of the mitrochondrion
•Surrounded by double membrane folded inner membrane to form cristae (site of electron transport chain)
•fluid matrix (contains chromosomal DNA, respiratory enzymes, lipids and proteins)
Describe the structure of chloroplasts
•Vesicular plastid with double membrane
•Thylakoids (flattened disks that stack to form grana (contains photostems with chlorophyll))
•Intergranal lamellae (tubes that attach thylakoids in adjacent grana)
•Stroma: liquid filled matrix
State the function of mitochondria.
Site of areobic respiration to form ATP
State the function of the chloroplast
site of photosynthesis
Describe the structure of golgi apparatus
Planar stack of membrane-bound, flattened sacs.
Molecules processed in cisternae.
Vesicles bud off trans face via exocytosis
State the functions of the golgi apparatus
•modifies and packages proteins for export
•synthesises glycoproteins
Describe the structure of lysosomes
Sac surrounded by a single membrane embedded H+ pump to maintain acidic conditions.
Contains digestive hydrolase enzymes.
Has a glycoprotein coat to protect cell interior.
State the functions of lysosomes
Digests contents of phagosomes.
Exocytosis of digestive enzymes.
What is exocytosis?
Where cells move materials from within the cell to the extracellular fluid.
Describe the function and structure of ribosomes
Formed with protein and rRNA.
Free in the cytoplasm or attached to endoplasmic reticulum (ER)
Is the site of protein synthesis via translation:
•large subunit: joins amino acids
•small subunit: contains mRNA binding site
Describe the structure and function of the endoplasmic reticulum (ER)
Has cisternae (network of tubules and flattened sacs extended from cell membrane through cytoplasm and connects to nuclear envelope)
Rough ER = many ribosomes attached for proteinsynthesis and transport.
Smooth ER = lipid synthesis
Describe the structure of the cell wall in bacteria cells
Made of polysaccharide murein
Describe the structure of the cell wall in plant cells
Made of cellulose microfibrils.
Plasmodesmata that allow molecules to pass between cells.
Middle lamella acts like a boundary between adjacent cell walls.
Describe the function of the cell wall
•Provides mechanical strength and support.
•Acts as a physical barrier against pathogens
•Part of the apoplast pathway in plants to allow easy diffusion of water
Describe the structure of the vacuole
Surrounded by a single membrane.
Contains cell sap, mineral ions, water, enzymes and soluble pigments.
Describe the function of the vacuole.
•Controls tugor pressure
•Absorbs and hydrolyses potentially harmful substances to detoxify cytoplasm.
Explain some common cell adaptations
•Folded membrane/ microvilli increase the surface area for better diffusion.
•Some cells have many mitochondria to make large amounts of ATP for active transport.
•Some cells have walls that are one cell thick to reduce the distance of the diffusion pathway.
State the role of plasmids in prokaryotes
Small ring of DNA that carries non-essential genes.
Can be exchanged between baceterial cells via conjunction.
State the role of flagella in prokaryotes
helps it move around
State 4 roles of the capsule in prokaryotes
•Prevents dessication
•Acts as a food reserve
•Provides mechanical protection against phagocytosis & external chemicals
•Sticks cells together
Contrast eukaryotic and prokaryotic cells
Prokaryotic cells are small and always unicellular whereas eukaryotic cells are larger cells that are often multicellular.
Prokaryotic cells have smaller ribosomes but eukaryotic cells have larger ribosomes.
Describe the structure of a viral particle
Linear genetic material (RNA or DNA) and viral enzymes.
Surrounded by a capsid
No cytoplasm
What is a capsid?
A protein coat made of capsomeres
Describe the structure of an enveloped virus
Simple virus surrounded by matrix protein.
Matrix protein surrounded by envelope derived from cell membrane and host cell.
Attachment proteins on surface
State the role of the capsid on viral particles
•Protect nucleic acid from degradation
•Surface sites enable virus particles to bind and enter host cells to inject their genetic material
State the role of attachment proteins on viral particles
Enable viral particles to bind to complementary sites on the host cell
Describe how optical (light) microscopes work
- lenses focus rays of light and magnify the view of a thin slice of a specimen.
- Different structures absorb different wavelengths of light.
- Reflected light is transmitted to the observer via the objective lens and eyepiece.
Suggest advantages and limitations of using a light microscope
+can show living organisms
+affordable
+colour image
-2D image
-lower resolution than electron microscopes
Explain how a transmission electron microscope (TEM) works
- Pass a high energy beam of electrons through a thin slice of specimen.
- More dense structures appear darker since they absorb more electrons.
- Focus image onto fluorescent screen or photographic plate using magnetic lenses
Suggest the advantages and limitations of using a TEM
+electrons have shorter wavelength than light = higher resolution
+higher magnification
-Requires a vacuum
-Heavy and expensive
-No colour image
Describe how a scanning electron microscope (SEM) works
- Focus a beam of electrons onto a specimens surface using electromagnetic issues.
- Reflected electrons hit a collecting device and are amplified to produce an imagine on a photographic plate
Suggest the advantages and limitations of using a SEM
+3D image
+electrons have a shorter wavelength than light = higher resolution.
-Requires vacuum
-Only shows outer surface
Define magnification
Factor by which the image is larger than the actual specimen
Define resolution
Smallest seperation distance at which 2 seperate structures can be distinguished from eachother
Explain how to use the eyepiece graticule and stage micrometre to measure the size of a structure.
- Place the micrometre on the stage to calibrate eyepiece graticule.
- Line up the scales on graticule and micrometre then count how many graticule divisions are in 100um on the micrometre.
- Length of 1 divison = 100um / number of divisions.
- Use calibrated values to calculate actual length of structure
Outline what happens during ultracentrifugation
- Mince and homogenize tissue to break open cells and release organelles.
- Filter homogenate to remove debris.
- Perform differential centrifugation.
a. spin homogenate in centrifuge.
b. most dense organelles form a pellet.
c. filter off the supernatant and spin again at higher speed
State the order of sedimentation of organelles during differential centrifugation
most dense > least dense.
nucleus > mitochondria > lysosomes > rough endoplasmic reticulum > plasma membrane > smooth endoplasmic reticulum > ribosomes
Explain why fractionated cells are kept in a cold, buffered, isotonic solution
cold: slows the action of hydrolase enzymes
buffered: maintain a constant pH
isotonic: prevent shrinking of organelles
State what the cell cycle is and outline its stages
Cycle of division with immediate growth periods.
1. interphase
2. meiosis or mitosis (nuclear division)
3. cytokinesis (cytoplasmic division)
Why does the cell cycle not occur in some cells
After differentiation, some cells lose the ability to divide (neurons)
State the difference between cell divison and mitosis
Cell cycle includes growth period between division.
Mitosis is only 10% of the cell cycle and only refers to nuclear division
What happens during interphase?
G¹ = cell synthesises proteins for replication and cell size doubles.
S = DNA replicates - chromosomes consist of 2 sister chromatids joined at a centrometre.
G² = organelles divide
What is the purpose of mitosis?
produces 2 identical daughter cells for:
•Growth
•Cell replacement/ tissue repair
•Asexual reproduction
Name the stages of mitosis
Prophase
Metaphase
Anaphase
Telophase
Outline what happens during prophase
- Chromosomes condense and become visible (x shaped: 2 chromatids joined at centrometre)
- Centrioles move to opposite ends of the cell and mitotic spindle fibres form.
- Nuclear envelope + nucleolus break down so chromosomes are free in the cytoplasm
Outline what happens during metaphase
Sister chromatids line up at the cell equator, attached to the mitotic spindle by their centrometres
Outline what happens during anaphase
(requires energy from ATP hydrolase)
- Spindle fibres contract so centrometres divide.
- sister chromatids seperate into 2 distinct chromosomes and are pulled to opposite poles of the cell (looks like V shapes facing eachother)
- Spindle fibres break down
Outline what happens during telophase
- Chromosomes decondense and become invisible again
- New nuclear envelopes form around each set of chromosomes so there are 2 nuclei, each with 1 copy of each chromosome
Explain the procedure for a root tip squash experiment
- Prepare a temporary mount of root tissue.
- Focus a light microscope on the slide. Count total number of cells in the field of view and number of cells in each stage of mitosis.
- Calculate mitotic index (number of cells undergoing mitosis)
Explain how to prepare a temporary mount of root tissue
- Place the root in hydrochloric acid to halt cell division and hydrolyse the middle lamella
- Stain the root tip with dye that binds to the chromosomes
- Macerate tissue in water using mounted needle
- Use mounted needle at 45° to press down the coverslip and obtain a single layer of cells.
Name 2 dyes that can bind to chromosomes
toluidine blue (blue)
acetic orcein (purple red)
Why is only the root tip used when calculating mitotic index
Meristematic cells at root tip are undergoing mitosis
Cells further from root tip are enlongating rather than dividing
What are proto-oncongenes
Genes that code for proteins to stimulate cell cycle to progress from one stage to the next
How can tumour suppressor genes and proto-oncongenes cause cancer?
•Tumour suppressor: no production of a protein needed to slow the cell cycle
•Proto-oncongenes: form permanently-activated oncongenes
•Disruption to cell cycle > uncontrolled cell division > tumour
Suggest how cancer treatments control the rate of cell division
Disrupt the cell cycle
•Prevent DNA replication
•disrupt spindle formation = inhibit metaphase/anaphase
!can also damage healthy cells
How do prokaryotic cells replicate?
- DNA loop replicates and both copies stay attached to cell membrane. Plasmids replicate in cytoplasm
- Cell elongates, seperating the 2 DNA loops
- Cell membrane contracts & septum forms
- Cell splits into 2 identical progeny cells, each with 1 copy of the DNA loop but a variable number of plasmids
Estimate the exponential growth of bacteria within 8 hours. Assume binary fission occurs once every 20 minutes and there is 1 bacterium at the start
8 × 60 = 480 minutes
480/20=24 divisions
2²⁴
Why are viruses classed as non living
They are acellular: no cytoplasm, no metabolism & cant self replicate
Outline how viruses replicate
- Attachment proteins attach to receptors on host cell membranes
- Enveloped viruses fuse with cell membrane or move in via endocytosis & release DNA/RNA into cytoplasm or viruses inject DNA/RNA.
- host cell uses viral gentic information to synthesise new viral proteins/ nucleic acid
- Components of new viral particle released from the cell
How do new viral particles leave the host cell?
a. Bud off & use cell membrane to form envelope
b. Cause lysis of host cell
Why is it so difficult to develop effective treatments against viruses?
Replicate inside living cells = difficult to kill them without killing host cells
Describe the fluid mosaic model of membranes
fluid: phospholipid bilayer where individual phospholipids can move: membrane has flexible shape
mosaic: extrinsic + intrinsic proteins of different sizes and shapes are embedded
Explain the role of cholesterol in membranes
steroid molecule in some plasma membranes; connects phospholipids & reduces fluidity to make bilayer more stable
Explain the role of glycolipids in membranes
cell signalling + cell recognition
Explain the functions of extrinsic proteins in membranes
•binding sites/receptors (for hormones)
•antigens (glycoproteins)
•bind cells together
•involved in cell signalling
Explain the functions of intrinsic proteins in membranes
•electron carriers (respiration/photosynthesis)
•channel proteins (facilitated diffusion)
•carrier proteins (facilitated diffusion/active transport)
Explain the functions of membranes within cells
•Provide internal transport system.
•Selectively permeable to regulate passage of molecules into / out of organelles
•Provide reaction surface
•Isolate organelles from cytoplasm for specific metabolic reactions
Explain functions of cell-surface membranes
•Isolates cytoplasm from extracellular environment
•Selectively permeable to regulate transport of substances
•Involved in cell signalling/ cell recognition
Name 3 factors that affect membrane permeability
•Temperature: high temp denatures membrane proteins or phospholipid molecules have more kinetic energy and move further apart
•pH: changes tertiary structure of membrane proteins
•Use of solvent: may dissolve membrane
Define osmosis
water diffuses across a semi permeable membranes from an area of higher water potential to an area of lower water potential until a dynamic equilibrium is established
What is water potential?
•pressure created by water molecules in kPa
•water potential of water at 25°C & 100kPa: 0
•more solute=more negative water potential
How does osmosis affect plant and animal cells?
•osmosis into cells:
plant: protoplast swells = cell turgid
animal: lysis
•osmosis out of cell
plant: protoplast shrinks = cell flaccid
animal: crenation
Suggest how a student could produce a desired concentration of a solution from stock solution
•volume of stock solution = required concentration × final volume needed / concentration of stock solution
•volume of distilled water = final volume needed - volume of stock solution
Define simple diffusion
•Passive process requires no energy from ATP hydrolysis
•Net movement of small, lipid soluble molecules directly through the bilayer from a high to low concentration
Define faciliated diffusion
•Passive process
•Specific channel or carrier proteins with complementary binding sites transport large and/or polar molecules/ions down concentration gradient
Explain how channel proteins work
hydrophilic channels bind to specific ions = 1 side of the protein closes and the other opens
Explain how carrier proteins work
binds to complementary molecule = conformational change releases molecule on the other side of the membrane
Name 5 factors that affect the rate of diffusion
• Temperature
• Diffusion distance
• Surface area
• Size of molecule
• Difference in concentration
State Fick’s law
surface area × difference in concentration / diffusion distance
How are cells adapted to maximise the rate of transport across their membranes
•many carrier/channel proteins
•folded membrane which increases the surface area
Explain the difference between the shape of graph of concentration (x-axis) against rate (y-axis) for simple vs faciliated diffusion
simple diffusion: straight diagonal line; rate of diffusion increases proportionally as concentration increases
facilitated diffusion: straight diagonal line that levels off when channel/carrier proteins are saturated
Define active transport
Active transport: ATP hydrolysis releases phosphate group that binds to carrier protein, causing it to change shape.
Specific carrier protein transports molecules/ions from an area of low concentration to an area of high concentration
Compare active transport and facilitated diffusion
•Both may involve carrier proteins
•Active transport requires energy from ATP hydrolysis whereas faciliated diffusion is a passive process
•Faciliated diffusion may also involve channel proteins
Define co-transport
Movement of a substance against its concentration gradient is coupled with the movement of another substance moving down the concentration gradient.
Substances bind to complementary intrinsic protein:
symport: transports substances in the same direction
antiport: transports substances in opposite direction (sodium-potassium pump)
Explain how co-transport is involved in the absorption of glucose/amino acids in the small intestine
- Na+ actively transported out of epithelial cells into bloodstream
- Na+ concentration lower in epithelial cells than in lumen gut
- Transport of glucose/amino acids from lumen to epithelial cells is coupled with facilitated diffusion of Na+ down electrochemical gradient
What is an antigen?
Cell surface molecule that stimulates the immune response
What are the most common types of antigens
(glyco)proteins, (glyco)lipid or polysaccharides
What does the immune system do that is triggered by antigens
identifies non-self or self = enables identification of cells from other organisms (pathogens, toxins, abnormal body cells)
How does phagocytosis destroy pathogens
- phagocyte moves towards pathogen via chemotaxis
- phagocyte engulfs pathogen via endocytosis to form a phagosome
- Lysozomes in phagosome digest pathogen
- Phagocyte absorbs the products from pathogen hydrolysis
Explain the role of antigen presenting cells (APCs)
Macrophage displayes antigen from pathogen on its surface (after hydrolysis in phagocytosis)
Enhances recognition by T-helper cells which cannot directly interface with pathogen/antigens in body fluid
Give 2 differences of non specific and specific immune responses
nonspecific (inflammation, phagocytosis) = same for all pathogens
specific (B & T lymphocytes) = complementary pathogen
nonspecific = immediate
specific = time lag
Name the 2 types of specific immune response
•cell mediated
•humoral
Outline the process of cell meditated response
- complementary T-helper lymphocytes bind to foreign antigens on APC
- Release cytokines that stimulate:
a. clonal expansion of T helper cells (rapid mitosis): they become memory cells or trigger humoral response
b. clonal expansion of cytotoxic T cells secrete enzyme perforin to destroy infected cells
Outline the process of the humoral response
- Complementary T-helper lymphocytes bind to foreign antigen on antigen-presenting T cells.
- Release cytokines that stimulate clonal expansion (rapid mitosis) of complementary B lymphocytes
- B cells differentiate into plasma cells
- Plasma cells secrete antibodies with complementary variable region to antigen
What is the structure of an antibody
proteins secreted by plasma cells
Quaternary structure: 2 ‘light chains’ held together by 2 disulfide bridges, 2 longer ‘heavy chains’
Binding sites on variable region of light chains have specific tertiary structure complementary to an antigen
The rest of the molecule is known as the constant region
How do antibodies lead to the destruction of a pathogen
Formation of an antibody-antigen complex results in agglutination, which enhances phagocytosis
What are the monoclonal antibodies
Antibodies produced from a single clone of B cells
What are memory cells
•Specialised T helper/ B cells produced from primary immune response
•Remain in low levels of the blood
•Can divide rapidly by mitosis if organism encounters the same pathogen again
Contrast the primary and secondary immune response
Secondary response:
has faster rate of antibody production
shorter time of lag between exposure and antibody production and primary response is the opposite
What causes antigen variability
- Random genetic mutation changes DNA base sequence
- Results in different sequence of codons on mRNA
- Different primary structure of antigen = H-bonds, ionic bonds & disulfide bridges form in different places in tertiary structure
- Different shape of antigen
Explain how antigen variability affects the incidence of disease
•Memory cells no longer complementary to antigen = individual not immune = can catch a disease more than once
•Many varieties of a pathogen = difficult to develop vaccine containing all amtigen types
Contrast passive and active immunity
Passive:
no memory cells & antibodies not replaced when broken down (short term)
immediate
antibodies from external response
Active
memory cells produced (long term)
time lag
lymphocytes produce antibodies
Explain the principles of vaccination
- Vaccine contains dead/inactive form of a pathogen or antigen
- triggers primary immune system
- memory cells are produced and remain in blood stream so secondary response is rapid
- pathogen destroyed before symptoms caused
What is herd immunity
Vaccination large proportion of the population reduces available carriers of the pathogen
Protects individuals who have not been vaccinated (weak immune systems)
Suggest some ethical issues surrounding the use of vaccines
•production may involve use of animals
•potentially dangerous side effects
•clinical tests may be fatal
•compulsory vs opt-out
Describe the structure of HIV
•Genetic material (2×RNA) & viral enzymes surrounded by capsid
•Surrounded by viral envelope derived from host cell membrane
•GP120 attachment proteins on surface
How does HIV result in the symptoms of AIDS
- Attachment proteins bind to complementary CD4 receptor on T helper cells
- HIV particles replicate inside T helper cells, killing or damaging them
- AIDS develop when there are too few T helper cells for the immune system to function
- Individuals can not destroy other pathogens and suffer from secondary diseases/infections
Why are antibiotics ineffective against viruses
Antibiotics often work by damaging murein cell walls to cause omsotic lysis. Viruses have no cell wall.
Viruses replicate inside host cells so it is difficult to destroy them without damaging normal body cells
Suggest the clinical uses of monoclonal antibodies
•Pregnancy tests by detecting HCG hormones in urine
•Diagnostic procedures (ELISA test)
•Targeted treatment by attaching drug to antibody so that it only binds to cells due to specificity of tertiary structure of binding site
Explain the principle of an indirect ELISA test
detects presence of an antibody against a specific antigen
1. Antigens bind to bottom of test plate
2. Antibodies in sample bind to antigen. Wash away excess
3. Secondary antibody with “reporter enzyme” attached binds to primary antibodies from the sample
4. Add substrate for reporter enzyme. Positive result = colour change
Suggest some ethical uses surrounding the use of monoclonal antibodies
•Production involves animals
•Drug trials against arthritis & leukaemia resulted in multiple organ failure
Define hypotonic
More water outside of cell
Define hypertonic
more water inside the cell
Define endocytosis
The movement of a substance into the cell by a vesicle (form of packaging used by cells)
Define exocytosis
The movement of a substance out of the cell by a vesicle
What does cholera do to chlorine ions
causes ion channels to open so chloride ions flood the lumen of the intestine.
loss of chloride ions raise the water potential and increase of chloride ions in lumen lowers the water potential.
Causes water potential gradient so water moves by osmosis from tissues to intestine making you severely dehydrated
