2 CELLS AS THE BASIS OF LIFE Flashcards
(96 cards)
1
Q
What are living things composed of?
A
Living things are composed of one or more cells (unicellular or multicellular).
2
Q
What is the cell theory?
A
- Cells are the smallest structural and functional units of life.
- Cells originate from existing cells through cell division.
- Cells contain hereditary material.
3
Q
What is a cell membrane, and what is its purpose?
A
A fluid boundary that separates a cell from its surroundings. It permits the exchange of matter and energy with the external environment. It imports nutrients and excretes waste to maintain the cell’s stable internal environment.
4
Q
What is selective permeability?
A
- Selective permeability is the cell membrane’s ability to allow only SOME materials (e.g. glucose, amino acids, and lipids) to cross it more readily than others.
- Ensures the cell maintains a constant internal environment (homeostasis).
5
Q
What is the cell membrane composed of?
A
Lipids, proteins, and carbohydrates.
6
Q
What is the phospholipid composed of?
A
Hydrophobic fatty acid tails and a hydrophilic phosphate head.
7
Q
What happens to the cell membrane when it is placed in water?
A
Phospholipid molecules assemble into a bilayer with the hydrophobic tails directed to the centre and the hydrophobic heads directed to the surface. This allows the phospholipid bilayer to act as a stable boundary between two aqueous compartments.
8
Q
Describe movement of phospholipid molecules in the cell membrane
A
- Phospholipid molecules are closely packed but still move freely within the bilayer.
- Most drift laterally
- Some flip-flop transversely across the membrane.
9
Q
What is the role of sterols in the cell membrane?
A
Regulate fluidity to prevent the membrane from becoming too permeable or impermeable.
10
Q
What is the role of cholesterol in animal cells?
A
Regulates fluidity at warm temps by preventing the membrane from becoming too permeable. They do this by restricting lateral movement and stops the membrane from becoming impermeable at cooler temperatures by preventing them from packing tightly.
11
Q
How do proteins bind to the cell membrane?
A
Proteins with hydrophilic side chains bind to the hydrophilic surface (peripheral proteins), while those with hydrophobic side chains penetrate the hydrophobic core (integral proteins).
12
Q
What are the different functions of proteins in the cell membrane?
A
- Transport of materials
- Enzymatic activity — catalysing specific metabolic reactions.
- Signal transduction — act as receptors for signal molecules.
- Cell-cell recognition — essential for normal development and immunity.
- Intercellular joining
- Attachment to the cytoskeleton and ECM.
13
Q
What is the purpose of carbohydrates in the cell membrane?
A
Carbohydrates can be chemically bonded to lipids to form glycolipids of glycoproteins on external surface. Both play an essential role in cell-cell recognition.
14
Q
What are the 2 types of cells?
A
prokaryotic and eukaryotic.
15
Q
Compare the hereditary material in prokaryotes and eukaryotes.
A
Hereditary material in prokaryotes is a single, circular, and double-stranded DNA molecule called the bacterial chromosome. Whereas in eukaryotes, hereditary material is on two or more linear chromosomes, each composed of a single DNA molecules wrapped tightly around histones.
16
Q
Where are chromosomes stored in prokaryotes vs. eukaryotes?
A
In prokaryotes, chromosomes are concentrated in the nucleoid, whereas in eukaryotes, they are stored in the membrane-bound nucleus.
17
Q
What are the 3 layers of a prokaryotic cell?
A
- Cell membrane (innermost layer): facilitates some metabolic processes like respiration, photosynthesis, exchange of nutrients, respiratory gases, and waste between the cell and its environment.
- Cell wall (middle layer): a rigid barrier that provides mechanical protection.
- Capsule (outermost layer): a thick layer of slime/gel secreted by the cell membrane that protects bacteria against host immune cells and viruses. It also regulates the concentration and uptake of essential ions and water.
18
Q
What is the cytoplasm?
A
The cytoplasm in enclosed by the cell membrane and is filled with cytosol, which contains water and various solutes like ions, enzymes, substrates, lipids, carbohydrates, DNA and RNA.
19
Q
Describe the nucleus
A
- Contains most of the genes.
- Enclosed by a nuclear envelope, a double lipid bi-layer membrane.
- Contains nuclear pores that regulate the entry and exit of macro-molecules like RNAs and proteins.
- DNA in the nucleus are organised into chromosomes, each made of chromatin, which is a mixture containing one long DNA molecule and a protein that help it coil tightly.
- Contains a nucleolus, where rRNA is synthesised from genes in the DNA.
20
Q
How do proteins exit the nucleus?
A
- Proteins imported from the cytoplasm are assembled with rRNA into large and small subunits of ribosomes in the nucleolus.
- These ribosomal subunits exit the nucleus through the nuclear pores to the cytoplasm and assemble into a ribosome.
- Once in the cytoplasm, the small and large subunits join together when they find an mRNA strand, forming a complete ribosome.
21
Q
Describe ribosomes
A
- Made of ribosomal RNAs and proteins.
- Not membrane-bound.
- Carries out protein synthesis
- There are two types of ribosomes: free ribosomes suspended in the cytosol, and bound ribosomes: attached to the outside of the rough endoplasmic reticulum.
22
Q
Describe the endoplasmic reticulum and its function
A
- Consists of a network of fluid-filled, membrane-bound sacs (cisternae).
- Rough ER specialises in synthesising and secreting proteins.
- Smooth ER specialises in synthesising lipids and membranes.
- Two distinct regions: Rough ER is studded with ribosomes on its surface, while Smooth ER lacks ribosomes.
- As polypeptide chain grows, it is threaded into ER lumen through a pore, where the new chain folds into a functional shape.
- Folded proteins depart from the ER wrapped in a membrane-bound vesicle (transport vesicles).
23
Q
Describe the Golgi Body and its function
A
- Consists of associated, flattened sacs (cisternae) stacked loosely.
- Packages, modifies, and secretes proteins to other destinations.
- Two sides: cis face (receiving) and trans face (sending).
- Vesicle from ER binds and adds its membrane and contents to the golgi by fusing with the membrane on cis face.
- Products are modified from the cis region to trans region.
- Trans face produces vesicles that pinch off and travel to other sites.
24
Q
Describe lysosomes and their function
A
- Enveloped by a single lipid bilayer membrane.
- Filled with a sac of digestive enzymes that eukaryotes use to digest macromolecules.
- Rough ER makes digestive enzymes and lysosomal membrane and then transfers it to the Golgi for further processing.
- Breaks down cellular waste and debris, destroys pathogens, and participates in programmed cell death (apoptosis).
- Performs intracellular digestion like phagocytosis, where a food vacuole is formed and fuses with a lysosome, whose enzymes digest the food.
- Example: human cells carry out phagocytosis in macrophages (white blood cells) that defend the body by engulfing/destroying bacteria and other pathogen envaders.
25
Describe vacuoles and their function
- Enveloped in a single lipid bilayer with selective permeability.
- Contains ions, simple, sugars, and amino acids.
- Functions as a food vacuole or as a contractile vacuole that pumps excess water out of a cell to maintains the cell's solute concentration.
- Vacuoles help protect a plant against herbivores by storing toxic compounds, while some contain pigments to attract pollinators.
- For example: White blood cells store histamine and heparine in vacuoles for release during the immune reponse.
26
Describe mitochondrion and its function
- The site of cellular respiration; converts glucose into ATP, the cell’s main energy currency.
- Enclosed by a double membrane with a unique collection of proteins.
- Cigar shaped.
- Has circular DNA.
- Smooth outer membrane and an inner membrane with infoldings (cristae) that provide a large surface area to enhance productivity of aerobic respiration.
- Contains enzymes on the cristae that catalyse some of the steps of aerobic respiration.
27
Describe chloroplasts and their function
- Lens shaped found in leaves and other green organs that facilitate photosynthesis (transformation of light energy into chemical energy).
- Enclosed by a double membrane.
- Has circular DNA
- Contain a green pigment (chlorophyll) in thykaloid disks, enzymes and other molecules that drive the photosynthetic production of carbohydrates.
- Arranged close to the cell boundary, where they absorb light most effectively.
28
Describe the cytoskeleton and its function
- A network of protein microfibres and microtubules extending throughout the cytoplasm.
- Gives mechanical support to the cell and maintain its shape, which is important fr animal cells as they lack cell walls.
- Assists in the development of spindle fibres during cell division.
- Provides anchorage for many organelles and enzymes.
- Can be quickly dismantled and reassembled in a new location, changing the cell's shape.
- Three main types of fibres: microtubules (thickest), microfilaments (thinnest), and intermediate filaments.
29
Compare and contrast plant and animal cells
- Both have a nucleus, mitochondria, cytoskeleton, smooth and rough ER, Golgi, vesicles, vacuoles, lysosomes, and cell membrane.
- Plant cells have a larger vacuole as they store more water.
- Plant cells have chloroplasts and are enveloped in a cell wall.
30
What are autotrophs?
Living things that synthesise their food source using energy and inorganic molecules in the environment.
31
What are heterotrophs?
Living things that synthesise their food source from living or formerly living things.
32
What is photosynthesis?
- A chemical process that uses light energy and chlorophyll to transform carbon dioxide and water (inorganic) into glucose and oxygen (organic) (chemical energy).
- Creates a chemical molecule that becomes a source of POTENTIAL energy.
33
Why do cells need energy?
- To maintain life processes like metabolism and growth.
- Example: protein and DNA synthesis, to power motor proteins, and active transport of substances across membranes.
34
What is respiration?
- A catabolic process that releases energy cells use to synthesise ATP.
- Respiration energy, or more precisely cellular respiration, is the process by which cells extract energy from food molecules like glucose.
- Carried out in several small steps, where each step in the metabolic pathway is catalysed by a specific enzyme.
35
Describe aerobic respiration
- Aerobic respiration occurs when oxygen levels are high. It breaks down glucose into carbon dioxide and water in the presence of oxygen (catabolic).
- Releases energy from glucose, which is used to synthesise ATP.
- Energy is lost as 60% heat and 40% ATP.
- ATP lost will be reused (e.g. for movement, active transport, or cell division).
- Remaining energy is used to make ATP by the cell.
- Most efficient in producing the most ATP.
36
Describe anaerobic respiration
- Anaerobic respiration/fermentation occurs when oxygen levels are low, only partially breaks down glucose into either lactic acid or ethanol.
37
Why does anaerobic respiration produce less ATP?
- Glucose is only partially broken down due to the lack of oxygen.
- Limits energy extraction to glycolysis.
- Aerobic: oxygen fully oxidised glucose through Krebs cycle and electron transport chain.
- Oxygen acts as a final electron acceptor, allowing for efficient ATP production.
- Aerobic: 36-38 ATP
- Anaerobic: 2 ATP
38
Describe the two types of anaerobic respiration
- Lactic acid fermentation partially breaks down glucose into lactic acid (e.g. in muscle cells during physical exertion/aerobic exercise).
- Alcohol fermentation partially breaks down glucose into ethanol and carbon dioxide.
39
Why does respiration occur all of the time, but photosynthesis doesn't?
- Respiration happens all the time because all living cells need a constant supply of energy to survive (e.g. for movement, repair, cell division, etc).
- Photosynthesis only happens when there is light, because it needs sunlight to convert carbon dioxide and water into glucose and oxygen.
40
Describe energy coupling
- The use of energy released from exergonic reactions (e.g. cellular respiration) to power endergonic reactions (e.g. active transport, synthesis of macromolecules) in cells.
- Mediated by adenosine triphosphate (ATP).
41
What is ATP and its structure?
- ATP is an energy-storage molecule in cells.
- Has three phosphate groups.
- Allows cells to power various processes like movement and protein synthesis
- Small, soluble, and accessible across entire cell.
42
What is ATP hydrolysis?
- ATP hydrolysis occurs, where the bond between the second and third phosphate groups is broken to form adenosine diphosphate (ADP) and inorganic phosphate (Pi).
- This chemical bond is unstable and high in energy.
- There is a net release of energy in this process which is used by cells to maintain life processes.
- Some energy gets stored by [ADP + Pi] molecule to be used to make ATP.
43
What is ATP synthesis?
- Occurs when an inorganic phosphate is added to ADP.
- The energy to add a 3rd phosphate group to ADP comes from respiration.
44
Where does ATP synthesis occur in prokaryotes and eukaryotes?
- Prokaryotes: cytoplasm and cell membrane.
- Eukaryotes: primarily in mitochondria, some in cytoplasm.
45
What factors influence photosynthesis?
- Temperature
- CO2 concentration
- Light intensity
- Colour of light
- Water availability
46
What are the two types of reactions?
- Exergonic: products have less energy than the reactants because energy is released (produces heat or can be used to make ATP).
- Endergonic: products have more energy than reactants because the reaction absorbs energy.
47
Why is energy released when atoms are joined together?
- Stronger bonds are formed in the products than bonds broken in the reactants, leading to a net energy release.
- Energy output: ATP and heat.
48
Why are metabolic pathways broken into small regulated steps?
1. Energy is released in small amounts, allowing ~40% to be captured as ATP and reducing heat damage to the cell.
2. Each step can be individually regulated by enzymes to control the overall reaction speed.
3. Intermediate products can be used in other essential cellular processes. Example: citrate from the Krebs cycle can be used to make fatty acids, and G3P from glycolysis can help synthesise lipids and amino acids.
49
What roles do intermediates and negative feedback play in metabolic pathways?
- Intermediate compounds regulate metabolic pathways by activating or inhibiting key enzymes (helps the cell adjust the pathway depending on what in needs e.g. slow down or speed up).
- Example: The intermediate fructose-1,6-biphosphate can activate an enzyme later in the pathway, helping the process continue efficiently.
- Negative feedback is a mode of metabolic control where one or more end products will bind to and inhibit upstream enzymes (allosteric) that catalyse reactions early in the same pathway (stops the cell from wasting energy and resources).
- Example: As isoleucine accumulates, it slows down its synthesis by inhibiting the enzyme for the first step of the pathway.
50
What are the common features of organelles ?
- Large SA/a lot of folded membranes.
- High SA:V ratio to maximise efficiencies of processes.
- Membrane-bound channels and transporters for exchange with 'outside' cytosol.
- Many enzymes all playing their single specific part (conveyer belt effect).
- Biochemical reaction occurs in multiple and sequential steps.
51
What factors affect biochemical processes?
- Temperature
- Light intensity and colour
- pH
- Water availability
- Substrate and enzyme availability
52
Which molecules can and cannot readily cross the membrane?
- Hydrophobic (non-polar) molecules (e.g. oxygen, CO2, and steroid hormones) dissolve in the lipid bilayer and pass through the membrane rapidly.
- Hydrophilic (polar) molecules (e.g. ions, amino acids, and water) do no not readily cross the membrane
53
How do hydrophilic molecules cross the membrane?
- Channel proteins have a hydrophilic core that hydrophilic molecules or ions use as a tunnel.
- Other transport proteins called carrier proteins, bind to specific molecules and change shape to shuttle them across the membrane.
54
What is diffusion?
The tendency for particles to spread out evenly into the available space.
55
How do particles diffuse (what region to what region)?
From a region of higher concentration to lower concentration, down the concentration gradient until the particles reach dynamic equilibrium.
56
What is passive transport?
Movement of ions and other molecules within a cell along along its concentration gradient, without any external energy.
57
What is osmosis?
Osmosis is a type of passive transport where water diffuses across the selectively permeable membrane.
58
How does osmosis occur?
- Water molecules bind to hydrophilic solutes, which reduces the concentration of free water molecules.
- Water diffuses passively across a membrane from the region of lower solute concentration to the region of higher solute concentration until the solute concentration is equal on both sides.
59
What is tonicity and describe the different types.
- Tonicity is the ability of a surrounding solution to cause a cell to lose or gain water.
- Isotonic: solute concentration is the same inside the cell (constant cell volume).
- Hypertonic: solute concentration is greater than inside the cell (cell volume decreases, plasmolysis).
- Hypotonic: solute concentration is less than inside the cell (cell volume increases and may cause the cell to burst, lysis).
60
What is facilitated diffusion?
- A passive transport mechanism where molecules move across a cell membrane with the help of specific protein channels or carrier proteins.
- No energy input required, as it occurs down the concentration gradient (high to low).
61
What is active transport?
- When materials are transported against their concentration gradient (low to high), requiring energy input (ATP).
62
How does active transport occur?
- A substrate binds to a specific transport protein that changes shape using ATP.
- Change in shape causes the protein to transport the solute against its concentration gradient.
63
Compare how small and large molecules pass the cell membrane
- Small molecules enter/leave the cell through the lipid bilayer or transport proteins.
- Large molecules (e.g. proteins) cross the membrane in bulk via vesicles in an active process called bulk transport.
64
Describe the two types of bulk transport
- Endocytosis: cell engulfs material from outside its membrane, forming a vesicle.
- Exocytosis: vesicle fuses with the cell membrane, releasing its contents outside of the cell.
65
Why is it preferable for a cell to be small?
Smaller cells have a higher SA:V ratio, making them efficient at exchanging materials because they have more surface area to work with in relation to their size.
- When a cell's volume increases, its material requirements and waste production increase, increasing the requirement for cellular structures to adequately exchange materials/energy with the surroundings.
66
What happens when cells become too large?
- When cells become too large, metabolic reaction rate is slowed, and waste materials begin accumulating in the cytoplasm.
- To exchange materials efficiently, cells must divide when they become too large.
67
Why does DNA need to replicate before cells divide?
To ensure that each new daughter cell receives a complete copy of the parent cell's genetic information (genome).
68
What is a sister chromatid?
- Identical copies of a single chromosome formed during DNA replication in preparation for cell division.
- Joined together at a region called the centromere.
69
Describe the stages of mitosis
Prophase:
1. Chromatin fibres condense into discrete chromosomes.
2. The nucleoli disappears and the nuclear envelope fragments.
3. Centrosomes produce spindle fibres that extend outwards.
Metaphase:
1. Spindle fibres attach to the kinetochores and organise the chromosomes on the metaphase plate.
Anaphase:
1. Cohesin proteins cleave, and the two sister chromatids part suddenly, becoming an independent chromosome.
2. Two ends of the cell now have an identical and complete collection of chromosomes.
Telophase:
1. Two daughter nuclei form.
2. Nuclear enveloped are assembled around the chromosomes from fragments of the parent cell's envelope.
2. Nucleoli reappear, chromosomes become less condense, and the remaining spindle microtubules are broken down.
70
Describe cytokinesis is animal cells and plant cells
Animal cells:
- Cytokinesis occurs through a cleavage.
1. Cleavage furrow appears on the cell's surface near the old metaphase plate.
2. The cleavage furrow deepens from the contractions of the contractile ring until the parent cell is pinched in two, producing two wholly separated cells.
Plant cells:
- Semi-rigid wall prevent cell to change shape.
1. Cytoplasm is partitioned from the inside out by constructing a new cell wall (cell plate) between the two daughter nuclei.
- New cell wall is constructed by materials transported in vesicles along the cytoskeleton from the Golgi body.
2. Cell plate begins to grow outwards until its membrane fuses with the cell membrane.
3. New cell wall develops as the cell plate matures, dividing the parent cell into two daughter cells.
71
Describe binary fission
1. Initiated when the DNA of the bacterial chromosome begins to replicate at a specific place (origin of replication), producing two new origins.
2. One origin moves rapidly toward the opposite end of the cell and the bacterium has reached twice its initial size.
2. Proteins cause its cell membrane to pinch inward, dividing the parent bacterial cell into two daughter cells.
72
How are genes inherited by offspring?
During fertilisation, male and female gametes, sperm and egg fuse together, forming a zygote that develops into offspring with genes passed on from both parents.
73
Compare homologous chromosomes and sister chromatids
- Homologous chromosomes are two chromosomes of a pair from each parent with the same length, centromere position, and gene loci.
- Sister chromatids are identical copies of a single chromosome from one parent formed after DNA replication.
74
Describe the stages of meiosis I
1. Prophase I
- Nuclear envelope breaks down.
- Chromosomes condense.
- Each chromosome pairs with its homolog.
- Crossing over occurs.
2. Metaphase I
- Pairs of homologous chromosomes are arranged at metaphase plate.
- Independent assortment occurs.
- Both chromatids of one homolog are attached to spindle fibres from the opposite pole.
3. Anaphase I
- Homologs are moved toward opposite ends by shortening spindle fibres.
- Sister chromatids of each chromosome move as a unit toward the same pole.
4. Telophase I
- Each half has a haploid set of duplicated chromosomes .
- Sister chromatids include regions of non-sister chromatid DNA from crossing over.
5. Cytokinesis I
- Chromosomes decondense.
- Nuclear membrane reforms.
- Forms two haploid daughter cells.
75
Describe the stages of meiosis II
1. Prophase II
- Spindle fibres form.
- Duplicated chromosomes move toward the metaphase II plate via spindle fibres.
2. Metaphase II
- Kinetochores of sister chromatids are attached to spindle fibres.
3. Anaphase II
- Proteins holding the sister chromatids allow them to separate and move toward opposite poles.
- Each chromatid is now an individual chromosome.
4. Telophase II
- Nuclei form.
- Chromosomes condense.
5. Cytokinesis II
- Divides cells into two, forming four haploid cells that mature into either sperm or egg.
76
What are the 3 mechanisms that contribute to genetic variation?
1. Crossing over
2. Independent assortment
3. Random fertilisation
77
Describe the steps of crossing over
1. Homologous chromosomes pair up during Prophase I to form a tetrad (4 chromatids total).
2. Non-sister chromatids align closely in a process called synapsis.
3. Chiasmata (crossing points) form where chromatids break and exchange segments.
4. Enzymes help cut and rejoin DNA, swapping genetic material.
5. Result: Recombinant chromatids with ew allele combinations, increasing genetic variation.
78
Describe the steps of independent assortment
1. During Metaphase I, homologous chromosome pairs line up randomly at the equator of the cell.
2. Each pair aligns independently of the others - maternal or paternal chromosomes can go to either side.
3. In Anaphase I, homologous chromosomes are pulled to opposite poles.
4. This creates gametes with different combinations of maternal and paternal chromosomes.
5. Result: Genetic variation due to many possible chromosome combinations.
79
What are the 3 checkpoints in the cell cycle, and what is their main purpose?
- Found in the G1, G2, and M phases.
- A control point where stop and go-ahead signals regulate the cell cycle.
- Regulated by both internal and external signals.
80
What is the purpose of the cell cycle?
- Increase in cell number.
- Doubling amount of DNA.
- Alternating of cell division and enlargement.
81
Describe the main events of the cell cycle (G0, G1, S, G2, M, and Cytokinesis)
G0: Cells are removed from the cycle and are not dividing, though functioning.
G1: Cell growth, ribosome synthesis, and protein synthesis.
S: DNA replication, histone synthesis, and phospholipid synthesis.
G2: Developing and preparing organelles for division.
M: Mitosis; division of the nucleus.
Cytokinesis: Division of the cytoplasm and the cell divides.
82
Summarise the 3 checkpoints
1. End of G1 → S (Checkpoint 1)
- No damage/mutations to chromosomes?
- Right quantity of free DNA nucleotides, DNA polymerase, and ATP?
- Hormones & growth factors, SA:V ration, contact inhibition, density.
- Cylin E/Cdk regulates tumour suppressor Retinoblastoma protein.
- If any are not optimal, the cell will not enter S phase.
2. Towards end of G2 (checkpoint 2) → enter into Mitosis (Prophase)
- Cell size large enough?
- DNA replication successfully completed?
- DNA mutations corrected (p53)?
- Cyclin B phosphorylated to produce MPF (reaches a max level)
- If any are not optimal, the cell will not enter Mitosis.
3. Metaphase checkpoint (checkpoint 3)
- Sister chromosomes joined by a centromere?
- Centromere is attached to spindle fibres?
- Cyclin expression stops, MPF levels decrease reaching a 'lower' level.
- Triggering exit (into Anaphase) & out of mitosis and into cytokinesis.
83
State and explain the two types of regulatory molecules in the cell cycle
- Protein kinases and cyclins
- Synthesised by protooncogenes and tumour suppressor genes. Mutations in these genes can cause cancer.
Protein kinase:
- An enzyme that activates or inactivates other proteins by phosphorylation
(e.g. Cdks).
- Phosphorylation: When Cdks transfer a phosphate group to a protein, altering its tertiary structure, and signals the cell to move to the next stage.
- At a constant concentration, but inactive most of the time.
- To be active, Cdks must bind to a specific cyclin (cyclically fluctuating in concentration).
- Cylin-dependent kinases (Cdks): Kinases that are bound to cyclins (e.g. MPF). Its activity rises and falls with changes in the concentration of its cyclin partner.
- Once Cdks bind to cyclins, they form a cyclin Cdk complex, which stimulates the cell to move to next stage.
- Cyclins eventually dissociate from Cdks which prevents a cell from entering next stage.
84
State examples of external and internal factor that regulate the cell cycle
External:
- Growth factors/hormones
- Contact inhibition
- Nutrient dependence
- Anchorage
- Cell size
- Density dependence
Internal:
- Activation of specific gene expression.
- Each protein has a unique function (e.g. TF, receptor, pump)
85
What is anchorage dependence?
Animals will stop dividing if they are not physically attached to the extracellular matrix, or another physical surface.
86
What is density dependence?
Animal cells stop dividing once a single layer of cells has formed. More cell division can be forced by death or removal of cells from this monolayer.
- Cells are stimulated to divide to close gaps (contact inhibition).
- Cells close in contact with one another will usually not divide.
87
What is nutrient dependence?
Cells require sufficient nutrient supplies of specific nutrients (e.g. glucose, fatty acids).
88
How does cell size regulate the cell cycle?
When the cell increaes, its SA:V ratio decreases, stimulating the cell to divide.
89
Explain growth factors
Growth factors will bind to specific target receptor proteins that are embedded in the cell membrane, initiating a metabolic pathway which stimulates growth and triggers the cell to pass checkpoint 1 into S phase.
Example: Human Growth Hormone (HGH) is secreted by pituitary gland in the brain and travels to target cells in the muscles and liver. It binds to its receptors and initiates cell division. HGH is responsible for dwarfism and gigantism.
90
How does Cyclin E regulate the cell cycle?
- Internal factor
- Accumulates in G1, interacts and activates CDK2.
- Cyclin E + Cdks → cyclin E + phosphate (phosphorylated)
- This inhibits the action of suppressor proteins (e.g. Retinoblastoma protein) that are preventing the cell from entering S phase.
- Activates gene products that activate entry into S phase.
91
How does Cyclin B regulate the cell cycle?
- Internal factor
- Produced during S phase and G2 phase.
- Cyclin B +Cdks → MPF (maturation promoting factor).
- Low in concentrations but increases when required.
- MPF activates: proteins for chromosome condensation, degration of nuclear membrane, and assembly of spindle fibres.
- When the cell reaches a 'max' level, the cell is ready to pass checkpoint 2.
- If everything is optimal, the cell enters PROPHASE.
92
Role of tumour suppressor genes and proto-oncogenes
- Tumour suppressor genes: Holds the cell in its current phase.
- Proto-oncogenes: Pushes the cell into the next phase.
93
How does the p53 protein regulate the cell cycle?
- Acts as a tumor suppressor, which regulates cell division by keeping cells from growing and dividing (proliferating) too fast or in an uncontrolled way.
- Binds to errors on DNA and flags them to be fixed by a DNA polymerase.
- If it is not fixed, the cell will directly go into programmed cell death, so it aborts and ends the cell cycles, causing cell death (apoptosis).
94
What is MPF degradation and how does it regulate the cell cycle?
- After entry into MITOSIS, MPF is degraded and Cyclin B levels decrease back to zero.
- As MPF levels decrease, it reacher a certain level which then triggers TELOPHASE to start.
- It is like an internal hour-glass within the cell that ensures enough time can pass for PROPHASE and METAPHASE to complete properly.
95
When are MPF levels high and low during mitosis?
MPF levels are the highest at the beginning of mitosis, and the lowest near the end of mitosis.
96
What causes cancer?
- Uncontrolled cell division, leading to tumour formation.
- This may be due to an accumulation of events or alterations in genetic material that disrupt the cell cycle.
- This means that the checkpoints may not be functioning correctly
