Chapter 1: Cell Biology Flashcards
1
Q
What are the three postulates of Cell Theory?
A
- All living organisms are composed of cells
- Cells are the smallest possible units of life
- Cells arise from pre-existing cells
2
Q
Draw
A
Represent by means of labelled, accurate diagram or graph, using a pencil. A ruler [straight edge] should be used for straight lines. Diagrams should be drawn to scale.
Diagrams should not be drawn faintly as they will not show clearly in scans.
There should be no gaps, overlaps or multiple lines.
Labelling lines should be drawn using a ruler and they should point precisely to the structure being labelled.
3
Q
What are four modern additions to Cell Theory?
A
- Surrounded by membrane which separates the cell contents from everything else outside
- Contain genetic material which stores all of the instructions needed for the cell’s activities
- Many of these activities are chemical reactions, catalysed by enzymes produced inside the cell
- Have their own energy release system that powers all of the cell’s activities
4
Q
What are the seven basic functions all living things must carry out to survive?
A
- Metabolism- chemical reactions that occur inside the cell for the purpose of releasing energy (e.g. cell respiration)
- Reproduction- producing offspring (sexually or asexually)
- Sensitivity- the ability to react to internal and external stimuli
- Homeostasis- keeping the internal conditions of the organism stable and relatively constant
- Excretion- the ability to remove waste products that occur as a consequence of metabolism
- Nutrition- the ability to obtain food in order to provide the energy and the materials needed for growth
- Growth- an increase in size which is irreversible
Mnemonic: MR SHENG
5
Q
How does Paramecium fulfill the seven basic functions of life?
A
Metabolism: produces enzymes which catalase many different chemical reactions in the cytoplasm
Reproduction: reproduces asexually using meiosis and gametes
Sensitivity: reacts to stimuli, e.g. reverses its direction of movement when it touches a solid object.
Homeostasis: keeps internal conditions within limits, e.g. expels excess water using contaractile vacuoles
Excretion: expels waste products of metabolism, e.g. CO2 from respiration diffuses out of the cell
Nutrition: feeds on smaller organisms by ingesting and digesting them in vesicles (endocytosis)
Growth: increases in size and dry mass by accumulating organic matter and minerals from its food
6
Q
How does Chlamydomonas fulfill the seven basic functions of life?
A
Metabolism: produces enzymes which catalase many different chemical reactions in the cytoplasm
Reproduction: reproduces asexually using meiosis and gametes
Sensitivity: reacts to stimuli, e.g. senses where the brightest light is with its eyespot and swims towards it
Homeostasis: keeps internal conditions within limits, e.g. expels excess water using contaractile vacuoles
Excretion: expels waste products of metabolism, e.g. oxygen from photosynthesis diffuses out of the cell
Nutrition: produces its own food by photosynthesis using a chloroplast that occupies much of the cell
Growth: increases in size and dry mass due to photosynthesis and absorption of minerals
7
Q
How does Chlamydomonas fulfill the seven basic functions of life?
A
Metabolism: produces enzymes which catalase many different chemical reactions in the cytoplasm
Reproduction: reproduces asexually using meiosis and gametes
Sensitivity: reacts to stimuli, e.g. senses where the brightest light is with its eyespot and swims towards it
Homeostasis: keeps internal conditions within limits, e.g. expels excess water using contaractile vacuoles
Excretion: expels waste products of metabolism, e.g. oxygen from photosynthesis diffuses out of the cell
Nutrition: produces its own food by photosynthesis using a chloroplast that occupies much of the cell
Growth: increases in size and dry mass due to photosynthesis and absorption of minerals
8
Q
As a cell grows larger its surface area to volume ratio becomes ________.
A
As a cell grows larger its surface area to volume ratio becomes SMALLER.
9
Q
What rate depends on the surface area of the cell?
A
The rate of material exchange (the rate at which materials enter or leave a cell) depends on the surface area of the cell
10
Q
What rate depends on the volume of a cell?
A
The rate of metabolism (the rate at which materials are used or produced) depends on the volume of the cell
11
Q
As a cell grows, ____1____ increases faster than ____2____, leading to a ____3____ surface area to volume ratio.
A
As a cell grows, VOLUME increases faster than SURFACE AREA, leading to a DECREASED surface area to volume ratio.
12
Q
As a cell grows, ____1____ increases faster than ____2____, leading to a ____3____ surface area to volume ratio.
A
As a cell grows, VOLUME increases faster than SURFACE AREA, leading to a DECREASED surface area to volume ratio.
13
Q
What are two advantages to being multicellular rather than unicellular?
A
Being multicellular allows:
- The organism to be larger
- Cell differentiation- where different groups of cells (tissues) become specialized for different functions
14
Q
What is the formula for calculating the linear magnification of a drawing or image?
A
Magnification= Image size (with ruler)/ Actual size (according to scale bar)
Hint: MIA
15
Q
What is the formula for calculating the linear magnification of a drawing or image?
A
Magnification
16
Q
What is the formula for calculating the actual size of a magnified specimen?
A
Actual size= Image size (with ruler)/ Magnification
17
Q
What three conventions should be followed when attempting to draw microscopic structures?
A
- A title should be included to identify the specimen
- A magnification or scale should be included to indicate relative size
- Identifiable structures should be clearly labelled
18
Q
Define emergent property
A
A property which a collection or complex system has but what individual members do not have
19
Q
How do emergent properties arise?
A
Emergent properties arise from the interaction of the component parts of a complex structure. Multicellular organisms have properties that emerge from interaction of their cellular components.
20
Q
With what famous phrase are emergent properties sometimes summed up?
A
“The whole is greater than the sum of its parts.” - Aristotle
21
Q
Define organelle
A
A discrete structure within a cell, with a specific function
22
Q
Define tissue
A
A group of cells similar to each other, along with their associated intercellular substances, which perform the same function within a multicellular organism
23
Q
Define organ
A
A group of tissues which work together as a single unit to perform a particular function within a multicellular organism
24
Q
Define organ system
A
A group of organs, vessels, glands, other tissues, and/or pathways which work together to perform a body function
within a multicellular organism
25
What is an example of emergent properties?
Each cell in a tiger is a unit of life that has distinctive properties such as sensitivity to light in retina cells, but all of a tiger's cells combined give additional emergent properties--for example the tiger can hunt and kill and have a profound ecological effect on its ecosystem
26
What do all the cells of an organism have in common?
All cells of an organism share an identical genome--each cell contains the entire set of genetic instructions for that organism
27
What is differentiation and how does it occur?
Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature. During differentiation a cell uses only the genes that it needs to follow its pathway of development. Other genes are unused. The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate. Once a pathway of development has begun in a cell, it is usually fixed and the cell cannot change to a different pathway. The cell is said to be 'commited'.
28
What is an example of differentiation?
The genes for making hemoglobin are only expressed in developing red blood cells
29
In what two forms can DNA be packaged within the nucleus of an eukaryotic cell?
1. Euchromatin- Active genes are usually packaged in an expanded form called euchromatin that is accessible to transcriptional machinery
2. Heterochromatin- Inactive genes are typically packaged in a more condensed form called heterochromatin (saves space, not transcribed)
30
Define stem cells
Cells that have the capacity to divide and differentiate along different pathways
31
What two qualities do stem cells have?
1. Self Renewal- They can continuously divide and replicate
| 2. Potency- They have the capacity to differentiate into specialised cell types
32
What are two places stem cells can be found?
1. Human embryos consist entirely of stem cells in their early stages, but gradually the cells in the embryo commit themselves to a pattern of differentiation. Once committed, a cell may still divide several more times, but all of the cells formed will differentiate in the same way and so they are no longer stem cells.
2. Small numbers of cells persist as stem cells and are still present in the adult body. They are mostly found in human tissues, including, bone marrow, skin and liver. They give some human tissues considerable powers of regeneration and repair, though, they do not have as great a capacity to differentiate in different ways as embryonic stem cells.
33
What are three examples of tissue that lack the stem cells needed for effective repair?
1. Brain
2. Kidney
3. Heart
34
What are the four main types of stem cells?
1. Totipotent- Can form any cell type, as well as extra-embryonic (placental) tissue (e.g. zygote)
2. Pluripotent- Can form any cell type (e.g. embryonic stem cells)
3. Multipotent- Can differentiate into a number of closely related cell types (e.g. haematopoeitic adult stem cells)
4. Unipotent- Can not differentiate, but are capable of self renewal (e.g. progenitor cells, muscle stem cells)
35
Define ethics
Moral principles that alow us to decide whether something is morally right or wrong
36
What is the main argument in favour of theurapeutic use of stem cells?
The main argument in favour of therapeutic use of stem cells is that the health and quality of life of patients suffering from otherwise incurable conditions may be greatly improved
37
What is the main ethical argument against stem cell therapies?
Ethical issues concerning stem cells taken from specially created embryos. Some argue that an embryo is a human life even at its earliest stage and if the embryo dies as a result of the procedure it is immoral, because a life has been ended and benefits from therapies using embryonic stem cells do not justify the taking of a life.
38
What are four arguments in favour of stem cells taken from specially created embryos?
1. Early stage embryos are little more than balls of cells that have yet to develop the essential features of a human life
2. Early stage embryos lack a nervous system so do not feel or suffer in other ways during stem cell procedures
3. If embryos are produced deliberately, no individual that would otherwise have had the chance of living is denied the chance of life
4. Large numbers of embryos produced by IVF are never implanted and do not get the chance of life; rather than kill these embryos it is better to use stem cells from them to treat diseases and save lives.
39
What are two examples of therapeutic stem cell therapy and how do they work?
1. Stargardt's macular dystrophy: Stargardt's disease is an inherited form of juvenile macular degeneration that causes progressive vision loss to the point of blindness. It is caused by a gene mutation that impairs energy transport in retinal photoreceptor cells, causing them to degenerate. It can be treated by replacing the dead cells in the retina with functioning ones derived from stem cells.
2. Parkinson's Disease: Parkinson's disease is a degenerative disorder of the central nervous system caused by the death of dopamine-secreting cells in the midbrain. Dopamine is a neurotransmitter responsible for transmitting signals involved in the production of smooth, purposeful movements. Consequently, individuals with Parkinson’s disease typically exhibit tremors, rigidity, slowness of movement and postural instability. It can be treated by replacing dead nerve cells with living, dopamine-producing ones.
40
What are the three sources stem cells can be derived from?
1. Embryos (may be specially created by therapeutic cloning)
2. Umbilical cord blood or placenta of a new-born baby
3. Certain adult tissues like the bone marrow (cells are not pluripotent)
41
What are two arificial stem cell techniques, how do they work and what are the ethical concerns with them?
1. Somatic cell nuclear transfer (SCNT): SCNT involves the creation of embryonic clones by fusing a diploid nucleus with an enucleated egg cell (therapeutic cloning). More embryos are created by this process than needed, raising ethical concerns about the exigency of excess embryos.
2. Nuclear reprogramming- In nuclear reprogramming a change in the gene expression profile of a cell is induced in order to transform it into a different cell type (transdifferentiation). This process involves the use of oncogenic retroviruses and transgenes, increasing the risk of health consequences (i.e. cancer).
42
Define resolution
The ability of the microscope to show two close objects separately in the image
43
Define ultrastructure
The detailed structure of a cell revealed by the electron microscope
44
What are the nine main parts of a light microscope?
1. Eye piece
2. Nose piece
3. Objective lens
4. Stage
5. Condenser lens
6. Diaphram
7. Coarse focusing know
8. Fine focusing knob
9. Lamp
45
How many micrometres are there in a millimetre?
1000um= 1mm
46
What three levels of magnification does a typical school light microscope have?
1. Times 40 (low power)
2. Times 100 (medium power)
3. Times 400 (high power)
47
How much higher is the resolution of electron microscopes versus light microscopes?
Electron microscopes have a resolution that is 200 times greater than light microscopes
48
What two types can cells be divided into acroding to their structure?
1. Prokaryotic
| 2. Eukaryotic
49
What are two distinctive features of prokaryotic cells?
1. Not compartmentalized
| 2. Do not have any membrane-bound organelle (i.e. nucleus, mitochondria)
50
What nine key features do prokaryotic cells typically have and what are they?
1. Cell wall- rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis)
2. Plasma membrane- semi-permeable and selective barrier surrounding the cell
3. Cytoplasm- internal fluid component of the cell
4. Nucleoid- region of cytoplasm containing naked DNA (DNA strand is circular and called a genophore)
5. Ribosomes-
complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryote ribosome = 70S)
6. Pili- hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)
7. Flagellum- long, slender projections containing a motor protein that enables movement (singular: flagellum)
8. Plasmids- autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer)
9. Slime capsule- a thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis
51
How do prokaryotic cells divide?
Prokaryotic cells divide by a process called binary fission--this simply means splitting in two. The bacterial chromosome is replicated so there are two identical copies. These are moved to opposite ends of the cell and the wall and plasma membrane are then pulled inwards so the cell pinches apart to form two identical cells.
Fun Fact: Some prokaryotes can double in volume and divide by binary fission every 30 minutes.
52
What are the five organelles with a single membrane found in eukaryotic cells?
1. Rough endoplasmic reticulum
2. Smooth endoplasmic reticulum
3. Golgi apparatus
4. Lysosomes
5. Vesicles and vacuoles
53
What three organells with a double membrane are found in eukaryotic cells?
1. Nucleus
2. Mitochondrion
3. Chloroplast
54
What is a key advantage of compartmentalization?
Enzymes and substrates used in a process can be concentrated in a small area, with pH and other conditions at optimum levels and with no other enzymes that might disrupt the process
55
What two organelles are present in plant cells but not animal cells?
1. Chloroplasts
| 2. Cell walls
56
What kingdom do prokaryotes belong to?
Monera
57
What two domains can prokaryotes be classified into and where can they be found?
1. Archaebacteria: found in extreme environments like high temperatures, salt concentrations or pH (i.e. extremophiles)
2. Eubacteria: traditional bacteria including most known pathogenic forms (e.g. E. coli, S. aureus, etc.)
58
What is binary fission and how does it work?
Binary fission is a form of asexual reproduction used by prokaryotic cells. In the process of binary fission:
1. Circular DNA is copied in response to a replication signal
2. Two DNA loops attach to the membrane
3. The membrane elongates and pinches off (cytokinesis), forming two cells
59
Define endosymbiosis
Type of symbiosis (mutually beneficial relationship) in which one organism lives inside the other, the two typically behaving as a single organism. It is believed to be the means by which such organelles as mitochondria and chloroplasts arose within eukaryotic cells
60
What four distinct kingdoms can eukaryotes be divided into and how can each kingdom be distinguished?
1. Protista- unicellular organisms; or multicellular organisms without specialised tissue
2. Fungi- have a cell wall made of chitin and obtain nutrition via heterotrophic absorption
3. Plantae- have a cell wall made of cellulose and obtain nutrition autotrophically (via photosynthesis)
4. Animalia- no cell wall and obtain nutrition via heterotrophic ingestion
61
What three organelles are found in prokaryotes and eukaryotes?
1. Ribosomes
2. Cytoskeleton
3. Plasma membrane
62
What is the structure and function of a ribosome?
Structure: Two subunits made of RNA and protein; larger in eukaryotes (80S) than prokaryotes (70S)
Function: Site of polypeptide synthesis (this process is called translation)
63
What is the structure and function of the cytoskeleton?
Structure: A filamentous scaffolding within the cytoplasm (fluid portion of the cytoplasm is the cytosol)
Function: Provides internal structure and mediates intracellular transport (less developed in prokaryotes)
64
What is the structure and function of the plasma membrane?
Structure: Phospholipid bilayer embedded with proteins (not an organelle per se, but a vital structure)
Function: Semi-permeable and selective barrier surrounding the cell
65
What six organelles are found in all eukaryotic cells?
1. Nucleus
2. Endoplasmic reticulum
3. Golgi apparatus
4. Mitochondrion
5. Peroxisome
6. Centrosome
66
What is the structure and function of the nucleus?
Structure: Double membrane structure with pores; contains an inner region called a nucleolus
Function: Stores genetic material (DNA) as chromatin; nucleolus is site of ribosome assembly
67
What is the structure and function of the endoplasmic reticulum?
Structure: A membrane network that may be bare (smooth ER) or studded with ribosomes (rough ER)
Function: Transports materials between organelles (smooth ER = lipids ; rough ER = proteins)
68
What is the structure and function of Golgi apparatus?
Structure: An assembly of vesicles and folded membranes located near the cell membrane
Function: Involved in the sorting, storing, modification and export of secretory products
69
What is the structure and function of a mitochondrion?
Structure: Double membrane structure, inner membrane highly folded into internal cristae
Function: Site of aerobic respiration (ATP production)
70
What is the structure and function of a peroxisome?
Structure: Membranous sac containing a variety of catabolic enzymes
Function: Catalyses breakdown of toxic substances (e.g. H2O2) and other metabolites
71
What is the structure and function of a centrosome?
Structure: Microtubule organising centre (contains paired centrioles in animal cells but not plant cells)
Function: Radiating microtubules form spindle fibres and contribute to cell division (mitosis / meiosis)
72
What three organelles are typically only found in plant cells?
1. Chloroplast
2. Vacuole (large and central)
3. Cell wall
73
What is the structure and function of a chloroplast?
Structure: Double membrane structure with internal stacks of membranous discs (thylakoids)
Function: Site of photosynthesis – manufactured organic molecules are stored in various plastids
74
What is the structure and function of the vacuole?
Structure: Fluid-filled internal cavity surrounded by a membrane (tonoplast)
Function: Maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)
75
What is the structure and function of the cell wall?
Structure: External outer covering made of cellulose (not an organelle per se, but a vital structure)
Function: Provides support and mechanical strength; prevents excess water uptake
76
What organelle is typically only found in animal cells?
1. Lysosome
77
What is the structure and function of a lysosome?
Structure: Membranous sacs filled with hydrolytic enzymes
Function: Breakdown / hydrolysis of macromolecules (presence in plant cells is subject to debate)
78
How do electron microscopes work?
Electron microscopes use electron beams focused by electromagnets to magnify and resolve microscopic specimens
79
What are the two most common types of electron microscopes and what are they used for?
1. Transmission electron microscopes (TEM): generate high resolution cross-sections of objects
2. Scanning electron microscopes (SEM): display enhanced depth to map the surface of objects in 3D
80
What are the two key advantages when comparing electron microscopes to light microscopes?
1. They have a much higher range of magnification (can detect smaller structures)
2. They have a much higher resolution (can provide clearer and more detailed images)
81
What is the main disadvantage of electron microscopes when compared to light microscopes?
1. They cannot display living specimens in natural colours
82
Define micrograph
A photo or digital image taken through a microscope to show a magnified image of a specimen
83
How can the relative abundance of mitochondria be used to deduce cell function?
Cells with many mitochondria typically undertake energy-consuming processes (e.g. neurons, muscle cells)
84
How can the relative abundance of endoplasmic reticulum networks be used to deduce cell function?
Cells with extensive ER networks undertake secretory activities (e.g. plasma cells, exocrine gland cells)
85
How can the relative abundance of lysosomes be used to deduce cell function?
Cells rich in lysosomes tend to undertake digestive processes (e.g. phagocytes)
86
How can the relative abundance of chloroplasts be used to deduce cell function?
Cells with chloroplasts undergo photosynthesis (e.g. plant leaf tissue but not root tissue)
87
What are the four key features prokaryotic cells and eukaryotic cells differ in?
1. DNA (composition and structure)
2. Organelles (types present and relative sizes)
3. Reproduction (mode differs according to chromosome structure)
4. Average size (exceptions may exist)
Mnemonic: DORA
88
What are three differences in the DNA of prokaryotic versus eukaryotic cells?
Prokaryotes:
1. DNA is naked
2. DNA is circular
3. Usually no introns
Eukaryotes:
1. DNA bound to protein
2. DNA is linear
3. Usually has introns
89
What are three differences in the organelles of prokaryotic versus eukaryotic cells?
Prokaryotes:
1. No nucleus
2. No membrane-bound
3. 70S ribosomes
Eukaryotes:
1. Has a nucleus
2. Membrane-bound
3. 80S ribosomes
90
What are two differences in the reproduction of prokaryotic versus eukaryotic cells?
Prokaryotes:
1. Binary fission
2. Single chromosome (haploid)
Eukaryotes:
1. Mitosis and meiosis
2. Chromosomes paired (diploid or more)
91
What is the difference in the average size of prokaryotic versus eukaryotic cells?
Prokaryotes:
1. Smaller (~ 1- 5um)
Eukaryotes:
1. Larger (~ 10- 100um)
92
What are eight differences that separate plant versus animal cells?
Plant:
1. Have plastids (e.g. chloroplast)
2. Have a cell wall (made of cellulose)
3. Have a large, central vacuole
4. May have plasmodesmata
5. Do not have centrioles
6. Do not have cholesterol in cell membrane
7. Store excess glucose as starch
8. Generally have a fixed, regular shape
Animal:
1. Do not have plastids
2. Do not have a cell wall
3. Have small, temporary vacuoles
4. Do not have plasmodesmata
5. Have paired centrioles within centrosome
6. Have cholesterol in the cell membrane
7. Store excess glucose as glycogen
8. Generally have an amorphous shape
93
What was the first membrane structure model that attempted to describe the position of proteins within the bilayer?
The Davson-Danielli Model
94
What did the Davson Danielli Model consist of?
Danielli and Davson proposed a model whereby two layers of protein flanked a central phospholipid bilayer. The model was described as a 'lipo-protein sandwich’, as the lipid layer was sandwiched between two protein layers. The dark segments seen under electron microscope were identified (wrongly) as representing the two protein layers.
95
What were the four main problems with the Davson-Danielli model?
1. It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
2. It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
3. It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores)
4. The temperatures at which membranes solidified did not correlate with those expected under the proposed model
96
What three main pieces of evidence falsified the Davson-Danielli model?
1. Freeze-fracture electron micrographs showed that globular proteins were present in the centre of the phospholipid bilayer
2. Analysis of membrane proteins showed that parts of their surfaces were hydrophobic, so they would be positioned in the bilayer and in some cases would extend from one side to the other
3. Fusion of cells with membrane proteins tagged with different coloured fluorescent markers showed that these proteins can move within the membrane as the colours became mixed within a few minutes of cell fusion
97
What evidence from the 1950s seemed to support the Davson-Danielli model?
High magnification electron micrographs were first produced in the 1950s. In these micrographs membranes appeared as two dark lines separated by a lighter band. This seemed to fit the Davson-Danielli model, as proteins usually appear darker than phospholipids in electron micrographs. The electron micrograph below shows membranes both at the surface of cells and around vesicles with the appearance that seemed to back up the Davson-Danielli model.
98
What three conditions do polar amino acids cause or create?
1. Polar amino acids on the surface of proteins make them water soluble
2. Polar amino acids create channels through which hydrophobic substances can diffuse. Positively charged R groups allow negatively charged ions through and vice versa.
3. Polar amino acids cause parts of membrane proteins to protrude from the membrane. Transmembrane proteins have two such regions.
99
What two conditions do non-polar amino acids cause or create?
1. Non-polar amino acids in the centre of water-soluble proteins stabilize their structure
2. Non-polar amino acids cause proteins to remain embedded in membranes
100
What three kinds of proteins occupy the membrane and in what position?
1. Integral proteins: embedded in the phospholipid bilayer and are typically transmembrane (they span across the bilayer)
2. Peripheral proteins: temporarily attached by non-convalent interactions and associate with one outer surface of the membrane
3. Glycoproteins: have sugar units attached on the outer surface of the membrane
101
What are phospholipids and what is an important feature of phospholipid molecules?
Phospholipids are the basic component of all biological membranes. Phospholipid molecules are amphipathic. This means that part of the molecule is attracted to water (hydrophilic) and part is not attracted to water (hydrophobic).
102
What is the structure of phospholipids?
- Consist of a polar head (hydrophilic) composed of a glycerol and a phosphate molecule
- Consist of two non-polar tails (hydrophobic) composed of fatty acid (hydrocarbon) chains
103
How are phospholipids arranged in membranes?
- Phospholipids spontaneously arrange into a bilayer
- The hydrophobic regions face inwards and are shielded from the surrounding polar fluids
- The hydrophilic head regions associate with the cytosolic and extracellular fluids respectively
104
What are four main properties of the phospholipid bilayer?
1. The bilayer held together by weak hydrophobic interactions between the tails
2. Hydrophilic / hydrophobic layers restrict the passage of many substances
3. Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
4. This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocytosis)
105
What is the structure of membrane proteins?
The amino acids of a membrane protein are localised according to polarity:
- Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer
- Polar (hydrophilic) amino acids are located internally and face aqueous solutions
106
What two tertiary structures do transmembrane proteins typically adopt?
1. Single helices / helical bundles
| 2. Beta barrels (common in channel proteins)
107
What three kinds of structures do integral proteins typically adopt and for what functions?
1. a-helix: recognition, receptors
2. Helical bundle: enzymes, transporters, receptors
3. B-barrel: transporters (channel proteins)
108
What three functions do peripheral proteins typically carry out?
1. Enzymes
2. Anchorage
3. Transporters (carriers)
109
What are the six key functions of membrane proteins?
1. Junctions: Serve to connect and join two cells together
2. Enzymes: Fixing to membranes localises metabolic pathways
3. Transport: Responsible for facilitated diffusion and active transport
4. Recognition: May function as markers for cellular identification
5. Anchorage: Attachment points for cytoskeleton and extracellular matrix
6. Transduction: Function as receptors for peptide hormones
Mnemonic: Jet Rat
110
In what type of cell is cholesterol found?
Cholesterol is a component of animal cell membranes. It is absent in plant cells, as these plasma membranes are surrounded and supported by a rigid cell wall made of cellulose.
111
What is the structure of cholesterol?
- Cholesterol is an amphipathic molecule (like phospholipids), meaning it has both hydrophilic and hydrophobic regions
- Cholesterol’s hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate heads of phospholipids
- The remainder (and majority) of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails
112
What are four ways cholesterol interacts with the fatty acid tails of phospholipids to moderate the properties of the membrane?
1. Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity
2. It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross
3. It functions to separate phospholipid tails and so prevent crystallisation of the membrane
4. It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein
113
Why is it important for cholesterol to restrict the movement of phospholipid molecules in animal cells?
This is important, as animal cells need to maintain concentration differences of these ions across their membranes, so diffusion through the membrane must be restricted.
114
What are six examples of membrane proteins and their functions?
1. Insulin receptor: an integral protein that is a hormone receptor
2. Cadherin: an integral protein used for cell-to-cell adhesion
3. Cytochrome c: a peripheral protein used for electron transport
4. Cytochrome oxidase: an integral protein that is an immobilized enzyme
5. Nicotinic acetylcholine receptor: an integral protein that is both a receptor for a neurotransmitter and a channel for facilitated diffusion of sodium ions
6. Calcium pump: an integral protein for active transport of calcium ions
115
What is passive transport?
The movement of particles along a concentration gradient (from a region of higher concentration to a region of lower concentration). It does not require the expenditure of energy (ATP hydrolysis).
116
What are two key qualities that cellular membranes possess?
1. Semi-permeable: only certain materials may freely cross; large and charged substances are typically blocked
2. Selective: membrane proteins may regulate the passage of material that cannot freely cross
117
What are the three main types of passive transport and what do each of them entail?
1. Simple diffusion-
movement of small or non-polar (lipophilic) molecules (e.g. O2, CO2, glycerol, etc.)
2. Osmosis- movement of water molecules (dependent on solute concentrations)
3. Facilitated diffusion-
movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)
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What two conditions need to be established for diffusion to occur?
1. There has to be a concentration gradient
2. The membrane must be permeable to the particle. For example, membranes are not permeable to cellulose so it does not diffuse across.
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What three factors can the rate of diffusion be influenced by?
1. Temperature: affects kinetic energy of particles in solution
2. Molecular size: larger particles are subjected to greater resistance within a fluid medium
3. Steepness of gradient: rate of diffusion will be greater with a higher concentration gradient
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What is facilitated diffusion and when is it used?
Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein. It is utilized by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions).
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What two types of transport proteins is facilitated diffusion mediated by?
1. Channel proteins
| 2. Carrier proteins
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What are carrier proteins? (four points)
1. Integral glycoproteins which bind a solute and undergo a conformational change to translocated the solute across the membrane
2. Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
3. Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
4. Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1000 molecules per second)
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What are channel proteins?
1. Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
2. Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
3. Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
4. Channel proteins have a much faster rate of transport than carrier proteins
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What is the difference between carrier and channel proteins? (ten points)
1. Solutes diffuse through the pore of channel proteins, whereas career proteins bind solutes on one side of membrane and release it on the other side
2. Compared with channel proteins, carrier proteins have very slow transport rates (on the order of 1000 solute molecules per second)
3. Unlike carrier proteins, channel proteins contain a pore, which facilitates the solute transportation
4. Unlike channel proteins, carrier proteins have alternate solute-bound conformations
5. Channel proteins are lipoproteins, while carrier proteins are glycoproteins
6. Carrier proteins can mediate both active and passive transport, while channel proteins can mediate only passive transport
7. Channel proteins are synthesized on ribosomes bound to endoplasmic reticulum, while carrier proteins are synthesized on free ribosomes in the cytoplasm
8. Carrier proteins can transport molecules or ions against the concentration gradient, while channel protein cannot
9. Carrier proteins move across the membrane, whereas channel proteins do not move while transporting molecules or ions
10. Channel proteins only pass water soluble molecules, while carrier proteins transport both water soluble and insoluble substances
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What are potassium channels?
Integral proteins with a hydrophilic inner pore via which potassium ions may be transported
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How are potassium channels structured?
Comprised of four transmembrane subunits, while the inner pore contains a selectivity filter at its narrowest region that restricts passage of alternative ions
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What does the opening and closing of potassium channels depend on?
Typically voltage-gated and cycle between an opened and closed conformation depending on the transmembrane voltage:
- They are closed when the axon is polarized
- They are open in response to depolarization of the axon membrane
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How is osmosis different from diffusion?
The passive movement of water across membranes is different from diffusion across membranes because water is the solvent. The direction in which water moves is due to the concentration of the solutes, rather than the concentration of water molecules, so it is called osmosis, rather than diffusion.
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Define solvent
A liquid in which particles dissolve
130
Define solute
Dissolved particles
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Define osmosis
The passive movement of water molecules from a region of lower solute concentration to a region of higher solute concentration, across a partially permeable membrane
132
Define osmolarity
The number of moles of solute particles per unit volume of solution.
Note: Pure water has an osmolarity of zero. The greater the concentration of solutes, the higher the osmolarity.
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If two solutions at equal pressure but with different osmolarity are separated by a partially permeable membrane, water will move by osmosis from the solution with the ____1____ osmolarity to the solution with the ____2____ osmolarity.
1. lower
| 2. higher
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Define hypertonic
Solutions with a relatively higher osmolarity (high solute concentration)
135
Define hypotonic
Solutions with a relatively lower osmolarity
136
Plant cells lose water if their osmolarity is ____1____ than that of the solution
1. lower
| i. e. the solution is hypertonic
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Plant cells absorb water from a surrounding solution if their osmolarity is ____1____ than that of the solution
1. higher
| i. e. the solution is hypotonic
138
Why must osmosis be prevented in donor organs?
Osmosis can cause cells in human tissues or organs to swell up and burst (lysis), or to shrink due to gain or loss of water by osmosis (crenation)
139
How is osmosis in donor organs prevented?
To prevent osmosis in donor organs, tissues or organs used in medical procedures such as kidney transplant must be bathed in a solution with the same osmolarity as human cytoplasm
- A solution of salts called isotonic saline is used for some procedures
- Donor organs are surrounded by isotonic slush when they are being transported, with the low temperatures helping to keep them in a healthy state
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Define active transport
The movement of substances across membranes using energy from ATP. Active transport can move substances against the concentration gradient--from a region of lower to a region of higher concentration
141
What are two important characteristics of protein pumps in the membrane?
1. Each pump only transports particular substances, so cells can control what is absorbed and what is expelled
2. Pumps work in a specific direction--the substance can only enter the pump on one side and can only exit on the other side
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What are the four steps involved in particle exchange via a protein pump?
1. Particle enters the pump from the side with a lower concentration
2. Particle binds to a specific site. Other types of particle cannot bind.
3. Energy from ATP is used to change the shape of the pump.
4. Particle is released on the side with a higher concentration and the pump then returns to its original shape.
143
What is a sodium-potassium pump?
An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)
144
What are the six key steps involved in ion exchange via a sodium-potassium pump?
1. Three sodium ions bind to intracellular sites on the sodium-potassium pump
2. A phosphate group is transferred to the pump via the hydrolysis of ATP
3. The pump undergoes a conformational change, translocating sodium across the membrane
4. The conformational change exposes two potassium binding sites on the extracellular surface of the pump
5. The phosphate group is released which causes the pump to return to its original conformation
6. This translocates the potassium across the membrane, completing the ion exchange
145
Define endocytosis
The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane
146
Define exocytosis
The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane
147
What are the three steps involved in endocytosis?
1. Part of the plasma membrane is pulled inwards
2. A droplet of fluid becomes enclosed when a vesicle is pinched off
3. Vesicles can then move through the cytoplasm carrying their contents
148
What are the three steps involved in exocytosis?
1. Vesicles fuse with the plasma membrane
2. The contents of the vesicle are expelled
3. The membrane then flattens out again
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What are the four steps involved in vesicular transport of proteins from the rough ER to the Golgi apparatus to the plasma membrane?
1. Proteins are synthesized by ribosomes and then enter the rough endoplasmic reticulum
2. Vesicles bud off from the rER and carry the proteins to the Golgi apparatus
3. The Golgi apparatus modifies the proteins
4. Vesicles bud off from the Golgi apparatus and carry the modified proteins to the plasma membrane
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What do we know and not know about the origins of the first cells?
There is no evidence that living cells can be formed on Earth today except by division of pre-existing cells. Spontaneous generation of cells is not currently possible. The general principle that cells are only formed by division of pre-existing cells can be used to trace life back to its origins. One of the great challenges in biology is to understand how the first cells evolved from non-living matter and why spontaneous generation could take place then but not now. The universality of the genetic code suggests strongly that all life evolved from the same original cells.
151
What did the most famous of Pasteur's experiments prove?
The general principle that cells can only come from pre-existing ones
152
How did Pasteur prove that cells can only come from pre-existing ones?
- Broths were stored in vessels that contained long tubings (swan neck ducts) that did not allow external dust particles to pass
- The broths were boiled to kill any micro-organisms present in the growth medium (sterilisation)
- Growth only occurred in the broth if the flask was broken open, exposing the contents to contaminants from the outside
- From this it was concluded that emergent bacterial growth came from external contaminants and did not spontaneously occur
153
What is explained by endosymbiotic theory?
The origin of eukaryotic cells
154
How are eukaryotic cells believed to have originated?
Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by endocytosis (more specifically) phagocytosis
The engulfed prokaryotic cell remained undigested as it contributed new functionality to the engulfing cell (e.g. photosynthesis)
Over generations, the engulfed cell lost some of its independent utility and became a supplemental organelle
155
What evidence supports the extracellular origins of mitochondria and chloroplasts?
Membranes (they have double membranes, as expected when cells are taken into a vesicle by endocytosis)
Antibiotics (they are susceptible to antibiotics which indicates that organelles may have bacterial origins)
Division (they grow and divide like cells)
DNA (they have a naked loop of DNA like prokaryotes)
Ribosomes (they synthesize some of their own proteins using 70S ribosomes, like prokaryotes)
Mnmemonic: MAD DR (mad doctor)
156
What are the differences between chromatin and chromosome? (three points)
Chromatin:
1. DNA is usually loosely packed within the nucleus as unravelled chromatin
2. In this unravelled form, the DNA is accessible to transcriptional machinery and so genetic information can be translated
3. DNA is organised as chromatin in all non-dividing cells and throughout the process of interphase
Chromosome:
1. DNA is temporarily packaged into a tightly wound and condensed chromosome prior to division (via supercoiling)
2. In this condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery
3. DNA is organised as chromosomes during the process of mitosis (condense in prophase, decondense in telophase)
157
What are the four distinct stages of mitosis?
1. Prophase
2. Metaphase
3. Anaphase
4. Telophase
158
What is interphase?
The resting phase between mitotic divisions of a cell.
159
What happens during prophase? (four stages)
1. The chromosomes become shorter and fatter by coiling. To become short enough they have to coil repeatedly. This is called supercoiling.
2. The nucleolus breaks down.
3. Microtubules grow from structures called microtubule organizing centres (MTOC) to form a spindle-shaped array that links the poles of the cell.
4. At the end of prophase the nuclear membrane breaks down.
160
What happens during metaphase? (three stages)
1. Microtubules continue to grow and attach to the centromeres on each chromosome.
2. The two attachment points on opposite sides of each centromere allow the chromatids of a chromosome to attach to microtubules from different poles.
3. The microtubules are all put under tension to test whether the attachment is correct. This happens by shortening of the microtubules at the centromere. If the attachment is correct, the chromosomes remain on the equator of the cell.
161
What happens during anaphase? (two stages)
1. At the start of anaphase, each centromere divides, allowing the pairs of sister chromatids to separate.
2. The spindle microtubules pull them rapidly towards the poles of the cell. Mitosis produces two genetically identical nuclei because sister chromatids are pulled to opposite poles. This is ensured by the way that the spindle microtubules were attached in metaphase.
162
What is telophase? (three stages)
The chromatids have reached the poles and are now called chromosomes.
1. At each pole the chromosomes are pulled into a tight group near the MTOC and a nuclear membrane reforms around them.
2. The chromosomes uncoil and a nucleolus is formed.
3. By this stage of mitosis the cell is usually already dividing and the two daughter cells enter interphase again.
163
What is mitotic index ?
The ratio between the number of cells in mitosis in a tissue and the total number of observed cells.
Mitotic index= (number of cells in mitosis) / (total number of cells)
164
What is the mitotic index used for? What does a high index indicate?
The mitotic index is used by doctors to predict how rapidly a tumour will grow and therefore what treatment is needed. A high index indicates a fast-growing tumour.
165
What is cytokinesis?
Cytokinesis is the division of the cytoplasm to form two cells. It occurs after mitosis and is different in plant and animal cells.
166
How is cytokinesis different in plant and animal cells?
- In plant cells a new cell wall is formed across the equator of the cell, with the plasma membrane on both sides. This divides the cell in two.
- In animal cells the plasma membrane at the equator is pulled inwards until it meets in the centre of the cell, dividing it in two.
167
What is the cell cycle?
The sequence of events between one cell division and the next. It has two main phases: interphase and cell division.
168
What four metabolic reactions occur during interphase?
- Cell respiration (also occurs during cell division)
- DNA replication in the nucleus and protein synthesis in the cytoplasm (only happens during interphase)
- Number of mitochondria in the cytoplasm increase, as they grow and divide
- In plant cells the numbers of chloroplasts increase, as they grow and divide
169
What three phases does interphase consist of?
1. G1 phase
2. S phase
3. G2 phase
170
What happens during S phase?
The cell replicates all the genetic material in its nucleus, so that after mitosis both the new cells have a complete set of genes. Some do not progress beyond G1, because they are never going to divide so do not need to prepare for mitosis.
171
Define cytokinesis
The process of cytoplasmic division, whereby the cell splits into two identical daughter cells
172
What are cyclins? What is the function of cyclins?
Cyclins are a family of regulatory proteins that control the progression of the cell cycle.
In other words, they ensure that tasks are performed at the correct time and that the cell only moves on to the next stage of the cycle when it is appropriate.
Cyclins activate cyclin dependent kinases (CDKs), which control cell cycle processes through phosphorylation.
In other words, cyclins bind to enzymes called cyclin-dependent kinases. These kinases then become active and attach phosphate groups to other proteins in the cell. The attachment of phosphate triggers the other proteins to become active and carry out tasks specific to one of the phases of the cell cycle.
173
How many main types of cyclin are there in human cells?
Four
174
What is oncogenesis?
The formation of tumours
175
Define oncogenes
A gene that has the potential to cause cancer
176
How does oncogenesis start?
The process starts with mutations in genes involved in the control of the cell cycle called oncogenes
177
What is the risk of tumour formation?
Mutations have to occur in several oncogenes in the same cell for control to be lost. The chance of this is very small but the body contains billions of cells, an one of which could have mutations in its oncogenes, so the overall risk is significant.
178
What increases the risk of tumour formation?
Anything that increases the chance of mutations will increase the risk of tumour formation. Some chemical substances cause mutations. These chemicals are called mutagens. Ionizing radiation also causes mutations and therefore tumours.
179
Define mutagen
An agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery)
180
Define cancers
Diseases caused by the growth of tumours
181
Define carcinogen
Mutagens that lead to the formation of cancer
182
What is metastasis?
Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour
183
What is a benign tumour?
Tumour cells that remain in their original location
184
What is a malignant tumour?
Tumour cells that spread and invade neighbouring tissue
185
How are secondary tumours treated?
Secondary tumours are made up of the same type of cell as the primary tumour – this affects the type of treatment required
E.g. If breast cancer spread to the liver, the patient has secondary breast cancer of the liver (treat with breast cancer drugs)
186
Does smoking cause cancer?
There is a positive correlation between cigarette smoking and death rate due to cancer. The more cigarettes smoked per day the higher the chance of developing cancer of the lung and some other organs. Although this correlation does not by itself prove that smoking causes cancer, there is also evidence that chemicals in tobacco smoke are mutagenic and therefore carcinogenic (cancer-causing).
