IB Cells

Question 1

Ribosomes Structure

Answer 1

Two subunits made of RNA and protein; larger in eukaryotes (80S) than prokaryotes (70S)

Question 2

Ribosomes Function

Answer 2

Site of polypeptide synthesis (this process is called translation)

Question 3

Cytoskeleton Structure

Answer 3

A filamentous scaffolding within the cytoplasm

Question 4

Cytoskeleton Function

Answer 4

Provides internal structure and mediates intracellular transport

Question 5

Plasma membrane Structure

Answer 5

Phospholipid bilayer embedded with proteins

Question 6

Plasma membrane Function

Answer 6

Semi-permeable and selective barrier surrounding the cell

Question 7

Nucleus Function

Answer 7

Stores genetic material (DNA) as chromatin; nucleolus is site of ribosome assembly

Question 8

Nucleus Structure

Answer 8

Double membrane structure with pores; contains an inner region called a nucleolus

Question 9

Endoplasmic Reticulum Structure

Answer 9

A membrane network that may be bare (smooth ER) or studded with ribosomes (rough ER)

Question 10

Endoplasmic Reticulum Function

Answer 10

Transports materials between organelles

(smooth ER = lipids ; rough ER = proteins)

Question 11

Golgi Apparatus Structure

Answer 11

An assembly of vesicles and folded membranes located near the cell membrane

Question 12

Golgi Apparatus Function

Answer 12

Involved in the sorting, storing, modification and export of secretory products

Question 13

Mitochondrion Structure

Answer 13

Double membrane structure, inner membrane highly folded into internal cristae

Question 14

Mitochondrion Function

Answer 14

site of aerobic respiration (ATP production)

Question 15

Centrosome Structure

Answer 15

Two perpendicular centrioles, (contains paired centrioles in animal cells but not plant cells)

Question 16

Centrosome Function

Answer 16

Radiating microtubules form spindle fibres and contribute to cell division (mitosis / meiosis)

Question 17

Chloroplast Structure

Answer 17

Double membrane structure with internal stacks of membranous discs (thylakoids)

Question 18

Chloroplast Function

Answer 18

Site of photosynthesis – manufactured organic molecules are stored in various plastids

Question 19

Cell Wall Structure

Answer 19

External outer covering made of cellulose (not an organelle per se, but a vital structure)

Question 20

Cell Wall Function

Answer 20

Provides support and mechanical strength; prevents excess water uptake

Question 21

Vacuole Structure

Answer 21

Fluid-filled internal cavity surrounded by a membrane (tonoplast)

Question 22

Vacuole Function

Answer 22

maintains hydrostatic pressure (animal cells may have small, temporary vacuoles)

Question 23

Lysosome Structure

Answer 23

Membranous sacs filled with hydrolytic enzymes

Question 24

Lysosome Function

Answer 24

Breakdown / hydrolysis of macromolecules (presence in plant cells is subject to debate)

Question 25

Principles of cell theory

Answer 25

1. All living things are composed of cells (or cell products)
2. The cell is the smallest unit of life
3. Cells only arise from pre-existing cells

Question 26

3 exceptions to cell theory and why

Answer 26

Striated muscle fibres:
Muscle cells fuse to form fibres that may be very long (>300mm)
Consequently, they have multiple nuclei despite being surrounded by a single, continuous plasma membrane
Challenges the idea that cells always function as autonomous units

Aseptate fungal hyphae:
Fungi may have filamentous structures called hyphae, which are separated into cells by internal walls called septa
Some fungi are not partitioned by septa and hence have a continuous cytoplasm along the length of the hyphae
Challenges the idea that living structures are composed of discrete cells

Giant Algae
Certain species of unicellular algae may grow to very large sizes (e.g. Acetabularia may exceed 7 cm in length)
Challenges the idea that larger organisms are always made of many microscopic cells

Question 27

All life functions of a cell

Answer 27

Metabolism – Living things undertake essential chemical reactions
Reproduction – Living things produce offspring, either sexually or asexually
Sensitivity – Living things are responsive to internal and external stimuli
Homeostasis – Living things maintain a stable internal environment
Excretion – Living things exhibit the removal of waste products
Nutrition – Living things exchange materials and gases with the environment
Growth – Living things can move and change shape or size

MR SHENG

Question 28

State how Paramecium (heterotroph) carries out all life functions

Answer 28

Paramecia are surrounded by small hairs called cilia which allow it to move (responsiveness)
Paramecia engulf food via a specialised membranous feeding groove called a cytostome (nutrition)
Food particles are enclosed within small vacuoles that contain enzymes for digestion (metabolism)
Solid wastes are removed via an anal pore, while liquid wastes are pumped out via contractile vacoules (excretion)
Essential gases enter (e.g. O2) and exit (e.g. CO2) the cell via diffusion (homeostasis)
Paramecia divide asexually (fission) although horizontal gene transfer can occur via conjugation (reproduction)

Question 29

State how Scenedesmus (autotroph) carry out, (nutrition/excretion), (metabolism), (reproduction), and (responsivness)

Answer 29

Scenedesmus exchange gases and other essential materials via diffusion (nutrition / excretion)
Chlorophyll pigments allow organic molecules to be produced via photosynthesis (metabolism)
Daughter cells form as non-motile autospores via the internal asexual division of the parent cell (reproduction)
Scenedesmus may exist as unicells or form colonies for protection (responsiveness)

Question 30

Why does SA:Volume limit cell size? What is an adaptation in the human body to increase SA

Answer 30

Cells need to produce chemical energy (via metabolism) to survive and this requires the exchange of materials with the environment

As a cell grows, volume (units3) increases faster than surface area (units2), leading to a decreased SA:Vol ratio

If metabolic rate exceeds the rate of exchange of vital materials and wastes (low SA:Vol ratio), the cell will eventually die. Hence growing cells tend to divide and remain small in order to maintain a high SA:Vol ratio suitable for survival

Adaptation:

Intestinal tissue of the digestive tract may form a ruffled structure (villi) to increase the surface area of the inner lining

or

Alveoli within the lung increase the total membrane surface

Question 31

Formula for magnification

Answer 31

Magnification = Image size (with ruler) ÷ Actual size (according to scale bar)

Question 32

What are emergent properties

Answer 32

Emergent properties arise when the interaction of individual component produce new functions

Question 33

What is cell differentiation and how does it occur?

Answer 33

Differentiation is the process during development whereby newly formed cells become more specialised and distinct from one another as they mature

All cells of an organism share an identical genome – each cell contains the entire set of genetic instructions for that organism. The activation of different instructions (genes) within a given cell by chemical signals will cause it to differentiate. (Some genes are expressed while other are not)

Question 34

What is chromatin, euchromatin and heterochromatin?

Answer 34

Within the nucleus of a eukaryotic cell, DNA is packaged with proteins to form chromatin

Active genes are usually packaged in an expanded form called euchromatin that is accessible to transcriptional machinery
Inactive genes are typically packaged in a more condensed form called heterochromatin (saves space, not transcribed)

Question 35

What are stem cells?

Answer 35

Stem cells are unspecialised cells that have two key qualities:

1. Self Renewal – They can continuously divide and replicate
2. Potency – They have the capacity to differentiate into specialised cell types

Question 36

What are the types of stem cells and where are they present in development?

Answer 36

Totipotent – Can form any cell type, as well as extra-embryonic (placental) tissue (e.g. zygote)

Pluripotent – Can form any cell type (e.g. embryonic stem cells)

Multipotent – Can differentiate into a number of closely related cell types (e.g. haematopoeitic adult stem cells)

Unipotent – Can not differentiate, but are capable of self renewal (e.g. progenitor cells, muscle stem cells)

Question 37

Give the general outline of stem cell therapy

Answer 37

The use of biochemical solutions to trigger the differentiation of stem cells into the desired cell type

Surgical implantation of cells into the patient’s own tissue

Suppression of host immune system to prevent rejection of cells (if stem cells are from foreign source)

Careful monitoring of new cells to ensure they do not become cancerous

Question 38

Outline two examples of stem cells treating disease. Briefly state what the disease is.

Answer 38

1. Stargardt’s Disease

An inherited form of juvenile macular degeneration that causes progressive vision loss to the point of blindness
Caused by a gene mutation that impairs energy transport in retinal photoreceptor cells, causing them to degenerate
Treated by replacing dead cells in the retina with functioning ones derived from stem cells

2. Parkinson’s Disease

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
Treated by replacing dead nerve cells with living, dopamine-producing ones

Question 39

What are the three sources for stem cells

Answer 39

Stem cells can be derived from one of three sources:

Embryos (may be specially created by therapeutic cloning)

Umbilical cord blood or placenta of a new-born baby

Certain adult tissues like the bone marrow (cells are not pluripotent)

Question 40

What are possible ethical considerations of harvesting and using stem cells?

Answer 40

Using multipotent adult tissue may be effective for certain conditions, but is limited in its scope of application
Stem cells derived from umbilical cord blood need to be stored and preserved at cost, raising issues of availability and access
The greatest yield of pluripotent stem cells comes from embryos, but requires the destruction of a potential living organism

Question 41

What is Somatic cell nuclear transfer (SCNT), state an advantage and disadvantage

Answer 41

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 excess embryos

Question 42

What is Nuclear reprogramming, state an advantage and disadvantage.

Answer 42

Induce a change in the gene expression profile of a cell in order to transform it into a different cell type (transdifferentiation)
Involves the use of oncogenic retroviruses and transgenes, increasing the risk of health consequences (i.e. cancer)

Question 43

What are the sizes of these structures?

(Eukaryotic cell (plant)
Eukaryotic cell (animal)
Organelle (e.g. mitochondrion)
Prokaryotic cell (bacteria)
Virus

Answer 43

Eukaryotic cell (plant) = ~100 μm
Eukaryotic cell (animal) = ~10 – 50 μm
Organelle (e.g. mitochondrion) = ~1 – 10 μm
Prokaryotic cell (bacteria) = ~1 – 5 μm
Virus = ~100 nm

Question 44

How do TEM and SEM microscopes produces an image?

Answer 44

Use electromagnets to focus electrons resulting in significantly greater magnifications and resolutions relative to light microscopes

Transmission electron microscopes (TEM) pass electrons through specimen to generate a cross-section
Scanning electron microscopes (SEM) scatter electrons over a surface to differentiate depth and map in 3D

Question 45

How do light microscopes produce an image

Answer 45

Use lenses to bend light and magnify images by a factor of roughly 100-fold

Question 46

Reasons why a electron microscope may be preferred to a light microscope or vice versa

Answer 46

Light Microscopes

Can be used to view living specimens in natural colour
Chemical dyes and fluorescent labelling may be applied to resolve specific structures

Electron Microscopes

Can be used to view dead specimens in monochrome (although false colour rendering may be applied)
Much higher resolution and magnification

Question 47

State the structures and function of the structure typically found in prokaryotic cells

Answer 47

Cytoplasm – internal fluid component of the cell
Nucleoid – region of the cytoplasm where the DNA is located (DNA strand is circular and called a genophore)
Plasmids – autonomous circular DNA molecules that may be transferred between bacteria (horizontal gene transfer)
Ribosomes – complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryote ribosome = 70S)
Cell membrane – Semi-permeable and selective barrier surrounding the cell
Cell wall – rigid outer covering made of peptidoglycan; maintains shape and prevents bursting (lysis)
Slime capsule – a thick polysaccharide layer used for protection against dessication (drying out) and phagocytosis
Flagella – Long, slender projections containing a motor protein that enables movement (singular: flagellum)
Pili – Hair-like extensions that enable adherence to surfaces (attachment pili) or mediate bacterial conjugation (sex pili)

Question 48

Outline binary fission

Answer 48

Binary fission is a form of asexual reproduction used by prokaryotic cells

In the process of binary fission:

The circular DNA is copied in response to a replication signal
The two DNA loops attach to the membrane
The membrane elongates and pinches off (cytokinesis), forming two cells

Question 49

Contrast plant and animal cells

Answer 49

Plants | Animals

Plastids present | lack of plastids
Cell wall made of cellulose | Lack of cell wall
Large central vacuole | small temporary vacuole
Store glucose as starch | Store glucose as glycogen

Question 50

Contrast the structure of eukaryotes and prokaryotes

Answer 50

Prokaryotes | eukaryotes

DNA is naked | DNA is bound to protein
DNA is circular | DNA is linear
No nucleus | Nucleus present
No membrane bound organelles | Has membrane bound organelles
70S Ribosomes | 80S Ribosomes

Question 51

Describe the structure of phospholipids

Answer 51

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

Question 52

State the properties of the phospholipid bilayer

Answer 52

The bilayer is held together by weak hydrophobic interactions between the tails
Hydrophilic / hydrophobic layers restrict the passage of many substances
Individual phospholipids can move within the bilayer, allowing for membrane fluidity and flexibility
This fluidity allows for the spontaneous breaking and reforming of membranes (endocytosis / exocyto

Question 53

Functions of membrane proteins

Answer 53

Junctions – Serve to connect and join two cells together
Enzymes – Fixing to membranes localises metabolic pathways
Transport – Responsible for facilitated diffusion and active transport
Recognition – May function as markers for cellular identification
Anchorage – Attachment points for cytoskeleton and extracellular matrix
Transduction – Function as receptors for peptide hormones

JET RAT

Question 54

Differentiate between Integral and Peripheral proteins in a membrane

Answer 54

Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)

Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane

Question 55

What structure do integral proteins have

Answer 55

Single helices / helical bundles
Beta barrels (common in channel proteins)

Question 56

Explain the function and alignment of cholesterol in the membrane

Answer 56

Cholesterol functions to immobilise the outer surface of the membrane, reducing fluidity

It makes the membrane less permeable to very small water-soluble molecules that would otherwise freely cross

It functions to separate phospholipid tails and so prevent crystallisation of the membrane

It helps secure peripheral proteins by forming high density lipid rafts capable of anchoring the protein

At high temperatures it stabilises the membrane and raises the melting point

At low temperatures it intercalates between the phospholipids and prevents clustering

Cholesterol’s hydroxyl (-OH) group is hydrophilic and aligns towards the phosphate heads of phospholipids

The remainder of the molecule (steroid ring and hydrocarbon tail) is hydrophobic and associates with the phospholipid tails

Question 57

What was the evidence that lead to the proposal of the Davison-Danielli model? What are the limitations of the model?

Answer 57

When viewed under a transmission electron microscope, membranes exhibit a characteristic ‘trilaminar’ appearance
Trilaminar = 3 layers (two dark outer layers and a lighter inner region)

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

There were a number of problems with the lipo-protein sandwich model proposed by Davson and Danielli:

-It assumed all membranes were of a uniform thickness and would have a constant lipid-protein ratio
-It assumed all membranes would have symmetrical internal and external surfaces (i.e. not bifacial)
-It did not account for the permeability of certain substances (did not recognise the need for hydrophilic pores)
-The temperatures at which membranes solidified did not correlate with those expected under the proposed model

Question 58

Based on what evidence was Davison and Danielli model falsified. What model was proposed instead?

Answer 58

Membrane proteins were discovered to be insoluble in water (indicating hydrophobic surfaces) and varied in size

Such proteins would not be able to form a uniform and continuous layer around the outer surface of a membrane

Fluorescent antibody tagging of membrane proteins showed they were mobile and not fixed in place

Membrane proteins from two different cells were tagged with red and green fluorescent markers respectively
When the two cells were fused, the markers became mixed throughout the membrane of the fused cell
This demonstrated that the membrane proteins could move and did not form a static layer (as per Davson-Danielli)

Freeze fracturing was used to split open the membrane and revealed irregular rough surfaces within the membrane

These rough surfaces were interpreted as being transmembrane proteins, demonstrating that proteins were not solely localised to the outside of the membrane structure

New Model:

In light of these limitations, a new model was proposed by Seymour Singer and Garth Nicolson in 1972

According to this model, proteins were embedded within the lipid bilayer rather than existing as separate layers

This model, known as the fluid-mosaic model

Question 59

Describe passive transport

Answer 59

Passive transport involves the movement of material along a concentration gradient (high concentration ⇒ low concentration)

Because materials are moving down a concentration gradient, it does not require the expenditure of energy (ATP hydrolysis)

Question 60

Describe active transport

Answer 60

Active transport involves the movement of materials against a concentration gradient (low concentration ⇒ high concentration)

Because materials are moving against the gradient, it requires the expenditure of energy (e.g. ATP hydrolysis)

Question 61

Describe the two types of active transport

Answer 61

There are two main types of active transport:

Primary (direct) active transport – Involves the direct use of metabolic energy (e.g. ATP hydrolysis) to mediate transport
Secondary (indirect) active transport – Involves coupling the molecule with another moving along an electrochemical gradient

Question 62

Describe the three types of passive transport

Answer 62

There are three main types of passive transport:

Simple diffusion – movement of small or lipophilic molecules (e.g. O2, CO2, etc.)

Diffusion is the net movement of molecules from a region of high concentration to a region of low concentration

This directional movement along a gradient is passive and will continue until molecules become evenly dispersed (equilibrium)
Small and non-polar (lipophilic) molecules will be able to freely diffuse across cell membranes (e.g. O2, CO2, glycerol)

Osmosis – movement of water molecules (dependent on solute concentrations)

Osmosis is the net movement of water molecules across a semi-permeable membrane from a region of low solute concentration to a region of high solute concentration (until equilibrium is reached)

Water is considered the universal solvent – it will associate with, and dissolve, polar or charged molecules (solutes)
Because solutes cannot cross a cell membrane unaided, water will move to equalise the two solutions
At a higher solute concentration there are less free water molecules in solution as water is associated with the solute
Osmosis is essentially the diffusion of free water molecules and hence occurs from regions of low solute concentration

Facilitated diffusion – movement of large or charged molecules via membrane proteins (e.g. ions, sucrose, etc.)

Question 63

What factors affect diffusion?

Answer 63

Temperature (affects kinetic energy of particles in solution)
Molecular size (larger particles are subjected to greater resistance within a fluid medium)
Steepness of gradient (rate of diffusion will be greater with a higher concentration gradient)

Question 64

What is the effect of placing animal and plant tissue in hypo and hyper - tonic solutions

Answer 64

In hypertonic solutions, water will leave the cell causing it to shrivel (crenation)
In hypotonic solutions, water will enter the cell causing it to swell and potentially burst (lysis)

In plant tissues, the effects of uncontrolled osmosis are moderated by the presence of an inflexible cell wall

In hypertonic solutions, the cytoplasm will shrink (plasmolysis) but the cell wall will maintain a structured shape
In hypotonic solutions, the cytoplasm will expand but be unable to rupture within the constraints of the cell wall (turgor)

Question 65

Describe Facilitated diffusion and provide an example

Answer 65

Facilitated diffusion is the passive movement of molecules across the cell membrane via the aid of a membrane protein

It is utilised by molecules that are unable to freely cross the phospholipid bilayer (e.g. large, polar molecules and ions)
This process is mediated by two distinct types of transport proteins – channel proteins and carrier proteins

Carrier Proteins

Integral glycoproteins which bind a solute and undergo a conformational change to translocate the solute across the membrane
Carrier proteins will only bind a specific molecule via an attachment similar to an enzyme-substrate interaction
Carrier proteins may move molecules against concentration gradients in the presence of ATP (i.e. are used in active transport)
Carrier proteins have a much slower rate of transport than channel proteins (by an order of ~1,000 molecules per second)

Channel Proteins

Integral lipoproteins which contain a pore via which ions may cross from one side of the membrane to the other
Channel proteins are ion-selective and may be gated to regulate the passage of ions in response to certain stimuli
Channel proteins only move molecules along a concentration gradient (i.e. are not used in active transport)
Channel proteins have a much faster rate of transport than carrier proteins

Potassium Channels

The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within
When the neuron fires, these ions swap locations via facilitated diffusion via sodium and potassium channels

Question 66

Describe active transport and provide an example

Answer 66

Active transport uses energy to move molecules against a concentration gradient

This energy may either be generated by:

The direct hydrolysis of ATP (primary active transport)
Indirectly coupling transport with another molecule that is moving along its gradient (secondary active transport)

Active transport involves the use of carrier proteins (called protein pumps due to their use of energy)

A specific solute will bind to the protein pump on one side of the membrane
The hydrolysis of ATP (to ADP + Pi) causes a conformational change in the protein pump
The solute molecule is consequently translocated across the membrane (against the gradient) and released

Sodium-Potassium Pump

The axons of nerve cells transmit electrical impulses by translocating ions to create a voltage difference across the membrane

At rest, the sodium-potassium pump expels sodium ions from the nerve cell, while potassium ions are accumulated within

An integral protein that exchanges 3 sodium ions (moves out of cell) with two potassium ions (moves into cell)

The process of ion exchange against the gradient is energy-dependent and involves a number of key steps:

Three sodium ions bind to intracellular sites on the sodium-potassium pump
A phosphate group is transferred to the pump via the hydrolysis of ATP
The pump undergoes a conformational change, translocating sodium across the membrane
The conformational change exposes two potassium binding sites on the extracellular surface of the pump
The phosphate group is released which causes the pump to return to its original conformation
This translocates the potassium across the membrane, completing the ion exchange

Question 67

Describe Vesicular transport within cells

Answer 67

Endoplasmic Reticulum

The endoplasmic reticulum is a membranous network that is responsible for synthesising secretory materials

Rough ER is embedded with ribosomes and synthesises proteins destined for extracellular use
Smooth ER is involved in lipid synthesis

Materials are transported from the ER when the membrane bulges and then buds to create a vesicle surrounding the material

Golgi Apparatus

The vesicle is then transported to the Golgi apparatus and fuses to the internal (cis) face of the complex

Materials move via vesicles from the internal cis face of the Golgi to the externally oriented trans face
While within the Golgi apparatus, materials may be structurally modified (e.g. truncated, glycosylated, etc.)

Material sorted within the Golgi apparatus will either be secreted externally or may be transported to the lysosome

Plasma Membrane

Vesicles containing materials destined for extracellular use will be transported to the plasma membrane

The vesicle will fuse with the cell membrane and its materials will be expelled into the extracellular fluid

Materials sorted by the Golgi apparatus may be either:

Released immediately into the extracellular fluid or stored within an intracellular vesicle for a delayed release in response to a cellular signal

Question 68

Describe endocytosis providing two examples

Answer 68

The process by which large substances (or bulk amounts of smaller substances) enter the cell without crossing the membrane

An invagination of the membrane forms a flask-like depression which envelopes the extracellular material
The invagination is then sealed off to form an intracellular vesicle containing the material

There are two main types of endocytosis:

Phagocytosis – The process by which solid substances are ingested (usually to be transported to the lysosome)
Pinocytosis – The process by which liquids / dissolved substances are ingested (allows faster entry than via protein channels)

Question 69

Describe exocytosis

Answer 69

The process by which large substances (or bulk amounts of small substances) exit the cell without crossing the membrane

Vesicles fuse with the plasma membrane, expelling their contents into the extracellular environment
The process of exocytosis adds vesicular phospholipids to the cell membrane, replacing those lost when vesicles are formed via endocytosis

Question 70

Describe co-transport providing two examples

Answer 70

If the two molecules are transported in the same direction it is called symport
If the two molecules are transported in opposite directions it is called antiport

The sodium-potassium pump is an example of an antiporter as sodium and potassium are pumped in opposite directions

This is primary active transport as both molecules are pumped against their gradient and require ATP hydrolysis

Glucose uptake in the kidneys is an example of symport as its movement is coupled to the parallel transport of sodium

This is secondary active transport as the sodium is moving passively down an electrochemical gradient

Question 71

What is abiogenesis? List the stages

Answer 71

The theory that living cells arose from non-living matter is known as abiogenesis

This process is theorised to have occurred over four key stages:

There was non-living synthesis of simple organic molecules (from primordial inorganic molecules)
These simple organic molecules became assembled into more complex polymers
Certain polymers formed the capacity to self-replicate (enabling inheritance)
These molecules became packaged into membranes with an internal chemistry different from their surroundings (protobionts)

Question 72

Describe the research that demonstrated the synthesis of simple organic molecules by non-living organisms.

Answer 72

Miller-Urey Experiment

The non-living synthesis of simple organic molecules has been demonstrated by the Miller-Urey experiment

Stanley Miller and Harold Urey recreated the postulated conditions of pre-biotic Earth using a closed system of flasks and tubes

Water was boiled to vapour to reflect the high temperatures common to Earth’s original conditions
The vapour was mixed with a variety of gases (including H2, CH4, NH3) to create a reducing atmosphere (no oxygen)
This mixture was then exposed to an electrical discharge (simulating the effects of lightning as an energy source for reactions)
The mixture was then allowed to cool (concentrating components) and left for a period of ~1 week
After this time, the condensed mixture was analysed and found to contain traces of simple organic molecules

Question 73

What is biogenesis? Why is it the only viable theory?

Answer 73

Cells can only be formed by the division of pre-existing cells (biogenesis)

The chemical processes that contributed to the initial formation of biological life required specific conditions to proceedThis included a reducing atmosphere and high temperatures (>100ºC) or electrical discharges to catalyse chemical reactions. These conditions do not commonly exist on modern Earth and hence living cells cannot arise independently by abiogenesis

Question 74

Describe the experiment that proved biogenesis

Answer 74

The law of biogenesis is largely attributed to Louis Pasteur, who demonstrated that emergent bacterial growth in nutrient broths was due to contamination by pre-existing cells

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

Question 75

State the evidence for endosymbiosis

Answer 75

Membranes (double membrane bound)
Antibiotics (susceptibility)
Division (mode of replication (fission-like process))
DNA (presence of own naked circular DNA)
Ribosomes (size of 70S)

MAD DR

Question 76

Describe endosymbiosis

Answer 76

an endosymbiont is a cell which lives inside another cell with mutual benefit

Eukaryotic cells are believed to have evolved from early prokaryotes that were engulfed by 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

Question 77

What is the cell cycle? List the events and outline what occurs in each briefly

Answer 77

The cell cycle is an ordered set of events which culminates in the division of a cell into two daughter cells

It can be roughly divided into two main phases:

Interphase

The stage in the development of a cell between two successive divisions

This phase of the cell cycle is a continuum of three distinct stages:

G1 – First intermediate gap stage in which the cell grows and prepares for DNA replication
S – Synthesis stage in which DNA is replicated
G2 – Second intermediate gap stage in which the cell finishes growing and prepares for cell division

M phase

The period of the cell cycle in which the cell and contents divide to create two genetically identical daughter cells

This phase is comprised of two distinct stages:

Mitosis – Nuclear division, whereby DNA (as condensed chromosomes) is separated into two identical nuclei
Cytokinesis – Cytoplasmic division, whereby cellular contents are segregated and the cell splits into two

Question 78

List what occurs in Interphase

Answer 78

DNA replication – DNA is copied during the S phase of interphase
Organelle duplication – Organelles must be duplicated for twin daughter cells
Cell growth – Cytoplasmic volume must increase prior to division
Transcription / translation – Key proteins and enzymes must be synthesised
Obtain nutrients – Vital cellular materials must be present before division
Respiration (cellular) – ATP production is needed to drive the division process

DOCTOR

Question 79

Distinguish between Chromatin and Chromosome

Answer 79

Chromatin:

DNA is usually loosely packed within the nucleus as unravelled chromatin
In this unravelled form, the DNA is accessible to transcriptional machinery and so genetic information can be translated
DNA is organised as chromatin in all non-dividing cells and throughout the process of interphase

Chromosome:

DNA is temporarily packaged into a tightly wound and condensed chromosome prior to division (via supercoiling)
In this condensed form, the DNA is able to be easily segregated however is inaccessible to transcriptional machinery
DNA is organised as chromosomes during the process of mitosis (condense in prophase, decondense in telophase)

Question 80

Distinguish between Chromatid and Chromosome

Answer 80

A chromosome is the condensed form of DNA which is visible during mitosis (via microscopy)

As the DNA is replicated during the S phase of interphase, the chromosome will initially contain two identical DNA strands

These genetically identical strands are called sister chromatids and are held together by a central region called the centromere

Question 81

What is cytokinesis?

Answer 81

Cytokinesis is the process of cytoplasmic division, whereby the cell splits into two identical daughter cells

Question 82

Outline cytokinesis in animals

Answer 82

After anaphase, microtubule filaments form a concentric ring around the centre of the cell
The microfilaments constrict to form a cleavage furrow, which deepens from the periphery towards the centre
When the furrow meets in the centre, the cell becomes completely pinched off and two cells are formed

Question 83

Outline cytokinesis in plants

Answer 83

After anaphase, carbohydrate-rich vesicles form in a row at the centre of the cell (equatorial plane)
The vesicles fuse together and an early cell plate begins to form within the middle of the cell
The cell plate extends outwards and fuses with the cell wall, dividing the cell into two distinct daughter cells

Question 84

Formal for mitotic index

Answer 84

Cells in mitosis / total number of cells

Question 85

Outline how Cyclin controls the cell cycle

Answer 85

Cyclins are a family of regulatory proteins that control the progression of the cell cycle

Cyclins activate cyclin dependent kinases (CDKs), which control cell cycle processes through phosphorylation

When a cyclin and CDK form a complex, the complex will bind to a target protein and modify it via phosphorylation
The phosphorylated target protein will trigger some specific event within the cell cycle (e.g. centrosome duplication, etc.)
After the event has occurred, the cyclin is degraded and the CDK is rendered inactive again

Question 86

Outline the factors that lead to cancer

Answer 86

Mutagens

A mutagen is an agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery)

Mutagens may be physical, chemical or biological in origin:

Physical – Sources of radiation including X-rays (ionising), ultraviolet (UV) light and radioactive decay
Chemical – DNA interacting substances including reactive oxygen species (ROS) and metals (e.g. arsenic)
Biological – Viruses, certain bacteria and mobile genetic elements (transposons)

Mutagens that lead to the formation of cancer are further classified as carcinogens

Oncogenes

An oncogene is a gene that has the potential to cause cancer

Most cancers are caused by mutations to two basic classes of genes – proto-oncogenes and tumour suppressor genes

Proto-oncogenes code for proteins that stimulate the cell cycle and promote cell growth and proliferation
Tumour suppressor genes code for proteins that repress cell cycle progression and promote apoptosis

When a proto-oncogene is mutated or subjected to increased expression it becomes a cancer-causing oncogene

Tumour suppressor genes are sometimes referred to as anti-oncogenes, as their normal function prevents cancer

Question 87

Describe and explain metastasis

Answer 87

Tumour cells may either remain in their original location (benign) or spread and invade neighbouring tissue (malignant)

Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour

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)

Question 88

What are the functions of mitosis

Answer 88

Tissue repair / replacement
Damaged or aged cells replaced with identical healthy ones

Organismal growth
Multicellular organisms derive new cells via mitotis

Asexual reproduction
Vegetative propagation in plants occurs via mitotic division

Development (of embryos)
Zygotes undergo mitosis and differentiate to become embryos

TOAD

Question 89

What is the G-zero stage?

Answer 89

Not all cells are continually replicating – some cells may enter into a non-dividing G0 stage

These cells may either be dormant (quiescent) or ageing and deteriorating (senescent)

Cells enter the G0 phase from the G1 phase; quiescent cells may re-enter G1 at a later time (senescent cells do not)

Normally, cells will only divide a finite time before reaching senescence (a typical human cell will divide ~ 40 - 60 times)

Specialised cells will often permanently enter G0, as differentiation has prevented their capacity for further division

Neurons are examples of cells that have been arrested in a G0 state – these cells are amitotic (cannot divide)