Topic 2 - Cell Theory Flashcards
Discuss the evidence for the three parts of the cell theory
1) Through the use of the microscope, Hooke described cells in 1665, Van Leewenhoek observed the first living cells, referring to them as animalcules. Schneider stated that plants were made of individual, separate beings
2) we’ve not been able to find any living entity that is not made of at least one cell
3) Pasteur proved that living organisms cannot spontaneously reappear after he sterilised chicken broth. Only after being exposed to pre-existing cells was life able to re-establish itself in the sterilised chicken broth
Outline the cell theory
- all organisms are composed of one or more cells
- cells are the smallest units of life
- all cells come from pre-existing cells
Why are unicellular organisms considered alive?
They carry out the functions of life
1mm =’X’
10^-3meters
1 micrometer = ‘x’
10^-3 millimetres
1 nanometer =’X’
10^-3 micrometers
What is the size of a molecule?
1 nanometer
What is the thickness of a cell membrane?
10 nanometers
What is the size of a virus?
100 nanometers
What is the size of a Bacteria?
1 micrometer
What is the size of a eukaryotic cell?
Up to 100 micrometers
2.16 Explain the importance of the surface area to volume ratio as a factor limiting cell size
As the organism gets bigger its surface area : volume ratio decreases and so this rule is a limiting factor for cell size. As the cell gets bigger the ratio decreases. If the ratio decreases the rate of exchange decreases
2.1.7
How do multicellular organisms show emergent properties?
Cells form tissues, tissues form organs, organs form organ systems, and organ systems form multicellular organisms. (The whole is greater that the composition of its parts). It’s the cells working together as a unit that allows an organ to perform its function.
2.1.8 Explain that cells in multicellular organisms differentiate to carry out specialized functions by expressing some of their genes but not others
As a general principle then we find that the larger a multicellular organisms become the more diversity and differentiated specialisms there are within the organism.
Rather than all cells carrying out all functions, tissues and organs specialise to particular functions. These organs and systems are then integrated to give the whole organism (with its emergent properties).
Differentiation: Cells within a multi cellular organism specialise their function.
2.1.9 What are the two key qualities of a stem cell?
Self renewal: They can continuously divide and replicate
Potency: they have the capacity to differentiate into any type of cell
2.1.10 Outline one therapeutic use of stem cells
- Retinal cells: Replace dead cells in retina to cure diseases like glaucoma and macular degeneration
- Skin cells: Graft new skin cells to replace damaged cells in severe burn victims
- Nerve cells: Repair damage caused by spinal injuries to enable paralysed victims to regain movement
- Blood cells: Bone marrow transplants for cancer patients who are immuno-compromised as a result of chemotherapy
2.2.2 What is the function of the cell wall?
Protects the cell from the outside environment and maintains the shape of the cell. It also prevents the cell from bursting if internal pressure arises.
What is the function of the Plasma membrane?
Semi-permeable barrier that controls the entry and exit of substances
What is the function of the cytoplasm?
Fluid component which contains the enzymes needed for all metabolic reactions
What is the function of the pili?
Hair-like extensions found on bacteria which can serve one of two roles:
Attachment pili: Shorter in length, they allow bacteria to adhere to one another or to available surfaces
Sex pili: Longer in length, they allow for the exchange of genetic material (plasmids) via a process called bacterial conjugation
What is the function of the flagella?
Long, slender projection containing a motor protein which spins the flagella like a propellor, enabling movement
What is the function of the ribosomes (prokaryotic cells)?
Complexes of RNA and protein that are responsible for polypeptide synthesis (prokaryotic ribosomes are smaller than eukaryotes - 70S)
What is the function of the nucleoid?
Region of the cytoplasm containing the naked DNA (genetic material) that controls the cell and will be passed on to daughter cells
2.2.4 How do prokaryotic cells divide? What is the name of this process?
Binary Fission
A method of asexual reproduction involving the splitting of the parent organism into two separate organisms
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 separate cells
2.3.2What is the function of ribosomes (eukaryotic cells)?
Site of protein synthesis. They translate RNA to produce proteins.
Found either floating free in cytoplasm or attached to rough endoplasmic reticulum.
Describe the function of the rough endoplasmic reticulum.
A system of membranes involved in the transport of materials between organelles studded with ribosomes and involved in the synthesis and transport of proteins destined for secretion
Describe the function of the lysosome.
Site of hydrolysis / digestion / breakdown of macromolecules
Describe the function of the mitochondrion.
Site of aerobic respiration, which produces large quantities of chemical energy (ATP) from organic compounds
Describe the function of the nucleus.
Contains hereditary material (DNA) and thus controls cell activities (via transcription) and mitosis (via DNA replication)
2.3.4 Compare prokaryotic and eukaryotic cells.
- Both have a cell membrane
- Both contain ribosomes
- Both have DNA and cytoplasm
Prokaryotes DNA circular and found in the nucleoid (in cytoplasm). Eukaryotes DNA is linear and found in nucleus.
Prokaryotes have no membrane bound organelles, eukaryotes do.
Prokaryotes have 70S ribosomes, eukaryotes have 80S ribosomes.
Prokaryotes reproduce asexually through binary fission, eukaryotes reproduce asexually through mitosis or sexually through meiosis.
Prokaryotes DNA - haploid
Eukaryotes DNA - diploid
Prokaryotic cells 1-5 micrometers
Eukaryotic cells 10-100 micrometers
2.3.5
State three differences between plant and animal cells
Animal cells only have a plasma membrane and no cells wall,whereas plant cells have both.
Plants have chloroplasts and animals cells don’t.
Animals store glycogen as a carbohydrate resource, plants store starch.
Animal cells do not usually contain vacuoles (if present they are small and temporary) whereas plant cells have large permanent vacuoles.
Animal cells can change shale due to lack of a cell wall and are usually rounded, whereas plan cells have a fixed shape, maintained by the presence of a cell wall.
2.3.6
Outline two roles of extra cellular components
Plant cells
The cell wall in plants is made from cellulose secreted from the cell, which serves the following functions:
Provides support and mechanical strength for the cell (maintains cell shape)
Prevents excessive water uptake by maintaining a stable, turgid state
Serves as a barrier against infection by pathogens
Animal cells
The extracellular matrix (ECM) is made from glycoproteins secreted from the cell, which serve the following functions:
Provides support and anchorage for cells
Segregates tissues from one another
Regulates intercellular communication by sequestering growth factors
2.4.2 Explain how the hydrophilic and hydrophobic properties of phospholipids help to maintain the structure of cell membranes
Consist of a polar head (hydrophilic) made from glycerol and phosphate
Consist of two non-polar fatty acid tails (hydrophobic)
Arrangement in Membrane
Phospholipids spontaneously arrange in a bilayer
Hydrophobic tail regions face inwards and are shielded from the surrounding polar fluid while the two hydrophilic head regions associate with the cytosolic and extracellular environments respectively
Structural Properties of Phospholipid Bilayer
Phospholipids are held together in a bilayer by hydrophobic interactions (weak associations)
Hydrophilic / hydrophobic layers restrict entry and exit of substances
Phospholipids allow for membrane fluidity / flexibility (important for functionality)
Phospholipids with short or unsaturated fatty acids are more fluid
Phospholipids can move horizontally or occasionally laterally to increase fluidity
Fluidity allows for the breaking / remaking of membranes (exocytosis / endocytosis)
2.4.3
List the functions of membrane proteins
Membrane proteins can act as hormone binding sites, electron carriers, pumps for active transport, channels for passive transport and also enzymes. In addition they can be used for cell to cell communication as well as cell adhesion.
2.4.4
Define diffusions
The net movement of particles from a region of high concentration to a region of low concentration (along the gradient) until equilibrium
2.4.4
Define osmosis
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
2.4.5 Explain passive transport across membranes in terms of simple diffusion
The plasma membrane is semi-permeable and selective in what can cross
Substances that move along the concentration gradient (high to low) undergo passive transport and do not require the expenditure of energy (ATP)
Small, non-polar (lipophilic) molecules can freely diffuse across the membrane
2.4.5 Explain passive transport across membranes in terms of facilitated diffusion
The plasma membrane is semi-permeable and selective in what can cross
Substances that move along the concentration gradient (high to low) undergo passive transport and do not require the expenditure of energy (ATP)
Larger, polar substances (ions, macromolecules) cannot freely diffuse and require the assistance of transport proteins (carrier proteins and channel proteins) to facilitate their movement (facilitated diffusion)
2.4.6
Explain the role of protein pumps and ATP in active transport across membranes
Active transport is the passage of materials against a concentration gradient (from low to high)
This process requires the use of protein pumps which use the energy from ATP to translocate the molecules against the gradient
The hydrolysis of ATP causes a conformational change in the protein pump resulting in the forced movement of the substance
Protein pumps are specific for a given molecule, allowing for movement to be regulated (e.g. to maintain chemical or electrical gradients)
An example of an active transport mechanism is the Na+/K+ pump which is involved in the generation of nerve impulses
2.4.7 Explain how vesicles are used to transport materials within a cell between the endoplasmic reticulum, Golgi apparatus and plasma membrane
Polypeptides destined for secretion contain an initial target sequence (a signal recognition peptide) which directs the ribosome to the endoplasmic reticulum
The polypeptide continues to be synthesised by the ribosome into the lumen of the ER, where the signal sequence is removed from the nascent chain
The polypeptide within the rough ER is transferred to the golgi apparatus via a vesicle, which forms from the budding of the membrane
The polypeptide moves via vesicles from the cis face of the golgi to the trans face and may be modified along the way (e.g. glycosylated, truncated, etc.)
The polypeptide is finally transferred via a vesicle to the plasma membrane, whereby it is either immediately released (constitutive secretion) or stored for a delayed release in response to some cellular signal (regulatory secretion = for a more concentrated and more sustained effect)
2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and reform during endocytosis
The membrane is principally held together by the relatively weak hydrophobic associations between phospholipids
This association allows for membrane fluidity and flexibility, as the phospholipids (and to a lesser extent the proteins) can move about to some extent
This allows for the breaking and remaking of membranes, allowing larger substances access into and out of the cell (this is an active process)
Endocytosis
The process by which large substances (or bulk amounts of smaller substances) enter the cell without travelling across the plasma membrane
An invagination of the membrane forms a flask-like depression which envelopes the material; the invagination is then sealed off forming a vesicle
There are two main types of endocytosis:
- Phagocytosis
The process by which solid substances (e.g. food particles, foreign pathogens) are ingested (usually to be transported to the lysosome for break down)
- Pinocytosis
The process by which liquids / solutions (e.g. dissolved substances) are ingested by the cell (allows quick entry for large amounts of substance)
2.4.8 Describe how the fluidity of the membrane allows it to change shape, break and reform during exocytosis
The process by which large substances exit the cell without travelling across the plasma membrane
Vesicles (usually derived from the golgi) fuse with the plasma membrane expelling their contents into the extracellular environment
2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis
The cell cycle is an ordered set of events that culminates in cell growth and division into two daughter cells
It can roughly be divided into two main stages:
Interphase
The stage in the development of the cell between two successive M phases
This phase of the cell cycle is a continuum of 3 distinct stages (G1, S, G2), whereby the cell grows and matures (G1), copies its DNA (S) and prepares for division (G2)
Sometimes cells will leave the cell cycle and enter into a quiescent state (G0), whereby it becomes amitotic and no longer divides
M phase
The periods of nuclear division (mitosis) and cytoplasmic division (cytokinesis)
2.5.2 What are tumours (cancers) as a result of?
The cell cycle is controlled by a complex chemical control system that responds to signals both inside and outside of the cell
Tumor suppressor genes produce proteins which inhibit cell division, while proto-oncogenes produce proteins that promote growth and division
Mutations to these genes result in uncontrolled cell division, resulting in the formation of a tumour
Tumours can grow in size which causes damage local tissue; they may also spread to other parts of the body (malignant tumours)
Diseases caused by the growth of tumours are collectively known as cancers
2.5.3 What occurs during interphase?
Interphase is an active period in the life of a cell - many events need to occur before a cell can successfully undergo division:
Protein synthesis: The cell needs to synthesise key proteins and enzymes to enable it to grow, copy its contents and then divide
ATP production: The cell will need to generate sufficient quantities of ATP in order to successfully divide
Increase number of organelles: The cell needs to ensure both daughter cells will have the necessary numbers of organelles needed to survive
DNA replication: The genetic material must be faithfully duplicated before division (this occurs during the S phase)
As none of these processes can occur during the M phase, interphase contains growth checkpoints to ensure division is viable
G1: A checkpoint stage before DNA replication during which the cell grows, duplicates organelles, synthesises proteins and produces ATP
S: The stage during which DNA is replicated
G2: A checkpoint stage before division during which the copied DNA is checked for fidelity (mutations) and final metabolic reactions occur
2.5.4 Describe events that occur in the four stages of mitosis
Prophase
DNA supercoils, causing chromosomes to condense and become visible under a light microscope
As DNA was replicated during interphase, the chromosomes are each comprised of two genetically identical sister chromatids joined at a centromere
The centrosomes move to opposite poles of the cell and spindle fibres begin to form between them (in animals, each centrosome contains 2 centrioles)
The nuclear membrane is broken down and disappears
Metaphase
Spindle fibres from the two centrosomes attach to the centromere of each chromosome
Contraction of the microtubule spindle fibres cause the chromosomes to line up separately along the centre of the cell (equatorial plane)
Anaphase
Continued contraction of the spindle fibres cause the two sister chromatids to separate and move to the opposite poles of the cell
Once the two chromatids in a single chromosome separate, each constitutes a chromosome in its own right
Telophase
Once the two sets of identical chromosomes arrive at the poles, the spindle fibres dissolve and a new nuclear membrane reforms around each set of chromosomes
The chromosomes decondense and are no longer visible under a light microscope
The division of the cell into two daughter cells (cytokinesis) occurs concurrently with telophase
2.5.5 Explain how mitosis produces two genetically identical nuclei
During interphase (the S phase) the DNA was replicated to produce two copies of genetic material
These two identical DNA molecules are identified as sister chromatids and are held together by a single centromere
During the events of mitosis (as described in 2.5.4), the sister chromatids are separated and drawn to opposite poles of the cell
When the cell divides (cytokinesis), the two resulting nuclei will each contain one of each chromatid pair and thus be genetically identical
2.5.6 When would a cell undergo mitosis?
Growth: Multicellular organisms increase their size by increasing their number of cells through mitosis
Asexual reproduction: Certain eukaryotic organisms may reproduce asexually by mitosis (e.g. vegetative reproduction)
Tissue Repair: Damaged tissue can recover by replacing dead or damaged cells
Embryonic development: A fertilised egg (zygote) will undergo mitosis and differentiation in order to develop into an embryo
