CELLS - TOPIC 2 (Cell Structure Chapter 3) Flashcards
METHODS OF STUDYING CELLS
What is cell fractionation
The process where cells are broken up and the different organelles they contain separated out
Homogenisation steps
- the tissue to be studied, cut into small pieces + placed into a cold, buffered, isotonic solution
- cells are broken up in a homogeniser (to release the organelles from the cells)
- the fluid is then called a homogenate
- The homogenate is then filtered ( to remove any complete cells and large pieces of debris)
- A suspension of homogenate is then placed in a test tube and then centrifuged
Why are cells broken up in a homogeniser?
To break open cells
To release the organelles from the cells (breaking the cell membrane)
Why is the homogenate filtered ?
To remove any complete cells and large pieces of debris e.g. cell wall/membrane
Explain the necessary tissue conditions (cold, isotonic, buffered)
Cold - to reduce enzyme activity (to prevent digestion of organelles)
Isotonic - to prevent water being lost or gained, prevents osmosis so no shrinkage of organelles, Same water potential to prevent bursting of the organelle.
Buffered - to maintain a constant pH to prevent protein from denaturing.
What is ultracentrifugation?
The process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge
Ultracentrifugation process
- the tube of filtrate is placed in the centrifuge and spun at a low speed
- The heaviest organelles, nuclei, are forced to the bottom of the tube, where they form a thin sediment
- These larger fragments are then removed and the supernatant (fluid at the top of the test tube) is re-spun at a faster speed than before
- The next heaviest organelles, the mitochondria, are forced to the bottom of the tube
- The process is continued in this way so that, at each increase in speed, the next heaviest organelle is sedimented and separated out
-process continues until there is no sediment left in the tube
THE ELECTRON MICROSCOPE
What is magnification ?
The number of times an image is enlarged compared with the real size of the object
How do you calculate for magnification ?
Magnification = size of image / real size
What is resolution ?
The ability to distinguish between two points that are close together
Why is it not possible to determine the identity of structures labelled X using an optical microscope?
- Low resolution
- Because wavelength of light is too long
(Can with an electron microscope because ….
1. High resolution
2. Wavelength of electrons is shorter)
When to use a light microscope?
- used for specimens above 200nm
- Light microscopes shine light through the specimen
- The specimens can be living (and therefore can be moving) or dead
- Light microscopes are useful for looking at whole cells, small plant and animal organisms, tissues within organs
What are the two types of electron microscopes ?
- TEM ( transmission electron microscope)
- SEM ( scanning electron microscope )
TEM key points:
- The electrons are focussed onto the specimen by electromagnets
- The electrons are passed through (transmitted) the specimen
- specimen must be very thin
- Produces a flat 2D image
( parts of specimen that absorb = appear dark
parts of specimen that transmit = appear bright ) - higher resolution than SEM
Why is the resolving power of the TEM not always being achieved in practice?
- difficulties in preparing the specimen limit the resolution that can be achieved
- a higher energy electron beam is required and this may destroy the specimen
What are the main limitations of TEM?
- the whole system must be in a vacuum and therefore living specimens cannot be observed
- a ‘complex’ staining process is required and even then the image is not in colour
- the specimen must be extremely thin
- The image may contain artefacts (look like real structure, but are the results of preserving and staining )
SEM key points :
- a beam of electrons bounce off specimen, so not as thin as TEM
- in colour
- 3D
What are the limitations of SEM ?
- same limitations as TEM ( except specimens do not need to be as thin, electrons bounce off rather than penetrate the specimen, means we can get a 3D image)
- lower resolving power than TEM
When using a light microscope, how can we measure the size of objects?
Using an eyepiece graticule ( a scale is etched on the glass disc, the scale is visible when looking down the eyepiece of the microscope
What is the calculation of 1 graticule division ?
1 graticule division = no. of micrometers/ no. of graticule division
How to find the measurement of the length of the object ?
graticule divisions X magnification factor = measurement (um - micrometre)
(The specimen slide would be used to replace the stage micrometer and the eyepiece graticule at the same magnification would be used to measure the length of the object)
EUKARYOTIC CELL STRUCTURE
What is the structure of the nucleus ?
- has a nuclear envelope- a double membrane that surrounds the nucleus
The nuclear envelope has spaces within it called nuclear pores
Has nuclear pores - allows the passage of larger molecules - Has a nucleoplasm - is the granular, jelly-like material that makes up the bulk of the nucleus
- Has a nucleolus- a small spherical region within the nucleoplasm, which consists of DNA, RNA + proteins
- Has chromosomes - consist of protein-bound, linear DNA
What is the function of the nucleus?
- store the genetic information of a cell
- controls gene expression, protein synthesis and storing DNA
- protein synthesis and ribosome production occurs in the nucleolus
What is the structure of mitochondria?
- oval shaped structure with double membrane : inner +outer membrane
- Cristae ( extensions of inner membrane) : provides a large surface area for the attachment of enzymes and other proteins involved in respiration
- The Matrix (makes up the remainder of the mitochondrion - formed from folding of inner membrane) : contains protein, lipids, ribosomes and DNA that allows the mitochondria to control the production of some of their own proteins
Many enzymes involved in respiration are found in the Matrix
What is the function of mitochondria?
- site of aerobic respiration which produces ATP, a molecule essential for cellular activity
- cells containing large amounts of mega contain many mitochondria
What is the structure of chloroplasts?
- has a double membrane - the chloroplast envelope, that surrounds the gel -like stroma
-stroma : a fluid-filled matrix where the second stage of photosynthesis takes place. Within the stroma are a number of other structures, such as starch grains - within the stroma are fluid-filled sacs called thylakoids (within thylakoids are chlorophyll), which stack up to form grana
- lamellae (thin pieces of thylakoid membrane) links the grana together
What is the function of chloroplasts?
- the granal membranes provide a large surface area for the attachment of chlorophyll, electron carriers + enzymes that carry out the first stage of photosynthesis
- The fluid of the stroma possesses all the enzymes needed to make the sugars in the second stage of photosynthesis
- Chloroplasts contain both DNA + ribosomes so they can quickly + easily manufacture some of the proteins needed for photosynthesis
What is the endoplasmic reticulum ?
An elaborate, three-dimensional system of sheet-like membranes, spreading through the cytoplasm of the cells
- it is continuous with the outer nuclear membrane
- the membranes enclose a network of tubules and flattened sacs called cisternae
What is the structure of the RER (rough endoplasmic reticulum) ?
- It has ribosomes attached along the outer surface system of flattened membrane-bound sacs (listernae)
What is the function of the RER (rough endoplasmic reticulum) ?
- provides a large surface area for the rate of protein synthesis and glycoproteins
- provides a pathway for the transport of materials, especially proteins, throughout the cell
What is the structure of the SER (Smooth endoplasmic reticulum) ?
- lacks ribosomes on its surface
- is often more tubular in appearance
- typically attached to RER and linked to the nuclear membrane
What is the function of SER (smooth endoplasmic reticulum) ?
-large surface area to synthesise, store + transport lipids
- synthesises, stores and transports carbohydrates
What is the structure of the Golgi apparatus ?
- a system of fluid filled sacs often with vesicles at the ends (the stack of membranes that make up flattened sacs : cisternae)
- The vesicles are small detached fluid -filled pockets found at the edges of the complex
What is the function of the Golgi apparatus?
- forms lysosomes
- transports, modifies and stores lipids
- Add carbohydrate to proteins to form glycoproteins
- produce secretary enzymes, such as those secreted by the pancreas
- secrete carbohydrates, such as those used in making cell walls in plants
- packages proteins
What is the structure of lysosomes?
-formed when the vesicles produced by the Golgi apparatus contain enzymes (e.g proteases and lipases)
- surrounded by the membrane and contain digestive enzymes called lysozymes
- Acidic pH is necessary for lysozymes to function
What is the function of lysosomes?
-lysosomes fuse with vesicles to release hydrolytic enzymes called lysozymes
- responsible for digesting invading cells, old, unwanted parts of the cell so that the useful chemicals they are made of can be re-used
- completely break down cells after they have died
-hydrolyse material ingested by phagocytic cells (white blood cells +bacteria) - release enzymes to the outside of the cell in order to destroy material around the cell
What is the structure of ribosomes ?
-very small structures of a large subunit and a small subunit
- consist of ribosomes proteins + ribosomal RNA
- not surrounded by a separate membrane
- often associated with another organelle called RER, but float freely within the cytoplasm
What is the function of ribosomes ?
-responsible for synthesising proteins from messenger RNA during translation
- In process, the codons in mlCNA are used to produce a specific sequence of amino acids ~~> this amino acid is then folded into a protein
What is the structure of cell walls?
-rigid structures surrounding the cell membrane of plant, algae + fungal cells
- consists of microfibrils of the polysaccharide cellulose, embedded in the matrix
- in plants +algae, cell wall is composed mainly of cellulose
- Fungal cells: composed in chitin
-thin layer : the middle lamella, marks the boundary between adjacent cell walls and cements adjacent cells together
What is the function of cell wall?
- helps maintain cell shape by providing structural support - stops cell bursting when osmosis takes place
- Allows water to pass along it, so contributes to movement of water through the plant
- provides the cell with protection against invading pathogens
What is the structure of the cell membrane?
-consists of a phospholipid bilayer
-hydrophilic heads form inner +outer surface of the membrane
-hydrophobic tails form inside of membrane
- cholesterol molecules are embedded between the phospholipids to prevent too much movement
-channel proteins and carrier proteins are found within the bilayer
What is the function of cell membrane?
- are physical barriers
-they prevent movement of foreign bodies and other molecules in and out of the cell - they are partially permeable
-substances can be transported across the cell membrane by diffusion, osmosis and active transport
Functions of glycoproteins and glycolipids:
-responding to insulin in liver cells, resulting in the cell absorbing glucose from the bloodstream
-establishing blood type
-immune responses
-responding to neurotransmitters involved in nervous responses
What is the structure of permanent cell vacuole ?
It is a permanent pocket of cell sap, a solution of sugars (salt +water), surrounded by a membrane called the tonoplast
What is the function of permanent cell vacuole ?
-maintains osmotic pressure inside the cell, ensures plant cells remain turgid ~~> stops plant wilting
- stores unwanted chemicals that are discarded by the cell
~~> If needed, they can provide an emergency food store for the plant
CELL SPECIALISATION AND ORGANISATION
What is cell differentiation ?
-the process where a cell changes to become specialised for its job
What cell remains undifferentiated?
-only stem cells
What are the different types of specialised organelles?
-sperm cells
-white blood cells
-red blood cells
-palisade cells
How is sperm cell adapted to its function?
-high amounts of mitochondria so they can propel themselves to the egg cell, releasing energy from respiration
How are white blood cells adapted to their function ?
Have high amounts of lysosomes, so they can break down invading pathogens
How are red blood cells adapted to their function?
No nucleus ~~> carry more oxygen
How are palisade cells adapted to their function?
High number of chloroplasts so they can photosynthesise effectively
What are cells ?
The building blocks of life
What are tissues?
A collection of cells working together for one function
What are some examples of tissues ?
-epithelial tissues : they line the surfaces of organs and often have a protective or secretory function
- xylem : is used to transport water and mineral ions throughout the plant and also gives mechanical support
What is an organ ?
Collection of tissues working together to perform a particular function
What are some examples of organs?
-stomach : made up of muscle tissues to churn and mix the stomach content. Epithelium tissue to protect the stomach wall and produce secretions
-leaf : palisade mesophyll made up of leaf palisade cells that carry out photosynthesis. Xylem +phloem used to transport substances
-arteries +veins
Why are capillaries not organs but arteries and veins are?
Capillaries are made up of just one tissue (epithelium), whereas arteries +veins are made up of many tissues (epithelial, muscle …)
What are organ systems?
A collection of organs working together to perform a particular function
What are examples of organ systems?
- Digestive system :digests and processes food. Organs include stomach, oesophagus, pancreas, liver …
- respiratory system :used for breathing + gas exchange. Organs include trachea,
bronchi, lungs. - circulatory system :pumps and circulates blood. Organs include heart, arteries and veins
PROKARYOTIC CELLS AND VIRUSES
What are the organelles in prokaryotic cells?
-cell wall
- plasma membrane
-mesosomes
-no nucleus
-plasmids
-ribosomes
-flagellum
-some bacteria have slime capsule
What is the structure of the cell wall in a prokaryotic cell?
A mesh work of a tough protein called murein (not cellulose)
What is the structure of the plasma membrane ?
Has the same structure and function in both eukaryotic and prokaryotic cells
What are mesosomes and the function of them in prokaryotic cells
-They are inner foldings of the cell membrane
- provide a large surface area for the attachment of enzymes involved in respiration
What is there instead of the nucleus in a prokaryote
The genetic material is a single circular chromosome
What are plasmids and the function of them in a prokaryote
-small loops of DNA
- contain genes that can be passed between prokaryotes (e.g. genes for antibiotic resistance)
What is the difference in ribosomes for eukaryotic and prokaryotic cells?
Smaller ribosomes in prokaryotic than eukaryotic
What is flagellum and the function of it in prokaryotic cells?
“Whip-like” structure
Rotates to allow the bacterium to move
What is the function of a slime capsule in a prokaryote
-helps prevent desiccation (drying out)
- sticks cells together
-protects the cell against the action of a host’s digestive enzymes
What is a virus?
They are non-cellular infectious particles that straddle the boundary between ‘living’ and ‘non-living’.
They are smaller than prokaryotic cells
What is the structure of a virus?
- a nucleic acid core (genomes either DNA/RNA)
- a protein coat called a ‘capsid’
- some have an outer layer called a lipid envelope, (formed usually membrane phosphate of a cell they were made in )
-are parasitic (only reproduce by infecting living cells +using protein building machinery to produce new virus particles) - the lipid envelope or capside will have attached proteins present
MITOSIS
What must replaced cells be to the original cell in mitosis ?
Genetically identical to the original cell
Parent cell= original cell
Daughter cell= new cell
What is mitosis needed for ?
1) growth
2) repair
3) differentiation
What are the 5 stages of mitosis (cytokinesis not part of it )?
1) interphase
2) prophase
3) metaphase
4) anaphase
5) telophase
How many stages does interphase have, and what are they ?
3 stages - G1 (first growth stage), S (synthesis) and G2 (Second growth stage)
What happens in interphase ?
G1 - cell is active + carries out its metabolic function
[proteins from which cell organelles are synthesised are produced]
S- DNA replicates (and two sister chromatids form from each chromosome)
G2 - mitochondria divides and the cell continues to grow until mitosis
What happens in prophase?
- chromosomes shorten + thicken (condense) and do become visible (when stained)
- nuclear envelope disintegrates
- nucleolus disappears
- chromosomes appear as 2 sister chromatids joined at the centromere
What happens in metaphase?
-spindle forms
[chromosomes made up by 2 chromatids, and are joined by a centromere]
-chromosomes line up on the equator
- the pair of chromatids become attached to the spindle fibres by their centromeres
What happens in anaphase ?
-the spindle fibres contract to pull the chromatids by the centromeres to opposite poles (there are identical chromosomes at the end of each cell)
- centromere splits
What happens in telephase?
- chromatids reach the poles
- nuclear envelope reforms
- nucleolus reforms and spindle disintegrates
- chromosomes uncoil, become long and thin
What happens in cytokinesis ?
- cytoplasm divides
- two new daughter cells are formed
What are the stages of virus replication ?
- Attachment proteins bind to complementary receptors on the surface of a host cell
- The virus injects in DNA/RNA into host cell
- Host cell uses its nucleic acid to produce new viral particles
- New viral particles are assembled
- Host cells burst open, releasing all new viral particles at once
- The exiting of viruses damages their host cells, causing disease
What is binary fission?
The process where prokaryotic cells divide + reproduce
What are the stages of binary fission ?
- The single, circular DNA molecule undergoes DNA replication
- Any plasmids present undergo DNA replication
- Parent cell divides into 2 cells, cytoplasm roughly halved between the 2 daughter cells
- 2 daughter cells each contain a single copy of the circular DNA molecule and a variable number of plasmids
Why are daughter cells genetically identical even if they have a different number of plasmid copies ?
They contain the same genetic code
How do daughter cells die ?
If a daughter cell receives the single circular DNA molecule or at least one copy of the plasmid, they die
The cell cycle
What is the cell cycle ?
- Only some cells in multicellular organisms retain the ability to divide.
-Those that do not divide continuously, but undergoes a regular cell cycle of division separated by periods of cell is known as the cell cycle.
What are the three stages of the cell cycle?
- Interphase
- Nuclear division ( when the nucleus divides either into two (mitosis) or four (meiosis) )
- division of the cytoplasm (cytokinesis)
Cancer and cell division
How is a tumour formed ?
Cancerous cells divide repeatedly and uncontrollably, forming a tumour
How does cancer arise ?
Due to uncontrolled mitosis
How does cancer start ?
When changes occur in the genes that control cell division
What is a mutation ?
A change in any gene
What is an oncogene?
If the mutated gene is one that causes cancer, it is referred to as an oncogene
What do most mutations usually result in ?
Early cell death or result in the cell being destroyed in the body’s immune system
What happens to mutations that result in the generation of cancerous cells ?
Do not result in early cell death or in the cell being destroyed by the body’s immune system,
This means harmful mutation occurring in the original cell can be passed on to all that cell’s descendants
What are carcinogens ?
Are any agents that may cause cancer (UV light, tobacco smoke )
What is a benign tumour ?
A tumour that does not spread from their original site, does not cause cancer
What is a malignant tumour ?
A tumour that spreads across the body, invading and destroying other tissues , causing cancer
What are the stages in the development of cancer ?
- Oncogenes arise due to carcinogens
- Cancerous cells do not respond to signals from other cells do continue to divide
- Mitosis
- Cancerous cells not removed by immune system
- Rapid mitosis
- Tumour gets bigger
- Tumour supplied with blood + lymph vessels. If malignant tumour, tumour cells spread in blood + lymph to other parts of the body.
- Metastasis. Tumour cells invade other tissues. Secondary cancer forms throughout the body.
What are the most current cancer treatments/ drugs work by controlling the rate of mitosis?
- Methotrexate : inhibits the synthesis of DNA nucleotides in cells
- Vincristine and taxol : prevent the format of the mitotic spindle
