cell structure- chapter 3 Flashcards
what is the magnification equation
size of image/size of real object
what is the resolution
the minimum distance apart that two objects can be in order for them to appear as separate items
what is cell fractionation
the process where cells are broken up and the different organelles that they contain are separated out
before cell fractionation, why is the tissue placed in a cold buffered solution of the same water potential as the tissue
cold= reduce enzyme activity that might break down the organelles
same water potential= prevent organelles bursting or shrinking as a result of osmotic gain or loss of water
buffered= so the pH doesn’t fluctuate, as any change in pH could alter the structure of organelles or affect the functioning enzymes
what are the two stages to cell fractionation and explain them
homogenation= cells are broken up by a homogeniser (blender) which releases the organelles from the cell. the resultant fluid, known as homogenate, is then filtered to remove any complete cells and large pieces of debris
ultracentrifugation= process by which the fragments in the filtered homogenate are separated in a machine called a centrifuge. This spins tubes of homogenate at very high speeds in order to create a centrifugal forces
what is the process of ultracentrifugation of animal cells
- the tube of filtrate is placed in the centrifuge and spun at a slow speed
- the heaviest organelles, the nuclei, are forced to the bottom of the tube, where they form a thin sediment or pellet
- the fluid at the top of the tube (supernatant) is removed, leaving the sediment of the nuclei
- the supernatant is transferred to another tube and spun in the centrifuge at a faster speed than before
- the next heaviest organelles, the mitochondria, are forced to the bottom of the tube
- the process is continued this way so that, at each increase in speed, the next heaviest organelle is sedimented and separated out
what are two advantages of the electron microscope
- the electron beam has a very short wavelength and the microscope can therefore resolve objects as well- high resolving power
- as electrons are negatively charged the beam can be focused using electromagnets
what are the two types of electron microscope
TEM (transmission electron microscope) and SEM (scanning electron microscope)
what is a TEM microscope
- consists of an electron gun that produces a beam of electrons that is focused onto the specimen by a condenser electromagnet
- the beam passes through a thin section of the specimen
- parts of the specimen absorb electrons and therefore appear darker
- other parts allow electrons to pass through and so appear bright
- an image is produced on a screen and this can be photographed to give a photomicrograph
why cant the resolving power of the TEM is 0.1nm always be achieved
- difficulties 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 thing
- the image may contain artefacts- theyre things that result from the way the specimen is prepared, artefacts may appear on the finished photomicrograph but are not part of the natural specimen
what are the limitations of an SEM
all the same as TEM except that the specimens need to not be extremely thin as electrons do no penetrate
explain an SEM
- directs a beam of electrons on to the surface of the specimen from above, rather than penetrating it from below
- the beam is then passed back and forth across a portion of the specimen in a regular pattern
- the electrons are scattered by the specimen and the pattern of this scattering depends on the contours of the specimen surface
- you can build up a 3-D image by computer analysis of the pattern of scattered electrons and secondary electrons produces
- the basic SEM had a lower resolving power than a TEM
what is an eyepiece graticule
- can measure the size of objects
- the graticule is a glass disc that is placed in the eyepiece of a microscope
- a scale is etched on the glass disc
what is a stage micrometer
needed to calibrate an eyepiece graticule
what does the nucleus contain
the hereditary material and controls the cells activities
what are 5 structures in the nucleus
nuclear envelope, nuclear pores, nucleoplasm, chromosomes and nucleolus
what does the nuclear envelope do
- is a double membrane that surrounds the nucleus
-Its outer membrane is continuous with the endoplasmic reticulum of the cell and often has ribosomes on its surface - It controls the entry and exit of materials in and out of the nucleus and contains the reactions taking place within it
what do the nuclear pores do
- allow the passage of large molecules, such as messenger RNA, out of the nucleus
- There are typically around 3000 pores in each nucleus
what is the nucleoplasm do
the granular, jelly-like material that makes up the bulk of the nucleus
what are the chromosomes in the nucleus
consists of protein-bound, linear DNA
what is the nucleolus
- a small spherical region within the nucleoplasm
- It manufactures ribosomal RNA and assembles the ribosomes
- There may be more than one nucleolus in a nucleus.
what are the 3 functions of the nucleus
- act as the control centre of the cell through the production of mRNA and tRNA and hence protein synthesis
- retain the genetic material of the cell in the form of DNA and chromosomes
- manufacture ribosomal RNA and ribosomes
what structures are within the mitochondrion
double membrane
cristae
matrix
what is the double membrane of the mitochondria
controls the entry and exit of material
inner of the two is folded to form extensions known as cristae
what is the cristae
- extensions of the inner membrane, which in some species extend across the whole width of the mitochondrion
- These provide a large surface area for the attachment of enzymes and other proteins involved in respiration
what is the matrix
- makes up the remainder of the mitochondrion
- It contains protein, lipids, ribosomes and DNA that allows the mitochondria to control the production of some their own proteins
- Many enzymes involved in respiration are found in the matrix
explain what the mitochondrias do
- the sites of the aerobic stages of respiration (the Krebs cycle and the oxidative phosphorylation pathway).
-they are responsible for the production of the energy-carrier molecule, ATP, from respiratory substrates such as glucose - Because of this, the number and size of the mitochondria, and the number of their cristae, are high in cells that have a high level of metabolic activity and therefore require a plentiful supply of ATP
- Examples of metabolically active cells include muscle and epithelial cells; cells in the intestines require a lot of ATP in the process of absorbing substances from the intestines by active transport
what are chloroplasts
the organelles that carry out photosynthesis
what structures does the chloroplast have
chloroplast envelope
grana
thylakoids
chlorophyll
stroma
what is the chloroplast envelope
- a double plasma membrane that surrounds the organelle
-It is highly selective in what it allows to enter and leave the chloroplast
what is the grana
- are stacks of up to 100 disc-like structures called thylakoids
- within the thylakoids is the photosynthetic pigment called chlorophyll
- Some thylakoids have tubular extensions that join up with thylakoids in adjacent grana
- The grana are where the first stage of photosynthesis (light absorption) takes place
what is the stroma
- a fluid-filled matrix where the second stage of photosynthesis (synthesis of sugars) takes place
- Within the stroma are a number of other structures, such as starch grains
how are chloroplasts adapted to their function of harvesting sunlight and carrying out photosynthesis
- The granal membranes provide a large surface area for the attachment of chlorophyll, electron carriers and enzymes that carry out the first stage of photosynthesis. These chemicals are attached to the membrane in a highly ordered fashion
- The fluid of the stroma possesses all the enzymes needed to make sugars in the second stage of photosynthesis
- Chloroplasts contain both DNA and ribosomes so they can quickly and easily manufacture some of the proteins needed for photosynthesi
what is the endoplasmic reticulum
- 3D 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 - There are two types (RER, SER)
what is the RER and its functions
- has ribosomes present on the outer surfaces of the membranes
1. provide a large surface area for the synthesis of proteins and glycoproteins
2. provide a pathway for the transport of materials, especially proteins, throughout the cell.
what is the SER and its functions
- lacks ribosomes on its surface and is often more tubular in appearance
1. synthesise, store and transport lipids
2. synthesise, store and transport carbohydrates
what are the functions of the golgi apparatus
- add carbohydrate to proteins to form glycoproteins
- produce secretory enzymes, such as those secreted by the pancreas
- secrete carbohydrates, such as those used in making cell walls in plants
- transport, modify and store lipids
- form lysosomes
what is the golgi apparatus and explain what it does
- in almost all eukaryotic cells
- consists of a stack of membranes that make up flattened sacs, or cisternae, with small rounded hollow structures called vesicles
- the proteins and lipids produced by the ER are passed through the Golgi in strict sequence.
- Golgi modifies these proteins often adding non-protein components, such as carbohydrate, to them
- ‘labels’ them, allowing them to be accurately sorted and sent to their correct destinations
- Once sorted, the modified proteins and lipids are transported in Golgi vesicles which are regularly pinched off from the ends of the Golgi cisternae
- These vesicles may move to the cell surface, where they fuse with the membrane and release their contents to the outside
what are the functions of lysosomes
-hydrolyse material ingested by phagocytic cells, such as white blood cells and bacteria
- release enzymes to the outside of the cell (exocytosis) in order to destroy material around the cell
- digest worn out organelles so that the useful chemicals they are made of can be re-used
- completely break down cells after they have died (autolysis).
what are lysosomes
- formed when the vesicles produced by the Golgi apparatus contain enzymes such as proteases and lipases
-contain lysozymes, enzymes that hydrolyse the cell walls of certain bacteria - As many as 50 such enzymes may be contained in a single lysosome
- lysosomes isolate these enzymes from the rest of the cell before releasing them, either to the outside or into a phagocytic vesicle within the cell
what are ribosomes
- small cytoplasmic granules found in all cells
-they may occur in the cytoplasm or be associated with the RER - there are two types, depending on the cells in which they are found:
1. 80S
2. 70S
what is the cell wall
-characteristic of all plant cells, the cell wall consists of microfibrils of the polysaccharide cellulose, embedded in a matrix
- Cellulose microfibrils have considerable strength and so contribute to the overall strength of the cell wall.
what are 2 features of the cell wal
-They consist of a number of polysaccharides, such as cellulose
- There is a thin layer, called the middle lamella, which marks the boundary between adjacent cell walls and cements adjacent cells together
what are the functions of the cell wall
- to provide mechanical strength in order to prevent the cell bursting under the pressure created by the osmotic entry of water
-to give mechanical strength to the plant as a whole
-to allow water to pass along it and so contribute to the movement of water through the plant.
what are the cell walls of algae made up of
what are the cell walls of fungi made up of
cellulose or glycoproteins, or a mixture
chitin
what are the functions of plant vacuoles
- They support herbaceous plants, and herbaceous parts of woody plants, by making cells turgid
- The sugars and amino acids may act as a temporary food store
-The pigments may colour petals to attract pollinating insects
what is the vacuole
-A fluid-filled sac bounded by a single membrane may be termed a vacuole
-The single membrane around it is called the tonoplast.
-A plant vacuole contains a solution of mineral salts, sugars, amino acids, wastes and sometimes pigments such as anthocyanins
what does mitosis produce
what does meiosis produce
two daughter cells that have the same number of chromosomes as the parent cell
produces four daughter cells, each with half the number of chromosomes of the parent cell
what is mitosis always preceded by (and describe)
interphase
- period of considerable cellular activity that includes a very important event, the replication of DNA
- The two copies of DNA after replication remain joined at a place called the centromere
what are the 4 stages of mitosis
prophase, metaphase, anaphase, telophase and cytokinesis
what happens in prophase
- the chromosomes first become visible, initially as long thin threads, which later shorten and thicken
-Animal cells contain two cylindrical organelles called centrioles, each of which moves to opposite ends (called poles) of the cell - From each of the centrioles, spindle fibres develop, which span the cell from pole to pole
- Collectively, these spindle fibres are called the spindle apparatus
- The nucleolus disappears and the nuclear envelope breaks down, leaving the chromosomes free in the cytoplasm of the cell
- These chromosomes are drawn towards the equator of the cell by the spindle fibres attached to the centromere
what happens in metaphase
- the chromosomes are seen to be made up of two chromatids
- each chromatid is an identical copy of DNA from the parent cell
- The chromatids are joined by the centromere
- It is to this centromere that some microtubules from the poles are attached, and the chromosomes are pulled along the spindle apparatus and arrange themselves across the equator of the cell
what happens in anaphase
- the centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosome apart
- The chromatids move rapidly to their respective, opposite poles of the cell and we now refer to them as chromosomes
- The energy for the process is provided by mitochondria, which gather around the spindle fibres
- If cells are treated with chemicals that destroy the spindle, the chromosomes remain at the equator, unable to reach the poles
what happens in telophase and cytokinesis
- the chromosomes reach their respective poles and become longer and thinner, finally disappearing altogether, leaving only widely spread chromatin
- The spindle fibres disintegrate and the nuclear envelope and nucleolus re-form
- Finally the cytoplasm divides in a process called cytokinesis
what is binary fission
- cell division in prokaryotic cells
- circular DNA molecule replicates and both copies attach to the cell membrane
- plasmids also replicate
- cell membrane begins to grown between the two DNA molecules and begins to pinch inward, dividing cytoplasm into two
- a new cell wall forms between the two molecules of DNA, dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids
how do viruses replicate
- by attaching to their host cell with the attachment proteins on their surface
- They then inject their nucleic acid into the host cell
- The genetic information on the injected viral nucleic acid then provides the ‘instructions’ for the host cell’s metabolic processes to start producing the viral components, nucleic acid, enzymes and structural proteins, which are then assembled into new viruses
