cell structure Flashcards
what is cell fractionation?
Cell fractionation is an important technique that allows scientists to study the functions of organelles
what is cell theory?
Cells are basic structural functional organisational units of both single celled and multicellular organisms
électron micrograph?
photograph of an image seen using an electron microscope
photomicrograph?
photograph of an image seen using an optical microscope
What is linear magnification?
Microscopes produce linear magnification meaning if a specimen is seen magnified x100 it appears to be 100 times wider and 100 times longer than it really is
one m is what in mm?
1 metre is divided into 1000mm
one mm is how many in micrometres?
one millimetre is divided into 1000 micrometres
one micrometres is how many nanometres?
1 micromètre is divided into 1000nm
stains can kill us - true or false
true
what is resolution?
Resolution is defined as the minimum distance between two objects where they can still be seen as two separate objects
features of light microscope
Poor resolution due to the long wavelength of light
Living samples can be examined and a colour image is obtained
Relatively cheap
Easy to use
Able to be used to study whole living specimens
what’s the magnification of a light microscope?
x1500 to x2000
resolution of light microscopes?
Optical microscopes use visible light, has a wavelength of between 400 and 700nm so structures closer together than 200nm will appear as one object
electrons have a wavelength of how much?
Beam of electrons have a wavelength of about 0.004nm
what happens in an electron microscope?
Electrons are fired from a cathode and focus by magnets on to a screen or photographic plate
resolution of an electron microscope
A beam of electrons has a very short wavelength - a high resolution meaning small organelles and internal structures can be visualised
how is an image created from an electron microscope?
Image is created using an electromagnet to focus the beam of negatively charged electrons
why must EM be in a vacuum?
Electrons are absorbed by air therefore EM must be in a vacuum
can living organisms be examined by an EM
no
does EM create coloured images?
Image is also black and white, do have to add a stain to add any colour
two types of EM
TEM and SEM
what happens in TEM?
Specimen has to be chemically fixed by being dehydrated and stained
Beam of electrons passes through the specimen which stained with metal salts
Some electrons pass throughout and are focused on the screen
Extremely thin specimens are stained and placed in a vacuum
Electron gun produces a beam of electrons that passes through the specimen
The electromagnet will focus the beam and these transmit/pass through the specimen
Some parts of the specimen absorb the electrons and this makes them appear darker
Some parts won’t and they’ll look lighter
features of TEM
2D black and white image is formed
High magnification and resolution
Electrons pass through the specimen to create an image
Resolution - up to 0.5nm
The image produced is 2D and shows detailed images of the internal structure of cells
features of SEM
High magnification and resolution
Electrons bounce off the surface of the specimen to create an image
Gives a 3D image with magnification from x15 up to x200 000
Image is black and white but computer software programmes can add false colour
Specimen has to be placed in vacuum and often coated with a fine film of metal
3-10nm
Specimen does not need t be thin as the electrons are not transmitting through
Electrons are beamed onto the surface and the electrons are scattered (reflect back) in different ways depending on the contours of your specimen
This is what produces a 3D image of the surface of the specimen
features of both EM
Are large and very expensive
Need a great deal of skill and training to use
what is optical microscope determined by?
determined by the wavelength of light
what is an electron microscope determined by?
determined by the wavelength of the beam of electrons
what is magnification?
Magnification refers to how many times larger the image is compared to the object
Magnification = image size/object size
what are artefacts?
Artefacts are false visible details that aren’t part of the specimen being observed, such as air bubbles or fingerprints.
what are artefacts caused by?
Caused by:
Treatment process
Creation of sample
Dehydration
staining/addition of heavy metals
how can you observe prepared specimens?
Dehydrating the specimens
Samples need to be preserved with chemicals such as formaldehyde
Water is removed and replaced with wax/resin plastic to retain shape and longevity
Embedding them in wax to prevent distortion during slicing
Using a special instrument to make very thin slices called sections - to allow light to pass through - these are stained and mounted in a special chemical to preserve them
what are the four main types of sample preparations?
1) Dry mount
2) Wet mount
3) Squash slide
4) Smear slide
dry mount meaning
when thin slices or whole specimens are viewed, with just the coverslip place on top eg/ plant tissue or hair
wet mounts meaning
when the specimens are added to water or a stain before you have the coverslip lowered on top with a mounted needle to prevent air bubbles from forming - aquatic organisms could be viewed this way
squash slide meaning
wet mounts which you then push down on the coverslip to squash the sample to ensure you have a thin layer of cells to enable light to pass through - used when creating a root tip squash sample to view the chromosomes in mitosis
smear slide meaning
created by placing a drop of the sample at one end of the slide and using the edge of another slide (held at an angle) to smear the sample across the first slide to create a smooth, thin, even coated specimen - cover slip is placed on top after smearing - used when examining blood cells in a blood sample
what is a stain?
Stains are coloured chemicals that bind to molecules in or on the specimen making the specimen easy to use
Stains can be used for many purposes
what purposes can stains be used for?
Charge attraction (eg/ methylene blue - positively charged binds to negatively charged materials at cells
Charge repulsion
Structural difference
Gram positive - crystal violet
Gram negative - safranin dye
Methylene blue is an all-purpose stain
Some cell components and cells are difficult to see unless they are stained a more obvious colour
what is the definition of differential staining?
technique which involves many chemical stains being used to stain different parts of a cell in different colours - bind to specific cell structures- makes it visually obvious what your looking at
what is an eyepiece graticule?
Inside light microscope , in the eye piece there is actually a scale that you can insert on a glass disc which is called the eyepiece graticule
eyepiece graticule preparation
You can use this scale to measure the size of the object your looing at but there are different lenses on your microscope which are going to be causing different magnifications
At each magnification, the value of one of the divisions on your your eyepiece graticule will be different so you have to know how to calibrate it at every magnification
Each time you change the objective lens and the magnification, you have to calibrate the eyepiece and work out what each eye piece unit is worth
how to calibrate an eyepiece graticule
In order to calibrate the eyepiece graticule you need a stage micrometer which is a glass slide which has a ruler on it
You would place it on the stage of your microscope and then look through your eyepiece and then move your stage micrometer so that it is aligned right next to your eyepiece graticule
Bottom scale is the scale on your stage micrometer and that has been aligned with your eyepiece graticule
Once you’ve got them aligned, count how many divisions on the eyepiece graticule fit into one division on the micrometer scale
Each division on the micromete is 10mcrometers so this can be used to calculate what one division on the eyepiece graticule is at that current magnification
The stage graticule will correspond to a number of eyepiece divisions
The stage graticule is 1mm or 1000 micrometers
Each eyepiece division will be 1000/number micrometers
what classes as a eukaryote?
animals,plants and fungi
what classes as a prokaryote?
archaea and bacteria
features of eukaryotic cells
In all eukaryotic cells, the DNA is contained in a membrane-bound nucleus
Membrane-bound - means surrounded by a membrane
Nucleus actually have a double membrane
Some eukaryotic cells can lose their nucleus as they develop, for example red blood cells in humans but vast majority of eukaryotic cells have a nucleus throughout their lifespan
Second key feature of eukaryotic cells is that their DNA is tightly wrapped around proteins called histones
Together the DNA and histone proteins form chromosomes
By tightly coiling their DNA into chromosomes, eukaryotic cells can pack a great deal of DNA into their nucleus
Another key feature of eukaryotes is that their DNA is a linear molecule
The word linear means that the ends of the DNA molecule in a chromosome are not joined together to form a loop
which membrane bound organelles are present in eukaryotes?
Membrane-bound organelles are present in eukaryotes - eg/ golgi apparatus, mitochondria, cytoplasm, endoplasmic reticulum
not membrane-bound organelles in eukaryotes
Eukaryotic cells also contain other organelles that are not membrane-bound
Eg - ribosomes which are involved in protein synthesis
Ribosomes in eukaryotic cells are larger than those in prokaryotes - eukaryotic cells contain 80S ribosomes
What does the S in 80S and 70S stand for?
S - unity showing how quickly organelles move in a centrifuge
function of the cell surface (plasma) membrane
Eukaryotic cells are surrounded by a cell-surface membrane which helps to control the molecules that can pass in and out of the cell
general function of cell wall
In plants and fungi, the cell membrane is surrounded by a cell wall - cell wall helps to maintain the structure of these cells
what are plant cell walls made from?
polysaccharide cellulose
what are fungi cell walls made out of
polysaccharide chitin
differences between prokaryotic cells and eukaryotic cells
rokaryotic cells are much smaller than eukaryotic cells
Have no membrane bound organelles at all
This means that in prokaryotic cells, DNA is found in the cytoplasm rather than in a nucleus
In prokaryotic cells, DNA is arranged into a circular chromosome with no free ends
DNA in prokaryotes is not bound to histone proteins
Bacterial cells contain small loops of DNA called plasmids
info about plasmids
Plasmids usually contain a relatively small number of genes - but these can include genes which make the bacterium resistant to antibiotics so plasmids are very important for bacteria
Eukaryotic cells do not contain plasmids
general info about ribosomes
Ribosomes in prokaryotic cells are smaller than in eukaryotic cells - have a size of 70S whereas eukaryotic ribosomes have a size of 80S
bacteria structure
Prokaryotic cells such as bacteria are surrounded by a cell wall - in bacteria, this is made from peptidoglycan which is also called murein
Peptidoglycan - polymer formed between peptides and polysaccharide molecules
Bacteria cell wall helps to maintain structure of the cell eg/ - if water moves into the bacterial cell by osmosis, cell wall prevents cell from bursting
Some bacteria produce a slime capsule on the outside of hte cell wall - slime capsule can help to protect the bacteria from phagocytosis in WBCs
Some have a flagellum which help them to move
Prokaryotic flagellum has a different structure to that found in eukaryotes
Some bacteria also have fine protein strands on their surface - these are called pili which help bacteria to attach to surfaces and also to attach to other bacteria
When two bacteria are attached, DNA can be transferred from one bacterium to another
Bacteria also contain lipid droplets and glycogen granules - act as nutrient stores for the bacterial cells
Sometimes, using the electron microscope, we can see infoldings in the cell membrane of prokaryotic cells - these are called mesosomes
Initially scientists though ribosomes played a role in respiration but they now believe that mesosomes are actually artefacts that are created when bacterial cells are prepared for electron microscopy
what proteins are used as?
Enzymes are proteins
Enzymes are required for all the chemical reactions taking place in cells
Other proteins play a structural role in cells - for example, moving organelles to where they are needed
Some proteins act as transport molecules - eg/ haemoglobin which transports oxygen in mammals
general info on protein synthesis
Protein synthesis is one of the most important functions taking place in cells
Involves several different organelles
The instructions for encoding the amino acid sequence of a protein are contained within the gene for that protein - these genes are part of the chromosomes which we find in the nucleus
To synthesise a protein, the genetic information encoded by that gene is converted to messenger RNA or mRNA
This process is called transcription
This mRNA then leaves the nucleus
Now a ribosome reads the information contained in the mRNA and synthesises the protein molecule - this is called translation
If protein remains in cytoplasm , eg cellular enzyme, then translation will take places on a free ribosome in the cytoplasm
However some proteins are secreted from cells - eg/ digestive enzymes and antibodies
Secreted proteins are translated on a ribosome attached to RER
These proteins then make their way through the rough ER and the golgi apparatus before leaving the cell
structure of a nucleus
Has a nuclear envelope which is this double membrane layer and the holes that you can see within it are the nuclear pores and that is what the mRNA can pass out of after transcription
Nucleoplasm - this granular, jelly like material in the middle
Chromosomes - found inside of the nucleus, are protein bound so wrapped around histone proteins and they are linear
Nucleolus - smaller sphere inside of the nucleus which is the site of rRNA production and makes ribosomes
Nuclear envelope separates the contents of the nucleus from the rest of the cell
In some regions outer and inner membranes fuse together - at some points, some dissolved substances and ribosomes can pass through
Pores enable larger substance such as mRNA to leave the nucleus
Substances such as steroid hormones may enter the nucleus from the cytoplasm via these pores
Contains DNA wound into linear chromosomes
detail on nucleolus
Nucleolus - smaller sphere inside of the nucleus which is the site of rRNA production and makes ribosomes
Nucleolus region is where ribosomes are formed from rRNA and proteins including an enzymatic site of peptidyl transferase which catalyses the condensation reaction between amino acids to form peptide bonds
Nucleolus does not have a membrane around it, contains RNA
Chromatine is the genetic material - consisting of DNA wound around histone protein
When cell is not dividing, chromatin is spread out or extended, when cell is about to divide, chromatin condenses and coils tightly into chromosomes
function of nucleus
In summary, the nucleus:
Control centre of the cell
Stores the organism’s genome
Transmits genetic information
Provides instructions for protein synthesis
Function of nucleus
Site of DNA replication and transcription ( first stage of protein synthesis when mRNA is created )
Contains the DNA for each cell
Site of ribosome synthesis
genetic material in prokaryotes
Prokaryotic cell - no nucleus - instead of a nucleus there is a single circular DNA molecule free in the cytoplasm which is not protein bound (NOT ATTACHED TO PROTEINS)
Contains DNA wrapped around histone proteins to form chromatin, chromatin forms chromosomes
All is isolated from cytoplasm by the nuclear envelope which is the double membrane
After transcription, messenger RNA/ mRNA leaves the nucleus via nuclear pores but DNA/ chromosomes are too long to fit
Plasmids are additional loops of DNA, only have a few genes on them and its where you’d find the genes for antibiotic resistance and bacteria either don’t have them or have them in varying numbers
flagella
Found on some eukaryotic cells
Whip like tail structure
Function: for mobility and sometimes as a sensory organelle for chemical stimuli
cilia
Hairlike projections out of cells
Can either be stationary cilia or mobile cilia
Mobile cilia help move substances in a sweeping motion
Stationary cilia are important in sensory organs such as the nose
centrioles
Made up of microtubules
Occurs in pairs to form a centrosome
Involved in the production of spindle fibre which is essential in organsising the position of chromosomes in mitosis and meiosis (cell division)
Consists of two bundles of microtubules at right angles to each other
Microtubules are made of tubulin protein subunits and are arranged to form a cylinder
Before a cell divides the spindle made of threads of tubulin forms from the centrioles
Chromosomes attach to the middle part of the spindle and motor proteins walk along the tubulin threads pulling the chromosomes to opposite ends of the cell
Centrioles are involved in formation of cilia and undulipodia
Before cilia form, the centrioles multiply and line up beneath the cell surface membrane
Microtubules then sprout outwards from each centriole forming a cilium or undulipodium
cytoskeleton
A network of fibres found within the cytoplasm all over a cell
Consists of microfilaments, microtubules and intermediate fibres
Provides mechanical strength to cells, helps maintain the shape and stability of a cell and many organelles are bound to the cytoskeleton
Microfilaments are responsible for cell movement
Microtubules are responsible for creating a scaffold-like structure
Intermediate fibres help to provide mechanical strength
Microfilaments made of subunits of the protein actin
Each microfilament is about 7nm in diameter
Intermediate filaments about 10nm in diameter
Microtubules about 18-30nm in diameter
Cytoskeletal motor proteins, myosins, kinesins and dyneins and molecular motors - are also enzymes and have a site that binds to and allows hydrolysis of ATP as their energy source
Protein microfilaments within the cytoplasm fibe support, mechanical strength, keep the cells shape stable and allow cell movemen
Microtubules provide shape and support to cells and help substances and organelles to move through the cytoplasm within a cell
Form the track along which motor proteins
Endoplasmic Reticulum
- Rough and smooth ER both have folded membranes called cisternae
- The only difference is that the rough have ribosomes on the cisternae
- RER - has the function of protein synthesis for proteins destined to leave the cell, the proteins are transported through the RER and into the secretory vesicles
- SER - where lipids and carbohydrates are synthesised and stored
RER
A system of membranes containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane
is the intracellular system - cisternae form channels for transporting substances from one area of a cell to another
Provides a large surface area for ribosomes which assemble amino acids into proteins
These proteins then actively pass through the membrane into the cisterna and are transported to the golgi apparatus for modification and packaging
SER
A system of membranes containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane
Contains enzymes that catalyse reactions involved with lipid metabolism such as:
Synthesis of cholesterol, lipids/phospholipids needed by the cell, steroid hormones
Its involved with the absorption, synthesis and transport of lipids from the gut
Golgi apparatus and vesicles
Secretory vesicles bring material to and from the folgi apparatus
Stack of membrane bound flattened sacs
Proteins are modified:
Adding sugar molecules to make gylcoproteins, lipid molecules to make lipoprotiens, being folded into their 3D shape
Proteins are packaged into vesicles that are pinched off and then: stored in the cell, move to plasma membrane either to be incorporated into plasma membrane or exported outside of cell
Folded membranes making the cisternae
Secretory vesicles that pinch off from the cisternae
function : add carbohydrates to proteins to form glycoproteins, produce secretory enzymes, secrete carbohydrates, transport, modify and store lipids, form lysosomes, molecules are ‘labelled’ with their destination
Finished products are transported to cell surface membrane in secretory vesicles here they fuse with the membrane and the contents are released
Lysosomes
Can be created by golgi apparatus
They are vesicles/bags of digestive enzymes - can contain 50 different enzymes
Function: hydrolyse phagocytic cells (bacteria), completely break down dead cells (autolysis)
Small bags formed from the golgi apparatus and each is surrounded by a single membrane
Contain powerful hydrolytic (digestive) enzymes
Are abundant in phagocytic cells such as neutrophils and macrophages (types of white blood cells) that can ingest and digest invading pathogens such as bacteria
Have two key features
Lysosomes contain powerful digestive enzymes such as proteases - these digest large molecules into smaller soluble molecules
The internal fluid in a lysosome is acidic - thats because lysosomal enzymes have an optimum pH which is acidic
Firstly, lysosomes play a key role in phagosytosis for example in white blood cells
Firstly wbcs forms a vacuole around the bacteria - this vacuole is called a phagosome which is the now fused by lysosomes
Lysosomal enzymes now digest the bacteria and the soluble digestion products now pass into the cytoplasm
Lysosomes can also be used to destroy organelles that are damaged or no longer functional
In this case organelle is surrounded by a vacuole and lysosomes fuse with the vacoule membrane and again lysosomal enzymes now digest the organelle
Digestion products are absorbed into the cytoplasm and can be reused to make new organelles
Sometimes lysosomes transfer their enzymes outside of the cell by exocytosis, eg/ to digest and remove unwanted proteins or dead cells
function:
lysosomes keep the powerful hydrolytic enzymes separate from the rest of the cell
Lysosomes can engulf old cell organelles and foreign matter, digest them and return the digested components to the cell for reuse
what are cilia and undulipodia
These are protrusions from the cell and are surrounded by the cell surface membrane
Each contains microtubules
Are formed from centrioles
The epithelial cells lining your airways each have many hundreds of cilia that beat and move the band of mucus
Nearly all cell types in the body have one cillium that acts as antenna - contains receptors and allows cell to detect signals about its immediate environment
Only type of human cell to have an undulipodium (a longer cilium) is a spermatozoon- undulipodium enables spermatozoon to move
lysosomes
Lysosomes fusing with phagosomes in phagocytosis which releases the digestive enzymes to hydrolyse/destroy pathogens
After whatever the digestive enzymes are breaking down, the lysosomes will then fuse with the cell membrane and that will release its contents to the outside of the cell
mitochondria
2-5 micrometers
Surrounded by two membranes with a fluid filled space between them
Inner membrane is highly folded into cristae and inner part of mitochondrion is a fluid filled matrix
Double membrane bound organelle
Inner membrane is folded to form cristae
Fluid centre called rhe mitochondrial matrix - site of some of the stages of aerobic respiration
Contain their own ribosomes and loops of DNA so they can create the enzymes necessary for respiration inside of the organelle itself
function : site of aerobic respiration, site of ATP production, contains DNA to code for the enzymes needed for respiration
Found in all eukaryoti organisms
Produces ATP
In aerobic respiration, the carbohydrate glucose is broken down to carbon dioxide and water
The energy contained in the chemical bonds of glucose is transferred to ATP
(COME BACK TO THIS)
Mitochondria have a double membrane - outer/inner mitochondrial membrane
In between these membranes we have the intermembrane space
Within mitochondria we have a fluid called the matrix
The inner membrane is highly folded - these folds are called cristae
By folding the inner membrane, we have a great deal of surface area
In mitochondria we also find a loop of mitochondrial DNA
The DNA contains the genes for some of the enzymes involved in aerobic respiration
Mitochondria also contains mitochondrial ribosomes - these synthesis the proteins encoded by the mitochondrial DNA
We tend to find larger numbers of mitochondria in cells which require a lot of energy - the mitochondria in these cells often obtain larger number of cristae which provides increased surface area
Function
Site of ATP production during anaerobic respiration
Self replicating, so more can be made if cells energy needs increase
Abundant in cells where much metabolic activity takes place
organelles without membranes
Ribosomes and cytoskeleton including centrioles are not covered by membranes
ribosomes
Made in the nucleolus as two separate subunits which pass through the nuclear envelope into the cell cytoplasm and then combine
Some remain free in the cytoplasm and some attach to the endoplasmic reticulul
Made of ribosomal RNA
Very very small, spherical organelles about 20nm in diameter made up of two sub units of protein and rRNA
80S - large ribosomes found in the cytoplasm of eukaryotic cells (25nm)
70S - smaller ribosomes found in prokaryotic cells, mitochondria and chloroplasts
Function: where protein synthesis occurs
Ribosomes bound to exterior of RR are mainly for synthesising proteins that will be exported outside the cell
Ribosomes that are free in cytoplasm, either singly or in clusters are primarily the site of assembly of proteins that will be used inside the cell
cytoplasm
Formed of cytosol - jelly like substance that is mostly water
Contains enzymes, substrates, cofactors/coenzymes and mineral ions involved in biochemical reaction
chloroplasts
Are large organelles (4-10 micrometres )
Surrounded by a double membrane
Inner membrane is continuous with stacks of flattened membrane stacks called thylakoids which contain chlorophyll
Each stack or pile of thylakoids is called a granum (plural: grana)
The fluid filled matrix is called the stroma
Chloroplasts contain loops of DNA and starch grains
Contains thylakoids (folded membranes embedded with pigment)
Stacks are called grana for plural or granum for singular
Fluid filled stroma contains enzymes for photosynthesis
Found in plants
Site of photosynthesis
details of chloroplasts
First stage of photosynthesis when light energy is trapped by chlorophyll and used to make ATP occurs in the grana
Water is also split to supply hydrogen ions
Second stage: when hydrogen reduces carbon dioxide, using energy from ATP to make carbohydrates occurs in the stroma
Dont find chloroplasts in parts of the plant that do not photosynthsise eg/ roots
The energy that was trapped in photosynthesis is now contained within the chemical bonds in the glucose molecule
Photosynthesis actually consists of two dets of reactions
The light dependent reactions use chlorophyll to harvest light energy - then energy is then transferred into the chemical bonds of other molecules such as ATP - only take place in the presence of light
Light independent reactions - where glucose is formed - do not require light - take place during both light and dark conditions
Chloroplasts are surrounded by a double membrane - these membranes control which molecules enter and leav the chloroplast
Inside the chloroplast we find membrane bound flattened discs - these discs are called thylakoids - where the light dependent reactions take place
Thylakoids contain the chlorophyll and enzymes needed for the light dependent reactions
Thylakoids are stacked uo on top of ech other
A stak of thyalkoids is called granum and light can be absorbed more efficiently
Thylakoids on different grana are connected to eahother by flattened membrsnes called lamellae - play a role in light dependent reactions and allow chemicals ro pass through the grana
Chloroplasts contain fluid material called the stroma- where light independent reactions take place
Stroma contains the enzymes needed for the light dependent reactions
This glucose can be converted to the polysaccharide starch which is stored in the chloroplast as starch granules
Chloroplasts also contain a loop of DNA which contains genes which encode some of the proteins needed for protein synthesis
Chloroplasts contain ribosomes which synthesis the proteins encoded by the chloroplast DNA
extra detail on cell wall
In plant and fungi cells - not in animal cells
Plants - made of microfibrils of the cellulose polymer
Fungi - made of chitin, a nitrogen-containing polysaccharide
Provide structural strength to the cell
vacuole
Surrounded by a membrane called the tonoplast and contains fluid
Only plant cells have a large permanent vacuole
Its filled with water and solutes and maintains cell stability because when full it pushes against the cell wall making cell turgid
If all plant cells are turgid then it helps to support the plant
details on cell/plasma membrane
Found in all cells
Made up of phospholipid bilayer - molecules embed within and attached on the outside (proteins,carbohydrates, cholesterol)
Phospholipid bilayer - 2 layers of phospholipids with a hydrophilic heads and hydrophobic tails region
In water, this structure naturally forms
Limits molecule ion transport based on site of charge - may contain transmembrane protein to be selectively permeable to specific substance
Cholesterol affects the fluidity and therefore the permeability of the membrane
Controls the entrance and exit of molecules
production and secretion of proteins
1) Polypeptide chains are synthesised on the RER (ribosomes on the outside)
2) These polypeptide chains move to the cisternae in the RER and are packaged into vesicles to be sent to the Golgi apparatus for further modification via the cytoskeleton
3) In the golgi apparatus, the proteins are modified and packaged into vesicles
4) The secretory vesicles carry the proteins to the cell surface membrane where it fuses and releases the protein by exocytosis
detail on slime capsule
Prokaryotic cell: the capsule is a slimy layer made of protein on the very outside
Function: prevents the bacteria from desiccating (drying out) and it also helps to cover the antigens to make it harder for the host immune system to detect the bacteria - protects the bacteria against the host’s immune system
more on nucleus
Contains a material called nucleoplasm
Nucleoplasm contains molecules such as nucleotides and enzymes which are needed for DNA nad RNA synthesis
Surrounding the nucleus we have a double membrane called the nuclear envelope - consists of two phospholipid bilayers
Within nuclear envelope we find nuclear pores - is to allowe molecules to enter or leave the nucleus
RNA nucleotides enter the nucleus through nuclear pores from the cytoplasm and these nucleotides are used in the nucleus to synthesise messenger RNA - messenger RNA then leaves the nucleus via nuclear pore and undergoes translation on a ribosome
Outer membrane of the nuclear envelope is continuous with the RER
RER plays a significant role in protein synthesis
Nucleus contains dark material called chromatin - consists of DNA coiled around protein called histones - together DNA and histone proteins form chromosomes
Chromosomes are not visible inside the nucleus under the cell is undergoing mitosis or meiosis
Inside nucleus theres a region which is darker than the rest - nucleolus - is where a special type of RNA is produced - this is called rRNA - forms part of the structure of ribosomes
Nucleolus is also where ribsome subunits are assembled
During translation the genetic information in the mRNA is used to synthesise the protein required
cytoskeleton
Runs through the cytoplasm
Three components: microfilaments, microtubules, microfibres
Microfilaments have the narrowest diameter and microtubules have the greatest diameter
Intermediate fibres have a diameter in between the other two
Microfilaments - narrow fibres containing the protein actin, these atin fibres can contract, are involved in cell movement, play a role during cell division
Last part of cell division is called cytokinesis
During cytokinesis the cell membrane is pulled inwards dividing the cytoplams into two - this process involves the action of microfilaments
Intermediate fibres are formed from a number of different proteins and the rold of this is to strengthen the cell
Microtubules are framed from subunits of the protein tubulin - tubulin subunits assemble to form tubulin polymers which then form hollow microtubules
Microtubules are involved in the movement of organelles, form the spindle fibres which are involved in the movement of chromosomes during meiosis and mitosis
By forming a complex network in the cytoplasm, microtubules help to determine the shape of cells
Centrioles, cilia and flagella
Can find a pair of centrioles in lot of different eukaryotic ells including animal cells
Certain relatively simple plants/algae can have centrioles
We dont find them in flowering plants or most fungi
Centrioles are made up of microtubules
Centrioles lie at right angles to eachther and found near the nucleus
Together we call pair of centrioles the centrosome
Firstly during mitosis and meiosis, centrioles play a role in the assembly of the spindle fibres
Spindle fibres are also formed from microtubules
How these microtubules are arranged into spindle fibres may be organised by the centrioles
Plant cells also form spindle fibres during cell division but flowering plant cells do not contain centrioles so this means centrioles cannot be essential for spindle assembly
Centrioles play another role in eukaryotes - in cilia and flagella
