Cell Structure Flashcards
What is magnification?
- Magnification is how much bigger an image appears compared to the original object – produce linear magnification
- If a magnification is x100, it appears 100 times wider and longer than it is
What is resolution?
- Resolution is the ability of an optical instrument to produce an image that shows more fine detail so objects are visibly distinct
- Limited by diffraction (wavelength of light)
What are the features of an electron microscope?
¥ Two types of electron microscopes: transmission electron microscopes and scanning electron microscopes
¥ Uses a beam of electrons that are focused onto the screen
¥ Both have a vacuum - ensures electrons remain in a straight line
¥ Preparation includes fixation (using chemicals/freezing), staining, dehydration
What are the features of a transmission electron microscope?
- Specimen must be chemically fixed by being dehydrated, stained with metal salts and set in resin – must be dead
- Form a 2D grey-scale image – called an electron micrograph
- Can have a magnification of up to 2 million, newer generations can go up to 50 million
- Resolving power: 0.5nm
What are the features of a scanning electron microscope?
- The specimen is whole (but dead) and coated in a fine film of metal, then placed in a vacuum but can be sliced to expose the inside
- This results in a 3D image with a magnification from x15 to x200,000
- The image is black and white but computer software programmes can add false colour
- Resolving power: 3-10nm
How is the eyepiece graticule used?
• The scale is arbitrary – the measure is epu (eyepiece units)
• The scale graticule – a microscopic ruler on a special slide – is 1mm/1000m long, with 100 divisions and each division is 0.01mm/10m
• The scale graticule is used only to calibrate the eyepiece graticule
1. Insert an eyepiece graticule (with 100 divisions) into the x10 (magnification of the eyepiece lens) eyepiece of your microscope.
2. Use the low-power objective (x4), to bring the stage graticule (place on the microscope stage) into focus – the total magnification is now x40.
3. Align the graticules and check the value of one eyepiece division at x4 magnification.
4. If the 1mm of the stage graticule corresponds with 40 divisions, each is 25m (1000/40)
5. Repeat for different magnifications.
Calculations
• Total magnification = magnifying power of the objective lens x magnifying power of the eyepiece lens
• Magnification= Image size / Actual size (M=I/A)
• To find the actual size of a structure:
1. Measure the widest part of the structure on the photomicrograph, in mm.
2. Convert the measurement into m (micrometres) by multiplying by 1000.
3. Divide the measurement by the magnification.
Describe features of eukaryotic cells.
¥ All animals, plants, fungal and protoctist cells are eukaryotic – have a true nucleus
¥ Has membrane bound organelles, other than the nucleus – i.e. mitochondria, Golgi apparatus, endoplasmic reticulum (ER)
¥ Has a nucleus surrounded by a nuclear envelope
¥ Has a nucleus that contains DNA organised and wound into linear chromosomes
¥ Has a nucleolus inside the nucleus, containing RNA, where chromosomes unwind, ribosome are also made at the nucleolus
¥ Has organelles that are suspended in a jelly-like cytoplasm
¥ Has a cytoskeleton – a network of protein filaments (actin or microtubules) within the cytoplasm that move organelles from place to place inside the cell; allow some cells to move; allow contraction of muscle cells
¥ Has a plasma (cell surface/cytoplasmic) membrane
¥ Has small vesicles
¥ Has ribosomes – organelles without membranes where proteins are assembled
What are benefits of being membrane-bound?
¥ This keeps organelles separate from the rest of the cell so that it is a discrete compartment - compartmentalisation
¥ Useful for specific reaction conditions, seletively-permeable
¥ Membrane-bound organelles are a feature of eukaryotic cells, but not of prokaryotic cells
What is the purpose of organelles?
¥ In every cell, there are various organelles each with a specific function
¥ This provides a division of labour, which means that every cell can carry out its many functions efficiently
¥ Organelles with membranes: vacuoles/cilia/undulipodia/lysosomes/SER/RER/nucleus/Golgi apparatus/mitochondria/chloroplast
¥ Organelles without membranes: ribosomes/centrioles/cytoskeleton/cell wall
What is the structure and function of the nucleus?
Structure
• The nucleus is surrounded by a double membrane – a nuclear envelope, which has pores
• It also contains the nucleolus, which does not have a membrane and contains RNA
• Chromatin is genetic material, consisting of DNA wound around histone proteins
• When the cell is not dividing chromatin is extended (spread out) but when it is about to divide (mitosis/meiosis), the chromatin condenses and coils tightly into chromosomes, which make up nearly all of a person’s genome
• Contains coded genetic information (DNA – which is too big to leave the nucleus)
• Nucleolus is an area within the nucleus, made of proteins and RNA
Function
• The nuclear envelope – separates the contents of the nucleus with the rest of the cell to protect DNA from damage in the cytoplasm/in some regions the outer and inner membranes fuse together – this is where some dissolved substances and ribosomes can pass through
• The pores of the nuclear envelope – enables large substances (mRNA) to leave the nucleus/allows substances (steroid hormones) to enter the nucleus from the cytoplasm
• Nucleolus – manufactures ribosomes (makes rRNA from RNA) for protein synthesis
• Chromosomes (coils and condensed chromatin) – contain a person’s genome
• The nucleus (overall) – control centre of the cell/stores the organism’s genome/transmits genetic information/provides instructions for protein synthesis/controls metabolic activity
What is the structure and function of the rough endoplasmic reticulum (RER)?
Structure
• System of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane
• Ribosomes bound to the surface
Function
• RER is the intracellular transport system – cisternae form channels for transporting substances from one area to another
• Responsible for the synthesis of proteins
• Provides a large surface area for the ribosomes – assembles amino acids into proteins, which actively pass through the membrane into the cisternae, then to the Golgi apparatus, where they are modified and packaged
• Secretory cells (release hormones or enzymes) have more RER
What is the structure and function of the smooth endoplasmic reticulum (SER)?
Structure
• System of membranes, containing fluid-filled cavities (cisternae) that are continuous with the nuclear membrane
• No ribosomes on the surface
Function
• Contains enzymes that catalyse reactions involved with lipid metabolism – synthesis of cholesterol/lipids/phospholipids/steroid hormones – as needed by the cell
• Involved in absorption/synthesis/transport of lipids (from the gut)
• Responsible for lipid/carbohydrate synthesis and storage
What is the structure and function of the Golgi apparatus?
Structure
• Compact structure of cisternae
• Consists of a stack of membrane-bound flattened sacs
• Secretory vesicles bring materials to and from the Golgi apparatus
Function
• Proteins are modified by adding 1) sugar molecules to make glycoproteins 2) adding lipid molecules to make lipoproteins 3) being folded into their 3D shape
• Proteins are packaged into vesicles that a pinched off then stored in the cells OR moved to the plasma membrane – either to be incorporated into the plasma membrane or to be exported outside of the cell
What is the structure and function of the mitochondrion?
Structure
• May be spherical/rod-shaped/branched
• Surrounded by two membranes (double membrane) with fluid filled space between them
• Inner membrane is highly folded to form cristae – NB: not ‘a’ cristae
• Membrane that forms the cristae contains the enzymes used in aerobic respiration
• Interior part is a fluid-filled matrix
• Intermembrane space between the two membranes
• Contains mitochondrial DNA (mtDNA)
Function
• Site of ATP (energy currency) production during aerobic respiration
• Abundant in cells, where much metabolic activity takes place (i.e. the liver cells/ the synapses between neurons where neurotransmitter is synthesised/released)
• mtDNA allows them to produce their own enzymes and self-replicate
What is the structure and function of the chloroplasts?
Structure
• Are large organelles: 4-10m long
• Found only in plant cells and in some protoctists (in green parts)
• Surrounded by double membrane/envelope
• Contains loops of DNA and starch grains
• Inner network of membranes is continuous, forming stacks of flattened membrane sacs – thylakoids (look like piled plates), which contain chlorophyll
• Each stack/pile of thylakoids is called a granum, which contain chlorophyll pigments (pl. grana) – the lamellae connect the two stacks of grana on opposite sides together
• The fluid enclosed is called the stroma
• Strach made in photosynthesis is present as starch grains
• Contain own DNA and ribosomes
Function
• Site of photosynthesis
• 1st stage (of photosynthesis): light-dependent reaction occurs in the grana (sectioning lengthways to see grana) as it contains chlorophyll
• 2nd stage: hydrogen reduces carbon dioxide, using energy from ATP, to make carbohydrates – occurs in the stoma
• Chloroplasts abundant in leaf cells – especially the palisade mesophyll layer
• Can make their own proteins from their DNA
• Inner membranes provide a large surface area, needed for proteins during photosynthesis
What is the structure and function of the vacuole?
Structure
¥ Surrounded by a membrane called the tonoplast
¥ Contains fluid made of water and solutes
Function
¥ Only plant cells have a large/permanent vacuole
¥ Maintains cell stability – as when it is full it pushes against the cell wall, making the cell turgid
¥ Turgid plant cells help support the plant – especially non-woody plants
What is the structure and function of the lysosomes?
Structure
• Specialised forms of vesicles
• Small bags – formed from the Golgi apparatus
• Each surrounded by a single membrane
• Contain powerful hydrolytic (digestive) enzymes
• Abundant in phagocytic cells (neutrophils/macrophages) that can ingest and digest invading pathogens (bacteria)
Function
• Keep the powerful hydrolytic enzymes separate from the rest of the cell
• Digest foreign matter (pathogens, engulfed by phagocytes) and return the digested components to the cell to reuse (immune system)
• Responsible for breaking down waste material in cells, including organelles (apoptosis)
What is the structure and function of the cilia/undulipodia?
Structure
• Are protrusions from the cell and are surrounded by the plasma membrane
• Each contains microtubules
• Formed in the centrioles
• Cilia (hair-like) can be mobile or stationary and contains two central microtubules (black circles), surrounded by nine pairs of microtubules arranged like a wheel (9+2 arrangement)
• Pairs or parallel microtubules slide over each other to create the beating motion
Function
• Epithelial cells in the trachea lining have hundreds of cilia that move the mucus
• Nearly all cell types in the body have one cilium that acts as an antenna – contains receptors/allows the cell to detect signals about its immediate environment
• Only type of human cell to have an endophoria (kind of a flagellum) – longer cilium – is a spermatozoon: enables it to move
• Flagella (whip-like) used for cell mobility, sometimes sense chemical changes in environment
• Cilia used in sensory organs (stationary), beat rhythmically to create a current and cause fluids/objects adjacent to the cell to move (mobile) – i.e. lungs, ovaries
What is the structure and function of the ribosomes?
Structure
• Small spherical organelles: found in the cytoplasm, RER, mitochondria, chloroplasts and prokaryotic cells
• About 20nm in diameter
• Made of ribosomal RNA
• Made in the nucleolus – in two separate sub-units, which pass through the nuclear envelope into the cell cytoplasm and then combine
• Some remain free in the cytoplasm and some attach to the ER – making RER
Function
• Bound to the exterior of the RER – used to synthesise proteins that will be exported outside the cell
• Free in the cytoplasm (either singly or in clusters) – are primarily the site of assembly of proteins that will be used inside the cell (site of translation in protein synthesis)
What is the structure and function of the centrioles?
Structure
• Component of the cytoskeleton (not found in most fungi and flowering plants)
• Centrioles consist of two bundles of microtubules at right angles to each other, forming the centrosome
• Made of tubulin protein subunits – arranged to form a cylinder
• Have a nucleus
• Usually absent from cells of (higher) plants but may be present in some unicellular green algae (Chylamdomonas)
Function
• Centrosomes are involved in the assembly and organisation of spindle fibres during cell division
• 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
• Involved in the formation of cilia and undulipodium – before cilia forms, centrioles multiply and line up beneath the plasma membrane, microtubules sprout out from each centriole forming the cilia/undulipodium
What is the structure and function of the cellulose cell wall?
Structure
• Made from bundles of cellulose fibres, making it strong
• Not found in animal cells – fungi have cell walls made of chitin (not cellulose)
Function
• Can prevent the cell from bursting when turgid (swollen)
• Provides strength and support
• Maintain cell’s shape and rigidity
• Contribute to the strength and the support of the whole plant
• Freely permeable and allow solutions (solvent/solutes) to pass through
• Acts as a defence mechanism, protecting contents from invading pathogens
What is the structure and function of the cytoskeleton?
Structure
• Network of protein structures within the cytoplasm
• Organelles held in place by cytoskeleton and this controls cell and organelle movement
• Consists of rod-like microfilaments, intermediate microfilaments, microtubules
Function
• Provides mechanical strengthen to cells, aids transport within cell and enables cell movement
How is a protein made and secreted?
- The gene that has the coded instructions for a protein (i.e. insulin), housed on chromatin in the nucleus, is transcribed into mRNA
- Multiple copies of the mRNA are made and they pass through the pores in the nuclear envelope to the ribosomes.
- At the ribosomes, the instructions are translated and the protein molecules are assembled.
- The protein molecules pass into the cisternae of the RER and along the hollow sacs.
- Vesicles with the protein inside are pinched off from the RER and pass, via the microtubules and motor proteins, to the Golgi apparatus.
- The vesicles fuse with the Golgi apparatus, where the protein molecules may be modified for release.
- Inside vesicles pinched off from the Golgi apparatus, these molecules pass to the plasma membrane.
- The vesicles and the plasma membrane fuse and the protein is released to the outside of the cell.
What are features of prokaryotic cells?
No nucleus Circular DNA (plasmids) Non-membrane bound organelles Smaller ribosomes Peptidoglycan cell walls Divide by binary fission
How to light microscopes work?
Compound light microscopes have an objective lens – near the specimen and an eyepiece lens – through which the specimen is viewed
Where are light microscopes used?
Schools, laboratories, hospitals
Magnification of a light microscope
x1500 (sometimes x2000)
Resolution of a light microscope
maximum resolution is 200nm
Advantages of a light microscope
relatively cheap to buy and operate/easy to use/small and portable – used in the field and in laboratories, hospitals and schools, able to be used to study whole living (and dead) specimens, easily available, increased magnification can be achieved easily, natural colour seen, simple sample preparation
Disadvantages of a ight microscope
resolution is a limiting factor, low contrast
How to laser-scanning confocal microscopes work?
Use laser light is moved across the specimen to scan an object point by point and a 2D image is produced but by assembling images from different focal planes (on a computer), the pixel image into one image, a 3D image can also be made
When a laser-scanning confocal microscopes used?
Used in the drug development and the medical profession (non-invasive) – i.e. to observe fungal filaments within the cornea – results in a swift/early diagnosis
Advantages of a laser-scanning confocal microscope
high resolution (due to removal of excess light and thin sections) /high contrast/depth selectivity/can observe living specimens
Disadvantages of LSCM
expensive/quite slow – scanning everything point by point does this
Advantages of electron microscopes
high resolution/give clear and highly magnified images/more detail of the cell ultrastructure
Disadvantages of electron microscopes
large/expensive/need lots of training and skill to use/specimens must be dead (viewed in a vacuum)/the stain may be hazardous for the user/can only be used inside a carefully controlled environment in a dedicated space/specimen can be damaged by the beam/artefacts (bubbles, loss of continuity in membranes, distortion of organelles) – but as technique improves these can be eliminated/complex sample preparation, which can distort the sample
When are electron microscopes used?
- Has enabled scientists to establish the structure of organelles by making and examining several sections through an organelle to build up a 3D picture of it
- Enabled biochemistry to discover the functions of the organelles and compare healthy and diseased cells
How is an optical microscope used?
- Place slide onto the stage and clip it into place, ensuring the part that wants to be viewed is directly above the hole in the stage.
- Rotate the nosepiece to get the smallest power objective lens over the specimen.
- Adjust the coarse focus knob, while looking into the eyepiece, until the image is clear.
- While viewing the image, adjust the iris diaphragm for optimum white light (usually comes from below/reflected from a mirror).
- Now rotate the nosepiece for another objective lens and repeat the steps.
Discuss staining
- Many specimens are colourless/transparent - these are unstained speimens
- Stains are coloured chemicals that bind to molecules in or on the specimen, making it easier to see
- Increased contrast allows components to be visible so they can be identified
- Risk management: many stains are toxic or irritants
Differential staining
Stains that bind to specific cell structures that would otherwise be difficult to identify so by staining them differently, they can easily be identified within a single preparation. Used to identify different cellular components and cell types.
How are slides prepared?
- Solid specimens are viewed whole or cut into thin slices with a sharp blade (sectioning), the specimen is placed on the slide and a cover slip is placed on top
- If the specimens are suspended in a liquid, the cover slip is placed on from an angle to avoid air bubbles
- Squash slides (root tip) – a wet mount is prepared, then tissue is used to gently press down the cover slip, carefully so the cover slip does not break when pressed
- Smear slides – a slide at 45 degrees is placed in the sample (blood), which is pushed to the other side of the slide to create a thin/even coating, a coverslip is placed on top
- Thin slices are used because the specimen can only be seen if light passes through
How to draw biological drawing?
Use a sharp HB pencil, include a title, state magnification, indicate the scale, label the structures with straight lines that are parallel to the top of the page and do not cross nor have arrowheads, use a ruler and pencil for labels, don’t sketch, use clear lines, use white unlined paper, use most of the page, do not shade, ensure proportions are correct
1mm = ? micrometers
1000
1 micrometer = ? nm
1000
Similarities of prokaryotes to eukaryotes
plasma membrane/cytoplasm/ribosomes
Differences of prokaryotes to eukaryotes
much smaller/do not have a nucleus/much less well-developed cytoskeleton with no centrioles/no membrane-bound organelles (no mitochondria/SER/RER/chloroplasts/Golgi apparatus)/cell wall made of peptidoglycan/smaller ribosomes/naked DNA that floats free in the cytoplasm as a loop
Ultrastructure
features that can be seen using an electron microscopy
Cytoskeleton
a network of protein filaments (actin or microtubules) within the cytoplasm that move organelles from place to place inside the cell; allow some cells to move; allow contraction of muscle cells
