Cell Structure (1) Flashcards
What is viewed under a light microscope and why
Specimens can be viewed under a light microscope; this allows some details of cellular material to be observed
What are permanent slides
Such slides are produced by cutting very thin layers of tissue which are stained and permanently mounted on a glass slide for repeated use
How to prepare a temporary slide for liquid specimen, purpose of coverslip
Add a few drops containing the liquid sample to a clean slide using a pipette
Lower a coverslip over the specimen and gently press down to remove air bubbles
Coverslips protect the microscope lens from liquids and help to prevent drying out
How to prepare a temporary slide for solid specimen
Use scissors or a scalpel to cut a small sample of tissue, and peel away or cut a very thin layer of cells from the tissue sample
The preparation method always needs to ensure that samples are thin enough to allow light to pass through
Place the sample onto a slide
A drop of water may be added at this point
Apply iodine stain
Gently lower a coverslip over the specimen and press down to remove any air bubbles
How to prepare a slide using human cells
Brush teeth thoroughly with normal toothbrush and toothpaste
This removes bacteria from teeth so they don’t obscure the view of the cheek cells
Take a sterile cotton swab and gently scrape the inside cheek surface of the mouth for 5-10 seconds
Smear the cotton swab on the centre of the microscope slide for 2-3 seconds
Add a drop of methylene blue solution
Methylene blue stains negatively charged molecules in the cell, including DNA and RNA
This causes the nucleus and mitochondria to appear darker than their surroundings
Place a coverslip on top
Lay the coverslip down at one edge and then gently lower the other edge until it is flat
This reduces bubble formation under the coverslip
Absorb any excess solution by allowing a paper towel to touch one side of the coverslip
Why are specimens stained
The cytoplasm and other cell structures may be transparent or difficult to distinguish; stains allow them to be viewed clearly under a light microscope
What are the stains and respective uses
Iodine - Stains starch blue-black, and colours nuclei and plant cell walls pale yellow
Crystal violet - Stains cell walls purple
Methylene Blue - Stains animal cell nuclei blue
Congo red - Is not taken up by cells and stains background red to provide contrast with cells present
What are biological drawings
Line drawings that show specific features that have been observed when the specimen was viewed
What are the rules and conventions when drawing biological drawings
The drawing must have a title
The magnification under which the observations shown by the drawing are made should be recorded if possible
A scale bar may be used
A sharp pencil should be used
Drawings should be on plain white paper
Lines should be clear, single lines without sketching
No shading should be used
The drawing should take up as much of the space on the page as possible
Well-defined structures should be drawn
Only visible structures should be drawn, and the drawing should look like the specimen
The drawing should be made with proper proportions
Structures should be clearly labelled with label lines that:
Do not cross
Do not have arrowheads
Connect directly to the part of the drawing being labelled
Are on one side of the drawing
Are drawn with a ruler
Difference between plan drawings and drawings of cells
Drawings of cells are typically made when visualizing cells at a higher magnification power, whereas plan drawings are typically made of tissues viewed under lower magnifications (individual cells are never drawn in a plan diagram)
What is magnification
Magnification is the number of times that a real-life specimen has been enlarged to give a larger view/image
What is the eyepiece graticule
The eyepiece graticule is an engraved ruler that is visible when looking through the eyepiece of a microscope
Eyepiece graticules are often divided into 100 smaller divisions known as graticule divisions, or eyepiece units
What does each eyepiece graticule division represent and how is it calibrated
The values of the divisions in an eyepiece graticule vary depending on the magnification used, so the graticule needs to be calibrated every time an object is viewed
The calibration is done using a stage micrometer; this is a slide that contains a tiny ruler with an accurate known scale
1 graticule division = number of micrometers divided by number of divisions
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What are the purpose of stage micrometer and eye piece graticule
An eyepiece graticule and stage micrometer are used to measure the size of an object when viewed under a microscope
How to calculate the size of a specimen
The calibrated eyepiece graticule can be used to measure the length of an object
The number of graticule divisions covered by an object need to be multiplied by the magnification factor:
graticule divisions covered by object x magnification factor = length of object (µm)
What is a light microscope made up of
A light microscope has two types of lens which allow it to achieve different levels of magnification:
An eyepiece lens, which often has a magnification of x10
A series of objective lenses, each with a different magnification, e.g. x4, x10, x40 and x100
How do I calculate the total magnification of a specimen being viewed
To calculate the total magnification of a specimen being viewed, the magnification of the eyepiece lens and the objective lens are multiplied together:
total magnification = eyepiece lens magnification x objective lens magnification
What is resolution and what happens if it is too low
The resolution of a microscope is its ability to distinguish two separate points on an image as separate objects; this determines the ability of a microscope to show detail
If resolution is too low then two separate objects will be observed as one point, and an image will appear blurry, or an object will not be visible at all
The resolution of a microscope limits the magnification that it can usefully achieve; there is no point in increasing the magnification to a higher level if the resolution is poor
How is the resolution of a light microscope limited
The resolution of a light microscope is limited by the wavelength of light
Visible light falls within a set range of light wavelengths; 400-700 nm
The resolution of a light microscope cannot be smaller than half the wavelength of visible light
The shortest wavelength of visible light is 400 nm, so the maximum resolution of a light microscope is 200 nm
E.g. the structure of a phospholipid bilayer cannot be observed under a light microscope due to low resolution:
The width of the phospholipid bilayer is about 10 nm
The maximum resolution of a light microscope is 200 nm, so any points that are separated by a distance of less than 200 nm, such as the 10 nm phospholipid bilayer, cannot be resolved by a light microscope and therefore will not be distinguishable as separate objects
What is the max resolution of electron microscope and why
0.5nm since electrons have much smaller wavelength than light
How does a light microscope operate
ght microscopes are used for specimens larger than 200 nm
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, and tissues within organs such as in leaves or skin
How does an electron microscope operate
Electron microscopes, both scanning and transmission, are used for specimens larger than 0.5 nm
Electron microscopes fire a beam of electrons at the specimen
The electrons are picked up by an electromagnetic lens which then shows the image
Electron microscopy requires the specimen to be dead, meaning that they can only be used to capture a snapshot in time, and not active life processes as they occur
Electron microscopes are useful for looking at organelles, viruses and DNA as well as looking at whole cells in more detail
Difference between transmission and scanning electron microscopes
Transmission electron microscopes (TEM) fire electrons through a specimen
Scanning electron microscopes (SEM) bounce electrons off the surface of a specimen
Differences between electron and light microscopes
- Electron microscope
Large machines that are permanently installed in laboratories
Need to create a vacuum for electrons to travel through
Specimen preparation is complex
Maximum magnification of x500 000
Maximum resolution of 0.5 nm
Specimens are always dead
- Light Microscopes
Small and portable
No vacuum required
Specimen preparation can be simple
Maximum magnification of x2000
Maximum resolution of 200 nm
Specimens can be living or dead
What can cells be divided into
Eukaryotic and Prokaryotic
Difference between Eukaryotic and Prokaryotic cells in their untrastructure
Eukaryotic cells have a more complex ultrastructure than prokaryotic cells
What is ultrastructure
Internal structure of cells
What does the cytoplasm of eukaryotic cells contain
The cytoplasm of eukaryotic cells is divided up into membrane-bound compartments called organelles
What is the cell membrane
All cells are surrounded by a cell surface membrane which separates the inside of cells from their surroundings
Cell surface membranes controls the exchange of materials between the internal cell environment and the external environment
The membrane is described as being partially permeable as it allows the passage of some substances and not others
The cell membrane is formed from a phospholipid bilayer and spans a diameter of around 10 nm
Cell surface membranes separate cell contents from the surrounding environment and control the passage of substances into and out of cells
What is the nucleus
Present in all eukaryotic cells, the nucleus is a large organelle that is separated from the cytoplasm by a double membrane
The nucleus contains the DNA, which is arranged into chromosomes
Chromosomes contain DNA and proteins, which are collectively referred to as chromatin
The nuclear membrane is known as the nuclear envelope, and contains many pores
Nuclear pores are important channels for allowing mRNA and ribosomes to travel out of the nucleus, as well as allowing enzymes and signalling molecules to move in
The nucleus contains a region known as the nucleolus, which is the site of ribosome production
The nucleus of a cell contains the genetic material
What are the rough and smooth endoplasmic membrane
The endoplasmic reticulum (ER) is made up of a series of membranes that form flattened sacs within the cell cytoplasm
The ER is linked with the nuclear envelope
There are two distinct types of ER, with different roles within the cell
- The rough endoplasmic reticulum (RER)
Continuous folds of membrane that are linked with the nuclear envelope
The surface of the RER is covered in ribosomes
The role of the RER is to process proteins that are produced on the ribosomes
- The smooth endoplasmic reticulum (SER)
The SER does not have ribosomes on the surface
It is involved in the production of lipids, and of steroid hormones such as oestrogen and testosterone
The RER has ribosomes on its outer surface and is continuous with the nuclear envelope, while the SER lacks ribosomes
What is the golgi body
The Golgi body is often referred to as the Golgi apparatus or the Golgi complex
It consists of a series of flattened sacs of membrane
It can be clearly distinguished from the ER, as it is not connected to other membrane-bound compartments, and it has a distinctive ‘wifi symbol’ appearance
Its role is to modify proteins and package them into vesicles
The Golgi body processes proteins and packages them into vesicles
What is the mitochondria
Mitochondria (singular mitochondrion) are relatively large organelles surrounded by a double-membrane
They are smaller than the nucleus and chloroplasts, but can be seen with a light microscope
The inner membrane is folded to form cristae
Mitochondria are the site of aerobic respiration within eukaryotic cells
The mitochondrial matrix contains enzymes needed for aerobic respiration
Small, circular pieces of DNA, known as mitochondrial DNA, and ribosomes are also found in the matrix
This allows the production of proteins required for respiration
Mitochondria have a highly folded inner membrane; this provides a large surface area for embedded proteins that are involved with aerobic respiration
What is a ribosome
Ribosomes are found in the cytoplasm of all cells or as part of the rough endoplasmic reticulum in eukaryotic cells
Each ribosome is a complex of ribosomal RNA (rRNA) and proteins
80S ribosomes (composed of 60S and 40S subunits) are found in eukaryotic cells
Smaller, 70S ribosomes (composed of 50S and 30S subunits) are found in prokaryotes, mitochondria and chloroplasts
Ribosomes are the site of translation during protein synthesis
Ribosomes are formed in the nucleolus and are composed of almost equal amounts of RNA and protein
What are vesicles
Vesicles are small, membrane-bound sacs used by cells for transport and storage
They can be pinched off the ends of the Golgi body; these are known as Golgi vesicles
They can fuse with the cell surface membrane to allow exocytosis, or bud from the membrane during endocytosis
Vesicles carry out transport and storage of substances within cells
What is a lysosome
Lysosomes contain hydrolytic enzymes for the breakdown of biological molecules
Lysosomes are specialised vesicles which contain hydrolytic enzymes
Hydrolytic enzymes break down biological molecules, e.g.
Waste materials, such as worn-out organelles
Engulfed pathogens during phagocytosis
Cell debris during apoptosis (programmed cell death)
What are centrioles and where are they not found
Centrioles are hollow fibres made of microtubules
Two centrioles at right angles to each other form a centrosome, which organises the spindle fibres during cell division
Note that centrioles are not found in flowering plants and fungi
Centrioles are involved with the movement of chromosomes during cell division
What are microtubules
Microtubules are hollow tubes made of tubulin protein
α and β tubulin proteins combine to form dimers, which are then joined into protofilaments
Thirteen protofilaments in a cylinder make a microtubule
Microtubules make up the cytoskeleton of the cell
The cytoskeleton is used to provide support and movement of the cell
Microtubules are tubes of protein that are involved with the structure of cell cytoskeletons
What are cilia
Cilia are hair-like projections made from microtubules
They can be found of the surface of some cells where they Allow the movement of substances over the cell surface
E.g. ciliated epithelial cells in the airways waft mucus away from the lungs
What are microvilli
Microvilli are cell membrane projections that increase the surface area for absorption
Microvilli are found in parts of the body that carry out absorption, e.g.
The lining of the small intestine
The kidney tubules
Microvilli increase the surface area of the cell surface membrane
What is the cell wall
Cell walls are outside cell surface membranes and offer structural support to some types of cell
Structural support is provided by the polysaccharide cellulose in plants, and by chitin in fungi
Cell walls are freely permeable and do not play a role in controlling the movement of substances into and out of cells
Plant cell walls contain cellulose
What are chloroplasts
Chloroplasts are larger than mitochondria, and are also surrounded by a double-membrane
Membrane-bound compartments called thylakoids stack together to form structures called grana
Grana are joined together by lamellae
Photosynthetic pigments such as chlorophyll are found in the membranes of the thylakoids, where their role is to absorb light energy for photosynthesis
Chloroplasts contain small circular pieces of DNA and ribosomes used to synthesise proteins needed in chloroplast replication and photosynthesis
What are plasmodesmata
Plasmodesmata are bridges of cytoplasm between neighbouring plant cells
They allow the transfer of substances between plant cells
Plasmodesmata mean that the cytoplasm of neighbouring plant cells is continuous; this allows substances to move easily between cells, e.g. sucrose can move easily from the surrounding cells into the phloem
What is a large permanent vacuole
Large permanent vacuoles are found in plant cells, where they store cell sap and provide additional structural support to cells
Vacuoles are sometimes found in animal cells, but these will be small and temporary
Vacuoles are surrounded by the tonoplast, which is a partially permeable membrane
Large permanent vacuoles store cell sap inside plant cells
What are images taken by microscopes called
Micrographs
How to identify cells in animal cell micrographs
t is possible to identify organelles in micrographs of animal cells on the basis of their shape, location, and size relative to other organelles, e.g.
The nucleus will always be the largest organelle
Mitochondria are the next largest, and are often cylindrical with a folded inner membrane
Note that mitochondria are not always cylindrical, but can also be circular; their shape will depend on their age, and on the angle at which they were sliced during specimen preparation
RER will be near the nucleus, and ribosomes can sometime be seen
Lysosomes and vesicles will be smaller than mitochondria
How to interpret plant cell micrographs
Plant cell micrographs can be interpreted using the same techniques as animal cells
Large, seemingly empty spaces inside cells will be vacuoles
The nucleus will be the largest dark region in the cell
Chloroplasts are the next-largest organelles, and grana are often visible
What are the common structures between plant and animal cells
Cell surface membrane
Cytoplasm
Nucleus
Mitochondria
Rough and smooth endoplasmic reticulum
Golgi bodies
Vesicles and lysosomes
Ribosomes
Microtubules
What structures are exclusive to plant cells compared to animal cells
Cellulose cell wall
Large permanent vacuoles
Chloroplasts
Plasmodesmata
What are the structures found in animal cells and not plant cells
Centrioles
Microvilli
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What do cells need to survive
All organisms require a constant supply of energy to maintain their cells and stay alive
Where is energy required in organisms
In anabolic reactions to build larger molecules from smaller molecules
To move substances across the cell membrane in active transport, or to move substances within the cell
In animals, energy is required
For muscle contraction
In the conduction of nerve impulses
What does ATP do
ATP from respiration is used to transfer energy in all energy-requiring processes in cells
Energy is released from ATP when it is broken down to ADP and inorganic phosphate
This process is reversed during respiration to make ATP and maintain a supply of energy
What is ATP
Adenosine Triphosphate (ATP) is a nucleotide (DNA and RNA are nucleotides)
What common organism has prokaryotic cells
Bacteria
What is difference in cellular structure between prokaryotes and eukaryotes
Their genetic material is free in the cytoplasm and is circular
Eukaryotic genetic material is packaged as linear chromosomes in the nucleus
Prokaryotes lack membrane-bound organelles
This means that they do not have any internal structures surrounded by membrane, e.g. a nucleus or mitochondria
They are many times smaller than eukaryotic cells
Prokaryotic cells are usually 1-5 μm in diameter, while eukaryotic plant cells can be 10-100 μm across
Their ribosomes are structurally smaller (70 S) in comparison to those found in eukaryotic cells (80 S)
Their cell walls are made of peptidoglycan rather than cellulose or chitin
Are prokaryotes unicellular
True
Are eukaryotes unicellular
False - can function together in multicellular organisms
What structures are always present in prokaryotes
Cell wall (peptidoglycan)
Cell surface membrane
Cytoplasm
Circular DNA
Ribosomes
What structures are sometimes present in prokaryotes and uses
Flagellum - motion
Capsule - protection
Infolding of cell surface membrane - allow photosynthesis or carry out nitrogen fixation
Plasmid - small circle of DNA
Pili - for attachment to other cells or surfaces for sexual reproduction
Features of prokaryotes in size, genetic material, cell division, ribosomes, cell wall and organelles
- Size
0.5-5 μm
- Genetic material
Circular chromosome in the cytoplasm
Not associated with proteins
- Cell division
Binary fission
No spindle fibres involved
- Ribosomes
70S
- Organelles
No membrane-bound organelles
- Cell wall
Made of peptidoglycan
Features of eukaryotes
- Size
Up to 100 μm
- Genetic material
Linear chromosomes in the nucleus
Associated with histone proteins
- Cell division
Mitosis or meiosis
Chromosomes are separated by spindle fibres
- Ribosomes
80S
- Organelles
Multiple membrane-bound organelles, e.g. a nucleus, mitochondria, chloroplasts
- Cell wall
Made of cellulose in plants, or chitin in fungi
What are viruses and are they cells
Viruses are non-cellular particles that infect living cells
Note that viruses are not cells, and they are not considered to be living organisms, so are referred to as ‘particles’
Size of a virus
20-300nm
What do viruses have structurally
A nucleic acid core made of either DNA or RNA
A protein coat called a capsid
What do viruses sometimes have structurally
Some viruses have an outer layer called an envelope; this forms from the membrane phospholipids of the host cell in which they were produced
How do viruses reproduce
Viruses can only reproduce by infecting living cells and using their protein-building machinery to produce new viral particles
Viruses use attachment proteins on their surface to bind to and infect their host cells
