Section 2 - Cells: 3. Cell structure Flashcards
Steps to prepare a microscope slide.
- Cut a piece of onion and peel off one thin layer using tweezers
- this gives one layer of cells, so it is thin enough to be used on a light microscope. - Place the layer of onion on the slide and add two drops of the necessary solution.
- Plant cells = Iodine, to stain nucleus, cytoplasm and cell walls.
- Animal cells = Methylene Blue
- Already stained (eg, red onion) = water. - Gently lower a coverslip over the specimen using a mounted needle.
- Angle avoids air bubbles (artifacts). - Use a paper towel to pat dry and excess liquid.
Steps to setting up a microscope.
- Place the slide on the stage at the lowest point
- altered with the coarse focusing wheel. - Set the objective lens to the lowest magnification
- For a greater field of view. - Focus the image with the course, then fine focusing wheels.
- Change the magnification and repeat focusing
- Total magnification = objective x eyepiece
Magnification
How many times larger the image is than the actual object
What is Resolution in microscopy
The minimum distance apart that two objects must be to appear as separate.
Depends on the wavelength of the image used (0.2micrometers for visible light)
Microscopes have a limit of resolution, magnification beyond which makes the image appear blurry.
What is the equation to calculate magnification
Magnification = Image / Actual
How to do a scientific drawing (7 key features)
1) Examine significant features to include
2) Only draw what you see
3) Draw in pencil
4) Use single lines (no shading)
5) Give clear title (+ Magnification)
6) Label with straight lines
7) Underline scientific names
What is a light microscope (optical microscope)
Uses visible light as a form of radiation to view specimens. The light passes through the specimen on the slide and through a set of lenses to the eyepiece, where the image can be viewed.
- x40 / x100 / x400 magnification
What are the limitations of a light microscope
- The specimen must be thin to allow light to pass through.
- The longer wavelength of light mean that it has a limited resolution.
What is a Scanning electron microscope
Uses a beam of electrons as a form of radiation. The electrons a directed onto the specimen from above, and computer analysis allows a 3D image to be built based on the scattered electrons.
- Resolving power = 20nm
What are the limitations of a SEM
- Specimen must be dead (in a vacuum)
- Complex staining process is required, although image is not in colour.
- Only shows the outer surface of the specimen
What is a Transmission electron microscope
Uses a beam of electrons as a form of radiation. The electrons are passed through the specimen from below, some of which are absorbed by parts of the specimen, appearing darker on the image. 2D image (photomicrograph) is produced that can be layered into 3D but it is complicated and time consuming to do.
- Resolving power = 0.1nm (greatest of all)
What are the limitations of a TEM
- Specimen must be dead (in a vacuum)
- High resolving power may not be achieved
(due to difficulty preparing the specimen and high energy electrons may destroy specimen) - Complicated staining process needed
- Specimen must be extremely thin
(microtome needed to prep)
What is an eye piece graticule and how do you calibrate it
An eyepiece graticule is a ruler in the eyepiece of a microscope
used to measure specimens.
To calibrate:
- Set the microscope to the desired magnification.
- Place the stage micrometre on the stage and count the amount of divisions for a given length, and divide to give the real length of one division.
- Use this to measure the length of specimens and of the field of view by multiplying the number of divisions by the scale.
Remember to recalibrate for every magnification.
What is cell fractionation and why is it needed
Cell fractionation is a method to separate organelles from cells, allowing them to be studied.
Cells are broken down and the different organelles are separated.
What is homogenation
When cells are broken down with a homogeniser (blender) to release the organelles inside it
What is a homogenate
Fluid produced by homogenisation, containing the organelles form inside a cell
What is ultracentrifugation
The process where fragments in the homogenate are separated by a centrifuge (spun round)
What is the supernatant in cell fractionation
The fluid that is removed and re-spun in the centrifuge during ultracentrifugation, to remove lighter organelles
What is the sediment in cell fractionation
The layer at the bottom of the tube containing the heaviest organelles after homogenisation (each sediment produced will contain a different group of organelles of similar weights)
What is the process of cell fractionation, ultracentrifugation and homogenization
1) Chop up the fresh tissue (eg. liver) and place in an ice cold, isotonic buffer solution.
2) put chopped tissue in a homogenizer (blender) to break open cells
3) Filter the mixture to remove debris
4) spin the mixture in a centrifuge
5) the supernatant (liquid at the top) is removed to be re-spun, and the pellet (sediments at the bottom) is removed, containing the largest organelles, such as the nucleus.
6) This process is repeated, spinning the supernatant at higher and higher speeds, separating into different organelles of different sizes.
Why is an ice cold, isotonic buffer solution used in cell fractionation
Ice cold:
Reduces enzyme activity
Isotonic:
Prevents osmosis
Buffer:
Maintain pH
What is a eukaryotic cell
A cell present in complex living organisms, containing a nucleus and membrane bound organelles
What are the main components of a eukaryotic cell
- Nucleus
- Mitochondria
- Endoplasmic reticulum (SER/RER)
- Golgi apparatus
- Lysosomes
- Ribosomes
- Chloroplast (only plant)
- Cell wall (only plant)
- Vacuoles (only plant)
What is the function of the nucleus
- Stores the DNA
- Site of DNA transcription leading to protein synthesis (production of mRNA)
- Helps cells produce ribosomes (rRNA)
What is the structure of the nucleus
-Surrounded by nuclear envelope: double membrane that controls the entry and exit of materials
- Nuclear pores in the membrane: allows RNA to leave
- Nucleoplasm: Jelly-like substance within the nucleus
- Chromosomes: contain protein bound DNA
- Nucleolus: Small spherical section where rRNA and ribosomes are formed (multiple in the nucleus)
What is the function of the mitochondria
- Site of the aerobic stages of respiration
- Stores calcium
- Produces heat by proton leaking
- Activates enzymes to breakdown dead cells
What is the structure of the mitochondria
- Double membrane: controls the entry and exit of materials
- Cristae: extensions of inner membrane providing a large surface area to attach membranes and proteins for respiration
- Matrix: space inside the mitochondria, containing proteins, lipids, ribosomes, DNA (to allow production of some proteins) and enzymes
What is the function of the smooth endoplasmic reticulum
Synthesis of lipids and carbohydrates
What is the function of the rough endoplasmic reticulum
Synthesis of proteins and glycoproteins, as well as ribosomes
What is the structure of the smooth endoplasmic reticulum
Smooth system of sheet like membranes, spreading through the cytoplasm, and connected to the nucleus.
Contains flattened sacs and tubes called cisternae.
What is the structure of the rough endoplasmic reticulum
System of sheet like membranes, spreading through the cytoplasm, covered in ribosomes, and connected to the nucleus.
Contains flattened sacs and tubes called cisternae.
What is the function of the Golgi apparatus
Packaging and modifying:
- Add carbohydrates to proteins to form glycoproteins
- Produce secretory enzymes (eg. those secreted by the pancreas)
- Packages substances into vesicles so they can be transported
- Secrete carbohydrates (eg. those used in making plant cell walls)
- Transport, modify and store lipids
- Form lysosomes
What is the structure of the Golgi apparatus
Similar the the SER, with flattened sacs called cisternae surrounded by membranes. Sections can break off and form vesicles to transport substances
What is function of the Lysosomes
- Digest unwanted materials so they can be removed from the cells (proteins, carbs and lipids): so are highly acidic
- Vital for immune system: digest pathogens in phagocytosis
What is the structure of the Lysosomes
Vesicles produced by the Golgi apparatus, containing enzymes such as protease, lipase and lysozymes
What is the function of the Ribosomes
Site of translation for protein synthesis
What is the structure of the Ribosomes
Made up of two subunits, between which the mRNA is passed during translation
- 80s: larger (eukaryotes)
- 70s: smaller (prokaryotes, mitochondria and chloroplast)
What is the function of the Chloroplast
- Site of photosynthesis
- Chlorophyll captures certain wavelengths of light to trigger chemical reactions
What is the structure of the Chloroplast
- Outer and inner membrane separated by intermembrane space
- Thylakoids: Disc like structures containing chlorophyll
- Grana: stacks of Thylakoids
- Intergranal lamella: bars connecting the grana
- Stroma: fluid filled matrix surrounding the grana
What is the function of the cell wall of plants
- Rigid structure helps maintain the cells chape (made up of cellulose)
- Gives the plant structural support (turgid pressure in cells)
- Acts as a semi-permeable membrane
- Allows water to pass along it, contributing to water movements within the plant as a whole
What is the structure of the cell wall of plants
- Middle lamella: Thin layer on the outside that marks the boundary with outer cells, and cements adjacent cells together
- Contain polysaccharides to provide strength (cellulose in plants, chitin in fungi)
What is the function of the Vacuole
- Filled with water and dissolved substances, so applies applies turgid pressure to the cell walls, giving the cells structure
- Contains a temporary store of sugars and amino acids
- Pigments within them may colour petals and attract pollinators
What is the structure of the Vacuole
Fluid-filled sacs surrounded by a single membrane called a tonoplast
What is the advantage of cell specialisation in multicellular organisms
Allows each cell to be suited for it’s role and the conditions it works in, so the whole organism can function more effectively
What is a tissue
A collections of similar cells that perform a specific function (eg. epithelial tissues)
What is an organ
A group of tissues that are coordinated to perform one major function, although can usually carry out multiple other functions (eg. the stomach)
What is an organ system
A collection of organs that work together as a single unit to perform particular functions more effectively (eg. the digestive system)
What is a prokaryotic cell
A cell with no nucleus or membrane bound organelles (eg. bacteria)
What are the main features of a prokaryotic cell (bacterial cell)
- Capsule
- Cell wall
- Cell-surface membrane
- Plasmid
- Genetic material
- Ribosomes (70s)
- Cytoplasm
- Flagellum
What is a flagellum and what is it’s function
Long, tail like projections on certain prokaryotic cells used for movement (may be more than one)
What is a capsule and what is it’s function
An outer mucilaginous layer on bacterial cells, protecting the cells and allowing them to stick together for protection
What is the difference between the cell wall of a plant cell, fungal cell and bacterial cell
- Plant cell = cellulose
- Fungus = Chitin
- Bacteria = Murein
What are plasmids in bacterial cells, and what is it’s function
Small, circular pieces of DNA that posses genes which may aid the survival of the bacterial in harmful conditions (eg. produce enzymes to break down antibiotics)
How is the genetic material held in a bacterial cell
In circular strands of DNA, loose within the cytoplasm
What are the stages of the cell cycle
- Interphase: G1, S, G2
- Nuclear division:
Mitosis (2): prophase, metaphase, anaphase, telophase
Meiosis (4) - Cytokinesis
What happens during the interphase stage of the the cell cycle
The cell carries out it’s regular function while it grows and begins to replicate, in 3 stages:
- G1: Organelle replication
- S: DNA synthesis and chromosome replication
- G2: Chromosomes are checked and repaired
What is Mitosis
Cell replication that produces two daughter cells that have the same number of chromosomes as the parent
What is Meiosis
Cell replication that produces four daughter cells, each with half the number of chromosomes of the parent
Are the daughter cells genetically identical to each other and the parent, after mitosis or meiosis
Yes for mitosis, apart from the rare event of mutations
No for meiosis
What are the 4 stages of mitosis
- Prophase
- Metaphase
- Anaphase
- Telophase
What happens during prophase of mitosis
- Chromosomes first become visible
- Nuclear envelope breaks down leaving the chromosomes free in the cytoplasm
What happens during metaphase of mitosis
- Centrioles (type of organelles) move to the poles and spindle fibres develop from them
- The chromosomes arrange themselves across the equator of the cell, with the spindles attached to the centromere
What happens during anaphase of mitosis
- The centromere divides into two, and the spindle fiberes pull the individual chromatids to each pole of the cell
- The separate chromatids are now referred to as chromosomes
What happens during telophase of mitosis
- The chromosomes reach their two poles and become longer and thinner, until they are no longer visible, leaving only widely spread chromatin (Complex of DNA, RNA and protein)
- Spindle fibres disintegrate and the nuclear envelopes and nucleolus reform
What happens during cytokinesis in the cell cycle
The cell divides into two (mitosis) or four (meiosis) as the cytoplasm divides
What are the main roles of mitosis
- Growth: organisms can grow as mitosis produced genetically identical cells
- Repair: cells with identical structures can replaced those that are damaged or or lost
- Reproduction: in single celled organisms
How is cell division carried out in prokaryotic cells
Binary fission:
- Circular DNA replicates and both copies attract to the cell membrane
- Plasmid replicates
- Membrane grows between the two copies of DNA, pinching together
- Cell wall forms between the the two DNA molecules, splitting the cell
How do viruses replicate
Viruses are non-living, so can’t carry out cell division, so they attach to a host cell and inject nucleic acid into it, containing genetic info so the host produces more viruses
What is Cancer
A group of diseases caused uncontrolled cell growth and division
What is the process that results in cancer
- Damage to or a mutation in the genes that regulate mitosis
- Uncontrolled growth and cell division
- A group of abnormal cells develops and expands (tumour)
- Tumour can be benign or malignant
- Malignant tumours spread as abnormal tissue replaces healthy tissue, which is life-threatening
What are the key features of a benign tumour
- Grow large in size
- Grow slowly
- Normal nucleus
- Cells produce adhesion causing them to stick together in the tissue, so don’t spread
- Surrounded by capsule of dense tissue so remain compact, with a localised effect
- Can be removed with surgery
- Rarely reoccur after treatment
What are the key features of a malignant tumour
- Grow large in size
- Grow rapidly
- Dark nucleus containing lots of DNA
- Cells don’t produce adhesion, so spread throughout the body
- Not surrounded by capsule, so grow finger-like projections into the surrounding tissue
- Replaces normal tissue, so is life threatening
- Often have systematic effects such as fatigue or weight loss
- Removal involves radiotherapy, chemotherapy, and/or surgery
- Frequently reoccur after treatment
What are the main causes of cancer
- Genetics
- Environmental factors: eg. Smoking, pollution, UV radiation, viruses
What are the main ways to prevent cancer
- Low fat, high fibre diet
- Vitamins
- Calcium
- Daily exercise
- Avoiding carcinogens (substances that can cause cancer, eg. cigarettes)
