Module 2 Section 1: Cell Structure Flashcards
Structure of the cytoskeleton
Microfilaments
Microtubules
Intermediate fibres
Functions of the cytoskeleton
Providing mechanical strength to cells
Aiding transport within cells
Enabling cell movement
What are Microfilaments
Fibres made from the protein actin.
They are responsible for movement of the cell and cytoplasm during Cytokinesis
What are Microtubules
Formed by the globular protein tubulin.
They polymerise to form tubes that determine the shape of the cell.
They also act as tracks for organelles moving around the cell
What are Intermediate fibres
Gives strength to cells and helps maintain integrity
Structure of the nucleus
Double nuclear envelope
Nuclear pores
Nucleolus
Chromatin
What is the double nuclear envelope
A double membrane which compartmentalises the Nucleus and prevents damage.
Protects the DNA
What are Nuclear Pores
Allows molecules to enter (e.g. nucleotides for DNA replication) and leave the nucleus
e.g. mRNA leaves the nucleus
What is the Nucleolus
Site of ribosome production.
Composed of RNA and proteins
What is chromatin
Chromatin is the DNA (with associated histone proteins).
Contains the Genetic Code which controls the activity of the cell
Function of the nucleus
Controls all the activity of the cell.
Where the Genetic Code (DNA) of the cell is stored, replicated, and copied into RNA (transcribed).
The Nucleus is attached to the Rough ER so the mRNA can easily get to ribosomes
Structure of the rough endoplasmic reticulum
Stacks of membrane bound (fluid filled) sacs which form sheets called cisternae
Rough ER: attached to the nucleus and covered with ribosomes.
Consists of an interconnected system of flattened sacs
Function of rough endoplasmic reticulum
Site of protein synthesis
Structure of smooth endoplasmic reticulum
Stacks of membrane bound (fluid filled) sacs which form sheets called cisternae
A system of interconnected tubules
Function of smooth endoplasmic reticulum
responsible for carbohydrate & lipid synthesis, and storage
Structure of ribosomes
A 2 subunit organelle.
Made from RNA and protein.
Not membrane bound.
Very small organelles: about 22nm in diameter.
Found free floating in the cytoplasm or attached to the Rough ER
Function of ribosomes
These are where protein is made.
They assemble amino acids into proteins in chains using mRNA
Structure of mitochondria
Oval shaped.
Surrounded by two membranes (double membrane).
The inner membrane forms finger-like structure called cristae which increases the surface area.
The solution inside is called a matrix which contains enzymes for respiration.
Mitochondrial DNA – Small amounts of DNA, enable mitochondrion to reproduce and create enzymes
Function of mitochondria
Site of aerobic respiration.
As a result of respiration, they release ATP (energy carrier in cells)
Structure of Golgi apparatus
Stacks of flattened, membrane bound sacs
(cisternae).
These are continuously formed from the ER at one end and budding off as Golgi vesicles at the other
Function of Golgi apparatus
Allows internal transport.
Receives proteins from the RER
Modifies and processes molecules (such as new lipids and proteins) and packages them into vesicles.
These may be secretory vesicles (if the proteins need to leave the cell) or lysosomes (which stay in the cell).
Makes lysosomes
Lipid synthesis
Structure of lysosomes
Spherical sacs surrounded by a single membrane
Function of lysosomes
They contain powerful hydrolytic digestive enzymes known as lysozymes.
Their role is to break down worn out components of the cell or digest invading cells.
Structure of centrioles
A component of the cytoskeleton, composed of many microtubules.
Small hollow cylinders that occur in pairs next to the nucleus in animal cells only.
Each centriole contains a ring of 9 microtubules.
Function of centrioles
Makes a copy of itself during cell division and then helps to form the spindle in cell division
Structure of cilia
‘Hair like’ extensions that protrude from some animal cell types.
In cross section they have an outer membrane and a ring of nine pairs of protein microtubules inside with two microtubules in the middle.
Known as a 9 +2 arrangement.
Arrangement allows movement
Function of cilia
sensory function (e.g.nose), beat creating a current to move fluid/mucous/objects
For locomotion
Structure of flagella
Similar to cilia but longer.
They protrude from the cell surface and are surrounded by the plasma membrane.
Like cilia they have a 9 +2 arrangement
Function of flagella
(Whip-like) enables a cells mobility.
The microtubules contract to make the flagellum
move.
Propel cells forward e.g. sperm cells
Structure of chloroplasts
Double membrane which encloses the stroma.
Stroma contains: Starch grains, lipid stores, DNA, RNA, ribosomes.
Series of membrane-bound flattened sacs called thylakoids in the stroma. Thylakoids stacked together are called grana.
Grana are linked together by lamellae.
The grana contain chlorophyll
Function of chloroplasts
Photosynthetic reactions
Structure of cell surface membrane
The membrane found on the surface of animal cells and inside the cell wall of plant and prokaryotic cells
A phospholipid bilayer.
Composed of proteins and lipids
Function of cell surface membrane
Regulates the movement of substances into and out of the cell.
Contains receptor molecules which allow it to respond to chemicals such as hormones
Structure of cellulose cell wall
Made of B-cellulose microfibrils– complex carbohydrate
Cell wall is fully permeable to substances
Thin layer called the middle lamella which marks the boundary between adjacent cell walls and ‘cements’ adjacent cells together
Function of cellulose cell wall
Gives the plant mechanical strength
Gives the plant cell support and it’s shape
Contents of plant cell can ‘push’ against the cell wall (turgid cell).
This gives the cell (and the whole plant) good support
Structure of large permanent vacuole
Single membrane bound (membrane is called a tonoplast).
Contains a fluid called cell sap (solution of mineral salts, sugars, amino acids, wastes etc).
Selectively permeable barrier
Function of large permanent vacuole
Stores cell cap
Support herbaceous plants by making cells turgid.
Helps maintain shape and gives support by maintaining turgor pressure.
Sugars and amino acids act as a temporary food store
Explain the interrelationship between organelles
The section of DNA encoded with the protein is transcribed onto mRNA
The mRNA leaves the nucleus via the nuclear pores and arrives at the rough endoplasmic reticulum and attaches onto the ribosome and amino acids are used to form proteins
The protein is packed into a vesicle ad moves along the microtubules to the Golgi apparatus and fuses into the Golgi
The protein moves through the cisternae of the Golgi and is processed and structurally modified ( e.g. sugar is added )
The modified protein is repackaged into a secretory vesicle and the vesicle leaves the Golgi
The vesicles travels along the microtubules and fuses with the plasma membrane and is released by exocytosis
Characteristics of a prokaryotic cell
Extremely small (less than 2 micrometres in diameter)
DNA is circular
No nucleus - DNA free in cytoplasm
Cell wall made of a polysaccharide, but not cellulose or chitin
Few organelles and no membrane bound organelles
Flagella (when present) made of protein flagellin, arranged in a helix
Small ribosomes
e.g. E.coli bacterium
Characteristics of a eukaryotic cell
larger cells (about 10-100 micrometres in diameter)
DNA is linear
Nucleus present - DNA is inside nucleus
No cell wall (in animals), cellulose cell wall (in plants) or chitin cell wall (in fungi)
Many organelles - mitochondria and other membrane bound organelles present
Flagella (when present) made of microtubule proteins arranged in a 9 + 2 arrangement
Larger ribosomes
e.g. human skin cell
How to calculate magnification
Magnification = image size/ object size
What is magnification
Magnifications how much bigger the images in the specimen
What is resolution
Resolution is how detailed the image is
It’s how well a microscope distinguishes between two points are close together
If a microscope lens can’t separate two objects, then increasing the magnification won’t help
Light microscope
Light microscopes use light
They have a lower resolution than electron microscopes – they have a maximum resolution of about 0.2 micrometers. So they are usually used to look at whole cells or tissues
The maximum useful magnification of a light microscope is about X 1500
How laser scanning confocal microscope’s work
These are a special type of light microscope, laser scanning confocal microscope to use laser beams (intense beams of light) to scan specimen, which is usually tagged with a fluorescent dye
The laser causes the dye to fluoresce – give off light
This light is then focus through a pinhole onto a detector
The detectors hooked up to a computer, which generates an image – the image can be 3-D
Pinhole means that any out of focus light is blocked, so these microscopes produce a much clearer image of a normal light microscope
Can be used to look at objects different depths in thick specimens
What are electron microscopes
Electron microscopes use electrons instead of light to form an image they have a higher resolution than light microscopes so give more detailed images
There are two kinds of electron microscopes:
Transmission electron microscope ( TEM )
Scanning electron microscope ( SEM )
How do transmission electron microscope’s work
They use electromagnets to focus a beam of electrons, which is then transmitted through specimen
Denser parts of the specimen absorb more electrons, which makes them look dark on the image you end up with.
TEMs Are good because they provide high resolution images ( So they can be used to look at a range of organelles ) but they can only be used on thin specimens
How do scanning electron microscope to work
They scan a beam of electrons across the specimen
This knocks off electrons from the specimen, which are gathered in the cathode ray tube to form an image
The images produced show the surface of the specimen and can be 3D
But they give a lower resolution than images with TEMs
Compare maximum resolution and magnification between light microscopes, TEMs and SEMs
Maximum resolution:
Light: 0.2 micrometers
TEM: 0.0002 micrometers
SEM: 0.002 micrometers
Maximum magnification:
Light: x1500
TEM: can be more than x 1,000,000
SEM: usually less than x 500,000
Why do we use stains for microscopes
In light microscopes and TEM, the beam of light of electrons passes through the object
The image is produced because some parts of the object absorb more light ( or electrons ) than others
Sometimes the object in view is completely transparent which makes the whole thing look white as all the light or electrons pass straight through
To avoid this, the object is stained
How to stain for light microscopes
Staining for light microscopes include using dyes
Common stains include methylene blue and eosin
The stain is taken up by some parts of the object more than others - the contrast makes the different parts show up
What do the different stains do for light microscopes
Different stains make different things show up
Methylene blue stains DNA
Eosin stains the cell cytoplasm
More than one stain can be used at once
Staining examples for electron microscopes
Objects are dipped in a solution of heavy metals e.g. lead
The metal ions scatter the electrons, creating a contrast
Some parts of the object show up darker than others
What is a slide
Strip of clean glass or plastic
Usually flat but some have a small dip or well in the centre
You can either prepare a slide as a wet mount or dry mount
How to prepare a dry mount
The specimen needs to let light through it in order to be able to be seen clearly under the microscope
A thick specimen must be split into a thin slice to use on the slide
Use forceps to pick up the specimen and put it in the middle of the slide
Put a cover slip on top
How to prepare a wet mount
Pipette a small drop of water on the slide
Use forceps to place the specimen onto the drop of water
Stand the coverslip up right next to the water droplet, then tilt and lower it so it covers the specimen, try to avoid air bubbles under it as they will obstruct the view
Once the cover slip is in position, a stain can be added
Place a drop of stain next to one edge of the cover slip
Put a bit of paper towel next to the opposite edge, the stain will get drawn under the slip, across the specimen
How to use a light microscope
Clip the slide onto the stage
Select lowest powered objective lens
Use coarse adjustment knob to bring the stage up to just below objective lens
Look down eyepiece and use coarse wheel to lower the stage until a roughly clear image appears
Adjust the fine focus wheel until a clear image is formed
Then turn up the magnification to look at the image at a higher magnification
What is the eyepiece graticule
Can be used to know the size of the specimen
The graticule is fitted onto the eyepiece. It’s a transparent ruler with numbers, but no units
The stage micrometer is placed on the stage - it is a microscope slide with an accurate scale ( has units ) and is used to work out the value of the divisions on the eyepiece graticule at a particular magnification
This means when the stage micrometer is replaced with the slide containing the specimen, you’ll be able to measure the size of the specimen
What is a bacterial cell wall made up of
Peptidoglycan
Labelled cell
