1.6. Plant And Animal Cells As Seen With An Electron Microscope Flashcards
Define ultrastructure
The fine (detailed) structure of a cell as revealed by the electron microscope
What does a cell surface membrane look like using an electron microscope at high magnifications?
It can be seen to have three layers- two dark (heavily stained) layers surrounding a narrow pale interior.
What are microvilli
are small fingerlike extensions of a cell surface membrane which increase the surface area of the cell surface membrane for more efficient absorption or secretion
What is a cell that the microvilli are typical
Epithelial cells are cells that cover the surfaces of structures.
Give examples of when the increased surface area by the microvilli helped the cells
For reabsorption in the proximal convoluted tubules of the kidney
For absorption of digested food into cells lining the gut
_________ is the largest cell organelle
Nucleus
What is a nuclear envelope?
The nucleus is surrounded by two membranes, forming a structure called the nuclear envelope.
The outer membrane of the nuclear envelope is continuous with
the endoplasmic reticulum
The nuclear envelope has many small pores called
the nuclear pores
What is the function of the nuclear pores
These allow and control the exchange between the nucleus and the cytoplasm
Examples of substances leaving the nucleus through the pores are
messenger RNA (mRNA), transfer RNA (tRNA) and ribosomes for protein synthesis
Examples of substances entering through the nuclear pores are
proteins (to help make ribosomes), nucleotides, ATP (adenosine triphosphate), and some hormones such as thyroid hormone T3
Chromosomes contain
DNA, the genetic material
DNA is organized into
functional units called
Genes control
the activities of the cell and inheritance
Hoe much DNA does a human cell have
of about 2 metres
The DNA molecules are so long; How are they prevented from being tangled
They are folded into a more compact shape by combining with proteins especially histones.
The combination of DNA and proteins is known as
Chromatin
When a cell is abput to divide,
the nucleus will divide first so that each new cell has its own nucleus
How many nucleoli is it most common to have
One
What is the function of a nucleolus
To make ribosomes using the information in its own DNA
How does the nucleolus make ribosomes
The nucleolus contains a core of DNA from one or more chromosomes which contain the genes that code for making tRNA. Around the core are less dense regions where the ribosomal subunits are assembled, combining the rRNA with ribosomal proteins imported from the cytoplasm.
The more ribosome a cell makes
The larger its nucleolus
The nucleolus as a structure disappears when
ribosome synthesis ceases and the different parts that had come together during the manufacturing of ribosomes separate.
The existence of many of the cell structures had not been suspected until cells were seen using a light microscope. An example of one of these structures is
The endoplasmic reticulum
The membranes of ER form
flattened compartments called sacs or cisternae.
Processes can take inside the cisternae
separated from the cytoplasm
What are the two types of endoplasmic reticulum?
The rough endoplasmic reticulum (RER) and the smooth endoplasmic reticulum (SER).
Why did the rough endoplasmic reticulum get its name
Because it is covered with many tiny organelles called Ribosomes
How do the ribosomes look like through a light microscope?
They are just visible as black dots
Ribosomes are the site of
protein synthesis
They can be found
free on the cytoplasm as well as on the ER
What is the function of SER
to make lipids and stereoids, such as cholesterol and the reproductive hormones oestrogen and testosterone
SER is a major storage site for
calcium ions
Why is SER found in large amounts in muscle cells?
It is because calcium ions are required in muscle contraction
What is SER involved in liver
In drug metabolism
At very high magnifications using an electron microscope, Ribosomes can be seen to consist
two subunits: a large subunit and a small subunit
What are S units
These are Svedberg units. These are a measure of how rapidly substances sediment in a high-speed centrifuge (ultracentrifuge).
Eukaryotic ribosomes are
80s ribosomes
Prokaryotic ribosomes are
70s ribosomes
What kind of ribosomes do mitochondria and chloroplasts contain
70s ribosomes
What does ribosomes and mitochondria having 70s ribosomes indicate
Their prokaryotic origins
Ribosomes allow all the molecules involved in protein synthesis such as
mRNA, tRNA, amino acids and regulatory proteins to gather together in one place
Ribosomes are made up of
roughly equal amounts of ribosomal RNA (rRNA) and proteins.
What is the Golgi apparatus
The Golgi apparatus consists of a stack of flattened sacs, constantly forming at one end (from the vesicles which bud off from ER) and breaking up into Golgi vesicles at the other end.
What is Golgi apparatus also known as
Golgi body, golgi complex
What are Golgi vesicles
They are vesicles that carry their contents to other parts of the cells, often to the cell surface membrane for secretion.
What is a unique feature of Golgi apparatus
The Golgi vesicles chemically modify the molecule it transports e.g. sugars may be added to protein to make glycoproteins.
What does Golgi apparatus do?
The Golgi apparatus collects and processes molecules particularly proteins from the RER. (It contains hundreds of enzymes for this purpose.) After processing the molecules can be transported to other parts of the cell or out of the cell.
Releasing molecules from the cell is called _________ and the pathway followed by the molecules is called the __________.
secretion…secretory pathway
What are some examples of the functions of the Golgi apparatus
Golgi vesicles are used to make lysosomes
Sugars are added to proteins to make glycoproteins
Sugars are added to lipids to make glycolipids
During plant cell division; Golgi enzymes are involved in synthesis of new cells.
In the gut and gas exchange systems, cells called goblet cells release a substance called mucin from the golgi apparatus. Mucin is one of the main components of mucus
What is the importance of glycolipids and glycoproteins
These are important components of membranes and are important in cell signaling.
What are lysosomes
A spherical organelle found in eukaryotic cells that contains digestive enzymes and has a variety of destructive dunctions such as removal of old cell organelles.
What is the diameter of lysosomes in animal cells?
0.1-0.5 micrometer
What may act as lysosome in plant cell
The large central vacuole, although lysosomes similar to animal cells are also present.
Lysosomes contain _______ enzymes
digestive
What is an example of digestive enzymes in lysosomes
hydrolases, proteases, lipases, nucleases (to break down nucleic acids)
Why does the hydrolases have its name
It is because it carries out hydrolysis.
Lysosomes are responsible for
breakdown (digestion) of unwanted substances and structures such as old organelles or even whole cells.
Why are the digestive enzymes isolated from the rest of the cell through a membrane
to prevent damage (to the cell and its organelles)
Hydrolysis works fastest in an acidic environment, so the contents of the lysosomes are acidic pH 4-5 compared with the 6.5-7 in the cytoplasm.
How many digestive enzymes does the lysosome have
60+
Where are the digestive enzymes produced
synthesized on the RER and delivered to lysosomes through Golgi apparatus
what are the four categories that the activity of lysosomes can be split into?
Getting rid of unwanted cell components
Endocytosis
Exocytosis
Self digestion
Elaborate more on lysosome’s categories of activities: getting rid of unwanted cell components
Lysosomes can engulf and destroy unwanted cell components such as molecules or organelles that are located inside the cell
Elaborate more on lysosome’s categories of activities: endocytosis
Material may be taken into the cell by endocytosis, for example when white blood cells engulf bacteria. Lysosomes may fuse with the endocytic vacuoles formed and released and their enzymes to digest the contents (in order to extract nutrients)
Elaborate more on lysosome’s categories of activities: exocytosis
Lysosomal enzymes may be released from the cell for extracellular digestion. An example is the replacement of cartilage by bone during development. The heads of sperms contain a special lysosome, acrosome, for digesting a path through the layers of cells surrounding the egg just before fertilization.
Elaborate more on lysosome’s categories of activities: Self digestion
The contents of lysosomes are sometimes released into the cytoplasm. This results in the whole cell being digested (a process called autolysis). This may be part of normal development, as when a tadpole tail is reabsorbed during metamorphosis or when a uterus is restored to its normal size after pregnancy. It also occurs after the death of an individual as membranes lose their partial permeability
What is the usual diameterof Mitochondria
About 1 micrometre
What shapes can mitochondria be of
various shapes, often sausage shapes
Mitochondria are sorrounded by
two membranes(an envelope)
The inner membrane is folded to form
finger like cristae (singular:crista) which project into the interior of the mitochondris which is called the matrix
The space between two membranes of mitochondria is called
The intermembrane space
The number of mitochondria is
very variable as they are responsibe for aerobic respiration so cells with high demad for energy such as liver and muscle cells contain a larer number of mitochondria.
A liver cell may contain as many as ______ mitochondria
2000
If your exercise more your muscles will make
more mitochondria
What are some functions of the mitochondria
The main function of mitochondris is to carry out aerobic respiration although they have other functions suc as synthesis of lipids.
Describe what happens in mitochondria regarding energy
During respiration, a series of reactions take place in which energy is released from energy rich molecules such as sugars and fats. Most if the energy is transferred to molecules of ATP.
The purpose of respiration is
to make ATP
What is ATP
The energy carrying molecule found in all living cells. It is known as the universal energy carrier.
What are cristae
folds of the inner membrane of the mitochondrial envelope on which are found stalked particles of ATP synthase and electron transport chains associated with aerobic respiration
Where does respiration take place in mitochondrion?
The reactions of respiration takes place in the solution in the matrix and in the inner membrane(cristae).
What the matrix of a mitochondria contains
The matrix contains enzyme in solution, including those of Krebs cycle.
What is found in cristae
Electron carriers
Wha happens to ATP once it is made in the mitochondia
Once made, ATP leaves the mitochondrion and as it is a small soluble molecule, it can spread rapidly to sall parts of the cells where the energy is needed.
How is the energy released from mitochondria
By breaking the molecule down to ADP(adenosine diphosphate). This is hydrolysis reaction. The ADP can then be recycled in a mitochondrion for conversion back to ATP during aerobic respiration.
What are microtubules
They are tiny tubes made of protein called tubulin and found in most eukaryotic cells.
What are some of microtubules functions
They have a large variety of functions such as cell support and determining cell shape.
The ‘spindle’ on which chromatids and chromosomes separate during nuclear division is made of microtubules
What is the size of microtubules
25nm in diameter
What makes up the cytoskeleton
Microtubules, actin filaments and intermediate filaments
What is a cytoskeleton
An essential structural component of cells which helps determine cell shape.
Microtubules are made of
a protein called tubulin. There are two forms of tubulins: alpha tubulins and beta tubulins. Alpha and beta tubulin molecules combine to form dimers (double molecules). These dimers then join together end to end to make long protofilaments. Thirteen protofilaments line alongside each other in a ring to form a cylinder with hollow center. This cylinder is called microtubule
Apart from the mechanical function, what are some of the other functions of microtubules
Secretory vesicles and other organelles and cell components can be moved along the outside surface of the microtubules forming an intracellular transport system, as in the movement of Golgi vesicles during exocytosis
During nuclear division, a spindle made of microtubules is used for the separation of chromatids or chromosomes
Microtubules form a part of the structure of centrioles
,icrotubules form an essential part of the mechanism involved in the beating movements of cilia and flagella
What is polymerisation
The process by which giant molecules are made by joining together many identical subunits. e.g. protofilaments from dimers of tubulins
What does the microtubules organizing centers(MTOC) do
The assembly of microtubules from tubulin molecules is controlled by MTOC. Because of their simple construction, microtubules can be formed and broken down at MTOCs according to need.
What is ADP (Adenosine diphosphate)
The molecule that is converted to ATP by addition of a phosphate (a reaction known as phosphorylation) during cell respiration; the enzyme responsible is ATP synthase, the reaction requires energy.
What did the extra resolution obtained through electron microscope show about centrioles
That just outside the nucleus there are really two centrioles and not one as it appears in the light microscope.
How are the centrioles placed?
Just outside the nucleus, they lie close togheter at right angles to each other in a region known as centosome
Centrioles and the centrosome are _______ from most plant cells
absent
What is a centriole
It is a hollow cylinder of about 500 nm long formed from a ring of short microtubules. Each centriole containsnine triplets of microtubules.
What is a centrosome
The main microtubule organising centre (MTOC) in animal cells
What is a belief held till recent about MTOC, and what do the recent studies suggest
Until recently, it was believed that centrioles acted as MTOCs for the assembly of microtubules that make up spindle during nuclear division. It is now known that this is done by centrosome but does not involve the centrioles
What is the use of centrioles
Centrioles are needed for the production of cilia and flagella. Centrioles are found at the base ofcilia and flagella, where they are known as basal bodies. The centrioles act as MTOCs. The microtubules hat extend from the basal bodies into the cilia and flagella are essential for the beaing movements of these organelles
What is cilia
They are whip like structures projecting from the surface of many eukaryotic cells (many animal cells and cells of many unicellular organisms) which beat causing locomotion or the movement of fluid across the cell surface
What is flagella
They are whip like structures projecting from the surface of many eukaryotic cells (many animal cells and cells of many unicellular organisms) which beat causing locomotion or the movement of fluid across the cell surface
What is the difference between flagella and cilia
Flagella are longer and found one or two per cell while cilia are short and often numerous
Cilia and flagella are sorrunded by
an extension of the cell surface membrane
Cilia and flagella are extremely complicated structures composed of
over 600 different polypeptides
This complexity of cilia and flagella lead to a
very fine control of how they beat
Describe the structure of ca cilium
Cilia have two central microtubulue and a ring of ning of nine microtubule doublets(MTDs) around it(the outside) This is referred to as 9+2 structure.
What is the difference between A and B microtubule
A microtubule is a complete ring of 13 protofilaments while the B microtubules is an incomplete ring with only 10 protofilaments. Each A microtubule also has inner and outer arms that are made of the protein dyenin.
Each MTD contains
an A and B microtubule
What do the inner and outer arms of A microtubules do
They connect with the B microtubules of neighboring MTDs during beating
If you imagine the microtubules in three dimesions
There are two rows of several hundred dynein arms along the outside of each A microtubule.
The whole cylindrical structure in cilia inside the cell surface membrane is called
Axoneme
At the base of each cilium and flagellum
is a structure called the basal body which is identical to the structure to the centriole. We now know that centrioles replicate themselves to produce these basal bodies, and that cilia and flagella grow from the basal bodies.
Cilia can be found in
the resitarory tract
How is the beating motion of cilia and flagella caused?
It is caused by the dynein arms making contact with and moving along neighboring microtubules. This produces the force needed for cilia to beat. As neighboring MTDs slide past each other the beating motion is converted into bending by other parts of the cilium
What are the functions of cilia and flagella
If the cell is attached to something so that it cannot move, fluid will move past through the cell. If the cell is not attached, the cell will move through with the fluid. Single celled organisms can therefore use the action of cilia and flagella for locomotion.
In vertebrates, beating cilia are found in some epithelial cells such as the lining of the airways which maintain the flow of mucus that removes debris such as dust and bacteria from the respiratory tracks.
How many cilia are found in the lining of the airway
10 million cilia per mm^2
Describe the structure of the choroplasts
Chloroplasts tend to have elongated dhape and a diameter of about 3 to 10 micrometers
What surrounds the chloroplast
They are surrounded by two membranes which form the chloroplast envelope
What is the main function of chloroplasts?
To carry out photosynthesis
Describe photosynthesis
Durin the first stage of photosynthesis (the light dependent stage), light energy is absorbed by photosynthetic pigments, particularly chlorophyll.
The second stage of photosynthesis (the light independent stage) uses the energy and reducing power generated during the first stage to convert carbon dioxide to sugars. This takes place in stroma.
Where are the photosynthetic membranes found?
The pigments are found on the membranes of chloroplasts
How are the sugars made in photosynthesis stored in plants?
They may be stored in the form of starch grains in the stroma.
Where are lipid droplets found and what do they appear as using an electron microscope?
They are found in the stroma and appear as black spheres using electron microscope
What is the use of lipid droplets in the stroma
They are reserves of lipid for making membranes or are formed from the breakdown of internal membranes as the chloroplast ages
What are thylakoids
The membrane system in chloroplasts contains of membrane bound, fluid filled sac which spread out like sheets in three dimensions (flattened) and is the site of light dependent reactions of photosynthesis in a chloroplast.
What are grana
In places, thylakoids form flat, disc like structures that stack u like pile of coins, forming structures called grana.
Where does the name grana come from
From their appearance in the light microscope, grana mean grains
