Lecture 3 and 4- Eukaryotic cells Flashcards
Compartmentalisation of eukaryotic cells, functions and major components
What are organelles?
Membraneous compartments and other structures that lack membranes but possess distinctive shapes and functions (such as ribosomes)
What defines the roles of different organelles?
The chemical reactions that they can carry out
What are the three main features of the nucleus?
It contains most of the cell’s DNA
It replicates genetic material
Site of the 1st steps in decoding genetic information
Site of genetic control of the cells activities
Assembly of ribosomes
What are the general functions of the mitochondria?
Power house of the cell
Energy stored in the bonds of carbohydrates and fatty acids is converted into a more useful form, ATP
What are lysosomes and vacuoles?
Cellular digestive systems in which large molecules are hydrolyzed into usable monomers
What are two purposes of membranes surrounding organelles?
- Keep organelle’s molecules away from other molecules in the cell
- Regulates the movement of raw materials and products into and out of the organelle
Why was the development of membrane bound organelles important in eukaryotic cells?
It gives the ability for eukaryotic cells to specialise and hence form organs and tissues of a complex, multicellular body
What can stains tell us about organelles?
Their chemical composition
Explain, in 4 points, the process of cell fractionation.
- Homogenize tissue in grinder (destroys plasma membrane)
- Cell homegenate contains large and small organelles
- Centrifuge to separate based on size or density
- Heaviest components sink to the bottom
How is information stored within a cell?
In a sequence of nucleotides in DNA molecules.
Most DNA resides in the nucleus
What organelle is usually the largest organelle in a eukaryotic cell?
The nucleus
What is the approximate diameter of a nucleus?
5 micrometers
What occurs in the nucleolus?
The assembly of ribosomes from RNA and specific proteins
What is nucleus surrounded by?
Two membranes that form the nuclear envelope
By what distance are the nuclear membranes separated by?
10-20nm
How many nuclear pores are there on a nucleus?
3500
What is the function of nuclear pores?
To connect the interior of the nucleus with the cytoplasm
What is the structure of the nuclear pores?
- Composed of 100 different proteins interacting hydrophobically
- Each pore is surrounded by a pore complex
What is the structure of a pore complex?
8 large protein aggregates arranged in an octagon surrounding the nuclear pore.
What is a nuclear basket?
Protein fibrils on the nuclear side form a basket like structure
How do molecules pass through nuclear pores?
Molecules less than 10,000 daltons freely diffuse
Molecules up to 50,000 daltons diffuse slowly
Larger molecules require a short amino acid sequence called a nuclear localisation signal
What evidence is there for the nuclear localisation signal?
- The sequence appears in nuclear proteins but not proteins that remain in the cytoplasm
- If removed, the protein remains in the cytoplasm
- If added to a protein, it enters the cytoplasm
- Some viruses have the signal sequence and enter the cytoplasm, others do not.
How does a signal sequence result in passage through a nuclear pore?
Three-dimensional structure binds noncovalently to a protein receptor
The receptor changes shape so the pore stretches and the protein passes through
What organelle is the nuclear envelope continuous with?
The endoplasmic reticulum
What is the name of the fibrous complex that occurs when DNA combines with a protein?
Chromatin
What does chromatin aggregate into before cell division?
chromosomes
What surrounds chromatin?
Water and dissolved substances called the nucleoplasm
What organises the chromatin?
The nuclear matrix (structural proteins)
What is chromatin attached to at the edge of the nucleus?
The nuclear lamina- formed from lamins into filaments
What is the purpose of the lamina?
Maintain the shape of the nucleus by attaching to the nuclear envelope and chromatin
Where are ribosomes found within prokaryotic cells?
Free floating
Where are ribosomes found within eukaryotic cells?
The cytoplasm- free or attached to the endoplasmic reticulum,
and inside the mitochondria and chloroplasts
What are ribosomes?
The sites where proteins are synthesised under the direction of the nucleic acids
How are eukaryotic and prokaryotic ribosomes similar?
They are made up of two different-sized subunits
What type of molecule are ribosomes made up of?
ribosomal RNA bound noncovalently to 50 proteins
What are the components of the endomembrane system?
The endoplasmic reticulum, golgi body, lysosomes, vesicles, nucleus
How large is the endomembrane system?
Much of the volume of the cell is taken up by this system
What is the endomembrane system?
A group of interrelated organelles
How is the endoplasmic reticulum viewed?
Through an electron mircograph
What is the structure of the endoplasmic reticulum?
A network of interconnected membranes braching through the cytoplasm forming tubes and flattened sacs
What is the lumen?
The interior of the endoplasmic reticulum
How much of the volume of the cell can the ER enclose?
Up to 10%
What is the rough endoplasmic reticulum?
The ER that is studded with ribosomes that are temporarily attatched
What are the functions of the rough ER?
- Segregates certain newly synthesised proteins and transports them to other locations within the cell
- Inside, proteins can be chemically modified so alter function/destination
What do the ribosomes attached to the ER do?
Synthesize proteins that function outside of the cytosol (proteins transported out of cell, or into membrane or endomembrane system)
Where do proteins synthesized by ribosomes attached to the ER go after they are synthesized?
Into the lumen of the ER
How do proteins enter the ER after being synthesized by ribosomes on its surface?
A sequence of amino acids on the protein act as an RER localisation signal
What happens to proteins after they have entered the RER?
They undergo changes- including: tertiary folding and formation of disulfide bridges
What type of molecule do proteins become after gaining carbohydrate groups in the RER?
Glycoproteins- these can act as an addressing system in lysosomes
What is the structure of the smooth endoplasmic reticulum?
More tubular (less like flattened sacs) Lack ribosomes
What happens to proteins that were synthesized in the RER inside the lumen of the SER?
They are chemically modified
What are the three main roles of the SER?
- Chemically modifying small molecules taken in by the cell (drugs, pesticides)
- Site of hydrolysis of glycogen in animal cells
- Site of lipid and steroid synthesis
What do cells that synthesize a lot of proteins have lots of?
Endoplasmic reticulum
Give examples of cells with many endoplasmic reticulum
Glandular cells- secrete digestive enzymes
White blood cells- secrete antibodies
What type of cell has many smooth endoplasmic reticulum?
Liver cells- these modify molecules that enter the body from the digestive system
What is the shape of the Golgi apparatus?
Flattened membranous stacks piled like saucers with small membrane-enclosed vesicles
What are the name of the membranous sacs that make up the golgi apparatus?
Cisternae
How long is the golgi apparatus?
1 micrometer
What are the three main roles of the golgi apparatus?
- Receive proteins from ER and further modify them
- Concentrate, package and sort proteins before sending (cellular or extracellular)
- Synthesis of some polysaccharides in the plant wall
Where is the golgi apparatus in plants, protists, fungi and many invertebrates within the cell?
Scattered throughout the cytoplasm
How is the golgi apparatus differently arranged in vertebrate cells?
Fewer stacks form larger, single, more complex golgi body
What are the three functionally distinct parts of the golgi body?
Bottom
Middle
Top
What does the bottom cisternae of the golgi apparatus lie closest to and what region is it?
Closest to nucleus/patch of RER
The cis region
What does the middle cisternae of the golgi apparatus make up?
The medial region
What does the top cisternae of the golgi apparatus lie closest to and what region is it in?
Lies closest to the surface of the cell
The trans region
How are the three parts of the golgi apparatus different?
They contain different enzymes and perform different functions
How do proteins remain segregated from the cytoplasm from the ER to the golgi apparatus?
The ER buds off a membranous vesicle containing the protein
What happens once a vesicle reaches the golgi apparatus?
The membranes fuse and releases protein
Where do vesicles that bud off of the trans regions go?
Carry their contents away from the golgi apparatus
Where do lysosomes originate from?
The golgi apparatus
What do lysosomes contain?
Digestive enzymes
What are lysosomes the site of?
Macromolecules (proteins, polysaccharides, nucleic acids, lipids) are hydrolysed into their monomers
How big are lysosomes?
1 micometer in diameter
How many lysosomes are their in a cell?
Dozens, depending on needs
How do food and foreign objects enter the cell?
Phagocytosis- pocket that forms in membrane is called phagosome
What is the function of lysosomes?
Site of break down of food and foreign objects
How does the lysosome digest a phagosome?
The primary lysosome fuses its membrane with the phagosome. This forms a secondary lysosome in which digestion occurs.
What makes the secondary lysosome break down its contents?
- Digestive enzymes
- Slightly more acidic environment
What happens to the products of lysosome break down?
They diffuse through the lysosome membrane and provide raw materials for other cell processes
What happens after a secondary lysosome is finished?
It fuses with the plasma membrane and releases undigested contents into the environment
What is the process by which lysosomes digest the cells own materials?
Autophagy
What cells don’t have lysosomes?
Plant cells- the vacuole may function equivalently
What do cells use energy for?
Growth, reproduction, movement
Where does the break down of fuel molecules start in eukaryotic cells?
The cytosol
Where do partially degraded molecules enter?
The mitochondria
What is the main function of the mitochondria?
Convert potential chemical energy of fuel molecules into a form the cell can use (ATP)
What is cellular respiration?
The process of converting fuel molecules and oxygen into ATP
What is the diameter and length of mitochondria?
1.5 micrometers diameter
2-8 micrometers long
How many mitochondria are there in cells?
One (some unicellular protists) Hundred thousand (large egg cells)
Typical humans liver cell- more than 1000
How many membranes do mitochondria have?
2- the outer and inner
What does the outer membrane of the mitochondria do?
Smooth and protective
Offers little resistance to movement of substances in and out
What is the structure of the inner membrane of the mitochondria?
Folds inwards in many places (larger surface area)
Folds–> regular shelf like structures called cristae
What is embedded in the folds of the inner mitochondrial membrane?
Large protein complexes involved in cellular respiration
What space is enclosed by inner and outer mitochondrial membranes?
The mitochondrial matrix
What does the mitochondrial matrix contain?
Enzymes
Ribosomes and DNA to make proteins needed for cellular respiration
What are plastids?
A class of organelles Found only in plants and certain protists
What is chloroplast?
A type of plastid
Contains green pigment chlorophyll
Site of photosynthesis
What happens during photosynthesis?
Light energy is converted into chemical energy of bonds between atoms
What is the purpose of photosynthetic products?
Provide food for photosynthetic organisms and the organisms that eat them
What is the structure of a chloroplast?
Variable size/shape
Two membranes
Internal membranes that differ among organisms
What do the internal membranes of chloroplasts look like?
Stacks of flat, hollow pita bread called grana
What do grana consist of?
A series of flat, closely packed circular compartments called thylakoids
What do thylakoid membranes contain?
Phospholipids
Proteins
Light harvesting pigments
How are thylakoids connected?
Thlakoids of one granum are connected to those of other grana (membrane network)
What is the fluid surrounding grana?
Stroma
What is in the stroma?
Ribosomes and DNA
What do ribosomes and DNA in the stroma do?
Synthesise some proteins that make up the chloroplast
What are two other types of plastids?
Chromoplasts
Leucoplasts
What colour are chromoplasts?
Red, orange and /or yellow pigments
What do chromoplasts do?
Give colour to plant organs such as flowers
What do leucoplasts do?
Storage deposits for starches and fats
What are peroxisomes?
Organelles that collect toxic peroxides (hydrogen peroxide for example)
Where do toxic peroxides come from?
Unavoidable by-products of cellular chemical reactions
How big are peroxisomes?
0.2 to 1.7 micrometers
What type of membrane do peroxisomes have?
Single membrane
What is inside a peroxisome?
Granular interior containing specialized enzymes
Where can peroxisomes be found?
Found (at one time or another) in (at least some cells of) all eukaryotic species
What similar to peroxisome structure can be found only in plant cells?
Glyoxysome
Where are glyoxysomes most prominent?
Young plants where stored lipids are converted to carbohydrates
What have vacuoles?
Many eukaryotic cells- plants and protists in particular
What do vacuoles contain?
Aqueous solutions containing dissolved substances
What are the 4 functions of plant vacuoles?
- Storage
- Structure
- Reproduction
- Digestion
How does storage in vacuoles contribute to the survival of plants?
Toxic and waste by products are stored in vacuoles which is poisonous/distasteful and deters animals
How does the vacuole contribute to plant structure?
90% of cell volume is taken up
Vacuole causes water to enter the cell which swells, providing turgor (stiffness) to support plant
How do vacuoles help plants to reproduce?
Some pigments (blue and pink) called anthocyanins are contained in the vacuole and attract pollinating/dispersal animals
Where are food vacuoles found?
Some simple ancient groups of eukaryotes (single celled protists, sponges)
What are food vacuoles for?
No digestive system, cells engulf food particles by phagocytosis, generating food vacuole which fuses with a lysosome for digestion
Where are contractile vacuoles found?
Freshwater protists
What do contractile vacuoles do?
Get rid of excess water that rushes into the cell because of solute potential
How does the contractile vacuole expel water?
It expands as water enters, then abruptly contracts.
Forces water through special pore structure
What is the cytoskeleton?
Long thin fibers
What are the roles of the cytoskeleton?
- Supports shape
- Movement
- Position organelles
- Tracks or support for motor proteins
- Anchors cell in place
What two methods show if a structure or process causes a function?
- Inhibition
- Mutation
How does inhibition prove a structure or process causes a function?
Drug inhibits the structure or process- if the function does not occur then the structure/process is probably a causative factor
How does mutation prove a structure or process causes a function?
Look at cells that lack the structure or process and see if the function still occurs
What drug breaks apart the microfilaments of the cytoskeleton of an amoeboid cell?
Cytochalasin B
What happens when an amoeboid cell is treated with cytochalasin B?
It rounds up and does not move
What three components make up the cytoskeleton?
Microfilaments
Intermediate filaments
Microtubules
How do microfilaments exist?
Single, in bundles or in networks
What is the diameter of a microfilament?
7 nanometers
What are the two major roles of microfilaments?
Help entire cell or parts move
Determine and stabilize cell shape
What are microfilaments made from?
Actin- a protein that exists in several forms
What is the structure of actin in microfilaments?
Extensively folded with plus and minus ends which interact to form long, double helix chains
What is the polymerisation of actin in microfilaments?
Reversible
What does actin produce in non-muscle cells?
Cytoplasmic streaming
Pinching that divides cells
Psuedopodia (false feet that enable movement)
What is cytoplasmic streaming?
Flowing movement of cytoplasm
How do actin filaments support cell shape?
Microfilaments form a meshwork inside plasma membrane which cross link and forms rigid structure
Where do microfilaments form a meshwork?
Microvilli that line human intestine
How many types of intermediate filaments are there?
At least 50- specific to cell type
How many molecular classes do intermediate filaments fall into?
6, based on amino acid structure
What are intermediate filaments composed of?
Fibrous proteins in keratin family
How are intermediate filaments arranged?
Tough, rope-like assemblages, 8-12 nm in diameter
What are the two structural functions of intermediate filaments?
Stabilize cell structure
Resist tension
What do some intermediate filaments do?
Radiate from nucleus and maintain position of nucleus and organelles
Give an example of intermediate filaments.
Lamins of nuclear lamina
Give an example of intermediate filaments maintaining rigidity of body surface tissues.
Connecting spot welds, desmosomes, between adjacent cells
What are microtubules?
Long, hollow, unbranched cylinders
What is the diameter if microtubules?
25nm
What are the two roles of microtubules?
- Form rigid internal skeleton in some cells
- Framework along which motor proteins can move structures
What are microtubules assembled from?
Tubulin (a protein)- made up of alpha and beta tubulin
What is tubulin?
A dimer- made of two monomers
What is the structure of a microtubule?
13 chains of tubulin dimers surround central cavity with a + and - end
Why are tubulin known as dynamic structures?
Tubulin dimers can be added or subtracted rapidly, mainly at the + end (lengthens and shortens)
Where do microtubules radiate from?
Microtubule organising center
What do microtubules do in plants?
Help control arrangement of cellulose fibers of the cell wall
What are motor proteins?
Specialized molecules that use energy to change their shape and move
What are microtubules essential in during cell division?
Distributing daughter chromosomes
What are eukaryotic cilia and flagella made from?
Specialized microtubules- idential internal structure, differ only in length and beating pattern
What are some characteristics of cilia?
Shorter than flagella
Present in great numbers
Beat stiffly in one direction, recovery flexibly in other direction
What are some characteristics of eukaryotic flagella?
Longer than cilia
Found singly or in pairs
Waves of ending propogates from one end to the other
What is the structure of eukaryotic cilia/flagella?
9 + 2 array
9 fused pairs of microtubules (doublets) to form outer cylinder
1 unfused pair up the middle
What is at the base of every cilia/flagella?
Basal body in the cytoplasm
What are centrioles?
Found in microtubule organising center- almost idential to basal bodies.
Not found in plants or protists
What are centrioles involved in?
Formation of mitotic spindle to which chromosomes attach
What is the movement of cilia/flagella the result of?
Sliding microtubule doublets past each other
What motor protein causes microtubules to slide past each other?
Dynein
How does dynein work?
It is attached to one microtubule doublet and then binds to its neighbor and changes shape
What motor protein carries vesicles from one part of a cell to another?
Kinesin
How does kinesin move proteins to other parts of a cell?
Binds to vesicle or organelle and then walks it along microtubule by changing shape
What molecule moves attached organelles towards the plus end of the microtubule?
kinesin
What molecule moves attached organelles towards the minus end of the microtubule?
Dynein
