cells [part 2]
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Part 2
Dr. M. Azzopardi
Organelles with membranes
1. Nucleus2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]4. Lysosomes5. Mitochondria
6. Chloroplasts7. Peroxisome8. Vacuoles
CHLOROPLASTScontain chlorophyll and carotenoid pigmentsfunction: carry out photosynthesis
What is a ‘granum’ [plural: grana]?
a stack of thylakoid membranes
thylakoids : an internal membrane system consisting of flattened sacs
granum
Grana under the electron microscope
Chloroplasts contain:DNA 70S ribosomes
QUESTION: [DEC, 1986]
List four similarities between a mitochondrion and a chloroplast. (4) Both have:i) a double membraneii) 70S ribosomesiii) their own DNAiv) electron transport chains / ATP synthase
Plastids have a double membraneoccur only in plants
Proplastids are simple, generally colorless
undifferentiated plastids
e.g. Amyloplasts store starch
Plastids can change from one type to another
Function of plastids:the site of manufacture & storage of important chemical compounds used by the cell
Various types of plastid in plants1. Chloroplasts – for photosynthesis2. Leucoplasts – colourless; for storage
3. Amyloplasts – contain starch
4. Chromoplasts – are red, orange or yellow plastids; in fruit & flowers
Organelles with membranes
1. Nucleus2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]4. Lysosomes5. Mitochondria6. Chloroplasts
7. Peroxisome8. Vacuoles
Peroxisomes: also called microbodiesspherical organelles bounded by a single
membranecontain catalase
Peroxisomes in a liver cell.
What is the function of catalase?An enzyme that catalyses the decomposition of hydrogen peroxide to the harmless products water and oxygen
H2O2
is a by-product of certain reactions of metabolism e.g. lactic acid breakdown in liver cells
Catalase in potato breaks H2O2.
Question: SEP, 2006Explain the following observations regarding cell organelles.
Peroxisomes contain oxidative enzymes. (2)
Peroxisomes contain enzymes that degrade fatty acids and amino acids, producing hydrogen peroxide. They contain catalase which brings about oxidation reactions.
Organelles with membranes
1. Nucleus2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]4. Lysosomes5. Mitochondria6. Chloroplasts7. Peroxisome
8. Vacuoles
8. VACUOLES
animal cells contain: relatively small
vacuoles e.g. food vacuoles phagocytic vacuoles
a vacuole is a fluid-filled sac bounded by a single membrane
Plant cells have: a large central vacuole
bounded by a membrane: tonoplast
What is ‘cell sap’?The fluid present in plant cell vacuoles.
Cell sap is a concentrated solution of:mineral saltssugarsorganic acidsoxygencarbon dioxidepigments some waste products
Non-membranous structures
1. Ribosomes2. Cytoskeleton3. Centrioles
have specialised functionsnot called organelles : lack a membrane
1. RIBOSOMES the sites of protein
synthesis
each ribosome consists of two subunits: a large a small one
Ribosomes are made up of roughly equal amounts of:
rRNA (ribosomal RNA) protein
rRNA is made in the nucleolus
+
A ribosome builds a protein
Two types of ribosomes:
70S in: prokaryotes mitochondria chloroplasts
80S in: eukaryotes
Ribosomes are located :
bound to endoplasmic reticulum
Cell membrane
Microtubule
Microfilament
Mitochondrionfree in cytoplasm
2. The cytoskeleton
is in the form of a network of fibres extending throughout the cytoplasm
was once thought to be unique to eukaryotes, but recent research has identified the prokaryotic cytoskeleton
Why don’t organelles collect at base of a cell by gravity?
kept in place by the cytoskeleton
Functions of the cytoskeleton:1. intracellular transport of organelles2. establishing cell shape3. providing mechanical strength4. chromosome separation in mitosis and
meiosis
The cytoskeletonis made up of three kinds of protein
filaments:
3. Microtubule
1. Microfilament
2. Intermediate filament
Microfilaments:Structure: thin filament made up of globular protein
actin
Function of microfilaments: play a major role in:
• cytoplasmic streaming movement in plants
• amoeboid movement
• muscle contraction
Actin is required to split the cytoplasm
Intermediate filaments:Structure: made up of the fibrous protein
keratin highly stable
Function: resist pulling anchorage of:
nucleus other organelles
Microtubules:Structure: are straight, unbranched
hollow cylinders, 25 nm wide & usually quite short in length
made of the protein tubulin
Microtubules are constantly being built up & broken down
Microtubules:Function: determine the overall shape of the cell form mitotic spindle intracellular transport [e.g. mitochondria &
lysosomes]
3. CENTRIOLESsmall hollow cylindersoccur in pairsusually located at 90 to each
other near the nuclear membraneFound in:
animal cells most protists
Absent in: plant cells fungi
Centrioles are composed of
nine triplets of microtubules arranged in a 9+0 arrangement
triplet
microtubules
What is the ‘centrosome’? the region surrounding the pair of centrioles
in all animal cellsNo centrosomes in
plant cells
Function of centrioles:help organise the mitotic spindlespindle itself is made of microtubules
Centrioles : replicate themselves at the beginning of nuclear
division the two pairs migrate to opposite poles of the
spindle spindle:
the structure on which the
chromosomes line up
Label
rough ER
plasma membraneribosomeperoxisome
smooth ER
nucleolus
cytoplasmnuclear membrane
mitochondrion
EUKARYOTIC FLAGELLA (UNDULIPODIA) & CILIA
Flagella & cilia: whiplike structures push or pull the cell through
its aquatic environment
move surrounding liquid over the surface of the cell
e.g. cilia move a Paramecium; human sperm moves by a flagellum
e.g. cilia move mucus in trachea; flagella in sponges beat to create a water current for respiration
sponge
5 µm
Direction of swimming
(a) Motion of flagella: snake-like
Direction of organism’s movement
Power stroke Recovery stroke
(b) Motion of cilia 15 µm
Cilia in Paramecium
Differences between cilia & flagellaCILIA FLAGELLA Short LongerUsually many present Usually one or two
presentMove with stiff power stroke and flexible recovery stroke
Movement is snake-like
Flagella & cilia are enclosed by a plasma membrane:
made of microtubules in “9 + 2” array
Axoneme: the central strand of a cilium or flagellum, composed of an array of microtubules, typically in 9+2 arrangement.
Basal body / kinetosome:Connects cilium or flagellum just below the plasma membrane
Model of axoneme
showing microtubules in
“9 + 2” array
Model of axoneme
The outer nine sets are often referred to as doublet microtubules.
Microtubules in the axoneme & basal body
9+0 array of microtubules
Longitudinal section of cilium
Cross section of cilium
Cross section of basal body
Triplet
9+2 array of microtubules
Central microtubules
Outer microtubule doublet
The basal body:is derived from a centriolecontrols the direction of the movement of cilia
Axoneme
Basal body
What microtubules are found in the basal body?the nine microtubule
doublets [9+0] each doublet is
accompanied by another microtubule, making nine sets of three microtubules
the central, unfused microtubules do not extend into the basal body
Microtubule doublets in cilia & flagella are linked by proteins
Nexin
Inner-arm dynein
Outer-arm dynein
Roles of nexin & dynein:
are motor complexes which produce the force needed for bending.
inter-doublet linkage that prevents microtubules in the outer layer of axonemes from movement with respect to each other.
Dynein arms:
Nexin:
What is the ‘radial spoke’?
The radial spoke is another protein complex:
the radial spoke projects from each set of outer doublets toward the central microtubules
thought to be important in regulating the motion of the axoneme
Radial spokes
What causes the motion of cilia and flagella?
Motion results from the sliding of the microtubules past each other driven by a motor protein called
dynein which can undergo changes in its shape driven by energy from ATP
Nexin cross-links the doublets
preventing them from sliding: thus
cilium bends
1. What is a flagellum? (1)A flagellum is a whip-like organelle that pulls or pushes the cell through its aqueous environment.
Question: [MAY, 2005]
2. Briefly describe the structure of a eukaryotic flagellum. (3)
A eukaryotic flagellum is surrounded by the plasma membrane and contains a 9+2 array of microtubules. Nine fused pairs of microtubules, called doublets, form an outer cylinder and one pair of unfused microtubules runs up the centre. A spoke radiates from one microtubule of each pair and connects the doublet at the centre of the structure. In the cytoplasm at the base of each flagellum is an organelle called a basal body. The nine microtubule doublets extend into the basal body. The protein called dynein is permanently attached to one microtubule and moves it with respect to a neighbouring one.
Question: [MAY, 2005]
Role of dyneindynein molecules attached to one
microtubule bind to a neighbouring microtubule
as the dynein molecules change shape, they move the microtubule past its neighbour
Motor proteins drive vesicles along microtubules
dynein & another motor protein, kinesin, are responsible for carrying protein-laden vesicles from one part of the cell to another
CELL WALLS: extracellular structures
A cell wallis found in:
1. plant cells2. prokaryotes3. fungi
is a relatively rigid wall: surrounds the cell is secreted by the living cell
chemical composition is different
Composition of cell wall:
Two types of cell wall
laid down during cell division may be laid down later
in life when cell expansion is complete
Primary & secondary cell walls
Thin primary wall:No support
Thick secondary wall:Provides support
In some cells, e.g. mesophyll cells, the primary wall remains the only wall
Middle lamella:holds adjacent cells
together
composed of sticky, gel-like magnesium & calcium pectate [pectins]
Plasmodesmata: cytoplasmic connections that form when the new wall is laid down
form where cell wall is not thickened further
Pits
CellulosePectatesHemicelluloses
Often impregnated with other substances:
Lignin is deposited in wood cells
Suberin makes cells waterproof
Composition of:
Polysaccharides in primary cell wall:
Cellulose
Cellulose is built from glucose
1 cellulose molecule = about 3000 glucose molecules
Microfibrils run in all directionsallowing for considerable stretching during
cell growth
Microfibrils
Lignin replaces pectins in the secondary cell wall
Lignin: adds strength to
cell walls makes cell walls
inflexible
Waxy suberinin cork tissue (tree bark)
Ligninin wood
Lignin in secondary cell walls is the main supporting material of trees
cements & anchors cellulose fibres together
acts as a very hard & rigid matrix, giving the cell wall extra tensile strength and particularly compressional strength which prevents buckling
protects the cells from physical and chemical damage
Three major roles of the cell wallProvides support for the cell and limits its
volume by remaining rigid.
Acts as a barrier to infection by fungi and other organisms that can cause plant diseases.
It contributes to plant form by growing as
plant cells expand.
Junctions between cells
What are ‘Junctions?structures that allow cells to connect
togetheroccur in multicellular organisms
In plant cellsPlasmodesmata
(singular = plasmodesma)
In animal cellsTight junctionsDesmosomesGap junctions
Plasmodesmata are living connections between neighbouring plant
cells which run through very fine pores in the walls
How do plasmodesmata make communication & coordination between plant cells easier? Molecules & ions do not have to cross a cell
surface membrane
Junctions in animal cells
Tight junctions: are barriers that prevent or reduce fluid movements in the spaces between cells
at the tight junction the outer parts of adjacent membranes are fused
e.g. in bladder prevent urine from leaking out
Desmosomes hold cells together e.g.epithelial cells are equivalent to spot welding in metal engineering dense fibrous material loops in and out of the
desmosome region
Gap junctions: tiny open channelsin the plasma membrane through which small
molecules and ions may pass
occur in a wide variety of cells, including certain muscle and nerve cells
TOPIC OUTLINE
A) THE CELL THEORYB) PROKARYOTIC CELL STRUCTUREC) CELL FUNCTION LIMITS CELL SIZED) EUKARYOTIC CELLS
E) THE ENDOSYMBIOTIC THEORY
How did eukaryotic cells originate?
eukaryotic cells appeared about 1.5 billion years ago
Endosymbiosis theory explains how eukaryotes could evolve from prokaryotes
The Endosymbiotic Theory proposes that:some of today’s eukaryotic organelles
evolved by a symbiosis arising between two cells that were each free-living
Symbiosis is a
close relationship between organisms of different species that live together
A small prokaryote:-
was ingested but not digested
divided at the same rate as the larger one
successive generations continued to be
inhabited by the smaller one
this is called ENDOSYMBIOSIS – ‘living within’
another cell or organism
What happened to the cells engulfed?Became mitochondria & chloroplasts
What happened to most of the genes of the mitochondria over the billion and a half years in which they have
existed as endosymbionts?
Most of their genes have been transferred to the chromosomes of the host cells – but not all
Evidence supporting Endosymbiosis Theory
Mitochondria and chloroplasts: have two membranes possess circular DNA possess 70S ribosomes are about the size of a prokaryotic cell divide by binary fission as bacteria not
mitosis
Essay Titles1. Organelles in cells are regarded as analogous to
organs in multicellular organisms. Comment on the validity of this statement. [MAY, 2004]
2. Are the cells of unicellular eukaryotes any different from those found in multicellular eukaryotes? Discuss. [SEP, 2004]
3. Compare and contrast the structure of prokaryotic and eukaryotic cells.
[SEP, 2007]
THE END
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