Cells [part 2]

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