Part 1

Dr. M. Azzopardi

Syllabus Requirements:

3.2 Cell structureand function

3.2.1 The cell as the basic unit of living things.

Comparison of the principal features of prokaryotic and eukaryotic cells.The structure of a generalised plant and animal cell as revealed by both light and electron microscopy. Organelles should include the nucleus and nuclear envelope, nucleolus, centrioles, basal bodies, eukaryotic flagella (undulipodia), endoplasmic reticulum, ribosomes, Golgi apparatus, lysosomes, peroxisomes, mitochondria, chloroplasts and cytoskeleton.

TOPIC OUTLINE

A)THE CELL THEORYB) PROKARYOTIC CELL STRUCTUREC) CELL FUNCTION LIMITS CELL SIZED) EUKARYOTIC CELLS E) THE ENDOSYMBIOTIC THEORY

The cell theory has three key points:

1. All living things are made up of one or more cells.

2. Each cell is a basic living unit.

3. A new cell arises only from cells that already exist.

Amoeba

Cells can be cultured to produce more cells:

In vivo – inside organism

In vitro / Ex-vivo – outside organism

Cell line: cells grown in tissue culture and representing generations of a primary culture.

Cells can be:Prokaryotic Eukaryotic

10 m

A eukaryotic cell is generally 10x greater

i) Prokaryotic very primitive include the bacteria lack:

a nucleus membrane-enclosed internal compartments

ii) Eukaryotic

more advanced include protists, plants, fungi and animals

What about viruses?

Viruses have no cellular structure but however, depend upon cells for reproduction

Tobacco mosaic virus causes disease in plants

All cells have 4 features in common:

1. Genetic material (DNA)2. Cytoplasm:

a semifluid matrix containing organelles3. Ribosomes

synthesise proteins4. A plasma membrane

DNA

TOPIC OUTLINE

A) THE CELL THEORY

B) PROKARYOTIC CELL STRUCTUREC) CELL FUNCTION LIMITS CELL SIZED) EUKARYOTIC CELLS E) THE ENDOSYMBIOTIC THEORY

The Kingdom Prokaryotae:Includes the bacteria

Strepto - chains

Shapes and arrangements of bacteria

Staphylo - clusters

Curved

Spiral

Rod-shaped Spherical Diplo – in pairs

Prokaryote Cell Structure

Prokaryote Cell Structure

Cell wall is strong and rigid due to peptidoglycan /murein

Peptidoglycan is: unique to prokaryotes made of polysaccharide chains cross-linked at

regular intervals by short chains of amino acids

Functions of the cell wall:

1. Protects the cell

2. Maintains shape

3. Prevents the cell from bursting when the cell absorbs water

Two natural groups of bacteria due to differences in their wall structure depending

on whether they take up Gram’s stain:

Gram positive[purple]

Gram negative[red]

Gram positive bacteria

Gram negative bacteria

take up the stainhave a thick cell wall

do not take up the stain have a thin cell wall a membrane covers

peptidoglycan

Hans Christian Gram developed the stain in 1883

e.g. Staphylococcus, Bacillus, Lactobacillus

attacked by: penicillin lysozyme (enzyme in tear

fluid)

Gram positive bacteria:

Gram negative bacteria: e.g. Salmonella, E. coli,

Azotobacternot attacked by

penicillin & lysozyme

How does penicillin kill Gram positive bacteria?

Penicillin: interferes with the cross-

linking in the peptidoglycan of growing cells

makes the walls: weaker more likely to burst

when water enters by osmosis

Lysozyme attacks bonds in peptidoglycan wall

Bond between NAM and NAG

breaks

Explain why penicillin and lysozyme have no effect on

gram-negative bacteria.

Attacked by penicillin & lysozyme

G + G - Not attacked by penicillin & lysozyme

The outer membrane protects bacteria

from lysozyme and attack by penicillin

Penicillin has no effect on plant and animal cells. Why?

No peptidoglycan wall present.

Plant cellAnimal cell

Plasma membrane in bacteriaregulates movement of materials into and

out of the cell

Cytoplasm is an aqueous substance containing

organelles various substances

Cytoplasm & Cytosol compared

consists of: organellescytosol

makes up of about 70% of the cell volume

composed of water, salts & some macromolecules such as protein enzymes

Functions of infoldings of the cell membrane

an artifact

nitrogen fixation

contain photosynthetic pigments e.g. bacteriochlorophyll

mesosome

No infoldings in eukaryotes.

Mesosomes: infoldings of the cell surface membrane

produced by the chemical fixation techniques used to prepare samples for electron microscopy

in the 1960s: several functions were proposed for these structures

by the late 1970s: they were recognized as artifacts nowadays: no longer considered to be part of the

normal structure of bacterial cells

Genetic materialbacterial DNA is: is a single circular molecule, 1mm long usually coiled attached to the plasma membrane

Bacterial DNA is: concentrated in a region of the cell called

nucleoid not physically separated from the cytoplasm

by a membrane

Nucleoid

Bacteria reproduce:

Asexually by Binary Fission

Ribosomes (70S)are the site of protein synthesis

are complexes of : RNA & proteins

smaller than those of eukaryotes

A svedberg unit (symbol S) is: a unit for sedimentation rate technically a measure of time, and

is defined as exactly 10−13 seconds

Capsules are slimy or gummy secretions of

certain bacteria function:

1. unite bacteria into colonies2. offer protection against white

blood cells3. prevent cells from drying out

Rigid capsule

Slime layer

Electron micrograph of a colony of

Staphylococcus aureus

Sporessome bacteria form

endospores (spores produced inside cells)

are: thick- walled long lived resistant to particularly: heat drought short-wave radiations

Flagella (singular flagellum)

some are rigid, though shaped into a wave

a single flagellum is a made of a protein called flagellin

one or more are present for motility

Differences between a flagellum in prokaryotes and in eukaryotes:

much simpler in structurenot made of microtubules

Flagellum in a eukaryote: beats

Flagellum in a prokaryote: rotates

Flagella propel the cell along by:

rotating at the base, providing a corkscrew-like motion rather than a beat

Pili (singular pilus) are fine protein rods:

shorter & thinner than flagella

are concerned with adherence

Bacterial pili have long been recognised as mediators of initial host–pathogen

interactions important for the progression of Gram-negative bacterial diseases.

project from the walls of Gram negative bacteria

Diagram shows how bacteria:

with pili may resist being flushed out with urine

without pili are flushed out with urine

Sex-pili help bacteria join to each other

to exchange genetic material

‘sex’ in genetic sense NOT directly linked to reproduction as in most eukaryotes

Transfer of plasmid

A pilus projecting from surface of a

Gram negative bacterium

Fimbriaesimilar to pili but shorter

help cells to adhere to surfaces e.g. animal cells for:

food protection

Plasmids tiny circles of self-replicating DNA found in some

species of bacteria in addition to the circular DNA

a bacterium may contain: dozens or hundreds of copies of a plasmid

A plasmid possess a few genes which give extra survival advantage

plasmid genes are known to:1. produce an enzyme which breaks down

penicillin, thus bacterium is resistant to penicillin

2. confer resistance to disinfectants3. confer ability to use complex chemicals as

food, such as hydrocarbons, with potential applications in clearing oil spills

When a bacterium dies, the plasmids liberated into the environment may transform

other bacteriaBacterial transformation is the process by which

bacterial cells take up naked DNA molecules.

Transformation occurs:Naturally in some species of bacteria

By artificial meansin other cells

[by genetic engineering]

QUESTION: [SEP, 2002]

Which type of cell is evolutionarily more primitive, eukaryotic or prokaryotic? Briefly give reasons for your answer. (2)

Prokaryotic. Lacks membrane-bound organelles.DNA is not wound around histones – not organised.

TOPIC OUTLINE

A) THE CELL THEORYB) PROKARYOTIC CELL STRUCTURE

C) CELL FUNCTION LIMITS CELL SIZED) EUKARYOTIC CELLS E) THE ENDOSYMBIOTIC THEORY

Why are cells small?

Volume determines the amount of chemical activity in the cell per unit time.

Surface area determines the amount of substances that can pass the cell boundary per unit time.

because they need a high surface area-to-volume ratio

As size increases, SA:Vol ratio decreases

Why do large organisms consist of many cells rather than one

large cell?

exchange would be limited by the:1. distance from the centre of the

cell to its surface 2. surface area of the cell

TOPIC OUTLINE

A) THE CELL THEORYB) PROKARYOTIC CELL STRUCTUREC) CELL FUNCTION LIMITS CELL SIZE

D)EUKARYOTIC CELLS E) THE ENDOSYMBIOTIC THEORY

Eukaryotic cells have membrane-bound organelles

the evolution of compartments was an important development:

enabled eukaryotic cells to specialise, forming the organs and tissues of complex multicellular organisms

Compartamentalisation makes it possible to have an

environment within the organelle which is biochemically distinct from the cytoplasm

metabolic pathways & processes are containedby having the enzymes & cofactors together: it makes for a more energy efficient process by keeping them within a membrane it can ensure

that metabolic processes can occur safely that would otherwise be harmful or interfere with the activity within the cytosol

If there were no compartmentsthe free-floating molecules would basically

wander around the cell until they bumped into the right reactants works only for

small cells

BUTas cells get bigger, the chance of two particular molecules

meeting goes way down

protoplasm = nucleus + cytoplasm

cytoplasm

nucleus

Plasma membrane

is a transparent semisolid or gelatinous fluid

contains:1. organelles 2. cytosol

Animal Cells Plant Cells

CentriolesLysosomes

Cell membraneRibosomes

NucleusEndoplasmic reticulum

Golgi apparatusVacuoles

MitochondriaCytoskeletonPeroxisomes

Cell WallChloroplasts

Central Vacuole

Venn Diagrams

Animal Cell

Microvilli: only in animal cellsare finger-like extensions of the cell surface

membrane form a fringe called a brush border

Role of microvilli:increase the surface area by

as much as 25 x

absorption or secretion at base of microvillus

microvillus

Microvilli can contract as they have actin filaments within them

What is the importance of

microvilli being able to contract?

To facilitate absorption

An

a specialised subunit within a cell that has a specific function, and it is usually separately enclosed

is:

although certain structures e.g. ribosomes do not have a membrane, they are still considered as organelles

Organelles with membranes

1. Nucleus2. Endoplasmic

reticulum3. Golgi apparatus

[Golgi complex]4. Lysosomes5. Mitochondria6. Chloroplasts7. Peroxisomes8. Vacuoles

1. Ribosomes2. Cytoskeleton3. Centrioles

Non-membranous organelles

QUESTION: [MAY, 2003]List three advantages of eukaryotic organisation over prokaryotic organisation.

(3)a) Can reach larger sizes thus provides protection

from predators;b) Division of labour (each organ has its own role and

each cell can perform a different function) leads to greater efficiency;

c) Allows compartamentalisation for example membranes surrounding different organelles allow each organelle to have its own set of chemicals and chemical reactions.

Organelles with membranes1. Nucleus2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]4. Lysosomes5. Mitochondria6. Chloroplasts7. Peroxisomes8. Vacuoles

Nucleus: largest organellecontrols the activities of the cell as it contains

DNAfound in all eukaryotic cells

except:

red blood cells

phloem sieve element

The nucleus is surrounded by:a nuclear envelope

has pores to allow communication between the nucleus & the cytoplasm

The outer membrane of the nuclear envelope is continuous with:

the endoplasmic reticulum (ER) may be covered with ribosomes

The nucleus contains:

[one or more nucleoli]

chromatin

NUCLEUS

nucleolus

What does ‘chromatin’ mean?

“coloured material” & refers to the fact that it is easily stained

Neutrophils in a blood smear

Chromatin is composed of mainly:nucleolus

chromatin

1. coils of DNA

2. bound to basic proteins called histones

chromatin

Nucleosomes: bead-like structures which are in turn regularly packed in the chromatin

DNA is wound around the histones which form nucleosomes:

one nucleosome is composed of 8 histone

proteins

How much DNA in each nucleus?

Chromatin organisation of eukaryotes

stains intensely

is seen as dark patches near the nuclear envelope

stains lightly

located towards the centre of the nucleus

HeterochromatinEuchromatin

Heterochromatin: tightly coiled chromatin

Euchromatin: loosely coiled chromatin

During nuclear division, chromatin

Because chromatin condenses into more tightly coiled threads called chromosomes

stains more intensely becomes more

conspicuous

WHY?

Nucleolusappears as a rounded, darkly stained

structure inside the nucleus

stains intensely as it contains large amounts of: DNA & RNA

manufactures ribosomal RNA (rRNA)

Organelles with membranes

1. Nucleus

2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]4. Lysosomes5. Mitochondria6. Chloroplasts7. Peroxisomes8. Vacuoles

ENDOPLASMIC RETICULUM (ER)consists of flattened cavities – cisternae:

made up of parallel membranes two types of ER:

Rough ER lined with ribosomes

Smooth ERno ribosomes

Role of the rough ER

RER is concerned with the transport of proteins which are made by the

ribosomes on its surface

Proteins:enter into the ER via a channel are transported through the cisternaeare usually being modified en route

Proteins from the RER commonly travel to the Golgi apparatus:

Flow of proteins :

Proteins from the Golgi apparatus can be:

1. secreted from the cell 2. passed on to other

organelles in the same cell

Smooth ER is made up of tubular

cavities occupies a smaller area

compared to the rough ER

Smooth ER

Rough ER

[occurs away from the nucleus]

Functions of the smooth ER:

1. lipid synthesis e.g. in the epithelium

of the intestine, the smooth ER makes lipids from fatty acids and glycerol absorbed from the gut and passes them to the Golgi apparatus for export

Functions of the smooth ER:

2. makes steroids (a type of lipid) some steroids are hormones e.g.

testosterone

3. site for the hydrolysis of glycogen

4. chemically modifies small molecules taken in by the cell this is especially true for drugs and

pesticides

5. proteins synthesised on the rough ER are chemically modified within the lumen of the smooth ER

Organelles with membranes

1. Nucleus2. Endoplasmic reticulum

3. Golgi apparatus 4. Lysosomes5. Mitochondria6. Chloroplasts7. Peroxisomes8. Vacuoles

GOLGI APPARATUS /GOLGI BODY /

GOLGI COMPLEX

The Golgi apparatus consists of two components:

2. Golgi vesicles

1. Cisternae

Lumen

[stack of flattened, membrane-bound sacs]

Functions of the Golgi apparatus: 1) it receives proteins from the ER and

chemically modifies them e.g.:

carbohydrate is added to proteins to form glycoproteins

the slime, wax, gum & mucilage secretions of many cells are released by the Golgi apparatus

1) it receives proteins from the ER and chemically modifies them e.g. in hormone secretion

New cisternae are constantly formed at one end of the Golgi & pinched off at another

Functions of the Golgi apparatus:2) proteins are concentrated, packaged, and

sorted before being sent to their cellular or extracellular destinations

3) forms lysosomes

Organelles with membranes

1. Nucleus2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]

4. Lysosomes5. Mitochondria6. Chloroplasts7. Peroxisomes8. Vacuoles

Lysosomes originate from the Golgi apparatus:

Golgi apparatus

LYSOSOME A lysosome is surrounded by a single membrane

Lysosomes are:

simple sacs that contain digestive enzymes such as:

ProteasesNucleasesLipases

carry out hydrolysis reactions (splitting by adding water)

work best in an acid environment

Enzymes in the lysosome:

Question: SEP, 2006Explain the following observations regarding cell organelles.

The interior of a lysosome (pH 4.8) is more acidic than the surrounding cytosol (pH 7). (2)

A lysosome is full of degrading enzymes whose optimum pH is acidic.

Four Functions of lysosomes:1. phagocytosis 2. autophagy – unwanted structures within the cell

are digested3. release of enzymes outside the cell (exocytosis)

1 23

Four Functions of lysosomes:

autolysis – self-digestion of a cell

1

2 3

Golgi apparatus

LYSOSOME

4

autophagy exocytosis

phagocytosis

Organelles with membranes

1. Nucleus2. Endoplasmic reticulum3. Golgi apparatus [Golgi complex]4. Lysosomes

5. Mitochondria6. Chloroplasts7. Peroxisomes8. Vacuoles

Mitochondria occur in all eukaryotic cells large organelles with a double membrane function: aerobic respiration

inner membrane is folded forming cristae

inner membrane

outer membrane

matrix

crista

MITOCHONDRIA have :

DNA

ATP synthase

70S ribosomes