Cell organelles

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Present in all cells – except prokaryotes and mature erythrocytes

Green alga Microsterias has only one per cell

Amoeba (Chaos chaos) has up to 500

000 per cell

Liver cells have about 1000 each

Sperm cell has 25

Accumulate in cells with high metabolic activity

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Surrounded by two membranes

Inner membrane with numerous narrow folds

(cristae) directed towards the matrix

Cristae enlarge the inner membrane surface area

The two membranes create two compartments

Matrix (central aqueous phase) and

Intermembrane space

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The matrix contains:  A circular

DNA molecule and ribosomes  Enzymes

for cellular respiration  Divalent cations

(Mg++, Mn++ ) as enzyme

cofactors

Mitochondria

Contain DNA, thus autonomous – divide like

bacteria

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Closely associated with aerobic respiration

Releasing energy in a useful form by the oxidation of foodstuffs

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The cristae vary in size, shape and number

(different in plants and animals)

Cristae increase the surface where electron transfer

occurs

Large numbers of small knobs on stalks occur on the

cristae – F0F1-ATP-ase (“elementary particles”)

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F0F1-ATP-ase is a transmembrane protein with ATPase properties

(synthesizes or hydrolyses ATP)

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F0F1-ATP-ase consists of

9 polypeptide chains of which – 4

hydrophobic (within the membrane)

5 combined as the spherical head

protruding from the membrane into the matrix

The catalytic centre is one of the 5

polypeptides in the head

The proton gradient across

the

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An extended network supporting fibres Three types: Actin filaments (thin – 6nm) Intermediate filaments (7-11 nm) Microtubules (thicker – 22 nm)

Functions: Supporting framework for organelles

Involved in cell movements “Anchors” the

plasmalemma Surfaces for intracellular

transport

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14

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Two threadlike chains

each consisting of a string of

globular proteins

wound around each other

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Muscle contraction – in co-operation

with myosin

Cell movements via assembly/

disassembly of actin chains

Structural support for plasmalemma iv.

Supporting elements for microvilli

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” Like actin, but three strings of globular proteins

” Common in epithelial cells

supply mechanical strength/rigidity

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Keratin is an example (Hair, nails,

epithelium)

Microtubules

” Hollow tubes, walls constructed from the

protein tubulin

” Two types: α-tubulin and β-tubulin Cross-section: 13 tubulin-molecules in a circle

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” Spindle formation during mitosis

“ Reinforcement of cytoplasm – “anchoring” of organelles

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bodies originate from microtubules

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“  Centrioles occur in pairs

the one perpendicular to the other.

“  Occur near the nucleus

in a region of the cytoplasm known as the “centrosome”

They have the 9+0 arrangement of tubules “  Give rise to basal bodies

cilia and flagella originate from the basal bodies. “  Determine the “plane” of cell division in human and other animal cells

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” Originate from basal bodies that in turn

originate from centrioles

” Cilia: Hairlike structures on surfaces of

some cell types.

“ Flagella similar but longer and whip-like. ” Both are covered by the plasmalemma. “ Note the 9+2 arrangement of tubules!!

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” Propelling of cells along a fluid (eg sperm

cells with flagella)

” Moving of fluids along stationary cells

Trachea

dust-laden mucus is moved upwards to the

pharynx by cilia.

Oviducts

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egg cell is moved along by the action of cilia – in the opposite direction that the sperm cell “swims”.

Origin of mitochondria &

chloroplast in eukaryote cells The endosymbiotic theory

“Primitive” eukaryotic cells did not have

mitochondria or chloroplasts

This theory postulates that mitochondria and chloroplasts were originally prokaryotes that invaded such “primitive” eukaryotic cells

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These organelles were obtained by endosymbiosis  Chloroplasts are said to be derived from a photosynthetic bacterium

The host cell was then able to photosynthesize

Mitochondria are said to be derived from an aerobic bacterium

the host cell could then carry out aerobic

respiration

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“The bacterium is engulfed by means of

phagocytosis

“Following the engulfment the bacterium is

surrounded by two membranes

its own, as well as the one originating from the plasmalemma of the host cell (endocytotic vesicle).

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” The symbiotic coexistence was more advantageous for both partners and the bacteria developed into mitochondria

“  All eukaryotes have mitochondria

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Mitochondria and chloroplasts have their own

ribosomes, similar to those of prokaryotes

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The ribosomes of mitochondria and chloroplasts are sensitive to the same antibiotics that interfere with prokaryotic ribosomes

Division of mitochondria and chloroplasts is similar to

those of prokaryotes

Mitochondria and chloroplasts, like prokaryotes, do

not have microtubules

The DNA of the organelles is circular as those of

prokaryotes

Mitochondria and chloroplasts are similar to bacteria

regarding size and shape

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Some genetic sequences of chloroplasts

correspond to those of cyanobacteria

Some genetic sequences of mitochondria

correspond to those of aerobic bacteria

Both organelles are surrounded by a double

membrane

The outer membrane derived from the endocytotic vesicle

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membrane of the prokaryote

Dimensions in cell biology

Micrometer ( µm) = 10-6 m

Nanometer (nm) = 10  -9 m

Angstrom-unit (A) = 0.1 nm (The

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Dimensions in cell biology

»  Diam. of cell =

40 µm

Mitochondrion (0.7 » µm)

(not to scale) »

Lysosome (0.2 »

µm) Atom = 0.1 nm

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Chloroplast (1 µm))

»

Protein = 10 nm

»

DNA = 2 nm in diam.

Cell size

Cells are between 5 – 20 – 100 µm in diameter

Thus visible with a light microscope

Smallest cells – Mycoplasms (0.1 µm)

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Only visible with the electron microscope

Living cells can most probably not be smaller

Or else there would not be enough room for all the

proteins/molecules necessary to perform the

functions of life

Cell size (continued)

Human egg cell: 130   µm. The

size of a full stop at the end of a

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algae-like Nitella and Acetabularia – up to 5 cm long!

Why did cells remain small?

Maximum size of cells

Most cells have only a single nucleus – the nucleus can only “manage” a limited volume of cytoplasm

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The distances substances could move

by means of diffusion (a very slow

process)  The ratio of the surface area of the cell to the volume of the cell

Why are cells small?

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Nucleus

Diffusion

Waste products

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Diameter of a cell

Surface (µm2)

Volume

(µm3)

0.1 µm

0.06

0.001

20 µm

2 400

8 000

Increase

200x

40 000 X

8 million X

Surface of a sphere = 4.Π.r2

Volume of a sphere = 4/3.Π.r3 36

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With an increase in cell size, cell volume increases to a much greater extent than the surface area

The surface area can no longer provide

for sufficient import and export of products

Diffusion is too slow to cope with the increasing distances in the cell The nucleus can no longer “manage” the increasing bulk of cytoplasm

Some large cells

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Methods for studying cells

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Resolving power (resolution) of a

microscope

Is the ability of the microscope to distinguish two points close to each other as separate points

Above the resolving power of the microscope the two points will appear as one

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Light rays have long wavelengths

This leads to a rather poor

resolution

(200 nm)

The ultra-structure of the cell can

not be studied

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It is  “cheap” and easy to work with

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Employs electron beams instead of

light rays

Magnetic lenses focus the electrons

Much better resolution (0.2 – 0.3 nm)

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But cannot be used for living material

Very expensive

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Human eye – 100 000 nm (100

um)

Light microscope – 200 nm TEM- 0.2 nm

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SEM

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components

Homogenize tissue to break cells

Cell content ends up in a buffer medium

Centrifuge at progressive higher g-values

Nuclei precipitate (100 x g)

Chloroplasts (1 000 x g)

Mitochondria (10 000 x g)

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Ribosomes (100 000 x g)

Aqueous remaining solution = Cytosol

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Each fraction is then studied with regard to

Their chemical composition

Enzymes

Reactions

In this way it was determined that mitochondria contain

52 different enzymes, which are all involved in some way in cellular respiration

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