Cell organelles

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Ø  Cytoplasm is intersected by repeatedly folded plates of

membranes

Ø  Enclosing spaces of varying shapes and sizes

Ø  ER lacking in mature red blood cells

Ø  Effectively divide the cytosol into compartments – vesicles,

tubules, flat sac-like cisternae Two types

Ø  Granular (rough) ER (GER) Agranular (smooth) ER (AER)

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Ø ER membranes similar in structure to the

plasmalemma

Ø Continuous with the plasmalemma, although

thinner

Ø ER membranes also continuous with the outer

membrane of the nuclear membrane

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Ø  Has large numbers of ribosomes attached to the cytosolic side of membrane

Ø  Ribosomes (consisting of two subunits) are attached with the larger subunit to the ER

Ø  The amount of GER in a cell correlates with the rate of protein synthesis of the cell

Ø  E.g. the pancreas produces many enzymes – thus large amounts of GER

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Ø Small (20 nm) solid particles without membranes

Ø Also present in prokaryotes

Ø Consisting of a larger and a smaller subunit

Ø Each unit consists of ribosomal RNA and

proteins

Ø Subunits assembled in nucleolus (in nucleus)

Ø Ribosomes free in cytosol or attached to the

GER

Ø Attach to mRNA chain to form a polysome

Ø As a polysome it synthesizes a protein 6

30S (40S)

50S (60S)

70S (80S)

50S(60S)

Numbers in brackets = Eukaryotic ribosomes S=Svedberg unit= rate of sedimentation in centrifuge The greater the S number, the greater the rate

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The synthesis of proteins

Canalising the proteins to the lumen of cisternae Ribosomes arranged in groups on mRNA as polysomes

Polysomes may occur free in the cytosol

Polysomes attached to ER membranes by means of their large subunits

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Proteins synthesized by free ribosomes in the cytosol,

remain in the cytosol

Proteins synthesized by ribosomes on the ER can be

Integrated in the membrane structure – they never reach the ER cavities

Or immediately be transferred to the ER cavities

Proteins in the cavities are transported to various

positions in the cell via the ER cavities

Or via the Golgi bodies transported out of the cell

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Ø Smooth ER – without

ribosomes

Ø Tubular as opposed to the flat, sac-like cisternae

Ø Amount per cell varies depending on the function of the cell

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Ø Biosynthesis of steroid hormones

Testosterone – Leydig cells in testes Progesterone – Corpusluteumin ovaries Corticosteroids – Adrenal cortex

Ø Synthesis of phospholipids (liver)

Ø Detoxification of poisons – alcohol & drugs like

morphine/barbiturates (liver)

leads to cirrhosis of the liver due to formation

of excessive amounts of ER/enzymes

Ø In muscle cells

for contractions

regulates [Ca++] as ‘trigger’

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Ø  All Golgi bodies of a cell are collectively known as

the Golgi apparatus (Golgi complex)

Ø  A Golgi body consists of a stack of 3-12 flattened

cisternae surrounded by a membrane

Ø  Cisternae often appear much wider on the sides than

in the middle

Immature face Forming face Cis face Convex side

Mature face Disappearing face Trans face

Concave face 14

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Ø  Vesicles and secretory vesicles are associated with the

cisternae

Ø  Vesicles are small (40 nm) spheres – pinch off from

the edges of mature cisternae

Ø  Secretory vesicles are larger (± 500), contain

different kinds of secretions

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Ø  Proteins within the ER cavities are eventually secreted

Ø  Parts of the rough ER become smooth, pinching off

small transport vesicles

Ø  Transport vesicles fuse with the cisternum at the

immature side of the Golgi body

Ø  Proteins now enclosed in the cisternae of the Golgi

body

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and the Golgi complex

Ø In the cisternae of the Golgi body, a protein becomes concentrated and modified (function and destination)

Ø E.g. pro-insulin (81 amino acids) is changed to

insulin (51 amino acids) and a residual peptide

Ø Secretory vesicles (insulin) or vesicle with digestive enzymes bud off at mature side

Ø Secretory vesicles move to surface of the cell

Ø Small vesicles remain inside cell as lysosomes

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and the Golgi complex

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Ø  Sharing (fusion) and transfer of membranes between

organelles are common in eukaryotic cells

Ø  E.g.: plasmalemma, nuclear membrane & ER form a

continous system

Ø  Membrane flow: a regular “flow” of membranes

between different organelles and plasmalemma

Ø  The original concept of organelles being more or less separate and autonomous, does not hold true any more

Ø  All these components are part of an endomembrane system involved in the synthesis of cell products, their modification in storage and eventually their final deposit (e.g. secretion) 20

Ø Small membrane bound vesicles

Ø Lower pH (5.5) than the cytosol (pH 7.0)

Ø Pinch off from Golgi body

Ø Contain digestive enzymes (hydrolytic enzymes)

Ø Function: intracellular digestion

Ø Primary lysosome = high pH

Ø Secondary lysosome = low pH

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Ø Digest contents of food vesicles

Ø White blood cells digest engulfed bacteria

Ø Digest old and worn-out cell components – cell

rejuvenation

Ø Digest worn-out cells – auto-digestion or autolysis

q Disappearance of a tadpole’s tail and the membranes between the fingers of a human embryo

q Acrosome of the sperm cell digest the layer surrounding the egg cell

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Ø Every cell contains a large specialised lysosome – the acrosome

Ø When the acrosome

touches the egg

cell, the enzymes of the acrosome are released – to digest the layer

surrounding the egg cell

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Two types present

PEROXISOMES

and

GLYOXYSOMES

Ø Both are enzyme-bearing, membrane bound vesicles

Ø The distribution of enzymes into microbodies is one of the principal ways in eukaryotic cells to organize cell

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metabolism

Ø Resemble lysosomes, also enzyme-bearing

vesicles

Ø But they contain specific enzymes

Ø Enzymes that transfer H2 to O2 forming H2O2

(hydrogen peroxide)

Ø H2O2 is poisonous – will immediately be broken

down to water (H2O) and O2 by enzyme catalase

Ø Abundant in liver cells

Metabolize alcohol – thus detoxifying it

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Ø Similar in structure to lysosomes and peroxisomes

Ø Microbodies present in lipid-storing seeds

Ø Concerned with the digestion of lipids

Ø During germination, enzymes in the glyoxysomes convert lipids to sugars

Ø This process will last just long enough for the seedling to emerge and start to photosynthesize

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