Chapter 2: Cell Structure & OrganellesStructure & Functions of Organelles

bY SHALENI KAVIRAJAN

STPM BIOLOGY

GOLGI APPARATUS are stacks of flattened, hollow cavities enclosed by

membranes, which are often continuous with the membranes of the ER

usually located near to the nucleus & ER

these stacks of flattened sacs / cavities are called cisternae. Each sac in the organelle contains enzymes that modify proteins

Functions in modification, assembly, packaging, storage and secretion of substances.

Golgi Apparatus

Cis-end(convex) receives newly manufactured proteins from ER, stores & modifies the proteins through glycosylation & adds sugar molecules to form glycoprotein.The modified proteins are then transported into vesicles that bud off at the trans-end(concave) carrying secretion of protein, glycoprotein / lipid. The contents are released to the cell’s exterior by exocytosis.

LYSOSOMES

special vesicles(spherical) which are formed by the Golgi apparatus.

The membrane(single layer of lipoprotein) is resistant to digestion & prevents the release of enzymes which would destroy the cell contents

Contain high conc. of hydrolytic enzymes(ribonucleas, deoxyribonucleas, carbohydrase, protease, lipase & phosphatase) at pH value of 4 to 5(acidic)

functions in : 1) cellular digestion 2) autophagy or disposal of damaged cell/worn out components like mitochondria 3) breakdown of a whole cell/autolysis (ex: happens in the tail of tadpole where the digestion products are used for building lungs & adult skin). For this reason, they are sometimes called “suicide sacs.”

are known to contain over 40 different enzymes that can digest almost anything in the cell,including proteins, RNA, DNA, and carbohydrates.

also appear to perform other digestive processes, such as those connected with phagocytosis and pinocytosis.

Lysosomes also helps in destroying bacteria invasion

RIBOSOMES Found attached to the ER forming RER Occur singly or group together forming polysomes consist of rRna(produced in the nucleolus) and

proteins each ribosome is made of 2 non-identical subunits –

large & small (spheroid in shape)

Provide site for synthesis of proteins(formation of peptide bonds - amino acid are joined to form polypeptide/protein)

Ribosome Structure – Front View Large Subunit { Stalk, Centra Ridge, Wing} Small Subunit { Platform, Cleft,, Head}

CHLOROPLAST large plastids found only in the photosynthetic tissues of

plant cell these are the double-membrane bound organelles

has a system of membranous flattened sacs(thylakoids) stacked to form granum(where chlorophyll is located).

site of photosynthesis

Intergranal lamella connects one thylakoid of a granum to another granum. The fluid outside the thylakoid is the stroma.

It(grana & thylakoid membrane) provides large surface area for chlorophyll & accessory pigments to be located – traps light energy & carry out production of ATP & NADPH

stroma – has enzymes to carry out light-independent reaction(Calvin cycle) forming high energy organic molecules such as ; sugar molecules

CYTOSKELETON The network of filamentous proteins structures

within the cell that determines the 3-D shape of animal cells & give certain firmness in the plant cells

The primary constituents of the cytoskeleton are microtubules, microfilaments and intermediate filaments

MICROTUBULES are fine, hollow tubes about 25nm

contains 2 types of globular proteins, and tubulin. made up of 13 rows of globular proteins arranged to form a hollow tube

due to continuous polymerising and depolymerising the length of microtubules changes thus causing it(the cell) to be unstable sometimes

Functions in :i. cell wall formationii. Serve as tracks for intracellular movement of organelles/vesicles (causes movement)iii. Main component of cilia & flagella in cell motility(movement)iv. gives shape & mechanical support to the cell

MICROFILAMENT

long, thin, contractile rods(6-7 nm in diameter) that appear to be responsible for the movement of cells (both external and internal movement).

composed of one/two types of protein-actin(found in skeletal muscles as thin filaments) & myosin(found in skeletal muscles as thick filaments)

Microfilament consists of two intertwined strands of actin

Exists in bundles & found in layers in the cytoplasm dynamic - can change their length very

quickly(depends on their location & functions)

Functions in:

i. assist in the cleavage process during cytokinesis

ii. role in cellular movementsExample: cytoplasmic streaming, cell motility – phagocytosis & pinocytosis

iii. maintains cell shape

INTERMEDIATE FILAMENTS diameter between 8 – 12 nm ( bigger than microfilaments

but smaller than microtubules) found only in animal cells made up of 4 long strands of - helix coiled fibrous

proteins, consisting only secondary coiled polypeptide very stable & branched, forming a network of

cytoskeleton in the cytoplasm & nucleus has its own arrangement and types of protein

ex: cells in the skin epithelium/forming the nail, hair & horn have keratin filaments which differs from those in the muscle and nerve cells

Functions in:i. maintains shape of the cellii. distribute the organelles & support them in the cytoplasm