Endoplasmic reticulum (EM) (x94000 app.)

4.2.6 Endoplasmic reticulum - an elaborate system of membranes forming a cytoplasmic skeleton - an extension of the out nuclear membrane Cisternae: flattened sacs formed by the membranes Rough endoplasmic reticulum: membranes are lined with ribosomes Rough ER is concerned with protein synthesis and is abundant with cells which are rapidly growing or secreting enzymes Smooth endoplasmic reticulum: membranes lack ribosomes

Functions of the ER: 1 Provides a large surface area for chemical reactions 2 Provides a pathway for the transport of materials through the cell 3 Produces proteins, especially enzymes 4 produces lipids and steroids 5 Collects and storing synthesized materials 6 Provides a structural skeleton to maintain cellular shape (e.g. smooth ER of a rod cell of retina)

Golgi apparatus of an olfactory bulb cell Golgi apparatus (EM) (x30000 app.)

4.2.7 Golgi apparatus (dictyosome) - similar to smooth ER but is more compact vesicles: small membranous sacs pinching off from the cisterna - normally only one Golgi apparatus in animal cells but a large number of stacks known as dictyosomes in plant cells - well developed in secretory cells and neurones for secretory functions

Functions: 1 adding carbohydrates to proteins to form glycoprotiens 2 produce secretory enzymes, e.g. pancreatic enzymes 3 produce new cell walls by secreting carbohydrates 4 transport and store lipids 5 form lysosomes

4.2.8 Lysosomes - bounded by a single membrane which contains enzymes (mostly hydrolases) - abundant in secretary cells & in phagocytic white blood cells

1 Digest material which the cell consumes from the outside by phogocytosis, e.g. white blood cells digest bacteria, amoeba digests food 2 Digest part of a cell or worn-out organelles, autolysis for dead cells 3 Release enzymes outside the cell (exocytosis) to break down other cells, e.g. reabsorption of tail of tadpoles during metamorphosis Therefore, they are abundant in secretory cells and phagocytic cells.

- small spherical membrane-bounded bodies; slightly granular structure 4.2.9 Microbodies - small spherical membrane-bounded bodies; slightly granular structure - contains enzymes (mostly catalase) to break down H2O2: 2 H2O2  2 H2O2 + 2O2 - H2O2 is a potentially toxic by-product of many biochemical reactions in cell, particularly active cells like the liver cells A B C D E nucleus A: heterochromatin B: euchrromatin C: nucleolus D: nuclear pores E: microbody

4.2.10 Vacuoles - tonoplast: single membrane around the vacuole - containing a solution of mineral salts, sugars, amino acids, wastes and sometimes anthocyanins - large and central in plants ; numerous but small in animal cells, e.g. food vacuole, phagocytic vacuole and contractile vacuole

Functions: 1 temporary food reservoir of sugars & amino acids 2 anthocyanins (coloured pigments) attract pollinating insects, animals for fruit dispersal 3 temporary stores for wastes, e.g. tannins, and to be removed through leaf cells when leaves fall 4 occasionally contain hydrolytic enzymes to act as lysosomes 5 provide turgidity to herbaceous plants & seedlings

Non-membranous Structures 4.2.11 Ribosomes - small cytoplasmic granules of RNA molecules for protein synthesis - around 20 um in diameter in eukaryotic cells (80S) but slightly smaller in prokaryotic cells (70S) - account for 20% of the mass of a cell polysomes: ribosomes occur in groups assciated with rough ER or occur freely within cytoplasm

4.2.12 Storage granules - soluble form as sugars in vacuoles of plant cells - insoluble forms: starch grains - in chloroplasts & cytoplasm; in specialized leucoplast (amyloplast) glycogen granules – in animals cells lipid droplets – both plant and animal cells Leucoplast are colourless plastids (double membraned organelles in plants)