THE ENDOMEMBRANE SYSTEM
- it is a grouping of some membrane organelles which function in close coordination with each other, i.e, endoplasmic reticulum,
Golgi compiex, lysosomes and vacuoles.
- Plastids, mitochondria, peroxisomes, glyoxisomes, etc., are not part of this system.
Endoplasmic Reticulum (ER)
ER was discovered by Porter and Thompson in 1945.
- ER is an interconnected system of membrane lined channels that run through cytoplasm.
- It is complicated organelle that exists in three forms - cisternae, vesicles and tubules. It is of two types- smooth endoplasmic | reticulum (SER, without ribosomes) and rough endoplasmic reticulum (RER, with ribosomes).
- RER provides surface for protein synthesis.
- The proteins in ER lumen are processed and are packed in membrane bound vesicles for storage.
- SER is involved in fat and steroidal hormone synthesis.
The Golgi apparatus was discovered by Camillo Golgi in 1898 and was also named after him.
- It is a parallel arrangement and interconnected system of a cluster of smooth membranous disc-shaped sacs or cisternae.
- Golgi complex is made up of four parts: cisternae, tubules, vesicles and Goigian vacuoles.
- One face of Golgi apparatus is convex or forming (cis face) while other is concave side or maturing face (trans face).
- Golgi apparatus helps in processing, packaging, transport and release of secretory proteins.
- Glycosylation of proteins and lipids to form glycoproteins and glycolipids.
- They act as primary lysosomes as they store digestive enzymes obtained through ER in the inactive state.
- Transformation and recycling of plasma membrane takes place here.
These were first reported by Christian de Duve in 1955 through fractionation technique. They are found in all animal cells except RBCs. In plants and fungi, their function is taken over by vacuoles.
- These are single membrane bound, small vesicular organelles containing hydrolytic enzymes.
- The organelle passes through various stages and shows polymorphism. There are four types of lysosomes (i) primary lysosome,(ii) secondary lysosome, (iii) residual bodies and (iv) autophagic vacuoles.
- They help in digestion of food obtained through phagocytosis.
- Harmful and unwanted materials are disposed off by lysosomes. Thus, lysosomes are called disposal bags or units.
- Ininjured and dead cells, the lysosome membrane ruptures spontaneously releasing the digestive enzymes or acid hydrolases that lyse the weakened cells. Therefore, these are called as ‘suicide bags’ of the cell.
Vacuoles are non-cytoplasmic areas present inside the cytoplasm.
- These are surrounded by single membrane called tonoplast.
- These are formed by expansion and pinching off from endoplasmic reticulum. They are small in animal cells and iarge in fungal and plant cells.
- Depending upon the contents and function, these are of four types - sap
vacuoles, food vacuoles, contractile vacuoles and air vacuoles.
- Vacuoles maintain osmotic pressure and turgidity and also help in osmoregulation.
- They provide buoyancy, mechanical strength and protection.
- In plants, the tonoplast facilitates the transport of a number of ions and other materials against concentration gradient.
Ribosomes were discovered by Robinson and Brown (1953) in plant cells and George Palade (1955) in animal cells,
- They are naked ribonucleoprotein protoplasmic particles (RNP) and are not surrounded by any membrane.
- Ribosomes are made up of two subunits; larger and smaller and Mg2+ is required for binding of two subunits,
- Eukaryotes have 805 ribosomes with 60S and 40S subunits and prokaryotes have 70S ribosome with 50S and 30S subunits.
- Chemically ribosomes are made up of two parts : proteins and rRNA.
- It has four sites for specific attachments : mRNAs binding site, aminoacy! site, peptidy! site and exit site.
- Ribosomes are sites for polypeptide or protein synthesis and therefore, act as protein factories.
- Free ribosomes synthesise structural and enzymatic proteins for use inside the cell.
- Provide protection to newly synthesised polypeptide by enclosing it in groove of larger subunit of ribosome.
Mitochondria are semi-autonomous organelles which were first discovered by Kolliker in 1850.
- Mitochondria are cylindrical, double membranous structure. A mitochondrion has two membranes: the outer membrane is smooth and has porin proteins and the inner mitochondrial membrane produces numerous infoldings called cristae.
- The cristae and the inner face of the inner membrane is studded with mushroom-like projections called Fo — Fy particles or oxysomes which are related to ATP synthesis during oxidative phosphorylation.
- Mitochondria are called power house of celll because they help in cellular respiration and energy generation.
- Mitochondria are the sites of aerobic respiration.
- Mitochondria help in synthesis of chlorophyll, cytochromes, pyrimidines, fatty acids, amino acids, etc.
- The term plastid was given by E.Haeckel in 1866.
- Pilastids develop from colourless precursors called proplastids. Proplastids have ability to divide and differentiate into various types of plastids.
- Plastids are pigment containing cell organelles found in all plant cell and in euglenoids. There are three types of plastids on the basis of pigments - chloroplasts, chromoplasts and leucoplasts.
- Chloroplasts are chlorophyll containing greenish plastids responsible for trapping light energy during photosynthesis. They are semi-autonomous organelles having their own DNA, enzymes and lipids. Chromoplasts are non-photosynthetic plastids but are variously coloured because of the presence of carotenoid pigments and leucoplasts are colourless plastids.
- Chloroplasts are the centre of photosynthesis.
- Chromoplasts provide colours to the flowers and fruits while leucoplasts store various nutrients like, proteins, fats, etc.
- Nucleus is the largest cell organelle, first discovered by Leeuwenhoek in red blood corpuscles of fish.
- Nucleus was first studied by Robert Brown in orchid root cells.
- The nucleus is a double membrane bound dense body that controls cellular metabolism and transmission to the posterity.
- Nuclear envelope consists of two unit membranes separated by perinuclear space.
- Nucleoplasm is transparent, semifluid and colloidal substance which fills the nucleus. It contains enzymes required for synthesis and functioning of DNA, RNA and nucleoproteins, etc.
- Nuclear matrix contains nucleolus and chromatin. Chromatin is the hereditary part of the nucleus and is differentiated into euchromatin and heterochromatin.
- Nucleclus is a naked, round or slightly irregular structure that is attached to the chromatin at a specific region called nucleolar organiser region (NOR).
- |t controls cellular activities like - growth, development, reproduction and metabolism. Ribosomes are formed in nucleolus part of the nucleus.
- It stores genetic information in DNA and transmits it to the next generation.
- it controls the synthesis of RNAs (mRNA, rRNA and tRNA).
They are microbodies, discovered by Christian de Duve and are found in both plant and animal cells.
- These are spherical, sac-like structures, bounded by a single membrane. These contain enzymes for peroxide biosynthesis.
- They are in close association with endoplasmic reticulum, mitochondria and chloroplasts.
- Animal peroxisomes metabolise a number of toxic substances like nitrite, phenol, etc.
- Plant peroxisomes found in photosynthetic cells perform photorespiration.
Glyoxysomes appear in germinating oil seeds and cells of some fungi till stored fat is consumed.
- These are microbodies that contain enzymes for B-oxidation of fatty acids and glyoxylate pathway.
- These have a single covering membrane and are considered to be special peroxisomes.
- They help in lipid storage.
- They also function in photorespiration and nitrogen fixation.
They form structural framework inside the cell and occur only in eukaryotic cells.
- An elaborate network of filamentous proteinaceous structures present in the cytoplasm is called cytoskeleton.
- Microtubules, microfilaments and intermediate filaments collectively form cytoskeleton.
- Cytoskeleton maintains shape of the cell and its extensions, regulate orientation and distribution of cell organelles.
- Microtubules help in the spindle and astral ray formation during cell division. They also form cilia and flagella.
- Microfilaments are involved in cytoplasmic streaming. Intermediate filaments helps in the formation of nuclear matrix.
Cilia and Flagella
They are fine, hair-like movable protoplasmic processes of the cells.
- Cilia and flagella are contractile outgrowths of the cell membrane. Cilia are small structures whereas flagella are comparatively longer.
- They have a microtubular composition referred to as 9 + 2 arrangement.
Both are capable of producing a current in fluid medium for locomotion and they also act as sensory organs of the cell.
Centrosome and Centrioles
Centrosome is an organelle found in animal cells only, containing two cylindrical structures called centrioles. They are surrounded by amorphous pericentriolar materials.
Both the centrioles lie perpendicular to each other and they also possess microtubular structures of 9 + O arrangement.
They polymerise microtubules for the formation of spindle fibre and astral rays during cell division. They also determine poles during the same.