1. Membraneous Cell Organelles
1.1 Nucleus
It is the largest organelle and can be seen by a light microscope. It is found in most cells except mammalian red blood cells and mature phloem sieve tubes. Spherical in shape and sometimes oval, it spans about 5-10 microns. It contains DNA and RNA and controls cell activities. A double membrane called the nuclear envelope, surrounding the nucleus, consists of numerous pores for transport of large macromolecules and ribosomal subunits to the cytoplasm. The nuclear envelope is perforated by pores which allow exchange of substances between the nucleus and cytoplasm. The double membrane is separated by a perinuclear space. The outer membrane is sometimes continuous with the endoplasmic reticulum (ER).
Within the nucleus is a matrix composed of proteins, metabolites and ions making up the nucleoplasm. The nucleus contains chromatin (composed of DNA bound to basic proteins) which is found in the nucleoplasm. Euchromatin is loosely coiled chromatin dispersedly located towards the centre of the nucleus and is generally active (found in cells in which a variety of genes are being expressed). Heterochromatin are tightly coiled and stained intensely, occurring near the nuclear envelope. The nucleus contains the nucleolus, which is a spherical structure within itself and is composed of DNA, RNA and protein. The function of the nucleolus is to manufacture rRNA.
1.2 Endoplasmic Reticulum (ER) – Rough & Smooth
Endoplasmic means within the cytoplasm. Reticulum means network. The ER consists of a network of membranous sacs called cisternae. It is very extensive and accounts for more than half of the total membrane in a eukaryotic cell. Rough ER (RER) consists of flattened, membrane-bound sac. The presence of ribosomes on its outer surface makes it rough. The RER is the site of protein synthesis and the protein made here is for export. Smooth ER (SER) lacks ribosomes and is more tubular. SER is sometimes called agranular ER. It functions in diverse metabolic processes including synthesis of lipids, metabolism of carbohydrates and detoxification of drugs and poisons. Completion of secondary and tertiary structures and the modification of proteins occur in the ER (e.g. addition of carbohydrates and assembly of multi-chain proteins). SER produce steroids like hormones and synthesizes phospholipids. The ER forms a transport system to move materials from one part to another. Muscle cells have a special type of ER called the sacroplasmic reticulum.
1.3 Golgi Apparatus
The GA consists of a stack of flattened and curved membranous sacs. It is where post-transitional modifications are complete and proteins are packaged for export or storage. The GA modifies polypeptides received from the RER and add carbohydrates most of the time. They are involved in the process of manufacturing, warehousing, sorting and shipping of materials. GA adds direction for destination of protein package which is very important in the secretory cells of the pancreas. New cisternae are constantly being formed at one end of the stack by fusion of vesicles (cis end). While at the trans end, cisternae break up into vesicles.
1.4 Lysosomes
They are membrane bound vesicles (0.05-0.5 microns) found in phagocytic cells and are characteristically homogeneous in appearance. They contain hydrolytic enzymes that can digest most biological macromolecules. The lysosome contents are acidic and the enzymes have a low optimum pH (pH5). The enzymes have to be kept apart from other cells or they would destroy it. The enzymes are not active in neutral environments. The lysosome main function is the breaking down of molecules.
It is formed from the budding of vesicle from the trans face of the GA. During phagocytosis, a bacterium is taken up into the cytoplasm inside a vesicle. The lysosome then fuses and discharges its contents into this vesicle to digest the bacterium. Autophagy (autolysis) is a process whereby the lysosome engulfs damaged organelles and recycles the molecular ingredients of the organelles. Inherited disorders such as lysosomal storage disease affecting lysosomal metabolism are e.g. Tay Sachs disease where lipid-digesting enzyme is inactive, resulting in accumulation of lipids leading to impairment of the brain.
1.5 Vacuoles
Vacuoles and vesicles are both membrane bounded sacs within cells but vacuoles are larger that vesicles. Vacuoles are found in both plant and animal cells; small vacuoles found in animal cells. Mature plant cells contain large central vacuoles enclosed by a membrane called the tonoplast. Vacuoles are filled with a fluid which is comprised of concentrated solution of mineral salts, sugars, organic acids and pigments. Plant vacuoles are places to store organic compounds and can also act as a disposal site for metabolic by-products that would endanger the cell if left accumulated in the cytosol. They may contain coloured pigments which attract pollinating insects and some contain poisonous or unpalatable compounds to prevent predators from eating them. In plants, as vacuoles absorb water, it elongates, contributing to the increase in size of the cell with minimal investment in new cytoplasm, increasing the ratio of membrane surface to cytoplasmic volume.
1.6 Peroxisome
They are specialized metabolic compartments bounded by a single membrane, in which they contain enzymes that transfer hydrogen from various substrates to oxygen, producing hydrogen peroxide as a by-product. Peroxisomes in the liver detoxify alcohol and other harmful compounds by transferring hydrogen from the poisons to oxygen. The hydrogen peroxide formed is itself toxic but the peroxisome contains an enzyme that converts it to water. Unlike lysosomes, peroxisomes do not bud from the endomembrane system. They grow by incorporating lipids and proteins made in the cytosol and increase in number by splitting into two when they reach a certain size.
1.7 Mitochondria
Mitochondria are found in both plants and animals and they are the sites of cellular respiration, a process that generates ATP. They are not considered part of the endomembrane system because their membrane proteins are not synthesized by the ER. They are about 1.5-10 microns in length. They are smaller than chloroplasts and are not visible under LM. Each mitochondrion is bounded by an outer membrane surrounding an inner membrane that folds into scaffolding (cristae). The outer and inner membranes are separated by an inter-membranous space of 8 nm. The mitochondrial matrix is gel-like and homogeneous and contains enzymes, DNA, RNA and ribosomes. Mitochondrial DNA is double-stranded and circular. Mitochondria are the power house of the cell and contains the molecular machinery for the conversion of energy from the breakdown of glucose into ATP.
The structure of the mitochondria is for compartmentalization for the establishment of a proton gradient for cellular respiration. The inner membrane is organized into folds which increase the surface area for the attachment of enzymes required for cellular respiration. The inner fluid matrix is the site of the Krebs cycle.
1.8 Chloroplasts
Chloroplasts are plastids found in plants. It is a double layered membrane organelle present in photosynthetic cells and are 2-5 microns in length. The inner membrane of the chloroplast encloses a semi-fluid material called stroma. The disc-like structures are called thylakoids which stack up to form grana. The photosynthetic pigments are arranged on to grana and are connected to each other within the stroma by lamella (thylakoid membranes that are not stacked up to form grana).
2. Non-Membranous Cell Organelles
2.1 Ribosomes
They are about 20 nm in length and are found in both animals and plants. They may occur free in the cytoplasm or bound to the surface membrane of the ER. Ribosomes are the site of protein synthesis. Cells that have a high rate of protein synthesis have a few million ribosomes. Most of the proteins made by free ribosomes will function within the cells while those made on bound ribosomes will be exported out of the cell.
2.2 Centrioles
They are found only in animal cells and there is usually one pair except for dividing cells which will have 2 pairs. Centrioles are made up of microtubules which are tube-like structures made up of tubulin. Centrioles are small hollow cylinders (0.3-0.5 microns long and 0.2 microns in diameter). They are located near the nucleus in the region called the centrosome. Each centriole consists of 9 triplets of microtubules. Before each cell division, each centriole replicates itself and participates in cell division as microtubule organizing centres. During mitosis, a pair of centriole moves to opposite poles of the cell to become organizing centres for the mitotic spindle.
2.3 Cytoskeleton
2.3.1 Microtubules
They are found in the cytoplasm and are straight hollow rods measuring about 25 nm in diameter. The wall of the hollow tubule is constructed from a globular protein called tubulin. Microtubules elongate by adding tubulin molecules to its ends. It shapes and supports the cell and serve as tracks along which organelles can move. They are also involved in the separation of chromosomes during cell division. Centrioles, cilia and flagella are made of microtubules.
Both cilia and flagella are slender extensions of the plasma membrane. Each cilium and flagellum contains a ring of nine fused pairs of microtubules with an unfused pair in the centre of the ring. The main difference between the two lie in the number, length and direction of force they generate. Cilia are short and numerous and they provide a force in the direction parallel to the plasma membrane. Examples of cilia are those lining the trachea which conducts air into the lungs as they sweep out mucus and trapped particles. Flagella are long and few in number and provide a force perpendicular to the plasma membrane.
2.3.2 Microfilaments
They are made of solid rods of about 7 nm in diameter and are also called actin filaments because they are built from actin. They are twisted double chain of actin subunits. They are involved with the movement of vesicles, granules and cytoplasmic organelles. They help to support the cell and maintain the shape of the cell.
2.3.3 Intermediate Filaments
They are so called because the diameter which at 8-12 nm, is larger than the diameter of a microfilament but smaller than that of a microtubule. Intermediate filaments are made up of keratin. They are especially important in reinforcing the shape of the cell and fixing the position of certain organelles. The nucleus commonly sits in a cage made of intermediate filaments fixed in location by branches of the filaments that extend into the cytoplasm.
2.4 Plant Cell Wall
This is a feature that distinguishes a plant from an animal cell. It is found external to the plasma membrane. It protects the plant cell, maintains its shape and prevents excessive uptake or loss of water. A young plant first excretes a relatively thin and flexible wall called a primary cell wall. Between primary walls of adjacent cells is the middle lamella, a thin layer rich in polysaccharides called pectins. The middle lamella glues the cells together. As the cell matures, it adds a secondary cell wall between the plasma membrane and the primary wall.
2.5 Inter-cellular Junctions
Many cells are integrated into one functional organism. Neighbouring cells often adhere, interact and communicate through special patches of direct physical contact.
2.5.1 Junctions in Plants
Cell walls in plants are perforated with channels called plasmodesmata. The plasma membrane of adjacent cells is continuous through a plasmodesma. Water and small solutes can pass through from cell to cell.
2.5.2 Junctions in Animals
There are 3 types of inter-cellular junctions namely desmosomes, tight junctions and gap junctions. The mobility of animals will cause their tissues to stretch, compress and bend. If their vital organs are not to tear apart under stresses of movement, their cells must adhere firmly to one another. Thus their tissues have junctions called desmosomes which hold adjacent cells together. Animal bodies contain tubes and sacs that must hold their contents without leaking, thus they need spaces between these cells to be sealed with tight junctions. Many cells communicate through protein channels that directly connect the insides of adjacent cells and these cell to cell channels are called gap junctions.
