Question

Describe the chemical composition of each structures below. (Note that the chemical composition for many of the structures listed is a phospholipid bilayer.)

cell wall

chromatin

cytosol-

chloroplasts

chromoplasts

cytoskeleton

Golgi apparatus

leucoplasts

mitochondria

nuclear envelope

plasma membrane

peroxisomes

ribosomes

rough endoplasmic reticulum

smooth endoplasmic reticulum

tonoplast

Answer 1

Phospholipid bilayer:

• The plasma membrane is composed mainly of phospholipids, which consist of fatty acids and alcohol.
• The phospholipids in the plasma membrane are arranged in two layers, called a phospholipid bilayer.
• Each phospholipid molecule has a head and two tails. The head “loves” water (hydrophilic) and the tails “hate” water (hydrophobic).
• The water-hating tails are on the interior of the membrane, whereas the water-loving heads point outwards, toward either the cytoplasm or the fluid that surrounds the cell.
• Molecules that are hydrophobic can easily pass through the plasma membrane, if they are small enough, because they are water-hating like the interior of the membrane.
• Molecules that are hydrophilic, cannot pass through the plasma membrane without help—because they are water-loving like the exterior of the membrane, and are therefore excluded from the interior of the membrane.

Chemical composition of cell wall:

• Cell wall, made of major carbohydrates they are cellulose, hemicellulose and pectin.
• The cellulose microfibrils are linked via hemicellulosic tethers to form the cellulose-hemicellulose network, which is embedded in the pectin matrix.
• The outer part of the primary cell wall of the plant epidermis is usually impregnated with cutin and wax, forming a permeability barrier known as the plant cuticle.
• Secondary cell walls contain additional compounds that modify their mechanical properties and permeability. The major polymers that make up wood (largely secondary cell walls) include:
  • cellulose, 35-50%
• xylan 20-35%, a type of hemicellulose
• lignin10-25%, a complex phenolic polymer that penetrates the spaces in the cell wall between cellulose, hemicellulose and pectin components, driving out water and strengthening the wall.
• The relative composition of carbohydrates, secondary compounds and proteins varies between plants and between the cell type and age.
• Plant cells walls also contain numerous enzymes, such as hydrolases, esterases, peroxidases, and transglycosylases, that cut, trim and cross-link wall polymers.
• Secondary walls - especially in grasses - may also contain microscopic silica crystals, which may strengthen the wall and protect it from herbivores.

Chemical composition of Chromatin:

• Chromatin is composed of DNA, RNA and proteins
• Constitution of DNA, RNA and protein vary in different organisms and in the different cells of the same organism.

The primary functions of chromatin are 1) to package DNA into a more compact, denser shape, 2) to reinforce the DNA macromolecule to allow mitosis, 3) to prevent DNA damage, and 4) to control gene expression and DNA replication.

Chemical composition of cytosol:

• The cytosol or cytoplasmic matrix is the liquid found inside cells. It constitutes most of the intracellular fluid.
• The cytosol consists mostly of water, dissolved ions, small molecules, and large water-soluble molecules (such as proteins).

Chemical composition of chloroplasts:

• Chloroplasts contain proteins, lipids, and chlorophyll, carotenoids, RNA and DNA. Starch granules or osmiophillic droplets are also present in it.
• Proteins form a part of chloroplast membrane and lamellae, and are also present in the matrix in the form of enzymes.
• Lipids are mainly phospholipids, fats, sterols and waxes found in the lamellae and wall of the plastid.

Chemical compositon of Cytoskeleton:

Cells to maintain their structure and shape has cytoskeleton

• Three main components: microtubules, intermediate filaments, and microfilaments.
• Microtubules are made of protein tubulin
• Intermediate filaments are thin thread like proteins. Eg keratin
• Microfilaments are made up of protein actin.

Chemical composition of Golgi complex:

• Golgi complex in some cells it is compact and limited, in others it is reticular.
• Golgi complex contains phospholipids in the form of phosphatidylcholins, whereas that of plant cells contains phosphatidic acid and phosphatidyl- glycerol.
• The Golgi apparatus also contains a variety of enzymes, such as glycosyl trans­ferases oxireductases, etc.

Chemical composition of leucoplasts:

• leucoplasts are not green and are located in non-photosynthetic tissues of plants, such as roots, bulbs and seeds. They may be specialized for bulk storage of starch, lipid or protein and are then known as amyloplasts, elaioplasts, or proteinoplasts.

Chemical composition of Mitocondria:

• A mitochondrion contains outer and inner membranes composed of phospholipid bilayers and proteins.
• It contains large numbers of integral membrane proteins called porins.
• Mitochondria has its own DNA and proteins for its function.

Chemical composition of Nuclear envelope:

• is the lipid bilayer membrane which surrounds the genetic material and nucleolus in eukaryotic cells.
• The nuclear membrane consists of two lipid bilayers—the inner nuclear membrane, and the outer nuclear membrane. The space between the membranes is called the perinuclear space.
• Nesprin proteins connect cytoskeletal filaments to the nucleoskeleto. Nesprin-mediated connections to the cytoskeleton helps nuclear positioning.
• Nuclear membrane has many holes called nuclear pores.

Chemical composition of plasmamembrane:

• The cell membrane is a biological membrane that separates the interior of all cells from the outside environment.
• It consists of a lipid bilayer with embedded proteins.
• The cell membrane consists of three classes of amphipathic lipids: phospholipids, glycolipids, and sterols. The amount of each depends upon the type of cell,

Chemical composition of peroxisomes:

• It is a type of organelle known as microbody
• They are involved in catabolism of fatty acids
• Peroxisomes are also made of phospholipid bilayer

Chemical Composition of Ribosomes:

• Ribosomes consist of two major components: the small ribosomal subunit, which reads the RNA, and the large subunit, which joins amino acids to form a polypeptide chain.
• Each subunit is composed of one or more ribosomal RNA (rRNA) molecules and a variety of ribosomal proteins.
• The ribosomes and associated molecules are also known as the translational apparatus.

Chemical Composition of rough and smooth endoplasmic reticulam:

• The general structure of the endoplasmic reticulum is a network of membranes called cisternae. These sac-like structures are held together by the cytoskeleton.
• It is also made of phospholipid bilayer.
• Endoplasmic reticulam with ribosomes attached to its surface forms rough endoplasmic reticulam.

Chemical composition of tonoplast:

• The single layered membrane surrounding the vacuole is tonoplast.
• Composed of lipid, proteins and carbohydrate.
• But the, composition of this membrane may vary to plasma membrane or any other membrane of the cell.