Plants have a cell wall that is made up of cellulose. It is a nonliving structure. In addition to being permeable to water and other plant nutrients, the wall supports and protects the thin inner membrane from damage. It also gives support to the non-woody plant organs. The large vacuole in plant cells is involved in the elongation of the plant.
Cytoplasm
Cytoplasm occupies most of the internal part of the cell. It has a consistency of an egg white and has the role of anchoring the organelles. Cytoplasm consists of about 80% water, the rest mainly protein and dissolved salts. Apart supporting the organelle, it also contains grains of stored food: starch in plants, glycogen in animals.
Nucleus
The nucleus is enclosed in a nuclear membrane that has pores for the exchange of substances with the cytoplasm. The nucleus controls the all the activities of the cell and indeed of the entire organism. One of its major roles is the synthesis of proteins that takes place in the cytoplasm. Within the nucleus are one or more nucleoli (singular nucleolus) which make the ribosomal RNA.
Ribosomes
Ribosomes are minute organelles that are found in the cytoplasm of the cell.
Their role is to assemble amino acids into proteins in the rough endoplasmic
reticulum.
Endoplasmic Reticulum (ER)
Consists of smooth and rough endoplasmic reticulum (ER). The smooth
ER does not have ribosomes while the rough ER has ribosomes.
The ER is composed of channels that run throughout the cytoplasm that are used for
transporting substances. It is the major site in the synthesis of protein in the cell.
Golgi Apparatus
The Golgi bodies are vesicles that are particularly abundant in secretory cells such as those of the pancreas and mammary cells. Their main function is the storage and transportation of cell secretions received from the ER. Complex compound such as lipoproteins, glycoproteins and carbohydrates are manufactured in the Golgi bodies. In addition, the Golgi bodies act as transit warehouses where substances are stored and then sorted out before being sent to their destination.
Lysosomes
These are sacs, which bud off the ER or Golgi apparatus and contain many different types of enzymes. They are important in digesting bacteria and worn out organelles and any other debris in the body. Body secretions such as sweat, saliva, etc., contain lysosomes.
Vesicles and Vacuoles
These are membrane bound sacs within the cell. Vacuoles are larger than vesicles and contain enzymes for digesting food particles as in Amoebae. Contractile vacuoles are used to pump water out of the cell. Plant cells have large permanent vacuole while those of animals are small and temporary.
Mitochondria
The mitochondria consist of an inner and an outer membrane. It is in the mitochondrion where carbohydrates are broken down to release energy, carbon dioxide and water. Carbon dioxide is a waste product that is excreted through the lungs.
Chloroplasts
These plant bodies contain a green pigment called chlorophyll. Like the mitochondria chloroplasts consist of two membranes, an outer and an inner membrane. Chloroplasts are the major centres for the synthesis of photosynthesis, which is the process by which plants make carbohydrates. During photosynthesis oxygen, the gas we breathe in is produced, showing that plants must have existed before the animals.
TISSUES
Tissues are a group of cells with a common structure and function. Tissues can be classified into four main categories: epithelial tissue, which forms coverings and linings; connective tissue, which provides support; nervous tissue, which offers rapid communication networks between cells; and muscular tissue, which provides motion. The nerve tissue and muscle tissue will not be examined here because they will be discussed elsewhere.
Epithelium
Epithelium
The epithelium covers the outside of the body and lines organs and cavities within the body. The epithelial tissue occurs in sheets of tightly packed cells with little material between them. This tight packing is consistent with the function of the epithelium as a barrier protecting against mechanical injury, invading pathogens and fluid loss.
Different types of epithelia are classified by the shape of the cells and the number of layers that they form. A layer of epithelium that is one cell thick is called single epithelium, and a layer of two or more cells thick is called stratified epithelium. A pseudostratified epithelium is single-celled but appears stratified because the nuclei are arranged one above the other giving the illusion of stratification.
Flat epithelial cells are called squamous. Squamous epithelial cells in the top layer of the skin accumulate a hard protein called keratin and become so thin that they flake off. Squamous cells lining the mouth, oesophagus and vagina, however, do not produce keratin. Epithelium can also be cube shaped (cuboidal), or tall (columnar).
In addition to protecting the organs they line, some epithelia are specialised for absorbing or secreting chemical substances. Epithelial cells that line the lumen (cavity) of the small intestine absorb nutrients. Another kind of epithelium called mucous membrane consists of mucous glands with goblet cells that secrete a slimy solution that lubricates the surface and prevents it from drying. The digestive and respiratory tubes are lined with mucous membranes consisting of cells that secrete mucous. The free surfaces of some mucous membranes have beating cilia that move mucous along the surface. The ciliated epithelium of the respiratory system keeps the lungs clear by trapping dust, pollen and other particles.
Connective Tissue
The main function of the connective tissue is to bind and support other tissues. The cells are sparse and scattered through an extracellular matrix. This nonliving matrix consists of a web of protein fibres embedded in a homogenous ground substance that may be liquid, jellylike or solid. The main connective tissue in the vertebrate body is the loose connective tissue. It functions by binding epithelia to underlying tissues and as packing material to hold organs in place. There are three kinds of connective tissue fibres: collagenous fibres, elastic fibres and reticular fibres.
Collagenous fibres are made of collagen, the most abundant protein in the animal kingdom. Each collagen fibre consists of a bundle of several fibrils. Under the light microscope, collagen fibres appear thick, wavy and white. They have great tensile strength, that is, they cannot easily tear.
Elastic fibres are long yellowish threads that are made up of a protein called elastin. Unlike the collagen fibre that resists stretching, the elastic fibre stretches, much like a rubber band.
The reticular fibres are branched forming a tightly woven fabric that joins the connective tissue to adjoining tissues.
Fibrous Connective Tissue
The fibrous connective tissue is a dense tissue reinforced with collagen fibres. It is found in tendons that attach muscles to bones and in the ligaments that join bones together at joints.
Adipose Tissue
Adipose tissue is a specialised form of loose connective tissue that stores fat in adipose cells distributed throughout the matrix. Adipose fat insulates the body and stores fuel molecules that can be burnt to release energy.
Blood is a connective tissue that consists of a complex mixture of different cell types suspended in a matrix called plasma. Blood cells are ‘wandering’ connective tissue because they circulate through the body. Other connective tissue cells are ‘fixed’ because they do not move. An adult has 5 litres of blood.
Cartilage
Cartilage is a connective tissue that cushions organs and forms structural framework to keep tubular organs from collapsing such as in the ear, the nose and in the respiratory passages. In joints, cartilage can sustain weight while allowing bones to move against one other. Cartilage also forms the skeleton in the embryo, and it is gradually replaced with bone, which is much harder tissue. Cartilage has one cell type, the chondrocytes, lodged in spaces within a collagen matrix called lacunae. The chondrocytes have large nuclei and extensive endoplasmic reticulum. Cartilage contains also the protein fibre elastin. The strong network of collagen and elastin give cartilage great flexibility.
Cartilage is covered with a thick tough shell of collagen. It lacks nerves and blood vessels. Because it lacks blood vessels, the nutrients reach the chondrocytes by diffusion.
Bone is a connective tissue that provides maximum strength with minimum weight and protects other tissues and organs. The skull protects the brain; the ribs guard the lungs and heart and the long bones of the arms and legs house the marrow. Bones also provide points of attachment for muscles. Bone consists mainly of calcium and phosphate mineral salts.
Bone contains cells called osteocytes that are found in spaces called lacunae within the matrix. Long narrow passageways called canaliculi connect the lacunae, and osteocytes send out extensions that touch each other through the canaliculi. The canaliculi and lacunae are arranged around large channels called haversian canals.
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