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Lysosomes and Peroxisomes (the cellular digestive system)
Lysosomes and peroxisomes are often referred to as the garbage disposal system of a cell. Both organelles are somewhat spherical, bound by a single membrane, and rich in digestive enzymes, naturally-occurring proteins that speed up biochemical processes. For example, lysosomes can contain more than three dozen enzymes for degrading proteins, nucleic acids, and certain sugars called polysaccharides. Here we can see the importance behind compartmentalization of the eukaryotic cell. The cell could not house such destructive enzymes if they were not contained in a membrane-bound system.
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Centrioles
Centrioles help in the formation of mitotic appratus. Two centrioles are present in the animal cells. They are also found in some fungi and algae cells.
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Vacuoles
Vacuoles store food and waste. Some vacuoles store extra water. They are often described as liquid filled space and are surrounded by a membrane. Some cells, most notably Amoeba have contractile vacuoles, which are able to pump water out of the cell if there is too much water.
The tissue
Biological tissue is a collection of interconnected cells that perform a similar function within an organism.
The study of tissues is known as histology, or, in connection with disease, histopathology.
The classical tools for studying the tissues are the wax block, the tissue stain, and the optical microscope, though developments in electron microscopy, immunofluorescence, and frozen sections have all added to the sum of knowledge in the last couple of decades.
With these tools, the classical appearances of the tissues can be examined in health and disease, enabling considerable refinement of clinical diagnosis and prognosis.
Animal tissues
There are four basic types of tissue in the body of all animals, including the human body and lower multicellular organisms such as insects. These compose all the organs, structures and other contents.
Epithelium
Tissues composed of layers of cells that cover organ surfaces such as surface of the skin and inner lining of digestive tract. The tissues serve for protection, secretion, and absorption.
Connective tissue
As the name suggests, connective tissue holds everything together. Blood is considered a connective tissue. These tissues contain extensive extracellular matrix.
Muscle tissue
Muscle cells contain contractile filaments that move past each other and change the size of the cell. Muscle tissue also is separated into three distinct categories: visceral or smooth muscle, which is found in the inner linings of organs; skeletal muscle, which is found attached to bone in order for mobility to take place; and cardiac muscle which is found in the heart.
Nervous tissue
Cells forming the brain, spinal cord and peripheral nervous system
Epithelium is a tissue composed of a layer of cells. Epithelium lines both the outside (skin) and the inside (e.g. intestine) of organisms. The outermost layer of our skin is composed of dead squamous epithelial cells, as are the mucous membranes lining the inside of mouths and body cavities. Other epithelial cells line the insides of the lungs, the gastrointestinal tract, the reproductive and urinary tracts, and make up the exocrine and endocrine glands.
Functions of epithelial cells include secretion, absorption, protection, transcellular transport, sensation detection, and selective permeability. Endothelium (the inner lining of blood vessels) is not related to epithelium apart from by name
Connective tissue is any type of biological tissue with an extensive extracellular matrix and often serves to support, bind together, and protect organs. There are four basic types:
Bone contains specialized cells called osteocytes embedded in a mineralized extracellular matrix, and functions for general support.
Blood functions in transport. Its extracellular matrix is the blood plasma, which transports dissolved nutrients, hormones, and carbon dioxide in the form of bicarbonate. The main cellular component is red blood cells.
Cartilage makes up virtually the entire skeleton in the osteichthyes. In most other vertebrates, it is found primarily in joints, where it provides cushioning. The extracellular matrix of cartilage is composed primarily of collagen.
Connective tissue proper
Dense connective tissue or Fibrous connective tissue forms ligaments and tendons. Its densely packed collagen fibers have great tensile strength.
Loose connective tissue or Areolar connective tissue holds organs and epithelia in place, and has a variety of proteinaceous fibers, including collagen and elastin. It is also important in inflammation.
Reticular connective tissue is a network of reticular fibers (fine collagen) that form a soft skeleton to support the lymphoid organs (lymph nodes, bone marrow, and spleen.)
Adipose tissue contains adipocytes, used for cushioning, insulation, lubrication (primarily in the pericardium) and energy storage.