The body and its Constituents- CELL AND TISSUES

Cells

Cells are the basic structural, functional and biological unit of all known living organisms.

Cell membrane

This membrane serves to separate and protect a cell from its surrounding environment and is made mostly from

·        A double layer of proteins and lipids,

·        Fat-like molecules.

Embedded within this membrane are a variety of other molecules that act as channels and pumps, moving different molecules into and out of the cell.

Mitochondria

Mitochondria are the cell’s power producers. It converts energy into forms that are usable by the cell. It is located in the cytoplasm. Mitochondria are also involved in cell processes, like cell division and growth and cell death.

Endoplasmic reticulum

The endoplasmic reticulum is formed an interconnected network of membrane vesicles. The endoplasmic reticulum is classified into two types,

·        Rough endoplasmic reticulum (RER) the rough endoplasmic reticulum is studded with ribosomes on the cytosolic face. These are the sites of protein synthesis. The rough endoplasmic reticulum is predominantly found in hepatocytes where protein synthesis occurs actively

·        Smooth endoplasmic reticulum (SER) the smooth endoplasmic reticulum is a smooth network without the ribosomes. The smooth endoplasmic reticulum is concerned with lipid metabolism, carbohydrate metabolism, and detoxification. The smooth endoplasmic reticulum is abundantly found in mammalian liver and gonad cells.

Nucleus

The nucleus is a membrane-enclosed organelle. It organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The function of the nucleus is to maintain the integrity of these genes and to control the activities of the cell by regulating gene expression, the control center of the cell.

Cytoplasm

The cytoplasm is basically the substance that fills the cell. It is a Jelly-like material that is 80% water and is usually clear in color. Cytoplasm, which can also be referred to as cytosol, means cell substance. The cytoplasm is found inside the cell membrane, surrounding the nuclear envelope and the cytoplasmic organelles.

Lipid bilayer

The lipid bilayer is a thin polar membrane made of two layers of lipid molecules that keep ions, proteins and other molecules where they are needed and prevents them from diffusing into areas where they should not be. The lipid the layer is made up of three types of lipid molecules:

·        Phospholipids (75%),

·        Cholesterol (20%)

·        Glycolipids (5%).

Membrane proteins

Membrane proteins are

Integral proteins extend through the lipid layer into the cytosol of the cell. Thus some of the small molecules can pass from the extracellular fluid through to the intracellular fluid.

Peripheral proteins do not go through the lipid layer. They are more associated with the polar heads of both outer and inner surfaces of the membrane.

Functions of the plasma membrane

The cell membrane gives shape to the cell. The primary function of the plasma membrane is to protect the cell from its surroundings. Composed of a phospholipid bilayer with embedded proteins, the plasma membrane is selectively permeable to ions and organic molecules and regulates the movement of substances in and out of cells.

Osmosis

Osmosis is the movement of solution from an area of high volume to an area of low volume through a selectively permeable membrane.

Solutions are described as  

Isotonic: - an isotonic solution refers to two solutions having the same osmotic pressure across a semipermeable membrane. This state allows for the free movement of water across the membrane without changing the concentration of solutes on either side

Hypotonic: - A hypotonic solution is any solution that has a lower osmotic pressure than another solution. In the biological fields, this generally refers to a solution that has less solute and more water than another solution.

Hypertonic: -a hypertonic solution is a particular type of solution that has a greater concentration of solutes on the outside of a cell when compared with the inside of a cell.

Diffusion

Diffusion is the net movement of molecules from an area of high concentration to an area of low concentration.

Facilitated diffusion

Facilitated diffusion is a type of passive transport that allows substances to cross membranes with the assistance of special transport.

Active transport

Inactive transport, the high energy bond in ATP (adenosine triphosphate) provides the energy needed to move ions or molecules across the membrane.

Secondary active transport

Secondary active transport is the transport of molecules across the cell membrane utilizing energy in other forms than ATP. This energy comes from the electrochemical gradient created by pumping ions out of the cell. This Co-Transport can be either via antiport or symport.

Endocytosis and exocytosis

Endocytosis is an energy-using process by which cells absorb molecules (such as proteins) by engulfing them. Endocytosis occurs in three different ways:

 I Phagocytosis: Pseudopodia engulf the particle to be imported to create a food vacuole. Once inside the cell, a lysozyme containing digestive enzymes will fuse with the food vacuole.

Ii Pinocytosis: The cell membrane pinches in to engulf a portion of extracellular fluid containing solutes required by the cell. This process is non-specific; any solutes in the solution will be engulfed.

Iii Receptor-mediated endocytosis: This process allows the intake of large quantities of molecules that may not be in high concentration in the extracellular fluid.

Tissues

Tissues are made up of large numbers of cells and are classified according to their size, shape, and functions. With each tissue type, there are wide variations in their cellular morphology as well as their function. Generally, tissue types are made up of similar cells carrying out related functions, for example, the epidermis of the face and the lining of the mouth are the same tissue type and have related functions. There are four main types of tissues

 • Epithelial tissue

• Nervous tissue

• Connective tissue

• Muscle tissue.

Epithelial tissue

Epithelial tissue also known as epithelia, is located in the covering of external and internal surfaces of the body, the hollow organs, and tubes, it is also found in the glands.

1)      Simple squamous epithelia.

2)      Simple cuboidal epithelia.

3)      Simple columnar epithelia.

4)      Stratified squamous epithelia.

5)      Stratified cuboidal epithelia.

6)      Stratified columnar epithelia.

THE FUNCTION OF THE EPITHELIUM

The function of the epithelium is to provide protection and impermeability to the covered structure. The cells are closely packed and the matrix (the intracellular substance) is minimal. There is usually a basement membrane on which the cells lie.

Nervous tissue

Nervous tissue is made up of neurons and glial cells. The function of the nervous tissue is to receive and to transmit neural impulses (reception and transmission of information).

There are two types of tissue

Excitable cells (the neurons – they initiate, receive, conduct and transmit information)

Non-excitable cells (the glial cells – these support the neurons). A neuron (the basic unit of nervous tissue) consists of two major parts, the cell body containing the neuron’s nucleus, cytoplasm and other organelles. Axons usually terminate at a synapse through which the signal is sent to the next cell, usually through a dendrite.

Connective tissue

There are a number of varieties of connective tissue, it is the most abundant type of tissue; the typical function of connective tissue is to fill empty spaces among other body tissues.

This function is associated with the ability of the cells of the connective tissue to secrete substances that compose extracellular material, such as collagen and elastic fibers, creating a significant spacing between these cells.

Connective tissue (excluding blood) is found in organs supporting specialized tissues.

Adipose tissue is found supporting the kidneys, brain and the eyes and is related to energy intake and expenditure.

Lymphoid tissue contains reticular cells and white blood cells and is found in lymph tissue in the lymph nodes and all lymphatic organs.

Dense connective tissue, fibrous tissue (made up of closely packed collagen fibers with little matrix) is found in ligaments, periosteum, muscle fascia and tendons.

Blood is a fluid connective tissue.

Cartilage is found as hyaline cartilage on the ends of the bones that form joints, the costal cartilage attaching the ribs to the sternum, forming part of the trachea, larynx, and bronchi.

Bone cells (the osteocytes) are surrounded by a matrix of collagen fibers with added strength provided by the calcium and phosphate.

Muscle tissue

Muscle tissues are tissues made of cells that permit contractions and as such generate movement. The function of the muscle tissue is to pull bones (skeletal striated muscle), to contract and move viscera and vessels (smooth muscle) as well as making the heartbeat (cardiac striated muscle). Each time we move, our heartbeats, we breathe, ingest food or urinate, muscle is involved. The muscle cells have internal structures called sarcomeres where there are myosin and actin molecules that work in creating contraction and movement.

There are three kinds of muscle in the body:

Skeletal

Cardiac muscle is not under voluntary control; the special cells of the Sino-atrial node are responsible for sending out impulses causing cardiac contraction

Smooth muscle is also known as striated muscle, it is a voluntary muscle.