The Cell (Biochemical and Biophysical Aspects)1

The cell is the basic structural and functional unit of the living organisms. Cells are composed of characteristic parts which coordinate together to perform a specific role. They are divided into two categories based on their organizational features.

The eukaryotic cell has three important parts (Fig. 1.1):

The cytoplasm is composed of:

It is the soluble portion of the cell (cell sap). Several enzymes, coenzymes, inorganic ions, amino acids, nucleotides, metabolites, RNA molecules are present in the cytosol. It is the site of many chemical (metabolic) reactions required for existence of cells.

Mitochondria are membrane bound cell organelles. Each cell contains few to several thousand number of mitochondria. They vary from cell to cell, more in metabolic active cells like liver, kidney, etc. They are spherical, oval, rod-shaped and vary in size.

It consists of two membranes:

It is a network of membranes that form flattened sacs or tubules called cisterne. It is of two types:

(Golgi bodies/Golgi apparatus)

Structure: It consists of 3–20 flattened membrane sacs with bulging edges called Golgi sacs. It was named after its discoverer Carmillo Golgi. Each cell may have one or more Golgi complexes.

Lysosomes are spherical vesicles surrounded by membranes. They are present in all cells except erythrocytes. They are derived from Golgi complex. They may vary in number and are 0.4 to 0.9μ in diameter. They contain a variety of hydrolytic and degradative enzymes. Acid phosphatase is a marker enzyme of lysosomes. They are called as suicidal bags of the cell.

Peroxisomes are also membrane bound vesicles. They are smaller, spherical 0.5–1.5μ in diameter. Absence of peroxisomes leads to Zellweger's syndrome.

It is a network of three different kinds of protein filaments which extend throughout the cell in the cytosol:

Ribosomes are tiny cell fractions which are packages of ribosomal RNA (rRNA) and proteins. Each ribosome has two subunits bigger and smaller. They are produced in nucleolus present in the nucleus separately, exit the nucleus and join together in the cytosol.

There are two types of ribosomes:

(Note: Ribosomes are also located within mitochondria where they are involved in synthesizing mitochondrial proteins).4

Prokaryotes: Its ribosomes (70s) have two subunits:

Eukaryotes: Its ribosomes (80s) also have two subunits:

They are sites of protein synthesis.

It is the largest cellular organelle. It contains DNA which possesses the genetic information.

Nuclear envelope: It is a double membrane structure. It separates the nucleus from the cytoplasm. It contains nuclear pores which permit the movement of RNAs and protein across the nuclear envelope.

Nucleolus: It is a dense body and is the site of synthesis of rRNAs.

Nucleoplasm: It is the ground material of the nucleus. It contains various enzymes like DNA polymerases and RNA polymerases.

It is an envelope covering the cell. It separates cellular contents from the external environment.

It is composed of lipid, protein and carbohydrates. Its structure was described as fluid mosaic model by Singer and Nicolson in 1972. It is essentially composed of a lipid bilayer. The lipids present are phospholipids, glycolipids and cholesterol. The hydrophobic (nonpolar) regions of the lipids face each other at the core of the bilayer, and hydrophilic (polar) regions face outside. Proteins are irregularly embedded in the lipid bilayer. Proteins are globular proteins and are of two types:

Fundamental laws of chemistry and physics apply equally well to biological systems. Hence related biophysical principles are described here.

Water is the most abundant component of the body. It contains about 60% of total body weight. The cell contains approximately 80% of water. Molecular formula of water is H₂O and its molecular weight is 18. Its structure resembles an irregular tetrahedron with oxygen in the center. It is a polar molecule. It is an ideal biological solvent and modifies the properties of biomolecules. It is very essential to life and water balance should be maintained to have normal and healthy life.

It refers to the negative log of the hydrogen ion concentration.

The term pH was introduced by Sorensen in 1909 who defined pH as mentioned above. It refers to relative acidity. pH scale is 1–14. pH of water is 7. pH lower than 7 denotes high concentration of H⁺ (acidic). pH higher than 7 denotes lower concentration of H⁺ (Alkaline).

pH of any solution can be determined by several methods:

Concept of acid-base chemistry is based on Lowry Bronsted theory.

An acid may be defined as:

A base can be defined as:

It is a combination of acid and base.

A salt is a compound of a metal radical (or a metal acting radical) and an acid radical.

A buffer is one that tends to maintain constant hydrogen ion concentration when acid or alkali is added to it, i.e. it resists a change in pH on addition of acid or alkali.

It is a mixture of:

Principal physiological buffers:

This equation states that pH is dependent on the ratio of the concentration of the base to acid.

It is the movement of solute molecules across a permeable membrane from high concentration to lower concentration until they get uniformly distributed.

When NaCl (solute) is added to water (solvent), it rapidly spreads and forms a homogeneous solution. Diffusion is more rapid in gases than in liquids.

Osmosis is the passage of solvent (water) to the solution through semipermeable membrane.

Flow of solvent occurs from a solution of low concentration to a solution of high concentration through a semipermeable membrane. It occurs in the direction opposite to that of diffusion.

It is the excess pressure that must be applied to a solution to prevent the passage of solvent into the solution when both are separated by a semipermeable membrane. It is the force required to oppose osmosis.

It depends on the number of solute particles and not on their nature (colligative property).

It is a technique for separating colloidal particles from small ions using a cellophane, collodion or parchment dialyzing membrane.

Non-diffusible ions occurring on one side of semipermeable membrane increase the concentration of oppositely charged diffusible ions, by decreasing their diffusion to the other side. They decrease similarly charged diffusible ions on the same side by increasing their diffusion to the opposite side. But the total cations and anions are equal on either side at equilibrium. This is known as Gibbs-Donnan membrane equilibrium.

Gibbs-Donnan effect is observed in the following:

The force with which the surface molecules are held together and form a membrane over the surface of the liquid is known as ‘surface tension’ of the liquid.

Drop weight method using stalagmometer.

Factors affecting surface tension:

It is the internal resistance offered by a liquid or a gas to flow.

By using Ostwald's viscosimeter.

Poise.

Solutes (substances) can be divided into two groups based on their passage through membrane.

Colloids are of two types depending on their ability to take up the dispersion medium.

It is the process of accumulation of a substance on the surface of another substance.

It is a process by which a substance is not only retained on the surface, but also penetrating to the interior of the material.

Some compounds bring about water insoluble substances soluble in water by absorption. For example, soaps of higher fatty acids, benzoic acid. These compounds are known as hydrotropic substances.

The phenomenon by which unstable atomic nucleus of some elements emits ionizing radiation during decay, is known as radioactivity.

There are three types of radiations: a rays, b rays and g rays.

The elements with the same atomic number but different atomic weight.

There are two types of isotopes:

Instruments to measure radioactivity:

Used for treatment of some diseases:

Used to estimate hormones by radioimmuno-assay, e.g. T₃, T₄, TSH, etc.10

Exposure to radioactivity is harmful for living organisms.

The role of the cell membrane is to allow compounds to enter or leave the cells in an orderly manner for the normal functions of the cell.

There are two main types of mechanisms for the transport of solutes (small molecules) through the membrane.

(Note: Membranes of host cells contain specific channels or ionophores. Channels are specific for inorganic ions like sodium, potassium, chloride, etc. Ionophores are specific for sufficiently small organic molecules like antibiotics).

The transport system is mainly divided into two types: