Why is ‘Cell’ called the fundamental unit of life and why its boundary line- the ‘Plasma Membrane’ so important?

Just as the basic unit of all non-living things is ‘ Atom’, the basic unit of all living organisms is ‘Cell’.

PS- The atom is still very smaller than the cell.

Now, the standard definition of the Cell says that “cell is the basic structural and functional unit of living organisms”.

By the word “structural” we mean that cells give shape or form to organs and consequently to the body parts.

By “functional” we mean that a cell contains the whole machinery (organelles) for performing various life processes required for sustaining life. A Cell processes nutrients to give energy and undergoes replication/ division to give rise to more cells. There are specialised cells to perform specific functions in Eukaryotes.

A typical Eukaryotic animal cell contains membrane- bound organelles like nucleus, mitochondria, ribosomes, endoplasmic reticulum(ER), Golgi apparatus which have specific functions to carry on various life processes.

There can be variations in number or presence/absence of a few organelles in different Specialized Cells, depending on the job they do.

Examples

1. Mature RBCs get rid of nuclei and all other cell organelles like mitochondria, Golgi apparatus and ER to free up space for more haemoglobin, which is an oxygen carrier.

2. Mature neurons lack centrioles which are responsible for mitotic cell division.

3. Muscle cells and liver cells have more number of mitochondria.

Some more interesting facts about the Cell:

– Human body is composed of about 200 different types of specialized cells.

– All cells are capable of carrying out certain basic functions like nutrition, respiration, growth and replication which are essential for the survival of cells.

– Since the cell carries out so many functions, so there is “division of labour” within the cell which means that different organelles of the cell perform different specific functions. For example- making of new material like protein synthesis by the organelle ribosomes, cellular metabolism/ respiration to release energy by mitochondria, DNA replication in nucleus of Eukaryotes, clearing up the waste substances from the cells by lysosomes etc.

Let us know about the Cell size, Cell shape, Cell volume, Cell number and Cell structure:

Cell size

– The cells are microscopic. In human, the cell size varies from 3 to 4 micron (leucocytes) to over 90 cm (nerve cells).

– The cell size is correlated with its function and not to the size of organism.

– In a large cell, the cytoplasm requires more proteins and consequently more RNA.

– The DNA content of the cell in a given organism remains constant.

– Small cells have more surface area per unit volume and with an increase in size this ratio decreases.

– Small cells are metabolically more active because they have greater surface area, so more exchange of materials occur with outside environment.

Cell shape

– The shapes of the cells are also related to their functions.

– The shapes also depend on the surface tension and viscosity of protoplasm, mutual pressure of the adjoining cells and rigidity of the cell membrane.

Cell volume

– The Cell volume is almost constant for a particular cell type and is independent of the size of the organism.

– The total mass of an organism depends on the number of cells present in its body and not on the volume of the cells. So, the cells of an elephant are not larger than any other tiny animal.

Cell number

– The number of cells is correlated with the size of organisms. So, small organisms have less number of cells than large organisms.

– The entire body of an adult animal or plant consists of a fixed number of cells and that remains the same in all members of the species. This phenomenon of cell or nuclear constancy is called “Eutely”.

– In human beings, the number of cells is around 100 trillion.

Cell structure

All cells have three major functional regions:

1. Cell membrane or Plasma membrane (cell wall also, in case of plants only)

2. Nucleus

3. Cytoplasm

Plasma membrane

What is the physical significance of Plasma Membrane?

– Plasma membrane is the outer boundary of the cell. It is present in all types of cells- both eukaryotic and prokaryotic cells, cells of plants, animals and microorganisms.

– It physically separates the cytoplasm from the surrounding cellular environment.

– Most cellular organelles like mitochondria, lysosomes, Golgi apparatus, nucleus, endoplasmic reticulum, and chloroplast (in plant cells only) are enclosed by the plasma membrane.

Is plasma membrane living or dead?

The plasma membrane is thin, elastic, living and selectively permeable membrane. It ranges from 6 to 10 nm. Chemically, membrane is 75% phospholipids and also contains proteins, cholesterol and polysaccharides.

Let’s know the ultrastructure of plasma membrane to know more about the living nature of plasma membrane.

Various models, to know the structure of plasma membrane, have been proposed by different scientists.

“Fluid Mosaic Model” of plasma membrane is well accepted which we take up as follows:

– The Fluid Mosaic Model was propounded by Singer and Nicholson.

– According to this, the plasma membrane is made up of a bilayer of phospholipids.

– Two types of proteins- Intrinsic or Integral and Extrinsic or Peripheral float about in the fluid phospholipid bilayer.

– Intrinsic proteins penetrate lipid bilayer partially or wholly.

– Extrinsic proteins are present either on the outer or inner surface of the lipid bilayer.

– The Lipids and Intrinsic proteins are amphipathic in nature ie. these molecules have both hydrophobic (non-polar) and hydrophilic (polar) groups.

– The proteins are present to serve as (a)enzymes (b)transport proteins or permeases (c)pumps (d)receptor proteins

– Lipids and proteins provide flexibility to the plasma membrane which helps in processes like Endocytosis.

– Plasma membrane is selectively permeable ie. it permits the entry and exit of some materials only in the cell.

– Substances allowed inside the cell include food, water, salts, oxygen, vitamins and hormones.

– Substances thrown out of the cell include nitrogenous waste and carbon dioxide, secretions like proteins, proenzymes, hormones, milk, tear, mucus, immunoglobulins (antibodies) etc.

– Since plasma membrane regulates the transport of various substances in and out of the cell, so it is living in nature. This is done to maintain the concentration of various substances and ions inside the cell.

Transport can be Passive or Active.

Passive transport

– Here, the particles or molecules move from a region of higher concentration to lower concentration through plasma membrane by DIFFUSION. So, this is also called “downhill transport”. Here, the movement occurs only due to the concentration gradient without consuming energy. The hydrophobic substances are readily transported by this method because these are soluble in lipids.

– Sometimes a carrier molecule called “Carrier Protein or Permeases” assist the transport (without the use of energy) and this is called “Facilitated transport”. It is helpful in the transport of hydrophilic nutrients like glucose and amino acids.

– When water molecules pass through the plasma membrane along the concentration gradient without the use of energy, the process is called OSMOSIS.

Active transport

– This is the movement of molecules or ions against the concentration gradient (Uphill movement) using energy (ATP) to counteract against gradient.

– The most important active transport in all animals is the sodium-potassium transport between cells and the surrounding extra cellular fluid. This transport is called “Sodium pump”.

Sodium pump

– The animal cell requires a high concentration of potassium ions inside the cells for protein synthesis by ribosomes and for certain enzymatic functions.

– The desirable potassium ion concentration is 20 to 50 times greater inside the cell than outside and sodium ion concentration maybe 10 times more outside the cell than inside.

How Sodium- Potassium ionic gradients are maintained or how the sodium pump works?

– There is a higher concentration of sodium ions outside the plasma membrane of the animal cell. The sodium ions are transported outside with the use of a carrier molecule- A Carrier Transport Complex is formed which utilises ATP and transports sodium ion outside the cell. Simultaneously, potassium ions are transported inside the cell by similar way.

– This unbalanced charge transfer leads to separation of charges across the plasma membrane. This difference helps in the Action Potential produced by nerve cells.

How are the macromolecules, solid food particles, etc transported inside the cell?

These are transported by the mechanism called ENDOCYTOSIS.

Depending on the nature of the substance, Endocytosis can be of the following types:

1. Pinocytosis (cell drinking)

– Pinocytosis is the process of “ingestion of fluid droplets & small solute particles” by the cell. The substances like protein, amino acids, which cannot enter by simple osmosis are ingested by pinocytosis.

– Here, the plasma membrane invaginates to form small vesicles or “Pinosomes”. The vesicles pinch off from the plasma membrane, move through the cytoplasm and fuse with the plasma membrane of the other side, there by discharging the contents.

– Pinocytosis is seen in microvilli of small intestine and in kidney cells.

2. Phagocytosis

– Phagocytosis is the process of “engulfing solid food particles” by cells through plasma membrane (as seen in protozoa also). Vesicles formed here are called “Phagosomes” (1 to 2 µm).

– Phagosomes move through cytoplasm and are dissolved and ingested by enzymes of lysosomes. The residues are ejected out of the plasma membrane by process called EPHAGY.

– In Phagocytosis, bacteria etc are engulfed.

3. Rhophaeocytosis

This is the transfer of small quantities of cytoplasm, together with their inclusions, from one cell to the other. This was demonstrated in bone marrow tissue.

PS- All types of Endocytosis processes occur by “Active Transport”.

FAQs

Q1. Can we call the plasma membrane as unit membrane?

Ans. Unit membrane means the limiting membrane of the cell and the organelles, viewed formerly as a three layered membrane, composed of inner lipid layer and two outer protein layers. This concept has been rejected as Fluid Mosaic model is the current accepted one.

Q2. Do proteins present in the plasma membrane give strength to the cell?

Ans. No

Q3. What is the meaning of hydrophilic?

Ans. Loving water

Q4. What is the meaning of hydrophobic?

Ans. Water fear