cell biology
Cell Biology

What is a Cell? Function and Structure

Cell Definition-

A cell is the basic functional and structural unit of the life which is necessary for the survival of the organism.

A cell is the individually capable of the –

  • Independent Existence.
  • Performing various metabolic activities like –
    1. Ingestion.
    2. Digestion.
    3. Absorption.
    4. Assimilation.
    5. Excretion.

Anything less than a cell structure is not capable to live independent-exitance. Hence, a cell is the basic fundamental and structural unit of the living organism.

History of a Cell-

The first form of the cell was discovered by the Robert Hooke in the dead cork of the plant, which was essentially dead named as the Cellulae as they looked like the beehive.

Later Anton Von Leeuwenhoek first saw and described the living cell and named it the animalcule.

 Later they were named as the Cell.

The invention of the microscope and the advancement of it into the electron microscope lead into the various structure (organelle) present in the cell.

Cell Theory

In 1938, Matthias Schleiden, a Botanist, examined a large number of plants and observed that a plant is made of the different types of the cell and together form the tissue.

At about the same time, Theodore Schwann (1939), Zoologist, studied the various type of animal cells and reported that they had a thin outer layer (today is known as the Plasma Membrane).

Later based on his studies on the plant he concluded that the Cell Wall is the Unique feature of the plant cells.

Later, M. Schleiden and T. Schwann together formulated the Cell Theory.

But they were unable to explain how the new cells are formed which was explained by the Rudolf Virchow, he said that all cells are formed from the pre-existing cells (Omnis cellula-e cellula).

He modified the theory given by Schleiden and Schwann and give the final shape to the cell theory.

Now, a days Cell theory can be explained as-

  • All living organisms are composed of cells and products of the cells.
  • All cells arise from the Pre-existing cells.

An Overview of the Cell

Cell Structure
Eukaryotic cell- image is under creatives commons, Image source

A typical plant cell contains the outer covering called the Cell Wall just beneath it, a Cell Membrane (Plasma Membrane) is present.

Note:- Cell Membrane is the universal structure and present in every living cell.

  • Cells that have the membrane-bound nuclei are called the Eukaryotic Cells, whereas cells that lack membrane-bound nuclei are called the Prokaryotic cells.
  • Besides the nucleus, Eukaryotes also have other membrane-bound structures called the Organelles like the Endoplasmic Reticulum (ER), Golgi Complex, Lysosomes, Mitochondria, Microbodies.
  • Prokaryotes lack such membrane-bound organelles.
  • A ribosome is the non-membranous organelle found in both the eukaryotic and the prokaryotic cells.
  • There are 2 more non-membranous organelles namely Nucleolus and Centriole (present only in the animal cell, and help in the cell division).

A eukaryotic cell has the four main component –

  1. Outer Covering – Cell wall + Cell Membrane (in case of Plant) and only Cell Membrane in the animal cell. (Not taking bacteria and virus here).
  2. Cytoplasm
  3. Cell Organelles
  4. Nucleus

Let’s see all them one by one-

Cell Wall

It is the Outermost dead covering present above the cell membrane and mainly made up of the cellulose (in plants). Present in the Algae, Plants, Fungi, and Bacteria.

In different organisms it is made from different components, eg- in plants it is made of cellulose, hemicellulose, pectins, and proteins.

So, what does a Cell Wall do?

Cell Wall is the dead and rigid covering so it provides the shape to the cell, protects the cell from the harsh chemicals, high pressure, and infections.

It is the permeable layer of the cell and allows all types of compounds to enter the cell.

Cell Wall contains mainly 2 layers, but in some cells 3 layers can also be found-

cell wall
Cell Wall
  • Primary wall– Thin and Elastic
  • Secondary wall– Rigid and thick
  • Tertiary Wall– Present in the few cells of the plants mainly in gymnosperms.

Cell Wall is connected with the help of the cementing layer called the middle lamella which is made up of the Ca and Mg Pectate.

 Cell Membrane

Also called as the Plasma Membrane or Bio-membrane. Biochemical investigation revealed that the cell membrane is made from the lipid (phospholipids), protein, and carbohydrates.

Unlike the Cell wall, the Cell membrane doesn’t possess the fixed shape i.e. it is Asymmetrical in structure, because of its fluid nature.

It is a universal structure and the structural cell membrane of the eukaryotes and prokaryotes is the same.

Plasma membrane
Cell membrane structure- source
  1. Phospholipids

The studies showed that the cell membrane is composed of the lipids that are arranged in a bilayer. Also, the lipids are arranged within the membrane with the polar head on the outer sides and the hydrophobic tails towards the inner part.

Because of this the non-polar tail of saturated and unsaturated hydrocarbons is protected from the aqueous environment.

Fact  In humans, the membrane of the erythrocyte has approximately 52 percent protein and 40 percent lipids.

Phospholipid layer provides fluidity to the plasma membrane because phospholipids are rich in unsaturated fatty acids which are liquid in nature.

Due to the fluid nature of the membrane, it can perform the following functions-

  • Cell Growth
  • Formation of the Inter-cellular Junctions
  • Endocytosis
  • Secretion
  • Cell division
The point to notice here is that, some amount of Cholesterol is also present between the bi-layer, which helps in the stability of the membrane structure, as it is more rigid than the phospholipid.

2. Proteins

Based on ease of extraction 2 types of protein is present in the plasma membrane-

a. Integral or intrinsic protein

This protein is tightly bound with the phospholipid. They can not be easily extracted from the phospholipid, without damaging the membrane.

Totally buried proteins work as the tunnel (channel) protein which provides the passage for the movement of water and soluble material across the membrane.

b. Peripheral or extrinsic protein

They are superficially arranged on the surface of the membrane and can be extracted easily with the slight electric shock without damaging the membrane. These proteins are related to enzymatic activities.

3. Carbohydrate

They are present on the outer surface of the cell membrane in the form of glycolipids and glycoproteins.

It is involved in the cell to cell recognition mechanism. Example- Sperm and ovum recognize each other during fertilization.

Note– The lipid-soluble compound can easily cross the cell membrane, but water-soluble compounds need the channels to cross the lipid bilayer.

Cytoplasm

The fluid present in between the cell membrane and the nucleus is cytoplasm.

The term “Cytoplasm” was given by the Strasburger.

The cytoplasm is made of 80% water, salts, and protein (enzymes). Cytoplasm provides the medium for many enzymatic reactions like glycolysis.

Cytoplasm can be divided into 2 parts-

  1. Cytosol– Liquid matrix of the cytoplasm present outside of the cell organelles. It is the mixture of the cytoskeleton filaments, dissolved molecules, and water.
  2. Cell Organelle

Cell Organelle

Metabolically active and living structure of cytoplasm is called organelles.

While each membranous organelle is distinct in terms of its structure and function, but many of them work together and their function is coordinated, is considered as the endomembrane system

Endomembrane System– It includes Golgi complexEndoplasmic Reticulum (ER)Lysosomes, and Vacuoles.

Since the function of the mitochondriachloroplast, and peroxysomes are not coordinated with the above components, these are not considered as part of the endomembrane system.

Endoplasmic Reticulum

While studying the Eukaryotic cells under the electron microscope, the network of the tiny tubular structures was found, which is scattered in the cytoplasm that is called the endoplasmic reticulum (ER).

cell organelle ER
Endoplasmic Reticulum- Source

Components of E.R.

  1. Cisternae– These are the long flattened and unbranched units arranged in stacks.
  2. Vesicles– These are oval membrane-bound structures.
  3. Tubules– These are irregular, often branched tubes bounded by a membrane. Tubules may be free or associated with the cisternae.

ER divides the intracellular space into two distinct compartments i.e. Luminal (inside ER) and extraluminal (outside ER) compartments.

The endoplasmic reticulum is of 2 types-

  1. Rough Endoplasmic Reticulum (R.E.R.)– It is granular, due to the presence of the ribosome. It is mainly associated with protein synthesis.
  2. Smooth Endoplasmic Reticulum (S.E.R.)- It is agranular, due to the absence of the ribosome its surface looks smooth and it is mainly associated with lipid synthesis.

Golgi Complex

Camillo Golgi (1898), first observed the densely stained reticulated structure near the nucleus. These were later named as the Golgi bodies.

golgi apparatus
Golgi ApparatusSource

Like ER they also have the three parts-

  1. Cisternae
  2. Tubules
  3. Vesicles

The main function of the Golgi apparatus is the packing and transportation, either to the intracellular targets or outside the cell.

A surface that is toward the nucleus is called the Cis or forming face, and the face opposite to the nucleus is called the Trans or Maturing face.

er and golgi body working together
A diagram showing the process of processing the compound received from ER.- Img source

Lysosome-

It is the enzyme-rich, membrane-bound ball-like structure, formed by the packaging of the Golgi apparatus.

Its membrane contains an H+ pump mechanism that produces acidic pH in the lumen of lysosomes.

Note- This H+ pump need ATP for its functioning.

Lysosomes are also called as the suicidal bag because sometimes all the lysosomes of a cell burst to dissolve the cell completely.

It happens in the tumor cell, virus attacked cells, or any unwanted cells of the body.

It is also observed that lysosomes are responsible for the digestion of the tail of the tadpole larva of a frog during metamorphosis.

Vacuoles

It is the membrane-bound space found in the cytoplasm of the plants.

It contains water, sap, excretory products, and other materials not useful for the cell.

In mature plant cells the vacuole occupies nearly 90% of the space.

As the plant doesn’t have the advanced excretory system like animals, so the majority of the waste of the cell is stored in the vacuoles of the cell.

The single membrane of the Vacuole is called the Tonoplast.

Mitochondria

The mitochondria is a semi-autonomous double-membrane-bound organelle found in most eukaryotic organisms.

Mitochondria
Mitochondria in a cell magnified.- Source

Some cells contain more than one mitochondria, while some cells however, lack mitochondria (for example, mature mammalian red blood cells).

Mitochondria contain the ribosome (70-s), own Nucleus called Nucleoid but is dependent on the genome of the cell for some protein that’s why it is called semi-autonomous organelle.

The main energy required for the cellular activities is formed by the mitochondria, so it is also called the energy house of the cell.

Its inner membrane folds inside to form compartments, which is called as Cristae, it is done to increase the surface area so that the enzymatic reaction could occur effectively.

Cristae contain a racket like oxysomes which are involved in the oxidative phosphorylation (formation of the ATP by the energy of oxidation).

Plastids

The plastid is a double membrane-bound organelle found in the cells of plants, algae, and some other eukaryotic organisms.

Plastids were discovered and named by Ernst Haeckel, but A. F. W. Schimper was the first to provide a clear definition.

Plastids are the site of manufacture and storage of important chemical compounds used by the cells of autotrophic eukaryotes.

They often contain pigments used in photosynthesis, and the types of pigments in a plastid determine the cell’s color.

Based on presence/absence and type of pigment, plasmids can be classified into 3 types-

1.Chromoplast-

They give colors to the different parts of plants like flowers and fruits, chlorophyll is either absent or present in a very trace amount.

Chromoplasts synthesize and store pigments such as orange carotene, yellow xanthophylls, and various other red pigments.

As such, their color varies depending on what pigment they contain. The main evolutionary purpose of chromoplasts is probably to attract pollinators or eaters of colored fruits, which help disperse seeds.

However, they are also found in roots such as carrots and sweet potatoes.

2. Leucoplasts-

Colorless plastids mainly involved in the storage and contain no pigment.

Leucoplasts sometimes differentiate into more specialized plastids –

Amyloplasts: for starch storage.

Elaioplasts: for storing fat.

Proteinoplasts: for storing protein.

3. Chloroplast-

It is the main plastid that is responsible for photosynthesis.

Chloroplast
Chloroplast- Source

Chloroplast contains a green color pigment chlorophyll and carotenoids, which are responsible for the trapping of the light energy.

Read the complete structure and function of the chloroplast.

The fluid present inside the membrane is called stroma, where the main enzymatic reactions of photosynthesis take place.

Because the stroma contains the most crucial enzymes (RuBisCO) required for photosynthesis.

A stalk of the thylakoid is present inside the stroma called granum (pl- grana).

Grana are interconnected with the help of the stroma lamellae.

Light reaction of the photosynthesis, occurs on the grana part (thylakoid) of the chloroplast. (absorption of the light and the photolysis of water to release H+ and O2).

Read all about the photosynthesis.

Ribosome

Also as the Palate particle or Protein factory of the cell.

It is mainly associated with protein synthesis, with the RNA.

Types of Ribosomes-

  1. 80-S – it is present in the eukaryotic cells.
  2. 70-S – it is present in the prokaryotic cells.

Svedberg unit or Sedimentation rate, which measures the density and size.

Centriole

A centriole is absent in the plant cell.

It plays an important role in the cell division by arranging the spindle fibers to the 2 opposite poles of the cell.

2 centrioles arranged them at the right angle to each other, thus forming a Centrosome, which is present just outside the nucleus.

Micro-bodies

There are several membranes bound minute vesicles that contain various enzymes, present in both plants and animals.

1. Peroxisomes-

It contains the catalase enzyme which is concerned with peroxide (H2O2) metabolism.

Catalase degrade peroxide into water and oxygen.

Peroxisomes prevent the green tissue of the plant from photo-oxidation (destruction of chlorophyll).

Peroxisomes are also involved in the β-oxidation of the fatty acids.

2. Glyoxysomes-

They are present only in the plants, especially in the fatty seeds (castor seeds, groundnut seeds, etc.).

They convert fats into carbohydrates.

Nucleus

The nucleus is a double membrane-bound organelle found in eukaryotic cells.

cell biology- Nucleus
Nucleus with the well defined NucleolusSource

Eukaryotes usually have a single nucleus, but a few cell types, such as mammalian red blood cells, have no nuclei, and a few others like osteoclasts, muscle cells have many.

The nucleus contains all of the cell’s genome (DNA), except for a small fraction of mitochondrial DNA.

The genome in the nucleus is organized as multiple long linear DNA molecules in a complex with a large variety of proteins, such as histones, to form chromosomes.

Note- Chromosomes are formed during the cell division, the rest of the time genome is present in the form of Chromatin fibers.

Genes are present in these chromosomes which are structured in such a way that whole-cell function can be controlled from the nucleus.

The nucleus maintains the integrity of genes and controls the activities of the cell by regulating gene expression.

The nucleus is, therefore, the control center of the cell.

The main structures making up the nucleus are-

Nuclear envelope- A double membrane that encloses the entire organelle and isolates its contents from the cellular cytoplasm.

Because the nuclear envelope is impermeable to large molecules, nuclear pores are required to regulate nuclear transport of molecules across the envelope.

The pores cross both nuclear membranes, providing a channel through which larger molecules must be actively transported by carrier proteins while allowing free movement of small molecules and ions.

Movement of large molecules such as proteins and RNA through the pores is required for both gene expression and the maintenance of chromosomes.

Nucleoplasm – It is the ground substance in the nucleus that adds mechanical support, much like the cytoskeleton, which supports the whole cell.

Chromatin fiber and Nucleolus are the main components of the nucleoplasm.

Chromatin fiber

Cell DNA
Coiling of the Chromatin material to form chromosome.

In the above picture you can see how coiling increases (from left to right), to form a chromosomes.

DNA during the interphase is present in the form of the chromatin.

Based on staining chromatin are of 2 types-

  1. Euchromatin– Light stained and diffused part of chromatin. Which is transcriptionally more active.
  2. Heterochromatin– Dark stained and condensed part of the chromatin. It is transcriptionally more active.

The nucleus does not contain any membrane-bound structures, its contents are not uniform, and several nuclear bodies exist, made up of unique proteins, RNA molecules, and particular parts of the chromosomes.

Nucleolus

It is non-membranous organelle, which is mainly involved in the assembly of ribosomes.

After being produced in the nucleolus, ribosomes are exported to the cytoplasm where they translate mRNA.

Note- Nucleolus is attached to the specific part of the chromosome called the Nucleolar Organiser Region (NOR).

Points to Remember

  • There are only 3 double membranous organelles Nucleus, Mitochondria, and Plastids.
  • There are only 3 non-membranous organelles Nucleolus, Centriole, and Ribosome.
  • Rest are single membranous organelle (ER, Golgi complex, Vacuole, Lysosome).

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