Cells

The most basic and functional unit of life is called a cell. These cells make up every organism on earth. First discovered by Robert Hooke  when examined some cork  , these were named cells because their structure looked like honeycomb cells. 

These cells are not functional on their own but need several organelles to do the heavy lifting. Each organelle has different functions but these come together to serve the common purpose of the cell.

I am going to document these organelles in detail, from their structure, functions to their correspondence with other cell organelles. But first we need to look at cell theory, which is the basis of modern biology.

Cell Theory

Two German scientists, Matthias Schleiden  and Theodor Schwann  worked together and made discoveries about cells and their role in all organisms.

Matthias Schleiden first proposed in 1839 that every plant is made up cells, while Schwann extended this statement to animals in the same year. However, they both disagreed on how cells come to be. Schleiden believed that cells came from free cell formation, i.e., they crystallized into existence out of nowhere. Schleiden’s claim was refuted by Rudolf Virchow  who gave the idea that cells came from pre-existing cells. Virchow is believed to have plagiarized  the work of a polish scientist Robert Remak  .

These gave us the three principles of cell theory.

Principles of Cell Theory

1. All organisms are composed of one or more cells.
2. The cell is the most basic unit of structure and organisation in organisms.
3. Cells come from pre-existing.

Classification of Cells

Cells, on the basis of their nuclear material, are divided into two types :

1. Prokaryotic Cells - Cells which don’t possess a well-defined nucleus and organelles are called prokaryotes. Here pro means primitive/before in Greek and karyon means kernel, sopro + karyon = prokaryote (primitive cell).

Prokaryotic cells are generally cells of bacteria, viruses, fungi or other such single-celled organisms like amoeba.

2.Eukaryotic Cells - Cells which have a well-defined, membrane-bound nucleus and membrane-bound organelles are called eukaryotes. Here eu means well/good in Greek and—as previously mentioned—karyon means kernel, so eu + karyon = eukaryote ( Well defined cell).

Eukaryotic cells are generally cells of complex, multi-cellular organisms like monkeys.

Difference in Prokaryotes and Eukaryotes

Sourced from wikipedia. 

Classification of Eukaryotic Cells

Eukaryotic cells are further divided according to the type of organism they belong to. These are of two types:

1. Animal Cells

2. Plant Cells

Animal Cells and Plant Cells

Eukaryotic cells belonging to members of the animal kingdom are called animal cells.
Similarly, cells belonging to members of the plant kingdom are called plant cells.

This diagram shows both an animal cell and a plant cell side by side. It may be observed that both have most of the organelles in common (some with a little difference) and a few unique ones. These organelles are what power these cells.

Cell Components

I will categorize others as “components” because they don’t quite fit the category of organelles. These are :

1. Cell Membrane
2. Cell Wall
3. Cytoplasm

Cell Membrane

The outermost-in animal cells and the innermost in plant cells- layer is called the cell membrane. It’s a selectively permeable membrane, as in it allows some substances such as H₂O , CO₂ etc., pass while blocking undesirable objects like hostile organisms.

Structure of the Cell Membrane

The cell membrane is made up of two layers of phospholipids, also called a bi-lipid layer, which have proteins between them.

The proteins which fully penetrate the bi-lipid layer are called intrinsic proteins, while the ones which only fully penetrate one of the two layers are called extrinsic proteins.

Functions of the Cell Membrane

• Intrinsic proteins help in the exchange of substances to and from the cell.
• Extrinsic proteins while not fully penetrating the bi-lipid layer, it also helps in transport of materials in times of distress. It also alerts the cell of the dangers outside like viruses, hostile bacteria etc.

Cell Wall

The outermost boundary of plant, algae and fungi cells is called a cell wall.

• It’s a dead component.
• Has high tensile strength.
• It’s freely permeable.

Plant cells are composed of cellulose, hemicellulose and pectin.

Cytoplasm

The area inside the cell is filled with a viscous, gelatinous fluid which is called cytoplasm. It helps in the transport of cellular substances like proteins, lysosomes etc. inside the cell.

Composition of Cytoplasm

Parts of Cytoplasm

1. Cytosol - The translucent fluid in which cell organelles are suspended, it make up 70% of the cytoplasm.

2. Cytoskeletal - The fibrous strands which give the cell a proper shape, adjust the cell organelles position and aid in the transport of vesicles. These are only found in eukaryotes.

Functions of Cytoplasm

• To give the cell a proper shape via the cytoskeletal.

• To help in dissolving in water-soluble substances.

• It aids in the transfer of cellular material inside the cell.

Cell Organelles

Cell organelles are sub-units of a cell which generally have a specialized function. There are quite a few of these, each with a different structure, size, function etc. The organelles covered here are going to be:

1. Common organelles

   • Nucleus
   • Endoplasmic Reticulum
   • Golgi Apparatus
   • Mitochondria
   • Lysosome
   • Vacuole

2. Organelles specific to plant cells

   1. Plastids

      • Chloroplast
      • Chrmoplast
      • Leucoplast

   2. Cell Wall

Nucleus

The nucleus is a spherical, double membrane-bound organelle located at the periphery in plant cells and at the center in animal cells, it’s the most important cell organelle in the cell as it controls all cellular functions.

• The nucleus is what differentiates eukaryotes and prokaryotes.
• It controls the metabolic activities of the cell.

Nuclear Membrane

This is a double layered membrane which separates the nucleus from the cytoplasm of the cell. This protects the cell from foreign objects and organisms.

Nuclear Envelopes & Pores

Nucleus pores are small openings in the nuclear membrane, these allow the entry and exit of macromolecules like ribosome. Envelopes are then, the pieces of the dashed membrane.

Chromatin

Long threads made up of DNA and proteins carrying genetic data necessary for cell division. These condense into gene-carrying chromosomes during cell division.

Nucleolus

The spherical structure present at the center of the nucleus. It’s not membrane bound and is responsible for the synthesis of ribosome.

Endoplasmic Reticulum

The Endoplasmic Reticulum is a contains membrane organelle made up of flattened sacs, covering the space around the nucleus.

It’s found in most eukaryotic cells.

According to appearance, the E.R. is divided into two types :

1. Rough Endoplasmic Reticulum

Named so due to appearance caused by embedded ribosomes which enable it to make protein.

2. Smooth Endoplasmic Reticulum

Not having embedded ribosomes gives this part of the E.R. a smooth appearance, due to the lack of ribosomes, it manufactures lipids.

Functions of the Endoplasmic Reticulum

1. Under Rough Endoplasmic Reticulum

   1. Synthesis of protein with the help of the ribosomes.
   2. Modification of proteins.
   3. Transportation of proteins to different sites of the cell via packaging in vesicles.

2. Under Smooth Endoplasmic Reticulum

   1. Synthesis of lipids.
   2. Transportation of lipidids.
   3. Secretion of hormones via the endocrine glands.
   4. Detoxification and filtering of consumed substances, specially during alcohol and drug overdose.

The Golgi Apparatus

The Golgi Apparatus or the golgi apparatus is a membrane-bound cell organelle made up of flat, stacked sacs called cistarnae. It’s found in most eukaryotic cells.

The Golgi Network

It has three components and two networks, namely:

1. Components

• Cis : The face closest to the E.R.

• Medial : Middle part of the golgi apparatus, located at medium distance from the E.R.

• Trans : The face closest to the E.R.

2. Networks

• Cis Network

• Trans Network

The networks belong to their respective components and are composed of multiple cistarnae. Here’s a youtube video  explaining the networks in detail.

Functions of the Golgi Apparatus

The proteins and lipids manufactured by the E.R. go into the golgi apparatus to be :

1. Modified : It modifies the proteins/lipids by adding other components such as other types of proteins and lipids.

2. Packaged & Transported : It packages its modified proteins/lipids into vesicles and transports them across the cell.

Since the vesicles forms out of the golgi itself, its essentially always forming.

Mitochondria

As you most probably already know, mitochondria is the powerhouse of the cell. 

It’s a vital organelle for eukaryotes (it’s only found in eukaryotic cells), it’s double membrane-bound, it uses oxygen (through aerobic respiration) to dissolve nutrients into energy that’s usable by the cell, into adenosine triphosphate or ATP.

Structure of the Mitochondria

It’s bean shaped with two membranes -

1. Inner Membrane

2. Outer Membrane

The inner membrane is folded towards the inside making a finger-like structure called cristae. These folds host a particle which is essential to the production of ATP. These are called F1 particles or oxysomes. That’s the reason these finger-like folds exist, having these increases the surface area on which the F1 particles are hosted, therefore increasing the ATP production.

It also houses a matrix in the inner membrane which is a liquid containing different types of enzymes, ribosomes and other proteins.

Lysosome

Lysosome is a membrane-bound organelle present in many eukaryotes. It carries various digestive enzymes inside its membrane for varied purposes. Its structure is quite simple, it’s just a membrane-bound spherical object filled with digestive enzymes.

Functions of the Lysosome

1. To digest foreign organisms which enter the cell.

2. To digest harmful foreign substances.

3. To release digestive enzymes into the cell if it becomes diseased and may harm surround cells, this is also why it’s called suicidal bag.

Vacuole

Vacuoles are some of the more noticeable category of organelles. They exist in both animal cells and plant cells. They’re generally formless and expand/contract as the cell needs it to. They are far bigger in plant cells than their animal cell counterparts, as can be seen here.

Functions of Vacuoles

They are mostly containers for different types of storage.

• Isolating materials that might be harmful or a threat to the cell.
• Containing waste products.
• Containing water in plant cells.
• Maintaining internal hydrostatic pressure or turgor within the cell during osmosis.

Plastids

Plastids are a group of double membrane-bound organelles found only in plant cells and algae. We’ll cover only 3 types of plastids out of the eight.

They are :

1. Chromoplast

2. Chromoplast

3. Leucoplast

Chromoplast

These kinds of plastids contain a non-green pigment (usually brownish to reddish), they have irregular shapes and they don’t change into other types of plastids.

• They give color to the non-green parts of a plant like the flowers, fruits, roots etc.

Chloroplast

Chloroplasts are perhaps the most important type of plastids. They contain chlorophyll, which gives plants their distinctive green color and enables them to manufacture food via photosynthesis.

The above diagram shows a chloroplast with the following :

1. Thylakoid : These are membrane-bound compartments which are shaped like sacs, (Thylakoid comes from Greek thylakos, which means sac) they are the sites of photosynthesis.

2. Grana : A bunch thylakoids when stacked together are called grana (singular : granum).

3. Stroma : Is the homogeneous in which grana exist.

4. Lamella : Lamella simply connects different grana.

Leucoplast

These are colorless, that is, they contain no pigment. They are spherical in shape and store food like starch, lipids and protein.

Functions of Plastids

1. Chloroplast

• Perform photosynthesis to manufacture food.
• Give color to leaves, shoots, stem etc.

2. Chromoplast

• Give color to flowers, fruits, roots etc. to attract animals for pollination and/or seed dispersal.

3. Leucoplast

• To store starch, lipids and protein.

Until next time!