What is Cell Structure?

The knowledge and concept about the structure of the cells have changed constantly over the past years. Primarily the scientist defines the cells as a simple membranous sac with fluids and some particles. Researches have now revealed that the cell is the more complex and basic unit of life. There are different types of cells and they also differ in shape and structure. The basic three parts of a cell are cell membrane, cytoplasm, and organelle.

Overview of Cell

Cells are defined as the fundamental and structural units of the living organism. They are the building blocks and basic unit of life. Protoplasm is the living matter of a cell that comprises molecules and compounds bound inside a plasma membrane sac. Organelles such as a nucleus, mitochondria, and Golgi bodies are present inside a eukaryotic cell. Cells efficiently work when the organelles work together in coordination.

Cell theory and history of Cell structure

Cell theory is a scientifically deduced theory first proposed in the mid-nineteenth century that states that living organisms are made up of cells, cells are the basic organizational unit of all organisms, and all cells originate from pre-existing cells. The study of cell structure and its basic functional components and processes is called cell biology. Some of the important researches and findings are:

1. The cell was discovered in 1665 by the scientist Robert Hooke.
2. Anton Van Leeuwenhoek observed a living cell in 1674.
3. In 1883, Robert Brown discovered the nucleus.
4. Felix Dujardin discovered the fluid content of a cell in 1835.
5. Matthias Schleiden found that all plants are made up of cells in 1838.
6. In 1839, J. E. Purkinje named the fluid content of cells as protoplasm.
7. Theodor Schwann proposed all animals are made up of cells in the year 1839.
8. Carl Heinrich Braun in 1845 proposed the cell is the basic unit of life.
9. Rudolf Virchow Proposed all cells arise from pre-existing cells in 1855.

Division of organisms based on Cell Number

Multicellular organism

The organism that is made up of multiple cells is called a multicellular organism. They have the tissue and organ level organization. Some of the examples are plants, animals, and fungus.

Unicellular organism

The organism made up of a single cell is called a unicellular organism. Such organisms show cellular level organization. For example, euglena, yeast, and paramecium are some of the examples.

Types of cells

Prokaryotic cell

Prokaryotic cells contain no nucleus and are reproduced by binary fusion. They have chromosomes and no membrane-bound organelles. They are usually in the size of 0.5-5 µm. Some of the examples are bacteria and blue-green algae.

Eukaryotic cell

Eukaryotic cells have a nucleus and membrane-bound organelles in them. There is more than one chromosome and the size ranges from 5-100 µm. Some of the examples are plants, fungi, protozoans, and animals. They reproduce sexually.

Plant cells

Plant cells are larger and have cell walls. Plant cells have a larger and greater number of vacuoles. Plastids are found in plant cells, and they lack centrioles.

Animal cells

They are smaller in size with no cell walls. They generally do not have vacuoles, and if present, they are too small in size. Plastids are absent, and centriole is present in the animal cells.

Components of the cells

Cell membrane

The cell membrane is also known as the plasma membrane that is the outer covering of the cells and separates the extracellular and intracellular materials. They protect the cells and maintain their structural integrity. The membrane is made up of double-layered phospholipid molecules.

Cell wall

The cell wall is found in plant cells, fungi, and bacteria. It is composed of cellulose and hemicellulose, pectin, chitin, and peptidoglycans. They are the outermost layer of a cell that provides a rigid structure and shape.

Nucleus

The nucleus is the integral and central component of the cell. They contain a chromatin reticulum that has the genetic material of the cell. The nucleus contains nucleolus where ribosomes are produced. A nuclear envelop acts as the membrane that restricts the entry of molecules inside the nucleus. Nucleoplasm is the liquid gel-like fluid as protoplasm that preserves other organelles.

Cytoplasm

The cytoplasm is the gel-like substance present inside the cell membrane. Most of the functions such as generation of cell growth factors, translation of proteins, and cell expansion are performed inside the cytoplasm by cytoplasmic organelles.

Cell organelles

Organelles found in the cytoplasm are called cytoplasmic organelles. For example, mitochondria, Golgi apparatus, and nucleus. Each of the organelles has a certain specific function that sustains the life processes.

Endoplasmic reticulum

The endoplasmic reticulum is the tubular network and vesicular structure which is the eukaryotic transport system that is connected as well to the nuclear membrane and cell membrane. The important role of the endoplasmic reticulum is the metabolism of carbohydrates, production of lipid, protein, and steroids. The ribosome over the endoplasmic reticulum is the rough endoplasmic reticulum, and the endoplasmic reticulum without ribosome is the smooth endoplasmic reticulum.

Mitochondria

The mitochondrion is known as the powerhouse of the cell. It is the rod-shaped small organelle having an inner layer having inward projection called cristae and matrix bounded with certain enzymes also the outer layer that protects and encloses the mitochondrion organelles. The mitochondrion produces adenosine triphosphate (ATP) that is used as an energy source. Mitochondrion maintains the cellular metabolism.

Golgi bodies

Golgi bodies are membranous sac-like structures having five to eight sacs called the cisternae. They are majorly packing the materials in the cells. They engulf the foreign bodies and protect the cells.

Ribosome

Ribosomes are the macromolecules present in the cells that synthesize protein with the help of ribonucleic acid.

Lysosome

The lysosome is a subcellular organelle present in virtually all types of eukaryotic cells (cells with a clearly defined nucleus) lysosome that digests macromolecules, old cell components, and microbes. The lysosome contains digestive enzymes also.

Chromosome

The chromosomes are formed due to the condensation of deoxyribonucleic acid and ribonucleic acid and store the genetic information. They play a vital role in sex determination having twenty-three pairs of chromosomes in humans.

Vacuoles

It is a single membrane sac that is present in a larger size in a plant cell. They consist of water, ions, and sugar. They maintain the osmotic pressure and rigidity in cells and stores waste materials.

Plastids

They are membrane sac structure that is found in plant cell which stores food. They consist of organelles such as chloroplast, leucoplast, and chromoplast that change the color of the cells.

Cytoskeleton

Cytoskeleton that provides shape to the cell consists of microtubules and microfilaments. Microtubules are structures formed by the protein tubulin, and microfilament is formed by actin.

Context and Applications

This topic is essential in the professional exams for both school level, undergraduate, and postgraduate courses, especially for bachelors and masters in botany, and bachelors and masters in plant physiology.

Practice Problems

Question 1: Name an organelle that plays as the main region of packaging for molecules that are spread over the whole cell.

1. Mitochondria
2. Plastids
3. Vacuole
4. Golgi apparatus

Answer: Option D is correct.

Explanation: The Golgi or Golgi body is also known as the Golgi apparatus. It is an organelle that is used as the principal packing area to distribute the molecules throughout the cell. It lies in the cytoplasm and near the cell nucleus at the reticulum of the endoplasm.

Question 2: Outermost layer of the cell is called as________________.

1. Plasma membrane
2. Cytoplasm
3. Nuclear membrane
4. None of the above.

Answer: Option A is correct.

Explanation: The outermost layer of the cell is the plasma membrane. The cell membrane is also known. It is the membrane that separates the cell interior from the external environment in all cells. A plasma membrane in bacterial and plant cells is connected to the cell wall on the outer side.

Question 3: Which of the one is known as blue-green algae?

1. Eukaryotes
2. Prokaryotes
3. Cyanobacteria
4. None of the above.

Answer: Option C is correct.

Explanation: Blue-green algae are prokaryotic and are also known as cyanobacteria. It has a lack of nuclei and it lacks membrane-bound.

Question 4: rRNA is synthesized by_______________.

1. Golgi apparatus
2. Nucleolus
3. Cytoplasm
4. Nucleolus

Answer: Option D is correct.

Explanation: The nucleolus is the region inside the nucleus where the synthesis of rRNA takes place. It is a small and spherical structure.

Question 5: Centriole take part in the formation of ___________.

1. Nucleus
2. Cell plate
3. Spindle
4. To start cell division

Answer: Option C is correct.

Explanation: The centrioles are a tubulin-composed cylindrical structure. They help in spindle formation.