Chapters

Cell Surface Membrane
Cell Wall
Nucleus
Mitochondria
Chloroplast
Ribosome
Endoplasmic Reticulum
Golgi Apparatus (Golgi Complex)
Large Permanent Vacuole
Vesicle
Lysosome
Centriole
Microtubules
Microvilli
Cilia
Flagella

In this article, we will discuss the cell structures present in the eukaryotic cells. So, let us get started.

Structure of an Eukaryotic Cell — Structure of a Eukaryotic Cell - Image Source: News Medical

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Cell Surface Membrane

A cell surface membrane surrounds every cell. The main objective of this membrane is to control the exchange of materials between the internal and external environment of the cell.
This membrane is partially permeable which means that it allows some materials to pass through it, whereas it prevents other materials to enter the cell.
The cell membrane is created from a phospholipid bilayer of phospholipids having a diameter of around 10 nm.
The cell membrane is also referred to as a plasma membrane.

Cell Wall

Unlike the cell membrane, which is a partially permeable membrane, the cell wall is a freely permeable membrane which means that it allows most substances to pass through it.
The cell wall is present outside the cell membrane.
The cell wall provides structural support to the plasma or cell membrane.
In-plant cells, the cell wall provides structural support to the cell membrane through polysaccharide cellulose.
In many bacterial cells, the cell wall provides structural support to the cell membrane through peptidoglycan.
Plasmodesmata which are narrow threads of cytoplasm that connect the cytoplasm of neighbouring plant cells are surrounded by the cell membrane.

Nucleus

The nucleus of the cell contains the genetic material of the cell known as chromatin.
Chromatin refers to the complex of DNA and histone proteins.
The nucleus is found in all eukaryotic cells.
The nucleus in the eukaryotic cells is relatively large.
A double membrane having several pores known as the nuclear envelope segregates the nucleus from the cytoplasm
Nuclear pores in the nucleus of the eukaryotic cell are vital channels that allow mRNA and ribosomes to move out of the nucleus.
Nuclear pores also enable enzymes like DNA polymerases and signalling molecules to move in.
The nucleus has chromatin. Chromosomes are made up of chromatin.
Generally, in eukaryotic cells, one can observe at least one or more darkly stained regions.
These regions are individually referred to as nucleolus and are termed as sites of ribosomes production.

Mitochondria

In eukaryotic cells, mitochondria (single: mitochondrion) are the site of aerobic respiration. Aerobic respiration is a type of respiration that requires oxygen.
We can observe mitochondria using a light microscope
Mitochondria are surrounded by the double membrane and the inner membrane of the mitochondria contains protein complexes that are critical for the later stages of aerobic respiration embedded within it.
The inner membrane of the mitochondria is folded to form cristae
The matrix created by the cristae has enzymes that are vital for aerobic respiration and the production of ATP
Mitochondrial DNA which are the circular pieces of DNA and ribosomes are also present in the matrix.
Mitochondrial DNA and ribosomes are required for replication.

Chloroplast

Chloroplasts are present in the green parts of the plant. The green colour is due to the photosynthetic pigment known as chlorophyll
Like mitochondria, a double membrane also surrounds the chlorophyll.
Chloroplasts are larger than mitochondria.
Structures known as grana are formed when membrane-bound compartments containing chlorophyll referred to as thylakoids are stacked
Lamellae refer to the thin and flat thylakoid membranes that join the grana together
Chloroplasts are the main site of photosynthesis which refers to the process by which plants make their food.
The light-dependent stage of photosynthesis occurs in thylakoids
The light-independent stage known as the Calvin cycle of photosynthesis occurs in the stroma.
Chloroplasts also have circular pieces of DNA and ribosomes which play an important role in the synthesis of proteins.
Protein synthesis is required in chloroplasts replication and photosynthesis.

Ribosome

Ribosomes are created in the nucleolus and are made up of equal amounts of protein and RNA.
These structures are present freely in the cytoplasm of all the cells or as a component of the rough endoplasmic reticulum in eukaryotic cells
Each ribosome is a composite of ribosomal RNA (rRNA) and proteins
In eukaryotic cells, 80S ribosomes made up of 60S and 40S subunits are present
70S ribosomes made up of 50S and 30S subunits are found in prokaryotes, chloroplasts, and mitochondria
Ribosomes are the site of translation, i.e. protein synthesis.

Endoplasmic Reticulum

Rough Endoplasmic Reticulum (RER)

It refers to a surface that is covered in ribosomes
It is created from continuous folds of membrane continuous with the nuclear envelope
RER processes proteins created by the ribosomes

Smooth Endoplasmic Reticulum

It does not contain ribosomes on the surface
Its main function is different from the RER (rough endoplasmic reticulum)
It plays an important role in producing, processing, and storing carbohydrates, lipids, and steroids.

Golgi Apparatus (Golgi Complex)

It refers to the flattened sacs of the membrane that resembles smooth endoplasmic reticulum
It alters proteins and packages them into lysosomes or vesicles

Large Permanent Vacuole

It refers to the sac in a plant cell that is surrounded by the tonoplast which is a selectively permeable membrane
In animal cells, vacuoles are temporary and small

Vesicle

Vesicle refers to a membrane-bound sac that plays an important role in transportation and storage.

Lysosome

Lysosome refers to a special form of vesicles containing hydrolytic enzymes.
Hydrolytic enzymes break down the biological molecules
These structures break down waste materials like worn-out organelles are employed extensively the cells in the immune system and programmed cell death known as apoptosis

Centriole

Centriole refers to the hollow fibres of microtubules.
A centrosome is created from two centrioles at right angles to each other.
During cell division, a centrosome organizes the spindle fibres.
These structures are absent in flowering plants and fungi

Microtubules

Microtubule makes up the cytoskeleton of the cell which is about 25 nm in diameter.
Composed of alpha (α) and beta (β) tubulin which combine to create dimers.
The dimers are then connected into protofilaments.
A microtubule is made up of 13 protofilaments in a cylinder
The cytoskeleton not only provides support but also helps in the movement of the cell.

Microvilli

Microvilli refer to the projections of the cell membrane that enhance the surface area for absorption.

Cilia

Cilia refer to the hair-like projections made up of microtubules
Cilia enable the movement of substances over the surface of the cell

Flagella

The structure of flagella resembles that of the cilia and are made of longer microtubules
Cilia contract to offer the movement of the cell, for instance, in sperm cells.

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