Eukaryotic Cell, Practice problems and FAQs

The universe has lots of living creatures which are diverse with respect to their size, morphology, appearance etc. You must be aware of the existence of bacteria which are unicellular organisms.

Have you ever wondered if the cells of bacteria and the cells composing your body are the same or not? 

The cells of bacteria and animals are much different. The bacterial cells are prokaryotic in nature, that is they lack a well-defined true nucleus, whereas the cells of protists, fungi, plants, animals are eukaryotic in nature. The term ‘eukaryote’ was derived from the Greek word ‘eukaryon’ where ‘eu’ means ‘advanced or well-defined’ and ‘karyon’ means ‘nucleus’. 

An eukaryotic cell has a more advanced internal organisation in comparison to a prokaryotic cell. So, can you guess who evolved later? Definitely, the eukaryotic cells appeared later in the course of evolution than the prokaryotic cells. 

Apart from having a well-defined nucleus, another distinctive feature of eukaryotic cells is the presence of membrane bound subcellular compartments known as cell organelles. Just like the different organs of a human body, the cell organelles of a eukaryotic cell help in division of labour within the cell and perform specific functions for the cell. Let’s peep into the eukaryotic cells and see what is inside.

 Table of contents

• Cell wall
• Cell membrane
• Cytoplasm
• Nucleus
• Endoplasmic reticulum
• Golgi apparatus
• Ribosomes
• Mitochondria
• Lysosome
• Vacuoles
• Plastid
• Cytoskeletal structure
• Centrosome
• Cilia and flagella
• Difference between plant and animal cell
• Practice problems
• FAQs

Cell wall

Eukaryotic cells such as the plant cells and fungal cells have a rigid outermost covering known as the cell wall. It is present outside the cell membrane and acts as a protective covering which also provides shape, rigidity and support to the cells. The cell wall is freely permeable in nature and allows the transport of substances in and out of the cell. Cell wall in plant cells is composed of cellulose, hemicellulose and pectin. In fungal cells the cell wall is composed of chitin and algal cell walls have cellulose, galactans, mannans and minerals like calcium carbonate.

Fig: Plant cell wall

Cell membrane or plasma membrane

Every cell (prokaryotic and eukaryotic) is surrounded by a thin membranous covering made up of lipids and proteins, known as the plasma membrane. In cells which possess a cell wall, the cell membrane lies internal to the cell wall but in cells which do not have a cell wall, such as animal cells, the cell membrane acts as the outermost covering. Being selectively permeable, the cell membrane helps to communicate with the external environment in a regulated manner and allows the entry and exit of selected substances across itself. 

According to the ‘Fluid mosaic model’ proposed by Singer and Nicholson, the cell membrane or plasma membrane is composed of a bilayer of phospholipids in which proteins are arranged in a mosaic fashion. Glycoproteins and glycolipids may be attached to the extrinsic membrane proteins.

Fig: Structure of cell membrane

Cytoplasm

Cytoplasm is the semi-solid, jelly-like matrix inside the cell which consists of cell organelles. The composition of the cytoplasm includes biomolecules such as amino acids, carbohydrates, proteins, enzymes, vitamins, nucleotides, tRNAs, etc. Proteins in the cytoplasm occur as colloidal substances which is why the ground substance of the cytoplasm is a crystallo colloidal system with water as the dispersion medium. The cytoplasm exists in a non-viscous and fluid sol form and a viscous gel form. The two forms are interchangeable and help in cytoplasmic movements.

The cytoplasm is the site for all biochemical reactions in the cell. The streaming movement of the cytoplasm helps in distribution of metabolites, enzymes, nutrients, etc. It also helps in movement of cell organelles, extension of pseudopodia in amoeboid cells, etc.

Fig: Cytoplasm

Nucleus

The nucleus is the brain of the cell and controls all the activities of the cell. In eukaryotic cells the nucleus is well defined with a nuclear membrane surrounding the nucleoplasm and separating it from the cytoplasm. The nuclear membrane has tiny nuclear pores which allow transport of substances in and out of the nucleus. The nucleoplasm is the transparent semi-fluid ground substance of the nucleus. It consists of a spherical nucleolus which is involved in the synthesis of rRNA. The nucleoplasm also consists of the genetic material in the form of chromatin fibres composed of DNA and histones.

The nucleus plays an important role in cell division and in the transfer of genetic information from one generation to the next.

Fig: Nucleus

Endoplasmic reticulum

The endoplasmic reticulum exists as a network of single membrane-bound tubules and vesicles and serves as the skeletal framework of the cell. It is found in all eukaryotic cells except mature RBCs. Endoplasmic reticulum can be of two types - 

• Rough endoplasmic reticulum has ribosomes on the surface
• Smooth endoplasmic reticulum does not have ribosomes on the surface

Fig: Types of endoplasmic reticulum

The ER helps in the intracellular transport of material. RER acts as a site for protein synthesis whereas SER acts as a surface for the synthesis of lipids and steroids. 

Golgi apparatus

The Golgi body or Golgi apparatus is composed of flattened sac like cisternae, tubules and vesicles. The cisternae are stacked parallel to each other and are concentrically arranged near the nucleus. The vesicles generally pinch off from the tubules and are involved in packaging and transport of material. The plant cells usually have several small golgi complexes known as ‘dictyosomes’.

Fig: Parts of a Golgi apparatus

The Golgi bodies are involved in packaging and modification of proteins, synthesis of glycolipids and glycoproteins and secretory activities of the cell.

Ribosomes

Ribosomes are the protein factories of the cell because they are the sites of protein synthesis in any cell. These are non-membrane bound organelles in the eukaryotic cells which are made up of rRNA and proteins. Eukaryotic ribosomes are of the 80S type, made up of two subunits - 60S and 40S. ‘S’ here stands for the Svedberg unit which is a measure of the ribosomes sedimentation coefficient.

Fig: Structure of ribosome

Mitochondria

Mitochondria are found in all aerobic eukaryotes. These are absent in mature RBCs and prokaryotic cells. Mitochondrion is a double membrane bound organelle in which the inner membrane is folded into finger-like projections called ‘cristae’ which increase the surface area of the membrane. The space between the inner and outer membranes is known as the intermembrane space or the perichondrial space. TThe inner membrane encloses a matrix.

The mitochondrion is called the ‘powerhouse of the cell’ because it is the site of aerobic cellular respiration and ATP synthesis in the cell. 

Fig: Internal structure of mitochondria

Lysosomes

Lysosomes in an eukaryotic cell are single membrane bound organelle which are formed by the pinching off of vesicles from the Golgi apparatus.  These protect the cell by engulfing the foreign bodies entering the cell. These are known as the cell’s suicidal bags since they contain hydrolytic enzymes that can destroy the cell components and degrade the cell under certain conditions. Lysosomes are rarely found in plant cells.

Fig: Intracellular digestion by lysosomes

Vacuoles

Vacuoles are fluid filled membrane bound organelles which are mainly associated with storage of substances. The fluid present within the vacuole is known as the cell sap whereas the membrane covering it is known as the tonoplast. A single large vacuole is centrally placed in plant cells whereas in animal cells, vacuoles are either absent or are small and few in number. Vacuoles help in storage of substances and in maintaining the turgidity of the cell.

Fig: Vacuole within a plant cell

Plastids

Plastids are double membrane bound organelles which are found only in plant cells. Based on the type of pigment present, plastids can be of three types - chloroplast, chromoplast and leucoplast.

Chloroplasts are the primary organelles for photosynthesis. They contain the green pigment, chlorophyll, which helps in trapping the sunlight, required for photosynthesis. These are only present in photosynthetic plants and algae.

Fig: Chloroplast

Chromoplasts are found in the coloured parts of the plant and contain red, orange and yellow coloured pigments such as carotenoids and xanthophylls. The colour of fruits and flowers is primarily due to the presence of chromoplasts in them.

Leucoplasts are non pigment containing plastids which are primarily responsible for the storage of starch, proteins or fats.

Cytoskeletal structures

The cytoskeleton is a network made up of three kinds of elements - microtubules, microfilaments and intermediate filaments. It is found in eukaryotic cells and is not permanent like our skeleton because its components can disassemble and reform. The centrosome, cilia and flagella are made up of elements of the cytoskeleton.

Fig: Cytoskeletal elements

Centrosome

Two perpendicularly placed centrioles linked together by interconnecting fibres, form a centrosome. Centrosomes are found in animal cells whereas plant cells have only centrioles. Centrioles are made up of a complex of proteins, producing microtubules embedded in a matrix known as pericentriolar material. The centrosome plays an important role in cell division. The centrioles help in formation of spindle fibres and form the basal bodies for locomotory structures such as cilia and flagella in animal cells.

Fig: Centrosome

Cilia and flagella

Cilia and flagella are the locomotory structures which are cell membrane extensions present on certain cells. Cilia are short, hair-like extensions of the plasma membrane that beat in a coordinated fashion to allow the movement. Multiple cilia occur throughout the surface of the cell.

Flagella are long, whip-like extensions of the cell membrane which are fewer in number and make lash like whips to allow cellular movement.

Cilia and flagella have a 9+2 arrangement of microtubules, that is 9 peripheral microtubules and two central microtubules.

Fig: Cilia and flagella

Difference between plant and animal cells

Both plant and animal cells are eukaryotic cells but still they have some differences on the basis of their structure and function. These are as follows-

Fig: Plant cell Vs Animal cell

Practice problems

Q1. Which of the following organelles are found in plant cells but not in animal cells?

a) Ribosomes
b) Mitochondria
c) Leucoplasts
d) Golgi bodies

Solution: Leucoplasts are plastids which do not contain any pigment and are present only in plant cells but are absent in animal cells. Their function is to store starch, proteins or lipids. 

Thus, the correct answer is option c.

Q2. Which of the given statements is not true about ribosomes?

a) They are bound by a single membrane
b) They are composed of RNA and proteins
c) They help in protein synthesis
d) The small subunit of the eukaryotic ribosome is 40S

Solution: Ribosomes are not bound by any membrane. They are made of RNA and proteins and are involved with protein synthesis. Eukaryotic cells have 80S ribosomes which are made up of a bigger 60S subunit and a smaller 40S subunit. Thus, the correct option is a.

Q3. Match the following:

a) A - 3, B - 4, C - 1, D - 2
b) A - 2, B -1, C - 4, D - 3
c) A - 1, B - 4, C - 2, D - 3
d) A - 4, B - 4, C - 3, D - 2

Solution: The chloroplast helps to trap light energy and acts as the site of photosynthesis in plant cells.

The lysosomes contain hydrolytic enzymes and help in intracellular digestion of foreign substances.

The vacuoles store water and other solutes and hence help in maintaining turgidity of the cells.

The Golgi apparatus is responsible for packaging and modification of proteins and helps in synthesis of glycolipids and glycoproteins.

Thus, the correct option is a.

Q4. The function of nucleolus is

a) to synthesise lipids
b) help in chromosome duplication
c) synthesise rRNA
d) to help in transport of molecules in and out of the cell

Solution: A spherical nucleolus lies within the nucleoplasm of the eukaryotic cells and is responsible for rRNA synthesis and assembly of the ribosomal subunits.

Thus, the correct option is c.

FAQs

Question 1. Is DNA found only within the nucleus of eukaryotic cells?
Answer: DNA is found in association with histone proteins within the nucleus of eukaryotic cells. Apart from this, the mitochondria and chloroplast of eukaryotes also contain DNA.

Question 2. Why are mitochondria and chloroplasts considered to be autonomous organelles?
Answer: Mitochondria and chloroplasts are considered to be autonomous organelles because they have their own DNA which can self replicate. These organelles also possess 70S ribosomes which enables them to synthesise their own proteins. Both mitochondria and chloroplasts are capable of dividing and multiplying on their own.

Question 3. Do mitochondria and chloroplasts have eukaryotic or prokaryotic ribosomes?
Answer: Like prokaryotes, mitochondria and chloroplasts have a 70S ribosome made up of 30S and 50S subunits.

Question 4. Why is being a eukaryote more advantageous over being a prokaryote?
Answer: Eukaryotic cells are more evolved than prokaryotic cells and have well defined nuclei and membrane bound organelles for specific functions of the cell. Thus, these cells are capable of maintaining unique and separate environments even within the cell which allows more specialisation of functions and makes them capable of performing complex metabolic functions which the prokaryotic cells cannot.

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