Eukaryotic Cell Structure and Functions

1. Eukaryotic Cell Structure & Functions

2. INTRODUCTION
 Eukaryotic Cells :
 The word Eukaryote which refer to any organism possessing eukaryotic cell is derived from the Greek word "
eu karyon " in which "eu" means "true" and "karyon" means "nucleus".
 A eukaryote is an organism whose cells contain complex structures enclosed within membranes.
 Eukaryotic organisms can be unicellular ( Protista) or multicellular ( Fungi, Plantae & Animalia) .
 Most eukaryotic cells also contain other membrane-bound organelles such as mitochondria, chloroplast,
endoplasmic reticulum and Golgi apparatus etc.

3. Types of eukaryotic cells
Figure 1: Eukaryotic cell - (a) Animal cell, (b) Plant cell
(a) (b)

4. Classification of cell organelles
 Classification of cell organelles on the basis of presence or absence of membrane into three categories
Cell organelles
• Endoplasmic reticulum (ER)
• Golgi apparatus
• Lysosomes
• Vacuoles
• Peroxisomes
• Mitochondria
• Chloroplast
• Nucleus
• Ribosomes
• Nucleolus
• Centrioles ( present in animal cell )
• Cytoskeleton
Single membrane Double membrane Without membrane

5. Cell membrane
 Outermost boundary of cytoplasm which separates the cell from external environment and controls the entrance & exit of
molecules or ions.
 It has highly selective permeable.
 Several types of models have been put forward to explain the structure of cell membrane.
 The most accepted model explaining the structure of cell membrane is fluid mosaic model, proposed by Singer and
Nicolson in 1972.
 Functions:
- Play important role in various
cellular activities, such as signal
transduction, membrane trafficking
as well as energy conversion.
Figure 2: Fluid mosaic model of plasma membrane

6. Endoplasmic reticulum
 Endoplasmic reticulum is a 3-dimentional, complicated and interconnected system of membrane-lined channels
that run through the cytoplasm.
 ER divides the intracellular space into two compartments luminal (inside the ER) and extra-luminal ( cytoplasm).
 The ER is composed of the following three kind of structures (i) cisternae, (ii) tubules, (iii) vesicles.
 Two types:
1. Rough endoplasmic reticulum (RER)
2. Smooth endoplasmic reticulum (SER)
 Functions:
RER- Synthesis of proteins.
SER- Synthesis of fatty acid and phospholipids.
Catabolism and detoxification of toxic substances.
 iii
Figure 3: Endoplasmic reticulum

7. Golgi Apparatus
 The Golgi apparatus is a membrane bound organelle found in most cells. It is responsible for packaging protein into
vesicles prior to secretion & therefore plays a key role in the secretary pathway.
 The Golgi is made of 5-8 folds called cisternae.
 It has two distinct faces i.e., forming face (cis region) and maturing face (trans region).
 The Golgi apparatus modifies proteins and lipids that is receives from the ER. This biochemical leaves the Golgi by
exocytosis before being delivered to different intracellular or extracellular.
 Functions :
- Processing of materials.
- Packaging of materials.
- Labeling and delivery of materials.
Figure 4: Golgi apparatus

8. Lysosomes
 Lysosomes are small vesicles which are bounded by single membrane and contain hydrolytic enzymes in the form of
minute crystalline or semicrystalline granules.
 They are also called acid hydrolyses because these digestive enzymes usually function in acidic medium or
pH of 4-5.
 Lysosomes are also called suicide bags because of the presence of large number of
digestive enzymes or acid hydrolyses in them.
 Lysosomes shows polymorphism.
 Functions:
- Digestion of intracellular and extracellular
materials
- Hormone secretion
- Autophagy
- kill or destroy their own cell through autolysis.
- formation of acrosome of sperm.
Figure 5: Lysosomes

9. Vacuole
Figure 6: Vacuole of a plant cell
 Vacuole is the membrane-bound space found in the cytoplasm.
 In plant cell the vacuoles can occupy up to 90% of the volume of cell.
 Mostly found in plant cell and fungi. However animal cells and protists also contain vacuoles.
 The vacuole is bound by a single membrane called tonoplast.
 Depending upon the contents & functions vacuoles are four
types - (i) Sap vacuoles, (ii) Contractile vacuoles (iii) Food vacuoles
(iv) Air vacuoles
 Functions:
- Maintain the turgor pressure of cell
- Vacuole store materials like water, food, nutrients and waste
Products.
- Molecular degradation and detoxification.

10. Ribosomes
 These are organelles without membrane, are granular and small dot like structure with a diameter of 15nm.
 They are made up to proteins (15%) and RNA (65%).
 Ribosome formed of two unequal subunits- one larger subunit (dome shaped) and another smaller
subunit (oblate-ellipsoid shape).
 Mg2+ ions is required for binding the two subunits.
-In prokaryotes- 70S (50S and 30S)
-In eukaryotes- 80S (60S and 40S)
 Types of ribosomes:
(I) Bound ribosomes- Attached to rough endoplasmic
reticulum.
(ii) Free ribosomes- Distributed in the cytoplasm.
 Function:
- Play an important role in protein synthesis.
Figure 7: Eukaryotic ribosomes

11. Mitochondria
 Mitochondria are double-membrane organelles that contain their own DNA and Ribosomes.
 Mitochondria are cell organelles of aerobic eukaryotes which take part in oxidative phosphorylation and
Krebs cycle.
 They are called power houses of cell because they are the release of energy in aerobic respiration.
 A mitochondrion has two membranes : The outer membrane and inner membrane.
 The inner membrane of mitochondria has many folds and
form layered structure called cristae.
 Functions:
- Production of energy.
- Synthesis of ATP.
- help in building certain part of blood & hormones
like testosterone and estrogen.
- Play important role in the process of apoptosis.
- Store and release of calcium.
Figure 8: Mitochondria

12. Plastids
 Plastids are semi-autonomous organelles having DNA and double membrane envelope which store and
synthesize various type of organic compounds.
 Depending upon their colour, plastids are of three main type-
Plastids
Chloroplast Chromoplast Leucoplasts
• Contain chlorophyll and
carotenoid pigment.
• Sites of CO2
assimilation.
• Fat soluble carotenoid
pigments present.
• Gives part of plant yellow,
orange or red colour .
• Protects against the
Photo-oxidation.
• Colourless plastids.
1. Amyloplasts : store starch
2. Elaioplasts : store lipid or fat
3. Aleuroplasts : store proteins

13. Chloroplast
 Functions:
- Most important function of the chloroplast
is to synthesize food through photosynthesis.
- Absorbs light energy & converts into chemical
energy .
- Produces ATP through photosynthesis.
- Produce NDPH & O2 by photolysis of water.
 Chloroplast are double-membrane organelles that contain their own DNA and Ribosomes.
 Chloroplasts are greenish plastids which possess photosynthetic pigments, chlorophyll and carotenoids, and take part in
the synthesis of food.
 Chloroplasts has two membranes : The outer membrane
and inner membrane.
 A chloroplast has three parts- Envelope, matrix (stroma),
& thylakoids.
Figure 9: Structure of Chloroplast

14. Peroxisomes
 Peroxisomes are spherical, sac-like structures, bounded single membrane.
 They contain digestive enzymes for breaking down toxic materials in the cell & oxidative enzymes for metabolic
activity.
 These contain 'peroxide' - producing enzymes (oxidases) and 'peroxide' - destroying enzymes (catalases).
 Functions:
- Perform β- oxidation process.
- Degradation of toxic substances.
- Major site of oxygen utilization in the cell.
- Participate in the formation of myelin.
- Play an important role in the formation of bile acids.
- Accelerate gluconeogenesis from fat.
Figure 10: Peroxisome

15. Nucleus
 Nucleus is a specialized double membrane bound protoplasmic body which contains all genetic information for
controlling cellular metabolism and transmission to the posterity.
 A nucleus in the non-dividing or metabolic phase is called interphase nucleus.
 Nucleus envelope consists of two concentric membranes
that are penetrated by nuclear pore complexes (NPC).
 Small round granular structure present in nucleus called
nucleolus.
 Functions:
- Storage of genetic information.
- Controls cellular activities like metabolism, protein
synthesis, growth and cell division.
- Synthesis of RNA.
Figure 11: Nucleus

16. Cytoskeleton
 It is complex network of structures of various sizes present throughout the cytoplasm.
 They are three types :- (i) Microtubule, (ii) Intermediate filaments, (iii) Microfilament.
1. Microtubules:
- Straight, hollow & tubular organelles without limiting
membrane.
- Determine the shape of cell.
- Form spindle fibers.
2. Intermediate filaments:
- Form network around the nucleus and extend to periphery
of the cell.
- Helps to maintain the shape of the cell.
3. Microfilaments:
- Long & fine thread like structures.
- Give structural strength to the cell.
- Responsible for cellular movements.
Figure 12: Types of Cytoskeletons