Cell and Its Organelles

Session Objectives

  • Name the prominent cellular organelles and state their functions in cells.
  • Explain the structure of the plasma membrane

What's inside a typical human cell?

Visual 2.1 Inside a human cell
Activity 2.1

From your basic knowledge about a typical human cell, enlist the various organelles present inside a cell.


Let's see each one in detail.

Nucleus

Visual 2.3 Structure of nucleus

Characteristic features of a nucleus

  • The nucleus is made up in large part of the chromosomes that carry a complete blueprint of all the individual characteristics of the animal. Each chromosome is made up of a giant molecule of DNA.
  • The DNA strand is wrapped around a core of histone proteins to form a nucleosome. The whole complex of DNA and proteins is called chromatin.
  • The nucleus of most cells contains a nucleolus, a patchwork of granules rich in RNA. They are the site of synthesis of ribosomes.
  • The interior of the nucleus has a skeleton of fine filaments that are attached to the nuclear membrane or envelope, which surrounds the nucleus.
  • The membrane is permeable only to small molecules. However, it contains nuclear pore complexes.

Mitochondria

Visual 2.5 Structure of mitochondria

Over a billion years ago, aerobic bacteria were engulfed by eukaryotic cells and evolved into mitochondria, providing the eukayotic cells with the ability to form ATP by oxidative phosphorylation.

Characteristic features of mitochondria

  • Each eukaryotic cell can have hundreds to thousands of mitochondria.
  • In mammals, they are generally depicted as sausage-shaped organelles, but their shape can be quite dynamic.
  • Each mitochondrion has an outer membrane, an intermembrane space, an inner membrane which is folded to form shelves (cristae), and a central matrix space.
  • Mitochondria have their own genome, but with much less DNA than in the nuclear genome.

Did you know?

Mitochondria are exclusively maternal in origin, since the zygote mitochondria are derived from the ovum.

Endoplasmic reticulum

They are either smooth or rough!!!

Visual 2.6 Structure of Endoplasmic reticulum
Visual 2.7 Structure of endoplasmic reticulum
Activity 2.2

Golgi apparatus

Visual 2.9 Structure of golgi apparatus

Characteristic features of Golgi apparatus

  • The Golgi apparatus is a collection of membrane-enclosed sacs (cisternae) that are stacked like dinner plates. One or more Golgi apparati are present in all eukaryotic cells, near the nucleus.
  • The Golgi apparatus is a polarized structure, with cis and trans sides.

What is the mechanism of Golgi vesicular traffic?

Membranous vesicles containing newly synthesized proteins bud off from the rough endoplasmic reticulum and fuse with cistern on the cis side of the golgi apparatus.

The proteins are then passed via other vesicles to the middle cisterns and finally to the cistern on the trans side, from which vesicles branch off into the cytoplasm.

From the trans Golgi, vesicles shuttle to the lysosomes and to the cell exterior.

Watch the video that explains the golgi vesicular traffic.

Visual 2.10 Golgi Apparatus

Lysosomes

Visual 2.12 Structure of lysosomes

About lysosomes

  • Lysosomes are large, irregular structures surrounded by membranes seen in the cytoplasm of a cell.
  • Their interior is more acidic than the rest of the cytoplasm, which is maintained by the action of a proton pump.
  • External materials such as endocytosed bacteria, worn-out cell components are digested inside the lysosomes, which contain over 40 types of hydrolytic enzymes.
Activity 2.3

What would happen if the lysosomes were to break open and release their contents into the cytoplasm of the cell? Will the lysosomal enzymes pose any threat to the cytoplasm or its contents?


Peroxisomes

Visual 2.13 Structure of peroxisomes

Characteristic features of Peroxisomes

  • Peroxisomes are 0.5 µm in diameter, are surrounded by a membrane, and contain enzymes that can either produce H2O2 (oxidases) or break it down (catalases).
  • Peroxisomes can form by budding of the endoplasmic reticulum, or by division.
  • The matrix of the peroxisomes contains more than 40 enzymes, which operate in concert with enzymes outside the peroxisome to catalyze a variety of anabolic and catabolic reactions (eg, breakdown of lipids).

Ribosomes

Visual 2.15 Structure of ribosomes

About Ribosomes

  • The ribosomes are complex structures, containing many different proteins and atleast 3 ribosomal RNAs.
  • Each ribosome is made up of a large subunit and a small subunit called, on the basis of their rates of sedimentation in the ultracentrifuge, the 60S and 40S subunits.
  • The ribosomes that get attached to the rough endoplasmic reticulum synthesize all transmembrane proteins, most secreted proteins, and most proteins that are stored in the Golgi apparatus, lysosomes and endosomes.
  • The free ribosomes synthesize cytoplasmic proteins such as hemoglobin and the proteins found in peroxisomes and mitochondria.

Centrosomes

Visual 2.16 Structure of centrosomes

Features of Centrosomes

  • The centrosome, which is usually seen near the nucleus of a typical cell, is made up of two centrioles and surrounding amorphous pericentriolar material.
  • The centrioles are short cylinders arranged so that they are at right angles to each other.
  • Microtubules in groups of three run longitudinally in the walls of each centriole.
  • The centrosomes are microtubule-organizing centers(MTOCs) that contain γ-tubulin.
  • When a cell divides, the centrosomes duplicate themselves, and the pairs move apart to the poles of the mitotic spindle, where they monitor the steps in cell division.
Visual 2.17 Centrosomes in mitosis

The cell, with all the organelles listed above, is surrounded by a selectively permeable membrane called the plasma membrane

APPLY & INTEGRATE

Integrate with Biochemistry to know the selectively permeable nature of the plasma membrane.

Let's learn about the plasma membrane in detail.

Plasma membrane

Visual 2.19 Structure of plasma membrane

Characteristic features of plasma membrane

  • The plasma membrane is generally is 7.5 nm (75 AO) thick.
  • The major lipids present in the membrane are phospholipids such as phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine.
  • The phospholipid molecules contain a head end and a tail end. The head end of the molecule contains the phosphate portion and is relatively soluble in water (polar, hydrophilic), and the tail ends are relatively insoluble (nonpolar, hydrophobic).
  • The hydrophilic ends of the molecules are exposed to the aqueous environment that bathes the exterior of the cells and the aqueous cytoplasm; the hydrophobic ends meet in the water-poor interior of the membrane.
  • Proteins, which makes up on average 50% of the mass of the membrane, exist as separate globular units and many pass through or are embedded in one leaflet of the membrane (integral proteins), whereas others (peripheral proteins) are associated with the inside or outside of the membrane.
  • Proteins in the membrane serve many functions. They act as cell adhesion molecules (anchor the cells to their neighbors or basal lamina), as pumps (active transport of ions across membrane), as carriers (facilitated diffusion), as ion channels (passage of ions into or out of the cell), as receptors (bind ligands or messenger molecules), and some act as enzymes (catalyze reactions at the membrane surface).
APPLY & INTEGRATE
Activity 2.4

The specialization of the cells in various organs is considerable, and no cell can be called "typical" of all cells in the body. However many organelles are common to most cells. Given below is the structure of a melanocyte.

Visual 2.20 Melanocyte

Q1. Where do you find these melanocytes and where do they derive from during embryogenesis?


   

BIG PICTURE

Yoshinori Ohsumi, a Japanese cell biologist, was awarded the Nobel Prize in Physiology or Medicine 2016, for his discoveries on how cells recycle their content, a process known as autophagy, a Greek term for "self-eating."

Lysosomes contain enzymes for digestion of cellular contents. A type of vesicle called autophagosome exists within the cell. As the autophagosome forms, it engulfs cellular contents, such as damaged proteins and organelles. Finally, it fuses with the lysosome, where the contents are degraded into smaller constituents.

Read more about the research through: Source link

Activity 2.5
Activity 2.6

End of Session

Leave your valuable feedback

DEEP DIVE
Topic Description Citation
Mitochondrial Briefing on the types, causes, symptoms and treatment of various mitochondrial diseases Mitochondrial disease. Wikipedia, the free encyclopedia. 23 May 2017; accessed through the link on 25 May 2017. Source link
Lysosomal storage diseases An extensive list on lysosomal storage diseases. LIST OF LYSOSOMAL DISORDERS.LDNZ. accessed on 25 May 2017. Source link
The cytoskeleton Description on the cytoskeleton of a cell The Cytoskeleton - Kimball's Biology Pages.07 Dec 2013. accessed on 05 July 2017.

External link Source link

EXAM PREP

Quick Retrieve

The video takes you on a quick tour inside the human cell and its organelles.

Visual 2.22 Biology: Cell Structure

Short Notes

With the help of a diagram, write briefly about:

  • Plasma membrane
  • Mitochondria
  • Endoplasmic reticulum
  • Lysosomes
  • Golgi apparatus

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