Cell : The Unit Of Life

The Cell is the fundamental unit of life, the smallest living structure capable of performing all the essential functions of life. This chapter delves into the various aspects of cell biology, including its structure, functions, and types.

Cell Theory:

All cells arise from pre-existing cells.

Prokaryotic Cells:

Example: Bacteria, archaea
Have ribosomes for protein synthesis.

Eukaryotic Cells:

Complex structure with a nucleus and membrane-bound organelles.
Example: Plants, animals, fungi, protists
Nucleus contains the genetic material (DNA).
Organelles perform specific functions (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus, chloroplasts).

Cell Organelles:

Endoplasmic Reticulum: Network of membranes involved in protein synthesis and lipid synthesis.
Ribosomes: Sites of protein synthesis.
Lysosomes: Contain digestive enzymes to break down cellular waste.
Vacuoles: Storage organelles for water, waste, and other substances.

Cell Membrane:

A semi-permeable barrier that surrounds the cell.
Composed of a phospholipid bilayer.

Cell Wall:

Found in plant cells and some prokaryotes.
Provides structural support and protection.
Made of cellulose in plant cells.

Key Points:

Prokaryotic cells are simpler than eukaryotic cells.
Eukaryotic cells have membrane-bound organelles.
The cell membrane regulates the movement of substances across the cell.

Exercise

1. Which of the following is not correct?

(a) Robert Brown discovered the cell.

(b) Schleiden and Schwann formulated the cell theory.

(c) Virchow explained that cells are formed from pre-existing cells.

(d) A unicellular organism carries out its life activities within a single cell.

Ans : The incorrect statement is (a) Robert Brown discovered the cell.

2. New cells generate from

(a) bacterial fermentation

(b) regeneration of old cells

(c) pre-existing cells

(d) abiotic materials

Ans : The correct answer is (c) pre-existing cells.

3. Match the following.

Column I Column II

(a) Cristae (i) Flat membranous sacs in stroma

(b) Cisternae (ii) Infoldings in mitochondria

(c) Thylakoids (iii) Disc-shaped sacs in Golgi apparatus

Ans :

(a) Cristae – (ii) Infoldings in mitochondria

(b) Cisternae – (iii) Disc-shaped sacs in Golgi apparatus

4. Which of the following is correct:

(a) Cells of all living organisms have a nucleus.

(b) Both animal and plant cells have a well defined cell wall.

(c) In prokaryotes, there are no membrane bound organelles.

(d) Cells are formed de novo from abiotic materials.

Ans : The correct answer is (c) In prokaryotes, there are no membrane-bound organelles.

5. What is a mesosome in a prokaryotic cell? Mention the functions that it performs

Ans :

Mesosomes

are convoluted membranous structures formed by invaginations of the plasma membrane in prokaryotic cells. While once thought to be functional organelles, they are now generally considered artifacts of the electron microscopy preparation process. However, they were previously hypothesized to play various roles in prokaryotic cells, including:

Cell division: Assisting in the formation of the septum during binary fission.
DNA replication: Providing attachment points for replicated DNA and aiding in its distribution to daughter cells.
Cellular respiration: Acting as a site for oxidative phosphorylation, similar to the cristae in mitochondria of eukaryotic cells.
Cell wall formation: Participating in the synthesis and assembly of the cell wall.

6. How do neutral solutes move across the plasma membrane? Can the polar molecules also move across it in the same way? If not, then how are these transported across the membrane?

Ans :

Neutral solutes can move across the plasma membrane through simple diffusion. This process involves the movement of molecules from a region of higher concentration to a region of lower concentration, without requiring energy input.

Polar molecules, on the other hand, cannot directly pass through the lipid bilayer of the plasma membrane due to their hydrophilic nature (attraction to water). They require facilitated diffusion or active transport to cross the membrane.

Facilitated diffusion: In this process, polar molecules move across the membrane with the help of membrane proteins called transport proteins. These proteins act as channels or carriers, allowing specific molecules to pass through the membrane down their concentration gradient without requiring energy.
Active transport: This process requires energy (usually in the form of ATP) to move polar molecules against their concentration gradient. It involves transport proteins that use energy to pump molecules across the membrane.

7. Name two cell-organelles that are double membrane bound. What are the characteristics of these two organelles? State their functions and draw labelled diagrams of both.

Ans :

The two cell organelles that are double membrane bound are:

Mitochondria:

Characteristics:
- Rod-shaped or oval-shaped structures.
- Surrounded by a smooth outer membrane and a folded inner membrane.
- The inner membrane is folded into cristae, which increase the surface area for energy production.
- Contains matrix, a fluid-filled space within the inner membrane.
Functions:
- Produces energy through cellular respiration, converting glucose into ATP (adenosine triphosphate).
- Plays a role in apoptosis (programmed cell death).
- Contains its own DNA, separate from the cell’s nuclear DNA.

8. What are the characteristics of prokaryotic cells?

Ans :

Prokaryotic cells are the simplest type of cell and are characterized by the following features:

Lack of a nucleus: Genetic material is free-floating in the cytoplasm.
No membrane-bound organelles: Organelles like mitochondria, chloroplasts, and the endoplasmic reticulum are absent.
Small size: Typically smaller than eukaryotic cells.
Ribosomes: 70S ribosomes are present for protein synthesis.
Plasmids: Small, extrachromosomal DNA molecules that can replicate independently.
Simple structure: Prokaryotes have a relatively simple internal organization compared to eukaryotic cells.

9. Multicellular organisms have division of labour. Explain.

Ans :

Multicellular organisms have division of labor because different specialized cells perform specific functions within the organism. This specialization allows for greater efficiency and complexity in the organism’s overall functioning.

For example, in humans:

Nerve cells are specialized for transmitting electrical signals and coordinating bodily functions.
Bone cells provide structural support and protection.
Blood cells transport oxygen, nutrients, and waste products.
Epithelial cells form protective coverings for organs and tissues.

10. Cell is the basic unit of life. Discuss in brief.

Ans :

The cell is the basic unit of life. This fundamental principle of biology means that all living organisms are composed of cells, and all life processes occur within cells.

Key characteristics of cells include:

Membrane-bound: Cells are surrounded by a plasma membrane that separates the internal environment from the external environment.
Genetic material: Cells contain genetic material (DNA or RNA) that stores and transmits hereditary information.
Metabolism: Cells are capable of carrying out metabolic processes, such as energy production, nutrient uptake, and waste removal.
Growth and reproduction: Cells can grow and divide to produce new cells, ensuring the continuation of life.
Response to stimuli: Cells can respond to changes in their environment, such as changes in temperature, light, or chemical signals.

11. What are nuclear pores? State their function

Ans :

Nuclear pores are large protein complexes embedded in the nuclear envelope of eukaryotic cells. They serve as channels that allow molecules to pass between the nucleus and the cytoplasm.

Functions of nuclear pores:

Selective transport: Nuclear pores regulate the movement of molecules, including proteins, RNA, and small molecules, between the nucleus and cytoplasm.
Communication: They facilitate communication between the nucleus and the rest of the cell, allowing for the exchange of essential molecules and signals.
Nucleocytoplasmic transport: Nuclear pores play a crucial role in nucleocytoplasmic transport, the process by which molecules are transported into and out of the nucleus.
Regulation of gene expression: Nuclear pores can control the movement of mRNA and regulatory proteins into and out of the nucleus, influencing gene expression.

12. Both lysosomes and vacuoles are endomembrane structures, yet they differ in terms of their functions. Comment.

Ans :

Lysosomes and vacuoles are both endomembrane structures, but they have distinct functions:

Lysosomes:

Function: Act as the “recycling centers” of the cell. They contain digestive enzymes that break down cellular waste products, debris, and foreign substances.
Key Role: Involved in autophagy (self-digestion), where the cell digests its own components, and phagocytosis (engulfing foreign particles).

Vacuoles:

Function: Primarily involved in storage and maintaining cellular turgor (internal pressure).
Key Roles:
- Store water, nutrients, ions, and waste products.
- Help maintain cell shape and turgidity in plant cells.
- Can also play a role in detoxification and defense.

13. Describe the structure of the following with the help of labelled diagrams. (i) Nucleus (ii) Centrosome

Ans :

Nucleus

Structure:

Nuclear Envelope: A double-membrane structure that surrounds the nucleus, separating it from the cytoplasm.
Nuclear Pores: Large protein complexes embedded in the nuclear envelope that allow molecules to pass between the nucleus and the cytoplasm.
Nucleoplasm: A gel-like substance that fills the interior of the nucleus.
Chromatin: The complex of DNA and proteins that make up the genetic material.
Nucleolus: A dense region within the nucleus that is involved in ribosome production.

Centrosome

Structure:

Centrioles: Two cylindrical structures composed of microtubules.
Pericentriolar material: A dense, amorphous substance surrounding the centrioles.

14. What is a centromere? How does the position of centromere form the basis of classification of chromosomes. Support your answer with a diagram showing the position of centromere on different types of chromosomes.

Ans :

It is essential for the proper segregation of chromosomes during mitosis and meiosis.

The position of the centromere along a chromosome can be used to classify chromosomes into different types:

Metacentric: The centromere is located at the middle of the chromosome, resulting in two equal-length arms.
Submetacentric: The centromere is located slightly off-center, creating one longer arm and one shorter arm.
Acrocentric: The centromere is located near one end of the chromosome, resulting in a very short arm and a long arm.
Telocentric: The centromere is located at the very end of the chromosome, resulting in a single arm.