cell

1. Discovery & History of Cells

The concept of cells was first introduced by Robert Hooke, an English scientist, in

1665. He used a simple microscope to observe slices of cork and noticed small

compartments that resembled a honeycomb. He named these compartments "cells,"

which means "little rooms" in Latin. It is important to note that Hooke was observing

dead plant cell walls, not living cells.

2. Cell Theory

The Cell Theory was proposed by two German biologists, **Matthias Jakob

Schleiden** and Theodor Schwann, in the 1830s. It was later confirmed by **Robert

Virchow** in 1855. The Cell Theory states:

1. All living organisms (plants and animals) are made up of cells.

2. Cells are the basic structural and functional units of all living organisms.

3. New cells arise from pre-existing cells through cell division.

4. Every organism starts its life as a single cell.

3. Instruments for Studying Cells

Cells are microscopic and cannot be seen with the naked eye. The study of cells has

been made possible through advanced microscopes, such as:

- Compound Microscope: Uses multiple lenses to magnify small objects.

- Electron Microscope: Uses beams of electrons to achieve much higher magnification

and resolution than light microscopes.

4. Unicellular & Multicellular Organisms

Living organisms can be classified based on the number of cells they possess:

- Unicellular Organisms: These organisms consist of a single cell. Examples include:

- Amoeba - Paramecium - Bacteria - Euglena

Unicellular organisms perform all life functions within that single cell.

- Multicellular Organisms: These organisms are made up of many cells. Most plants

and animals fall into this category. They start their life as a single fertilized egg cell

(zygote) and develop into a multicellular organism through cell division.

5. From Cell to Organism

In multicellular organisms, cells represent the lowest level of organization. Here’s how

they organize:

- Cells combine to form Tissues (groups of similar cells performing a specific

function).

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- Different tissues join to form Organs.

- Organs that work together form an Organ System.

- Various organ systems together make up a Living Organism.

6. Cell Shape & Size

Cells come in various shapes and sizes, which often relate to their functions:

- Spherical: Eggs of many animals.

- Spindle-shaped: Smooth muscle fibers.

- Elongated: Nerve cells.

- Oval: Red blood cells.

- Branched: Osteocytes (bone cells).

Some cells, like Amoeba and White Blood Cells, can change shape.

Cell Size

Cells are measured in micrometers (μm). The size of cells varies widely, from tiny

bacteria to large ostrich eggs. For example, red blood cells are among the smallest,

while nerve cells can be very long.

7. Parts of the Cell

A typical cell consists of three main parts:

1. Cell Membrane: A thin, flexible barrier that surrounds the cell, controlling the entry

and exit of substances. It is selectively permeable.

2. Cytoplasm: A jelly-like fluid inside the cell where chemical reactions occur. It

contains various organelles.

3. Nucleus: The control center of the cell, containing genetic material (DNA). It is

surrounded by a nuclear membrane and contains nucleoplasm, nucleolus, and

chromosomes.

8. Cell Organelles

Cell organelles are specialized structures within the cell that perform specific functions:

- Mitochondria: Known as the powerhouses of the cell, they produce energy through

respiration.

- Vacuoles: Sac-like structures that store substances. Plant cells have larger vacuoles

than animal cells.

- Endoplasmic Reticulum: A network involved in the synthesis and transport of cell

SCIENCE NOTES 2

products.

- Ribosomes: Sites of protein synthesis.

- Golgi Bodies: Involved in the secretion of substances like enzymes and hormones.

- Plastids: Found only in plant cells, they contain pigments for photosynthesis and

storage.

- Centrosome: Involved in cell division, found only in animal cells.

9. Comparison Between Plant & Animal Cells

While plant and animal cells share many similarities, they also have distinct differences:

Feature Plant Cells Animal Cells

Cell Wall Present (made of cellulose) Absent

Vacuoles Large central vacuole Small vacuoles

Plastids Present (chloroplasts) Absent

Shape Usually rectangular Irregular

10. Cell Division & Growth

Cell division is crucial for growth and repair. When a cell reaches a certain size, it

divides into two new cells, called daughter cells. The process involves:

1. The nucleus divides first.

2. The cytoplasm divides, forming two separate cells.

This process is essential for replacing dead or damaged cells and for reproduction in

unicellular organisms.

11. Cellular Respiration

Cellular respiration is a metabolic process that converts glucose into energy (ATP). One

molecule of glucose can generate about 30-32 ATP molecules, which the body uses

for various functions.