Cell Structure and Functions Notes

Introduction:

Our earth is inhabited by different kinds of living organisms, who look very different from each other. Living creatures' bodies are built up of tiny units known as cells. It is defined as follow:

Cell: The cell is the basic structural and functional unit of living organisms.

Discovery of cell:

The cell is discovered by Robert Hooke in 1665. He observed the cells in a cork slice with the help of a self-designed microscope (Cork cells are dead). He observes some tiny compartments like a honeycomb. Robert Hooke called these compartments "cells".
With the upgraded microscope, Leeuwenhoek (1674) found free-living cells in pond water.

Cell Theory:

Schleiden and Schwann (1839) formulated the cell theory.

"Cell theory states that the bodies of all organisms are made up of cells and their products so that cells are units of both structure and function of living organisms".

Fundamental features of cell theory:

All living organisms are composed of cells.
The cell is the fundamental unit of life.
All the new cells come from pre-existing cells.

Structure of cell:

The components of a cell include cell wall, cell membrane, cytoplasm, nucleus, and cell organelles.

1. Cell Membrane /Plasma Membrane:

The cell membrane offers support and protection to the cell.
It is the outer covering of a cell within which all other organelles, such as cytoplasm, nucleus are enclosed.
It is a permeable membrane that allows selected molecules to flow into and out of the cell.
It forms the wall-like structure between two cells as well as between the cells and its surrounding.

2. Cell Wall:

The most prominent part of the plant's cell structure. It is composed of cellulose, hemicellulose, and pectin.
Exclusively found in plant cells. It protects the plasma membrane as well as other cellular components.
It is a stiff and rigid structure that surrounds the cell membrane.
It provides shapes and support to the cells and protects them from mechanical shocks and injuries.

3. Cytoplasm:

The cytoplasm is a thick, jelly-like substance present inside the cell membrane.
The cytoplasm is where the majority of a cell's chemical processes take place.

Two parts of the cytoplasm are:

Cell organelle
Cytosol cell matrix

Cell organelle: Cells are composed of various cell organelles that perform certain specific functions to carry out life processes. Cell organelle such as:

Endoplasmic reticulum
Ribosome
Mitochondria
Lysosomes
Golgi apparatus
Vacuoles
Centrioles
Plastid, etc.

4. Nucleus:

The nucleus contains the hereditary material of the cell, the DNA.
Brain or Headquarter of the cell.
It sends signals to the cell to grow, mature, divide and die.
The nuclear envelope, which isolates the DNA from the rest of the cell, surrounds the nucleus.

Nucleolus: The nucleolus is the place where ribosome synthesis takes place. Also, it is involved in controlling cellular activities and cellular reproductions.
Nuclear Membrane: It protects the nucleus by forming a boundary between the nucleus and other cell organelles.
Chromosome: It plays a crucial role in determining the sex of an individual. Each human cells contains 23 pairs of chromosomes.

Cell Organelles

1. Endoplasmic Reticulum:

It is involved in the transportation of substances throughout the cell.
It is essential for glucose metabolism as well as the production of lipids, steroids, and proteins.

2. Golgi Bodies/Dictyosome:

It is called the cell's post office as it is involved in the transportation of materials within the cells.

3. Ribosomes:

Ribosomes are the cell's protein makers.
In eukaryotic cells, the ribosome is of the 80S (40S + 60S ) type.
In prokaryotic cells, the ribosome is of the 70S (50S + 30S) type.

4. Mitochondria: The powerhouse of the cell

It is a double membrane structure with an outer membrane bring smooth and porous whereas the inner membrane is thrown into a number of folds called cristae.
They contain their own DNA and ribosome.
Function: They are the sites of cellular respiration, hence providing energy for the vital activities of living cells.

5. Lysosomes:

They are small, spherical, sac-like structures that contain several digestion enzymes enclosed in a membrane.
Function: Help in the digestion of foreign substances and worn-out cell organelles.

6. Chloroplast:

Primary organelles for photosynthesis contain a pigment called chlorophyll.

7. Vacuoles:

Food, water, and other waste products are stored in the cell by vacuoles.

The function of Cell:

A cell performs these major functions essential for the growth and development of organisms.

Major functions of cells are:

Provide support and structure.
It facilitates growth by mitosis.
It allows the transport of substances and provides energy.

Types of cells:

The cell is primarily classified into two types:

Prokaryotic cells.
Eukaryotic cell.

FEATURES	PROKARYOTIC CELL	EUKARYOTIC CELL
Nucleus	No membrane-enclosed nucleus. Genetic material consists of one or more folded DNA strands without associated histones.	Enclosed in the nuclear membrane. Genetic material presents as more than one chromosome. DNA associated with histones (only 40-50%DNA).
Cell Wall	Cell wall present in all but the smallest prokaryotes when present contains muramic acid.	Variable, if present, does not contain muramic acid.
Plasma membrane	Present	Present
Size ranges	0.1-5 micrometer	5-100 micrometer
Organelles	Membrane-bounded organelles not found	Membrane-bound organelles present.
Ribosome	Present free in the cytoplasm. 70S type	Present free in the cytoplasm or associated with the endoplasmic reticulum. 80S type
Flagella	Sometimes present, made up of single-strand and without differentiation.	The flagella if present is double-stranded and shows differentiation.
Plasmids and Pilli	Found in many prokaryotes	Absent in eukaryotes.
Endoplasmic reticulum	Absent	Present
Protein Synthesis	Transcription and Translation take place in the cytoplasm	Transcription occurs in the nucleus and translation takes place in the cytoplasm.
Duration of cell cycle	Short, take 20-60 minutes to complete	Long, take place 12-24 hours to complete.
Spindle formation	The mitotic spindle is not formed in cell division	The mitotic spindle is formed in cell division.