Cell Structure and Cell Theory

Cells are the fundamental structural and functional units of all living organisms.
Schleiden and Schwann formulated the cell theory, and Rudolf Virchow explained that cells divide and new cells arise from pre-existing cells.

Cells are surrounded by a cell wall and a cell membrane.
The nucleus is a dense membrane-bound structure containing DNA.
Cells with membrane-bound nuclei are called eukaryotic, while cells without a membrane-bound nucleus are called prokaryotic.
Cytoplasm is a semi-fluid matrix occupying the volume of the cell.
Eukaryotic cells have other membrane-bound distinct structures called organelles, such as:
- Endoplasmic reticulum (ER)
- Golgi complex
- Lysosomes
- Mitochondria
- Microbodies
- Vacuoles

The lipid component of the cell membrane mainly consists of phosphoglycerides.
Cell membranes also possess protein and carbohydrate.
Membrane proteins can be classified as integral and peripheral.
The fluid mosaic model proposes that the quasi-fluid nature of lipid enables lateral movement of proteins within the overall bilayer.
The membrane is selectively permeable and enables transport of molecules across it.
Osmosis is the movement of water by diffusion from a higher to lower concentration.
Active transport is the energy-dependent process in which ATP is utilized to transport ions or molecules across the membrane against their concentration gradient.

The cell wall is a non-living rigid covering for the plasma membrane.
The cell wall provides shape, protection, and cell-to-cell interaction.
Cell walls are composed of different materials, such as cellulose, galactans, mannans, and minerals like calcium carbonate in algae.
In plants, the primary wall is capable of growth, and the secondary wall is formed on the inner side of the cell.
The middle lamella is a layer of calcium pectate that holds neighboring cells together.

Plastids are organelles present in plant cells that contain specific pigments.
Plastids can be classified into:
- Chloroplasts (containing chlorophyll and carotenoid pigments)
- Chromoplasts (storing fat-soluble carotenoid pigments)
- Leucoplasts (colorless plastids storing nutrients like carbohydrates, oils, and fats)

Chloroplasts are found in the mesophyll cells of leaves.
Chloroplasts are double membrane-bound and contain:
- Stroma (the space limited by the inner membrane)
- Grana (organized flattened membranous sacs)
- Stroma lamellae (connecting the thylakoids of different grana)
- Lumen (the space enclosed by the thylakoid membrane)
Stroma contains enzymes, double-stranded DNA molecules, and ribosomes.

Mitochondria are the sites of aerobic respiration and produce cellular energy in the form of ATP.
Mitochondria have an outer membrane and an inner membrane, forming cristae towards the matrix.
The matrix contains single circular DNA, a few RNA molecules, ribosomes, and protein synthesis components.

Prokaryotic cells are represented by bacteria, blue-green algae, mycoplasma, and PPLO.
Prokaryotic cells have a fluid matrix called cytoplasm and naked genetic material.
In addition to genomic DNA, many bacteria have small circular DNA called plasmids.
Prokaryotes have unique inclusions and a special membranous structure called the mesosome.
Bacteria can be classified into two groups based on the differences in their cell envelopes and response to Gram staining:
- Gram-positive bacteria
- Gram-negative bacteria

ER is a network of tiny tubular structures scattered in the cytoplasm.
ER divides the intracellular space into two distinct compartments: luminal and extra-luminal.
ER can be classified into:
- Rough endoplasmic reticulum (RER) with ribosomes on its surface
- Smooth endoplasmic reticulum (SER) without ribosomes
RER is involved in protein synthesis and secretion, while SER is the site for lipid synthesis.

Golgi apparatus is composed of many flat, disc-shaped sacs or cisternae.
Materials to be packaged in the form of vesicles from the ER fuse with the cis face of the Golgi apparatus and move towards the maturing face.
Glycoproteins and glycolipids are synthesized in the Golgi apparatus.

Vacuoles are membrane-bound spaces containing water, sap, excretory products, etc.
The vacuole is bound by a single membrane called Tonoplast.
In Amoeba, the contractile vacuole is important for excretion.
In many cells, food vacuoles are formed by engulfing food particles.