Cell : The Unit of Life

Robert Brown discovered the nucleus, not the cell. The other statements are correct according to the chapter: Schleiden and Schwann formulated the cell theory, Virchow stated that new cells arise from pre-existing cells, and a unicellular organism performs all life activities within one cell.

According to Virchow’s principle quoted in the chapter, new cells arise from pre-existing cells (*Omnis cellula-e cellula*).

From the chapter: cristae are the infoldings in mitochondria; cisternae are the disc-shaped sacs in Golgi apparatus; thylakoids are the flat membranous sacs in stroma.

The correct statement is that prokaryotes lack membrane-bound organelles. The other statements are incorrect: not all living cells have a nucleus, plant cells have a cell wall but animal cells do not, and cells do not arise de novo from abiotic materials.

A mesosome is a specialised differentiated form of the plasma membrane in prokaryotic cells. It is formed as infoldings of the cell membrane and may appear as vesicles, tubules and lamellae.

Functions of mesosome:
- helps in cell wall formation
- helps in DNA replication
- helps in distribution of DNA to daughter cells
- helps in respiration
- helps in secretion
- increases the surface area of the plasma membrane and its enzymatic content

Neutral solutes move across the plasma membrane by simple diffusion along the concentration gradient from higher concentration to lower concentration.

Polar molecules cannot pass through the nonpolar lipid bilayer in the same way. They are transported with the help of carrier proteins in the membrane.

If transport is against the concentration gradient, it requires energy in the form of ATP and is called active transport.

Two double membrane-bound organelles are mitochondria and chloroplasts.

### Mitochondria
- Double membrane: outer membrane and inner membrane
- Inner membrane forms folds called cristae
- Inner compartment contains the matrix
- Have their own circular DNA and 70S ribosomes
- Function: site of aerobic respiration and production of ATP

### Chloroplasts
- Double membrane: outer and inner membrane
- Inner space is the stroma
- Contain thylakoids arranged in stacks called grana
- Have their own circular DNA and 70S ribosomes
- Function: trapping light energy for photosynthesis

For diagrams, label the membranes, matrix/cristae in mitochondrion and stroma/thylakoids/grana in chloroplast.