● `color{violet}("Mitochondria")` (sing.: mitochondrion), `color{brown}("unless specifically stained, are not")` `color{brown}("easily visible under the microscope.")`

● The number of `color{violet}("mitochondria")` per cell is variable depending on the physiological activity of the `color{violet}("cells.")`

● In terms of `color{violet}("shape and size")` also, considerable `color{violet}("degree of variability")` is observed.

● Typically it is sausage-shaped or `color{violet}("cylindrical")` having a diameter of `color{violet}("0.2-1.0µm")` (average 0.5 µm) and length `color{violet}("1.0-4.1 µm.")`

● Each `color{violet}("mitochondrion")` is a double membrane-bound structure with the outer membrane and the inner membrane dividing its lumen distinctly into two aqueous compartments, i.e., the `color{violet}("outer compartment")` and the `color{violet}("inner compartment.")`

● The `color{violet}("inner compartment")` is called the `color{brown}("matrix.")`

● The `color{violet}("outer membrane")` forms the continuous limiting boundary of the `color{violet}("organelle.")`

● The `color{violet}("inner membrane")` forms a number of `color{violet}("infoldings")` called the `color{brown}("cristae (sing. : crista)")` towards the matrix.

● The `color{violet}("cristae")` increase the surface area.

● The two membranes have their own specific enzymes associated with the `color{violet}("mitochondrial function.")`

● `color{violet}("Mitochondria")` are the sites of `color{brown}("aerobic respiration.")`

● They produce `color{violet}("cellular energy")` in the form of `color{brown}("ATP,")` hence they are called `color{brown}("‘power houses’")` of the cell.

● The matrix also possesses `color{brown}("single circular DNA molecule")`, a few `color{violet}("RNA molecules")`, ribosomes (70S) and the components required for the `color{violet}("synthesis of proteins.")`

● The mitochondria divide by `color{brown}("fission.")`

● `color{brown}("Plastids")` are found in all plant cells and in `color{violet}("euglenoides.")`

● These are easily observed under the `color{violet}("microscope as they are large.")`

● They bear some specific `color{violet}(" pigments,")` thus imparting specific `color{violet}("colours to the plants")`

● Based on the type of `color{violet}("pigments plastids")` can be classified into `color{brown}("chloroplasts, chromoplasts")` and `color{brown}("leucoplasts.")`

● The `color{brown}("chloroplasts")` contain `color{brown}("chlorophyll")` and `color{brown}("carotenoid pigments")` which are responsible for trapping light energy essential for `color{violet}("photosynthesis.")`

● In the `color{brown}("chromoplasts")` fat soluble `color{brown}("carotenoid")` pigments like `color{brown}("carotene, xanthophylls")` and others are present.

● This gives the part of the plant a yellow, orange or red colour.

● The `color{brown}("leucoplasts")` are the colourless plastids of varied `color{violet}("shapes and sizes")` with stored nutrients:

`star` `color{brown}("Amyloplasts ")`store carbohydrates (starch), e.g., potato;
`star` `color{brown}("Elaioplasts")` store oils and fats whereas
`star` `color{brown}("Aleuroplasts")` store proteins.

● Majority of the `color{violet}("chloroplasts")` of the green plants are found in the `color{brown}("mesophyll cells")` of the leaves.

● These are `color{violet}("lens-shaped, oval, spherical, discoid")` or `color{violet}("even ribbon-like organelles")` having variable length (5-10mm) and width (2-4mm).

● Their number varies from 1 per cell of the `color{violet}("𝘊𝘩𝘭𝘢𝘮𝘺𝘥𝘰𝘮𝘰𝘯𝘢𝘴")`, a green alga to 20-40 per cell in the mesophyll.

● Like `color{violet}("mitochondria,")` the `color{violet}("chloroplasts")` are also double membrane bound.

● Of the two, the `color{violet}("inner chloroplast")` membrane is relatively less permeable.

● The space limited by the `color{violet}("inner membrane")` of the `color{violet}("chloroplast")` is called the `color{brown}("stroma.")`

● A number of `color{violet}("organised flattened membranous")` sacs called the `color{brown}("thylakoids,")` are present in the `color{violet}("stroma.")`

● `color{violet}("Thylakoids")` are arranged in stacks like the piles of coins called `color{brown}("grana (singular: granum)")` or the `color{brown}("intergranal thylakoids.")`

● In addition, there are `color{violet}("flat membranous tubules")` called the `color{brown}("stroma lamellae ")`connecting the thylakoids of the `color{violet}("different grana.")`

● The membrane of the `color{violet}("thylakoids enclose")` a space called a `color{brown}("lumen. ")`

● The `color{violet}("stroma")` of the `color{violet}("chloroplast")` contains enzymes required for the synthesis of carbohydrates and proteins.

● It also contains small, double-stranded circular DNA molecules and ribosomes.

● `color{violet}("Chlorophyll pigments")` are present in the `color{violet}("thylakoids.")`

● The `color{violet}(" ribosomes")` of the `color{violet}("chloroplasts")` are smaller (70S) than the cytoplasmic ribosomes (80S).

● `color{violet}("Ribosomes")` are the granular structures first observed under the `color{violet}("electron microscope")` as dense particles by `color{brown}("George Palade (1953)")`.

● They are composed of `color{violet}("ribonucleic acid (RNA) ")` and proteins and are not surrounded by any membrane.

● The `color{violet}("eukaryotic ribosomes")` are `color{brown}("80S")` while the `color{violet}("prokaryotic ribosomes")` are `color{brown}("70S.")`

● Here ‘S’ stands for the `color{brown}("sedimentation coefficient")`; it indirectly is a measure of density and size.

● Both 70S and 80S ribosomes are composed of `color{brown}("two subunits.")`