24.

(a) Why are xylem and phloem classified as complex tissues? Describe the structure of phloem.
(b) Describe the ultra-structure of chloroplast.
(c) State three functions of the placenta.

(a) Xylem and phloem classified as complex tissues because they are made up of more than one type of cell which have a common origin and work together to perform a special function. Structure of Phloem:

• Sieve tubes: living cells which are slender and elongated tubular cells. Transverse walls are obliquely placed and are perforated by a number of pores to form a sieve plate/ enucleated/ nucleus absent

• Companion cells: lie in association with sieve tube elements and assist in food translocation.

• Phloem parenchyma: living cylindrical parenchymatous cells with thin walls and dense cytoplasm.

• Phloem fibres: sclerenchymatous cells or bastfibres

(b) Ultra-structure of a Chloroplast: The chloroplast are double membrane bound organelles and are the site of photosynthesis The chloroplasts have a system of three membranes: the outer membrane, the inner membrane and the thylakoid system. The outer and the inner membrane of the chloroplast enclose a semi-gel-like fluid known as the stroma. This stroma makes up much of the volume of the chloroplast, the thylakoids system floats in the stroma.

Outer membrane - It is a semi-porous membrane and is permeable to small molecules and ions

Intermembrane Space - It is usually a thin intermembrane space about 10-20 nanometers and it is present between the outer and the inner membrane of the chloroplast.

Inner membrane - The inner membrane of the chloroplast forms a border to the stroma. It regulates passage of materials in and out of the chloroplast. In addition of regulation activity, the fatty acids, lipids and carotenoids are synthesized in the inner chloroplast membrane.

Stroma
Stroma is a alkaline, aqueous fluid which is protein rich and is present within the inner membrane of the chloroplast. The space outside the thylakoid space is called the stroma. The chloroplast DNA chlroplast ribosomes and the thylakoid sytem, starch granules and many proteins are found floating around the stroma.

Thylakoid System
The thylakoid system is suspended in the stroma. The thylakoid system is a collection of membranous sacks called thylakoids. The chlorophyll is found in the thylakoids and is the sight for the process of light reactions of photosynthesis to happen. The thylakoids are arranged in stacks known as grana.
Each granum contains around 10-20 thylakoids.

Important protein complexes which carry out light reaction of photosynthesis are embedded in the membranes of the thylakoids. The Photosystem I and the Photosystem II are complexes that harvest light with chlorophyll and carotenoids, they absorb the light energy and use it to energize the electrons.A large protein complex known as the ATP synthase controls the concentration gradient of the hydrogen ions in the thylakoid space to generate ATP energy and the hydrogen ions flow back into the stroma.

(c) Functions of the Placenta:

i. Transportation of nutrients from maternal blood to foetus.

ii. Helps in the exchange of oxygen and carbon dioxide between foetus and mother.

iii. Removes nitrogenous wastes from foetus by diffusion