Plants are capable of synthesising their energy source/food through the process of photosynthesis.
Plants take in carbon-dioxide and water which is then converted into carbohydrates in the presence of sunlight.
This kind of nutrition is known as autotrophic nutrition.
Photosynthesis requires energy in the form of light to drive the chemical reaction by which inorganic molecules are converted to glucose.
Light energy is absorbed by a green pigment called chlorophyll present in plant cells.
The carbon di-oxide needed for photosynthesis is absorbed from the air.
Tiny pores present on the lower side of the leaf - called stomata regulate the flow of carbon dioxide into the leaf. Gaseous exchange also occurs across the surface of stems and roots.
When a plant does not need to absorb CO2 the stomata actually closes. This avoids unecessary loss of water from the plant.
The water used in photosynthesis is taken up from the soil. While other elements such as nitrogen - which are essential for synthesis of protiens are taken up in the form of inorganic nitrates.
In the process of photosynthesis plants create glucose and release oxygen.
Below you can see the different ways in which a plants uses glucose.
1. Thousands of glucose molecules can be linked together to form starch. Starch is then stored inside plant cells as grains.
2.Glucose molecules are also used to form cellulose, which is used in building the cell wall of plants.
Plants absorb water and other nutrient from the soil through their roots.
In large plants, diffusion is not an efficient method of transporting these substances throughout the plant.
Plant transport systems need to move stored energy from leaves and raw material such as water from the roots.
Two specialised systems exist in plants for these purposes.
Xylem moves water and minerals absorbed from the soil.
While phloem moves the product of photosynthesis from leaves to other parts of the plant.
Xylem is actually a tissue which forms a continous system of water conducting channels reaching all parts of the plant - a bit like vessels in the human body.
At the roots of the plant, cells in contact with the soil take in ions. This creates a difference in concentration of these ions between the root and the soil.
To eliminate this difference, water from the soil moves into the root.
This creates a steady flow of water into the root xylem. The water is then steadily pushed upwards.
You might be wondering now, whether the pressure created at the roots is enough to push water up trees that can sometimes be meters long. That is a fair question.
To understand the mechanism through which plants push up water, we have to recall the tiny pores called stomata.
Evaporation of water from the stomata creates a suction which pulls the water up from the roots.
This process is known as transpiration.
The process of transportation of stored energy from the leaf to other parts of the plants is called translocation.
Translocation occurs with the help of the tissue called phloem.
Unlike the flow of water which can be achieved through suction, the transport of stored energy (food) requires expending energy.
The area of the plant where the food is made is called the source and the area where the food needs to be transported to is called the sink.
Energy from ATP is used to increase osmotic pressure in the phloem tissue.
Osmotic pressure is the pressure required to nullify osmosis. Remember in osmosis water moves from a higher concentration gradient to a lower concentration gradient.
Because of the increase in osmotic pressure in the phloem, water moves into it and the energy/food from the phloem moves to areas which have less pressure.
Excretion in plants is quite different from excretion in human beings.
Plants can store a lot of their waste in parts (leaves stems) which fall off the plant.
They can also store waste in cellular vacuoles or in the form of resins and gum.
Oxygen - which can be thought of as a waste product of photosynthesis is released through stomata.