Nutrition
“Strength doesn’t come from what you can do; it comes from overcoming the things you once thought you couldn’t.”
– Rikki Rogers
HELLO, STUDENTS. How are you all? Having a good week so far? I know you are! You have set up your study timetable and you have been working with it. You are now comfortable in 10th grade/fourth form and, therefore, life is bearable! You are becoming more organised, hence, less stressed! Life is good!
Last week, we concluded the lessons on diffusion and osmosis. This week, we will begin a new series of lessons. We will be looking, first, at nutrition in plants, then at nutrition in animals. We begin with the types of nutrition.
Three types of nutrition are listed below. The first two deal with plant and animal nutrition.
Autotrophic – Auto = self, trophic = feeding. The word in its simplest form means self-feeding. This type of nutrition is carried out by organisms that make their own food. The majority of these are green plants.
Heterotrophic – Hetero = different, trophic = feeding. This type of nutrition is carried out by all organisms that are not autotrophs, e.g., a lizard eating an insect or a human being eating a hamburger.
Saprophytic – These are organisms that feed on decaying organic matter, e.g., fungus feeding on stale bread.
AUTOTROPHIC NUTRITION
As shown by the definition, this type of nutrition is mainly carried out by plants and is known as PHOTOSYNTHESIS. Why can plants carry out autotrophic nutrition and animals cannot? The answer is quite simple. The plants have the elements that are required for the process to take place and animals do not. What are these elements?
These are:
Two types of raw materials. Two conditions that are suitable.
What is meant by ‘raw materials’? The raw materials are the compounds that are used by the plant to make their own food. These are:
1. Carbon dioxide (CO2)
2. Water (H2O)
What is meant by the ‘conditions’? These are the factors that must be present for the food to be made. For example, in cooking, some form of heat must be present in order for cooking to take place; in a similar way, photosynthesis cannot take place without these conditions. The conditions are:
1. Chlorophyll
2. Sunlight
WHERE DO THESE COME FROM?
Figure 1 below will answer this question. Figure 1
The diagram shows that the sun provides the sunlight, the carbon dioxide comes from the air around the plant, the water comes from the soil and the chlorophyll is provided by the leaf. Do you remember the name of the process by which carbon dioxide enters the leaf? Of course, you do! Did you say diffusion? If you did, you are correct! Now, do you remember the name of the process by which water enters the roots? Did you say osmosis? If you did, again you are correct! Keep it up! You are doing great!
Where does photosynthesis take place? It takes place in the green parts of the plant, predominantly the leaf.
WHAT FACTORS MAKE THE LEAF SUITABLE FOR PHOTOSYNTHESIS?
These factors can be divided into two groups: one that deals with the external features of the leaf, and the other that deals with the adaptations of its internal structure.
THE LEAF AS A UNIT (EXTERNAL FEATURES)
The stem is positively phototrophic. This means that the upper portion of the plant always faces the rays of the sun. The leaves are on the upper section (above the ground) of the plant, and so the leaves will always be exposed to sunlight, hence satisfying one of the conditions for photosynthesis.
The lamina is broad, providing a large surface area which is held perpendicular (at right angles) to the light source. This enables it to receive maximum sunlight.
The leaves are thin – so the cells carrying out the photosynthesis can receive a lot of light and gases can be exchanged rapidly.
The leaves on the tree are arranged in a mosaic. This minimises overlapping/shading and maximises exposure to light.
The leaf’s cuticle is transparent, allowing light to pass through. It is also waxy and this reduces water loss.
Adaptations of the internal structure of the leaf. Figure 2
1.Epidermis – Chloroplasts are not present in epidermal cells, hence they do not carry out photosynthesis. The epidermis allows sunlight to pass through to the palisade cells.
2. Palisade mesophyll: a. These cells are arranged side by side like a picket fence so that the cells can get maximum sunlight.
b. Chloroplasts are arranged in similar fashion; chloroplasts also move within the cytoplasm of the cells (cytoplasmic streaming). In doing this, all the chloroplasts can capture the maximum amount of sunlight.
c. Cells are tightly packed with chloroplasts.
3. Spongy mesophyll – Cells are loosely packed, causing air spaces to be created between them. The air spaces serve to hold the gases, carbon dioxide, oxygen; changes in their concentration (decreasing/increasing) create diffusion gradients, facilitating movement of these gases.
4. Stomata – These are tiny pores found mainly on the underside of the leaves which allow carbon dioxide to enter and allow oxygen to leave.
5.Guard cells – These serve to open and close the stomata.
6. Vascular bundle: a. Xylem – This transports water from the roots to the leaves.
b. Phloem – This transports manufactured food from the cells where it is manufactured to sites where it is used.
Next week, we will look at the process. See you then!