You must remember that “Digestion” has a specific meaning in Biology. It is the term used for the process that involves the chemical breakdown of large, insoluble food molecules into smaller, simpler molecules that can be absorbed into the blood. Many of the molecules in food are polymers – that is macromolecules made from long chains of repeating subunits. Examples of dietary macromolecules include proteins, polysaccharides and fats. These molecules are too large to be able to pass into the blood in the villi of the small intestine and so the body has evolved to chemically break them down into their constituent monomers or building blocks. Digestion is the process in the alimentary canal that achieves this.
Digestion reactions are also known as hydrolysis reactions because a molecule of water is required in the reaction to break the covalent bond holding the monomers together. These reactions are all catalysed (sped up) by specific molecules called digestive enzymes.
Why do different food types need different digestive enzymes to speed up their breakdown in the digestive system?
(If you are unsure, you need to revise the way enzymes work to catalyse reactions by a “lock and key” theory?)
Digestion of Carbohydrates
Many simple carbohydrates (e.g. glucose) do not need digesting. This is because they are already small enough to be absorbed into the blood directly in the ileum (small intestine). But larger disaccharide sugars (e.g. maltose and sucrose) do need to be broken down, as do all polysaccharides (e.g starch).
The family of enzymes that break down carbohydrates are called carbohydrases.
Starch is a large polysaccharide made up of many hundreds of glucose residues linked together. It is way too big to be able to cross the epithelial lining of the small intestine and so needs to be digested. This happens in a two-stage process. Firstly there is an enzyme amylase that can catalyse the following reaction:
starch + water ——-> maltose
Amylase is made in the salivary glands and so works in the mouth. But the main region for the digestion of starch is in the duodenum. This is because amylase is also made in the pancreas.
Maltose is a disaccharide molecule made of two glucose residues joined together. Maltose itself requires digesting to its constituent glucose molecules in order to be absorbed. So the second stage in the digestion of starch involves a second enzyme, maltase that is found embedded into the epithelial lining of the ileum. Maltase catalyses the breakdown of a molecule of maltose into two molecules of glucose which can be absorbed into the blood.
maltose + water ——> glucose
In the topic of sexual reproduction in plants, the final stage is often overlooked. I think it is helpful for students to think of this topic in several distinct stages.
- Flower Structure (hermaphrodite nature of most plants)
- Pollination (self v cross pollination; wind v insect pollinated flowers)
- Fertilisation (how does the pollen tube reach the egg cell to fertilise it?)
- Seed and Fruit formation (what forms what after fertilisation)
- Seed Dispersal (by animals, by wind, by water, by explosive means)
Once the seed has been dispersed there then follows a period of dormancy when nothing happens. In latitudes such as the UK this often is there to delay germination until the following spring when growing conditions become more favourable. The process of taking an inert seed and growing a new plant from it is called germination.
You don’t need to worry too much about the details of germination but there are a few vital parts of the process that GCSE candidates need to appreciate for A* marks. Firstly you should know the structure of a typical seed.
The seed coat or testa surrounds the seed and provides a tough waterproof container. Inside there are the embryonic plant (composed of a plumule and radicle), one or two seed leaves called cotyledons and a storage tissue called endosperm.
Germination starts when the seed starts to take up water by osmosis. There is an opening in the testa called the micropyle that allows water to move into the seed causing it to swell and thus rupture the seed coat to allow the embryo plant to emerge.
Water entering the seed also activates the embryo plant such that it starts to release digestive enzymes such as amylase. Amylase catalyses the digestion of starch into a simple sugar maltose. The endosperm and cotyledons contain energy stores in the form of starch, lipids and proteins and as these get broken down by the various enzymes, they provide the energy for the early growth of the seedling. The radicle emerges first and grows downwards (positive geotropism) and then the plumule or shoot grows upwards towards light (positive phototropism). Remember that throughout the early stages of this growth the energy required comes from stored food molecules in the seed. If you measure the mass of the plant during this phase, it would be decreasing. Only when the first leaves emerge above ground and the plant can start the process of photosynthesis will the overall mass start to increase.