Section 2 e) Summary

Flowering Plants
Plants don't need to eat, they get all of their nutrients by absorbing mineral ions from the soil, and carrying out photosynthesis.
Photosynthesis is the process by which plants generate glucose (which is stored as starch) using sunlight:

Carbon dioxide + Water -(sunlight)-> Glucose + Oxygen
6CO2 + 6H2O -(light energy)-> C6H12O6 + 6O2

They are able to do this because their cells contain organelles called chloroplasts, which contain a green pigment known as chlorophyll. Chlorophyll is the chemical that carries out photosynthesis.

Factors affecting photosynthesis:

• Temperature: As temp. increases, the particles move faster and the rate of photosynthesis is increased, but after reaching optimum temperature for the enzymes they begin to denature at any higher temp. and the rate of photosynthesis drops steeply.
• Light intensity: More intense light means more photosynthesis, but only up until a certain point as the number of chloroplasts is limited.
• Carbon dioxide concentration: Higher concentrations of CO2 mean more photosynthesis, but only up until a certain point as the number of chloroplasts is limited.
• Chlorophyll: Variagated leaves will photosynthesize less than single coloured leaves, lighter coloured leaves will photosynthesize less than darker ones due to the number of chloroplasts available to carry out photosynthesis

The diagram below shows the structure of a leaf

 Each layer has a different function:
The waxy cuticle protects the cell from damage, and prevents water loss.
The upper epidermis is thin and clear, and provides a layer of protection.
The palisade mesophyll is made up of column-shaped palisade cells, which are densly packed with chloroplasts to maximise absorption of light, and therefore photosynthesis.
The spongy mesophyll contains air pockets to increase diffusion in gas exchange and photosynthesis.
The lower epidermis contains stomata and guard cells which control photosynthesis by opening during the day, and closing at night to minimise water loss through evapotranspiration.
The lower wax cuticle provides protection to the underside of the leaf.

Plants don't just need glucose, though. They require mineral ions, which they can absorb from the soil using active transport and diffusion.

A variety of different experiments can be done to test different parts of a flowering plant's nutrition.

Experiment 1: Oxygen and water plants

1. Choose the variable you wish to change (temp, light intensity, etc.) 
2. Place containers of water and water plants of about the same size in the different conditions. 
3. Count the bubbles that are formed in a certain length of time. 

Experiment 2: Starch and light intensity

1. Put 3 leaves from the same plant of similar sizes in different light conditions: One in a dark room, one in direct sunlight, and one in shaded light. Leave for 48 hours.
2. Test leaves for starch by boiling each in water for 1 minute, then placing in ethanol, then returning it to the water and finally spreading on a petri dish. Add iodine solution to see which leaves test positive for starch 

Experiment 3: Chlorophyll
Test variagated leaves for starch. You can see the green parts test positive for starch while the white parts do not.

Experiment 4: Carbon dioxide
Place a plant in a sealed plastic bag with a container of sodalime (which removes CO2), then test for starch.

Experiment 5: Carbon dioxide
Place water plants in different light levels for 12 hours, with hydrogencarbonate indicator. At the start, indicator should be red, then change to purple for low levels of CO2 (In sunlight) and change to yellow for high levels of CO2 (In darkness)

Experiment 6: Mineral ions
Place cuttings of the same plant into different mineral ion solutions: One with a complete ion solution, each of the others missing one ion and one containing just water. Place them together in controlled conditions (light, temp, etc.), then after a week or two check the plants to see any changes.

Humans

Humans gain nutrients from eating food, which has to pass through the digestive tract.
The essential nutrients are:

These are represented in the correct proportions by the eatwell plate:

This gives a good indication of what proportions of food we should be eating, but the total energy intake varies from person to person. Generally, you need more energy if:

• You have more body mass
• You are a man
• You are pregnant
• You are active
• You are a teen. After puberty, energy requirements gradually decrease, and children require less energy than adults. 

Below is a diagram of the digestive system. This is the system within your body where food is digested and nutrients absorbed. But how does this work?

1. First, food is ingested through the mouth.

It is mechanically digested, by chewing, and chemically digested, by salivary amylase. The teeth break up the food to increase its surface area (aiding chemical digestion later on) and to make it easier to swallow in food bolus.
The food is then swallowed, (the epiglottus closes over the trachea to avoid food from falling into the lung) and it travels down the oesophagus through peristalsis: waves of contraction and relaxation of circular and longitudinal muscles. Peristalsis pushes the bolus of food into the stomach.

2. The stomach is a large muscular bag that contracts and relaxes to churn the food. The food is held here for 2-4 hours, during this time mixing with gastric juice (a combination of HCl and pepsin, a form of protease), which breaks down protein in the food. 

The stomach is lined with mucus-producing goblet cells, which helps to prevent the highly acidic HCl from damaging the stomach. Food then passes through into the duodenum.

3. The duodenum is the first part of the small intestine, it is where digestion is completed. Digestive enzymes (Carbohydrases, proteases and lipases) are secreted in pancreatic juice, from the pancreas. 

Bile is also released after being made in the liver and stored in the gall bladder. It emulsifies lipids to increase their surface area. The fully digested nutrients are then transported to the ileum.

4. The ileum is where absorption takes place. The surface of the intestine is folded into tiny villi, which increase the surface area for maximum absorption. It can take place passively, through diffusion, or via active transport. 

The villi contain capillaries and lacteals (lymph vessels) which absorb digested lipids, amino acids and glucose. After being absorbed, a process called assimilation takes place, where the nutrients are used or stored by the body. 

The above diagram is of two villi. 

Villi are small, hair-like protrusions in the lining of the small intestine. Their shape increases the surface area, helping to absorb nutrients more quickly. Each villus is covered in micro-villi, which further increase the surface area.
The walls of the villi are just one cell thick to decrease the distance and increase the rate of absorption.
Villi each contain a lacteal, a vessel connected to the lymphatic system. This absorbs fatty acids and glycerol, then transports them away from the small intestine.
Each villus contains a network of capillaries connected to blood vessels. Glucose and amino acids are absorbed into the bloodstream through them.

5. The remaining material is now passed on to the large intestines, the colon. Here, water and mineral ions are reabsorbed. The leftover undigested food, bacteria, etc. (faeces) is stored in the rectum, then egested via the anus.
The table below shows the most important digestive enzymes to know in this course:

Experiments can be done to determine the energy content of foods:

1. Take a food sample, and light it on fire. Hold beneath a quantity of water with a thermometer in it. 
2. If the sample goes out, quickly relight it. 
3. Note down the temperature rise

Use this equation to calculate the energy content:

energy transferred (J) = mass of water (g) × 4.2 (J/g°C) × temperature increase (°C)