Transport

Unicellular organisms

Simple unicellular organisms rely on diffusion as the means by which molecules move in and out of their cells. This is because they have a high surface area to volume ration which results in diffusion occurring quickly.

Multicellular organisms

Multicellular organisms are simply too large for diffusion to occur efficiently. They have a small surface area to volume ratio. Therefore, multicellular organisms have elaborate systems such as the respiratory system and circulatory system to transport molecules around the body to support themselves.

Water moving into the roots

Water moves into the root hair cells through osmosis. When soils take in nutrients and minerals from the soil by active transport, a low water potential is created. This low water potential means that water would diffuse into the roots. The roots have a large surface area to volume ratio, to absorb minerals and water at the greatest rate possible.

Lack of minerals in the soil, waterlogged soil and leaves removed

Without minerals in the soil such as nitrates, a plant would wilt. Nitrates make up amino acids which form proteins. Without protein, a plant cannot grow and support itself hence it wilts. Without magnesium, chlorophyll cannot be produced and a plant cannot photosynthesise to create glucose for growth also. The plant would also turn yellow.

A water logged soil would cause nutrients to become diluted. Diluted nutrients are not concentrated hence they cannot be taken up into the plant by active transport. There is no oxygen in waterlogged soil for plants to respire from.

A plant with its leaves removed would not be able to absorb water through osmosis in the root hair cells as transpiration is not occurring and providing a low water potential.

Minerals, nutrients, dissolved solutes and water are stored in the vacuole in cells before they are taken up through the xylem and distributed to other areas of the plants. Glucose is transported through the phloem from the leaves [where it is produced from photosynthesis] to other areas of the plant.

Phloem and Xylem

There are two types of transport tissue in a plant:

1. Phloem
2. Xylem

1. The phloem transports glucose from the leaves of a plant [and any green area with chlorophyll] to the other areas. The phloem vessels are thin, permeable so that glucose can diffuse in and out. They transport glucose in both directions as every cell requires it for respiration. The thin walls are made from lignin. The phloem is involved in TRANSLOCATION. This defines the movement of food substances from the stem to growing tissues around the plant.

1. On the other hand, the xylem only flows upwards [one direction]. It consists of hollow tubes constructed of dead cells. The walls are thick and made from cellulose. They are impermeable to allow no water or minerals to diffuse out. They transport the water, minerals and nutrients from the roots to the leaves.

Gas exchange [plants]

The role of diffusion in gas exchange is simple:

Gases move from an area dense in gas to an area less dense in gas. This is prevalent in gas exchange in the lungs. The oxygen in the alveoli diffuse into the bloodstream as it is less dense with oxygen. On the other hand, the carbon dioxide in the bloodstream diffuses into the alveoli as they are less dense with carbon dioxide.

Respiration is the conversion of oxygen and glucose into carbon dioxide and water. Oxygen has been exchanged with carbon dioxide.

On the other hand, photosynthesis is the conversion of carbon dioxide and light into oxygen and glucose.

Equation for photosynthesis:

Carbon dioxide and light = glucose and oxygen

The rate of photosynthesis is affected in three ways:

1. Light intensity
2. Temperature
3. Concentration of carbon dioxide in the air.

Temperature would affect photosynthesis as the enzymes involved in the chemical reactions during photosynthesis would not work efficiently in cool temperatures while becoming denatured in hot temperatures.

A high concentration of carbon dioxide would maximise the rate of photosynthesis.

A high light intensity would increase the rate of photosynthesis.


Image courtesy of BBC Bitesize.

Photosynthesis v respiration	Overall result
Dark	Respiration No photosynthesis	Oxygen taken in Carbon dioxide given out
Dim light	Photosynthesis rate equals respiration rate	Neither gas is taken in or given out, as each cancels the other out
Bright light	Photosynthesis rate greater than respiration rate	Carbon dioxide taken in Oxygen given out

Plants and how they are adapted for gas exchange

Plants are thin, enabling a short diffusion distance for carbon dioxide to be absorbed. There are stomata situated at the bottom of the leaf to allow the gases to diffuse into the leaves. When the guard cell (of the stomata) is turgid, it bends and opens, allowing the gas to diffuse into the leaf. 

Stomata in gas exchange

Photosynthesis produces oxygen as a byproduct. However, this oxygen is used in respiration. But, photosynthesis can only work in light therefore gas exchange only occurs during the day. As photosynthesis only occurs in the day, guard cells only absorb water during light hours. When they absorb water, they become turgid and the stomata opens. [stoma for singular]. During the night, the guard cells lose water through respiration, as only respiration continues during the night. Therefore, the guard cells become flaccid and close the stomata, preventing any gas from diffusing into the leaves.

An experiment to investigate the effect of light on the net release of carbon dioxide from a plant using hydrogen-carbonate indicator

Concentration	Indicator turns
Highest	Yellow
Higher	Orange
Atmospheric level	Red
Low	Magenta
Lowest	Purple

A leaf is placed in a stoppered boiling tube containing somehydrogen carbonate indicator solution. The effect of light intensity can then be investigated.

The table shows some typical results.

	Tube 1	Tube 2	Tube 3	Tube 4
Light turned on	✓	✓	✓	✓
Paper on tube	Black paper	Tissue paper	None	None
Leaf	Living	Living	Living	Dead (boiled)
Indicator colour at the end	Yellow	Magenta	Purple	Red
Carbon dioxide concentration	Highest	Low	Lowest	Atmospheric level
Respiration	✓	✓	✓	✗
Photosynthesis	✗	✓	✓✓	✗

• Tube 4 was a control. The results in tubes 3 and 4 show that the leaf has to be alive for the carbon dioxide concentration to change.
• Tubes 1, 2 and 3 show the effect of increasing the light intensity. The black paper stopped light reaching the leaf in tube 1, so only respiration could happen.
• The tissue paper stopped some of the light reaching the leaf in tube 2, and the leaf in tube 3 received the most light.
• Photosynthesis happened as well as respiration in tubes 2 and 3, so there was a net absorption of carbon dioxide.
• The rate of photosynthesis was greatest in the leaf in tube 3, and it had the greatest net absorption of carbon dioxide.

From this experiment we can see that the concentration of carbon dioxide produced fluctuated due to the light intensity varying. Gas exchange occurs at the greatest rate when respiration and photosynthesis occur at the same time.

Experiment to investigate photosynthesis

An experiment to investigate photosynthesis, showing the evolution of oxygen from a water plant, the production of starch and the requirements of light, carbon dioxide and chlorophyll.

Remember: Photosynthesis experiment = pondweed

This experiment will involve pondweed and a white leaved plant.

1. Add pondweed to a beaker of water with a known volume.
2. Add the white leaved plant to another beaker of water with the same volume.
3. Add the same volume of baking powder to increase the carbon dioxide levels.
4. Place two desk lamps 10cm away from each beaker.
5. Leave for five minutes so that the plants can accustom to the new light intensity.
6. After five minutes, count how many bubbles are emitted from each plant during a period of a minute.
7. After noting this down, move the desk lamps further away by 10cm.
8. Wait for five minutes then count how many bubbles are emitted from each plant for a period of a minute.
9. Repeat this process, moving the desk lamp back by 10cm then counting the number of bubbles.
10. Continue until the lamp is 50cm away from the plant[s].

Independent variable = light intensity
Dependent variable = bubbles omitted.
Controlled variables = size of each plant and temperature of the water.

Structure of a leaf for photosynthesis

In order for photosynthesis to occur efficiently in a plant and be most effective, the factors that influence photosynthesis must not be inhibited by a plants structure. A leaf is specifically adapted for photosynthesis in various ways both internal and external.

The veins that wrong along the leaf are for support and to transport water and the carbohydrates produced.

Light

1. The leaf is large and has a large surface area.
2. The leaf is covered in wax [gas exchange] this is transparent hence it does not reduce the intensity of the light entering the leaf.
3. The palisade cells are situated close to the top of the leaf and contain chlorophyll to ensure the maximum amount of light is absorbed.
4. The epidermis layer is thin and transparent to not block out the light.

Carbon Dioxide

1. The leaf is thin for a short diffusion distance.
2. The spongy mesophyll contains large air spaces for the carbon dioxide to diffuse through the leaf.
3. The stomata are situated near the bottom to ensure gas exchange is effective.
 

Mineral ions

These are significant for the growth of a plant. They are situated in the soil and are taken into the plant through active transport. Active transport is the taking in of molecules against the concentration gradient using energy released from respiration.

Nitrates are taken up to gel construct amino acids for protein. Without it, the plant will wilt.
Magnesium is used in chlorophyll. Without it, leaves would turn yellow.

Nutrition [plants]

Photosynthesis is the conversion of carbon dioxide and water into glucose and oxygen using light energy. The light energy is absorbed in the chlorophyll and divides the water into oxygen and hydrogen ions. The hydrogen ions bond with the carbon dioxide to form glucose.

The chemical energy in glucose is essential in growth and used in active transport. It is used in cellulose for cell walls also.

Carbon dioxide + water = glucose + oxygen

6CO2(g) + 6H2O(l) = C6H12O6(s) + 6O2(g)

Carbon dioxide relies upon the light intensity, carbon dioxide concentration and temperature in order to occur efficiently.

The following is credited to BBC Bitesize.

Limiting factors

Three factors can limit the speed of photosynthesis - light intensity, carbon dioxide concentration and temperature.

• 
  

  Light intensity
  Without enough light, a plant cannot photosynthesise very quickly, even if there is plenty of water and carbon dioxide. Increasing the light intensity will boost the speed of photosynthesis.
• 
  

  Carbon dioxide concentration
  Sometimes photosynthesis is limited by the concentration of carbon dioxide in the air. Even if there is plenty of light, a plant cannot photosynthesise if there is insufficient carbon dioxide.
• 
  

  Temperature
  If it gets too cold, the rate of photosynthesis will decrease. Plants cannot photosynthesise if it gets too hot.

If you plot the rate of photosynthesis against the levels of these three limiting factors, you get graphs like the ones above.

In practice, any one of these factors could limit the rate of photosynthesis.

Balanced diet

A balanced diet is the different components present in appropriate proportions needed to maintain a healthy body. It should inlude carbohydrates, lipids, proteins, vitamins a,c,d, iron, calcium, dietary fibre and water.

Carbohydrates: Constructed mostly of starch and glucose hence it is a source of immediate energy. Found in cereal, potatoes, rice and pasta. Note that starch is a store of glucose.

Proteins: Constructed of a unique sequence of amino acids. They are required for the growth and repair of tissue. Found mostly in eggs, lentils, pork and soy.

Lipids: Constructed of fatty acids and glycerol. This is used as a store of energy in the long term. It is primarily used to insulate the body and stop the loss of heat i.e. homeostasis. It is found in dairy products, cheese, milk, fish etc.

Vitamin A: Essential for the reproduction of cells - maintains immune system. It is an antioxidant found in fatty fish, milk, cheese and mango.

Vitamin C: Another antioxidant that helps fight infection. Found in orange, grapefruit and kiwi.

Vitamin D: Maintains healthy bones and teeth by aiding the absorption of calcium. Remember that absorption is the taking in of minerals, ions and other soluble molecules into the bloodstream. It is found in milk and other dairy products i.e. yoghurt. Eggs and fish are also rich in vitamin d. Sunlight is helpful in encouraging the body to produce vitamin d.

Calcium: Is essential to the growth and repair of bones and heart function. Found in dairy products.

Iron: A component of haemoglobin in red blood cells. This is what oxygen binds to. It is common in red meats.

Water: Useful in the chemical reactions of cells. Found in the sea, clouds, me, you, in glaciers and in your tap.

Dietary fibre: Keeps the bowels functioning well and reduces the risk of bowel cancer. Found in cereals and rice.

Activity levels, age and pregnancy requirements for energy:

When people are young, they are more active and are growing. As a result they require more energy than an elderly person who is less active and no longer grows. With an active lifestyle, more energy is utilised in movement and the development of cells. As a result, more energy is required as the cells are respiring more.

When a woman is pregnant, she is supporting not only her life but her baby's. Therefore she needs to consume the appropriate amount of energy for both of them. The baby's growth and development requires significant amounts of energy.

An experiment to investigate the energy content in a food sample

1. Get a food sample.
2. Weigh the food sample and record the mass.
3. Fill a boiling tube with 10cm^3 of cold water.
4. Using a thermometer, measure the temperature of the water.
5. Burn the food sample using a bunsen burner until it catches fire.
6. Hold the burning food under the water for 2 minutes.
7. Extinguish the fire.
8. Measure the new temperature of the water using the thermometer and calculate the temperature rise.
9. Using the equation [mass of water x 4.2J/oC x temperature rise] calculate the total energy transferred.
10. Divide the total energy transferred by the original mass of the sample to calculate the energy present in one gram.

Human digestion

Key terms:

1. Mastication
2. Peristalsis
3. Absorption
4. Assimilation
5. Ingestion
6. Digestion
7. Egestion
8. Excretion

1. Mastication is the physical break-up of food in the mouth. It is the chewing and crushing of the food in order to obtain a higher surface area. A higher surface area enables enzymes to break down the food more efficiently.

2. Peristalsis is the muscular contractions that move food along the gut i.e. small intestine. Peristalsis occurs to ensure food is moved along the gut. It first occurs in the oesophagus yet it continues in the small intestine.

3. Absorption is the taking-in of small, soluble molecules into the bloodstream [at the end of the small intestine called the ileum]. It is performed efficiently as the ileum is lined with villi. These increase the surface area greatly, allowing the soluble molecules to be absorbed at a high rate. They are one cell thick and also contain blood capillaries to ensure the diffusion distance is small. The blood has a lower concentration of the food molecules therefore diffusion occurs quickly. The bloodstream transports the molecules all around the body to various tissues.

4. Assimilation is the taking in of molecules into the cells where they are then utilised.

5. Ingestion is the consumption of food by an organism.

6. Digestion is the breakdown of large food molecules into smaller, soluble ones.

7. Egestion is the removal of undigested semi-solid waste as faeces.

8. Excretion is the removal of all waste products from the body. Not necessarily as faeces.

Structure of the human alimentary canal

1. Mouth is where the physical breakdown of food occurs. Mastication increases the surface area greatly and this aids the enzymes. Amylase is an enzyme released with saliva. It breaks down starch into maltose. The saliva itself lubricates the food to help swallowing.

2. The oesophagus is situated next to the trachea and it connects the mouth to the stomach. Food never enters the trachea as it is covered with the epiglottis when food is swallowed. Therefore food only enters the oesophagus. Peristalsis occurs in the oesophagus to ensure food is moved along.

3. The stomach contains hydrochloric acid. This is a powerful acid with a pH of 1-2. As a result, it breaks down food even further to maximise the surface area more. The food here is now chyme, a liquid solution.

4. At the start of the small intestine, enzymes are released. These enzymes catalyse reactions to break down large food molecules into smaller, soluble ones. Each enzyme is specific to the reaction of a particular food molecule. Enzymes work most efficiently in the right temperature, pH and when there is a large surface area. There is more about enzymes further down the page.

5. The end of the small intestine is called the ileum. This is where absorption occurs. The ileum is lined with millions of small lumps called villi. These maximise the surface area hence they increase the rate of diffusion of soluble molecules into the bloodstream.

6. The large intestine is where water from chyme is absorbed and transported around the body.

7a. The pancreas produces enzymes essential to the chemical breakdown of food molecules such as amylase, lipase and protease. It is connected to the start of the small intestine [where enzymes are released to catalyse the breakdown of food] by the common bile duct.

7b. Bile is produced in the liver and stored in the gall bladder. It emulsifies lipids to increase the surface area. This makes their breakdown more efficient and occur at a greater rate. Bile is released with the pancreatic juices i.e. the enzymes through the common bile duct. Bile is also essential in creating alkaline conditions for the enzymes to work in. Note that it is not released specifically to neutralise stomach acid, it simply neutralises the acidic food molecules which have come out of the stomach. 

The digestive enzymes

These enzymes catalyse the reactions of the chemical breakdown of food molecules into smaller soluble molecules. They are made from proteins and are specific to the type of molecule they catalyse the reaction for. Amylase is produced in the salivary glands and is released in the mouth along with saliva. It breaks down starch into maltose. Maltose is then broken down by maltase into glucose at the start of the small intestine.

Protease breaks down proteins into amino acids. It is produced in the pancreas and released through the common bile duct along with the other enzymes at the start of the small intestine.

Lipase breaks down lipids into fatty acids and glycerol. It is produced in the pancreas and released along with bile from the common bile duct at the start of the small enzymes. Bile emulsifies the lipids, increasing the surface area so that the enzyme can work more efficiently.