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AQA Categories Archives: 3.3 Organisms exchange substances with their environment

Mass transport in plants

Topic: Surface area to volume ratio , Gas exchange,  Digestion and absorption,  Mass transport in animals,  Mass transport in plants

Organisms exchange substances with their environment (AQA AS Biology) PART 5 of 5 TOPICS

 

 

Mass transport in plants:

Transpiration is the water loss through the leaves via the stomata. Cohesion forces between water molecules allows water to be constantly absorbed through the soil. Adhesion forces between the water molecules and the walls of the xylem causes narrowing of the xylem vessel for faster transpiration.

As well as water that needs to be transported to all parts of the plant, so does sugars and other nutrients. Sources are referred to as leaves as this is where the sugars are made and the sinks are referred to as shoots and root tips as this is where the nutrients need to go to. Translocation is the process involved to move the nutrients by the mass flow hypothesis in the phloem. Phloem cells are connected together to make phloem vessels by sieve plates with the pores called sieve tube elements. Cells known as companion cells aid with the translocation. Phloem loading takes place at the leaves where the sugars made pass through the companion cells by diffusion down their concentration gradient into the sieve tube elements. This lowers the water potential and so water from the neighbouring xylem vessel moves by osmosis down its concentration gradient into the phloem vessel. As a result of the water and sugars, high pressure is created and so the water and sugars move down the pressure gradient to the sink end. The sugars then go through various active transport methods depending on the cell type into the cells at the sink end. As the sugars pass through the companion cells a high water potential is created in the phloem vessel compared to the xylem vessel and so water moves down its concentration gradient into the xylem by osmosis but primarily due to transpiration. NB: A higher water potential in the phloem vessel near the sink end is established compared to the xylem vessel because the xylem contains nutrients such as nitrates and magnesium that have been absorbed through the soil making the water potential lower than the water potential in the phloem vessel as the nutrients are being taken out.

] That’s all that you need to know about mass transport in plants [

 

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Gas exchange

Organisms exchange substances with their environment (AQA AS Biology) PART 2 of 5 TOPICS

 

 

TOPICS: Surface area to volume ratio  Gas exchange  Digestion and absorption  Mass transport in animals  Mass transport in plants

Gas exchange:

Gas exchange is when oxygen is reached to the respiring cells and waste products are removed.

Single celled organisms such as amoeba have a thin membrane that is moist.

Exchange surfaces have a large surface area, a thin membrane, most are moist for gases to dissolve and a mechanism to maximise the diffusion gradient. Organisms that have their exchange systems inside reduces water so moisture remains. They have good blood supply if the organism is an animal.

There are four gas exchange systems that you need to know:

  • Lungs: Air enters from the nose and is cleaned by the cilia capturing dust particles and other irritants. It is warmed and moistened and is sent through the trachea then through the bronchus to the bronchioles and then to the alveoli. This is where the gas exchange takes place. Deoxygenated blood passes next to the alveoli which has a low concentration of oxygen compared to the alveoli producing a concentration gradient. Oxygen diffuses into the blood down its concentration gradient from the alveoli. CO2 leaves the deoxygenated blood down its concentration from the blood to the alveoli as the blood has a higher concentration of CO2 than the alveoli. This waste gas is then expired. NB: There are different thorax movements that you need to know which is described as the following – Inhalation causes the diaphragm to contract and flatten and the external intercostals muscles to contract pulling the rib cage up and out. This increases the volume of the thorax which decreases the pressure so air can move in down its pressure gradient. Exhalation causes the diaphragm to relax and lift and the internal intercostals muscles to contract pulling the rib cage down and in. This decreases the volume of the thorax which increases the pressure so air can move out down its concentration gradient. Alveoli are one cell thick for short diffusion pathway and there are many alveoli present in the lungs creating a large surface area. Constant ventilation replaces air which maintains the concentration gradient.
  • Fish: Water moves in through the mouth with a high concentration of oxygen than the blood. When the water passes through next to the deoxygenated blood a concentration gradient is created where oxygen diffuses into the blood down its concentration gradient. CO2 moves from the blood into the water down its concentration gradient because the blood has a higher concentration of CO2 compared to the water. The water with the waste gas moves out through the operculum. NB: The method on the direction of flow of blood and water must be known which is as follows – There is a counter current flow of blood and water meaning the movement of blood one way means the water will move the other way. This is because a steep concentration gradient is maintained for efficient gas exchange. The gill filaments that the gases diffuse through are two cells thick so the diffusion pathway is short and have a large surface area as they have gill lamellae .
  • Plants: Gases diffuse in and out of the leaf down their concentration gradients where oxygen diffuses in and CO2 diffuses out. The wind replaces the air accumulating the stomata which maintains a concentration gradient.
  • Insects: These organisms have a tracheal system which is similar to plants. Oxygen diffuses in through the spiracles into the tracheae and then into the trachieoles to the respiring cells. Waste gases then exit through the spiracles.

Xeromorphic plants such as xerophytes reduce water loss as they have a thicker cuticle than the normal plants and have sunken stomata which means water droplets can accumulate round the stomata reducing water loss as it prevents a concentration gradient being made. There are hairs round the stomata preventing water from leaving the stomata and the leaves are curled up which creates a moist environment further stopping the diffusion of water out of the stomata. NB: When water leaves through the stomata of any plant it does not leave by osmosis but by diffusion. This is because it is not crossing a semi-permeable membrane but is just simply moving from a high concentration to a low concentration.

The equation at the bottom should be known:

Pulmonary ventilation = Ventilation rate x Tidal volume

 

 

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Surface area to volume ratio

Topic: Surface area to volume ratio,  Gas exchange,  Digestion and absorption,  Mass transport in animals,  Mass transport in plants

Organisms exchange substances with their environment (AQA AS Biology) PART 1 of 5 TOPICS

 

 

Surface area to volume ratio:

The smaller the object the bigger the surface area to volume ratio. This means that there is a smaller diffusion pathway which increases the metabolic rate.

Surface area of sphere = 4πr2

Volume of sphere = (4/3)πr3

] That’s all that you need to know about surface area to volume ratio. NB: The equations must be known by heart [

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Mass transport in animals – Heart disease

Organisms exchange substances with their environment (AQA AS Biology) PART 4 of 5 TOPICS

 

 

TOPICS: Surface area to volume ratio  Gas exchange  Digestion and absorption  Mass transport in animals  Mass transport in plants

Mass transport in animals – Heart disease:

Coronary heart disease is where the arteries supplying the heart with oxygenated blood and nutrients (coronary arteries) are narrowed causing the blood supply to reduce. This means that the heart has to work harder to force blood through the narrowed vessels therefore blood pressure increases. The outcomes can be any of these:

  • Angina: This is chest pain due to severe shortage of blood to the heart muscle – cells do not die. The pain only occurs when the person is active and not at rest.
  • Heart attack (myocardial infarction): This is when a coronary artery is completely blocked by a thrombus (dried red blood cells). This causes the blood supply to reduce majorly leading to heart muscle cells dying which is fatal.
  • Heart failure: Some blockages are not as fatal as the thrombus that leads to heart attacks. Instead they will lead to weakening of the muscle which means the heart will be less efficient for pumping. Often there is an accumulation of blood on the right side of the heart creating an enlargement.

The main cause of coronary heart disease are:

  • Atherosclerosis: This is where fatty deposits of cholesterol and plaques of white blood cells block the artery. This deposit may even cause a rupture in the artery walls creating a thrombus which can break and block arteries in the heart as well as the brain.
  • Aneurysm: This is where the artery walls weaken causing it to collapse. This means that the blood pressure is not maintained throughout the artery. Sometimes the weakened walls can burst as a higher pressure is needed to move the blood leading to severe loss of blood (haemorrhage). This process which can occur in the brain is more commonly known as stroke.

Smoking is one of the top causes of heart disease.

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Digestion and absorption

Organisms exchange substances with their environment (AQA AS Biology) PART 3 of 5 TOPICS

 

 

TOPICS: Surface area to volume ratio  Gas exchange  Digestion and absorption  Mass transport in animals  Mass transport in plants

Digestion and absorption:

Large molecules are hydrolysed into smaller molecules so that they can be absorbed across the cell membrane.

The following molecules being digested and the absorption of the products must be known:

  • Carbohydrates: Amylase in the saliva hydrolyses starch into two maltose molecules. Pancreatic juices that are secreted into the stomach contain amylase too. Maltase is embedded in the intestinal wall which hydrolyses the maltose into two glucose molecules. This is then absorbed into the blood stream by co-transport which is explained in ‘Biological molecules AQA AS Biology PART 8 of 8 TOPICS: Inorganic ions’. NB: It is important that you say hydrolyses and not break down as you will not get the mark.
  • Lipids: Bile emulsifies fats in the stomach for digestion to happen faster. It is secreted by the liver. Pancreatic lipase hydrolyses the fats into monoglycerides and fatty acids. Monoglycerides and fatty acids combine with bile salts and phospholipids to form micelles. The bile slats and phospholipids allows the poorly soluble monoglycerides and fatty acids to the cells lining the intestinal walls where the non-polar nature of these molecules help them to diffuse through the lipid bi-layer.
  • Proteins: Amino peptidase from the pancreas is an exopeptidase where it hydrolyses the ends of the polypeptide to make amino acids. Trypsin is an endopeptidase where it hydrolyses the proteins into polypeptides in the middle. Cells from the intestinal wall secrete enzymes called dipeptidase which hydrolyse dipeptides into amino acids. This is then absorbed into the blood using co-transport which is similar to glucose and is further explained in ‘Biological molecules AQA AS Biology PART 8 of 8 TOPICS: Inorganic ions’.

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