Exchange And Transport Systems Flashcards
What is surface area to volume ratio?
The surface area to volume ratio of something is how large its surface area is in comparison to its volume. This can be done by dividing the surface area by the volume to get a ratio in the form n:1. The larger the object, the larger the surface area (in general). This is because as an object gets larger, less of it comparatively will be on the surface.
How do single-celled organisms complete gas exchange?
Single celled organisms are very small, and so have large surface area to volume ratios. They also have a short diffusion path to get into the cell (one cell membrane thick). This means that they can exchange substances with their environment simply by using diffusion.
Why can multicellular organisms generally not exchange substances with their environment just by diffusion?
Multicellular organisms are much larger than unicellular organisms, and so they have much lower surface area to volume ratios. This also, diffusion paths tend to be longer, because to get to the cells in the centre of the organism, materials need to get through many other cells first. This means that simple diffusion is not an effective method for gas exchange in these organisms, so mass transport systems must be developed.
What is an example of a multicellular organism that can use diffusion to exchange gases and explain why this is the case?
The flatworm can exchange gases just by diffusion, and as such, has no specialised gas exchange system. This is because it is very flat and so has a large surface area to volume ratio and a short diffusion path, making the rate of diffusion fast.
What is the relationship between surface area to volume ratio and metabolic rate and why?
As the surface area to volume ratio increases, the metabolic rate also increases (positive correlation/direct proportion). This is because organisms with a large surface area to volume ratio lose heat more quickly than those with lower SA:V ratios. This means that the smaller organisms must have high metabolic rates to maintain enough body heat to survive (as metabolic processes release heat).
How does gas exchange occur in fish?
Fish contain gills which extract lots of oxygen from water (it is particularly important that this is efficient as water has a lower % of oxygen than air). Gills are made of thin plates called gill filaments, which give a large surface area to volume ratio and even smaller structures called gill lamellae, which increase the ratio even more. The gill lamellae have a good network of capillaries and a thin surface, creating efficient gas exchange and fast diffusion.
What is the counter-current system?
The counter-current system describes the flow of water and blood over and in fish gills. It consists of water flowing over the gill filaments and blood flowing through the capillaries in the gill lamellae in the opposite direction. This means that the blood with the highest concentration of oxygen meets the water with the highest concentration of oxygen and the blood with the lowest concentration of oxygen meets the water with the lowest concentration of oxygen. This maintains a concentration gradient for oxygen across the entire gill filament, which means a high proportion of the oxygen in the water can be extracted and used by the fish.
Why is counter-current flow better than parallel flow?
Counter-current flow maintains a concentration gradient of oxygen across the entire gill filament, while parallel flow does not (this is when blood and water flow in the same direction). This is because with parallel flow the oxygen will diffuse from the water to the blood until equilibrium is reached, meaning oxygen can only diffuse over part of the gill filament. Therefore, parallel flow results in a lower concentration of oxygen in the blood than counter-current flow, so it is a less efficient system.
What is the structure of a leaf?
Leafs have, from top to bottom, a waxy cuticle, an upper epidermis, palisade mesophyll, spongy mesophyll with air spaces, xylem and phloem vessels, lower epidermis containing guard cells and stomata and another waxy cuticle.
How can the stomata be opened or closed?
Guard cells are on either side of a stoma. When the stomata are open, the guard cells are turgid (full of water). When the stomata are closed, the guard cells lose water (by osmosis), becoming flaccid. The stomata can be open or closed to control gas exchange and water loss.
How can dicotyledonous plants exchange gases with their environment?
Dicotyledonous plants exchange gases through the stomata on the underside of their leaves. When the stomata are open, oxygen and carbon dioxide can diffuse both in and out of the leaf through the stomata (the plant needs both for photosynthesis and respiration). The air spaces in the spongy mesophyll create concentration gradients to enable this to happen. However, this dies result in water loss through transpiration, so sometimes the stomata must be closed to limit water loss.
What are some adaptations of xerophytic (live in dry conditions) plants which limit water loss?
- having few stomata and stomata which are sunk in pits. This traps water vapour and reduces the concentration gradient of water, meaning the rate of transpiration is slowed
- thicker waxy cuticle to prevent water from evaporating off the leaves
- curled leaves with the stomata inside (this protects them from wind, which blows away water vapour, producing a steeper concentration gradient)
- a layer of ‘hairs’ on the epidermis to trap water vapour around the stomata, decreasing the concentration gradient of water.
How do terrestrial insects exchange gases with their environment?
Terrestrial insects have holes in their abdomen known as spiracles. These lead to tubes known as tracheae, which branch off into smaller tubes called tracheoles. Air enters the insect through the spiracles, then diffuses down a concentration gradient through the tracheae then the tracheoles. Oxygen then diffuses directly into cells and carbon dioxide out of cells due to their concentration gradients and the short diffusion path. When an insect isn’t getting enough oxygen, water enters the cells by osmosis, creating lower pressure in the tracheoles and trachea. This creates a pressure gradient with the outside air, forcing air into the insect. This increases rate of oxygen intake. Also, insects can ‘pump’ their abdomen to force air in and out of their spiracles, speeding up gas exchange.
How can terrestrial insects reduce water loss?
They have a waxy cuticle all over their bodies which prevents water loss by evaporation. They can also close their spiracles using muscles if necessary and they have tiny hairs around their spiracles, which reduce water loss.
Describe the structure of a xylem vessel.
Xylem vessels are long tubes consisting of stacks of specialised plant cells which have no cytoplasm or organelles. The cell walls of the xylem vessels have lignin, which is impermeable to water (but water adheres to it). Xylem vessels have no end walls and they have pits in the walls (sections of the xylem walls which have no lignin (just cellulose), which allow the lateral flow of water.
What is the function of a xylem vessel?
Xylem vessels are responsible for the mass transport of water and dissolved ions throughout the plant. Flow is unidirectional (from the roots to the leaves). The movement of water in this way through the xylem is called transpiration.
What is transpiration?
Transpiration is the loss of water vapour from the mesophyll layer of the leaf of a plant. The more water is lost by transpiration, the more water is pulled up the xylem (cohesion-tension hypothesis). Transpiration itself is a passive process but relies on energy from the sun to give the water molecules sufficient kinetic energy to move through the plant.
Relate the structure of a xylem vessel to its function.
- long cells end-to-end with no end walls - enables continuous columns of water
- no cytoplasm/organelles - nothing to obstruct the flow of water
- cellulose cell walls thickened with lignin - withstand tension (or else xylem could collapse) and there is adhesion between water and lignin, which helps create tension
Pits in walls - lignin is waterproof, so doesn’t allow water through, but the pits have no lignin so allow water to move laterally between xylem vessels and into phloem. This can help bypass blocked vessels.
What apparatus is used to measure transpiration rate?
Potometer
How does a potometer work?
The potometer is submerged in water to fill the capillary tube with water. A plant cutting is fixed to the potometer and a reservoir is filled with water. A bubble of air is introduced to the capillary tube and the apparatus is left for a set amount of time. The initial and final position of the air bubble shows the volume of water taken up by the plant, which indicates transpiration rate (assuming water taken up by the plant is proportional to water lost by transpiration). The tap of the reservoir can be opened to reset the experiment, moving the bubble back to its starting position.
Why might the volume of water taken up by a plant not equal the volume of water lost by the plant in the potometer experiment?
- water may be used in photosynthesis in the plant
- some water which appears to have been taken up by the plant may actually have diffused into the surrounding air if the equipment is not properly sealed
What factors influence transpiration rate and why?
- light intensity - higher light intensity increases transpiration as more stomata will be open and stomatal aperture will be wider, meaning more water is lost by transpiration
- temperature - higher temperature increases transpiration as water molecules have more kinetic energy so move faster
- humidity - the higher the humidity, the slower the transpiration rate as having more water vapour in the air decreases the water potential gradient
- air movement - an increase in air movement increases the rate of transpiration because the water molecules which diffuse out of the stomata are removed from the area around the leaf, thus increasing the water potential gradient.
What is digestion?
Digestion is the process of hydrolysing larger biological molecules into smaller molecules, which can be absorbed into the bloodstream and cross cell membranes to be used in the body’s processes or excreted.
What are the differences between physical/mechanical digestion and chemical digestion?
Mechanical digestion involves chewing, which increases the surface area for digestive enzyme action. Chemical digestion involves the action of digestive enzymes, during which, hydrolysis occurs and larger molecules are broken down into smaller ones.
How are carbohydrates digested?
Amylase hydrolyses starch into maltose. Amylase is produces by the salivary glands and the pancreas. Membrane-bound disaccharidases are attached to the cell membranes, and they hydrolyse the glycosidic bond which holds maltose together, producing alpha glucose. This glucose is small enough to pass through the cell membrane, and enters the cell by cotransport.
What are the names of the disaccharidases which hydrolyse sucrose, maltose and lactose?
Sucrose is hydrolysed by sucrase into glucose and fructose
Maltose is hydrolysed by maltase into two alpha glucose molecules
Lactose is hydrolysed by lactase into glucose and galactose
How are glucose molecules and amino acids transported into a cell?
They enter the cell by cotransport.
- Na+ ions are actively transported out of the epithelial cells in the ileum into the blood by the sodium-potassium pump
- ATP is hydrolysed to ADP and Pi to enable this to happen
- this creates a concentration gradient of Na+ ions (higher concentration of Na+ in the lumen of the ileum than in the epithelial cells).
- Na+ binds to the cotransporter protein alongside glucose/amino acid and the Na+ diffuses down its concentration gradient, enabling glucose/amino acids into the cell against their own concentration gradient.
- glucose diffuses out of the epithelial cells, into the blood by facilitated diffusion, through a channel protein down the concentration gradient of glucose.
Which enzymes catalase lipid digestion and what is produced?
Lipase enzymes catalyse the breakdown of lipids into monoglycerides and fatty acids. This involves the hydrolysis of the ester bonds in lipids. Lipases are mainly made in the pancreas and act in the small intestine.
What are bile salts?
Bile salts are produced by the liver and emulsify lipids, causing them to form small droplets. This increases the surface area for the action of lipases. Once lipases have acted, monoglycerides and fatty acids bind to bile salts, forming micelles.
What is the role of micelles in lipid digestion?
Micelles increase the surface area of the lipids for lipase action and transport the monoglycerides and fatty acids to the cell surface membrane.
How do monoglycerides and fatty acids cross the cell membrane?
They are lipid-soluble, so simple diffusion across the phospholipid bilayer.
What is emulsification?
Emulsification is the process which occurs during lipid digestion which increases the surface area of the lipid droplets for faster lipase action.
What happens once the fatty acids and monoglycerides are inside the epithelial cells?
The fatty acids and monoglycerides enter the smooth endoplasmic reticulum and Golgi apparatus, where the triglycerides are resynthesised and chylomicrons form from triglycerides, cholesterol, and lipoproteins. The chylomicrons are transported to the cell surface membrane by Golgi vesicles and they are released into the lymphatic vessel (lacteal).
What are the components of chylomicrons?
Triglycerides, cholesterol and lipoproteins.
What are the adaptations of the epithelial cells which make them well-adapted for absorption?
- the lining of the ileum is folded (villi), which increase the surface area for absorption
- the villi have thin walls (one epithelial cell layer thick), so there is a short diffusion path for the molecules being absorbed
- the epithelial cells have microvilli, increasing the surface area even more
- the villi have a good blood supply to carry away absorbed monomers, maintaining concentration gradients
What are endopeptidases?
Endopeptidases hydrolyse peptide bonds in the middle of the polypeptide to produce shorter polypeptides. This increases the number of ends on which the exopeptidases can work.
What are exopeptidases?
Exopeptidases hydrolyse peptide bonds at the ends of the polypeptide to produce single amino acids (or dipeptides if it is the last two amino acids in the chain).
What are dipeptidases?
Dipeptidases hydrolyse dipeptides into single amino acids. They are often found on the cell-surface membrane of epithelial cells in the small intestine.
How are amino acids absorbed into the epithelial cells?
Amino acids are absorbed using a type of cotransport in which sodium ions are actively transported out of the epithelial cells, into the ileum. They diffuse back into the cells through the sodium-dependent transporter proteins in the epithelial cell membranes, carrying the amino acids with them.
What is Visking tubing?
Visking tubing is a semi-permeable membrane which can be used to simulate the ileum in experiments involving digestion.
What are some adaptations of the lungs for rapid gas exchange?
- alveoli create a large surface area
- surrounding every alveolus is a network of capillaries
- walls of alveoli (epithelial cells) and wall of capillaries (endothelial cells) are very thin (one cell thick), so short diffusion pathway
- capillaries are narrow, so red blood cells have to squeeze through, which slows down blood flow, allowing more time for exchange to occur and reduces diffusion pathways further
- concentration gradients are maintained by the heart circulating the boood (moving away CO2 and brining O2 to lungs)
- breathing ventilates the lungs, so there is constant movement of O2 and CO2 in and out
What pathway do inspired gases take in humans?
As you breathe in, air enters the trachea, which splits into two bronchi (one bronchus leading to each lung). The bronchi then branch off into smaller bronchioles. The bronchioles end in alveoli.
What is the purpose of the branching in the human gas exchange system?
The branching increases the surface area to volume ratio of the system, allowing more gases to diffuse per unit time.
Describe what happens during inspiration.
The external intercostal muscles contract, while the internal intercostal muscles relax. The ribs are pulled upwards and outwards, increasing the volume of the thoracic cavity (thorax). The diaphragm contracts and flattens. The increased volume of the thorax causes a reduction of pressure in the lungs. Atmospheric pressure is greater than pulmonary pressure, so air is forced into the lungs.
Describe what happens during expiration.
The internal intercostal muscles contract while the external intercostal muscles relax. The ribs move downwards and inwards, decreasing the volume of the thorax. The diaphragm muscles relax, so the diaphragm is pushed up by the contents of the abdomen, so the volume of the thorax is decreased. This increases pressure in the lungs, making pulmonary pressure greater than atmospheric pressure, so air moves down the pressure gradient, out of the lungs.
Describe the exchange of gases at the alveoli.
Oxygen diffuses across the alveolar epithelium and the capillary endothelium from the alveoli to the blood (it hydrogen bonds to haemoglobin in the red blood cells). CO2 diffuses across the alveolar epithelium and the capillary endothelium from the blood plasma to the alveoli, to be expired.
