3.3 Organisms exchange substances with their environment Flashcards Preview

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3.3.1 Surface Area to Volume Ratio

How do you calculate the total surface area of an object?

Sum of surface area of all faces of shape


How do you calculate surface area for squares/rectangles, triangles, circles, spheres, cylinders?

Squares/Rectangles = length x width
Triangles = ½ x base x height
Circles = π x radius²
Spheres = 4 x π x radius²
Cylinder = (2 x (2 x π x radius)) + (length x width)


How do you calculate the volume of cubes/cuboids, prisms, spheres?

Cubes/Cuboids = length x width x height
Prisms = area of cross-section x length
Spheres = 4/3 x π x radiusᶟ


3.3.2 Gas Exchange

Why can’t insects use their body surface to exchange respiratory gases?

An efficient gas exchange surface would also leave them vulnerable to water loss as it would be an efficient water loss surface also


How do insects conserve water?

Rigid Exoskeleton – waterproof cuticle made of chitin
Small Surface Area: Volume – Minimises water loss area
Spiracles – Open and close to prevent water loss
Hairs around spiracles – trap humid air
Air sacs along tracheae – store oxygen if spiracles are closed for long time


How do the tracheae work in insects?

It is an internal network of tubes
Tracheae divide into tracheoles
Tracheoles branch throughout the body tissue of insects
Allows air to be brought directly to respiring tissues


How do spiracles work in insects?

Spiracles are tiny pores at end of tracheae
They allow respiratory gases in and out of insect
Valves control opening/closure of spiracle
When open, water can evaporate out of spiracles
They are closed most of the time to control water loss and only open to allow gas exchange
Whilst spiracles are closed, the level of oxygen in the tracheae decreases. Once level becomes too low, the spiracles open


Why aren’t insects bigger/ limitation of the tracheal system?

- Insects rely on diffusion rather than a transport system such as lungs. For diffusion to be adequate the diffusion distance must be short. This limits the size that insects can grow to
- Bigger insect = more cells = more demand for oxygen. Tracheal system could not meet these demands
- A larger insect = heavier exoskeleton = insect unable to move


How does the diffusion gradient allow gases to move in and out of the tracheal system?

During respiration, oxygen is used
Oxygen concentration at tracheole ends falls which creates a diffusion gradient
Oxygen therefore diffuses from atmosphere along the tracheae and tracheoles to the cells
Carbon Dioxide is produced by respiring cells which creates diffusion gradient in opposite direction
The carbon dioxide then diffuses out of the tracheoles and into the atmosphere


How does muscle contractions/mass transport allow gases to move in and out of tracheal system?

Abdominal pumping means contraction of insect muscles
This causes tracheae to be ‘squeezed’ and reduced in volume
Therefore, some air is expelled from tracheae
This is common in larger insects as it uses energy


How does water filled tracheoles allow gases to move in and out of tracheal system?

Anaerobic respiration produces lactate/lactic acid
Lactate is water soluble so lowers water potential of muscle cells
Water moves into muscle cells from tracheoles meaning the volume of tracheole ends decreases, drawing air in


How do fish exchange gas?

Water containing oxygen enters the fish through its mouth and passes out through the gills
Each gill is made of lots of thin plates called gill filaments, which increase the surface area for more efficient gas exchange
The gill filaments are covered in tiny structures called lamellae, which further increase the surface area
The lamellae have lots of blood capillaries and a thin surface layer of cells to speed up diffusion due to rich blood supply


How does counter-current flow help gas exchange in fish?

Blood flows through the lamellae in one direction and water flows over in the opposite direction
It maintains a large concentration gradient between the water and the blood.
The concentration of oxygen is always higher than that in the blood, so as much oxygen as possible diffuses from the water into the blood
Blood already loaded with oxygen meets water with maximum concentration of oxygen, so oxygen diffuses into the blood
Blood with low oxygen concentration meets water that has had most of its oxygen removed. Diffusion still happens and results in the maintenance of a favourable oxygen gradient across the whole gill which allows maximum oxygen diffusion


What is the equation for photosynthesis?

Carbon dioxide + water -> Glucose + oxygen

6CO2 + 6H2O -> C6H12O6 + 6O2


What is the equation for respiration?

Oxygen + glucose -> Carbon dioxide + water

6O2 + C6H12O6 -> 6CO2 + 6H2O


How are leaves adapted to exchange gas?

Large Surface Area
- Greater surface for diffusion
- Short diffusion pathway
Selectively permeable
- Controls what comes in and out
Diffusion gradient
- Large diffusion gradient = increased rate of diffusion


What structures do leaves have to facilitate efficient exchange?

- Small pores which allow gases in and out
- All cells are close to a stomatal pore so there is a short diffusion pathway
Air Spaces
- Interconnected air spaces throughout the mesophyll layer so gases can move around mesophyll cells
Spongy Mesophyll Layer
- Large surface area of mesophyll cells allows for maximum diffusion


What do stomata do?

Are tiny pores on the underside of leaves
Each stoma is surrounded by guard cells which control the opening and closing of stomata
Control the diffusion of gas and water vapour
In daytime: Photosynthesis occurring so needs lots of CO2 so stomata usually open
In night-time/dark: No photosynthesis so no need for CO2 so stomata closed


How can adaptations be detrimental to a plant?

Large SA of a leaf allows it to exchange gases and absorb sunlight
These features also promote desiccation (drying out)


What are xerophytes and how are they adapted to limit water loss?

Xerophytes are plants adapted to living in areas with short supply of water
Thick cuticle
- Barrier to evaporation
- Shiny surface reflects heat so lowers temperature
Sunken stomata
- Moist air trapped so lengthens diffusion pathway and reduces evaporation rate
Reduced SA:Vol ratio
- Less efficient diffusion
Hairs on leaves
- Traps heat
Rolled up leaves
- Traps heat and moisture


Features of human gas exchange system

- Lobed structures made up of a series of highly branched tubules (bronchioles) which end in tiny air sacs called alveoli
- Flexible airway supported by cartilage rings
- The cartilage prevents the trachea collapsing as air pressure inside falls when breathing in
- Muscular walls lined with ciliated epithelium and goblet cells
- Trachea splits into 2 bronchi
- Larger bronchi are supported by cartilage rings
- Lined with ciliated epithelial and goblet cells
- Produce mucus to trap dirt particles and have cilia that move the dirt-laden mucus towards throat
- Subdivisions of bronchi
- Muscular walls lined with epithelial cells allowing them to constrict to control airflow in and out of alveoli
- Minute air sacs at the end of bronchioles
- Collagen and elastic fibres between alveoli
- Lined with epithelium
- Elastic fibres allow alveoli to stretch as fill with air when breathing in. They then spring back during breathing out to expel CO2 rich air
- Alveolar membrane is the gas exchange surface


Why does gas exchange need to happen?

All aerobic organisms need a constant supply of O2 to release energy in the form of ATP during respiration
The CO2 produced needs to be removed as its build up could be harmful to the body


Why is the volume of O2 absorbed and volume of CO2 removed large in mammals?

They are relatively large organisms with a large volume of living cells
They maintain a high body temperature which is related to them having high metabolic and respiratory rates


Why is the site of gas exchange in mammals (lungs) located inside the body?

Air is not dense enough to support and protect these delicate structures
The body as a whole would otherwise lose a great deal of water and dry out


What is inspiration?

When air pressure of atmosphere is greater than air pressure inside the lungs, and air is forced into the lungs
Is an active process so requires energy


What is expiration?

When air pressure in lungs is greater than air pressure of atmosphere, and air is forced out of lungs
Is a largely passive process so does not require much energy


What three sets of muscles movement cause pressure changes in the lungs?

- A sheet of muscle that separates the thorax from the abdomen
Internal Intercostal Muscles
- Lie between the ribs whose contractions lead to expiration
External Intercostal Muscles
- Lie between the ribs whose contractions lead to inspiration


How do you calculate the pulmonary ventilation rate?

Pulmonary ventilation rate (dm3min-1) =

Tidal volume (dm3) x Breathing rate (min-1)


Describe the process of inspiration

External intercostal muscles contract, while internal intercostal muscles relax
Ribs are pulled upwards and outwards, increasing the volume of the thorax
The diaphragm muscles contract, causing it to flatten, which also increases the volume of the thorax
The increased volume of thorax results in reduction of pressure in the lungs
Atmospheric pressure is now greater than pulmonary pressure, and so air is forced into lungs


Describe the process of expiration

Internal intercostal muscles contract while external intercostal muscles relax
Ribs move downwards and inwards decreasing volume of the thorax
Diaphragm muscles relax and so it is pushed up again by contents of abdomen that were compressed during inspiration. Volume of the thorax is therefore further decreased
Decreased volume of thorax increases pressure in lungs
Pulmonary pressure now greater than atmospheric pressure, and so air is forced out of lungs


Where is the site of gas exchange in mammals?

The epithelium of the alveoli


What does there need to be to maintain a diffusion gradient?

Movement of both the environmental medium (e.g. air) and internal medium (e.g. blood)


Describe features of alveoli

300 million alveoli in each human lung
Total surface area around 70m2
Each alveolus lined with epithelial cells
Around each alveolus is a network of pulmonary capillaries, so narrow that red blood cells are flattened against the thin capillary walls order to fit through
These capillaries have walls that are only a single layer of cells thick


Why is the diffusion of gases between alveoli and blood very rapid?

Red blood cells are slowed as they pass through pulmonary capillaries, allowing more time for diffusion
The distance between alveolar air and red blood cells is reduced as the red blood cells are flattened against capillary walls
The walls of both alveoli and capillaries are very thin and therefore the distance over which diffusion takes place is short
Alveoli and pulmonary capillaries have a large total surface area
Breathing movements constantly ventilate the lungs, and the action of the heart constantly circulates blood around the alveoli. Together, these ensure that a steep concentration gradient of the gases to be exchanged is maintained
Blood flow through the pulmonary capillaries maintains a concentration gradient


What is a correlation?

Occurs when a change in one of two variables is reflected by a change in the other variable
A correlation does not mean that there is a causal link


What are the risk factors for lung disease?

Air pollution
Genetic make-up


What is emphysema?

A loss of elasticity preventing expansion and contraction
Common in smokers
Healthy lungs contain elastic tissue made from elastin (protein), so lungs stretch when we inhale and spring back when we exhale
In emphysemous lungs the elastin has been permanently stretched so lungs no longer able to expel all air from alveoli
SA of alveoli reduced as some alveoli burst so little gas exchange occurs


3.3.3 Digestion and absorption
What is the role of the oesophagus?

Carries food from the mouth to the stomach


What is the role of the stomach?

A muscular sac with an inner layer that produces enzymes
Its role is to store and digest food, especially proteins
It has glands that produce enzymes which digest protein


What is the role of the ileum?

A long muscular tube
Food is further digested in the ileum by enzymes that are produced by its walls and by glands that pour their secretions into it
The inner walls of the ileum are folded into villi, giving them a large SA
The surface area of the villi is further increased by millions of tiny projections called microvilli on epithelial cells of each villus
This adapts the ileum for its purpose of absorbing the products of digestion into the bloodstream


What is the role of the large intestine?

Absorbs water
Most of the water that is absorbed is water from the secretions of the may digestive glands


What is the role of the rectum?

The final section of the intestines
The faeces are stored here before periodically being removed via the anus in a process called egestion


What is the role of the salivary glands?

Situated near the mouth
They pass their secretions via a duct into the mouth
These secretions contain the enzyme amylase, which hydrolyses starch into maltose


What is the role of the pancreas?

A large gland situated below the stomach
It produces a secretion called pancreatic juice
This secretion contains proteases to hydrolyse proteins, lipase to hydrolyse lipids and amylase to hydrolyse starch


What is digestion?

The process in which large molecules are hydrolysed by enzymes into small molecules, which can be absorbed and assimilated


What are the two stages of human digestion?

Physical breakdown
Chemical digestion


What is physical breakdown?

If the food is large, it is broken down into smaller pieces by means of structures such as teeth
This makes it possible to ingest the food and also provides a large SA for chemical digestion
Food is churned by the muscles in stomach wall and this also physically breaks it up


What is chemical digestion?

Chemical digestion hydrolyses large, insoluble molecules into smaller, soluble ones
It is carried out by enzymes
All digestive enzymes function by hydrolysis which is the splitting up of molecules by adding water to chemical bonds that hold them together
Enzymes are specific so more than one enzyme is needed to hydrolyse large molecules


Name the three most important digestive enzymes and their roles

Carbohydrases – hydrolyse carbohydrates to monosaccharides
Lipases – hydrolyse lipids (fats and oils) into glycerol and fatty acids
Proteases – hydrolyse proteins to amino acids


Describe the process of carbohydrate digestion

The enzyme amylase is produced in the mouth and pancreas.
Amylase hydrolyses the alternate glycosidic bonds of the starch molecule to produce the disaccharide maltose
The maltose is then hydrolysed into the monosaccharide alpha glucose by the enzyme maltase
Maltase is produced in lining of ileum


What is the process of carbohydrate digestion (especially maltose) in humans?

Saliva enters mouth from salivary glands and is mixed with food during chewing
Saliva contains salivary amylase which starts to hydrolyse any starch in the food to maltose. Also contains mineral salts that maintain pH at neutral which is optimum pH for salivary amylase to work
Food is swallowed and enters stomach, where conditions are acidic. Acid denatures amylase and prevents further hydrolysis of the starch
Food is passed to small intestine, where it mixes with secretion from pancreas called pancreatic juice
Pancreatic juice contains pancreatic amylase. This continues hydrolysis of remaining starch to maltose. Alkaline salts are produced by both pancreas and intestinal wall to maintain pH at neutral so amylase can function
Muscles in intestine wall push food along ileum. Its epithelial lining produces disaccharide maltase. Maltase is not released into lumen of ileum but is part of cell-surface membranes of epithelial cells. So is referred to as membrane-bound disaccharidase
Maltase hydrolyses maltose from starch break down into alpha glucose


How is the carbohydrate sucrose digested in humans?

Sucrose is found in many natural foods especially fruits
Sucrase hydrolyses the single glycosidic bond in the sucrose molecule. This hydrolysis produces two monosaccharides glucose and fructose


How is the carbohydrate lactose digested in humans?

Lactose is found in milk and milk products e.g. yoghurt and cheese
Lactase hydrolyses the single glycosidic bond in the lactose molecule. This hydrolysis produces two monosaccharides glucose and galactose


Describe the process of lipid digestion

Lipids are hydrolysed by enzymes called lipases
Lipases are enzymes produced in pancreas that hydrolyse ester bond found in triglycerides to form fatty acids and monoglycerides (a glycerol molecule with a single fatty acid molecule attached)
Lipids are firstly split up into tiny droplets called micelles by bile salts, which are produced by the liver
This process is called emulsification and increases SA of the lipids, so action of lipases is speeded up


Describe the process of protein digestion

Proteins are large, complex molecules hydrolysed by a group of enzymes called peptidases (proteases)

- Hydrolyse the peptide bonds between amino acids in the central region of a protein molecule forming a series of peptide molecules
- Hydrolyse the peptide bonds on the terminal amino acids of the peptide molecules formed by endopeptidases. In this way they progressively release dipeptides and single amino acids
- Hydrolyse the bond between two amino acids of a dipeptide. Dipeptidases are membrane-bound being part of the cell-surface membrane of the epithelial cells lining the ileum


How is the ileum adapted for absorption?

Folded inner walls/villi and microvilli to increase surface area
Thin walls lined with epithelia cells
Lots of capillaries for constant blood supply
Muscular walls so can move which helps maintain a concentration gradient (movements mix food)


How are amino acids and glucose absorbed?

Amino acids and monosaccharides such as glucose are absorbed into the blood using co-transport
Na+ actively transported out of epithelial cell by Na+/K pump into the blood
Maintains higher concentration of Na+ in lumen than inside epithelial cells
Na+ diffuse into epithelial down concentration gradient
They carry with them amino acids or glucose
Glucose and amino acids pass into the blood plasma by facilitated diffusion


3.3.4 Mass transport

What is mass transport?

The bulk movement of materials from exchange surfaces to the cells


What are the features of efficient transport systems (e.g. circulatory systems)?

A suitable transport medium
- E.g. The Blood
- Normally liquid but can be gas
- Materials (e.g. oxygen, waste) can dissolve
Closed system of tubular vessels
- E.g. Blood Vessels
- Contains/ holds the medium
- Forms branching to all parts of the organism
- Ensures medium is close to cells
Mechanisms for movement of tissue fluid
- E.g. The Heart
- Requires a pressure difference in one part of the system and another
- Enables the medium to move


Why is a double circulatory system required in larger mammals?

Small SA: Volume
High level of activity so high metabolic rate
Maintain temperature via respiration
Therefore, blood passes through heart twice per complete circuit


Describe the role of the first part of double circulatory system (Pulmonary Circulation)

Pumps blood from heart to lungs
Oxygenates blood/removes CO2


Describe the role of the second part of the double circulatory system (Systemic Circulation)

Blood to the rest of the body
Increased pressure from heart


Structure of heart

Look at diagram


Describe the flow of blood

- Vena Cava carries deoxygenated blood from the head and body into the right atrium then into right ventricle through tri-cuspid valve
- Pulmonary artery carries deoxygenated blood through semi-lunar valve and to the lungs
- Pulmonary vein carries oxygenated blood from the lungs into left atrium then into left ventricle through bicuspid valve
- Aorta carries oxygenated blood through semi-lunar valve and to the head and body


What are the adaptations of the heart?

Coronary arteries
- Supply the heart with the O2 it requires to contract
Wall Thickness
- Left ventricle wall is much thicker
- Pumps blood around the body
- Atria only pump blood into ventricles so thinner
- Right ventricle pumps blood into lungs at relatively low pressure
- Volume in each is the same
- Between atrium and ventricle
- Between ventricles and vessels
- Prevent back flow of blood
- Pressure is higher on concave side of valve
- Pushes the flexible fibrous tissue together to form tight fit
- Causes the LUB DUB sound of heartbeat


What are the two main processes in the cardiac cycle?

Contraction – Systole
Relaxation – Diastole


Describe the process of diastole

- Blood returns to the atria of the heart through the pulmonary vein (from the lungs) and the vena cava (from the body)
- As the atria fill, the pressure in them increases
- When this pressure increases that of the ventricles, the atrioventricular valves open, allowing the blood to flow into the ventricles aided by gravity
- Muscular walls of both atria and ventricles are relaxed. Relaxation of ventricle walls causes them to recoil and reduces ventricular pressure
- This causes pressure to be lower in aorta and pulmonary artery, so semi-lunar valves in aorta and pulmonary artery close
- DUB sound of heartbeat


Describe the process of Atrial Systole

- Cardiac muscle of atria contracts
- Forces the remaining blood into the ventricles from the atria
- Ventricle walls remain relaxed


Describe the process of Ventricular Systole

- Ventricles begin to fill with blood and their walls contract
- This increases blood pressure within them, forcing shut the atrioventricular valves and preventing backflow of blood into the atria
- LUB sound of heartbeat
- With atrioventricular valves closed, pressure in ventricles rises further
- Once it increases that in aorta and pulmonary artery, blood is forced from ventricles into these vessels
- Ventricles have thicker walls than atria meaning they contract forcefully, creating high pressure necessary to pump blood around body


What is cardiac output?

Volume of blood pumped by one ventricle of the heart in one minute


How do you calculate cardiac output?

Cardiac Output = Heart Rate x Stroke Volume


What is haemoglobin?

A respiratory pigment used to transport oxygen
Is a protein


Describe the structure of haemoglobin

Has quaternary protein structure
Made up of 4 polypeptide chains (2x α polypeptide and 2x β polypeptide)
Each chain is attached to a haem group which contains a ferrous ion (Fe²⁺)
Each Fe²⁺ ion can combine with a single oxygen molecule
When combined with oxygen = oxyhaemoglobin


What must haemoglobin be able to do to be efficient?

Readily associate with oxygen at the exchange surface
Readily dissociate at the tissues


How does haemoglobin readily associate and dissociate with oxygen?

Depends on the affinity of haemoglobin (The attractive force binding atoms in molecules/chemical attraction)

High affinity: High attractive force so readily associates with oxygen

Low affinity: Associates with oxygen less easily but readily dissociates/releases oxygen


What does the affinity of haemoglobin found within an organism depend on?

1. The Environment
- How much oxygen is available?
- Low partial pressure of oxygen means need to hold onto oxygen, so haemoglobin has a high affinity for oxygen BUT, oxygen dissociates less easily as organisms have low metabolic rate
- High partial pressure means oxygen readily available so weak attraction of oxygen (low affinity for oxygen) BUT oxygen dissociates easily as organisms have high metabolic rate
2. Metabolic Rate
- How much oxygen is required by the organism


What does an oxygen dissociation curve show?

Shows the relationship between the saturation of haemoglobin with oxygen and the partial pressure of oxygen

Where pO2 is high (e.g. in lungs) haemoglobin has high affinity for oxygen so has a high saturation of oxygen
Where pO2 is low (e.g. in respiring tissues) haemoglobin has a low affinity for oxygen, so releases oxygen rather than combining with it. Therefore, low saturation of oxygen


What is positive cooperativity?

Binding of the first oxygen molecule makes binding of the second easier


What does a shift to the left on the oxygen dissociation show?

Higher oxygen affinity
Low partial pressure of oxygen so needs to readily associate with oxygen


What does a shift to the right on the oxygen dissociation curve show?

Lower oxygen affinity
High metabolic rate so higher oxygen demand
High oxygen availability


Describe the Bohr effect

When cells respire, they produce carbon dioxide, raising the partial pressure of carbon dioxide
This increases the rate of oxygen unloading (the rate at which oxyhaemoglobin dissociates to form haemoglobin and oxygen)
So, the dissociation curve shifts RIGHT
The saturation of blood with oxygen is lower meaning more oxygen is being released


How does CO2 affect the blood?

It lowers the pH of the blood
Need higher pH to bind to oxygen
Need lower pH to release oxygen


Describe the process of loading oxygen at the lungs

CO2 is constantly removed at the lungs
Low CO2 concentration
Therefore, pH is raised
Haemoglobin changes shape and the affinity of haemoglobin increases
Strong attraction for oxygen so easier loading
LEFT shift on oxygen dissociation curve as lungs have high affinity for oxygen


Describe the process of unloading at the respiring tissues

CO2 is taken in
High CO2 concentration
pH is lowered
Haemoglobin changes shape
Affinity of haemoglobin decreases
Weaker attraction for oxygen so easier unloading (oxygen readily dissociates)
More O2 is available for respiration
RIGHT shift as muscle cells have low affinity


What is the role of arteries?

Carry oxygenated blood from the heart to the rest of the body


What is the role of arterioles?

Contract to control blood flow from arteries to capillaries
Form a network throughout the body
Blood is directed to different areas of demand in body by muscles inside arterioles


What is the role of capillaries?

Link arterioles to veins


What is the role of venules?

Control blood flow from capillaries to veins


What is the role of veins?

Carry deoxygenated blood to the heart


What are the similarities of veins and arteries?

Tough Outer Layer
- Resist pressure changes from both within and outside
Muscle Layer
- Contract and control blood flow
- Vasodilation (Blood vessel widens)
- Vasoconstriction (Blood vessel narrows)
Elastic Layer
- Helps to maintain blood pressure by stretching and recoiling
Lumen (Cavity)
- Not a layer but a passage for the blood to travel through


How are arteries adapted?

- Thick and muscular walls to maintain high pressure
- Elastic tissue stretches and recoils as heart beats to also maintain high pressure
- Inner lining (endothelium) is folded allowing artery to stretch and also maintains high pressure
- All arteries carry oxygenated blood except for pulmonary arteries, which take deoxygenated blood to the lungs


How are veins adapted?

- Wider lumen
- Little elastic or muscle tissue as under low pressure
- Contain valves to stop the blood flowing backwards
- Blood flow through veins is helped by contraction of body muscles surrounding them
- All veins carry deoxygenated blood except for pulmonary veins which carry oxygenated blood to heart from lungs


How are capillaries adapted?

- Found near cells in exchange tissues so short diffusion pathway
- Walls only one cell thick to shorten diffusion pathway
- Large number of capillaries, to increase surface area for exchange. Networks of capillaries are called capillary beds


What is tissue fluid?

Watery liquid that allows the exchange of substances between blood and cells


What are the substances that tissue fluid contains?

Molecules required by cells
- Glucose, amino acids, fatty acids, ions, and oxygen
Waste produced
- Carbon dioxide, urea, and water


Unlike the blood, what does tissue fluid not contain?

Large molecules
- Red blood cells, Plasma proteins as they are too big to fit through capillary walls


Describe the process of formation of tissue fluid at ARTERIOLE

Formation of tissue fluid is a result of the balance between hydrostatic pressure (blood pressure) and water potential

1. Hydrostatic pressure inside the capillary is greater (due to narrowing of capillaries causing greater pressure) than hydrostatic pressure outside so fluid from blood is forced out of the capillaries
2. Blood contains plasma proteins that lower the water potential, so some water moves into capillaries
3. Hydrostatic pressure can only force small molecules out of the capillary so larger molecules and proteins remain in the blood
4. This type of filtration is known as ULTRA-FILTRATION


Describe the process of the return of tissue fluid at VENUOLE

1. Tissue fluid must then return to the blood plasma to remove waste
2. Water potential is lower inside the capillaries, so water moves back into capillaries
3. Hydrostatic pressure is reduced at the venule end, so tissue fluid is forced back in


What happens to some of the tissue fluid?

Not all tissue fluid can return via the capillaries so 10% enters the lymphatic system


What is the lymphatic system?

Separate system to the circulatory system made up of microscopic tubes called lymph capillaries.
Contains accumulated tissue fluid (lymph)
Drains back into the blood via ducts that join veins close to the heart


How is lymph moved?

Hydrostatic pressure of the tissue fluid
Contraction of body muscles squeezes lymph vessels


What is fluid called in the blood, surrounding the cells and in the lymphatic system called?

Fluid in the blood = Plasma
Fluid surrounding the cells = Tissue Fluid
Fluid in lymphatic system = Lymph


Why do plants require a mass transport system?

They are large
They are multicellular
Small SA: V
Cannot rely on diffusion


What molecules are transported in plants?



What are the two main vessels that enable this movement in plants?

Phloem (sugar)
Xylem (water)


Cross section of a leaf

See diagram


What is transpiration?

The loss of water (evaporation) from the stomata
If the humidity of the atmosphere is less than the air spaces next to the stomata, water will move


What are the factors affecting transpiration?

- Water particles have more kinetic energy
- More humid = less transpiration as more water in air
Light Intensity
- Stomata are open in light and close in dark due to photosynthesis
Air Movement
- Disperses the humid layer of air (increases rate of transpiration)


How are root hair cells adapted for transpiration?

Large Surface Area – Increased diffusion rate
Thin Cell Wall – Short diffusion pathway
No Chloroplasts – Underground so doesn’t need to photosynthesise
Large Vacuole – Helps toensure has water potential gradient
Mitochondria – for active transport energy from respiration


How are xylem vessels adapted for transpiration?

Hollow open ended tubes to allow water to pass through middle easily
Walls lined with lignin
Lignin forms springs or spirals around the vessel for support
Formed from dead cells


What is the cohesion tension theory?

The hypothesis used to explain how water can travel upwards against gravity in a plant


What is cohesion?

Hydrogen bonds between molecules form
This forms a continuous water column
As water moves up, the following molecule is pulled up – TRANSPIRATION PULL


What is adhesion?

Water is attracted to lignin in the xylem wall
It stops the water column from breaking and facilitates movement


How does water move upwards through the xylem?

Water is evaporated from the leaves due to higher water potential otside cells
Water moves out of the xylem creating a pull/tension on water in xylem
Water enters through the roots by osmosis
Water is in a continuous column
Cohesion due to hydrogen bonding
Column doesn’t break due to adhesion with xylem wall


How do you investigate transpiration using a potometer?

1. A leafy shoot is cut under water ensuring no water gets onto leaves
2. Fill the potometer completely with water, making sure there are no air bubbles
3. Using a rubber tube, the leafy shoot is fitted to the potometer under water
4. The potometer is removed from under the water and all joints sealed with waterproof jelly
5. An air bubble is introduced into the capillary tube
6. The distance moved by the air bubble in a given time is measured a number of times and mean is calculated
7. Using this mean value, the volume of water lost is calculated
8. Volume of water lost against the time in minutes can be plotted on a graph
9. Once the air bubble nears the junction of resevoir tube and capillary tube, the tap on resevoir is opened and syringe is pushed down until bubble is pushed back to start of scale on capillary tube. Measurements continue as before
10. Experiment repeated to compare values of water uptake in different conditions e.g different temperatures, humidity, light intensity, or different species of plants


What is translocation?

The process by which organic molecules and some mineral ions are transported from one part of a plant to another


What is the structure of the phloem?

- Phloem made up of sieve tube elements, long thin structures arranged end to end
- End walls are perforated to form sieve plates
- Assosiated with sieve tube elements are cells called companion cells
- After producing sugars during photosynthesis, the plant transports them from sies of production (sources) to places where they can be used directly or stored (sinks)
- Since sinks can be anywhere in plant, flow of translocation of molecules can be in any direction


What does phloem transport?

Organic molecules e.g. sucrose and amino acids
Inorganic ions e.g. potassium, chloride, phosphate and magnesium


What is the precise mechanism by which translocation is achieved?

Mass Flow Theory


Describe the first stage of mass flow theory

Transfer of sucrose into sieve elements from photosynthesising tissue
- Sucrose is manufactured from products of photosynthesis in cells with chloroplasts
- Sucrose diffuses down a concentration gradient by facilitated diffusion from the photosynthesising cells into the companion cells
- Hydrogen ions are actively transported from companion cells into the spaces within the cell walls using ATP
- These hydrogen ions then diffuse down a conc gradient through carrier proteins into the sieve tube elements
- Sucrose molecules are transported along with the hydrogen ions via co-transport. The protein carriers are therefore known as co-transport proteins


Describe the second stage of mass flow theory

Mass flow of sucrose through sieve tube elements
- The sucrose produced by photosynthesising cells is actively transported into the sieve tubes
- This causes sieve tubes to have a lower WP
- As xylem has higher WP, water moves from xylem into sieve tubes by osmosis, creating high hydrostatic pressure within them
- At the respiring cells, sucrose is either used up during respiration or converted to starch for storage
- These cells therefore have a low sucrose content and so sucrose is actively transported into them from sieve tubes lowering their WP
- Due to lowered WP, water also moves into these respiring cells from sieve tubes by osmosis
- Hydrostatic pressur to this region is lowered
- Due to water entering sieve tube elements at source and leaving a sink, there is high hydrostatic pressure at source and low at sink
- There is therefore a mass flow of sucrose solution down this hydrostatic gradient in sieve tubes


Describe the third stage of mass flow theory

Transport of sucrose from the sieve tube elements into the storage or other sink cells
- Sucrose is actively transported by companion cells, out of the sieve tubes and into the sink cells


Evidence SUPPORTING the mass flow theory

- There is pressure within the sieve tubes, shown by sap being released when they are cut
- Concentration of sucrose is higher in leaves (source) than in roots (sink)
- Downward flow in phloem occurs in daylight, but ceases when leaves are shaded, or at night
- Increases in sucrose levels in the leaf are followed by similar increases in sucrose levels in phloem a little later
- Metabolic poisons and/or lack of oxygen inhibit translocation of sucrose in phloem
- Companion cells possess many mitochondria and readily produce ATP


Evidence AGAINST the mass flow theory

- Function of sieve plates is unclear, as they would seem to hinder mass flow(has been suggested that they may have a structural function, he,ping to prevent tubes from bursting under pressure)
- Not all solutes move at same speed but should do if movement is by mass flow
- Sucrose is deleivered at more or less same rate to all regions, rather than going more quickly to the ones with the lowest sucrose concentration, which the mass flow theory would suggest


How is mass flow as a whole active?

Mass flow is a passive process however occurs as a result of active transport of sugars so the process as a whole is active
Therefore the rate is affected by temperature and metabolic poisons


How are ringing experiments used to investigate transport in plants?

Woody stems have outer protective layer of bark. On inside is a layer of phloem that extends all round the stem. Inside phloem layer is xylem
- Section of outer layers is removed around complete circumference of the woody stem while it is still attached to rest of plant
- After period of time, region of stem immediately above missing ring of tissue starts to swell
- Samples of liquid that has accumulated in this swollen region are found to be rich in sugars and other dissolved organic substances
- Some non-photosynthetic tissues in region below ring (towards roots) are found to wither and die, while those above ring grow
The observation suggest that rmoving phloem around stem led to:
- Sugars of phloem accumulating above the ring, leading to swelling
- Interruption of flow of sugars in region below ring and death of tissues below
- The phloem rather than xylem is tissue responsible for translocating sugars in plants
- As ring of tissue removed was not extended into xylem, its continuity had not been broken. If it were the tissue responsible for translocating sugars, you would not have expected sugars to accumulate above ring or tissues below to die


How are tracer experiments used to investigate transport in plants?

Radioactive isotopes are useful for tracing the movemt of substances in plants
- Isotope ¹⁴C can be used to make radioactively labelled carbon dioxide (¹⁴CO₂)
- If a plant is then grown in an atmosphere containing ¹⁴CO₂, the ¹⁴C isotope will be incorporated into the sugars produced during photosynthesis
- These radioactive sugars can then be traced as they move within the plant using autoradiography
- Autoradiography involves taking thin cross sections of a plant stem and placing them on piece of x-ray film
- The film becomes blackened where exposed to radiation produced by ¹⁴C in sugars
- Blackened regions found to correspond to where phloem tissue is in stem
- As other tissues do not blacken film, it follows that they do not carry sugars and phloem alone responsible for the translocation


What is other evidence that translocation of organic molecules occurs in phloem?

1. When phloem cut, solution of organic molecules flow out
2. Plants provided with radioactive CO2 can be shown to have radioactively labbeled carbon in phloem after short time
3. Aphids are type of insect that feed on plants. Have needle like mouthparts which penetrate phloem. Can therefore be used to extract contents of sieve tubes. Contents show daily variations in sucrose content of leaves that are mirrored later by identical changes in sucrose content of phloem
4. The removal of ring of phloem around whole circumference of stem leads to accumulation of sugar above ring and their dissapearance from below it