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OCR Categories Archives: Module 3: Exchange and Transport

Transport in Animals #2

Transport in Animals

Transport – the movement of substances such as oxygen, nutrients, hormones, waste and heat around the body

Three factors influence the need for transport systems:

  • Size
  • Surface area/volume ratio
  • Level of metabolic activity

Features of a good transport system:

  • A fluid or medium to carry substances around the body e.g. blood, lymph
  • A pump to create pressure to push fluid round the body
  • Exchange surfaces that enable substances to enter the fluid (blood) e.g. capillaries
  • Tubes or vessels to carry the fluid by mass flow
  • Two circuits, one for collection of oxygen and another for the delivery of oxygen

Single circulatory system – one in which the blood flows through the heart only once for each circuit of the body e.g. heart -> gills -> body -> heart

Double circulatory system – one in which the blood flows through the heart twice for each circuit in the body e.g. heart -> body -> heart -> lungs -> heart

 

  • The blood pressure must not be too high in the pulmonary circuit so as not to damage the delicate capillaries
  • The heart can increase the pressure after it returns to the lungs so that the blood circulates through the body quickly
  • The systemic circulation can carry higher pressured blood than the pulmonary circulation

Blood Vessels

 

Artery Adaptions:

  • Artery walls are thick to withstand high pressure
  • The lumen is small to maintain the high pressure
  • The inner layer consists of a thin layer of elastic tissue that allows the walls to stretch and recoil
  • The middle layer consists of a thick layer of smooth muscle
  • The outer layer is a thick layer of collagen and elastic tissue for strength and to support the recoil for maintaining pressure.

Arterioles:

  • Small blood vessels that distribute the blood from arteries to the capillaries
  • They have a layer of smooth muscle which contracts to increase resistance to flow and reduces the rate of blood flow
  • Constriction of arteriole walls is used to divert blood to regions of the body that are demanding oxygen

Capillaries:

  • Very thin walls consisting of a single layer of endothelium
  • Narrow lumen to squeeze red blood cells against the wall to aid in the transfer of oxygen and reduce the diffusion distance
  • Leaky walls to allow blood plasma and dissolved substances to leave the blood (e.g. in lymph)

Venules:

  • These collect blood from the capillary bed and lead into the veins
  • Consists of thin layers of muscle and elastic tissue as the pressure is relatively low

Veins:

  • Carry blood back to the heart
  • Large lumen in order to ease the flow of blood
  • Walls are thin as they do not need to stretch and recoil and are not actively constricted in order to reduce the blood flow
  • They contain valves to prevent back flow of blood and help the blood flow into the heart properly

Blood plasma and tissue fluid

Plasma is the fluid portion of the blood which may move into the body, containing dissolved substances such as O2 and CO2. Tissue fluid is formed by blood plasma leaking from the capillaries into the tissues.

  1. t the arteriole end of the capillary bed the blood is at high hydrostatic pressure (the pressure a fluid exerts when pushing against the sides of a vessel or container
  2. This high hydrostatic pressure pushes the plasma out of the capillaries between the tiny gaps between the cells in the walls
  3. The fluid contains dissolved nutrients and oxygen but the cells and the platelets are too large to leave the blood system
  4. Not all the fluid returns to the blood, some is directed into the lymphatic system, which drains the excess tissue fluid out of the tissues and returns it to the blood system in the subclavian vein
  5. The lymphatic system contains many more lymphocytes which are produces in the lymph nodes

Oncotic pressure – the pressure created by the osmotic effects of the solutes and causes the movement of tissue fluid into the blood (it has a negative figure and is also measured in kPa)

Structure of the Heart

 

Blood Pressure

Atria – These chambers have relatively thin walls as they do not need to create much pressure

Right ventricle – thicker walls than the atria but still not as thick as the left ventricle as the blood only needs to be pumped as far as the lungs which lie next to the heart in the chest cavity

Left ventricle – two or three times thicker than the right ventricle as an inordinate amount of pressure must be created here to overcome the resistance of the systemic circulation

Cardiac Muscle Structure

 

  • They are divided into contractile units called sarcomeres
  • There are numerous mitochondria between the myofibrils (muscle fibres)
  • The intercalated discs facilitate synchronised contraction

The Cardiac Cycle

Blood enters the aorta and pulmonary artery in a rapid spurt but must be delivered in an even flow to prevent damage so the structure of the artery walls come into play. The smooth muscle and elastic fibre layers allow stretch and recoil with each beat of the heart which them lowers the pressure. The further along the blood flows into the arterioles the less obvious the fluctuations tend to be. It is important to maintain blood pressure so that the blood can travel all the way around the body and deliver nutrients to all the tissues.

Coordination of the cardiac cycle

 

The heart is myogenic as it can initiate its own contraction.

If the contractions of the atria and ventricles are not synchronised it is known as fibrillation.

The sino-atrial node generates a wave of excitation at regular intervals (55-80x per min).

The wave spreads over the walls of both atria causing the cardiac muscle cells to contract during atrial systole.

At the top of the atrioventricular septum, the atrioventricular node delays the signal before conducing it down the purkyne tissue and spreads out through the walls of both ventricles.

This means that they contract from the apex upwards, forcing the blood in the correct direction.

Electrocardiograms

  • Wave P shows the excitation of the atria
  • QRS indicates the excitation o the ventricles
  • T shows diastole

 

Sinus rhythm – normal

Bradycardia – slow heart rate

Tachycardia – fast heart rate

Atrial fibrillation – atria beating faster than the ventricles

Ectopic heartbeat – irregular heart beat

Transport of Oxygen

Haemoglobin + oxygen à oxyhaemoglobin

Haemoglobin has a high affinity for oxygen and each of the four haem groups binds with one oxygen molecule

Oxygen is absorbed in the blood as it passes the alveoli in the lungs. The oxygen binds reversibly to the haemoglobin.

In the body tissue dissociation takes place and the oxyhaemoglobin releases the oxygen for the cells to use in aerobic respiration

The ability of haemoglobin to take up and release oxygen depends on the oxygen partial pressure. Association of oxygen and haemoglobin takes place when the partial pressure is high whereas the dissociation of oxygen and haemoglobin takes place when the partial pressure of oxygen is low.

Partial pressure symbol = pO2

Haemoglobin dissociation curve:

 

 

 

 

 

 

 

 

After the first oxygen binds to the haemoglobin the haemoglobin undergoes a slight conformational change which allows other oxygen molecules to more easily bond to its remaining haem groups.

Fetal haemoglobin lies to the left of the normal haemoglobin dissociation curve as fetal Haemoglobin has a higher oxygen affinity that normal haemoglobin as the fetal haemoglobin must cause the dissociation of the mother’s haemoglobin in the placenta and absorb oxygen from the surrounding fluid as well.

Transport of CO2

Carbon dioxide is transported in three ways:

  • 5% is dissolved directly in the plasma
  • 10% is combined directly with haemoglobin to form carbaminohaemoglobin
  • 85% is transported in the form of hydrogencarbonate ions

  1. Carbon dioxide diffuses into the red blood cell and combines with water to form carbonic acid (catalysed by carbonic anhydrase) CO2 + H2O à H2CO3
  2. The carbonic acid releases H+ ions and hydrogencarbonate ions.
  3. The hydrogen carbonate ions diffuse out of the cell into the plasma.
  4. Charge of the RBC is maintained by the chloride shift, where negative chloride ions move into the cell to adjust the charge.
  5. The hydrogen ions have the potential to make the RBC very acidic so the haemoglobin acts as a buffer and binds with the H+ ions to make haemoglobinic acid.

The Bohr Effect

Carbon dioxide concentration increases

The following increase in cell cytoplasm acidity causes changes in the tertiary structure of the haemoglobin and reduced its affinity for oxygen

This causes the dissociation of the oxygen to the tissues in order to provide them with it for respiration

This ensures that tissues that are respiring more such as contracting muscles get more oxygen in comparison to those not respiring as quickly/actively

The Bohr shift refers to the change in the haemoglobin dissociation curve which moves down and right when more CO2 is present.

 

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Transport in Plants #2

Transport in Plants

Plants need a transport system to:

  • Move water and minerals from the roots up to the leaves
  • Move sugars from the leaves to the rest of the plant

The Vascular Tissues

Water and soluble mineral ions travel upwards in the xylem tissue

Assimilates such as sugars travel in both directions in the phloem tissues

In a dicotyledonous plant (one that has two seed leaves) the xylem and phloem are present in the vascular bundle along with other types of tissue such as collenchyma and sclerenchyma.

Meristem – a layer of dividing cells, also called the pericycle

Cambium – the layer of meristem that divide to produce new xylem and phloem cells

Xylem

  • Long cell with thick lignified walls
  • The lignin waterproofs the walls of the xylem and causes the cell to die
  • The end walls of the xylem cells then decay and the contents leaves the cells
  • It is them a tube with no end walls called a xylem vessel
  • Lignin strengthens the vessel walls and prevents collapse when water levels are low
  • The spiralling patterns of lignin allow flexibility to the stems and branches of the plant
  • In some place the lignin is not complete and gaps called bordered pits form which connect the xylem vessel to other xylem vessels and to the parenchyma

Phloem

  • Made of sieve tube elements and companion cells
  • Sieve tube elements transport sugars and assimilates.
  • When mature the sieve tubes lack a nucleus and contain very few organelles
  • Companion cells subsidise their metabolic needs
  • The sieve tube does contain a smooth endoplasmic reticulum which can be found at the plasma membrane and near the plasmodesmata
  • All sieve tube cells have groups of pores at their ends called sieve plates
  • The pores are reinforced by platelets of a polysaccharide called callose
  • Companion cells are a specialised form of parenchyma cell

 

 

Transpiration

Transpiration is the loss of water vapour from the upper parts of the plant, particularly the leaves as some water may evaporate through the upper leaf surface but this loss is limited by the waxy cuticle. Most water vapour evaporates from the underside where the stomata open during the day for gaseous exchange.

  1. Water enters the leave through the xylem and moves by osmosis into the cells of the spongy mesophyll.
  2. Water evaporates from the cell wall of the spongy mesophyll
  3. Water vapour moves by diffusion out of the leaf through the open stomata. This relies of a difference between the water vapour potential of the leaf and the outside environment.

The importance of transpiration system:

  • Transports useful mineral ions up the plant
  • Maintains cell turgidity
  • Supplies water for growth, cell elongation and photosynthesis
  • Supplies water that, as it evaporates, keeps the plant cool on hot days

Factors that affect the rate of transpiration:

Light intensity – in light the stomata open more for gaseous exchange as the light is needed for photosynthesis

Temperature – high temperatures increase the rate of evaporation, increase the rate of diffusion through the stomata because the water molecules have more energy, decrease the relative water vapour potential in the air, allowing for more rapid diffusion of the water molecules out into the air from the leaf

Relative humidity – higher humidity decreases the rate of transpiration as there is a smaller water vapour potential gradient between the leaf and the environment

Air movement – increases the rate of transpiration as this carries away water vapour from the plant, increasing the water vapour potential gradient

Water availability – if water is less available the plant will preserve the water by closing the stomata

Measuring Transpiration rate:

The main route of water loss from a plant is through the stomata of the leaf.

It is not easy to measure the rate at which water vapour is lost from the leaves.

However, it is relatively easy to measure the rate at which a plant stem takes up water, and can give a good estimate of the rate of transpiration

The amount of water lost by the plant may be investigated experimentally using a potometer – a device that measures water movement through the plant  – assumption: Water loss = Water uptake

The Transpiration Stream

This is the movement of water from the soil through the plant to the air surrounding the leaves. The main driving force is the water potential gradient between the soil and the air in the leaf air spaces.

The root hair cells absorb water and minerals from the soil.

The water then moves across the root cortex down a water potential gradient to the endodermis of the vascular bundle. Water may also travel through the apoplast pathway as far as the endodermis but must enter the symplast pathway due to a casparian strip blocking the way.

The movement of water across the root is driven by an active process that occurs in the endodermis. The endodermis is a layer of cells surrounding the medulla and xylem. This layer of cells is also known as the starch sheath as it contains granules of starch.

The plasma membranes contain transporter proteins which actively pump mineral ions from the cytoplasm of the cortex cells into the medulla and xylem.

This makes the medulla and xylem more negative in terms of WP so the water moves in by osmosis.

Root pressure

This is the pressure when the endodermis actively moves water and minerals into the medulla with no way to go back. This pressure forces the water and minerals up into the xylem. It can push water up a few metres but cannot account solely for water getting to the top of tall trees.

Transpiration pull

The loss of water by evaporation from the leaves must be replaces by water from the xylem. Water molecules are attracted to each other by cohesion and so pull each other up into the leaf.

Because this mechanism involves cohesion between the water molecules and tension occurring to pull the chain up the xylem and into the leaf, it is therefore called the cohesion-tension theory.

Capillary Action

The same forces that hold water molecules together in cohesion also cause the water to adhere to the sides of the xylem vessels. This helps the water molecules crawl up the walls of the vessel towards the leaves again.

Adaptions of plants to the availability of water

Hydrophyte – a plant adapted to living in water or where the ground is very wet

Xerophyte – a plant adapted to living in dry conditions

Terrestrial Plants

Most terrestrial plants reduce water loss with:

  • A waxy cuticle on the leaf will reduce the water loss due to evaporation through the epidermis
  • The stomata are closed at night, when there is no light for photosynthesis
  • The stomata are often found on the under surface of the leaves not on the top surface, which reduces the evaporation due to direct heating from the sun
  • Deciduous plants lose their leaves in winter when the ground may be frozen and water less available and also when temperatures are too low for photosynthesis

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Other xerophytic features

  • Closing the stomata when water availability is low
  • Low water potential inside their leaf cells made by maintaining a high salt concentration in the cells
  • A very long tap root that can reach the water deep underground (the water table)

Many plants that live in humid areas may contain specialised structures at the tips or margins of their leaves called hydathodes which release water droplets which may then evaporate from the leaf surface.

Translocation

Translocation occurs in the phloem and is the movement of assimilates throughout the plant.

Source – a part of the plant that loads materials into the transport system e.g. leaves photosynthesise and the sugars made are moved to other parts of the plant

Sink – a part of the plant where materials are removed from the transport system e.g. the roots receive sugars and store them as starch. Other times of the year this starch can be converted back into sugars and used to grow the plant so the roots may also be a source.

Active loading

  1. ATP is used to pump H+ ions out of the companion cells.
  2. The concentration of H+ ions outside the cell increases and a gradient is formed.
  3. The hydrogen ions diffuse back into the cell with sucrose through facilitated diffusion. This is known as cotransport.
  4. The cotransport required ATP to move the H + ions out of the cell and to let the sucrose enter the cell against its concentration gradient.
  5. As the concentration of sucrose in the companion cell increases, it can diffuse out of the plasmodesmata and into the sieve tube elements of the phloem.

 

Sucrose is moved along the phloem by mass flow. It is moved with other assimilates such as amino acids in sap. The sap can be made to travel up or down the plant depending on the difference in hydrostatic pressure between the two ends of the tube. The sucrose and water enters the phloem at the source causing a concentration gradient that will push it towards the sink. The assimilates and water leaving at the sink lowers the hydrostatic pressure. Although the sap may be flowing in different directions in different sieve tube elements, the fact that it is always flowing the same way in one group of cells makes it mass flow.

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Exchange Surfaces

Exchange Surfaces

For exchange to be effective:

  • The surface area of an organism must be large compared with its volume.
  • Thin exchange surface to give a short diffusion pathway
  • Partially permeable to allow selected materials to cross
  • Movement of the environmental medium (e.g. air)
  • Movement of the internal medium (e.g. blood)

Gas exchange in single-celled organisms

Single-celled organisms have a large surface area to volume ratio so oxygen can be absorbed by diffusion across their body surface, which is covered by only a cell-surface membrane. Carbon dioxide from respiration diffuses out across their body surface in the same way.

Gas exchange in insects

Insects must balance the opposing needs of exchanging respiratory gases with reducing water loss (which occurs easily for terrestrial organisms).

To reduce water loss, terrestrial organisms have waterproof coverings and a small surface area to volume ratio to minimise the area over which water is lost. These features mean that insects cannot use their body surface to diffuse respiratory gases in the same way a single-celled organism does. Insread, they have developed an internal network of tubes called trachae, which are supported by strengthened rings to prevent them from collapsing.  These then divide into smaller tubes called tracheoles which extend throughout all the body tissues of the insect. In this way, oxygen can be brought directly to respiring tissues.

Respiratory gases move in and out of the tracheal system via a diffusion gradient (oxygen is used up in respiration so its concentration towards the end of the tracheoles falls) and ventilation (movement of muscles in insects can create mass movements of air in and out of the trachae).

Gases enter and leave trachae through tiny pores called spiracles, on the body’s surface. The spiracles may be opened and closed by a valve. When they are open, water can evaporate from the insect.

The tracheal system relies mostly on diffusion to exchange gases between the environment and cells. For diffusion to be effective the pathway must be short, as a result, this limits the size that insects can attain.

Gas exchange in fish

Fish have developed a specialised internal gas exchange surface (gills) because their waterproof covering and small surface area to volume ratio means their body surface is not adequate to supply and remove their respiratory gases via diffusion.

Gills are located behind the heads of fish and are made up of gill filaments which are stacked up in a pile. At right angles to te filaments are gill lamellae which increase the surface area of the gills.

Water is taken in through the mouth and forced over the gills and out through and opening on each side of the body. The flow of water over the gill lamellae and the flow of blood within them are in opposite directions (countercurrent flow). This is so that there is always a higher concentration of oxygen in the water than in the blood, so it diffuses into the blood along the whole length of the lamellae.

 

 

Gas exchange in a leaf

Circulatory system of a mammal

Features of transport systems:

  • A suitable medium in which to carry materials (e.g. blood)
  • A form of mass transport in which the transport medium is moved around in bulk
  • A cosed system of tubular vessels that contains the transport medium and forms a branching network to distribute it to all parts of the organism

  • A mechanism for moving the transport medium within vessels (e.g. muscular contraction)
  • A mechanism to maintain the mass flow movement in one direction (e.g. valves)
  • A means of controlling the flow of the transport medium to suit the needs of different parts of the organism

Artery structure related to function:

  • Thick muscle layer – can constrict and dilate to control the volume of blood passing through them
  • Thick elastic layer – stretching and recoil helps maintain high blood pressure and smooth the pressure surges created by the beating of the heart
  • Overall thickness – helps prevent vessel bursting under pressure
  • No valves – blood is under constant high pressure so does not tend to flow backwards

 

Tissue fluid and its formation

Tissue fluid is a watery liquid that contains dissolved oxygen and nutrients. It supplies these necessary solutes to the tissues and receives waste materials such as carbon dioxide in return. It is therefore the means by which materials are exchanged between blood and cells, and as such, it bathes all the cells of the body. It provides a mostly constant environment for the cells it surrounds.

Blood is pumped along arteries, into narrower arterioles and then narrower capillaries, creating hydrostatic pressure at the arterial end of the capillaries. This pressure forces tissue fluid out of the blood plasma however this pressure is opposed by two other forces:

  • Hydrostatic pressure of the tissue fluid outside the capillaries
  • Lower water potential of the blood, due to plasma proteins, pulling water back into the blood within the capillaries

The combined effect of these forces is to create an overall pressure that pushes tissue fluid out of the capillaries. This pressure is only enough to force small molecules out of the capillaries, leaving all cells and proteins in the blood. This type of filtration under pressure is known as ultrafiltration.

Return of tissue fluid to the circulatory system:
Once it has exchanged metabolic materials with the cells it bathes, tissue fluid must be returned to the circulatory system. Most tissue fluid returns to the blood plasma directly via the capillaries since the hydrostatic pressure within the capillaries has been reduced due to the loss of tissue fluid, as a result, by the time the blood has reached the venous end, its hydrostatic pressure is less than that of the tissue fluid outside it. In addition, the osmotic forces resulting from he proteins in the blood plasma pull water back into the capillaries.

The remainder of the tissue fluid is carried back via the lymphatic system.

Movement through the roots

Roots are composed of different tissues each with their own function.

 

  • Epidermis – a single layer of cells often with long extentions called root hairs which increase the surface area. A single plant may have 1010 root hairs.
  • Cortex – a thick layer of packing cells often containing stored starch.
  • Endodermis – a single layer of cells that surround the vascular tissue containing a waterproof layer called the casparian strip which allows the plant to control the movement of ions into the xylem.
  • Pericycle – a layer of undifferentiated meristematic (growing) cells.
  • Vascular tissue – this contains xylem and ploem cells which are continuous with the stem vascular bundles.

Water moves into the root hair cells by osmosis since there is a lower water potential in the cell than in the soil. The root hair cells are efficient surfaces for exchange because they provide a large surface area as they are long extentions and they occur in thousands on each root. They also have a thin cell wall and cell membrane so give a short osmotic pathway.

Water moves through the root via two pathways: the symplastic pathway and the apoplastic pathway.

The symplast pathway

This consists of the living cytoplasms of the cells in the root. Water is absorbed into the root hairs by osmosis since the cells have a lower water potential than the water in the soil. Water then diffuses from the epidermis through the root to the xylem, down a water potential gradient. The cytoplasms of all the cells in the root are connected by plasodesmata through holes in the cell walls, so there are no further membranes to cross until the water reaches the xylem, and so no further osmosis.

The apoplast pathway

This consists of cell walls between cells. The cells walls are quite thick and very open so water can simply diffuse through cell walls down the water potential gradient. There are no cell membranes to cross so it moved by diffusion, not osmosis. However, the apoplast pathway stops at the endodermis because of the waterproof casparian strip, which seals the cell walls. At this point water has to cross the cell membrane by osmosis and enter the symplast. This allows the plant to have some control over the uptake of water into the xylem. Around 90% of water transport through the root uses the apoplast pathway, as the available volume is greater.

The uptake of water by osmosis actually produces a force that pushes water up the xylem. This force is called root pressure which can be measured by placing a manometer over a cut stem. This force helps push water up short stems i.e. a few centimetres however longer distances like up trees would require a much greater pressure.

Movement through the stem (mass flow):

The xylem vessels form continuous pipes from the roots to the leaves. Water can move up through these pipes to a height of over 100m. Since the xylem vessels are dead, open tubes, no osmosis can occur within them, thus water moves by mass flow. The driving force for the movement is transpiration in the leaves. This causes low pressure in the leaves, so water is drawn up the stem, replacing the lost water. The column of water in the xylem vessels is therefore under tension. Fortunately, water has a high tensile strength due to the tendancy of water molecules to stick together by hydrogen bonding (cohesive), so the water column does not break under the tension. This mechanism of pulling water up a stem is sometimes called the cohesion-tension mechanism.

Root pressure also pushes water up from beneath. It arises because mineral ions are actively taken up into the xylem in the root. If transpiration is slow then these ions are not transported up the stem, they build up in the root xylem. This lowers the water potential in the root tissue and water is drawn into the root by osmosis, pushing the column of water upwards.

Movement through the leaves:

The xylem vessels ramify (branch) in the leaves to form a system of fine vessels called leaf veins. Water diffuses from the xylem vessels in the veins through the adjacent cells down its water potential gradient. As in the roots, it uses the symplast pathway through the living cytoplasm and the apoplast pathway through the non-living cell walls. Water evaporates from the spongy cells into the sub-stomatal air space and diffuses out through the stomata.

Each stomata is surrounded by guard cells which inlike the rest of the epidermal cells, contain chloroplasts which allow them to photosynthesise and produce ATP, which they use to drive active transport ion pumps, which mean they can qickly alter their water potential.

To open the stomata the guard cells pump ions into the cell which lowers the water potential so water enters by osmosis. The cells becoem turgid and bend apart so the stoma between them opens.

To close the stomata the guard cells pump ions out of the cell, which raises their water potential so water leaves by osmosis. The cells become flaccid and striaghten to the stoma between them closes.

Evaporation of water is an endothermic process since energy must be put in to turn water from a liquid to a gas. This energy is provided by the sun, in a process separate to that by which is provides light energy for photosynthesis.


Factors affecting transpiration

Temperature – high temperature increases the rate of evaporation of water from the surface of the spongy mesophyll cells because it increases the kinetic energy of the water molecules. This raises the Ψ in the sub-stomatal air space and means that the molecules are moving faster so tanspiration increases.

Humidity – high humidity means a higher Ψ in the air surrounding the stomata, so a lower Ψ gradient between the sub-stomatal air space and the eair outside,so less evaporation.

Air movement – wind blows away saturated air from around the stomata, replacing it with drier air with a lower Ψ , so increasing the Ψ gradient and increasing transpiration.

Light intensity – light stimulates plants to open their stomata to allow gas exchange for photosynthesis, which also increases the rate of transpiration as a side effect.

If plants are losing too much water and their cells are wilting, their stomata close to reduce transpiration. Therefore, long periods of light, heat or dry air could result in the stomata closing and decreasing the rate of transpiration.

Potometers are used in transpiration investigations. They do not actually measure the rate of traspiration but the rate of water uptake by the cut stem.

Adaptations to habitats

Xerophytes – adapted to dry habitat
Halophytes – adapted to salty habitat
Hydrophytes – adapted to freshwater habitat
Mesophytes – adapted to habitat with adequate water

Adaptations of xerophytes:

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Transport in Animals

Transport in Animals

(a) explain the need for transport systems in multicellular animals in terms of size, level of activity and surface area: volume ratio

(b) explain the meaning of the terms:

(c) explain the meaning of the terms:

d) describe, with the aid of diagrams and photographs, the external and internal structure of the mammalian heart

(e) explain, with the aid of diagrams the differences in the thickness of the walls of the different chambers of the heart in terms of their functions

(f) describe the cardiac cycle, with reference to the action of the valves in the heart

Atrial Systole/ Ventricular Diastole:

  1. Blood enters the atria and the pressure inside the atria increases
  2. The pressure is higher in the atria than the ventricles so the atrioventricular valves open
  3. The atria contract pushing blood into the ventricles

Atrial Diastole/ Ventricular Systole:

4.Blood enters the ventricles and the pressure inside the ventricles increases

  1. The pressure becomes higher in the ventricles than the atria forcing the atrioventricular valves to close to prevent the backflow of blood
  2. The pressure in the ventricles is also higher than in the major arteries (aorta and pulmonary artery), forcing the semi-lunar valves open
  3. The ventricles contract and blood is forced out into the arteries

Atrial Systole/ Ventricular Diastole:

  1. The atria and ventricles are relaxed
  2. The pressure is higher in the major arteries (aorta and pulmonary artery) so the semi-lunar valves close to prevent the backflow of blood
  3. The pressure is slightly higher in the atria than in the ventricles so the atrioventricular valves open
  4. Blood flows passively into ventricles from the atria

Calculating Heart Rate:

Heart Rate (beats per minute) = 60/Time taken for one cardiac cycle

(g) describe how heart action is coordinated with reference to the sinoatrial node (SAN), the atrioventricular node (AVN) and the Purkyne tissue

  1. The cardiac muscle is myogenic – the heart will contract and relax by itself
  2. The SAN initiates a wave of excitation in the right atrium which spreads over the walls of both atria causing atrial systole
  3. The wave of excitation spreads to the AVN, found at the top of the inter-ventricular septum, as there’s a band of fibres between the atria and ventricles which stops the wave of excitation passing to the ventricle walls.
  4. The AVN delays the wave of excitation to allow time for the atria to finish contracting and for blood to flow down the ventricles
  5. The AVN directs the wave of excitation to the Purkyne tissue, which runs down the inter-ventricular septum. The wave of excitation spreads upwards from the apex (base) of the ventricles causing ventricular systole, pushing blood upwards towards the arteries

(h) interpret and explain electrocardiogram (ECG) traces, with reference to normal and abnormal heart activity

Electrocardiograms are used to monitor the electrical activity of the heart by attaching a number of sensors to the skin. Some of the electrical activity generated passes through the tissues next to the  heart and then to the skin so that the sensors can pick up the electrical excitation made by the heart and convert this into a trace.

(i)describe, with the aid of diagrams and photographs, the structures and functions of arteries, veins and capillaries

The pressure in the blood decreases as it moves away from the heart because the blood vessel branches into more smaller vessels meaning they have a larger cross sectional area. Also there’s more friction in the capillaries therefore there is a lower pressure.

It’s important that the pressure is lower by the time the blood reaches the capillaries because they are only one cell thick so a high pressure would cause them to burst and become damaged. Also a low pressure means a slow flow rate allowing time for exchange. Furthermore capillaries do not have a lot of elastic tissue.

The pressure in the veins is very low so blood is returned to the heart by the muscle around it contracting and pumping the blood with the help from valves to prevent back-flow of blood.

 

(j) explain the differences between blood, tissue fluid and lymph

(k) describe how tissue fluid is formed from plasma

  • At the arterial end of a capillary, the hydrostatic pressure is high, therefore plasma moves out of the capillaries because the pressure is higher in the capillaries than outside, moving down the pressure gradient.
  • In the capillaries because the plasma proteins are too large to pass through, they stay in the plasma, making the water potential lower than that in the tissue fluid so tissue fluid moves back into the capillaries at the venous end.

(l) describe the role of haemoglobin in carrying oxygen and carbon dioxide

(m) describe and explain the significance of the dissociation curves of adult oxyhaemoglobin at different carbon dioxide levels (the Bohr effect)

Oxygen Dissociation Curve:

  • The first oxygen does not attach easily to the haem groups, due to it being in the centre of the haemoglobin molecule
  • The concentration of oxygen rises making it easier to oxygen to associate with the heam groups.
  • Once one oxygen molecule binds with a haem group, it causes a conformational change in shape of the haemoglobin molecule – this makes it easier for oxygen molecule to reach the haem group and associate with it
  • The next two oxygen molecules attach easily to the haem groups
  • The fourth oxygen molecule finds it difficult to associate with the last haem group. This means that 100% oxygen saturation is difficult even at high pO2 forming a sigmoid curve.

(n) explain the significance of the different affinities of fetal haemoglobin and adult haemoglobin for oxygen

  • Fetal haemoglobin has a higher affinity for oxygen than adult haemoglobin. This is because the fetal haemoglobin must be able to ‘pick up’ oxygen from the haemoglobin from its mother.
  • Placenta has low partial pressure of oxygen
  • At low partial pressure of oxygen, in the placenta, adult haemoglobin will dissociate
  • Fetal haemoglobin takes up oxygen in lower partial pressure of oxygen

 

 

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Transport in Plants

Transport in Plants

(a) explain the need for transport systems in multicellular plants in terms of size and surface area: volume ratio

Multicellular plants have a small surface area: volume ratio so diffusion would be too slow to provide necessary substances like water, minerals and sugars and to remove waste substances. Also multicellular plants are large so have a greater demand for substances. Therefore plants need transport systems to move substances to and from individual cells quickly.

(b)describe, with the aid of diagrams and photographs, the distribution of xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants

 

(c) describe, with the aid of diagrams and photographs, the structure and function of  xylem vessels, sieve tube elements and companion cells

(d) define the term transpiration

Transpiration is the evaporation of water from a plant’s surface, especially the leaves.

Transpiration involves 3 processes:

  1. Water leaves the xylem and passes to the mesopyll cells by osmosis.
  2. Water evaporates from the surface of the mesophyll cells, to form water vapour, into the air spaces.
  3. The water vapour potential in the leaf is higher than outside, so water molecules will diffuse out of the leaf.

(e) explain why transpiration is a consequence of gaseous exchange

It is a side effect of gas exchange as a plant needs to open its stomata to let in carbon dioxide so that it can produce glucose by photosynthesis, but it also lets water out as there’s a higher concentration of water inside the leaf than in the air outside water moves out of the leaf down the water potential gradient when the stomata is open.

(f) describe the factors that affect transpiration rate

(g) describe, with the aid of diagrams, how a potometer is used to estimate transpiration rates

A potometer is used to estimate the rate of water loss. It is not an exact measure, as it actually measures the rate of water uptake by a cut shoot. It is important to make sure there are no air bubbles inside the apparatus. Water lost by the leaf is replaced from the water in the capillary tube. The movement of the meniscus at the end of water column can be measured.

You need to remember:

  • Cut a shoot underwater to prevent air from entering the xylem
  • Cut a shoot at a slant to increase the surface area available for water uptake
  • Dry the leaves
  • Use non-wilting shoots
  • Allow time for equilibrium and for it to acclimatise
  • Note where meniscus is at start and end of time period

(h) explain, in terms of water potential, the movement of water between plant cells, and between plant cells and their environment

 

(i) describe, with the aid of diagrams, the pathway by which water is transported from the root cortex to the air surrounding the leaves, with reference to the Casparian strip, apoplast pathway, symplast pathway, xylem and stomata

 

  1. Water enters from the soil to the root hair and epidermis through osmosis – from a higher water potential (soil) to the most negative water potential (xylem)
  2. Water enters the cortex by the apoplast pathway between cell walls, the symplast pathway through plasmodesmata and the vacuolar pathway
  3. Water enters the endodermis which has a Casparian strip which blocks the apoplast pathway so water must be transported by the symplast pathway allowing selective mineral uptake
  4. Water enters the xylem and minerals are moved using active transport which reduces the water potential in the xylem creating a water potential gradient. Water can’t pass through to the cortex again as the endodermis is blocked

(j) explain the mechanism by which water is transported from the root cortex to the air surrounding the leaves, with reference to adhesion, cohesion and the transpiration stream

  1. Minerals are actively transported into the xylem vessels. This lowers the water potential in the xylem and water follows by osmosis.
  2. Root pressure pushes some of the water upwards.
  3. Water evaporates from the surface of the leaf by transpiration and water is lost.
  4. The water must be replaced as water moves out of the xylem into the leaf, creating a low hydrostatic pressure, and a pressure gradient, and thus tension.
  5. Water molecules are attracted to each other by forces of cohesion creating a continuous column of water so that water can be moved by mass flow, pulled upwards by tension from above.
  6. Water molecules are also attracted to the walls of the xylem by forces of adhesion and causing capillary action.

(k) describe with the aid of diagrams and photographs, how the leaves of some xerophytes are adapted to reduce water loss by transpiration

A xerophyte is a plant that is adapted to reduce water loss so that it can survive in very dry conditions e.g. marram grass, cacti etc.

 

(l) explain translocation as an energy-requiring process transporting assimilates especially sucrose, between sources (e.g. leaves) and sinks (e.g. roots, meristems)

Translocation is the movement of assimilates (e.g. sugars – sucrose) through the phloem tissue and is an energy-requiring process. It moves from source to sink.

  • Source: where the assimilates are made/came from (e.g. leaves)
  • Sink: where the assimilates are used/stored (e.g. roots, meristems)

(m) describe, with the aid of diagrams, the mechanism of transport in phloem involving active loading at the source and removal at the sink, and evidence for and against this mechanism

How are assimilates loaded into the phloem?

  1. ATP is used by the companion cells to actively transport hydrogen ions out of their cytoplasm and into the surrounding tissue
  2. This sets up a diffusion gradient as there are more hydrogen ions outside the cell than inside, and the hydrogen ions diffuse back into the companion cells.
  3. Diffusion happens through cotransporter proteins – they allow hydrogen ions to bring sucrose molecules into the companion cells.
  4. As the concentration of sucrose molecules builds inside the companion cells, they diffuse into the sieve tube element through the numerous plasmodesmata.

 

Movement of Sucrose Along the Phloem:

Evidence For and Against Mass Flow:

 

 

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