F211 Biology

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  • Created on: 04-04-12 17:07


Definition: the transport of assimilates throughout the plant, in the phloem tissue

A source: releases sucrose into the phloem

A sink: removes sucrose from the phloem

Sucrose is loaded into the phloem by an active process. ATP is used by the companion cekks to actively transport hydrogen ions out of their cytoplasm and into the surrounding tissue (diffusion gradient) then the hydrogen ions diffuse back into the companion cell.

This diffusion occurs through cotransporter proteins (enable the hydrogen ions to bring sucrose molcules into the companion cells)

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Translocation (2)

As the concentrations of sucrose build up inside the companion cells, they diffuse into the sieve tube elements through the plasmodesmata.

Movement of sucrose along the phloem

Sucrose entering sieve tube element reduces the water potential.

Therefore water molecules move inside the sieve tube by osmosis 

This increasing the hydrostatic pressure in sieve tube.

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Translocation (3)

At the sink:

Sucrose is used in the cells surrounding the phloem.

Sucrose may be converted to to starch for storage or used for respiration

This reduces the sucrose concentration in the cell 

Sucrose molecules move by diffusion or active transport from the sieve tube element into the surrounding cells.

This increases the water potential in the sieve tube element so water molecules move into the surrounding cells by osmosis

This reduced the hydrostatic pressure in the phloem at the sink.

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Translocation (4)

Along the phloem:

Moving down the hydrostatic pressure gradient and leaving phloem at the sink produces a flow of water along the phloem.

This flow carries sucrose and other assimilates along the phloem - mass flow.

Up or down the plant - dependant on where sugars are needed. 

Most obvious source = leaf

sugars made during photosynthesis are converted to sucrose and loaded into the phloem.

Other sources = roots

stored carbohydrates are released into the phloem

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Evidence for translocation

1. the phloem is used because: 

if the plant is supplied with radioactioactively labelled carbon dioxide - appears in the phloem

ringing a tree to remove the phloem results in sugars collecting above the ring

An aphid feeding on the plant stem can be used to show th aphid gets food from the phloem

2. it needs metabolic energy (ATP):

companion cells have many mitochondria

translocation can be stopped by using a poison that stops formation of ATP

the rate of flow is too hight not to have energy to drive it 

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Evidence for translocation (2)

3. it uses translocation:

the pH of the companion cells is higher than that of the surrounding cells

the concentration of sucrose is higher in the source than the sink

4. evidence against translocation?

not all the solutes in the phloem sap move at the same rate

sucrose is moved to different parts of the plant at the same rate rather than moving to areas with low concentration first

the role of the sieve plates is unclear

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Reducing water loss - xerophytes

definition: a plant that is adapted to reduce water loss so that it can survive in very dry conditions

If water loss is too great the plant may suffer loss of turgidity undergo plasmolysis - die

arid conditions = dry

Some plants have a low water potential in the leaf cells by having a large concentration of salt. The low water potential reduces the evaporation of water from cell surfaces.

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Reducing water loss - Marram Grass (Ammophila)

Small leaves (needle shape) = reduction of total surface area therefore less water is lost by transpiration.

Densely packed spongey mesophyll = reduction of cell surface area - less water will evaporate into the leaf space.

Thicker waxy cuticles reduce evaporation (holly leaves).

Hairs on the surface of the leaf trap a layer of air close to the surface of the leaf. This air can become saturated with moisture and reduce the diffusion of water vapour out of the stomata. This is because the water vapour potential has been reduced.

Pits containing stomata at the base also trap air so it becomes saturated with water vapour. Reducing the diffusion of water vapour out of the stomata. Water vapour potential has been reduced.

Closing stomata when water availability is low - reduce water uptake and need for water.

Rolling leaves so upper epidermis is not exposed - eliminate water potential gradient 

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Definition: loss of water by evaporation from the aerial parts of the plant.

Water must evaporate to form water vapour before it is lost. The water vapour will diffuse down the water vapour potential gradient. 

1. osmosis from xylem to mesophyll cells

2. evaporation from the surface of the mesophyll cells into intercellular spaces

3. diffusion of water vapour from intercellular spaces through the stomata

Transpiration Stream: as water moves up the xylem it must be replaced from below. Water moves up the xylem from the roots to replace water lost.

Useful because: water keeps cells turgid, water is required for photosynthesis in the leaves, water is required for plants to grow and elongate, the flow of water is needed to carry minerals up, evaporation is needed to keep the plant cool. 

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How to measure transpiration

potometer: estimates the rate of water loss.

Important there are no air bubbles

Water lost by the leaf is replaced from the water in the capillary tube. 

The movement of the meniscus at the end of the water column can be measured. 

(find a detailed diagram)

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Factors altering the rate of transpiration

Number of leaves: larger surface area over which water vapour can be lost

Number/Size/Position of stomata: many large stomata = water vapour lost more quickly. Stomata on lower surface water vapour lost slower

Presence of cuticle: waxy cuticle reduces evaporation from leaf surface.

Light: in light, stomata open to allow gaseous exchange for photosynthesis

Temperature: High = increase in rate of evaporation from cell surface (water vapour potential rises), increase rate of diffusion through stomata (increased kinetic energy), decrease in relative water vapour potential in the air allowing more rapid diffusion of molecules out of leaf.

Relative humidity: Higher = decrease the rate of water loss because there will be a smaller water vapour potential gradient between air inside and air outside. 

Air moving outside the leaf will carry water vapour that just diffused out of leave, this maintains a high water vapour potential gradient.

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Water uptake and movement up stem

cohesion: the attraction of water molecules for one another

adhesion: the attraction of water molecules to the walls of the xylem

the symplast pathway: moves water through the cell cytoplasm 

the apoplast pathway: moves water in the cell walls and between the cells.

epidermis contains root hair cells = increase surface area of the root. Absorb minerals from the soil by active transport using ATP. Minerals reduce the water potential of the cytoplasm. This makes water potential of cell lower than the soil . 

Water taken across the plasma membrane by osmosis down the water potential gradient.

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Movement across the root

The movement of water across the root is driven by an active process that occurs in the endodermis (layer of cells surrounding the xylem/ also known as starch sheath as it contains grains of starch - indication it uses energy).

The endodermis has specialised cells that have a waterproof strip - Casparian strip - blocks the apoplast pathway forcing water into the symplast pathway.

The endodermis cells move minerals by active transport from the cortex into the xylem . This decreases water potential in the xylem. Therefore water moves from cortex to xylem by osmosis.

This reduces water potential in cells outside endodermis, combined with water entering root hair cells this creates water potential gradient across whole cortex. Therefore water is moved along symplast pathway from the root hair cells across the cortex and into the xylem.

Water can also move across apoplast pathway across the cortex. This water moves through cells to join up with the symplast pathway.

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Role of the Casparian *****

1. blocks the apoplast pathway between cortex and the xylem.

2. ensures water and dissolved nitrate ions have to pass into cytoplasm

3. there are transported proteins in cell membranes so nitrates can be actively transported into xylem from cortex

4. This lowers water potential in the xylem so water from cortex follows into the xylem by osmosis

5. Once the water has entered the xylem it cannot pass back into the cortex via the apoplast pathway because it is blocked by casparian strip 

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How does water move up the stem

Root pressure: action of endodermis moving minerals into xylem by active transport drives water into xylem by osmosis. This forces water up the xylem. Root pressure can push water a few metres up but no further.

Transpiration pull: the loss of water by evaporation must be replaced by water coming up the xylem. water molecules attached to each other my cohesion.This creates a transpiration stream. The pull from above creates tension in the column of water so xylem vessels are strengthened by lignin (prevents xylem from collapsing under tension). Known as the cohesion-tension theory. Relies on the plant maintaining an unbroken. If one xylem vessel is broken then water column can be maintained through another vessel via pits.

Capillary Action: Adhesion - because xylem vessels are very narrow, these forces of attraction can pull water up the sides of the vessel, 

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How water leaves the leaf

most leave through stomata (tiny pores) 

tiny amount leave through waxy cuticle

water evaporates from the cell lining the caivity immediately below the guard cells

- this lowers the water potential in the cells causing water to enter them by osmosis. Eventually water leaves xylem and enters inner most cells.

A plasmodesma = fine strand of cytoplasm that links contents of adjacent cells

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Plant Cells and Water

Water potential is a measure of the tendency of water molecules to diffuse from one place to another. 

Water always moves from a region of high water potential down a water potential gradient to a region of low water potential

The water potential of pure water is 0 in a cell there is cytoplasm and sugars/salts which reduces water potential - fewer free water molecules 

Water potential of a plant cell is always negative. 

Pressure potential - the water inside the cell starts to exert pressure upon a cell wall

plasmolysis - water loss - cell surface membrane lose contact with the cell wall

The Vacuolar Pathway

similar to symplast pathway but the water is not confined to cytoplasm - can move through vacuoles 

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plant transport tissue that carries water from the roots to the rest of the plant. consists of hollow columns of dead cells lined end to end and reinforces with lignin. Provides support for the plant. 

Adaptions: tubes are narrow so the water column does not break easily and capillary action can be effective

pits in the lignified walls allow water to move sideways from one vessel to another

lignin deposited in the walls in a spiral, annular or reticulate patterns allow xylem to stretch as plants grow which enables plants to bend or branch.

The flow of water is not impeded because: there are no end walls, no cell contents, no nucleus or cytoplasm, lignin thickening prevents walls from collapsing

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Sieve Tubes:  contain very little cytoplasm and no nucleus. Lined end to end to from a tube in which plants transport sugars (sucrose) - dissolved in water to form sap. Contains cross walls at intervals that are perforated by many pores to allow sap to flow through.

Companion Cells: Between sieve tubes are small cells, each with a large nucleus and dense cytoplasm . Contain numerous cytoplasm to produce ATP. Carry out metabolic processes that are needed for the sieve tubes (using ATP as a source of energy to load sucrose into the sieve tubes) 

companion cells and sieve tubes connected by plasmodesmata 

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Transport in plants

Needs a transport system because it is multicellular large surface area and further distance to get the nutrients to

Vascular bundles - contain phloem and xylem and other tissues that give strength and support. 

Found at the centre of a young root 

Around vascular bundle is endodermis. Inside endodermis is a layer of meristem cells called pericycle.

Complete ring of vascular tissue around bark of tree gives strength and flexibility. 

In betwen xylem and phloem there is a layer of cambium - layer of meristem cells that will divide to produce new xylem and phloem. 

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Tissues in the Lungs

Cartilage: structural role - supports trachea and bronchi holding them open preventing collapse when the air pressure inside is low during inhalation. The incomplete ring provides flexibility.

Smooth Muscle: contracts=it will constrict the airway this makes the lumen narrower. Most obvious effect is on bronchioles. Controlling the flow of air is important when toxic particles are in the air - allergic reactio

Elastic Fibres: when smooth muscles contract it reduces diameter of lumen - deforms the elastic fibres. When smooth muscle relaxes the elastic fibres recoil to their original shape. This helps dilate the airway.

Goblet cells and glandular tissue: secrets mucus to trap particles in the air. Reduce the risk of infection

Ciliated epithelium: cilia move in a synchronised pattern to waft the mucus up the airway to the back of the throat. Mucus is swallowed and acidity in stomach kill bacteria 

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Measuring Lung Capacity

Inspiration (inhalation) and expiration (exhalation)

Tidal volume: the volume of air moved in and out of the lungs with each breath when at rest (approx. 0.5dm3) 

Vital capacity: largest volume of air that can be moved in and out of the lungs in any one breath (approx. 5dm3) 

Residual volume: the volume of air always present in the lungs (1.5dm3) 

Dead space: air in the bronchioles, bronchi and trachea. There is no air exchange between this air and blood.,

Inspiratory reserve volume: how much air you could inspire over the tidal volume - eg when exercising 

Expiratory reserve volume: how much air can be expired over and above the amount that is breathed in a tidal volume breath.

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Contains a large chamber filled with oxygen that floats on a tank of water.

A person breathes from a disposable mouthpiece attached to a tube connected to the chamber of oxygen.

Breathing in takes oxygen from the chamber, which then sinks down. 

Breathing out pushes air into the chamber which then floats up.

Movement recorded using a datalogger so a spirometer trace can be produced

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Measuring Oxygen uptake

Over a period of time of inhaling and exhaling the level of carbon dioxide will increase dangerously in a spirometer. 

To avoid this soda lime is used to absorb carbon dioxide. This means this total volume  of gas in the spirometer is reduced.

As the carbon dioxide this total reduction is equal to the amount of oxygen used.

volume (for 1 minute x60)


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Circulatory systems

single circulatory system = Fish 

heart -> gills -> body -> heart

Known as pulmonary circulation 

double circulatory system = mammals

heart -> body -> heart -> lungs -> heart

systemic circulation: carries oxygen and nutrients around body and tissues

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Advantages of double circulation

single circulatory:

  • blood pressure is reduced as blood passes through capillaries and gills
  • not flow very quickly to the rest of the body  
  • limits the rate at which oxygen and nutrients are delivered to respiring tissues

Double circulatory:

  • heart can increase pressure of the blood after it has passed through the lungs so blood can flow more quickly to the body tissues quicker
  • the systemic circulation can carry blood at a higher pressure than pulmonary circulation
  • the blood pressure must not be too high in the pulmonary circulation, otherwise it may damage the delicate capillaries. 
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Structure of the mammalian heart

muscular pump  that creates pressure due to the contraction of the left ventricle walls.

pumps deoxygenated blood to the lungs to be oxygenated.

2 pumping chambers = ventricles 

above ventricles = 2 thin walled chambers - atria 

coronary arteries lie over surface

right atria = deoxygenated blood from the vena cava 

left atria = oxygenated blood from the pulmonary vein 

blood flows through the atrioventricular valves  into the ventricles, when ventricles contract the valves fill up woth blood and remain closed this ensues blood flows upward into the major arteries and not back into the atria.  

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Structure of the mammalian heart (2)

Inside ventricles are string like tendinous cords. They attach the values to the walls of the ventricles and prevent flimsy valves from turning inside out, which would allow blood to flow back into the atria. 

Wall of the muscle called septum separates the ventricles from each other. This ensures that oxygenated blood in the left hand side and deoxygenated blood on the right. 

Deoxygenated blood pumped out of right ventricle leaves through the pulmonary artery lead to the lungs.

Oxygenated blook leaves through the aorta from the left ventricle which carries blood to many arteries for supply. 

At the base of major arteries there are semilunar valves which prevent blood returning to the heart when ventricles relax.

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Blood Pressure

Atria: muscle very thin this is because the chambers do not need a lot of pressure, function to push blood into ventricles.

Right Ventricle: thicker than atria, enables right ventricles to push blood out of heart. Thinner than left ventricle the right ventricle pumps deoxygenated blood to lungs, the blood does not need to travel far also lungs contain a lot of capillaries that are in close contact to alveoli, the alveoli have very fragile and thin walls so the pressure must be kept down so they don't burst capillaries to lungs.

Left Ventricle: 2/3 X thicker than right ventricle. Needs sufficient pressure to overcome resistance of the systemic circulation.

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The Cardiac Cycle

1. Filling Phase DIASTOLE

Both atria and ventricles relax. 

The blood flows into atria 

Then through open atrioventricular valves into ventricles

2. Atriole Contraction ATRIAL SYSTOLE

Both atria contract together - increase in pressure helps push blood into ventricles 

This stretches the walls of ventricles to make sure they are full

Once ventricles are full they begin to contract

Blood fills the atrioventricular valve flap shuts preventing blood going back into the aorta

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The Cardiac Cycle (2)

3. Ventricular Contraction VENTRICULAR DIASTOLE

Contractions start at the base of the ventricles so this pushes blood upwards to arteries.

Semilunar valves open and blood is pushed out of heart

Ventricles relax.

Cycle starts again

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Control of Cardiac Cycle

Need = myogenic could cause inefficient pumping if the chambers are not synchronised = mechanism to coordinate contractions of 4 chambers

Sinoatrial node= the heart's pacemaker. Small patch of tissue next to vena cava that sends out waves of electrical excitation at regular intervals to initiate contractions - starts heart beat. occurs 55-80 times a minute (SAN)

Atrioventricular node (contraction of the atria)= base of atria disc of tissue cannot conduct wave of excitation so another node is placed - AVN. Only route through disc of non conducting tissue. Wave of excitation is delayed which allows time for the atria to finish contracting and the blood to run into the ventricles before they begin to contract.

Purkyne tissue (contraction of ventricles) = specially adapted muscle fibres that conduct the wave of excitation from AVN down the septum to the ventricles. As excitation spreads upwards it causes ventricles to contract. Pushing blood up to arteries.

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monitor electrical activity of the heart

number of sensors on skin - sensors pick up electrical activity of heart and convert this to a trace.

shape of an ECG trace can indicate when part of the heart muscle is not healthy 

irregular = arrhythmia - if in fibrillation it indicates myocardial infarction 


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Blood Vessels

Open circulatory system = the blood is not always in a vessel

Closed circulatory sysytem = blood always remains in vessels

OC work for insects because they are small 

CC needed for larger animals - creates pressure needed to get round whole body quicker and remove carbon dioxide quicker

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Specific Blood Vessels


  • lumen relatively small to maintain high pressure 
  • Wall relatively thick and contains collagen, a fibrous protein, to give it strength to withstand high pressure
  • Wall has elastic tissue - allows wall to stretch and recoil when heart pumps . The recoil maintains the high pressure while heart relaxes.
  • Wall contains smooth muscles that can contract and constrict the artery. This narrows the lumen (used in arterioles to limit blood supply to one organ and redirect to another)
  • The endothelium is folded and can unfold when artery stretches


  • Lumen large to ease blood flow
  • Walls have thinner layers of collagen, smooth muscle and elastic tissue. Do not need to stretch and recoil.
  • Contain valves to help blood flow back into the heart and to prevent blood flowing the other way.
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Specific Blood Vessels (2)


  • Walls consist of single layer of flattened endothelium cells that reduces diffusion distance for exchange material.
  • The lumen is very narrow (7 micrometres). This ensures that the red blood cells are squeezed as they pass through helps them release oxygen because it pushes them closely to the capillary wall reducing diffusion path to tissues 
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Blood, tissue fluid and lymph

Blood consists of plasma (oxygen, co2, salts, glucose, fatty acids, amino acids, hormones and plasma proteins) The cells contain erythrocytes and leucocytes and platelets. 

Tissue fluid consists of oxygen, co2, salts, glucose, fatty acids, amino acids, hormones. Role =  transport oxygen and nutrient from blood to cells and carry carbon dioxide and waste products from the blood. 

When artery reach tissue it branches to many arterioles then into capillaries. These link up to venules to carry blood back to the veins. At the artery end of a capillary the blood is under high hydrostatic pressure. It will push blood fluid out of tiny gaps in capillary walls.

The fluid that leave the blood consists of blood plasma - tissue fluid

This fluid surrounds body cells, exchange of nutrients and gases occur - diffusion or facilitated diffusion.

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Blood, tissue fluid and lymph (2)

Tissue fluid also has hydrostatic pressure which pushes fluid back into capillaries . 

Water potential of the tissue fluid is less negative than that of blood this mean water tends to move back into blood by osmosis down water potential gradient.

At the vein end of the capillary  the blood has lost its hydrostatic pressure so with the combined effect of the hydrostatic pressure in tissue fluid and osmotic force of the plasma proteins is enough to pull the fluid back into the capillary. 

Formation of Lymph: not all tissue fluid moves back into capillary some is drained away to lymphatic system. Eventually rejoin in chest cavity

Lymph fluid: more carbon dioxide and wastes released, less oxygen and nutrients, more fatty materials from intestines. Contains many lymphocytes - produced in lymph nodes (filter bacteria and foreign materials from lymph fluid. 

Lymphocytes can engulf and destroy foreign particles. 

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Carriage of oxygen

Haemoglobin is oxygenated to become oxyhaemoglobin

The iron ion in haem group attracts and hold oxygen - affinity for oxygen 

1 haemoglobin can carry 4 oxygen (1 for each unit of haem)

Taking up oxygen: absorbed into blood in the lungs  diffusing through into blood plasma enter the red blood cells, taken up into haemoglobin. Maintains a steep diffusion gradient. Allows more oxygen into cell

Releasing oxygen: Body tissues cells need oxygen for aerobic respiration therefore oxyhaemoglobin must be able to release oxygen - DISSOCIATION

Amount of oxygen measured by the relative pressure that it contributes to a mixture of gases  (partial pressure or pO2) Also vcalled oxygen tension and is measured in units of pressure (kPa)

Oxyhaemoglobin dissociation curve - S shaped 

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Oxyhaemoglobin dissociation curve

At low tension, the haemoglobin does not readily take up oxygen molecules because the haem groups that attract the oxygen are in the centre of the haemoglobin. 

This makes it difficult for the oxygen to reach the haem group and associate with it. This accounts for the low saturation level if haemoglobin at low oxygen tensions.

As oxygen tension rises the diffusion gradient into the haemoglobin molecule increases. Eventually one oxygen molecules diffuses into the haemoglobin and associates with a haem group. 

Causes a change in shape of the haemoglobin molecule known as a conformational change. It allows more oxygen molecules to diffuse and associate relatively easily. This accounts for the steepness of the curve

Once the haemoglobin contains 3 oxygen molecules it becomes more difficult for the 4th oxygen to associate. The curve levels off as saturation approaches 100%  despite increasing oxygen tension.

The oxygen tension in lungs is sufficient enough to cause oxygen to dissociate readily

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Fetal haemoglobin

Higher affinity for oxygen than an adult haemoglobin. 

Fetal haemoglobin must be able to pick up oxygen from an environment that makes adult haemoglobin release oxygen.

Placenta - fetal haemoglobin must absorb oxygen from the fluid in the mother's blood. This reduces oxygen tension within the blood fluid, which in turn makes the maternal haemoglobin release oxygen. So oxygen dissociation curve for fetal haemoglobin is to the left of the cure for adult.

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Carriage of Carbon Dioxide

The Bohr Effect: refers to a change in the shape of the oxyhaemoglobin curve when carbon dioxide is present - this causes oxyhaemoglobin to release oxygen more readily. 

Carbon dioxide: 5% dissolved in plamsa, 10% combined directly with haemoglobin to form carbaminohaemoglobin, 85% transported in the form of hydrogencarbonate ions 

How are hydrogencarbonate ions formed: as CO2 diffuses into the blood some enters the red blood cells it combines with water t form a weak carbonic acid. Catalysed by carbonic anhydrase. 

The carbonic acid dissociates to release hydrogen ions and hydrogencarbonate ions

The hydrogencarbonate ions diffuse out of the red blood cells into the plasma. The charge in the RBC maintained by chloride ions (chloride shift). H ions cause RBS to become acidic to prevent this hydrogen ions taken up by haemoglobin to produce haemoglobinic acid. Haemoglobin acts as a buffer to maintain pH.

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Releasing Oxygen

The oxygen tensionof respiring tissue is lower than the lungs. As a result the oxyhaemoglobin begins to dissociate and release oxygen into the tissue. 

The hydrogen ions released from the dissociation of carbonic acid compete for the space taken up by the oxygen on the haemoglobin molecule. When carbon dioxide is present hydrogen ions displace the oxygen on the haemoglobin as a result the oxyhaemoglobin releases more oxygen to the tissues. 

When tissues contract more there will be more CO2 therefore more hydrogen ions will displace oxygen so more oxygen will be available to the respiring tissues. THE BOHR EFFECT 

This makes the oxyhaemoglobin dissociation curve shift downwards and to the right 

The Bohr effect results in oxygen being more readily released where more CO2 is produced for respiring tissues eg. exercise

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Great powerpoint! Thanks!

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