B3.2 Transport Theme: Form and Function Level of Organization : Organisms

B3.2.1: Adaptations of capillaries for exchange of materials between blood and the internal or external environment

Capillaries 

• Are small blood vessels which connect arteries to veins

• The function is to exchange materials between the blood and cells

Adaptations of Capillaries 

• Large Surface area, as capillaries are highly branched with narrow diameters

• Narrow lumen, which is wide enough for one red blood cell to pass through at a time

• Thin walls allow rapid exchange of materials by diffusion. Capillaries are typically one cell thick 

B3.2.2: Structure of arteries and veins 

Micrograph of arteries and veins 

• Arteries have a relatively thick wall and a narrow lumen 

• Veins have a relatively thin wall and wide lumen 

B3.2.2: Adaptations of arteries from the transport of blood away from the heart 

Arteries 

• Transport blood away from the heart 

• Adapted to withstand and maintain high blood pressure 

  • Thick wall allowing them to withstand high blood pressure 

  • Collagen in the outer wall of the artery strengthens the artery to withstand high blood pressure 

  • Smooth muscle in the artery can contract to maintain blood pressure between heart beats 

  • Elastic fibers in the wall allow the arteries to stretch and recoil as pressure increases and decreases due to heart beats 

  • The recoil helps to keep the blood moving in the artery 

  • A narrow lumen helps maintain high blood pressure 

  • The lumen is lined with smooth endothelial cells, which reduces friction as blood flows

B3.2.4: Measurement of pulse rates 

Measuring pulse rate 

• Pulse can be felt using fingertips at the radial artery in the wrist or the carotid artery in the neck 

• Determined by counting the number of beats per unit time 

• Technologies like smart watches and oximeters can also determine pulse rate 

B3.2.5: Adaptations of veins for the return of blood to the heart 

Veins 

• Return blood to the heart 

• Blood returning to the heart is moving slowly and is not under high pressure 

• Veins are adapted to return blood to the heart in the following ways:

  • Thin wall, which allows the vein to be compressed by the skeletal muscles. The compression moves blood back to the heart 

  • Wide lumen, which allows the vein to carry a large volume of blood 

B3.2.6: Causes and consequences of occlusion of the coronary arteries 

Atherosclerosis 

• Is the hardening and narrowing of the arteries due to the build of cholesterol, triglycerides, and other substances on artery walls 

Occlusion of coronary arteries 

• Coronary arteries branch off of the main artery, the aorta, and supply the heart with o2 and nutrients 

• The coronary arteries can be occluded (blocked) due to atherosclerosis

• Occlusion of the coronary arteries can lead to the death of heart tissue and heart attacks 

Causes of atherosclerosis 

• The inner lining of an artery is damaged due to high blood pressure. The response to the damaged artery includes:

  • Macrophages (A type of white blood cell) are attracted to sites of damage within the arteries and the macrophages release growth factors which stimulate the development of fibrous tissue 

  • The macrophages consume cholesterol and begin to form a plaque 

  • Over time plaque continues to grow and may occlude (block) the artery 

  • The plaque can break away from the artery and cause a blood clot 

The following risk factors are under our control 

• Obesity, which increases blood pressure and damages artery walls 

• Physical inactivity, which can lead to obesity

• Smoking, which increases blood pressure 

• A diet high in fats and cholesterol 

Notes on Correlation coefficients 

1. If the correlation coefficient is close to zero, it can be assumed that there is no relationship between the two variables. This can disprove a hypothesis

2. Correlation does not imply causation. Causation requires the change of one variable to be a direct result of the change of the other.

Saturated Fat and Coronary Heart DIsease 

• As sat fats intake increases there is an increase in the number of deaths caused by coronary heart disease

• Important to appreciate that correlation doesn't imply causation 

• Good Evidence to prove that sat fat does contribute to the risk of heart disease

B3.2.7: Transport of water from roots to leaves during transpiration 

Transpiration 

• Movement of the water up the plant and its evaporation from the leaves

• Water evaporates from the mesophyll cells, and diffuses out through the stomata

• Water is drawn out of the xylem, moving through cell walls, by capillary action, to replace water lost by mesophyll cells 

• The loss of water in the xylem creates a negative pressure in the leaf

• Water enters the root by osmosis and moves into the xylem in the roots, creating higher pressure 

• Water moves up the xylem from the roots to the leaves by transpiration pull

• Hydrogen bonds make water cohesive, allowing water to be pulled up the xylem 

• The adhesive properties of water allow it to form hydrogen bonds with the cellulose in the cell wall, helping to maintain the movement of water up the xylem 

B3.2.8: Adaptations of xylem vessels for transport of water 

Xylem 

• Function is to transport water and dissolved minerals up a plant 

  • Thickened cell walls of xylem tissue also provide structural support of the stems of plants 

  • Xylem cells form long hollow tubes, known as xylem vessels

    • Happens when cells are stacked on top of each other 

    • These dad, hollow cells have no end cell walls, forming a long hollow tube through them 

• Adaptations

  • No end walls between cells, allow from column of water to move up plat 

  • No cell contents or plasma membrane, allow unimpeded flow of the transpiration stream 

  • Pits, allow water to move between the xylem and adjacent cells 

  • Lignin strengthens the cell walls of xylem which allows the xylem vessels to resist the inward pressure created by transpiration 

B3.2.9: Distribution of tissues in a transverse section of the stem of the dicotyledonous plant 

Dicotyledonous Stem 

• Epidermis: provides protection for the stem 

• Corextex: provides structural support for the stem and stores starch in the root of plants 

• Vascular bundles: transport materials up and down the plant. The vascular tissue contains xylem and phloem tissue 

• Phloem: transports organic compounds, like sucrose and amino acids, up and down the stem of the plant 

B3.2.10: Distribution of tissues in a transverse section of the root of a dicotyledonous plant 

B3.2.11: Release and reuptake of tissue fluid in capillaries 

Tissue fluid

• Surrounds cells, enabling the exchange of materials between the blood and cells 

• Formed by the liquid part of the blood, plasma, and leaking out of capillaries 

• Release of tissue fluid includes

  • Blood leaves an artery (arterole) at high pressure and enters a capillary

  • The high hydrostatic pressure of the blood filters blood plasma through the gaps in the capillaries, forming tissue fluid 

• Reuptake of tissue fluid includes:

  • Blood pressure decreases as the blood moves along the capillary 

  • Plasma proteins decrease the osmotic potential of the blood

  • Most of the tissue fluid returns to the blood by osmosis due to oncotic pressure, which is higher than the hydrostatic pressure 

B3.2.12: Exchange of substances between tissue fluid and cells in tissues 

Blood plasma and tissue fluid 

• The high hydrostatic pressure of blood filters blood through the gaps of the capillary wall 

  • Large particles such as blood cells and proteins are too large to pass through the gaps in the walls of the capillary 

  • Small particles and the solutes dissolved in the blood plasma leave the blood and become tissue fluid 

• Both blood plasma and tissue fluid are composed of 

  • Dissolved nutrients, including glucose, amino acids, and fatty acids

  • dissolved oxygen

  • Metabolic wastes, including co2

  • White blood cells, which can move through the gaps in the capillary walls 

• Blood plasma components which are not present in tissue fluid include 

  • Red blood cells and platelets

  • Large plasma proteins 

TIssue Fluid and Cells 

• Contains a high concentration of nutrients and o2 and a low concentration of co2 and other metabolic waste 

• Metabolism in cells uses nutrients and O2, producing metabolic wastes. Therefore cells have a low concentration of o2 and nutrients, and a high concentration of metabolic wastes

• Oxygen and nutrients diffuse from the tissue fluid into cells

• Metabolic wastes such as carbon dioxide diffuse from cells into tissue fluid 

B3.2.13: Drainage of excess tissue fluid into lymph ducts 

Drainage of Tissue FLuid 

• Most of the tissue fluid returns to blood plasma

• The tissue fluid that does not directly re-enter the blood, is taken up by lymph ducts, and is known as lymph 

• Lymph travels through the lymphatic system and the fluid is returned to the blood via lymph nodes 

Adaptations of Lymph Vessels 

• Lymph ducts collect excess tissue fluid and return it to the blood 

• Lymph ducts have the following adaptations for moving lymph fluid:

  • Gaps in the wall of the lymph ducts which allow tissue fluid to enter

  • Think walls, which are compressed by skeletal muscles to move the lymph fluid

  • Valves which prevents the backflow of the lymph fluid 

• Lymph is returned to the blood via the thoracic duct, which drains into the subclavian vein 

B3.2.14: Differences between the single circulation of bony fish and the double circulation of mammals 

Circulation in Bony Fish 

• Bony fish have a single circulatory system as they have two chambered heart 

• The heart ventricle pumps blood to the gills 

• O2 and co2 are exchanged as blood passes through capillaries in the gills 

• The oxygenated blood leaves the gills and is transported to tissues 

• Gas exchange occurs as blood passes through body tissues 

• Deoxygenated blood returns to the heart

Circulation in Mammals 

• Mammals have a double circulatory system as they have a four chambered heart 

• The right side of the heart pumps blood to the lungs 

• O2 and co2 are exchanged as blood passes through capillaries in the lungs 

• The oxygenated blood returns to the left side of the heart and is pumped to the body  

• Gas exchange occurs as blood passes through body tissues 

• Deoxygenated blood returns to the heart

B3.2.15: Adaptations of the mammalian heart for delivering pressurized blood to the arteries

Mammalian circulatory system 

• Mammals have a double circulatory system 

Heart 

• Deoxygenated blood returns from the body in the (superior and inferior) vena cava.

• Blood enters the right atrium.

• Blood enters the right ventricle through an atrioventricular valve.

• The right ventricle contracts, forcing blood through a semilunar valve into the pulmonary artery.

• The pulmonary artery brings blood to the lungs where it is oxygenated.

• Oxygenated blood returns to the heart via the pulmonary vein.

• Blood enters the left atrium.

• Blood enters the left ventricle through an atrioventricular valve.

• The left ventricle contracts, forcing blood through a semilunar valve into the aorta.

• The aorta brings blood to the tissues, where it becomes deoxygenated.

Adaptation of the Heart 

• The heart is adapted to pump blood under pressure to the lungs and body at the same time. The following structures are adapted to the heart’s function 

  • Atria: receive blood from the blood from the body and lungs 

  • Ventricles: contain lots of cardiac muscle to pump blood to the lungs and the body 

  • Cardiac muscle: allows the heart to contract to create high pressure. The cardiac muscle from the left ventricle is much thicker than the right ventricle. The left ventricle requires high pressure to move blood around the body 

  • Pacemaker (sinoatrial node): initiates and controls the rate of heartbeat 

  • Atrioventricular valves: prevent the backflow of blood from the ventricles to the atria 

  • Semilunar valves : prevent the backflow of blood from the arteries to the ventricles 

  • Septum: prevent oxygenated and deoxygenated blood from mixing 

  • Arteries: move blood away from the heart at high pressure 

  • Veins: return blood back to the heart 

B3.2.16: Stages in the cardiac cycle 

Control of the cardiac cycle

• The medulla oblongata has two nerves connected to the sinoatrial node of the heart. The nerve control the rate at which the heart beats 

• Cardiac muscle is myogenic, as it contracts without stimulation 

  • The sinoatrial node (pacemaker) controls the rate of heart beat.

  • The atrioventricular node initiates an action potential (electrical signal) which rapidly spreads across the atria, causing atrial systole (contraction).

  • A layer of fibrous tissue prevents the action potential from travelling directly to the ventricles. There is a pause before the signal reaches the ventricles, so that the four chambers do not contract at the same time.

  • The action potential travels to the ventricles via the atrioventricular node.

• The action potential travels down the Purkinje fibers to the apex of the heart 

• The action potential travels up the walls of the ventricles, initiating ventricular systole from the apex, pumping all of the blood out of the ventricles

The cardiac cycles

• Systole is the contraction of the heart muscles

• Diastole is the relaxation of the heart muscles

• The left atrium and left ventricle are in diastole. Most of the blood flows directly through the atrium to the ventricle. 

• An action potential from the sinoatrial node causes the left atrium to enter the systole.

• Pressure increases in the atrium as it contracts, forcing all of the blood into the ventricle.

• An action potential from the atrioventricular node causes the ventricle to enter the systole.

• As the ventricle contracts, pressure increases in the ventricle, causing the atrioventricular valve to close, as pressure is higher in the ventricle than the atrium.

• The high blood pressure in the ventricle increases until the semilunar valve opens and blood moves into the aorta.

• The ventricle enters the diastole, and pressure in the ventricle decreases.

• When pressure is greater in the aorta than the ventricle, the semilunar valve closes.

• When pressure in the ventricle is lower than the atrium, the atrioventricular valves open.

Measuring Blood Pressure 

• Blood pressure measurements include two pressure measurements:

  • Systolic pressure, caused by ventricular systole

  • Diastolic pressure between ventricular contractions 

B3.2.17: Generation of root pressure in xylem vessels by active transport of mineral ions 

Roots absorb minerals 

• The concentration of mineral ions is higher inside the root cells than outside the root 

• The root cells will actively transport mineral ions into the root cells

Root absorb Water 

• Water enters the roots of a plant due to the high mineral ion solute concentration within root cells 

• High concentration of mineral ions within root cells (due to active transport) and a low concentration of mineral ions outside the root

• Water will pass through the plasma membrane of root hair cells by osmosis 

• Osmosis is the passive movement of water molecules from a low solute concentration to high concentration, through a partially permeable membrane 

• The plasma membrane of root cells contains aquaporins, which facilitate the rapid movement of water into the root cells

Symplastic and apoplastic pathways

• Water moves from the root hairs to the xylem by two pathways:

• Symplastic pathway: water moves through the cytoplasm of adjacent cells by osmosis

• Apoplastic pathway: water moves through the cell walls of plant cells by capillary action 

Root Pressure 

• Mineral ions are actively transported through the Casparian strip of the endodermis and into the xylem which creates a relatively low water potential in the xylem 

• Water moves into the xylem by osmosis, creating a positive root pressure that moves water up the xylem 

• Root pressure allows water to move up the xylem of plants when transpiration rates are low 

• Transpiration rate can be low due to high levels of humidity or the absence of leaves in deciduous trees

B3.2.18: Adaptations of phloem sieve tubes and companion cells for translocation of sap

 Translocation 

• Moves nutrients, such as sucrose and amino acids, up or down the stem of a plant through phloem tissue 

Phloem Tissue 

• Transports nutrient, such as sucrose and amino acids, up and down the stem of the plant

• Phloem tissue is composed of companion and sieve tubes 

• The cytoplasm of a sieve tube and its companion cell are linked through plasmodesmata

Sieve tubes

• Sieve element cells form long narrow sieve tubes 

• Are adapted to their function of translocating nutrients up and down the plant

  • Adaptations:

  • Reduced cytoplasm and no nucleus in the cell, allowing movement of cell sap 

  • plasma membrane protein pumps for active transport

  • Sieve plates: pores (appearing like a sieve in the cell walls between cells) allowing cell sap containing nutrients to flow from cell to cell 

  • Plasma membranes with protein pumps for active transport

  • Plasmodesmata: allows direct connections between the cytoplasm of the companion cell and sieve tube 

• Every cell in the sieve tube is connected to a companion cell, which provides metabolic support for the sieve tube cell 

Companion Cells

• Provide metabolic support for sieve tube element cells 

• Adapted to their function through

  • The cytoplasm of companion cells is directly connected to the cytoplasm of sieve tube cells via plasmodesmata 

  • Companion cells contain large numbers of mitochondria to provide sufficient ATP for active transport (loading) of nutrients tts into the phloem tissue 

  • Companion cells contain transport proteins from loading nutrients into the sieve tubes 

Translocation 

• Moves nutrients from a source,where they are produced or stored to a sink, where they are used to stored

• Sucrose is produced by the leaves and then is actively transported by companion cells into sieve tubes 

• Creates a high sucrose concentration in the phloem sieve tubes and is known as phloem loading 

• Water moves into the phloem sieve tubes from the xylem by osmosis

• Since water is incompressible, hydrostatic pressure builds up in the sieve tubes in the leaves

• Increasing hydrostatic pressure moves water and dissolved sucrose from source (leaves) to sink (roots)

• At the sink, the roots, sucrose, is actively transported into root cells. The sucrose is either used for respiration or stored as starch 

Notes on Translocation:

• Storage tissues can also be sources, and the stored starch is converted to sucrose and transported to sinks such as growing tissues.

• Other nutrients such as amino acids are also translocated through phloem tissue.

• Translocation can move nutrients up or down the phloem.