Maintaining a balance (booklet 2)

Maintaining a balance (booklet 2)

MAINTAINING A BALANCE YEAR 12 (BOOKLET 2) TRANSPORT DISSOLVED NUTRIENTS AND GASES Both plants and animals require a transport system to distribute food and oxygen to active cells and to remove carbon dioxide and any other waste products that may accumulate. Unicellular organisms rely on the processes of diffusion, osmosis and active transport of substances directly

between the surface of the organism and the environment. In most multicellular organisms, transport of substances in this way is not adequate, due to their large SA:V ratio. Therefore specialised transport systems have developed in complex plants and animals to carry substances. TRANSPORT DISSOLVED NUTRIENTS AND GASES The common features of a transport system are: 1. a suitable transport medium (fluid) 2. the presence of vessels in which substances can be carried 3. a driving mechanism to ensure that substances move in the correct

direction. The transport tissue in plants is known as the vascular tissue and consists of xylem and phloem. In mammals, the transport system is known as the cardiovascular system, made up of a pump (the heart) to move the blood and a series of vessels BLOOD AS A MEDIUM OF TRANSPORT Blood is a fluid transport medium that flows through the heart and blood vessels of the transport

(cardiovascular) system in all vertebrates and some invertebrates. If whole blood is spun in a centrifuge, it separates into its component parts: 45% cells and 55% watery plasma. COMPOSITION OF BLOOD Read pages 36-37 for information about the components of blood

ESTIMATING THE SIZE OF RED AND WHITE BLOOD CELLS perform a first-hand investigation using the light microscope and prepared slides to gather information to estimate the size of red and white blood cells and draw scaled diagrams of each In this investigation, students are required to estimate the size of blood cells. It is possible to do this using simple equipment such as a light microscope and a plastic ruler, or by using a mini-grid slide in place

of a ruler. The smaller or more precise calibrations give a more accurate estimate of the diameter field of view. HOW IT IS DONE: https://www.youtube.com/watch?v=kt_EK8MElYE CHEMICAL SUBSTANCES AND HOW THEY ARE TRANSPORTED IN BLOOD identify the form(s) in which each of the following is carried in mammalian blood: carbon dioxide; oxygen; water; salts; lipids; nitrogenous waste; other products of

digestion To maintain homeostasis, chemicals being transported in the blood must also be maintained at a particular concentration and carried in a specific form that will not affect the balance in the internal environment of the body. INTRODUCTION If the normal balance of substances in

the blood is altered, conditions such as low blood sugar levels or high blood pressure will arise. This brings on unpleasant and sometimes dangerous side effects. Its an indication that metabolic functioning has been compromised. Homeostasis relies on maintaining a balance of chemicals within the blood. OXYGEN TRANSPORT When oxygen diffuses across the respiratory surface of the lung

into the blood, most of it (98.5%) combines reversibly with haemoglobin inside red blood cells. The remaining 1.5% may travel dissolved in the plasma. Red blood cells are ideally adapted to carrying oxygen. They dont have a nucleus, providing plenty of space for haemoglobin. Haemoglobin has an affinity for oxygen. Each red blood cell contains approximately 250 million molecules of haemoglobin, resulting in a very high oxygen carrying capacity.

OXYGEN TRANSPORT The slightly flattened biconcave shape of red blood cells gives them a larger surface area to volume ratio for easy diffusion of oxygen across the surface. When blood in the lungs comes into contact with oxygen that has entered the body by diffusion, haemoglobin in the red blood cells binds with this oxygen, forming a compound called oxyhaemoglobin. This compound gives a

bright red colour to blood. OXYGEN TRANSPORT Most arteries carry bright red oxygenated blood, whereas most venous blood is a dark red colour. OXYGEN TRANSPORT - QUESTIONS

1. Why is oxygen needed in the body? 2. How is oxygen transported in the blood? CARBON DIOXIDE TRANSPORT When carbon dioxide enters the blood, most (70%) of it is transported in the form of hydrogen carbonate ions. This is formed in the red blood cells but is carried in the plasma. The remaining carbon dioxide is carried either dissolved in the plasma (7%) or is carried

combined with haemoglobin (23%). CARBON DIOXIDE TRANSPORT Carbon dioxide is produced as a waste product of respiration. Since carbon dioxide mixed with water forms carbonic acid, it is not ideal for all of the carbon dioxide to dissolve in the plasma, since this would affect the pH of blood.

Instead, a large portion of the carbon dioxide enters the red blood cells. Once there one of two things happen. CARBON DIOXIDE TRANSPORT 1. Most of the carbon dioxide mixes with water in the cytoplasm within the blood cells and forms carbonic acid. This is rapidly converted to hydrogen carbonate ions (bicarbonate ions). These ions then move out of the red blood cells into the blood plasma and 70% of carbon dioxide is transported in this form. This can be summarised as: Carbon dioxide + water carbonic acid hydrogen carbonate + buffered

hydrogen ions CO + HO HCO HCO + H CARBON DIOXIDE TRANSPORT 2. Some carbon dioxide binds to haemoglobin, forming carbaminohaemoglobin. Haemoglobin does not bind to carbon dioxide in the same way that it binds to oxygen. Oxygen binds to the iron atom of haemoglobin, whereas carbon

dioxide binds to the amino group of the protein part-the globin molecule. CARBON DIOXIDE TRANSPORT As with oxygen, this is a reversible reaction and many carbon dioxide molecules can combine with a single haemoglobin molecule. Only 23% of carbon dioxide is carried in this form. CARBON DIOXIDE TRANSPORT - QUESTION

1. How is carbon dioxide transported in the blood? CARBON DIOXIDE TRANSPORT - QUESTION WATER AND SALT Water is the medium of transport of all substances in the body. It forms the basis of the cytoplasm in all cells, the interstitial fluid (tissue fluids) surrounding the cells and blood and lymph. About 90% of blood plasma is water. The

other 10% is made up mostly of various kinds of protein molecules as well as other substances including hormones, vitamins, end products of digestion and salts. WATER AND SALT Salts are carried in blood as ions (charged particles) dissolved in the plasma. For example, the salt sodium chloride (NaCl) is carried as positively charged sodium ions (Na)

and negatively charged chloride ions (Cl) in solution of the watery medium of the plasma. WATER AND SALT Substances (such as salts) that become ions in solution are often referred to as electrolytes, because their capacity to conduct electricity. The balance of the electrolytes in our bodies is essential for normal function of our cells and our organs. Common electrolytes include sodium, potassium, chloride and

bicarbonate. LIPIDS AND OTHER PRODUCTS OF DIGESTION The aim of digestion is to break large molecules down to a size small enough for absorption through the intestine wall and into the bloodstream, so that they can be transported to cells in the body where they are required. LIPIDS AND OTHER PRODUCTS OF DIGESTION The digestion of large organic molecules

to their smaller end products is summarised below: Carbohydrates (simple sugars) Proteins glucose amino acids

Lipids (fats and oils) and glycerol fatty acids Nucleic Acids nucleotides LIPIDS AND OTHER PRODUCTS OF DIGESTION

Glucose and amino acids are water soluble and so they are transported in the bloodstream dissolved in the plasma, along with other soluble substances such as nitrogen bases, vitamins and glycerol, absorbed from the digestive tract. LIPIDS AND OTHER PRODUCTS OF DIGESTION Lipids pose a problem in terms of transport, since they are insoluble in water and therefore cannot be carried dissolved in plasma. They need to be

packaged into small droplets (micelles), which pass into the lymphatic system and then into the bloodstream. NITROGENOUS WASTES Nitrogenous wastes are harmful substances produced in the body as a result of the breakdown of proteins. These substances need to be transported in a diluted form, from cells where they are produced to the excretory organs where they can be eliminated from the body. Such wastes

in the form of ammonia, urea, uric acid and creatinine are all carried dissolved in blood plasma. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN explain the adaptive advantage of haemoglobin STRUCTURE OF HAEMOGLOBIN Haemoglobin is a protein made up of four polypeptide chains (called globins) and each is bonded to a haem (iron containing) group.

Each haem is a red pigment molecule and the iron necessary for haemoglobin formation is obtained from the diet. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN Since small amounts of iron are lost from the body regularly in waste products like urine and faeces, a regular supply of dietary iron is necessary to maintain haemoglobin in red

blood cells. A lack of iron in the diet may lead to a condition known as anaemia, where there are too few red blood cells or the blood cells that are present are unable to carry sufficient oxygen. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN Haemoglobin has the adaptive advantage of being able to increase the oxygen-carrying capacity of blood. Haemoglobin molecules each contain

four haem units, giving one haemoglobin molecule the ability to bond with four oxygen molecules. Because of this more oxygen can be carried in blood cells by haemoglobin (1000 million molecules of oxygen) than could be dissolved in plasma. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN Haemoglobin has a further adaptive advantage because its ability to bind oxygen increases once the first oxygen

molecule binds to it. The binding of each oxygen molecule causes it to change slightly in shape, making it easier for every subsequent oxygen molecule to bind to it. This increases the rate and efficiency of oxygen uptake. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN As a result, a very small increase in the oxygen concentration in the lungs can result in a large increase in the

oxygen saturation of blood. E.g. during exercise, we breath more deeply and rapidly, increasing the oxygen intake into the lungs and this causes an increased uptake of oxygen by haemoglobin. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN Another adaptive advantage of haemoglobin is that its capacity to release oxygen

increases when carbon dioxide is present. It is important for haemoglobin to combine with oxygen at respiratory surfaces, but equally important for it to release the oxygen in tissues where oxygen concentration is low. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN Metabolising cells release carbon dioxide, which combines with water to form carbonic acid and this lowers the pH. Haemoglobin has the adaptive advantage of a reducing affinity for oxygen at a lower pH and so it

releases the oxygen in these tissues where it is needed (known as the Bohr Effect). Once the haemoglobin has released oxygen, it has an increased ability to pick up carbon dioxide. THE ADAPTIVE ADVANTAGE OF HAEMOGLOBIN The fact that haemoglobin is enclosed in a red blood cell is also

an advantage because if it were simply dissolved in the plasma, oxygen would upset the osmotic balance of the plasma. OXYGEN, CARBON DIOXIDE AND CELL FUNCTIONING outline the need for oxygen in living cells and explain why removal of carbon dioxide from cells is essential Oxygen is necessary for cellular respiration, a process

by which cells obtain energy from glucose. Energy is needed for life- sustaining processes such as growth, repair of tissues, movement, excretion and reproduction. OXYGEN, CARBON DIOXIDE AND CELL FUNCTIONING Although glucose and other food molecules are energy rich, the energy stored in them must be converted into a form living cells can use for metabolism.

Oxygen combines with glucose in a sequence of enzyme controlled steps during cellular respiration to release chemical energy as ATP. OXYGEN, CARBON DIOXIDE AND CELL FUNCTIONING ATP is the form of chemical energy needed by living cells for their metabolism. This is called the oxidation of glucose and it takes place in all living cells.

OXYGEN, CARBON DIOXIDE AND CELL FUNCTIONING product of chemical respiration. When carbon dioxide reacts with water in cytoplasm of cells or in the plasma of blood, it forms carbonic acid. A buildup of carbonic acid is toxic as it lowers the pH of the cells and bloodstream, affecting the homeostatic balance within an organism. CO 2

Carbon dioxide is produced as a waste OXYGEN, CARBON DIOXIDE AND CELL FUNCTIONING A low (acidic) pH would prevent enzymes from functioning optimally and this affects cell functioning by reducing metabolic efficiency in the body. Therefore the quick removal of carbon dioxide is essential for the optimal functioning of enzymes.

Enzyme Substrate THE EFFECT OF CARBON DIOXIDE ON THE PH OF WATER perform a first-hand investigation to demonstrate the effect of dissolved carbon dioxide on the pH of water Page 45-47 TECHNOLOGY - MEASURING BLOOD GASES analyse information from secondary sources to identify current technologies that

allow measurement of oxygen saturation and carbon dioxide concentrations in blood and describe and explain the conditions under which these technologies are used This dot point was covered in your assessment task See page 49 for summary STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM compare the structure of arteries, capillaries and veins in relation to their function The transport or vascular system in mammals consists

of the heart, blood vessels and lymph vessels, as well as the fluids transported in them blood and lymph. These systems are interrelated. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Transport vessels all have some structural features in common: They are long, hollow structures that

consist of a lumen (cavity), surrounded by a wall. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Blood and blood vessels Blood is transported by arteries away from the heart, towards the tissues of the body. Blood is transported by veins

from the tissues in the body back to the heart. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Capillaries are tiny, thin-walled blood vessels in the tissues of the body that carry blood very close to the cells, linking the arteries and veins. Arteries branch to form arterioles that

lead directly into capillary networks in the tissues. Blood flows from these capillary networks into venules which join up to form veins so that blood can be returned to the heart. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Fluids that seep out of capillaries into the

surrounding tissues are returned to the bloodstream by the lymphatic system STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Arteries, veins and capillaries have a similar basic structure, but they differ in terms of the layers of tissue that make up the wall of each and the size of the lumen Each vessel is structurally modified to best carry out its specific

transport function. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM The walls of both arteries and veins consist of three layers: 1. Inner layer consists of a thin layer of endothelial cells 2. Middle layer made up of mostly smooth muscle and some elastic fibres

3. Outer layer made up of connective tissue STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM DRAW! Arteries The function of arteries is to carry blood away from the heart to the various parts of the body.

Since the blood is pumped out of the heart in regular bursts under high pressure, the walls of the arteries are thicker than those of veins, to withstand the force. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Major arteries close to the heart have thick layers of smooth muscle in their walls to allow them to withstand the increase in blood pressure as blood is pumped from the heart.

The smooth muscle also functions to adjust the diameter of the lumen and therefore regulates blood flow in the arteries. When the smooth muscle contracts, the size of the lumen is decreased (vasoconstriction) and blood flow slows down. When the smooth muscle relaxes (vasodilation), blood flow increases. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM The walls of arteries also have a large

proportion of elastic fibres in both the inner layer and in the middle layer surrounding the smooth muscle. This increased elasticity enables the arteries to expand to accommodate the increased volume of blood pumped with each heartbeat. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM When the heart relaxes, the elastic fibres allow the arteries to recoil

back to their original diameter, squeezing the blood forward and ensuring a continuous flow in one direction. In certain parts of the body where large arteries are near the surface of the skin, the expansion and recoil of the arteries can be felt by a pulse. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Veins Structurally, veins have walls that are thinner than those of arteries since the blood that they receive flows in under lower pressure. Its not pumped in.

The walls have very few elastic fibres as no stretch and recoil is necessary and the smooth muscle layer is much thinner. The lumen also has a wider diameter, for easy flow of blood. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Since blood seeps into veins, two mechanisms prevent the backflow of blood. 1. Many veins are situated between

large groups of muscles (e.g. in the legs and arms) and their thin walls allow them to be easily compressed. When muscles contract, the veins are compressed and this propels the blood towards the heart. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM 2. Veins have valvessmall pocket-like folds of

the endothelium lining the lumen of veins. These valves occur at regular intervals along the inside walls of veins. Valves work like one way swing doors they open to allow blood to flow through in one direction towards the heart, but the pressure of blood trying to flow backwards causes them to swing shut. These mechanisms are especially important in veins such as those in the legs, where blood flows against the force of gravity. STRUCTURE AND FUNCTIONING OF THE CIRCULATORY

SYSTEM Capillaries Capillaries are extremely tiny, microscopic vessels that bring the blood into close contact with the tissues, for the exchange of chemical substances between body cells and the bloodstream. The walls of capillaries consist of an endothelium lining the lumen, which is only one cell layer thick. Capillaries have no other layers in their walls.

STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Diffusion is a fairly slow, passive process and so the structure of capillaries is suited to slowing down the flow of blood. Capillaries have: Thin walls allow for the efficient diffusion of substances, so they dont have far to travel between the blood and body cells A small lumen force the RBCs to pass in a single file, slowing down their flow and

increasing their exposed surface area for gaseous exchange STRUCTURE AND FUNCTIONING OF THE CIRCULATORY SYSTEM Capillaries form an expansive network to spread blood flow over a large surface area so that no cells are far from the blood supply.

SUMMARY CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION describe the main changes in the chemical composition of the blood as it moves around the body and identify tissues in which these changes occur The circulatory system has vessels to ensure that blood can flow towards the heart (in veins) and away from the heart (in arteries). Arteries and veins often run alongside each other within the body. The circulatory system is involved in moving four basic groups of chemicals: 1. gases (carbon dioxide and oxygen)

2. nutrients 3. wastes 4. hormones (chemical signals). CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION Assisting metabolic functioning The chemical functioning of cells (metabolism) relies on the correct balance of chemical reactants being brought to cells

and the removal of wastes produced. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION Energy is the basis of all metabolic functioning. Energy production depends on the correct balance of nutrients such as glucose and the gas oxygen being transported to the sites where they are needed (body cells) from their source (glucose from the digestive system, oxygen from the lungs). Once the reactants reach the cells, cellular respiration occurs

producing ATP. Carbon dioxide and water are released as byproducts of this process. The carbon dioxide is a toxic waste product and must be removed. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION Nitrogenous wastes are the end products of protein breakdown that occurs during metabolic functioning. All wastes (nitrogenous wastes and carbon dioxide) are carried from their sites of production, via blood vessels, to organs where they

can be excreted. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION The circulatory system is also responsible for transporting hormones chemical messenger molecules produced by endocrine glands. These are ductless glands and so they secrete their secretions directly into the bloodstream, which transports them to their target

organs. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION The changing chemical composition of blood The chemical concentration of blood entering or leaving an organ depends on the function of that organ: External gaseous exchange occurs in the lungs deoxygenated blood releases carbon dioxide and picks up oxygen. The pulmonary vein then carries this oxygenated blood from the lungs to the heart, to be pumped to other organs of the body for cellular respiration.

You find an increase in oxygen and a decrease in carbon dioxide concentrations in blood that has passed through the lungs. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION Internal gaseous exchange occurs in all tissues of the body as a result of cellular respiration - oxygen combines with glucose to make energy and carbon dioxide is released as a waste product (which diffuses

into the blood capillaries in the tissues) You find a decrease in oxygen and an increase in carbon dioxide in blood that has passed through any organ other than the lungs. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION Absorption of nutrients into the bloodstream takes place in the digestive tract (particularly in the small intestine). You find an increase in digestive end products in blood that has passed through

organs of the digestive tract. These products of digestion travel in the bloodstream directly to the liver. CHANGES IN CHEMICAL COMPOSITION IN BLOOD DURING CIRCULATION Nitrogenous waste products are produced in the liver and excreted by the kidneys. You find: A decrease in digestive end products once blood has passed through the liver as this is the centre of food metabolism An increase in nitrogenous wastes in blood that has

passed through the liver, the organ where proteins are broken down to form nitrogenous wastes. A decrease in nitrogenous wastes in blood that has passed through the kidneys, which filter nitrogenous wastes out of the blood and excrete them. DONATED BLOOD AND ITS PRODUCTS analyse information from secondary sources to identify and describe the products extracted from donated blood and the uses of these products This dot point was covered in

your assessment task See page 58-60 ARTIFICIAL BLOOD AND ITS IMPORTANCE analyse and present information from secondary sources to report on progress in the production of artificial blood and use available evidence to propose reasons why such research is needed This dot point was covered in your assessment task See page 61-63 TRANSPORT OF NUTRIENTS IN PLANTS

describe current theories about processes responsible for the movement of materials through plants in xylem and phloem tissue The role of transport in plants is mainly to carry materials for photosynthesis to the cells and to move cell products away to other parts of the plant. In large plants, specialised vascular tissue has developed to serve this transport function. TRANSPORT OF NUTRIENTS IN PLANTS The vascular system in plants consists of two types of vessels:

Vessels Transport medium Diving mechanism From To Xylem Water and dissolved

inorganic salts Transpiration stream Roots (site of absorption) Leaves (for photosynthesis) Phloem Dissolved sugars

(organic nutrients) Pressure flow Movement in two directions The movement of materials from one part of the plant to another is known as translocation. TRANSPORT OF NUTRIENTS IN PLANTS Xylem

The transpiration stream theory (adhesion-cohesion-tension theory) accounts for the movement of water and mineral ions in the xylem. The passive transport of water (and ions) depends on transpiration and the physical forces of water. TRANSPORT OF NUTRIENTS IN PLANTS Water and mineral ions that have been absorbed from the roots move

across into the xylem. The continual influx of these substances, forces the solution upwards. Most of the upward movement results from the transpiration stream - water is drawn up the xylem to replace water loss from the leaves. TRANSPORT OF NUTRIENTS IN PLANTS Because of the properties of adhesion (the attraction between water molecules and the

walls of the xylem) and cohesion (the attraction of water molecules to each other), the water column forms a continuous stream and so it is pulled upwards to replace this water loss. TRANSPORT OF NUTRIENTS IN PLANTS The combination of adhesive and cohesive forces, together with the suction pull of transpiration create the transpiration stream. Mineral ions dissolved in the water are carried along by the transpiration stream and can move out by active transport to reach

the tissues where they are needed. TRANSPORT OF NUTRIENTS IN PLANTS Phloem The pressure flow theory (source-path-sink theory) accounts for the translocation of organic nutrients in the phloem. Translocation in phloem tissue moves products of photosynthesis (such as glucose, sucrose and amino acids) by active transport (requires energy). Materials are distributed

especially to growing points and reproductive structures TRANSPORT OF NUTRIENTS IN PLANTS The mechanism of flow is driven by an osmotic pressure gradient, generated by differences in sugar and water concentrations. Sugars are actively loaded into phloem sieve tubes at one end (the source). Two theories: 1. Symplastic loading: Sugars and nutrients move in the cytoplasm from the mesophyll cells to the sieve elements through plasmodesmata. 2. Apoplastic loading: Sugar and nutrients move

through the cell walls until they reach the sieve element. Then they cross the cell membrane to enter the phloem. TRANSPORT OF NUTRIENTS IN PLANTS Sugars are then actively unloading from phloem into surrounding tissues it at the other end (the sink). TRANSPORT OF NUTRIENTS IN PLANTS

The loading of sugar at the source (leaves) causes the phloem sap to become more concentrated. This attracts water to flow in by osmosis from the adjacent xylem tissue. This resulting pressure causes water and dissolved solutes to flow toward a sink. The active offloading of sugars at the sink (e.g. roots, flowers) also causes water to flow out of the phloem by osmosis. This reduces osmotic pressure at the sink

region. lower osmotic pressure due to the higher water conc. in the sink TRANSPORT OF NUTRIENTS IN PLANTS This difference in osmotic pressure between the source and the sink in the phloem drives the phloem sap to flow.

INVESTIGATING XYLEM AND PHLOEM TISSUE IN PLANTS (USING A LIGHT MICROSCOPE) choose equipment or resources to perform a first-hand investigation to gather firsthand data to draw transverse and longitudinal sections of phloem and xylem tissue Next term

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