2SS Transport Notes B1 I

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Sec 2 Transport in Living Things Notes B1(I) © Just Education Holdings Pte Ltd

Name: ____________________________________ Class Code: ______________ Date: __________ Diffusion Diffusion is the net movement of atoms or molecules from a region where there is a higher concentration to a region where there is a lower concentration of atoms or molecules. In the process, atoms or molecules are spread out evenly throughout the space available. It will continue until the two regions become homogeneous i.e. have the same concentration. For instance, the smell of cooking in the kitchen is spread to the living room by diffusion. Orange juice concentrate is also spread through a glass of water by the same process. Diffusion takes place as the particles of gases and liquids are moving randomly and are always knocking into one another. The particles are then evenly spread throughout the gas or liquid after some time.

JUST SCIENCE TUITION CENTRE Secondary 2 Notes B1(I)

Topic B1 Transport in Living Things

Region of high concentration of

molecules

Region of low concentration of

molecules

Direction of movement of molecules

Potassium permanganate crystals

Water particles Particles

leaving the crystals

Water particles and potassium permanganate particles become evenly mixed

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The particles in a gas are further apart and are moving at a higher speed as compared to those in liquids and solids. Hence, diffusion occurs the fastest in gases. It is the slowest in solids. Diffusion through a Membrane Diffusion also occurs through a membrane, a thin barrier which has many holes which allow small molecules to pass through. There are two types of membranes: (a) Permeable membrane

- Allows all molecules to pass through it - E.g. cellophane bag

(b) Semi-permeable membrane

- Allows only small molecules to pass through it but not the large ones. - E.g. Visking tubing

Permeable membrane

Water

Copper sulphate particles

Potassium iodide particles

Copper sulphate and potassium iodide particles are evenly distributed throughout on both sides

Glucose molecules Starch molecules

Long glass tube

Water (molecules not shown)

Visking tubing

Water

Partially permeable membrane

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Diffusion in Living Cells Diffusion allows for substances to be exchanged between living cells and their environment. All living cells have semi-permeable membranes. For instance, when carbon dioxide is used up during photosynthesis, it’s concentration inside the plant cells decreases. The concentration of carbon dioxide molecules is then lower than that in the surrounding air. Hence, carbon dioxide in the air diffuses into the plant cell. As photosynthesis continues to occur, oxygen and glucose are then produced. The higher concentrations of these substances then cause them to be diffused out from the cell. Osmosis Osmosis is the net movement of water molecules through a semi-permeable membrane from a region of higher concentration to a region of lower concentration of water molecules. It occurs in a semi-permeable membrane as the membrane only allows the small water molecules to pass through but not the other larger molecules. In the process, the larger molecules are trapped behind the membrane. Osmosis occurs as long as the concentrations of the two solutions on both sides of the membrane differ. Difference between Diffusion and Osmosis Diffusion: - Involves the movement of any atoms or molecules.

- Occurs with or without a membrane. Osmosis: - Involves only the movement of water molecules.

- Occurs through a semi-permeable membrane. Osmosis in Living Cells Water enters or leaves a living cell through osmosis. Water moves from one cell to another when the concentrations of the dissolved solutes in the two cells differ. For instance, when water is used up during photosynthesis, the concentration of water molecules in the root hair cells of plants is lower than that in the surrounding soil. Therefore, osmosis occurs and water from the soil enters the root hairs. As the concentration of water molecules differ from cell to cell, osmosis will continue to occur within the plant, until water reaches the xylem.

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Transport in Living Things All living organisms need to absorb useful substances into their cells and remove waste products out of the cells. In small organisms, e.g. single-cell organisms such as the amoeba, such substances are transported to and from their bodies by simple diffusion since their bodies are in direct contact with the surroundings. In bigger, multi-cellular organisms, the body cells are further away from the surface of the body and it will take a very long time for substances to be transported over the long distance by diffusion. Thus, a transport system is required for the substances to be transported quickly and efficiently to all the cells in a multi-cellular organism.

Water molecules

Sugar Solution

Sugar molecules

Pure Water

Partially permeable membrane

Water

Partially permeable cell membrane Oxygen diffuses in

Carbon dioxide diffuses out

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Transport System in Plants The transport system of all flowering plants consists of two tube-like structures: the xylem and the phloem. The diagram below represents the transverse section of a plant stem. (a) Xylem

- made of dead cells. - carries water and dissolved minerals from the roots to the other parts of the plants.

(b) Phloem

- made of living cells. - carries glucose produced in the leaves to the other parts of the plant.

Absorption of Mineral Salts and Water (into Root Hairs) Water is absorbed into the root hairs by osmosis due to the higher concentration of water in the soil. It is then transferred from one cell to another by osmosis. Mineral salts are ‘sucked’ into the root hairs by the energy produced from respiration of plants since the mineral salts cannot diffuse into the root hairs due to the lower concentration of dissolved salts in the soil.

Phloem

Cambium

Xylem

Vascular bundle

Xylem (Transports water and mineral salts)

Phloem (Transports manufactured food)

Root cap

Growing zone

Root hair

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Transport of Water and Mineral Salts (to the Xylem and then throughout the Plant)) After water enters the root hairs, it is carried to the xylem vessels from cell to cell by osmosis. On the other hand, the mineral salts are transported to the xylem through the cells by diffusion. The water and mineral salts are then moved up the xylem by a pulling force, which is produced when water evaporates from the leaves through the stomata in the process of transpiration. The water is absorbed into the cells in the other parts of the plant while the dissolved minerals are ‘pumped’ into the cells. This process in which water and minerals are moved up the xylem and then into the other plant cells is called active transport. As active transport moves the water and mineral salts up the xylem and throughout the plant, the absorption of water and the ‘sucking’ of dissolved mineral salts continues at the root hairs and the above cycle repeats.

To stem

Xylem vessel

Root cell

Cell sap Soil particles

Root hair Water

(3) Water is lost from the leaves through transpiration.

(2) Water and mineral salts are transported up the stem.

(1) Water and mineral salts are taken up by the roots.

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Transport of Food (via the Phloem) Food produced in the leaves, in the form of glucose, is transported to other parts of the plant by the phloem through diffusion due to the higher concentration of glucose in the leaves (oxygen is also transported throughout the plant in the same way). The dissolved food in the tube moves both upwards and downwards.

Sugar moves up the phloem vessels to the rest of the plant.

Sugar is made in the leaves by photosynthesis.

Sugar moves down the phloem vessels into the roots.

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2SS Transport Notes B1 I