Photosynthesis and transport in plants

Photosynthesis step by step

**Photosynthesis** (photo = light, synthesis = making) is how green plants make their own food. This is **autotrophic nutrition** (auto = self, trophos = food). It takes place in the **chloroplasts** of leaf cells. **Chlorophyll**, the green pigment inside chloroplasts, traps light.

1. 1
   Water and minerals reach the leaf from the roots through the xylem.
2. 2
   Carbon dioxide from the air enters the leaf through tiny pores called stomata.
3. 3
   Light reaction: chlorophyll in the chloroplast captures the energy of sunlight.

   This is how the sun's energy becomes chemical energy, stored in glucose. Your NIOS lesson says energy is released in photosynthesis; in fact it is stored here, and released later in respiration
4. 4
   Dark reaction: with the help of enzymes, carbon dioxide and water combine to form glucose.

   It does not need darkness: both reactions go on at the same time in the chloroplast
5. 5
   The cells use some glucose at once. The rest is changed into starch and stored.
6. 6
   Oxygen is formed as a by-product. It diffuses out into the air through the stomata.
7. 7★
   Overall: 6CO₂ + 12H₂O → C₆H₁₂O₆ + 6H₂O + 6O₂

   Write sunlight and chlorophyll over the arrow

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Why photosynthesis matters

• It makes the food for all living things. Animals eat plants, or eat animals that ate plants.
• It uses up the carbon dioxide that living things give out in respiration, so CO₂ does not pile up in the air.
• It gives out oxygen, which living things use for respiration.
• So autotrophs are called the producers: every food chain starts with them.

Xylem vs phloem: how water and food move

Transpiration pull: water evaporates as vapour from the stomata (this is transpiration). The loss creates a suction that pulls water up the xylem, like sucking a drink up a straw.

Activity: starch is made only in leaves kept in sunlight (practical)

1. 1
   Keep one potted plant in the dark for two days. This removes its stored starch (destarching). Keep a similar plant in sunlight for a few hours.
2. 2
   Pluck a leaf from the plant in sunlight. Dip it in boiling water for 1 to 2 minutes to soften it.
3. 3
   Put the leaf in a test tube of ethanol (alcohol). Stand the tube in the hot water: this is a water bath.

   Never heat ethanol over a flame: it catches fire easily
4. 4
   The ethanol removes the chlorophyll, so the leaf turns pale. The green colour would hide the result.
5. 5
   Dip the leaf in hot water again to soften it. Spread it on a white tile and cover it with iodine solution.
6. 6
   Repeat steps 2 to 5 with a leaf from the plant kept in the dark.
7. 7★
   Observe: the leaf from sunlight turns blue-black; the leaf from the dark does not. Conclusion: starch is made only in sunlight.

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Activity: oxygen is given out in photosynthesis (practical)

1. 1
   Fill a beaker with water and add a pinch of baking soda. It gives the plant extra carbon dioxide.
2. 2
   Tie a few twigs of a water plant, such as Hydrilla, together at the cut ends.
3. 3
   Put the twigs under an upside-down funnel in the beaker, cut ends towards the funnel's neck. Keep the funnel fully under water.
4. 4
   Fill a test tube with water to the brim. Close it with your thumb and turn it upside down over the funnel's stem, with no air bubbles.
5. 5
   Keep the set-up in sunlight, or under a table lamp, for 30 to 40 minutes. Count the bubbles rising each minute.
6. 6
   A gas collects at the top of the tube. Lift the tube out with your thumb over it. Bring a glowing matchstick near its mouth.
7. 7★
   Observe: the matchstick burns more brightly. Conclusion: the gas is oxygen, given out in photosynthesis.

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Activity: osmosis through a membrane (practical)

**Osmosis** is the movement of water through a **semipermeable membrane** (a thin layer that lets water pass but not dissolved sugar or salt). Water moves from where there is more water to where there is less. Root hairs take in water from the soil in this way.

1. 1
   Peel a raw potato. Cut its bottom flat so it stands up.
2. 2
   Scoop out a pit in the top centre. Make a hollow, not a hole through it.
3. 3
   Stand the potato in a dish of water, about half under water.
4. 4
   Half fill the pit with a strong sugar solution. Mark the level with a pen.
5. 5
   Leave it for about one hour, then look at the level again.

   With a thistle funnel, tie cellophane paper over its mouth: it works as the membrane
6. 6★
   Observe: the level in the pit rises. Reason: water moved from the dish, through the living potato cells, into the sugar solution. The cells act as a semipermeable membrane.

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