Photosynthesis
Photosynthesis is the process by which plant gain biomass, and therefore photosynthetic organisms are the main produces of food. It is an endothermic reaction where energy transferred by sunlight reaches the chloroplasts, and carbon dioxide and water are used to make glucose and oxygen. The glucose molecules form the polymer of starch, and in the cytoplasm, it is broken into sucrose, the form in which it travels through the plant.
Carbon Dioxide for photosynthesis comes from air. Leaves contain pores called stomata, which allow carbon dioxide to diffuse into the leaf. During the day, waters contain much water and turgid pressure causes the stoma to open. At night, the guard cells contain cell water and so the stoma is closed, ensuring that carbon dioxide does not escape.
The leaf is also adapted for photosynthesis in other ways.
- Waxy cuticle reduces water loss by evaporation and is a barrier to pathogens
- Upper Epidermis is relatively transparent to allow light to pass to the chloroplasts below
- Palisade Cells are made of elongated cells with many chloroplasts to maximise photosynthetic rate; close to surface
- Spongy cells are irregularly shaped with air spaces allowing for gas exchange between photosynthesising cells and stomata
- Guard Cells open and close stomata based on conditions
There are three limiting factors which affect the rate of photosynthesis:
- Light Intensity: as light intensity increases, the chloroplasts absorb more energy, increasing the rate of reaction. However, eventually other factors like CO2 concentration or temperature would be the limiting factor when it reaches maximum rate of photosynthesis
- CO2 Concentration: higher concentration means that the reaction will occur more frequently to produce more glucose to use up the reactant. Again, eventually other factors will be the limiting factor when it reaches maximum rate of photosynthesis
- Temperature: as a reaction, catalysed by reactions, the rate of reaction increases as temperature increases as there is an increased frequency of enzymes and substrates colliding.
- However, once it has reached optimum temperature, enzymes become denatured and they lose their shape, decreasing the rate of photosynthesis
Light intensity can be calculating to the inverse square law, as it is inversely proportional to the distance squared.
CORE PRACTICAL: Light Intensity and Photosynthesis
A – Add 20 algal balls to clear glass bottles
B – Add the same amount of indicator to each bottle and replace cap
C – Compare colour of tubes to pH chart to work out the starting pH
D – Set up a tank of water between the lamp and the tubes
E – Place one bottle in the dark
F – Measure distances from the lamp and place bottles at set intervals. Turn on lamp and wait for obvious colour changes in the bottles
G – Compare with pH chart to see change and calculate change in pH/hour
Absorption and Transportation of Substances
Water is needed to carry mineral ions, to maintain turgidity, to cool leaves and for photosynthesis. It enters the root hair call by diffusion or osmosis. There are many root hair cells and the extensions increase the surface area to volume ratio, meaning substances are absorbed quickly.
Water can enter the root by diffusion through cell walls or through osmosis as it passes through the semi-permeable membranes between cytoplasm of cells. Mineral Ions are needed to make substances, and since the concentration is greater in the root than in the soil, carrier proteins are needed to pump the ions into the cell by active transport.
Transpiration
- The flow of water into the root, up the stem and out of the leaves is called transpiration
- Xylem vessels form tiny pipes leading through the plant. The vessels are made of dead cells with no cytoplasm, giving space for water to flow through. The tiny pores allow water and ions to enter and leave the vessel. The lack of cell walls between cells means rate of transpiration is not slowed and the rings of lignin and thick walls ensure that is does not burst or collapse under high water pressure
- Water molecules have weak forces of attraction due to slight electric charge which causes hydrogen bonds to form between molecules
- As a result, an unbroken chain of water in the xylem, pulled up as water evaporates out of the leaf
- There are factors that affect the rate of transpiration. Wind blows water molecules from the stomata, low humidity increases the concentration gradient between the air and the stomata, higher temperature increases the energy of the water particles and greater light intensity means that the stomata widen. All of these factors increase transpiration as if more water is lost, water is pulled through the xylem in a shorter amount of time
- Transpiration rates can be measured using a potometer. A cutting is placed in the top of a capillary tube and sealed in with a rubber stopper. Water below fills a capillary tube. If an air bubble is introduced, the speed of its movement can be measured as this is the rate of water movement through the plant
Translocation
- Sugar is transported as sucrose in the phloem tissue, made of a sieve tube and a companion cell, by translocation
- Sucrose moves to the companion cells from leaves where is was produced during photosynthesis
- It then enters the sieve tube by active transport. The companion cell is adapted to this role as it has many mitochondria for active transport, and the pores allow for movement of substances
- In the phloem tissue, there is little cytoplasm allowing for more room for the central channel. Holes in the end of the cell walls allows for liquids to flow from one sieve cell to the next
- In the sieve tube, sugars can move in either direction, caused by pressure from companion cells moving substances in and out
- If it is moving towards the roots, it is stored in the root tissue as starch
Adaptations of Plants
In winter, many deciduous plants lose their leaves, preventing water loos when soil water is frozen. Conifers have needle-shaped leaves with a thick cuticle and a small surface area, creating less wind resistance and lose little water.
Plants can be adapted to warm climates in the following ways:
- Spines reduce surface area for water loss by transpiration or herbivores
- Trichomes reduce air flow near the leaf and trap water near the surface, reducing diffusion from the leaf to the air
- Waxy Cuticle reduces water loss by transpiration
- Stem can store water
- Stomata only open at night. CO2 is taken in during the day and stored for use at night
- Sunken Stomata reduces airflow and loss of water
Plant Hormones
A stimulus is a change in the environment that causes a response by an organism.
A tropism is responding to a stimulus by growing towards or away from it. Positive Phototropism is growing towards a light source to gain energy for photosynthesis to increase biomass. Auxins are produced in the tips of a shoot, where they cause elongation of cells. If a shoot is grown with light
coming from only one direction, auxins move downwards to the shaded side of the shoot. This makes the cells on the shaded side elongate more, which in turn causes the shoot to grow towards the light.
Auxins are also produced in root tips, where they have the opposite effect in order to cause positive gravitropism – growth in the direction of gravity, in order to anchor the plant and to reach moisture and nutrients underground. Auxins are pulled downwards by gravity and inhibit cell elongation. Cells on the other side continue growing downwards into the soil.
Gibberellins help seeds to germinate and start to grow shoots and roots. Ethene gas helps fruit to ripen.
| Hormone | What does it do? | What is its industrial use? |
| Auxin | Selective auxins kill only plants with broad leaves and not those with narrow leaves | Selective weed killers leave the crop e.g. wheat and only kill the weeds e.g. dandelions |
| Auxins promote root growth | Cuttings are placed in auxin rooting powders which encourages root growth | |
| Gibberellin | Involved in germination initiation | Growers can control germination rather than relying on photoperiodism |
| Causes some plants to produce fruit without seeds after pollination, and increases fruit size | Produces seedless or bigger fruit | |
| Ethene | Ripens fruit | Unripe fruit can be picked and ripened when necessary using ethene, so that it can be sold at any time of year |