Transport Over Larger Distances: Plants and Photosynthesis — AQA Combined Science: Synergy
This topic covers plant tissues, transport in plants, photosynthesis and the factors that affect it.
Plant structures and meristem tissue
Plant organs include roots, stems and leaves. Key tissues:
- Epidermis — protective covering.
- Palisade mesophyll — packed with chloroplasts for photosynthesis.
- Spongy mesophyll — air spaces for gas exchange.
- Xylem — carries water and minerals up from the roots.
- Phloem — carries dissolved sugars around the plant.
Meristem tissue (at the tips of roots and shoots) contains plant stem cells that can divide and differentiate throughout the plant's life.
Transpiration
Transpiration is the loss of water vapour from the leaves through stomata (pores controlled by guard cells). It pulls water up from the roots in the transpiration stream. The rate increases with higher temperature, more air movement, lower humidity and brighter light.
Translocation
Translocation is the movement of dissolved sugars (made in photosynthesis) through the phloem to where they are needed for respiration or storage.
Chlorophyll and plant pigments
Chlorophyll, found in chloroplasts, absorbs light for photosynthesis. Plants also contain other pigments. Chromatography can separate plant pigments.
Photosynthesis
Photosynthesis is endothermic — it transfers energy from light to the plant:
carbon dioxide + water → glucose + oxygen $$6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$$
The glucose is used for respiration, converted to starch for storage, used to make cellulose, lipids, and combined with nitrate ions to make amino acids.
Factors affecting the rate of photosynthesis
- Light intensity — more light, faster rate (until another factor limits it).
- Carbon dioxide concentration — more CO₂, faster rate.
- Temperature — increases rate up to an optimum, then enzymes denature.
A limiting factor is the one in shortest supply that holds back the rate. Light intensity follows the inverse square law (intensity ∝ 1/distance²).
Required practical: investigating the effect of light intensity on the rate of photosynthesis using pondweed.
Growers control these factors (light, CO₂, temperature) in greenhouses to increase yield, balancing cost against profit.
Plant diseases
Plants can be affected by diseases such as tobacco mosaic virus (mosaic leaf pattern) and rose black spot (fungal). Diseased plants photosynthesise less. Plants have physical, chemical and mechanical defences, and signs of disease include stunted growth, spots, discolouration and rot.
Exam tips
- Distinguish xylem (water, upward, transpiration) from phloem (sugars, translocation).
- Learn the photosynthesis equation and that it is endothermic.
- For limiting-factor graphs, explain which factor is limiting at each part.
- Apply the inverse square law to light intensity and distance.