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CET - Plant Transpo
OCR A level Biology F211 Transport in Plants
| Question | Answer |
|---|---|
| Why do plants need transport systems? | To get substances like water, minerals and sugar and to excrete waste substances. Multicellular; have a small SA:V ratio. Direct diffusion from outer surface to inside cells would be too slow. |
| What is the difference between xylem and phloem? | Xylem tissue transports water and mineral ions; phloem transports dissolved substances like sugars. Found throughout a plant and transport materials to all parts. Xylem also supports plant. |
| Where are xylem and phloem in the root? | In the centre to provide support for the root as it pushes through the soil. |
| Where are xylem and phloem in stems? | Near the outside to provide a sort of scaffolding that reduces bending. |
| Where are xylem and phloem in leaves? | Make up a network of veins which support the thin leaves. |
| What are xylem vessels formed from? | Long, tube like structures formed from cells (vessel elements) joined end to end. |
| How is xylem adapted for function? | No end walls, uninterrupted water flow. Dead cells so contain no cytoplasm. Lignified walls help support vessels and stop them from collapsing inwards. Lignin increases with age. Water and ions move through unlignified pores. |
| What does phloem tissue do? | Transport solutes, mainly sugars like sucrose, around plants. Purely transport – no support. |
| What is phloem formed from? | Formed from cells arranged in tubes. Contain phloem fibres, phloem parenchyma, sieve tube elements and companion cells. |
| Describe sieve tube elements. | Living cells, form tube for solute transport. Joined end to end to form sieve tubes. Sieve plates on end walls, holes allow solutes through. No nucleus, are connected to companion cells, thin layer of cytoplasm and a few organelles. |
| Describe companion cells? | Necessary because sieve tube elements cannot survive alone without a nucleus, so need companion cells. Carry out functions for themselves and their sieve cells, provide energy for things like active transport of solutes. |
| How does water enter a plant? | Enters the plant through root hair cells, and then passes through the root cortex including the endodermis, to reach the xylem. It is drawn into the roots down a water potential gradient. |
| How does water move from the roots to the xylem? | Water can move from the roots (vie the root cortex) into the xylem by two different paths: |
| Describe the symplast pathway? | Goes through the living parts of cells – the cytoplasm. The cytoplasm of neighbouring cells are connected by plasmodesmata (small gaps in the cell wall). |
| Describe the apoplast pathway? | Goes through non-living parts of cells – the cell walls, as they are very absorbent so water can simply pass through them, as well as passing through the spaces between them. |
| What happens when water in the apoplast pathway reaches the endodermis cells in the root? | Blocked by the waxy Casparian strip in cell walls. Water has to take the symplast pathway. Useful - water has to pass through cell wall, controls what substances can get through. |
| What happens when the water gets past the barrier? | Once past this barrier, water moves into the xylem. |
| Which is the main pathway used? | Apoplast – it provides the least resistance. |
| What happens at the leaves? | Water leaves xylem, moves into cells, mostly via apoplast. Evaporates through cell walls into spaces between cells. When stomata open, water moves down water potential gradient into surrounding air – transpiration. |
| How does water move up a plant, against gravity? | Transpiration occurs at top of the plant, creating tension, pulling more water up xylem. Cohesion of water molecules means whole column pulled upwards (adherence also helps). Water enters stem through root cortex cells. |
| What is transpiration? | Inevitable consequence of gas exchange. Plants open their stomata to let in carbon dioxide for photosynthesis. Lets water vapour out down water vapour potential gradient. |
| What are the main factors which affect transpiration rate? | Light, temperature, humidity, wind. |
| How are xerophytic plants adapted to reduce water loss? | Reduced stomata, and sunk in pits to shelter from the wind. Layer of hairs traps moist air around stomata. Curled leaves trap moist air. Spines instead of leaves reduce surface area for water loss. Thick waxy layer on epidermis. |
| What is translocation? | Movement of dissolved substances to where they are needed in the plants. These dissolved substances are assimilates, i.e. they are made by the plant. Requires energy, and happens in the phloem. Translocation moves substances from sources to sinks. |
| SOURCE. | Where substance is made; substance is at a high concentration. |
| SINK. | Where a substance is used up; substance is at a lower concentration. |
| How do enzymes maintain a concentration gradient from the source to the sink? | Changing the dissolved substances at the sink by breaking them down or converting them into something else. For example, in a potato glucose is converted into starch at the potato, the sink. |
| How do dissolved solutes get into sieve tubes of phloem at source? | Actively loaded by active transport. Lowers the water potential in the sieve tubes, so water enters the sieve tubes by osmosis. |
| What does this water entering sieve tubes do? | Creates high pressure inside sieve tubes at source end of the phloem. |
| What happens at sink end? | Solutes removed from the phloem and used up. Increases water potential in sieve tube, so water leaves tube by osmosis, lowering pressure inside sieve tubes. |
| What does the pressure gradient between source and sink end cause? | Solutes are pushed along sieve tube to where they are needed. |
| What evidence do we have to support mass flow? | Remove ring of bark, bulge forms above it. Aphids pierce phloem, sap flows out faster near leaves than further down stem. Metabolic inhibitor stops ATP production and translocation. Experimental model for mass flow. |
| What are the objections to mass flow? | Sugar travels to many different sinks, not just the one with the highest water potential as model suggests. Sieve plate would create a barrier to mass flow - a lot of pressure would be needed for solutes to get through at reasonable rate. |
Created by:
emm142
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