Unit 7 Essay

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AP Biology Unit 7 Study Guide

Ch 36 – Transport in Plants p. 748- 761
Ch 38 – Flowering plants, p. 783-4, 788- 94
Ch 39 – Plant responses to internal & external signals
Ch 40 – Animal Form and Function p. 839-47, Ch 44 p. 925-35
Ch 45 – The endocrine system p. 955- 72 Ch 36 – Transport in Plants p. 748- 761
1. Describe how is water transported across plant cells, addressing the following: selective permeability, proton pumps, water potential, aquaporins
Selective permeability – allow specific solutes to cross the membrane. Some channels are gated and open/close in response to certain stimuli
Proton pumps – use ATP to pump hydrogen ions out of the cell, generating an energy storing proton gradient and a membrane potential due to separation of charges. The gradient/potential is used by the plant cell to drive the transport of many solutes
Water potential – predicts the direction water will move when a plant cell is surrounded by a particular solution by taking into account solute concentration and physical pressure
Aquaporins – proteins in membrane which increase rate of water diffusion

2. Describe how is water & mineral absorption by roots enhanced by: root hairs, mycorrhizae, cortical cells
Root hairs ­– extensions of epidermal cells and account for much of the surface area of roots. Soil particles adhere tightly to the hairs as the soil solution flows into its hydrophilic walls and passes freely along the apoplast into the root cortex
Mycorrhizae – symbiotic structures consisting of plant’s roots united with hyphae of the fungi. The hyphae absorbs water and selected minerals, transferring much of these resources to the host plant

3. Describe the symbiotic relationship between mycorrhizae and plants
Mycorrhizae are symbiotic structures between the plant’s roots and the hyphae (filaments) of fungi. The hyphae absorbs water and selected materials, which are transferred to the host plant. The mycelium (network of hyphae) provide the mycorrhizae with an enormous surface area for absorption

4. Distinguish between xylem and phloem
Xylem conducts most of the water and minerals; xylem is made of tube-shaped cells (tracheids) strengthened with the polymer lignin
Cells of phloem distribute sugars, amino acids, and other organic products

5. Define transpiration, and describe how transpiration is the force pulling xylem sap upwards
Definition – loss of water vapor from leaves
As minerals are pumped into the stele and prevented from leaking out by the endodermis, water potential in the stele is lowered. Water flows in from the cortex by osmosis, resulting in root pressure, which forces fluid up the xylem

6. Describe the transpiration-cohesion-tension mechanism and distinguish how the various properties of water interact
Water vapor diffuses from the moist air spaces of the leaf to the drier air outside via stomata
Evaporation from the water film coating the mesophyll cells maintains the high humidity of the air spaces
The loss of water causes the water film to form menisci that become more and more concave as the rate of transpiration increases
A meniscus has a tension that is inversely proportional to the radios of the curved water surface, so as the water film recedes and its menisci become more concave, the tension of the water film increases
Tension is a negative pressure, a form that pulls water from locations where hydrostatic pressure is greater so the tension of water lining the air spaces of the leaf is the physical basis of transpirational pull, which draws water out of the xylem
Cohesion of water makes it possible to pull a column of sap from above without the water separating
Adhesion of water molecules by hydrogen bonds to the hydrophilic walls of the xylem cells makes it possible to fight gravity

7. Describe the role, structure and function of stomata, how they open and close, and their responses to environmental stresses
Role, structure, function – stoma allow gas exchange between the surrounding air and
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