Biology AS AQA Unit 2 Exchange and Transport

Made these notes for my year 12 summer exam to revise and read over. There are spelling mistakes in most of my files but due to the busy exam schedule I had no time to correct them (sorry).

Most files have more information than what is needed but I feel it helps you feel more confident walking into the exam if you have a greater knowledge background and may help when having to apply knowledge to questions. Good luck :)

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  • Created by: Chelcie
  • Created on: 02-09-13 12:46
Preview of Biology AS AQA Unit 2 Exchange and Transport

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Exchange and Transport
Water moves into the roots by
Water moves into the stem
through a water potential
The root cells have a thin cell wall
to give a shorter diffusion
Water and nitrates are taken in.
Nitrates are converted into
amino acids which make up
Nitrates and phosphates are
dissolved in the water in the
Dicotyledonous plants have two seed leaves (cotyledons), broad leaves and flower.
(Dicotyledonous root)
Plants constantly lose water through transpiration.
Each root hair is a long, thin extension of a root epidermal cell.
Root cells are efficient surfaces for water and mineral ions because:
1. They provide a large surface area.
2. They have a thin surface layer.
The soil solution is generally mostly water, so has a high water potential,
Whereas the root hairs have a lower water potential (sugars, amino acids and mineral ions dissolved
inside them).
As a result, water moves by osmosis from the soil solution to the root hair cells (down the
concentration gradient).
Afterwards the water goes across the root by the apoplastic pathway or the symplastic pathway:
Apoplastic Pathway
It is the pathway water takes as it is cohesively pulled into the endodermal cells.
This creates a tension that causes water to be drawn along the cell walls of the root cortex cells.
There is little resistance to this as the cellulose cell walls of these cells have many water filled spaces.

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Symplastic Pathway
Takes place across cytoplasm of cortex cells due to osmosis.
Plasmodesmata are tiny openings in the cell wall in which water passes through.
The plasmodesma is filled with a thin strand of cytoplasm which causes a continuum of cytoplasm in a
column, from the root hair cell to the xylem at the centre of the root.
Water entering the cell by osmosis increases the water potential of the root hair cell.…read more

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Main force is transpiration (from evaporation)
Out through stomata:
If the humidity of the external air is lower than the internal air of the stomata, water vapour
molecules will diffuse outwards if the stomata are open. This causes the guard cells to become
flaccid, lose their rigidity and close.
Water is replaced after the evaporation of water from the cell walls of the mesophyll.…read more

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Xylem Tissue
Dead cells joined together to form long empty tubes.
Water moves in via pits.
Lignin is waterproof and strong.
Transports water and mineral salts.
Movement in xylem is due to root pressure or capillaries.
Capillary method is the upward movement of a fluid in a narrow bore tube (xylem has narrow
The Stomatal Mechanism
Open stomata are where guard cells are turgid due to an osmotic influx of water. Potassium ions
actively transport into guard cells causing water potential to fall.…read more

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Stomata are open in the light and closed in the dark as photosynthesis only occurs in the light (and
requires the rapid diffusion of CO)
When stomata are open, water moves out of the leaf into the atmosphere.
Consequently, an increase in light intensity causes an increase in transpiration.
As the temperature increase both the water potential of air and the speed at which molecules move
increase.…read more

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MESOPHYTE ­ Normal conditions.
HALOPHYTE ­ Salt marsh conditions (effect of osmosis)
Are plants which are adapted to living in areas where their water losses due to transpiration may
exceed their water uptake.
The thicker the cuticle, the less water can escape.
This protects the lower epidermis where most stomata are found.…read more

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Need to conserve water as it easily evaporates from body surface. Therefore they need
waterproof coverings (rigid exoskeleton covered by a waterproof cuticle) and small SA/V
ratio (minimise area in which water is lost).
Respiratory gases move in and out of tracheal system along a diffusion gradient...
When cells are respiring, oxygen is used and its concentration towards the end of the
tracheoles falls.
This causes a diffusion gradient which causes oxygen to diffuse from the atmosphere,
along trachea and tracheoles and to the cells.…read more

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Transport medium to transport dissolved chemicals.
Mass transport system.
Unidirectional flow e.g. valves are then used.
Closed tubular system.
Mechanism for moving transport medium is that we have muscles/heart or physical
processes. E.g. evaporation.
Control of flow of transport medium.
Arteries: carry blood away from the heart and into arterioles.
Arterioles: smaller arteries that control blood flow from arteries to capillaries.
Capillaries: Tiny vessels that link arteries to veins.…read more

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Muscle layer is thick compared to veins:
This means smaller arteries can be constricted and dilated in order to control the volume of blood
passing through them.
The elastic layer is relatively thick compared to veins:
It is important that pressure is high to reach the extremities of the body.
Recoiling action of the muscle layer helps maintain high pressure.
Smooth pressure surges created by the beating of the heart.…read more

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To squeeze the red blood cells against the cells at the side of the capillary.
This reduces distance of diffusion pathway hence speeding up diffusion.
They are numerously and highly branched
Provides a large SA for diffusion which provides a fast rate of exchange (of substances).
Smooth inner surface
To reduce friction and smooth flow.
They have a narrow diameter and hence permeate tissues
This means that no cell is far from a capillary to exchange chemicals between blood and cells.…read more


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