OCR AS Level Biology: Transport in Plants

Reasons why plants need transport system

Size, metabolic rate, surface area to volume ratio

Why size affects a plant's need for a transport system

Large plants need to move substances up and down the entire length

Why metabolic demand affects a plant's need for a transport system

Internal and underground plant parts need oxygen and glucose to get to them, hormones need transporting to where they have an effect, mineral ions need transporting to make proteins for enzymes

Why surface area to volume ratio affects a plant's need for a transport system

Plants have a low surface area to volume ratio so diffusion is not enough to supply the plant cells with everything they need

Arrangement of vascular bundles in the stem

Around the edge for strength and support

Arrangement of vascular bundles in the root

In the middle to help withstand the tugging strains from the wind

Arrangement of vascular bundles in the leaf

Midrib of a leaf supports the structure of the leaf

Dicotyledonous plants

Plants that make seeds that contain two cotyledons

Cotyledons

Organs that act as food stores for developing embryos

Types of dicots

Herbaceous, woody

Structure of the xylem

Long hollow structures made of columns of cells fused together end to end, thick-walled parenchyma around the xylem vessels, lignified secondary walls, bordered pits

Role of thick walled parenchyma

To store food, to store tannins

Role of lignin in xylem

To provide mechanical strength

Arrangement of lignin in the xylem

Rings, spirals, solid tubes

Role of pits in the xylem

To be where the water leaves the xylem for other cells in the plant

Function of the xylem

To transport water and mineral ions, to support the plant

Structure of sieve tube elements

Many cells joined end to end to form a hollow structure, not lignified, sieve plates

Function of sieve tube elements

Main transporting vessels of organic solutes

Function of sieve plates

To let phloem contents flow through

Why mature phloem cells have no nucleus

Large pores appear in the cell walls, tonoplast and nucleus and other organelles break down, phloem fills with phloem sap

Structure of companion cells

Linked to sieve tube elements by plasmodesmata, nucleus and organelles present

Function of companion cells

To act as the life support system for the sieve tube cells

Structure of the phloem

Sieve tube elements, companion cells, fibres, sclereids

Sclereids

Cells with very thick cell walls

How to dissect stems to observe xylem

Put material in water containing a strongly coloured dye for 24 hours, rinse it, make clean transverse cut with a sharp blade on a white tile, xylem show up as spots, make a clean longitudinal cut, xylem show up as coloured lines

Limitations of dissection of stem to observe vascular bundles

Can't be adjusted to see phloem, dependent on sharp blade and steady hand, if they aren't cut in the right place you won't see any xylem

Process of transpiration

Water evaporates from the surface of mesophyll cells into air spaces in the leaf and moves out of the stomata by diffusion, evaporation lowers water potential of the cell, water moves into cell by osmosis through apoplast and symplast pathways, repeated across the leaf to the xylem, water moves out of xylem by osmosis, water molecules form hydrogen bonds with each other resulting in cohesive forces causing capillary action, water drawn up the xylem to replace water lost by evaporation by the transpiration pull, transpiration pull causes tension in xylem

Theory related to transpiration

Cohesion-tension theory

Capillary action

Water moving up a narrow tube against the force of gravity

What is transpiration an inevitable consequence of?

Gaseous exchange for photosynthesis

Evidence for the cohesion-tension theory

Trees shrink in diameter when transpiration is at its highest because of the higher tension in the xylem, broken xylem vessels take up air rather than letting water out, broken xylem vessels can't move water because the continuous stream has broken

How to measure transpiration rate

Potometer

Why is it difficult to measure transpiration directly?

Hard to condense and collect all water that evaporates from leaves without collecting water from the soil, hard to separate water from transpiration and water vapour from respiration

Precautions when setting up a potometer

All joints sealed with waterproof jelly, airtight, calibrated, cut the shoot at a slant, set up underwater

Which wall of the guard cell is more flexible?

Outer layer

Factors which will affect the rate of transpiration

Light intensity, relative humidity, temperature, air movement, soil-water availability

How does light intensity affect the rate of transpiration?

Increased light intensity opens more stomata, increases evaporation from the surfaces of the leaf

How does relative humidity affect the rate of transpiration?

High relative humidity will lower the rate of transpiration, reduced water potential gradient

How does temperature affect the rate of transpiration?

Increased temperature increases the kinetic energy of water molecules and increases rate of evaporation, increased temperature increases concentration of water vapour that the external air can hold

How does air movement affect the rate of transpiration?

Air movement blows away diffusion shells, increases water potential gradient

How does soil-water availability affect the rate of transpiration?

Dryness will put the plant under water stress and the rate of transpiration will be reduced

Things water is used for in plants

Turgor pressure, cell expansion due to turgor, cooling by evaporation, transport medium, photosynthesis

Adaptations of root hair cells

Microscopic size so can penetrate between soil particles, high surface area to volume ratio, thin surface layer, high concentration of solutes in the cytoplasm of root hair cells

Names of water pathways through the root

Symplast, apoplast, vacuolar

Symplast pathway

Water moves though continuous cytoplasm of plant cells through plasmodesmata by osmosis

Apoplast pathway

Water moves through the cell walls and intercellular spaces between cellulose fibres, water entering xylem pulls a thread of water through the cell walls through cohesive forces

Vacuolar pathway

Water moves through the vacuoles

How water moves into the xylem

Reaches endodermis and Casparian strip, apoplast pathway diverges with symplast pathway, water passes through selectively permeable membranes, water potential in xylem is lower than that of endodermal cells, returns to apoplast pathway, root pressure results in water being pushed up xylem

Why it is good that water must move through selectively permeable membranes before reaching the xylem

Removes toxic solutes because of lack of transport proteins for them

Thing endodermal cells do to maintain water potential gradients

Pump mineral ions into the xylem by active transport

Evidence for the role of active transport in root pressure

Cyanide causes root pressure to disappear, root pressure increases with a rise in temperature which suggests chemical reactions are involved, low levels of oxygen or glucose decreases root pressure, guttation

Guttation

The forcing of xylem sap out of the ends of cut stems or from special pores at the ends of leaves

Xerophytes

Plants that have adapted to be able to live and reproduce in places where there is little water availability

Adaptations of xerophytes

Thick waxy cuticle, sunken stomata, fewer stomata, reduced leaf area, hairy leaves, curled leaves, succulents, leaf loss, long roots, shallow roots with large surface area, dormancy, disaccharide trehalose

How a thick waxy cuticle reduces water loss

Prevents water loss through the cuticle

How sunken stomata reduce water loss

Reduces air movement, reduce water potential gradient

How fewer stomata reduces water loss

Reduce water loss by transpiration

How reduced leaf surface area reduces water loss

Reduced surface area to volume ratio

How hairy leaves reduce water loss

Trap water lost by transpiration, reduce water potential gradient

How curled leaves reduce water loss

Confines stomata to microclimates

How succulents are good for xerophytes

Water stored in specialised parenchyma tissue

How losing leaves reduces water loss

Water can't be lost through the leaves

How long roots reduce water loss

Allow them to reach water that is further down

How shallow roots with a large surface area work for xerophytes

Can absorb any available water before a rain shower evaporates

Adaptations of marram grass

Vertical and horizontal roots, stomatal pits, hairs, curled leaves

Adaptations of cacti

Vertical and horizontal roots, sunken stomata, reduced leaves, succulent

Hydrophytes

Plants that live in water and need adaptations to cope with growing in water

Why hydrophytes need to get rid of water

Need to float to get to light, air spaces need to be full of air

Adaptations of hydrophytes

Very thin or no waxy cuticle, lots of stomata, stomata always open, reduced structure, wide flat leaves, small roots, large surface area of stems and roots under the water, air sacs, aerenchyma, pneumatophores

How having a very thin or no waxy cuticle will help a hydrophyte

Water can be lost through the cuticle

Why can hydrophytes have their stomata open all the time?

No worries about loss of turgor

How wide, flat leaves help hydrophytes

Can capture more light

How small roots help hydrophytes

Water can diffuse directly into stem and leaf tissue

How having large surface areas of stem and root under water helps hydrophytes

Maximises the area for photosynthesis, maximises area for oxygen diffusion

How having air sacs helps hydrophytes

Enables leaves to float to the surface of the water

How aerenchyma helps hydrophytes

Makes leaves and stems more buoyant, low resistance internal pathway for movement of substances

How pneumatophores help hydrophytes

Roots that grow upwards to get to the air

Adaptations of water lilies

Wide flat leaves, stomata on the upside of the leaf, flexible stems

Main substance transported by translocation

Sucrose

Example of a source

Leaves

Example of sinks

Roots, meristems

Translocation

An energy requiring process that transports assimilates in the phloem between sources and sinks

Assimilates

Transported products of photosynthesis

Process of translocation

Hydrogen ions pumped out of companion cell using ATP, hydrogen ions return to companion cell via cotransport protein, sucrose cotransported, sucrose diffuses out of companion cell through plasmodesmata, water moves in by osmosis, carries assimilates, moves to areas of low pressure

Adaptations of companion cells

Infoldings in cell membranes to increase surface area for active transport, lots of mitochondria

How water and solutes are transported through a plant

Solute accumulation in phloem increases turgor pressure to force sap to regions of lower pressure, pressure differences transport the stuff

How the phloem is unloaded

Diffusion of sucrose into surrounding cells, sucrose moves rapidly to maintain concentration gradient, loss of solute in phloem increases water potential of phloem, water moves into surrounding cells by osmosis, surrounding cells can be xylem

Evidence for process of translocation

Microscopes show adaptations of companion cells, poisoned mitochondria stop translocation, flow is 10000 times faster than it would be if just done by diffusion, aphid's stylet show there is a positive pressure

Questions about translocation

Not all solutes move at same rate, role of sieve plates

What do potometers measure?

Water uptake

Why may potometer measurements not be representative of the rate of transpiration?

Not all of water uptaken is lost, uptake by a detached shoot is not the same as a whole plant