Transport in plants

Why do multicellular plants need transport systems

Metabolic demands, size and surface area to volume ratio

Metabolic demands

Nutrients obtain in one part of the plant need to be transported to other cells.

Surface area : volume ratio

Multicellular plants have a small SA:V ratio so they can’t rely on diffusion alone for transport.

Vascular system structure in the stem

Vascular bundles are found around the edge to give strength and support.

Vascular system structure in the roots

Vascular bundles are in the middle to help the plant withstand tugging strains.

Vascular system structure in dicot leaves

The midrib is the main vein carrying the vascular tissue and helps to support the structure of the leaf.

Xylem is always

Inside the phloem

Structure of xylem vessels

Long, hollow structures made by several columns of cells fusing together.

Xylem parenchyma

Thick walled cells pack around the xylem vessels, storing food and containing tannin deposits (chemical protection from predators).

Xylem fibres

Long cells with lignified secondary walls that provide extra mechanical strength.

Phloem structure

Living tissue with no organelles, containing phloem sap that transports organic solutes around the plant (up and down).

Sieve tube elements

The main transporting vessels of the phloem

Sieve plates

In the areas between cells, the walls become perforated to form sieve plates

Companion cells

Linked with sieve tube elements by plasmodesmata to perform cell functions.

Meristematic tissue

It is located between the the phloem and xylem tissues and produces stem cells for vascular growth.

Palisade cell specialisation

Contain many chloroplasts, rectangular, thin cell walls, large vacuole

Root hair cell specialisation

Increase surface area of the cell, thin cell wall, vacuole containing ions and sugars

Guard cell specialisation

Thick inner cell wall

Ribosomes

The site of protein synthesis

Rough endoplasmic reticulum

A network of membranes attached to the SER with ribosomes bound to the surface. It is responsible for the synthesis and transport of proteins.

Smooth endoplasmic reticulum

A network of membranes attached to the nucleus, not containing ribosomes. It is responsible for lipid and carbohydrates synthesis and storage.

Golgi apparatus

A structure formed of cisternae which puts proteins into vesicles (lysosomes or secretory)

Lysosome

A specialised vesicle that contains digestive enzymes to break down waste materials.

Mitochondria

Contain a double membrane with the inner membrane folded and containing enzymes to perform aerobic respiration.

Chloroplasts

They have a double membrane with the inner membrane forming a granum which contains chlorophyll for photosynthesis.

Vacuole

Membrane lines sacs containing cell sap to maintain cell turgor.

Peroxisome

Contains enzymes to break down hydrogen peroxide (protection against bacteria).

Nuclear envelope

A double membrane containing the DNA in the nucleus.

Magnification

The number of times bigger an image is than the actual object.

Resolution

The ability to distinguish between two points that are close together.

What is the difference in resolution between light and electron microscopes?

Electron microscopes have a higher resolution, as they used beams of electrons with short wavelengths. This makes it easier to distinguish between different structures.

Light microscopes (8)

Inexpensive, small, simple sample prep, no vacuum, colour, up to 2000x magnification, resolving power is 200nm, samples can be living

Transmission electron microscope

The beam is transmitted through the specimen - best resolution

Scanning electron microscope

The beam is sent across the surface and reflected electrons are collected - resolution is worse

Hydrogen bonding

Polar molecules (water) interact with each other as the positive and negative regions attract each other and form weak bonds.

High boiling point of water

It provides a constant environment for aquatic animals

Solid water is less dense than liquid

Ice floats, forming an insulating layer so that habitats don’t freeze.

Waters cohesive/adhesive properties

It is an efficient transport medium within living things because molecules stick together.

Water acts as a solvent

It is polar so can carry polar molecules dissolved in it and acts as a medium for chemical reactions (cytosol).

Water acts as a coolant

Maintains constant temperatures in cellular environments for enzyme activity.

Water absorption

Roots absorb water from the soil through root hairs with a high water potential by osmosis.

Movement up the stem

Water travels up the stem through xylem vessels by capillary action

Transpiration pull

As water evaporates from the leaves, it creates a suction force that pulls more water up the xylem.

Evaporation

Water reaches the leaves, where it evaporates from a mesophyll cell travels through air spaces and out of the stomata.

Apoplastic pathway

Water moves through cell walls and the spaces between cells - stops at the casparian strip.

Symplastic pathway

Water moves through the cytoplasm of cells through plasmodesmata.

Light as a limiting factor

Increasing light intensity gives increasing numbers of open stomata, increasing the rate of water vapour diffusing out.

Relative humidity as a limiting factor

A high relative humidity will lower the rate of transpiration because of the reduced water vapour potential gradient between the leaf and air.

Temperature as a limiting factor

Increase in kinetic energy of water molecules, increases rate of evaporation.

Air movement as a limiting factor

Wind increases the rate of transpiration because water vapour potential around stomata decreases, increasing diffusion gradient.

Soil-water availability

If it is very dry the plant will be under water stress so will close its stomata and the rate of transpiration will decrease.

Hydrophytes

Plants with adaptations that enable them to survive in wet habitats.

Xerophytes

Plants with adaptations that enable them to survive in dry habitats.

Hydrophyte adaptations (5)

Thin waxy cuticle, open stomata, wide flat leaves, small roots, air sacs

Xerophyte adaptations (5)

Thick waxy cuticle, sunken stomata/hairs, reduced stomata, long/wide roots, curled leaves (marram)

Properties of glucose (2)

Soluble, bonds store lots of energy

Alpha glucose structure

DUDD

Beta glucose structure

DUDU

Maltose structure

Glucose + glucose

Sucrose structure

Glucose + fructose

Lactose structure

Glucose + galactose

Condensation of sugars

Reaction between hydroxyl groups to form a glycosidic bond and water

Hydrolysis of sugars

Reaction between disaccharide and H20 to break glycosidic bond

Polysaccharides include: (2)

Starch (Amylose and amylopectin) and cellulose

Properties of starch

Made of a-glucose, insoluble, large, coiled

Amylose structure

Long, unbranched chain, 1,4-glycosidic bonds, helix

Amylopectin structure

Branched chain, 1,4 and 1,6 glycosidic bonds, easily broken down

Cellulose structure

B-glucose, 1,4-glycosidic bonds, monomers are flipped, long, unbranched chains, H-bonds between chains

Cellulose fibres

Chains form micro fibrils, micro fibrils form macro fibrils, macro fibrils form cellulose fibres

Adaptations/properties of cellulose

Long, unbranched chains, H-bonds add collective tensile strength, micro fibrils provide additional strength

Reducing sugars

Monosaccharides and some disaccharides

Non-reducing sugars

Disaccharides (sucrose) and all polysaccharides

Benedict’s test

Heat in gently boiling water for 5 mins, turns blue - brick red

Qualitative test for reducing sugars

Use a colorimeter to measure the absorbance of each solution

Testing non-reducing sugars

If results for Benedict’s test are blue, add HCl before heating and then sodium hydrogen carbonate before retesting

How does Benedict’s test work?

Reducing sugars react with Cu2+ ions in Benedict’s reagent, reducing them to brick-red Cu+ ions. For non-reducing sugars, the sugars are hydrolysed by the acid - turns sucrose to glucose and fructose.

Biosensors process

• Protein interacts with molecule under investigation

• Cause change in the transducer which produces a response

• This produces a visible signal which can be read (colour/electrical)

Diffusion

The net movement of particles from a region of higher concentration to a region of lower concentration.

Facilitated diffusion

Diffusion across a membrane through protein channels

Active transport

The movement of molecules into or out of a cell from a region of lower concentration to a region or higher concentration requiring energy (ATP) and carrier proteins.

How does active transport work?

• molecule being transported binds to receptors in the carrier protein

• On the inside of the cell, ATP binds to the carrier protein and is hydrolysed into ADP and phosphate.

• Binding of the phosphate molecule to the carrier protein causes the protein to change shape.

• The molecule is released from the protein and recombined with ADP to from ATP

Bulk transport

Large molecules are too large to move through a channel or carrier protein so they are transported by endocytosis and exocytosis.

Endocytosis

The bulk transport of material into cells - the cell-surface membrane invaginates and enfolds the material to form a vesicle

Exocytosis

The reverse of endocytosis - vesicles formed by the Golgi apparatus move towards and fuse with

Translocation

Plants transport organic compounds by active transport in the phloem from sources to sinks

Assimilates

The products of photosynthesis that are transported in the phloem

Actively loading (phloem)

• H+ ions are actively transported out of companion cells into surrounding source cells.

• H+ is co-transported along its concentration gradient back into companion cells with sucrose.

• Sucrose can then diffuse along its concentration gradient through plasmodesmata from companion cells to sieve tube elements.

Unloading (phloem)

• when solutes are actively loaded into sieve tube elements (STE) from companion cells at the source, the water potential decreases

• Water enters the STE from the xylem and companion cells by osmosis

• This increases hydrostatic pressure in the STE at the source

• At the sink solutes are actively removed from the STE

• This increases the water potential in STE at the sink.

• This creates a pressure gradient, pushing solutes from the source to areas of lower pressure at the sink

Communicable diseases in plants

Pathogens are spread directly - contact, pollen/seeds, spores in soil

Ring rot

• bacteria - clavibacter michiganensis

• Infects potatoes, tomatoes and aubergines

• Damages leaves, tubers and fruit

• No cure + lasts at least 2 years

tobacco mosaic virus

• viral

• Infects tobacco plants +

• Damages leaves

• Resistance crop strains

Potato blight

• protoctist (fungus-like) - phytophthora infestans

• Hyphae penetrate host cells

• Destroys leaves, tubers and fruit

• Resistant strains + chemical treatments

Black Sigatoka

• fungus - mycosphaerella fijiensis

• Banana disease

• Attacks leaves (black)

• Hyphae penetrate and digest cells

• Resistant strains + fungicide treatment

How plants recognise attack

• receptors in cell membrane recognise pathogen molecules

• Stimulates release of signalling molecules

• Causes nucleus to trigger cellular responses - defense chemicals + alarm signals

Physical defenses

Callose is a polysaccharide containing beta-1,3 linkages and beta-1,6 linkages between glucose monomers. Molecules act as barriers in cells walls + block sieve plates / plasmodesmata.

Insect repellents

Pine resin, citronella from lemon grass

Insecticides

• Pyrethrins from chrysanthemums act as neurotoxins

• Caffeine is toxic to insects and fungi

Antibacterial compounds

• phenols

• Cotton gossypol

• Defensins (proteins)

Antifungal compounds

• phenols

• Defensins

• Saponins against membranes

• Chitinases break down chitin cell wall (fungi)

Anti-oomycetes

Glucanases break down glucans in cell walls (protoctists)