Xylem transports water and ions, phloem transports products of photosynthesis
Xylem:
Transports water and ions from roots to the rest of the plant
Contains xylem vessels, tracheids, and xylem parenchyma
Water enters and leaves xylem vessels through pits
Phloem:
Transports organic molecules in plants
Contains sieve tubes and companion cells
Sieve tubes have little cytoplasm and few organelles
Companion cells support sieve tubes and aid in transport
Phloem sieve tube cells contain numerous mitochondria, rough ER, and a large dense nucleus
Companion cells and sieve tube elements are connected via plasmodesmata
End walls of phloem sieve tube cells are perforated by small pores called sieve plates
Strands of cytoplasm pass through these pores from one phloem sieve tube element to the next
Vascular bundles contain xylem, phloem, cambium, and other cells
Cambium is a meristematic tissue that can keep dividing by mitosis
Xylem cells include vessels and tracheids, while phloem cells include sieve tube elements and companion cells
Xylem cells are dead, while phloem cells are alive
Xylem cell walls are thick and contain lignin, while phloem cell walls are thin and contain cellulose
Xylem cell walls are impermeable, while phloem cell walls are permeable
Xylem cells have no cytoplasm, while phloem cells have cytoplasmic strands
Xylem transports water and mineral ions, while phloem transports products of photosynthesis like sucrose and amino acids
Direction of transport in xylem is from roots upwards, while in phloem it is to and from sites of photosynthesis/storage to growing regions and sites of storage
Water uptake by roots can occur through apoplast, symplast, and vacuolar routes
Ions are absorbed by plant cells through active transport and co-transport mechanisms
Root pressure can push water up the xylem against gravity
Cohesion-tension theory explains water movement in xylem
Transpiration is the main force that pulls water into roots and up stems
Factors affecting rate of water uptake in plants include temperature, humidity, wind speed, and light intensity
Hydrophytic adaptations include floating leaves and stems for plants living in water
Adaptations of hydrophytes include:
Floating leaves and stems
Large air-filled cavities in leaves and/or stems enable plants to float and act as reservoirs of oxygen/carbon dioxide
Increased leaf surface area for gas exchange and photosynthesis
Lack of protective tissues like waxy cuticles to reduce water loss
Roots are usually reduced in size and act mainly to anchor the plant
Translocation is the process of moving the products of photosynthesis from where they are made or stored to other parts of the plant
Carried out by the phloem
Mechanism of phloem transport is not well understood
Mass flow hypothesis (pressure flow hypothesis) proposed by Ernst Munch in 1930
Details of how scientists believe the mass flow hypothesis works in plants:
Glucose produced during photosynthesis is converted into sucrose
Sucrose is passed into phloem sieve tubes through various routes
Increased sucrose concentration in sieve tubes reduces Ψ of sieve tube contents
Water moves into sieve tubes from xylem through osmosis
Roots and growth points act as sinks where sucrose is unloaded and converted into glucose/starch/other carbohydrate
Alternative theories to explain translocation:
Electro-osmosis: sieve plates become charged due to movement of water and ions, attracting/repelling substances
Cytoplasmic streaming: strands of cytoplasm move within cells and in different directions
Protein contraction/peristalsis: protein microtubules in sieve tubes contract and push cytoplasm along in different directions
The lifecycle of a potato plant:
Leaves act as source in summer
Potato tubers act as sink
Buds grow in autumn/winter and become sink
Shoots grow in spring, tubers are source of energy/nutrients until leaves develop for photosynthesis
All parts of plant are sink during summer growth
Tubers act as source at night
Investigating translocation:
Ringing experiment: removing bark and phloem from stem affects transport
Use of radio-isotopes: exposing leaves to radioactive CO2 to track transport
Use of aphids: feeding on phloem contents to analyze composition
Circulatory system in animals:
Closed circulation system: blood travels through vessels with heart as pump
Open circulation system: blood bathes all cells and organs, no red blood cells to transport oxygen
Single circulation: blood passes through heart once in each circulation, found in fish
Double circulation: blood passes through heart twice in one circulation, found in humans and mammals
Blood vessels and heart structure:
Arteries take blood away from the heart
Veins take blood into the heart
Capillaries are site of gas exchange and tissue fluid formation
Arteries have thick tunica externa, muscle, and elastic tissue to carry blood at high pressure
Arterioles constrict and dilate to control blood flow to capillaries
Capillaries consist of a single layer of endothelial cells
Capillaries:
Consist of a single layer of endothelial cells and are a tissue rather than an organ
Site of gas exchange with a short diffusion path
Capillary beds have a massive surface area for diffusion
Pressure is lowered as blood passes through capillaries due to greater cross-sectional area compared to arterioles
Capillaries are narrow, leading to greater resistance and slower blood flow, allowing more time for gas exchange
Red blood cells have to bend to squeeze through capillaries