Organs and leaf structure• Different types of tissues are

arranged together to form organs.

• Structure: 2 parts (Petiole and Leaf Blade)

• Thin flat blade, large surface area

• Leaves contain all 3 types of permanent tissues

• Functions of the leaf: .............

Vascular tissue• Location: Long strip called leaf

veins

• Leaf has a main vein called the midrib.

• Numerous veins split off from the midrib in a branching pattern.

• Upper part of each vein consist of Xylem

Vascular Tissue• Below the Xylem tissue in the

veins, you will find Phloem.

• Transports .................. To regions of the plant where it is used for ..........or stored..

• Because Xylem and Phloem are lignified, the vein also provide the plant with support.

• The midrib runs along the petiole and into the stem, linking the leaf’s vascular tissue to the vascular tissue in the rest of the plant.

Dicotyledonous plantsA flowering plant that has two seed leaves

• Dicotyledonous plant tissues

• Plant tissues grouped into 2 main types

• Plant tissues can be identified by observing the thickness of the cell walls of the cells that make up the tissue.

• Dimensions of the cells, the way in which they are packed, the size of the intercellular air spaces and the size of the vacuoles

Dicotyledonous plant 3 Main organsRoots: divided into 3 main

regions

• Epidermis, single layer of cells on the surface of the root

• Cortex: layer of cells found below the epidermis (Thin walled parenchyma cells)

• Vascular tissue: xylem is arranged in the centre of the core in a cross-shape and is surrounded by phloem.

Dicotyledonous StemsDivided into 4 main regions

• Epidermis: the outer layer of cells that cover the stem

• Cortex: Collenchyma (support) parenchyma (storage)

• Vascular tissue: ring separating the cortex from the pith, Xylem and Phloem

• Pith: cylinder of parenchyma tissue found in the centre of stems.

Transpiration

• Water is absorbed by the roots and moves up through the stems to the leaves.

• Approximately 2% of the water that reaches the leaves is used by the plant in photosynthesis.

• 98% evaporates from the leaves (Transpiration)

• Most of the water evaporates through the stomata

• Approximately 10% will evaporate through the cuticle.

Diffusion

• Water reaches the leaf in xylem vessels and diffuses into the leaf tissue via osmosis.

• Water evaporates from the wet cell walls into the intercellular air spaces until they become saturated with water vapour.

• This moist air has direct contact with the air on the outside of the leaf through the stomata.

• Water vapour diffuses down a water vapour concentration gradient from a region of high to a low concentration in the atmosphere

Transpirational pull

• The flow of the column of water from the root to the leaf is maintained by transpirational pull.

• When one water molecule evaporates from the leaf it exerts a force that pulls another molecule to take its place

• Assisted by the properties of water:

cohesion (the intermolecular force that makes water molecules stick together) and

Adhesion (the tendency of the water molecules to stick to the walls of the xylem vessels)

There are three purposes of transpiration

• The plant relies on transpirational pull to move water to the leaves for photosynthesis

• The water absorbed by the roots contains minerals. As the water flows up the xylem it carries the minerals to the rest of the plant

• Evaporation from the leaf surface cools the leaf

Factors affecting the rate of transpiration

• WindOn still days the water vapour diffusing out of the leaf collects in layers called diffusion

shellsWhen these layers build up the concentration gradient is low and hence transpiration

decreases

On windy days the wind blows the water vapour away increasing the concentration gradient and hence transpiration increases

• HumidityWhen the humidity is high the concentration of water vapour in the atmosphere is high

and therefore the rate of transpiration decreases. When the humidity is low the rate of transpiration increases because the concentration

gradient is high.

• TemperatureWhen temperature increases the rate of transpiration increases. High

temperatures increase the kinetic energy of the water molecules and decrease the humidity.

• Light IntensityWhen the light intensity increases the rate of transpiration increases as

light causes the stomata to open widely

Water Loss and Leaf Structure

• In dry regions where water is scarce plants modify their leaf structure to prevent excessive water loss

• Plants can suffer from water stress if they lose more water to the air than they can gain from the soil.

• These adaptations occur at the stomata, cuticle and epidermis

• Wilting occurs if a plant looses more water than can be replaced by osmosis

• This causes the plant to droop as the cells are no longer turgid, instead they are flaccid

• Plants may recover but continuous water shortages will lead to permanent wilting and death

• May result from various factors such as

▪ Drought conditions

▪ Saturated soil conditions that smother and kill the root hairs

• High environmental temperatures (leaves wilt to avoid contact with direct sunlight)

• Extremely low environmental temperatures (vascular system of the plant cannot function)

• High salinity of ground water, which causes water to flow out of cells

• Guttation is the process of water loss through small pores (hydathodes) in the leaves

• These pores are found at the tips and along the edges of the leave

• Guttation usually occurs under environmental conditions that restrict transpiration, when the soil moisture is high and when the root pressure is increased

• The increased root pressure pushes water up from the roots into the leaves and stems

• This creates pressure on the xylem that forces droplets of water out of the hydathodes

• Morning dew and Guttation are not the same.

▪ Guttation only occurs at the tips or the edges of the leaf.

▪ Dew will cover the entire surface of the leave.

Uptake and transport of minerals from the root to the leaves• Plants need certain elements in order to grow optimally

• Plants obtain carbon, hydrogen and oxygen from the breakdown of carbon dioxide and water during photosynthesis

• Plants obtain minerals from the soil. These minerals are dissolved in the water as minerals salts (which are made up of mineral ions)

• If the concentration of mineral ions is higher in the soil and lower inside the cells they will move freely in by diffusion

• If this is not the case mineral ions can enter the root by active transport

Uptake of water by the root hairs

• Water is absorbed via osmosis into the cytoplasm and vacuole of root hair cells (single specialised cell of the root epidermis)

• Root hair adaptations to maximise absorption

▪ Cell walls lack cuticles so that water can enter easily

▪ Cell is a finger-like shape that increases the surface area and the hairs penetrate between the soil particles

▪ Cells have a large vacuole - the soil has a higher water potential than the cell and this allows water to enter by osmosis

Movement of water from the root hair cell to xylem vessels • Water diffuses into the root hair cell, increasing its water potential

• Water then moves out of the root hair cell into the outer tissue of the root cortex down a concentration gradient

• The water is drawn into the xylem tissue in the middle of the root

• Three possible routes along which water moves from the root hair cell to the xylem vessels in the root

▪ Apoplast route ▪ Symplast route▪ Vacuolar route

• Apoplast Route

▪ Water passes through the cell walls of the cells

➢This route is interrupted at the endodermis because these cell walls contain the Casparian strip (impermeable)

➢The endodermal cells secrete salts into the vascular tissue

➢This decreases the water potential of the vascular tissue

➢Hence water moves down the concentration gradient from the endodermal cells into the xylem vessels in the root

• Symplast Route

▪ Water passes through the cytoplasm of the cells

• Vacuolar Route

▪ Water passes through the vacuoles of cells as well as through the cytoplasm

Transport of water from root xylem to the leaves

• There are three factors that cause water to move from the root to the leaves namely;

▪ Transpirational Pull

▪ Root Pressure is the result of osmosis and the active pumping of salts into xylem tissue which maintains a concentration gradient along which the water flows

▪ Capillarity is the tendency of liquids to rise and fall in narrow tubes as a result of surface tension. Xylem tubes are extremely narrow

Uptake and Transport of Water and Minerals in Plants• A plant cell has a living region that contains the nucleus, cytoplasm and cell

membrane (semi permeable) called the protoplast

• The non living cell wall (made up of cellulose fibres and have spaces filled with water) surrounds the protoplast

• The cell membrane is a living component that plays a role in controlling what goes in and out of the cell.

• Molecules dissolved in water can moved passively across the cell membrane

• Larger molecules (sugar) needs to be carried across via active transport

Translocation of organic substances in plants

• Translocation refers to the movement of minerals and organic substances within a plant.

• It occurs in the vascular tissue: minerals transported through xylem and organic material through the phloem

• The movement of organic substances is from where carbohydrates are made (source e.g. leave) to a place where carbohydrates are stored (sink e.g. growing root)

• Carbohydrates are synthesised in the mesophyll cells of leaves and then actively transported to the phloem.

• Water then enters the cell due to a large influx of solute

• This entrance of water creates internal pressure inside the cell and this causes the plant sap to move down the sieve tubes.

• At the sink the solute is actively transported from the phloem to the surrounding tissues

• This sap is made up of water, sucrose, amino acids, plant hormones and minerals that are transferred from the xylem.

• Sucrose is used to transport carbohydrates as it is soluble

• Once it reaches the sink it can be converted to glucose and fructose, stored as starch or used as a raw material for the manufacture of more complex organic compounds (proteins and lipids)