Fruits

Fruit Types A fruit may be defined as a matured ovary There are two basic fruit types – dry or fleshy. These types arise from the development of the pericarp The pericarp may become dry and these form dry fruits The pericarp may also become soft, thick and fleshy – and these form fleshy fruits

Apples and Pears

Violet flower types

Plant Transport Mechanisms Guttation

Brown algae – Macrocystis and Laminaria - California

Giant Sequoia

Plant Transport Plant transport occurs at three levels: 1. The uptake and loss of water and solutes by individual cells, such as absorption of water and minerals from the soil by root cells. 2. Short-distance transport of substances from cell to cell at the level of tissues and organs, such as moving sugar from photosynthetic cells of leaf to the phloem sieve tubes 3. Long distance transport of sap within xylem and phloem at the level of the entire plant.

Transport at the Cellular Level Substances will tend to diffuse across the cells plasma membrane from areas of high concentration to areas of low concentration – this is down a concentration gradient and is done by passive transport. We can also get movement of substances across plasma membranes against their concentration gradient – this is movement uphill and requires the expenditure of energy. It is known as active transport. This almost always uses ATP for energy. Cotransport is a process where a transport protein couples the downhill movement of one substance (such as H+) to the uphill passage of another. It can be used by root cells to take in NO 3 nitrate or uptake of sugars like sucrose.

Water Potential The movement of water in and out of plant cells is driven by water potential. The net uptake or loss of water by a cell occurs by osmosis, the passive transport of water across a membrane. Water usually moves from hypotonic (low solute concentration) to hypertonic (high solute concentration) – this is what happens in animal cells. But plants have a rigid cell wall that provides physical pressure. So in plants the movement of water depends upon a combination of solute concentration and physical pressure known as water potential symbolized by the Greek letter psi Ψ

Plant cells have three basic compartments 1. Outside the cell is a thick cell wall that helps maintain the plant cells shape. It does not regulate the movement of material in and out of the cell – that is done by the plasma membrane. 2. The plasma membrane serves as the barrier between the cell wall and the cytosol – the cytoplasm inside the cell but outside of the organelles 3. Most mature plant cells have a large vacuole that contains cell sap. It may occupy 90% of the cell volume. It is surrounded by the tonoplast and regulates traffic between the cytosol and the cell sap.

Symplast and Apoplast Most plant cells have openings in the cell walls called plasmodesmata. The plasmodesmata connect the cytosol compartments of neighboring cells allowing easy movement of substances between cells. The connected cytoplasms of many cells is known as the symplast. The cell walls for a continuum of spaces between cells. This is known as the apoplast.

Lateral transport can happen in three ways: 1. Substances can move out of one cell and into another making repeated crossings of the plasma membranes. This is fairly slow and not that common. 2. Substances can move via the symplast – crossing a plasma membrane once and then traveling through many cells. 3. Substances can move via the apoplast, traveling between cell walls without ever entering into a cell.

Root with mycorrhizae

Transport in Xylem Once water and minerals reach the xylem via lateral transport they move up the xylem vessels via bulk flow the movement of a fluid driven by pressure. Xylem sap flows upward to the veins of leaves due to the pressure of transpiration – the loss of water vapor from the leaves and other aerial parts of the plant. An early botanical question was whether xylem sap was pushed up or pulled up the plant.

Water – pushed or pulled? Pushing of the xylem sap occurs via root pressure – root cells expend energy to pump mineral into the xylem. Minerals accumulate in the xylem sap lowering water potential there. Thus water flows into the xylem, generating a positive pressure that pushes fluid up the xylem. But root pressure can only push sap up a few meters and many plants generate no root pressure at all. How does water reach leaves of 100 m tall trees? Xylem sap is pulled up the plant via transpirational pull. Leaves actually generate the negative pressure necessary to bring water to them.

Guttation – from root pressure

Transpiration

Translocation The transport of food throughout a plant is known as translocation. Sugar from mesophyll cells in the leaves and other sources must be loaded before it can be moved. In some species, sugar moves all the way from mesophyll cells to sieve tube members via the symplast. In other species, sugars moves by a combination of symplast and apoplast. Often sieve tube members accumulate very high sucrose concentrations – 2 to 3 times higher than concentrations in the mesophyll – so phloem requires active transport using proton pumps

Translocation At the sink end of a sieve tube, the phloem unloads its sugar. Phloem unloading is a highly variable process; its mechanism depends upon the plant species and the type of organ. In any case, the concentration of sugar in the sink cells is lower than in the phloem because the sugar is either consumed or converted into insoluble polymers like starch. Phloem moves at up to 1 m/hour – too fast to be by diffusion. So phloem also moves via bulk flow – pressure drives it.