A LEVEL TOPIC 4 PLANTS

Chloroplast

A chlorophyll containing organelle found in the cells of plants and certain other organisms such as algae. Photosynthesis, the process in which light energy is converted to chemical energy, takes place inside chloroplasts.

Adhesion

A force resulting from attraction between molecules of different substances. In xylem, water molecules adhere to the molecules of lignin and to other substances which make up the walls of the vessel.

Cohesion

A force resulting from attraction between molecules of the same substance. Water molecules stick to each other by cohesion. As a result, transpiration pulls a continuous, unbroken water column up through the xylem in a plant stem.

Cytoplasm

A jelly-like fluid surrounded by the cell surface membrane. Ions, sugars and amino acids are dissolved in cytoplasm, and large molecules are suspended. It also contains a network of proteins, which help to give the cell its shape. Organelles such as mitochondria are suspended in the cytoplasm

Nucleus

A large organelle that contains a cell's genetic material. It is enclosed by a nuclear envelope. This is composed of a double membrane perforated by tiny pores. The nucleus contains DNA, which is packaged into chromosomes. This DNA contains genes, which code for the proteins made by the cell.

cell membrane

A membrane found either at the surface or inside a cell. Every cell in an animal or a plant is surrounded by a membrane, referred to as the cell surface membrane or plasma membrane. Cell cytoplasm also contains membranes, and organelles are surrounded by membranes. Cell membranes are very thin and consist mainly of a double layer of phospholipid molecules in which there are proteins.

Turgid

A plant cell is described as being turgid when the cell contents press against the cell wall. If a plant cell is surrounded by a solution with a high concentration of water molecules, water will move into the cell by osmosis. As a result, the volume of the cell contents will increase and press against the cell wall.

Phloem

A plant tissue that transports the products of photosynthesis. It consists of a system of tubes which takes these molecules from the leaves to other areas of the plant such s the developing flowers and fruits and the roots. Phloem provides a mass flow system for transport of the products of photosynthesis. Each tube is formed from a column of cells, sieve tube elements, with perforated end walls, sieve plates, to form a continuous tube.

Lignin

A polymer found in some plant cell walls. The main component of plant cell walls is cellulose. In sclerenchyma and xylem, the cellulose is impregnated with lignin. Lignin makes the walls stronger and more able to resist the forces on them. However, it also prevents water and salts getting through. As a result of being lignified the cell dies.

Amylose

A polysaccharide formed from a long straight chain of glucose molecules. This chain winds into a spiral shape. Starch consists of a mixture of amylose and amylopectin

Amylopectin

A polysaccharide formed of branched chains of glucose molecules. Starch consists of a mixture of amylopectin and amylose.

Cellulose

A polysaccharide which is an important component of plant cell walls. A molecule of cellulose consists of a long straight chain of beta-glucose molecules joined by glycosidic bonds. Hydrogen bonds form between neighbouring chains. These bonds hold the cellulose molecules together in bundles called microfibrils.

Vacuole

A region in a plant cell containing cell sap and surrounded by a cell membrane. A large vacuole is one of the characteristic features of a plant cell and distinguishes it from an animal cell. The cell sap in the vacuole contains dissolved substances such as sugars and mineral ions. Both of these are usually at a higher concentration than in the surrounding cytoplasm. Because of this, water moves into the cell and through the cytoplasm by osmosis. This keeps the cell turgid and helps it to support the plant

Pit

A region where a plant cell wall is very thin because only the first layer of cellulose has been laid down. Pits help substances to move between neighbouring plant cells.

Cell wall

A rigid layer surrounding a cell. Bacteria, fungi and plant cells have cell walls. Animal cells do not. In plants, the most abundant substance in the wall is cellulose. The cell walls of bacteria and fungi do not contain cellulose. They are made of other substances.

Pectin

A structural carbohydrate found in plant cell walls. Pectin consists of a mixture of polysaccharides, depending on its source. The majority of the structure is formed of repeating units of galacturonic acid. Pectins combine with calcium ions to form calcium pectates. Calcium pectates are found in and between plant cell walls where they help to cement cellulose fibres together.

cohesion-tension theory

A theory explaining the movement of water through the xylem of a plant from the roots to the leaves. Water molecules stick to each other by cohesion. As a result, transpiration pulls a continuous, unbroken water column up through the xylem. This water column is under tension as a result of being pulled up by transpiration and down by gravity.

Collenchyma

A type of supporting tissue found in plants. Collenchyma cells are slightly elongated and have their cell walls strengthened with extra cellulose at the corners.

Cellulase

An enzyme that digests cellulose into soluble sugar; this reaction involves the addition of water molecules so it is an example of hydrolysis. Cellulase is produced by many different microorganisms but it is not produced by animals. An animal such as a cow can digest cellulose because it has large numbers of bacteria and other microorganisms in its gut. It is these microorganisms that produce the necessary cellulase enzymes.

Amyloplast

An organelle found in plant cells which stores starch. Each amyloplast consists of a tiny grain of starch surrounded by a double layered cell membrane.

Eukaryote

An organism that has cells containing a nucleus. Eukaryotic cells also contain organelles such as chloroplasts and mitochondria, which are surrounded by membranes. Animals and plants are eukaryotes; bacteria are prokaryotes. Prokaryotic cells are very small and do not have nuclei or other membrane-bound organelles.

Xylem

Aplant tissue whose main function is to transport water and mineral ions from the roots, up through the stem to the leaves. It consists of a system of vessels. The walls of these vessels contain lignin. Lignin makes the walls stronger and more able to resist the forces on them. Xylem provides a mass flow system for transport of water and mineral ions.

Microfibrils

Cellulose is a polymer made up of long chains of beta-glucose molecules. In a plant cell wall, these cellulose molecules are arranged in bundles called microfibrils. The arrangement of the microfibrils makes the wall of the cell very strong and helps it to resist the forces acting on it.

vascular bundle

In the stems and leaves of young plants, particularly those which are not woody, the vascular tissue is found in bundles. These vascular bundles contain xylem vessels, which transport water and mineral ions up the stem to the leaves and phloem tubes. The phloem tubes transport the products of photosynthesis.

Sclerenchyma fibres

Long, thin plant cells which are tapered at both ends. Sclerenchyma fibres help to provide plants with support. They have cell walls which are thickened with lignin. They do not have living contents.

Tonoplast

Membrane that surrounds a vacuole.

vascular tissue

One of the three main types of tissue found in a plant. Vascular tissue consists of xylem and phloem. Xylem transports water and mineral ions from the roots, up through the stem to the leaves. Phloem transports the products of photosynthesis from the leaves to other parts of the plant.

Hemicellulose

Polysaccharides that have molecules made up of short chains of various sugars. They are important in plant cell walls where they help to bind cellulose microfibrils together.

Plasmodesmata

Small fluid filled channels through the cell walls of plant cells. Plasmodesmata allow dissolved molecules such as mineral ions to pass directly from the cytoplasm of one cell to the cytoplasm of the next. These molecules do not have to pass through cell membranes or cell walls

Chlorophyll

The green pigment found in plants and other organisms which photosynthesise. It is responsible for the capture of light energy.

biodigradable

The molecules of substances which are biodegradable can be broken down into smaller molecules and ions by microorganisms such as bacteria and fungi. Most paper and wood products are biodegradable but many plastics are not.

companion cells

These long thin cells lie alongside phloem sieve tubes and perform the metabolic functions to maintain the sieve tube cells which lack cell organelles and only have a thin layer of cytoplasm around the edge of the cell.