Active transport

This is the movement of molecules from the region of low concentration to the region of higher concentration i.e. movement against concentration gradient using energy.
Energy for this process is derived from respiration. Anything that affects the rate of respiration, also affects the active transport e.g. cyanides prevent ATP synthesis.
Active transport takes place by means of carrier molecules in the cell membranes which are protein. The carrier, on reaching the inner part of the membrane releases the molecules and is set free for further transportation.

Examples of active transport
1) Up take of mineral salts from soil by plant roots
2) Absorption of some food molecules e.g. glucose
3) Selective re absorption of molecules e.g. glucose

Importance of active transport
1) Used by plant roots or root hairs to absorb minerals from the soil.
2) Used in the absorption of food materials from the ileum into the blood stream
3) Used in the reabsorption of minerals in the kidney during urine formation
4) Used in the secretion and active uptake of ions in the fish gills from fresh water

This is the process by which animal cells take in liquid materials into their bodies. Thus it is said to be cell-drinking.

This is the process by which animal cells take in solid materials. The cell engulfs/invaginates or takes in solid materials and form a food vacuole where the food is digested. The food is absorbed into the cytoplasm and undigested particles are released. It requires energy

Importance of phagocytosis
1) Used by amoeba during feeding
2) White blood cells destroy pathogens by phagocytosis
3) Unicellular animals egest undigested material by phagocytosis.

This refers to the movement of materials from one part of the organism to another. In plants, it is called translocation. It involves diffusion, osmosis and active transport in simple organisms and active transport in simple organisms and transport systems in large higher organisms, (Vascular & circulatory systems).


All living things need a continuous exchange of certain substances between their cells and the environment e.g. oxygen, food, materials carbon dioxide, waste products.
In large complex animals, most of the cells are located far from the surface thus the need for a transport system.
Flat worms are flattened to shorter distance for transport.

Requirements of transport system
The materials to be transported
The medium of transport
The channels of transport

Materials to be transported
In animals, they include respiratory gases oxygen and carbon dioxide, nitrogenous excretory products e.g. uric acid, nutrients e.g. glucose, amino acid.
In plants, they include oxygen and carbon dioxide.
Mineral elements for plant growth
Manufactured food (autotrophs)
Absorbed food (in saprophytes)
Vitamins, amino acids auxins

The medium of transport
The medium of transport in plants and lower animals is water and it is blood in vertebrates and in a few invertebrates like arthropods, annelids (earth worm).

The channels of transport
In most animals, these are blood vessels, in others e.g. earth worms, it is the body cavity (coelom). In higher plants, there is a vascular system or system of xylem and phloem.

Circulation of blood in animals requires energy supplied from respiration used in pumping of the heart and muscle contractions.

Transporting tissue in plant is xylem and phloem. It involves movement of water, salts and organic molecules (manufactured food).

This Consists of xylem vessels and tracheids. Xylem vessels develop from cylindrical cells, arranged end to end, in which the cytoplasm die and cross- walls disappear leaving a dead empty tube. Through this
Water, mineral salts, move from roots, stems, up to leaves. Xylem vessels are strengthened by lignin in their walls.
This strength gives support to the soft tissue of roots, stems, and leaves: it also prevents collapse of the vessels under tension as sap pressure changes.

Structure of xylem

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They are similar to xylem vessels; except that they are typically 5 or 6 sided. In cross- section, instead of being open at each end, their tapering end walls are perforated by pit (tiny holes in lignified walls). Even xylem has pits in their walls.
Tracheids are more primitive- they are found in gymnosperms e.g. cypress where there is control of transpiration, for water does not move very fast through them.

Characteristics of xylem tubes
a) Consist of dead tubes
b) They are hollow
c) Its walls are lignified
d) Has no protein filaments
e) Has no cytoplasm
f) Transports water and salts
g) Transports water and mineral salts in one direction

Types of thickenings in the xylem vessels/tissues

  1. Annular lignification
  2. Spiral lignification
  3. Recticulate lignification

This Consists of sieve tubes and companion cells. The sieve tubes are formed from cylindrical cells arranged end to end. Unlike the xylem vessels, the cross walls do not disappear but develop perforations of enlarged pits forming sieve plates. The protoplast of a sieve tube / elements remains living; although its nucleus disintegrates as the cell differentiates. Each sieve tube is closely associated with companion cells which are complete cells.
The companion cells regulate a metabolic activity of the sieve tubes.

Characteristics of phloem tissue/tube
a) Consist of living cells
b) Have a thin cytoplasm
c) Associated with companion cells
d) Consist of sieve cross walls
e) Consist of protein filaments
f) Transport food materials
g) Transport materials in opposite direction

Structure of the phloem tissue
Sieve tubes have perforated cross walls called sieve plates. In between the plates are sieve pores which allow food substances to pass from one cell to another along the cytoplasmic filaments (protein filaments). Adjacent to sieve elements are the
companion cells which provide the sieve tube with energy to transport the food substances.

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Structured comparison between xylem and phloem

Both have cells without nucleus e.g. vessels and tracheids in xylem and sieve tubes in phloem.
Both are perforated, i.e. xylem is bordered with pits and phloem has sieve pores in the sieve plates
Both tissues are surrounded by parenchyma cells as packing tissues.

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Function comparison
In both, materials are transported in solution form.
In both, transport involves use of energy e.g. in xylem, transpiration pull depend on solar energy and in phloem it depends on respiratory energy.


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