Forms and Functions of Plants notes

MDCAT Biology

This chapter explains plant nutrition, photosynthesis, plant transport, growth regions, plant movements, and the functions of plant growth regulators. It also covers mineral deficiencies, transpiration, ascent of sap, photorespiration, tropisms, and adaptations of plants to environmental conditions.

Plant Nutrition and Essential Mineral Elements

Plants need water, carbon dioxide, oxygen, and mineral elements for growth and metabolism. Carbon dioxide is obtained from the air, while water and mineral salts are mainly absorbed from the soil by roots.

Mineral elements are divided into macronutrients and micronutrients according to the quantity required by plants. Both groups are essential because deficiency of any required element disturbs normal growth and development.

Macronutrients are required in relatively large quantities. Examples include nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, and sometimes carbon, hydrogen, and oxygen.
Micronutrients are required in very small quantities, commonly about 6 to 200 g per acre.
Examples of micronutrients are iron, manganese, copper, zinc, boron, molybdenum, chlorine, and nickel.
Magnesium is a macronutrient, not a micronutrient. It is a central component of the chlorophyll molecule.
Nitrogen is needed for amino acids, proteins, nucleic acids, chlorophyll, and many enzymes.
Phosphorus is a component of ATP, nucleic acids, phospholipids, and several phosphorylated compounds.
Chlorosis means yellowing of normally green leaves. It may result from deficiency of elements such as nitrogen, magnesium, iron, or sulphur.

Root Functions and Water Absorption

The root is usually the underground organ of a plant. It anchors the plant in the soil and provides a large surface for absorption of water and mineral salts. Roots may also store food, as in carrot, turnip, and sweet potato.

Water enters root hairs mainly by osmosis. Mineral ions may enter through active transport or diffusion, depending on their concentration and electrochemical gradients. The cortex, endodermis, and vascular tissues help move water towards the xylem.

The root performs anchoring, storage of food, absorption of water, and absorption of mineral salts.
Root hairs are unicellular extensions of epidermal cells that increase the absorbing surface area.
Water moves from the soil into root hairs because the cell sap usually has a lower water potential than the surrounding soil solution.
The endodermis contains Casparian strips, which regulate the movement of water and ions into the vascular cylinder.
Xylem transports water and dissolved mineral salts from roots towards the stem and leaves.
The phosphorus requirement of an adult man is about 800 mg per day. This is a nutritional value for humans, not a plant mineral requirement.

Photosynthesis and Pigments

Photosynthesis is the process by which green plants use light energy to manufacture carbohydrates from carbon dioxide and water. Oxygen is released as a by-product. The process occurs mainly in chloroplasts of green cells.

The light-dependent reactions occur on the thylakoid membranes. They produce ATP and reduced NADP. The light-independent reactions occur in the stroma and use ATP and reduced NADP to fix carbon dioxide.

Chlorophyll a is blue-green in colour and is universally present in all green plants.
Chlorophyll b is yellow-green and acts mainly as an accessory pigment in many plants.
Carotenoids are accessory pigments that absorb additional wavelengths and help protect chlorophyll from excessive light.
Each photosystem consists of a light-gathering antenna complex and a reaction centre.
Photosystem II is associated with photolysis of water and the release of oxygen. Photosystem I helps in the production of reduced NADP during non-cyclic photophosphorylation.
Synthesis of ATP during photosynthesis takes place at the thylakoid membrane by photophosphorylation.
The Calvin cycle occurs in the stroma of chloroplasts. Its major stages are carbon fixation, reduction, and regeneration of the carbon dioxide acceptor.
Rubisco, or ribulose bisphosphate carboxylase/oxygenase, is the enzyme responsible for carboxylation in the Calvin cycle.

Photorespiration and Environmental Factors

Rubisco can act as either a carboxylase or an oxygenase. During normal carbon fixation, it combines ribulose bisphosphate with carbon dioxide. During photorespiration, it combines ribulose bisphosphate with oxygen.

Photorespiration reduces the efficiency of photosynthesis because it consumes energy and releases previously fixed carbon dioxide. It becomes more likely when temperature is high and the carbon dioxide to oxygen ratio is low.

Photorespiration lowers the overall rate of carbon dioxide fixation.
High environmental temperature and a low carbon dioxide to oxygen ratio favour photorespiration.
When stomata close during hot or dry conditions, carbon dioxide concentration inside the leaf falls while oxygen concentration becomes relatively higher.
Rubisco is involved in both carbon fixation and photorespiration.
C3 plants show more photorespiration than C4 plants under conditions of high temperature and low internal carbon dioxide.
Photorespiration does not produce useful carbohydrate and reduces the net gain from photosynthesis.

Transpiration and Its Importance

Transpiration is the loss of water vapour from the aerial parts of a plant, mainly through stomata. It uses the property of the latent heat of vaporisation of water. Evaporation of water cools the plant surface.

Transpiration also creates a negative pressure in the leaves. This pressure helps pull water upward through the continuous water column in xylem vessels and tracheids.

Most transpiration occurs through stomata. Smaller amounts occur through the cuticle and lenticels.
The main force that draws water from the soil through the plant is transpiration pull.
Transpiration helps in the ascent of water and mineral salts from roots to leaves.
The latent heat of vaporisation allows evaporation of water to remove heat without an immediate rise in temperature.
Factors increasing transpiration include high temperature, dry air, wind, and strong light, within suitable limits.
High humidity reduces the water vapour gradient between the leaf and atmosphere and therefore reduces transpiration.
Stomata are controlled by guard cells, which regulate the exchange of gases and loss of water vapour.

Ascent of Sap and Root Pressure

Ascent of sap is the upward movement of water and dissolved mineral salts through the xylem. The widely accepted explanation for this movement in tall trees is the transpiration cohesion theory of Dixon.

According to this theory, evaporation from leaves produces transpiration pull. Cohesion between water molecules maintains a continuous water column, while adhesion helps water remain in contact with xylem walls.

The transpiration cohesion theory of Dixon is the most widely accepted explanation for ascent of sap in trees.
Cohesion is the attraction between water molecules.
Adhesion is the attraction between water molecules and the walls of xylem vessels and tracheids.
Transpiration pull is generated mainly at the leaf surface due to evaporation of water.
Root pressure is a positive pressure developed in the xylem of roots because of active ion accumulation and osmotic water entry.
Root pressure can cause guttation, especially when soil water is abundant and transpiration is low.
Guttation is the loss of liquid water droplets from leaf margins or tips through hydathodes.
Root pressure alone cannot explain the rise of water to the tops of very tall trees.

Plant Growth and Regions of Growth

Plant growth is a permanent and generally irreversible increase in size, dry mass, or cell number. Growth involves cell division, cell enlargement, and cell differentiation. Meristematic tissues contain actively dividing cells.

The tip of a root or shoot has different zones. The zone of cell division contains meristematic cells. The zone of elongation lies behind it, and the zone of maturation lies behind the region of elongation.

The phase behind the region of elongation in higher plants is the maturation phase.
The zone of cell division contains small cells with dense cytoplasm and prominent nuclei.
In the elongation zone, cells increase mainly in length and vacuole size.
In the maturation zone, cells differentiate and become specialized tissues.
Primary growth increases the length of roots and shoots and occurs through apical meristems.
Secondary growth increases the thickness of stems and roots through lateral meristems such as vascular cambium and cork cambium.
In the absence of light, elongation of internodes generally increases. This condition is called etiolation.
Senescence is a genetically controlled and active developmental process, not simply passive ageing. It is indicated in structures such as vessel and tracheid elements.

Plant Growth Regulators

Plant growth regulators are chemical substances that control growth and development in plants. They may promote or inhibit processes such as cell division, cell elongation, flowering, fruit ripening, dormancy, and senescence.

The term phytohormone is a general term for a naturally occurring plant hormone. It is not the name of one separate growth hormone. Major groups include auxins, gibberellins, cytokinins, abscisic acid, and ethylene.

Auxins promote cell elongation, apical dominance, root initiation, and tropic responses.
Gibberellins promote stem elongation, bolting, seed germination, and growth of some fruits.
Cytokinins promote cell division, stimulate lateral bud growth, and delay senescence in leaves.
Abscisic acid generally inhibits growth, promotes seed and bud dormancy, and causes stomatal closure during water stress.
Ethylene is a gaseous regulator that promotes fruit ripening, leaf abscission, and several senescence responses.
Auxin is produced mainly in shoot apices, young leaves, developing seeds, and fruits.
Growth hormones are specific substances such as auxin, gibberellin, and cytokinin, while phytohormones is the collective term for plant hormones.
The effect of a regulator depends on its concentration, the plant organ, developmental stage, and environmental conditions.

Plant Movements and Environmental Responses

Plant movements may be directional growth responses or non-directional movements. Tropisms depend on the direction of the stimulus. Nastic movements do not depend on the direction from which the stimulus comes.

Auxin distribution is involved in many growth movements. However, nyctinastic leaf movements are not considered movements directly related to auxin level. They are commonly associated with changes in turgor in specialized motor cells.

Phototropism is growth in response to light. Shoots usually show positive phototropism.
Geotropism, or gravitropism, is growth in response to gravity. Roots usually show positive geotropism, while shoots usually show negative geotropism.
Chemotropism is growth in response to chemicals. The growth of a pollen tube towards the egg is positive chemotropism.
Hydrotropism is growth in response to water. Roots generally show positive hydrotropism.
Thigmotropism is growth in response to touch, as seen in tendrils coiling around support.
Nastic movements are not dependent on the direction of the stimulus. Nyctinasty is the sleep movement of leaves in response to day and night conditions.
Auxin promotes unequal cell elongation in many tropic responses, producing bending of the organ.

Xerophyte Adaptations and Stomatal Control

Xerophytes are plants adapted to dry habitats. Their structures and physiological processes reduce water loss or increase water storage. These adaptations help maintain a favourable water balance in conditions of limited water supply.

Stomata are important in controlling both transpiration and gas exchange. Xerophytes often alter the timing and position of stomatal opening to reduce water loss.

An osmoregulatory adaptation in xerophytes is that their stomata are closed during daytime.
Some xerophytes open their stomata at night and take in carbon dioxide when evaporation is lower. This is associated with CAM photosynthesis.
Thick cuticles reduce water loss from the epidermal surface.
Sunken stomata and stomata surrounded by hairs trap moist air and reduce the diffusion gradient for water vapour.
Reduced leaves, spines, succulent tissues, and extensive root systems are common xerophytic adaptations.
Stomatal closure is promoted by abscisic acid during water shortage.
Closing stomata reduces transpiration but also limits the entry of carbon dioxide for photosynthesis.

Key terms

Macronutrient: A mineral element required by a plant in relatively large quantity.
Micronutrient: A mineral element required by a plant in a very small quantity, commonly about 6 to 200 g per acre.
Chlorosis: Yellowing of green plant tissue due to loss or reduced formation of chlorophyll.
Root hair: A thin unicellular extension of a root epidermal cell that absorbs water and mineral salts.
Photosystem: A light-harvesting unit made of an antenna complex and a reaction centre.
Rubisco: The enzyme that catalyses carbon dioxide fixation and can also combine ribulose bisphosphate with oxygen.
Photorespiration: A light-dependent process in which Rubisco uses oxygen and the rate of carbon dioxide fixation is reduced.
Transpiration: Loss of water vapour from the aerial parts of a plant, mainly through stomata.
Transpiration pull: The negative pressure generated by evaporation of water from leaves that draws water upward through xylem.
Root pressure: Positive pressure developed in roots that can push water upward and cause guttation.
Guttation: The loss of liquid water droplets from leaf margins or tips through hydathodes.
Cohesion: Attraction between water molecules that helps maintain a continuous water column in xylem.
Phytohormone: A naturally occurring plant hormone that regulates growth and development.
Auxin: A plant growth regulator that commonly promotes cell elongation, apical dominance, and tropic responses.
Vernalization: Induction or promotion of flowering by exposure to low temperature.
Senescence: An active developmental process involving ageing and the gradual decline of cells, organs, or the whole plant.
Tropism: A directional growth response of a plant organ to an external stimulus.
Chemotropism: Directional growth in response to a chemical stimulus, such as pollen tube growth towards the egg.

Test yourself on Forms and Functions of Plants

Free Forms and Functions of Plants MCQs with an explanation on every answer. No account needed.

Practise Forms and Functions of Plants MCQs

More for Forms and Functions of Plants in the MDCAT pack

A one-page revision sheet for this chapter
5 Forms and Functions of Plants mnemonics
Chapter-wise Ratta Cards and a Quiz Builder for your own tests

See the MDCAT packAlready have it? Open Revision

Biology shortcuts

Recognising acellular organisms

If an entity has no cellular organisation and depends on a host for replication, identify it as a virus. Viruses are non-cellular and are not placed among cellular organisms.

Check whether it has cytoplasm, ribosomes and independent metabolism.
Viruses contain nucleic acid inside a protein coat but lack cellular structure.
Example: An infectious particle without cytoplasm or ribosomes is a virus.
Answer: Non-cellular virus.

This shortcut does not apply to bacteria, which are cellular prokaryotes.

Separating prokaryotic and eukaryotic cells

Use ribosomes as the common feature. Both cell types have ribosomes, but only eukaryotes have a membrane-bound nucleus and membrane-bound organelles.

Look for the feature present in both groups.
Ribosomes occur in prokaryotes and eukaryotes for protein synthesis.
Example: Which structure is common to both? Ribosomes.
Answer: Ribosomes.

Do not use mitochondria, chloroplasts or a membrane-bound nucleus as common features.

15 more Biology shortcuts are in the MDCAT pack. Already have it? See all shortcuts