Plant Structure and Photosynthesis


1 Biology (AS90928 The Life Cycle Of Flowering Plants) Note on Plant Structure and Photosynthesis, created by mckenziedev on 07/09/2013.
Note by mckenziedev, updated more than 1 year ago
Created by mckenziedev about 10 years ago

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Flowers are the reproductive structures of plants. Their role is to attract pollinators, and to produce fruit and seeds

Protect the seeds and may also help with their distribution. Animals eat the fruit and later "distribute" the seed

Seeds contain embryonic plants. Provide nourishment for the embryo and help it to disperse to a new location

The site of photosynthesis. Each leaf is produced at a different angle to maximise exposure to sun

Holds the leaves towards the light and transports nutrients between the leaves and roots

Many including absorb water and nutrients, anchor plants to ground and store good and nutrients


Primary growth refers to the elongation of the plant. Primary growth occurs in apical meristematic cells. 'Apical' means 'tip' so apical meristematic tissues are found in the plant's extremes (buds and root tips). Meristematic cells are undifferentiated (don't have a specific function yet), rapidly dividing cells.

Growth at the meristem occurs in three stages: Cell division - cells multiply by the process of mitosis Cell enlargement - cells grow in both length and width as water fills the vacuole and the elastic cell stretches Cell differentiation - enlarged cells become specialised. At the upper meristem they may become leaves, etc. At the roots they may become root hairs, etc.


Secondary growth in plants is an increase in diameter of roots, stems and branches. Secondary growth is easiest to explain by looking inside a stem. In order to understand the process of secondary growth you need to be able to identify three different types of plant cells... Xylem moves water from the roots to the leaves Phloem moves food from the leaves to the rest of the plant Cambium contains lateral meristematic cells.


CambiumThe first stage in secondary growth occurs when the cambium in vascular bundles grows (by mitosis) into a complete cylinder around the stem. The cambium also thickens. Undifferentiated cells on both sides of the cambium become new tissue.

PhloemUndifferentiated cells on the outside of the cambium become secondary phloem. The initial phloem (called 'primary phloem') is soft and gets crushed. The remains of primary phloem cells, along with tissues produced by the cork cambium, becomes part of the bark. Bark is a protective, waterproof layer around the outside of the plant that prevents infection and water loss.

XylemUndifferentiated cells on the inside of the cambium become secondary xylem. Xylem tissue is hard and is not destroyed by the growth of cambium tissue. Primary xylem tissue remains with the plant and becomes wood. Growth rings within the plant are actually layers of old xylem tissue. During periods of rapid growth (i.e. spring) the size of xylem cells are larger, therefore the distance between the growth rings is greater when compared to the distance between them during periods of slow growth (i.e. winter and periods of drought).


Explain how environmental conditions could have caused differences in the thickness of annual growth rings.Higher/lower spring time temperatures, changes in rainfall volume (i.e. more rainfall increases growth producing a wider ring, drought decreases growth, producing a narrower ring.Variations in the amount of sunlight (i.e. crowding from neighboring trees) etc.

Roger is fencing off some paddocks on his farm. He decides to attach a gate to a tree. His farmhand suggests that this is not a good idea as in a few years the gate will have lifted off the ground as the tree grows.Outline why the farmhand is wrong. In your answer you should describe the likely impact the tree will have on the gate.The stem will not elongate at the point where the gate is attached. It is likely to thicken and "swallow" the gate hinges via the process of secondary growth.Explain how cells and tissues within the tree account for the growth of the tree you outlined above.Secondary growth is the result of the activity of the lateral meristem whereby the cambium thickens. Cells on the outside of the cambium become bark and secondary phloem . Cells on the inside of the cork cambium become secondary xylem. Secondary growth increases the girth of the plant root or stem, rather than its length. As long as the lateral meristems continue to produce new cells, the stem or root will continue to grow in diameter. In woody plants the process produces wood.


Plants are able to control their rate and direction of growth using hormones. Hormones also determine the formation of flowers, stems, leaves, the shedding of leaves, and the development and ripening of fruit. There are five major plant hormones, one of which is called auxin. Auxin is composed of indoleacetic acid (IAA). Auxin is produced in cells at the stem and root tips and transported to regions of growth. Auxin causes shoot cells to elongate but inhibits elongation in root cells.In 1880 Charles Darwin and his son Francis found that coleoptiles only bend towards the light when their tips are exposed. Coleoptiles are a sheath of tissue found in members of the grass family which cover the emerging shoot.

Short Description of AUXIN

Auxin hormone controls elongation (amongst other things) of cells in plants. Auxin is inhibited by light so after being produced in the tip it migrates down the side of them stem away from the light. This means the cells on this side elongate more than the other side resulting in a bending towards the LIGHT = positive phototropism (see below)


Phototropism is growth in which the direction is determined by the direction of a light source. A positive phototropism occurs when a plant grows towards light. Auxin is produced in the tips of the shoot and travels down the side of the plant facing away from the light. Auxin causes cells along this side to elongate, making the shoot bend towards the light.


Geotropism refers to a plant's response to gravity. If a seed is planted on its side or upside down, the plumule (shoot) and radical (root) always grow in the correct directions. In roots, auxin inhibits the elongation of cells as there is no light so auxin doesn't do anything.

A scientist attached a pot containing seeds to an instrument called a clinostat. A clinostat is a disc that rotates very slowly. In this experiment, it is set to rotate once every hour.Outline whether you would expect the seeds show any sign of either phototropic or geotropic response. Explain your answer.No, the shoots would not show any sign of response. The rotation of the clinostat prevents the shoots from being able to detect the true direction of light or gravity. Instead they would sense that light and gravity is all around them and therefore grow horizontally.


Nitrates Function: Used in proteins, DNA. Essential for growth.Deficiency Symptoms: Stunted growth, yellow leavesPhosphatesFunction: Needed for photosynthesis, DNA and organellesDeficiency Symptoms: Poor root growth, brown spots on leavesPotassiumFunction: Needed by enzymes in photosynthesis. Used in flowering/fruitingDeficiency Symptoms: Yellow leaves, dead areas, no fruit/flowers

Photosynthesis is the process that converts the sunlight (solar energy) into chemical energy to feed plants. The raw materials are CO2 and H2O. In the presence of sunlight and chlorophyll they react to form the product glucose and the waste product O2.


Concentration of Carbon DioxideAs CO2 concentration increases the rate of photosynthesis increases up to a maximum when the plant may run out of some other factor/some other limiting factor.Light IntensityAs light intensity increases the rate of photosynthesis increases up to a maximum where it levels out due to other limiting factors like not enough CO2.TemperatureAs temperature increases the rate of photosynthesis increases until 40 - 50 degrees celsius when the enzymes involved begin to denature and eventually the reactions will stop altogether.Concentration of Chlorophyll in The LeafThe greater the concentration of chlorophyll the more light can be absorbed.WaterNeeds to be sufficient water to perform at maximum possible if other factors are unlimited. Not usually a problem but could be in drought/desert conditions.

Waxy Cuticle is a waxy protective outer layer that prevents water lossOtherwise water would be evaporated from the top of the leaf and could become a limiting factor to photosynthesis, meaning that there could not be enough water for maximum rate of photosynthesis to occur.Epidermis is an almost transparent layer of cells that protect the interior tissues. These cells do not contain chloroplasts.Palisade Cells are tightly packed cells that are the main site of photosynthesis. Cells contain a high number of chloroplasts.Tightly packed so that it is hard for rays of sunlight to escape through them - they can capture maximum amount of sunlight for photosynthesis. Spongy Mesophyll is loosely arranged cells that facilitate the movement of oxygen, carbon dioxide and water vapour. Contain only a few chloroplasts.Contains a few chloroplasts in case any rays of sunlight manage to slip through the palisade cells. Loosely arranged so that gases have a path to diffuse quickly through. Will diffuse 10,000x faster through gas than through water.Vascular Tissue commonly known as leaf veins. Contains xylem and phloem tissues for the transport of water from the roots and glucose away from the leaf.Stomata are pore-like openings on the bottom of the leaf that allow gases to diffuse into the leaf.Allows the absorption and excretion of gases for photosynthesis.Guard Cells are located either side of the stomata and are able to open and close it.Allows the absorption and excretion of gases for photosynthesis when needed.

Gross Morphology Of A Plant


Auxin & Tropisms


Leaf Features In Relation To Photosynthesis

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