Unit 5: Plants

 Site of photosynthesis  Leaf cells absorb energy from sunlight through organelles called chloroplasts.  When solar energy is absorbed, the plant cell uses it to convert carbon dioxide and water into glucose and oxygen

 Gas exchange  Gas exchange between the interior of the plant and its environment  Epidermis contains pores through which gases can pass in and out.  In photosynthesis, the plant uses CO2 and releases O2 and H2O vapours through these pores.

 Storage  The sugars (glucose) that are produced during photosynthesis are stored in the leaves as well as other parts of the plants

 Protection from predators  Ex: cacti leaves are reduced to sharp spines; some leaves have surface hairs or toxins that prevent herbivores from eating it

 The epidermal cells are tightly packed in a single layer and covered by a waxy coating called a cuticle.

 The cuticle  prevents water loss  provides a physical barrier against bacteria, moulds and insects.

 The Epidermis  Epidermal cells do not contain chloroplasts, so they do not perform photosynthesis.  Epidermal cells are transparent, so light can pass through them to the cell within the leaf.

 Chloroplasts are found mainly in the cells of the mesophyll, which means ‘middle leaf’. This ground tissue is specialized for photosynthesis. There are two parts to the mesophyll: 

 palisade mesophyll (region directly under the upper epidermis). The tightly packed, elongated arrangement maximizes the amount of light the plant can collect for photosynthesis   spongy mesophyll (region directly beneath the palisade). The loosely packed layer with lots of air spaces allow for gas exchange between the mesophyll cells and the atmosphere through stomata. 

 A stoma (plural: stomata) is an opening in the epidermis of a leaf, through which gases pass in and out.  Two kidney-shaped cells, called guard cells, control the opening and closing of a stoma.  In terrestrial plants, most of the stomata are in the lower epidermal layer, below the spongy mesophyll.

 When stomata are open, carbon dioxide can enter the leaf and oxygen can escape. This helps the plant photosynthesize.  Ideally, plants would open their stomata whenever it was sunny. However, when stomata are open, water vapour can also escape.  How is this problematic?

 A very thin leaf could dry out and die very quickly on a sunny or windy day. Preventing too much water loss is therefore a major concern for many plants.

 If there is a good supply of water within the leaf, the guard cells expand and bend apart -- opening the stomata.  If there is a shortage of water, the guard cells become soft and collapse -- closing the stomata.

 Guard cells also have a mechanism that responds to light levels. This lets them close the stomata at night when carbon dioxide is not needed because there is not light for photosynthesis.

 In leaves, the vascular tissue (xylem and phloem) are arranged into veins that run through the spongy mesophyll.  Xylem carries water, mineral and nutrients upwards from the roots to the top of the plant.  Phloem carries food(sugars) from one part of a plant to another. It can move upwards or downwards.

Photosynthesis  Light is used with Carbon dioxide from the air and water from the root system to produce glucose and oxygen. This occurs during the day.  6CO 2 + 6H 2 O + Light Energy = C 6 H 12 O 6 + 6O 2 Cellular Respiration  The glucose is broken down (with oxygen from the air) to create ATP with carbon dioxide and water as by products.  C 6 H 12 O 6 +6O > 6CO 2 +6H 2 O+36ATP

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