GCSE BIOLOGY REVISION: ORGANISATION AND PLANT TISSUES
Principles of organisation
Cells are the basic building blocks of all living organisms.
A tissue is a group of cells with a similar structure and function.
Organs are aggregations of tissues performing specific functions.
Organs are organised into organ systems, which work together to form organisms.
Students should be able to develop an understanding of size and scale in relation to cells, tissues, organs and systems.
These concepts have been discussed in further details in a previous article.
Plant tissues
Students should be able to explain how the structures of plant tissues are related to their functions.
Plant tissues include:
• epidermal tissues
• palisade mesophyll
• spongy mesophyll
• xylem and phloem
• meristem tissue found at the growing tips of shoots and roots.
The leaf is a plant organ. Knowledge limited to epidermis, palisade and spongy mesophyll, xylem and phloem, and guard cells surrounding stomata.
Tissue
Structure relating to function
Epidermal tissue
Thin and transparent. This allows more light to reach into the palisade cells
Palisade mesophyll
Absorption of more light and the increased rate of photosynthesis
Spongy mesophyll
There are air spaces inside which allow carbon dioxide to diffuse through for photosynthesis
Xylem and phloem
Xylem- lose their end walls so a continuous hollow tube is formed. Walls are strong with lignin deposition as lignin gives strength and support
Phloem- presence of companion cells to carry out the living functions of the cell. Has sieve tubes which are specialised for transport as they have no nuclei, this means there is more room for movement of solutes. Additionally, cytoplasm connects from one cell to the next.
Meristem tissue
Undifferentiated cells, able to develop into whatever type of cell is required in the plant
Structure of the leaf and its adaptations for photosynthesis (top to bottom of leaf)
Structure
Purpose
Adaptations
Waxy cuticle
Makes the leaf waterproof
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Upper Epidermis
Provides a layer for the waxy cuticle to lie on
Thin and transparent to maximise the amount of light which can reach the mesophyll layers
Palisade Mesophyll
Photosynthesis
The cells in this layer contain many chloroplasts to maximise the rate of photosynthesis
The cells are column shaped and packed closely together to maximise the number of cells that can be packed into this layer and so maximise the rate of photosynthesis
Layer situated towards the upper surface of the leaf so that the photosynthesising cells are close to the light
Spongy Mesophyll
Gas exchange and photosynthesis
Gas exchange is where carbon dioxide diffuses into the spongy mesophyll cells, and oxygen diffuses out. This is vital for photosynthesis (see later topic)
Layer packed loosely so that gases can get between the cells.
Cells covered by a thin layer of water which the gases dissolve in as they move into and out of cells.
Cells also contain chloroplasts for photosynthesis but less than the palisade layer.
Lower epidermis
Provides a layer for the waxy cuticle to lie on
Has small pores called stomata which allow water to enter or exit the leaf by transpiration. Guard cells open and close the stomata to control the rate of transpiration and so water loss.
image- https://www.holganix.com/blog/plant-tissue-is-cooler-than-you-think
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