Plant Structure and Function PDF

# Topic 8: Plant Structure and Function ## Parts of a Plant - **Shoot System:** - **Flower:** Helps in reproduction. - **Leaf:** Performs photosynthesis. - **Fruit:** Protects the seeds. - **Stem:** Supports the plant. - **Root System:** - **Root:** Absorbs water and minerals....

# Topic 8: Plant Structure and Function ## Parts of a Plant - **Shoot System:** - **Flower:** Helps in reproduction. - **Leaf:** Performs photosynthesis. - **Fruit:** Protects the seeds. - **Stem:** Supports the plant. - **Root System:** - **Root:** Absorbs water and minerals. * The shoot above the soil and the roots in the soil. * Roots have root hair cells to increase surface area to absorb more water and anchor the plant with the soil particles. * The stems have vascular bundles made up of xylem and phloem which transport water, minerals and sucrose up and down the plant. * Leaves have specialised cells with many chloroplasts to be able to produce food by photosynthesis. * The undeveloped flower is enclosed in a bud. When the flower dries out it forms fruit containing seeds. ## Need for Specialisation Plants are all multicellular. As an organism gets larger there is a need for a transport system and specialisation to carry out certain functions. | Structure | Function | |---|---| | Root | Anchorage and Absorption of water | | Stem | Support the leaves and Transport of water and nutrients | | Leaf | Photosynthesis | | Flowers | Reproduction | ## The Leaf Usually a flattened structure of a higher plant, typically green and blade-like, that is attached to a stem directly or via a stalk (petiole). Leaves are the main organs of photosynthesis and transpiration. ### Functions of the Leaf 1. **Photosynthesis:** To produce glucose using carbon dioxide, water and energy from sunlight. 2. **Gas Exchange:** Stomata are small openings on the underside of the leaf, these regulate moisture, gas exchange and temperature (cooling effect as water vapour escapes through stomata). 3. **Transpiration:** They lose water through their leaves. As a result, water is pulled up throughout the plant from the roots upwards. 4. **Food Storage:** Leaves store food in form of sucrose for later use. ## Internal Structure of the Leaf * **Waxy Cuticle:** Made of wax. It is transparent, to let light pass through. Prevents water loss from the leaf. * **Upper Epidermis:** One cell layer thick. Contains no chloroplasts and are transparent. It helps protect the leaf by aiding in preventing water loss and providing an extra layer between the outside and inside of the leaf. * **Palisade layer:** Elongated cells that are packed with chloroplasts. Most photosynthesis takes place here. Found in the top part of the leaf to get as much light as possible. * **Spongy mesophyll Layer:** Has chloroplasts for some photosynthesis. Contain air spaces to let gases diffuse into the leaf. * **Stomata:** Small holes on the lower side of the leaf opened and closed by guard cells. They allow gasses to move in or out of the leaf and allows water to go out of the leaf by transpiration. * **Vascular Bundle:** Contains Xylem to transport water and minerals and phloem to transport sucrose. ## Photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy, normally from the Sun, into chemical energy that can be later released to fuel the organisms' activities (by respiration). $Water + Carbon Dioxide \rightarrow Glucose + Oxygen$ ## Need for a Transport System in Plants Plants do need a transport system to move food, water and minerals around. * Food is produced by the leaves by photosynthesis. * Water and minerals are obtained by the roots by osmosis. They use two different systems: * **Xylem** moves water and minerals from the roots upwards to the leaves. * **Phloem** moves food substances from leaves to the rest of the plant both upwards and downwards. Remember gas exchange happens throughout the plant. Stomata are found in leaves, stems and roots. ## Phloem and Xylem - **Phloem:** - Transportation of food and nutrients such as sucrose and amino acids from leaves to the roots, storage organs and growing parts of plant. This movement of substances is called Translocation. - Bidirectional movement of nutrients. Moves up or down the plant's stem. - These are living cells; elongated, tubular shape with thin cross walls with sieve plates. The sieve plates have pores at each end in the cross walls to increase the speed of translocation. - Living tissue with little cytoplasm but no nuclei. - Phloem occurs on outer side of the vascular bundle. - Forms vascular bundles with xylem. - **Xylem:** - Water and mineral transport from roots upwards to the leaves and flowers. This movement of water is called Transpiration. - Unidirectional; only moves upwards through the plant's stem. - Tubular shape with no cross walls which allows a continuous column of water + helps more rapid transport within the xylem vessels. - Dead, hollow continuous tubes with no cell contents. - Xylem occupy the centre of the vascular bundle. - Forms vascular bundles with phloem and gives mechanical strength to the plant due to presence of lignified cells (with lignin). The lignified cell wall also makes the xylem waterproof and prevent it from collapsing under the pressure of water transpiration. ## The Stem - The stem holds the leaves in the right position for photosynthesis. It also holds the flowers in the best position for pollination. - Within the stem there are the xylem and phloem. Water is transported in the xylem from the roots upwards to the rest of the plant while glucose is transported by the phloem to the rest of the plant; both upwards and downwards. - The outer layer of the stem is the epidermis. This gives protection to the stem. - The vascular bundles of the stem are found at the outer side of the stem. These give support and flexibility to the stem. The stem is packed with packing cells called cortex and pith. These store water and sugars. Pith cells are found in the centre of the stem while cortex cells are found around the vascular bundles. - Cambium tissues are the single-celled layer of meristematic (dividing) tissues that continually divides to form phloem tissues toward the outside and xylem tissues toward the inside. ## The Roots - Roots are found underground. Roots cells don't contain chloroplasts. The functions of the roots are to absorb water and minerals from the soil and the hold the plant firmly into the soil. - Roots can also be a storage organ to store starch for future to use. (E.g. Potato, carrot) - The epidermis is the outer layer of cells protecting the root. Root hairs increase the surface area for the absorption of water and minerals. The Vascular bundles (xylem and phloem) are found in the centre of the root for stability and anchorage. - Roots are filled with cortex cells that give support and store nutrients. ## Comparing the position of Xylem and Phloem in the stem and root (In Dicots) - **Stem Cross-Section:** - The xylem and phloem are arranged near the edge of the stem, with the phloem on the outside and the xylem on the inside. - This arrangement of the xylem in a ring helps: - the stem to resist compression (squashing) and bending forces caused by the plant's weight. - To give flexibility to the stem which prevents it from breaking with wind. - **Root Cross Section:** - Xylem and phloem found in the centre of the root with the xylem in the middle surrounded by the phloem. - This arrangement of the xylem in the middle helps: - to resist forces that could pull the plant out of the ground. - This means that the roots will provide anchorage and prevent the plant from being swept away. ## Root hair cells and osmosis - Plants absorb water from the soil by osmosis. Root hair cells are adapted for this by having a large surface area to speed up osmosis. - The absorbed water is transported through the roots to the rest of the plant where it is used for different purposes: - It is a reactant used in photosynthesis. - It supports leaves and shoots by keeping the cells turgid. - It cools the leaves by evaporation (transpiration). - It transports dissolved minerals around the plant. ## Factors causing water rising - **Root pressure:** Nutrients pumped into the xylem causes water to follow by osmosis. Osmosis pushes more water into the root cells. - **Capillarity:** Capillary action is the ability of water to flow in narrow spaces against gravity. (Cohesion water particles stick to other water particles and Adhesion water particles stick to other surfaces). - **Transpiration:** Transpiration is the process by which water is carried through plants from roots to the stomata on the underside of leaves, where it changes to vapour and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves which pulls the rest of the water column.