Plant tissues, organs and systems

Plant tissues

Structure of the Leaf

The leaf is a plant organ made up of several distinct tissue layers.

• Epidermal Tissues:

• Upper Epidermis: Transparent to allow light to pass through to the palisade layer. Covered in a waxy cuticle to reduce water loss.

• Lower Epidermis: Contains stomata (pores) to allow gas exchange.

• Palisade Mesophyll: * Found under epidermal tissue.

  • Packed with chloroplasts for maximum photosynthesis.

  • Cells are column-shaped to fit more into a small area.

• Spongy Mesophyll: * Contains air spaces to increase the surface area for the diffusion of gases (carbon dioxide in, oxygen out).

• Meristem Tissue: * Found at the growing tips of roots and shoots.

  • Able to differentiate into different types of plant cells which allows it to grow.

• Direct Light: If an exam question asks why the palisade layer is at the top, the answer is always "to absorb more light for photosynthesis."

• Diffusion: If asked why the spongy mesophyll has gaps, it is "to allow gases to diffuse easily to and from the cells."

• Organ vs. Tissue: Remember that the leaf is an organ, while palisade mesophyll is a tissue.

Plant organ system

The roots, stem and leaves form a plant organ system for transport of substances around the plant.

Root hair cells are adapted for the efficient uptake of water by osmosis, and mineral ions by active transport.(Large surface area and lots of mitochondria).

The role of stomata and guard cells are to control gas exchange and water loss.

• Xylem tissue made of dead cell that transports water and mineral ions from the roots to the stems and leaves. It is composed of hollow tubes strengthened by lignin(passive process)

• The transpiration stream is the movement of water up the xylem vessels to the rest of the plant. Water vapour diffuses out of the leaves through the stomata

• Factors Affecting Rate:

  • Temperature: Higher temperature = faster evaporation and diffusion = faster transpiration.

  • Humidity: Higher humidity = slower diffusion (due to low concentration gradient) = slower transpiration.

  • Air Movement (Wind): More wind = removes water vapour from leaf surface = speeds up the diffusion of more water vapour out of the leaf.

  • Light Intensity: More light = stomata open wider for CO2 for photosynthesis = more water evaporates.

The rate of transpiration is observed using a potometer, which involves placing a plant in a tube of water and measuring the distance travelled by the bubble.

• Phloem tissue made of living cells transports dissolved sugars from the leaves to the rest of the plant(occurs in both directions) through the process of translocation for respiration or storage.

• Structural Adaptations:

  • Sieve Plates: The end walls between phloem cells have pores (holes). These allow the sugary solution to flow easily from one cell to the next.

  • Companion Cells: Because phloem cells lack many organelles, they have "companion cells" next to them. These cells are packed with mitochondria to provide the energy needed for active translocation.

  • Two-Way Flow: Transport in the phloem can go both up and down the plant to wherever the sugar is needed most.

If a question asks why a plant wilts, it’s because it is losing water faster than it is taking it up. The cells lose turgor pressure and become flaccid.

Guard Cells and Stomata

The lower epidermis is specialised for gas exchange and controlling water loss.

• Stomata: Small openings that allow CO2 to enter and CO2 and water vapour to leave.

• Guard Cells: Control the opening and closing of stomata.

  • When the plant has lots of water: Guard cells become turgid and curve, opening the stomata.

  • When the plant is short of water: Guard cells become flaccid, closing the stomata to prevent further water loss.

  • When lot if water is available to the plant, the cells fill and change shape opening the stomata

  • This allows gases to be exchanged and more water to leave the plant