Week 18 BBC Article || Anatomy of the Plant

Adaptations of the leaf for photosynthesis and gaseous exchange

• leaves are adapted for photosynthesis & gaseous exchange
• stomata: small holes in the leaf that let CO2 in an oxygen out
• transpiration: water evaporates from leaves, leading to more water to be sucked up by the roots

Functions of leaves

• basic equation for photosynthesis - carbon dioxide and water → glucose and oxygen

Features of leaves

• large surface area - to capture light
• thin - short distance for CO2 to diffuse through
• chlorophyll - absorbs sunlight used for energy
• network of veins - support and for transporting materials
• stomata - see above

Structure of a leaf

• 
• made as efficient as possible through adapting over time

Functions of tissues of the leaf

• thin/transparent epidermis - allow more light to reach palisade mesophyll cells
• thin layer of wax - prevents infection and water loss
• palisade layer near top - increase rate of photosynthesis
• spongy layer - gases can diffuse in the leaf

Gas Exchange

• Carbon dioxide enters through the stomata and oxygen leaves
• water vapor also diffused out of the stomata

Absorbing light energy

• light absorption occurs in the palisade mesophyll
• palisade cells are packed close together

Transport in plants and the structure of specialised plant cells

• Types of transport systems in the plant:
  • xylem: moves water and mineral ions from roots to leaves
  • phloem: food (sugars and amino acids) is moved from the leaves through the plant
• Movement of food in the plant: translocation

Root Hair Cells

• plants use osmosis to absorb water from the soil
• absorb minerals by active transport (against concentration gradient)
• root hair cells: absorb water and minerals
  • Has a large surface area for maximizing rate of absorption
  • contain mitochondria so they can provide energy for active transport
• water has many purposes in a plant
  • reactant used in photosynthesis (remember light reaction cycle!
  • keeps leaves and shoots rigid
  • cools leaves by evaporation
  • transports dissolved minerals

Stomata

• Plants in dry conditions have a small amount of stomata and only on the bottom of the leaf’s surface to help prevent water loss
• plants regulate size of stomata with guard cells
  • guard cells become turgid when there is light and take in water through osmosis
  • in low light guard cells become flaccid
• size of the stomata opening helps control wilting

Plant transport tissues - Xylem and phloem

Xylem

• (xylem: moves water and mineral ions from roots to leaves) [copied from earlier]
• xylem is made of dead cells
• xylem forms a hollow tube
• strengthened by lignin - giving support to the plant
  • lignified cells = wood

Phloem

• (phloem: moves food from the leaves through the plant)
• sucrose is the transport sugar in the phloem
• phloem uses translocation; form of active transport
• parts of phloem
  • sieve tubes - for transport
  • no nuclei
  • tubes have perforated ends so the cytoplasm in the tubes connects one cell to the next
  • sucrose and amino acids are translocated in the cytoplasm of these tubes
  • companion cells - provides energy for active transport
  • attached to sieve tubes
  • sieve tubes depend on these

Transpiration

• transpiration: the plant opens its stomata to let in CO2, then water on the top layer of the spongy and palisade mesophyll evaporate and diffuse out of the cell
  • xylem provides water to replace the water that is lost
• (review) water is connected through hydrogen bonds, water molecules in the xylem are attracted to each other - a constant column of water is pulled up in the stem when transpiration occurs
• water travels through the xylem → replaced by water from the roots
• photosynthesis takes 5% of water from the plant

Water uptake and transport across the root

• root hairs are a lengthened part of epidermal cells in the roof
• water enters root hair cells through osmosis; minerals come through by active transport

Factors affecting transpiration

• temperature
• humidity
• air movement
• strength of light

Calculating a mean and principles of sampling

Investigating transpiration

• simple way to experiment with water loss in plants is to measure their change in mass
• variables that can be changed
  • air movement (ex. using a fan)
  • temperature
  • blocking the stomata (ex. using Vaseline)

Method

1. remove some leaves from a plant
2. find the mass of each leaf
3. suspend leaves
4. after a predetermined amount of time passes, re-measure mass of leaves

Analysis of results

• there will be variation in leaves, so repeat the experiment and calculate the mean of each data set

Water loss through the stomata

• scientists usually take a representative sample of the leaf’s stomata
  • to be representative it must:
  • include a decent amount of stomata spread throughout the slide
  • must be random and not be an area with few or concentrated stomata
• count stomata in this area with a microscope; repeat the experiment a couple of days and take the mean.

Measuring water uptake - Potometers

Investigating transpiration

• a potometer can be used to determine the uptake of water
  • finding the uptake of water can show the rate of transpiration
• potometer: tubing that is connected to the plant
  • water uptake is measured by recording the time that an air bubble takes to travel up a certain distance in the tube

Aim of the experiment

• AIM: to find the rate of water uptake in a plant
• 

Variables

• independent variable - time
• dependent variable - the distance moved by the bubble along the capillary tube
• control variables - temperature, air flow or draughts, water supply

Measuring effect of some environmental factors on water uptake

Investigating factors that affect the rate of transpiration

• factors that affect transpiration can also be observed through the potometer

Investigate distribution of stomata and guard cells

Counting stomata

• Why do scientists count stomata?
  • to investigate number of stomata
  • amount of closed or open stomata at a given time
  • to investigate adaptations that are made when put in certain conditions
  • effects of heightened CO2 concentration from pollution

Method 1 (for counting stomata)

1. put a drop of water onto a slide
2. tear the leaf where you’d like to examine so you can reveal the epidermis
3. examine

Method 2 (for counting stomata)

1. paint leaf’s surface with clear nail polish and let dry
2. peel off nail polish
3. examine

Recording the distribution

• density of stomata is recorded per unit area (typically sq mm)