Learning objectives

• • describe the general structure of a plant
  • distinguish between monocotyledonous and dicotyledonous plants
  • identify parts of the internal structure of a young dicotyledonous stem and root
  • compare internal structure of dicotyledonous stem and root
  • state the functions of xylem and phloem vessels
  • define transpiration
  • describe the functions of transpiration
  • investigate factors affecting rate of transpiration
  • describe adaptations of leaves to minimize water loss
  • define translocation
  • define translocation
  • describe the effect of ring barking

Diffusion

• is the movement of molecules from their region of higher concentration to their region of low concentration along the diffusion gradient
• the steeper the concentration gradient the faster the rate of diffusion
• occurs during gaseous exchange in the lungs, movement of soluble food from the ileum into blood stream etc.

 

Factors affecting diffusion

• temperature: higher the temperature the faster the rate of diffusion (molecules have more kinetic energy at higher temperature) and the opposites true
• size of particles: the smaller the particles the faster the rate of diffusion
• concentration: the higher the concentration the faster the rate of diffusion
• surface area to volume ratio: the larger the surface area the faster the rate of diffusion

 

Importance of gaseous and solute diffusion

• obtain many of their requirements
• get rid of their waste products carbon dioxide
• gas exchange for respiration

 

Osmosis

• is the movement of water molecules from the region of of high concentration to their region of low concentration through a permeable (partially permeable) membrane

 

Water Potential

• is the energy that freely moving water molecules have enabling them to diffuse from one region to another
• it is proportional to the concentration of water molecules
• The higher the concentration of water molecules the higher the water potential
• water molecules always move from a region of higher water potential to a region of lower water potential

 

Importance of osmosis

• water moves from the soil into the root hairs of plants by osmosis
• water moves from one cell to another in plants by osmosis eg in xylem vessels

 

Osmosis In Plant cells

Turgidity

• when a plant cell is placed in a solution which has a higher water potential that its cell sap;
• water will then enter the cell by osmosis.
• vacuole will force cytoplasm and the cell membrane against the cell wall.
• this pressure is acting on the cell wall is called turgor pressure
• helps keep stems of plants upright and keeps leaves firm and flat so that they can absorb sunlight

 

Flaccidity

• when a plant cell is placed in a solution with lower water potential than its cell sap;
• water then moves out by osmosis, the cytoplasm and the vacuole shrink.
• the cell membrane will pull away from the cell and the cell become placid;
• this is called plasmolysis

 

Active Transport

• The movement of molecules from an area of lower concentration to an area of higher concentration (against concentration gradient) by using energy.
• Active transport is involved in a number of processes occurring within an organism eg;
• Dissolved mineral salts by the root hairs,
• Glucose and amino acids by cells in the small intestine of humans.

 

Example of active transport

• absorption of glucose and amino acids by the villi into the small intestines.
• absorption mineral ions such as nitrates from the soil by the root of the plants.

 

Plant structure

plants have body made up of roots, stems, leaves and flowers. Cotyledon is a food storing leaf eg bean/peas

• Plants are classified into ;
• monocotyledonous
• dicotyledonous

Monocotyledonous

They have leaves with parallel veins, fibrous root system and seed embryo has a single cotyledon

dicotyledonous

They have leaves with branched veins,a tap/main root with side branches and seed embryo has two cotyledons

Differences between mocots and dicots

Monocots	Dicots
eg maize grain	bean seed
one cotyledon	two cotyledons
cotyledon is thin and lacks food materials	cotyledon is fresh and store food
endosperm present which stores food	endosperm is absent

Internal structure of stem and root

Stem

Root

Plant parts and functions

Nme	Function
Xylem vessels	water & dissolved minerals salts transport
Phloem tubes	transport soluble foods
Epidermis	absorption and protection
Root hairs	increase surface area for absorption of water and mineral salts
Cotex	storage and support
cambium	divides vascular bundles/tissues

Vascular Tissue

• Vascular tissue is a plant tissue that transports substances in a plant.
• It consists of two vessels.
• 1. Xylem vessel
  • A long narrow hollow vessel that transports water and minerals from roots to leaves.
  • It consists of elongated dead lignified cells that are attached end to end.
  • The vessel has pits where water and minerals enter the vessel.

 

• 2. Phloem vessel
  • A long narrow hollow vessel that transports nutrients from leaves to other parts of a plant.
  • It contains perforated sieve plates, which separate neighboring phloem cells.
  • Adjacent to the phloem cells are companion cells. They provide nutrients and energy to the
  phloem cell. They also assist in the transport of nutrients.

 

Transpiration

Is the process during which water evaporates from the surface of the leaf into the atmosphere.

Functions of transpiration

• provides a cooling effect on leaves
• create a transpiration stream ie the continous loss of water creates a stream in plant

Factors affecting transpiration

• surface area and stomata
• wind
• humidity
• temperature
• light intensity

Leaf adaptation to minimise water loss by transpiration

• reduced surface area
• distribution of stomata ie more stomata found in lower part of the leaf
• thickness of cuticle – some plants have needle shaped leaves or leaves reduced to spines
• Rolling of leaves – trapping water vapour through their surface.
• presence of hairs

 

Wilting

It occurs when the rate of transpiration exceeds the rate of water intake/absorption from the soil.Leaves use their strenght and hang down and the plant will eventually die

 

Translocation

is the movement of assimilates ie photosynthetis products in the phloem tubes throughtout the plants