Biology Section 3: From Cell to Organism: Focus on Plants

Maximizing Potential

Tissues are structured, organized groups of cells that work together to achieve a certain function

Unicellular organisms can only grow to a certain size before more specialized cells/tissues are needed to survive

Having tissues composed of a bunch of smaller cells allows for the quick transport of things in and out of the organisms (SA:V?)

For example, plants have large flat leaves to help with maximizing gas exchange and light absorption which is needed for photosynthesis

How Do the Small Survive?

1. Division of labour: In small, single-celled organisms, they have structures which they perform many functions at once whereas more complex organisms have specialized cells and tissues

2. Size: Unicellular organisms are limited due to their SA:V ratio in regards to transporting essential nutrients and gases, where as specialized systems which work specifically to take care of that purpose

3. Interdependence of cells: The life of a unicellular organism depends on one cell, it can easily die-off when faced with challenging conditions. Multicellular organism are more resilient due to their wide range of specialization

Specialized & Organized

Why do larger organisms have to be multicelled?

As organisms grow larger they must become multicellular

Different cells become SPECIALIZED to perform specific tasks

The organism must be ORGANIZED: OCTOS (organelle, cell, tissue, organ, system)

Dissecting the Leaf

Cuticle

Cells of the leaves and stem secrete a waxy substance

Resists attack from micro-organisms and helps to reduce water loss from the plant

Epidermis (Dermal tissue)

Outer layer of cells that covers all herbaceous plants

One layer thick and responsible for the exchange of matter an gases into and out of the plant (Carbon dioxide and Oxygen)

Xylem (one way)

Xylem tissue moves water and dissolved minerals from the roots of the stem to the leaves where they’re used for photosynthesis

Phloem (two way)

Phloem tissue transport sucrose and other dissolved sugars from the leaves to other parts of the plant. Formed from individual sieve tube cells

Stomata

Tiny pores created from guard cells. Stomata is used for gas exchange

Guard Cells

Regulate the size of the stomata (opening) to control H2O loss

The shape of the guard cells change to open or close the stomata

Spongy Mesophyll

Contains chloroplasts and carry out photosynthesis

Loosely packed with many air spaces around them - this structure helps in water & gas exchange with the environment

Peliside Cells

Main cells for photosynthesis contains LOTS OF CHLOROPLASTS

Why are most stomata found on the underside of a leaf?

Lower surface are not directly exposed to sun. This tends to reduce excessive water loss. It is a beneficial adaptation to check water loss in plants. Thus, the underside of leaves have more stomata

Reactions that involved gases

Photosynthesis

Occurs in the chloroplast

Energy is produced (glucose)

Greater volumes of gases exchanged in photosynthesis

Cellular Respiration

Occurs in the mitochondria

Energy produced (ATP)

Where does the Diffusion of Gases occur?

Between the environment and the plant

• Diffusion through the stomata

Within the plant

• Gasses move in and out of the intercellular spaces (spongy tissue)

• Passive transport

Let’s Talk About Stomata

Gas moves in and out of plants via diffusion

• O2 net movement out

• CO2 net movement in

During the daytime, CO2 gas enters the plant through openings on the bottom side of the leaf called stomata

Each stomata is surrounded by a pair of special cells called guard cells (work like elevator doors)

Let’s Talk About Guard Cells

Take in potassium by active transport (stimulated by light on the leaf)

• This ^ particles in cell - water then enters by osmosis and the guard cells swell, opening the stomata

When the guard cells are relaxed, they look like they’re deflated and they touch together (closing the stomata)

When water moves into the guard cells (via osmosis) they enlarge-look inflated - and they open allowing for CO2 to move in and oxygen to move out

• Water moving into the guard cells increase pressure inside called turgor pressure

Guarding the Goods

Guard cells are specialized cells that are at the opening of the stomata

When conditions are favorable they swell open (usually when there is plenty of sunlight or CO2)

• They swell open due to solutes travelling into the cells (which drag water with them)

• When conditions are not favorable (lack of water, too hot, minimal light) they deflate and close the opening to the stomata

Number and appearance depends on environmental conditions

• Hot, dry climates with low humidity have fewer stomata

• High humidity = more stomata

• Low Co2 levels = stomata open

• Normal levels of CO2 = stomata is relaxed

Solutes such as potassium salts, chloride ions, and additional sugars will diffuse across the membrane of the guards cells

Water will move into the guard cell to dilute it, which causes swelling

During this time the stomata opening is exposed to the environment

So why not leave the stomata open?

1. So plants are constantly losing water

2. This loss of water is called Transpiration

3. Without closing GC the plant would become very dehydrated

Beyond the Leaf

In the roots and stem gas exchange occurs in the outer layer of cells

Lenticles break through the bark (on woody plans) and allow air to diffuse through

Within the plant diffusion is used in the spongy tissue

Water Movement in Plants

Water moves from the “roots to the shoots”

Previously, we mentioned how water and nutrients move through the plant like water in a straw

The attraction of water molecules is called cohesion, this is how water droplets help to drag each other through the plant, due to water’s polar nature

Water droplets will also stick to the sides of the cells themselves, think of when a water droplet clings to a straw. This is called adhesion

Water transport in xylem vessels

Sugar transport in phloem vessels

Uptake of water in roots

More specifically…

• Properties of H2O that aid in water transport

• Root pressure

• Transpiration

Root Pressure

Root pressure is how water flows from the roots through the xylem

Water is forced from higher pressure roots (active transport), to the lower pressure spaces in the leaves

Root pressure works together with transpiration (evaporation of water through the leaves) because as water droplets leave the plant during photosynthesis/cell resp.

Another water molecule gets pulled up (by cohesion) to replace the lost water particle

Expanding on Transpiration

When the stomata of a plant open, not only do they exchange gas, but they also release tiny bits of water to the environment as well (evaporation)

If it is very hot out, transpiration will occur at high rates therefore water will be drawn up faster

This loss creates a tension or transpiration pull which draws water up the xylem

Some water will go to help continue the transpiration pull, while other water that enters the leaf will go to help making sugars in the process of photosynthesis

How Water Moves Through the Plant

H2O absorption in roots (Root hairs increase surface area for osmosis)

1. Root Pressure

   • soil outside root is hypotonic to root cell so water enters cell by osmosis

2. Cohesion of Water Molecules

   • Water sticks to other water molecules and forms a long chain of water molecules and pull each other along

3. Adhesion of Water Molecules

   • Water sticks to inside walls of xylem

4. Transpiration Pressure

   • Water flows out of stomata in leaves by diffusion

   • The air spaces created are filled in by new water molecules (adhesion and cohesion)

   • This pulls the whole column of water up the xylem

How do sugars move in Phloem?

Pressure Flow Theory

Sugars made in pallasade layer (spongy mesophyll) are actively transported from leaf cells to the phloem system

Pumping of sugars into the phloem makes the phloem hypertonic (lower concentration of water) to the cells around it

Water moves into phloem from the surrounding cells by osmosis

Water entering the phloem system creates pressure that moves sugar and water down phloem to rest of plant and roots