Biology: 2.1 - 2.3

What is a tissue?

A collection of similar cells which work together to complete a specific function

What is an organ?

An organ is a group of different tissue that work together to complete a specific function

3 main plant organs and function

Leaves - carry out photosynthesis, stems - support leaves and flowers, roots - absorb water and minerals from the soil and anchor the plant

3 extra plant organs

flowers, fruits, seeds

List al the parts of a leaf in order of position

Waxy cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, vascular bundle, lower epidermis, guard cell, stomata

Which parts of the leaf are tissues?

Upper epidermis, palisade mesophyll, spongy mesophyll, lower epidermis, xylem, phloem

Function of the waxy cuticle and upper epidermis

Waxy cuticle is waterproof so less water lost by evaporation (needed for photosynthesis), Waxy cuticle acts as a barrier to the entry of pathogenic bacteria and fungi so protects against disease, both are transparent so more light reaches the photosynthesising cells

Function of the palisade mesophyll

Each palisade cells has many chloroplasts containing chlorophyll so more light absorbed for photosynthesis, palisade cells are tightly packed together so more light absorbed for photosynthesis, under the epidermis and near the top of the leaf and close to the light source so more light absorbed for photosynthesis

Function of the spongy mesophyll

Many airspaces and a large surface area so faster diffusion of carbon dioxide for photosynthesis and oxygen between stomata and mesophyll cells

Function of vascular bundle (in spongy mesophyll)

Highly branched so close to all photosynthesising cells, contains the xylem and phloem

Function of stomata

Holes in the lower epidermis that allow gas exchange; carbon dioxide to diffuse into the leaf for photosynthesis and oxygen and water vapour to diffuse out of the leaf, usually closed at night to reduce water loss

Function of guard cells

2 surround each stoma - change shape to control the opening and closing of the stomata and therefore waterloss

Where is the meristem tissue found?

At the growing tips of roots and shoots

What does the meristem tissue do?

It is made up of rapidly dividing plant cells that grow and differentiate into all the other cell types needed and allow the plant to grow and replace dead/damaged cells

Why can unicellular organisms rely on diffusion, osmosis and active transport across their surface to obtain useful substances and remove waste products, and why don’t they need a transport system?

Large SA:volume ratio - increases rate of diffusion - short diffusion distance - low demand for substances

Why do multicellular organisms like plants need to have specialised exchange surfaces and transport systems?

small SA:volume ratio - large diffusion distance - high demand for substances

What does a plant need to transport to the leaves for photosynthesis?

Water from the soil so from the roots

What does a plant need to transport away from the leaves?

Glucose - sucrose is transported to where it’s used or stored e.g. meristem, roots, seeds, fruit, flowers

What is the function of the xylem tissue?

Transports water and mineral ions from roots to photosynthesising cells; thickened walls of xylem vessel help to hold the leaf flat and horizontal - more light absorbed for photosynthesis

What is the function of the phloem tissue?

Transport sucrose and amino acids from photosynthesising cells to where they’re used or stored

What kind of cells make up the xylem vessels?

Dead, hollow cells, arranged end-to-end - continuous vessels.

What do the xylem cell walls contain?

Lignin for strength/support and waterproofing

What happens to the cells in the xylem?

They die, lose their cytoplasm, organelles and end walls making them hollow so water can pass through the lumen

What are the holes in the xylem?

Pits that allow water out

What elements does the phloem have?

Sieve tube elements that have perforated end walls called sieve plates

What kind of cells make up the phloem?

Living cells that have fewer organelles, less cytoplasm and no nucleus

What are the key cells in the phloem?

Companion cells that help the sieve tube elements to function

Function of the sieve tube elements

Load sucrose and amino acids into and unload them out of the phloem vessels

What is transpiration?

The loss of water from the leaves of a plant

How does water leave the leaves?

It evaporates and the water vapour diffuses out the stomata

Why is transpiration described as a necessary evil?

Evil because it means the plant loses water which is needed for photosynthesis but necessary as the stomata have to be open for gas exchange (co2 diffusing in)

Which cells control water loss and gas exchange?

Guard cells which open and close the stomata

What causes the stomata to open?

Guard cells gain water by osmosis, swell and become turgid and bend outwards

What causes the stomata to close?

Guard cells lose water by osmosis, shrivel and become flaccid

What is the transpiration stream?

continuous column of water in xylem

Why is transpiration important?

It supplies water and mineral ions to cells in the leaf and stem for photosynthesis and other function - it provides water to keep cells turgid and plant upright - the larger surface area is good for absorbing light - allows evaporation from the leaf surface which cools the leaf

What is used to measure transpiration?

A mass or volume potometer

How does a mass potometer estimate the rate of transpiration?

1. Measure the initial mass

2. Leave for a certain length of time

3. Measure the final mass

4. Calculate the change in mass

5. Divide the change in mass by the time

How does a bubble/volume potometer estimate the rate of transpiration?

1. Measure the distance travelled by the bubble in a certain length of time

2. Work out the volume of water taken up in that time frame

3. Work out volume of water taken up using volume = 2PIr x distance

4. Work out the rate of uptake of water using rate = volume x time

Why does a volume potometer only give you an estimate?

It only measures the uptake of water rather than the loss - some water remains in the cells or is used in photosynthesis

How does high light intensity affect the rate of transpiration?

Increases - leaves need more co2 for PHS so stomata are open so water vapour is able to diffuse out

How does high temperature affect the rate of transpiration?

Increases - more water evaporates so more water vapour diffuses out the stomata and more kinetic energy

How does low humidity affect the rate of transpiration?

Increases - less water vapour in atmosphere so a steeper conc gradient so diffuses out faster

How does high wind speed affect the rate of transpiration?

Increases - water vapour blown away by wind so steeper conc gradient so diffuses out faster

What is translocation?

The transport of sugars and amino acids in the phloem from where they’re produced (sources) to where they’re used or stored (sinks)

Sources

leaves, stem, storage organs

Sinks

Fruit, flowers, seeds, young leaves, roots, meristem, storage organs

Main source and sink in summer

Lots of PHS so sugars transport from LEAVES TO ROOTS for respiration and storage

Main source and sink in spring

Sugars transported from STORAGE ORGANS TO MERISTEM for releasing energy for growth; also to flowers and fruit

Function of root hair cells

absorb water by osmosis and mineral ions by active transport from the soil