HL Biology (Form & Function) - Unit Test 1 Review

Gas exchange in plants, transpiration, tissue distribution (dicot plants), membranes and vesicles, proteins and transport

Epidermis (plant)

Single cell layer that covers and protects the outer surface of leaves, stems, and roots in plants.

Palisade Mesophyll Tissue

Single layer of tightly packed cells, rich in chloroplast to maximize photosynthesis.

Spongy Mesophyll Tissue

Loosely packed with moist cell walls to dissolve CO₂ and help water evaporate. There are air spaces between them where gas can circulate to maintain the concentration gradient.

Stomata

Small pores in the epidermis of leaves (mostly) and stems where gas exchange happens.

Guard Cells

Surround stomata, causing them to open when turgid (full of water) and close when flaccid (drained of most water).

Transpiration

The process by which water vapour escapes through stomata, diffusing outside with the concentration gradient. It’s impacted by various environmental factors.

Waxy Cuticle

Protective, hydrophobic layer that protects the plant, it’s secreted by the epidermis.

Main purposes of leaves

To maximize light and CO₂ in order to make glucose, and to regulate loss of water vapour to prevent dessication.

Dessication

When a plant loses too much water (vapour) and dries out.

Factors affecting rate of transpiration

Temperature, humidity, wind, light intensity

Temperature’s impact on transpiration

Evaporation is increased, creating a higher concentration of vapour inside the plant, causing it to diffuse outward at a higher rate.

Humidity’s impact on transpiration

There is a higher concentration of vapour outside the plant, causing diffusion to decrease with the concentration gradient.

Wind’s impact on transpiration

Vapour outside the plant is displaced, causing more of it to exit the plant with the concentration gradient.

Light intensity’s impact on transpiration

Sunlight causes stomata to open, and infrared rays increase temperature.

What stomata do when conditions are dry

Close early in the day

Xylem function

Carrying water and dissolved minerals from the roots to the stem & leaves

Phloem function

Distributing sugars and amino acids from photosynthesis throughout the plant

Stomatal density

Amount of stomata per unit area

Stomatal density depends on

Size and shape as well as species/environment (variety of other factors, but these are the main ones)

Osmosis

Passive transport of water through a membrane from an area of high concentration to an area of low concentration (with the gradient)

Process of stomata opening

Blue light from the sun triggers the guard cells to begin actively pumping potassium inside, causing the concentration to increase. This raises the solute potential while lowering the water potential, causing water to flow in through osmosis (with the concentration gradient). Turgor pressure increases as the vacuole fills with water, and the cells bend outward, opening the stoma.

Process of stomata closing

In darkness, the potassium ions leave the guard cell, lowering the solute potential and raising the water potential. The water then leaves through osmosis, allowing the cells to become flaccid as the vacuoles empty. Stomata then close to prevent desiccation.

Essential role of the stem

Supporting leaves in sunlight

Essential role of the root

Acts as an anchor for plants, and absorbs water and ions in the soil

Essential role of the leaf

Organ for photosynthesis, includes the leaf blade and stalk

Sources of replacement water during transpiration

Cell cytoplasm, water in spaces of nearby cell walls, xylem vessels

Transpiration is a result of…

Plant structure/nutrition and the gas exchange mechanism

Why water evaporation cools the plant

Energy is needed to break down the H-bonds when water vapour forms

The water stream in the xylem passively carries…

Dissolved ions to help with growth

All plant cells get water from this type of movement

Lateral movement

This type of pressure supports the leaf to give it more sunlight

Turgor pressure

Process by which xylem cells/tubes are created

They begin as normal cells with cellulose walls and living contents, then the end walls dissolve and the mature vessels become long/hollow tubes. Living contents are used up to bring cellulose to the inner surface of the walls, which is hardened by the deposition of lignin. This gives it the internal strength to remain upright despite suction force.

Mature xylem vessels are always

Non-living

The reason mature xylem vessels are non-living

Transportation must be passive

Reason for gaps in the xylem vessels

Water passage

3 Pathways for Water Transport

1. Apoplast

2. Symplast

3. Vacuolar

Apoplast pathway

Where the majority of water flow happens, within the free spaces between cellulose fibres, completely avoiding the living contents of the cells. This also includes the water filled spaces of dead cells and hollow xylem vessels. This route involves capillary action and diffusion.

Symplast pathway

Through the cytoplasm of cells and plasmodesmata (small cytoplasmic connections/channels between cells). It involves membrane regulated diffusion, but the organelles resist/slow the water flower, making it significantly slower than the apoplast pathway.

Vacuolar pathway

Osmosis between the vacuoles of cells, driven by the concentration gradient. It’s caused by the uptake of minerals/ions, and lets individual cells absorb water.

Order of layers in a plant root, from outer to inner

Root hairs, epidermis, cortex, endodermis/casparian strip, xylem and phloem

Endodermis

Layer of cells with a waxy strip integrated that blocks the apoplast pathway, forcing water to move symplastically. It’s selectivley permeable, acting as the ‘airport security’ of the root.

Casparian Strip

Waxy strip within the endodermis that blocks the apoplast pathway

Proton pumps

Protein pumps in the root hair cell membranes that use ATP to push H⁺ ions out into the soil, creating an electrochemical gradient where it’s more negative inside the cell and more positive outside. The inside now wants positive nutrients, allowing cations in the soil to enter passively.

How anions enter the roots against the gradient with the cations

Symport (secondary active transport), a buddy system of sorts

Unlignified pits

Act as pores in the xylem, a place for water to escape in case of blockage

Cortex function

Support and photosynthesis

Pith function

Bulks out the stem, which would otherwise be hollow

Cambium

Produces xylem and phloem cells

Differentiate the stem and root’s tissue distribution

The root has a larger cortex, causing the vascular bundles (xylem and phloem) to be more congested/smaller. In the stem, the xylem and phloem are about the same size and shape, while in the root, the xylem branches out like a star while the phloem surrounds it. The other components are mostly the same.

Triggers of root pressure

1. High humidity that limits diffusion

2. Nighttime/darkness that closes the stomata

3. Early spring (in deciduous trees), when leaf/stomatal surfaces haven’t fully formed

List the 5 steps of evaporation

1. Active transport of minerals

2. Water potential dropping

3. Osmotic water entry

4. Casparian strip blocking apoplast pathway

5. Positive hydrostatic pressure is generated

Step 1 of evaporation (AT of minerals)

Endodermal cells use protein pumps and ATP to push minerals into the xylem’s lumen.

Step 2 of evaporation (Water potential decrease)

The xylem sap becomes hypertonic relative to the cortex, creating a high solute concentration/potential.

Step 3 of evaporation (Osmotic water entry)

Water passively enters the xylem vessels, flooding them due to the high solute concentration in them caused by the minerals uptake.

Step 4 of evaporation (Casparian strip blocking apoplast)

Water and solutes are forced to move symplastically (through cells’ cytoplasm), preventing backwards leakage and trapping pressure.

Step 5 of evaporation (Pressure)

Hydrostatic pressure is generated by the water/solute accumulation in the enclosed space, and this root pressure pushes the sap upwards in the xylem.

Critical functions of evaporation

1. High tension/freezing that breaks the water column (air bubbles can also form and stop water flow, but root pressure can cause water to push through and dissolve these bubbles)

2. Sap bleeding caused by high humidity, resulting in the sap leaking out of the pores

Translocation

Bidirectional movement of sugars through phloem tissues

Phloem source area

Tissues that produce/release sugars

Phloem sink area

Tissues that consume/store sugars

The main adaptations of the phloem

1. Sieve tubes

2. Companion cells

Structure of sieve tubes

Long and narrow with plates (end walls) that have pores. Each are connected to a companion cell by cytoplasmic strands (plasmodesmata), which pass through the pits/gaps in the walls. They have no nucleus, and lack most organelles in general.

Structure of companion cells

Each connected to a sieve tube by plasmodesmata, which they regulate the cytoplasm of due to their lack of a nucleus.

Pressure-Flow Hypothesis

Companion cells’ role is to maintain conditions in tubes favourable to mass solute flow

Process of translocation

1. Sugars accumulate in mesophyll tissues of leaves (source area)

2. Sucrose then moves into the companion cells

3. It then moves into the sieve tubes with active transport

4. The concentration increases, raising solute potential

5. Water enters through osmosis

6. Hydrostatic pressure is generated in the tubes of the source area

Process of translocation in other living cells of the plant (often roots)

Sucrose is converted to insoluble starch deposits (in sink areas), it then flows out of the tubes, decreasing solute potential and causing water to diffuse out. This decreases the pressure.

Source and sink areas are

Dynamic/Variable

Reasons for source/sink areas to vary

1. Dormant periods

2. Seasonal changes

3. Use/conversion of starch stores

What drives the changes in mass flow in the phloem

Changes in pressure

In high temperatures cholesterol does what to the cell membrane of animal cells

Decreases fluidity by strengthening bonds between it and the surrounding phospholipids. This decreases permeability.

In low temperature cholesterol does what to the cell membrane of animal cells

Increases fluidity by forcing the phospholipids apart, maintaining space between them. This increases permeability.

Vesicle

A small membrane sac with a drop of fluid inside

2 things that cause vesicles to pinch off from the cell membrane

1. Proteins

2. ATP

Endocytosis

Something outside the cell is brought inside using a vesicle

Types of endocytosis

1. Phagocytosis (cell eating)

2. Pinocytosis (cell drinking)

Macrophages/white blood cells’ endocytotic function

Taking in debris from damaged/dying cells to dispose of it (ex. red blood cells)

Exocytosis

Something inside the cell is expelled using a vesicle that fuses with the cell membrane

Steps for proteins from the RER leaving the cell

1. Vesicles from RER carry proteins to the golgi apparatus

2. Proteins are modified at the golgi

3. Vesicles carrying the modified proteins are carried to the membrane where they then fuse with it

4. The membrane flattens again

Voltage gated protein channels

Protein channels that open/close when a certain threshold membrane potential is reached.

Steps of the Sodium/Potassium Channel (in axonal membrane of nerve cells)

1. The receptor receives the stimulus

2. Na channels open, depolarizing the axon’s interior

3. Membrane potential goes from -70 V to +40 V, initiating a nerve impulse/action potential

4. The impulse passes, leaving more positive charges inside

5. K channels open while the Na channels close

6. K exits with the electrochemical gradient into the tissue fluid outside the nerve cell

7. The axon becomes less positive and the K channels close with a ball and chain device

Structure of ‘ball and chain’ device on voltage gated channels

1. Chain is a flexible amino acid strand

2. Ball is a globular protein

Indirect active transport

When molecule/ion A moves with the concentration gradient, while another one (B) follows it using the potential energy released by A when it moves passively.

Steps of indirect active transport

1. AT creates a new electrochemical gradient by accumulating something (A) outside the cell

2. Passive/facilitated diffusion brings A back into the cell, releasing potential energy

3. Molecule B ‘hitches a ride’, following A into the cell using its previously released potential energy

Sodium dependent glucose cotransporter

Found in the small intestine and proximal tubules of nephrons in the kidney. Sodium is actively transported out of the cell, where it accumulates until it can follow the concentration gradient back in. This passive movement releases potential energy, which glucose then uses to enter the cell with sodium, against its own concentration gradient. This way, more glucose that can be used to make ATP can enter the cell without using ATP to do so.

Types of gated channel proteins

1. Voltage gated (specialized amino acid ‘sensor’ on it + ball/chain device to open and close it once specific voltage is reached)

2. Ligand gated (specific chemical binds to its receptor to open it, it closes when the chemical leaves)

Molecules that ‘glue’ animal cells together

Cell adhesion molecules (CAMs)

Function of CAMs

1. Maintain tissue structure/function

2. Stick together to form cell junctions

What CAMs bind to

1. Each other

2. Other CAMs

3. ECM (extracellular matrix)

Extracellular Matrix (ECM)

Non-cellular part of all tissues/organs that provides essential physical support for all cell parts. Ex. Bone tissue’s ECM is made up of collagen fibres and bone mineral

Cell Junction Classification

1. Cell-Cell (adherin proteins)

2. Cell-ECM (integrin proteins)

3. Anchoring (strengthen contact between cells)

4. Tight (seal gaps between cells)

5. Gap (link cytoplasms of adjacent cells to allow molecule transport between them)

6. Signal relaying (ex. synapses)