BIO SEM 1 UNIT 3

What is active transport, and why does it require energy?

The movement of molecules against a concentration gradient, using carrier proteins; it requires energy (ATP) because moving substances against their gradient does not happen spontaneously

Describe the steps of the P-type ATPase (primary active transport) mechanism.

1) The substrate binds to the carrier protein; 2) ATP is broken down and a phosphate group binds to the protein, causing it to change shape; 3) the substrate is released on the other side of the membrane, the phosphate is released, and the protein returns to its original shape

Give three examples of P-type transporters.

Na+/K+ ion pump, H+ ATPase pump, Ca2+ ATPase pump

What is co-transport (secondary active transport)?

Two substances are transported together: one moves down its concentration gradient (releasing energy) which drives the movement of the other against its own concentration gradient

Describe how glucose is absorbed in the small intestine via co-transport.

1) A Na+/K+ pump actively transports Na+ out of epithelial cells into the blood, creating a Na+ concentration gradient; 2) a Na+-glucose co-transporter allows Na+ to move back into the cell down its gradient, carrying glucose with it; 3) glucose then moves into the blood via facilitated diffusion

What is endocytosis?

A method of transporting large volumes of material into a cell, where the cell surface membrane wraps around material to form a vesicle inside the cell

What is pinocytosis, and give an example?

The endocytosis of liquid/dissolved substances — the membrane invaginates, forms a vesicle around the fluid, and detaches; e.g. absorption of fat droplets in the small intestine

What is phagocytosis, and give an example?

The endocytosis of large solid particles — the membrane forms pseudopodia that surround the particle, forming a food vacuole/phagosome, which then fuses with a lysosome for digestion; e.g. white blood cells engulfing pathogens

What is exocytosis, and what is exocytosis of a liquid called?

The transport of material out of a cell via a vesicle fusing with the cell surface membrane and releasing its contents; exocytosis of a liquid is called reverse pinocytosis

Compare simple diffusion, facilitated diffusion, and active transport in terms of energy use and direction of movement.

Simple and facilitated diffusion are passive (no energy) and move substances down a concentration gradient; active transport requires energy (ATP) and moves substances against a concentration gradient using carrier proteins

Why do multicellular organisms need specialised transport systems, unlike many unicellular organisms?

Diffusion alone is too slow over the larger distances in multicellular bodies; they have a lower surface area to volume ratio (less area to absorb nutrients relative to body size); and a higher metabolic rate, using nutrients/producing waste faster

What is the function of xylem tissue, and what types of cell does it contain?

Transports water and dissolved mineral ions around the plant (from roots to leaves); contains vessel elements and tracheids (dead, hollow, lignified cells), fibres (support), and parenchyma cells

What is lignin, and what is its role in xylem?

A waterproof substance that coats/strengthens the walls of vessel elements and tracheids, making xylem vessels rigid and impermeable

What are pits in xylem vessels, and what is their function?

Areas in the xylem cell wall that lack lignin, allowing water to move sideways between adjacent vessels/tracheids

Describe the symplast pathway of water movement through the root.

Water moves through the cytoplasm of root cells by osmosis, passing between cells via plasmodesmata

Describe the apoplast pathway of water movement through the root.

Water moves through the cell walls and the spaces between them (not through the cytoplasm), carrying dissolved substances down a pressure gradient

What is the Casparian strip, and why is it important?

A waterproof band of suberin/lignin in the cell walls of the endodermis; it blocks the apoplast pathway, forcing water to enter the symplast pathway (through the cytoplasm) before reaching the xylem

What is the cohesion-tension hypothesis (how does water move up the xylem)?

Water loss through transpiration creates tension (a pulling force) at the top of the xylem; because water molecules are cohesive (stick together via hydrogen bonds), the whole column of water in the xylem is pulled upward, drawing more water in from the roots

What is transpiration?

The evaporation of water from inside leaves followed by its diffusion out through the stomata into the surrounding air, down a water potential gradient

Describe how light causes stomata to open.

1) Light enables photosynthesis, producing ATP; 2) ATP is used to actively transport K+ ions into the guard cells; 3) this lowers the guard cells' water potential, so water enters by osmosis; 4) the guard cells swell, becoming turgid, and bend to open the stomatal pore

List the four factors affecting the rate of transpiration and their effects.

Light (more light = more open stomata = more transpiration); temperature (higher = more kinetic energy in water molecules = faster transpiration); humidity (lower humidity = steeper water potential gradient = faster transpiration); wind speed (higher wind = removes water vapour from around stomata = steeper gradient = faster transpiration)

What is a xerophyte, and list two example adaptations.

A plant adapted to live in dry conditions (e.g. cacti, marram grass); adaptations include a thick waxy cuticle (reduces evaporation), sunken stomata/rolled leaves/hairs (trap humid air around stomata), spines instead of leaves (reduce surface area for water loss), deep roots, or CAM photosynthesis (stomata open at night when cooler)

What is a halophyte, and how do they cope with their environment?

A plant adapted to live in salty conditions (e.g. mangroves); adaptations include accumulating salt in root cells (lowers water potential so water still enters), salt glands to excrete excess salt, and proline accumulation in vacuoles to lower cell water potential

What is a hydrophyte, and list two example adaptations.

A plant adapted to live in water (e.g. water lilies); adaptations include aerenchyma (air spaces for buoyancy and oxygen transport), flexible stems (bend with currents), breathing roots, and finely divided underwater leaves (reduce water resistance)

What is translocation, and what does phloem transport?

The movement of assimilates (e.g. sucrose, amino acids) through the phloem; phloem transports the products of photosynthesis around the plant

Describe the structure of phloem tissue.

Sieve tube elements are living cells with a thin layer of cytoplasm and no nucleus, joined end-to-end via sieve plates to form sieve tubes; companion cells are linked to sieve tube elements via plasmodesmata and carry out living functions for them

Define "source" and "sink" in the context of phloem transport.

Source = a part of the plant that produces/stores food (e.g. leaves), where assimilate concentration is high; sink = a part of the plant that uses food (e.g. roots, fruits, growing stems), where assimilate concentration is low

Describe the mass flow hypothesis of phloem transport.

At the source, assimilates are actively loaded into sieve tubes, lowering water potential so water enters by osmosis, raising pressure; at the sink, assimilates are removed, raising water potential so water leaves by osmosis, lowering pressure; this pressure difference drives mass flow of fluid from source to sink

Give two pieces of experimental evidence supporting the mass flow hypothesis.

Ringing experiments (removing a ring of bark/phloem causes sugar to accumulate above the ring, showing downward flow from the source); aphid experiments (phloem sap oozes faster from aphid mouthparts closer to the leaves, showing flow from leaves down the stem); radioactive tracer studies (using autoradiography to track labelled sugars moving from source to sink)

Give two pieces of evidence against the mass flow hypothesis.

Sieve plates would create a lot of resistance, requiring a much larger pressure gradient than predicted; assimilates can move in two directions simultaneously (e.g. down to roots and up to fruits), which mass flow's single pressure gradient cannot fully explain

Describe how sucrose is actively loaded into companion cells.

1) H+ ions are pumped out of the companion cell into surrounding tissue using active transport, creating an H+ concentration gradient; 2) H+ ions move back into the companion cell down this gradient via a co-transport protein; 3) sucrose binds to the same co-transport protein and is carried in alongside the H+ ions (sucrose can then move into sieve tubes via diffusion through plasmodesmata)

Compare xylem and phloem in terms of substances transported, direction, and pressure.

Xylem transports water/mineral ions, moves from roots to leaves only (one direction), and its fluid is under negative pressure (tension); phloem transports assimilates, moves from source to sink (can be in either direction depending on location), and its fluid is under positive pressure