Exchange and Transport

Question 1

Diffusion equation

Answer 1

diffusion= (SA x diff. in concentration)/ length of diffusion path

Question 2

Methods of transportation

Answer 2

Passively (no energy)- diffusion and osmosis Actively (energy)- active transport

Question 3

Gas exchange in single celled organisms

Answer 3

Their small SA:Volume ratio means that oxygen is absorbed by diffusion across the cell-surface membrane. If it has a cell wall this is permeable and so not a barrier to gas diffusion.

Question 4

Respiratory system in insects

Answer 4

Spiracle -> trachea -> tracheoles -> muscle fibre

Question 5

How gases diffuse in insects

Answer 5

Diffusion gradient- oxygen is used up by respiring cells at the end of the tracheoles, reducing its concentration. This creates a diffusion gradient from along the trachea. It works in the opposite direction for carbon dioxide. In water this occurs more quickly as diffusion is more rapid than in air. Ventilation- Muscle movements can create movements of air which speeds up respiratory exchange.

Question 6

Describe spiracles and their limitations

Answer 6

Tiny pores on the body surface, which can be opened/closed by a valve. When open water can evaporate, so they are mostly kept closed. Limits the size an insect can achieve as the diffusion pathway must stay short.

Question 7

Gill structure

Answer 7

Question 8

Countercurrent flow and parallel flow

Answer 8

Question 9

Gas exchange in a leaf

Answer 9

Question 10

Stomata structure

Answer 10

Question 11

Features of transport systems

Answer 11

• A medium to carry materials eg/ blood
• A form of mass transport in which the transport medium is moved aorund in bulk over large distances
• A closed system containing the transport medium
• A mechanism for moving the transport medium within vessels, which requires a pressure difference eg/ muscle contractions or water evaporation
• Mechanism to keep flow in one direction eg/ valves
• Means of controlling flow

Question 12

Mammalian transport system

Answer 12

Question 13

Types of blood vessel

Answer 13

• Arteries carry blood away from the heart and into arterioles
• Arterioles are smaller arteries that control blood flow from arteries to capillaries
• Capillaries are tiny vessels which link arterioles to venules
• Veins and venules carry blood from the capillaries back to the heart

Question 14

Basic structure of arteries and veins

Answer 14

Tough outer layer resists internal and external pressure changes

Muscle layer can contract to control blood flow

Elastic layer maintans blood pressure by stretching and springing back

Thin innner lining (endothelium) is smooth to prevent friction and thin for diffusion

Lumen is the cavity the blood flows through

Question 15

Artery structural function

Answer 15

• Thick muscle means smaller veins can be constricted and dilated to control the volume of blood passing through them.
• Thick elastic layer because pressure has to be kept high for blood to reach extremities. Stretches at systole and springs back at diastole. This maintains high pressure and smooths the pressure surges created by the heart beating.
• Overall wall thickness resists the vessel bursting under pressure
• No valves as the high pressure means the blood doesn’t flow backwards

Question 16

Arteriole structural function

Answer 16

Thicker muscle layer than arteries to control blood movement into capillaries

Thinner elastic layer because the pressure is lower

Question 17

Vein structural function

Answer 17

Thin muscle layer as it is not needed to control blood flow to tissues

Thin elastic layer due to low pressure. Pressure is too low to create a recoil action

Overall wall is thin because there is no risk of bursting. They can then also be flattened easily to aid blood flow.

Valves throughout to ensure blood doesn’t flow backwards. When muscles contract and veins are compressed, the valves ensure the pressurised blood only flows one way.

Question 18

Capillary structural function

Answer 18

Walls only consist of lining layer giving a short diffusion distance for rapid diffusion.

Numerous and highly branched for a large diffusion surface area.

Narrow diameter to permeate tissues so no cell is far from a capillary.

Narrow lumen means RBC are squeezed flat against the side reducing diffusion distance and increasing SA.

Spaces between endothelial cells allow WBC to escape to deal with infections in tissues.

Question 19

What is tissue fluid?

Answer 19

Watery liquid containing glucose, amino acids, fatty acids, salts and oxygen. It supplies tissues with these substances and receives carbon dioxide and other waste. It bathes all the cells of the body abd is the exchange fluid between blood and cells. It is formed from blood plasma.

Question 20

Tissue fluid formation

Answer 20

Blood being pumped into the capillaries creates hydrostatic pressure at the arterial end. This forces tissue fluid out of the blood plasma. This outward pressure is opposed by two other forces:

• Hydrostatic pressure of tissue fluid outside the capillaries
• Lower water potential of blood due to plasma proteins pulls water back into the blood within the capillaries

Only small molecules leave the capillaries leaving cells and proteins in the blood. This is called ultrafiltration.

Question 21

Tissue fluid returns to the circulatory system

Answer 21

Most tissue fluid returns to the blood plasma directly vie the capillaries. At the venous end, hydrostatic pressure in the capillaries is less than the tissue fluid so the tissue fluid is orced back into the capillaries. Also, osmotic forces due to water potentialpull water back into the capillaries.

Lymphatic system carries the remainder tissue fluid back to two ducts that join veins near the heart. The contents of the lymphatic system are moved by hydrostatic pressure and muscle contractions that squeeze the vessels (valves for same purpose as veins).

Question 22

Root hair diagram

Answer 22

Question 23

Water enters root hair cells

Answer 23

Large SA and thin surface layer which materials can move across easily. The soild has a high water potential as it is mostly water, while the root hair cells have a much lower water potential due to suagars, amino acids, and minerals dissolved in them. As a result, water moves by osmosis into the root hair cells down the water potential gradient.

Question 24

Apoplastic and symplastic pathway

Answer 24

Question 25

Passage of water into the xylem

Answer 25

When water reaches the endodermis by the apoplastic pathway it is forced into the protoplast of the cell to join the water that arrived by the symplastic pathway. This is due tue the waterproof Casparian strip.

Endodermal cells actively transport salts into the xylem, lowering the water potential. Water can then enter by osmosis. The pressure caused by these salts is called root pressure.

Question 26

What is the main force that pulls water up the stem of a plant?

Answer 26

Transpiration

Question 27

Water movement out of the stomata

Answer 27

If the stomata are open, water vapour molecules diffuse into the surrounding air. The water lost is replaced by water evaporating from the cell walls of the surrounding mesophyll cells.

Question 28

Water movement across a leaf

Answer 28

Water moves down a water potential gradient out of the leaf from the xylem, through the mesophyll to air through the stomata.

Question 29

Evidence supporting cohesion-tension theory

Answer 29

• Diameter of tree trunks changes according to the rate of transpiration. In they day the tree diameter is smaller than at night
• A broken xylem vessel means a plant can no longer draw up water because the continous column o water is broken so the water molecules can no longer stick together
• When a xylem is broke water doesn’t leak out, but air is drawn in, showing it is under tension and not pressure

Question 30

Energy needed for transpiration

Answer 30

As a [assive process it doesn’t directly need metabolic energy. However, energy in the form of heat from the sun is necessary to draw the water up.

Question 31

Factors affecting transpiration

Answer 31

1. Light. Photosynthesis only occurs in the light, and so stomata are mostly open in the light for gas exchange. Water leaves via the stomata, and so light increases rate of transpiration.
2. Temperature. This affects the water potential of air, as well as the speed at which water molecules move. An increase in temperature increases kinetic energy meaning water eaporates more readily from the leaf. It also decreases humidity i.e. water potential.
3. Humidity. This affects the diffusion gradient from the air in the leaf and the air outside the leaf. Less humid= more transpiration.
4. Air movement. Wind moves water from the leaves more easily by reducing the layer of moist air, increasing transpiration rate.

Question 32

Potometer

Answer 32

Question 33

Adaptations of xerophytic plants

Answer 33

• Thick cuticle (waterproof barrier)
• Rolled leaves traps a region of still air within the leaf, making a very small water potential gradient
• Hairy leaves trap moist air next to the leaf surface, reducing the water potential gradient
• Stomata in pits/grooves traps moist air
• Reduced SA:Volume ratio of leaves to reduce water loss, but must be balanced with are for photosynthesis