B3 - Gas Exchange and Transport

Gas Exchange

Organisms absorbing one gas and releasing another

Surface Area-to-Volume Ratio Gas Exchange

Smaller in larger organisms so they require specialized inner surfaces.

Features of Gas Exchange Surfaces

1. Permeable - O2 and CO2 can diffuse freely

2. Large - Surface area is large in relation to volume

3. Moist - Covered by film of moisture for gases to dissolve

4. Thin - Gasses diffuse short distance

Concentration Gradients in Small Organisms

• Cell respiration maintains gradients

• Oxygen is continuously used and CO2 is produced, so oxygen concentration within remains lower than outside

Concentration Gradients in Large Organisms

Pumping is required to maintain gradients.

• Air or water adjacent to gas exchange surface is replaced through process of ventilation

Airway for Ventilating Lungs

Nose, mouth, trachea, bronchi and bronchioles

Ventilation Air Pressure

Gas always flows from high pressure to low pressure. Muscle contractions cause air pressure changes in the thorax that pull extra air into the alveoli then push it out again.

Inspiration (Inhaling)

• External intercostal muscles contract, moving ribcage up and out

• Diaphragm contracts, flattening and moving down

• Increased thorax volume

• Pressure inside thorax drops below atmospheric pressure

• Air flows into the lungs until air pressure rises

Exhalation (Exhaling)

• Internal intercostal muscles contract, moving ribcage down and in

• Abdominal muscles contract, pushing diaphragm up

• Decreased thorax volume

• Air flows out from the lungs

Lung Adaptations for Efficiency

• Airways

• Large surface area

• Extensive capillary bed

• Short distance for diffusion

• Most surface with surfactant

Airways Adaptation

Consist of branching bronchioles ending in alveolar ducts.

Large Surface Area Adaptation

Large number of alveoli (300 million) creating surface area 40x greater than outside body

Extensive Capillary Bed Adaptation

The surface area of the networks of blood capillaries is almost as large as the alveoli themselves

Short Distance for Diffusion

Single layers of extremely thin cells so air and blood are a short distance apart. Less than a micrometre

Moist Surfactant Adaptation

Fluid secreted by cells in alveolus walls that keep the lining moist allowing gas to dissolve. Reduces surface tension and prevents sides from sticking together causing lung collapse.

Ventilation Rate

The number of times that air is drawn in or expelled per minute

Tidal Volume

Volume of fresh air inhaled or volume of stale air exhaled with each ventilation

Vital Capacity

The total volume of air that can be exhaled after maximum inhalation.

Inspiratory Reserve Volume

Amount of air a person can inhale forcefully after normal tidal inhalation.

Expiratory Reserve Volume

Amount of air a person can exhale forcefully after normal tidal exhalation

Lung Volumes

Spirometer

Device used to measure lung volumes and flow rates into and out of lungs.

Waxy Cuticle Leaf Adaptations

Upper and lower surface of leaves is covered in waterproof wax. Reduces water loss but also prevents gas movement

Guard Cell Leaf Adaptations

Pairs of guard cells in epidermis and can open or close pores. Pores are called stomas. Stomas are normally closed at night when photosynthesis isn’t happening.

Air Space Leaf Adaptations

Stomata connect the air outside to a network of air spaces in the spongy mesophyll allowing for diffusion.

Spongy Mesophyll Leaf Adaptations

Inner tissue of leaf with extensive air spaces. Provides large total surface area. Photosynthesis maintains the concentration gradients.

Vein Leaf Adaptations

Xylem vessels located in veins replace water lost by evaporation.

Leaf Tissues

Water Evaporation

When hydrogens bonds between molecules break and become water vapour.

Transpiration

Loss of water vapour from the leaves and stems of plants. Is affected by environmental factors.

Factors Affecting Transpiration

• Temperature (positive correlation): More energy available to break hydrogen bonds

• Humidity (negative correlation): Reduces concentration gradient of water vapour

• Wind: Increases transpiration by moving pockets of air, but stomata close with strong wind

Stomatal Density

Number of stomata per unit area of leaf surface.

Measuring Stomatal Density

1. Sample of epidermis peeled from leaf and examined with miroscope

2. Colourless nail varnish painted to form a cast. When dried it is peeled off and examined with microscope

3. Surface of leaf is photographed and micrograph is used for stomatal counts

Stomatal Density = (Mean number of stomata)/(area of field view)

Capillary Structure

Adaptations of Capillaries

• Large surface area: Narrowest blood vessels. Capillary network has huge total length with narrow diameter

• Thin walls with pores. Allows for small/medium molecules to pass through

• Fenestrations: Large pores allowing larger volumes of tissue fluid to be produced. Speeds up exchange between tissue cells and blood.

Arteries

Carry pulses of high pressure blood away from the heart to every organ of the body

Veins

Carry stream of low pressure blood from the organs back to the heart.

Structural Features of Arteries

• Thicker walls

• Narrower lumen

• Circular in section

• Inner surface corregated

• Fibres visible in the wall

Structural Features of Veins

• Thinner wall

• Wider lumen

• Circular/flattened

• Inner surface smooth

• No or few fibres visible

Layers of Arteries

• Tunica externa - outer coat, connective tissues with tough collegen to prevent swelling despite high pressure

• Tunica media - thick layer containing smooth muscle and elastic fibres for pumping

• Tunica intima - smooth endothelium lining the artery and reducing resistance to flow

• Lumen - space in which the blood flows

Pulse Oximeter

Digital way method to measure pulse using LEDs shining light into finger and detectors measure how much light is absorbed depending on the amount of blood in the tissues

Layers of Veins

• Tunica Externa: tough outer coat of protective tissue

• Tunica Media: Thin layer with few elastic or collagen fibres because there is low blood pressures

• Tunica Intima: smooth endothelium to reduce resistance

• Lumen: relatively wide space to accommodate slow moving blood

Vein Valves

Because of low pressure there is a risk of backflow. Valves prevent this. Each consists of 3 pocket-shaped flaps that are closed by blood flowing backwards and opened by blood flowing towards the heart.

Atheroma

Plaque caused by the deposition of lipids including fats and cholesterol in the walls of arteries. They narrow the lumen, restricting blood flow.

Occlusion

Total blockage of the artery caused by the build up of plaque (atheroma)

Coronary Heart Disease

Medical conditions due to narrow or blocked arteries. When blood flow to regions of the heart wall is restricted, causing pain in the chest (angina) or shortness of breath.

Risk Factors of Coronary Heart Disease

• High blood pressure (hypertension)

• Smoking

• Obesity

• Inactive lifestyle

• Family history

• Old age

• High blood cholesterol

• Diabetes

Capillary Action

Water lost by evaporation is replaced by water drawn through pores between cellulose molecules in leaf walls. This is due to adhesion of water to cellulose and cohesion between water molecules.

Xylem Vessels

In leaf vein where water is drawn through. Tensions are generated inside when water is drawn out of them.

Transpiration Pull

Cohesion of water allows for tension to be transmitted down the columns of water down to the roots. Allows passive transport against the force of gravity. Creates a rope-like resistance due to hydrogen bonding.

Adaptations of Xylem Vessels

• Wall thickening and lignification - Thickened walls impregnated with polymer lignin. Prevents collapse when low pressure

• Lack of end walls and cell contents - creates continuous tubes so flow of xylem sap is unimpeded

• Pits for entry and exit of water - Wall thickenings are impermeable so there are gaps allowing water to enter and exit.

Dicotyledons (dicots)

Plants with two embryo leaves in their seeds

Distribution of Tissues in Dicotyledonous Plants

• Epidermis: single layer of cells with waxy cuticle to reduce water loss

• Phloem: thin-walled cells that transport sugar and food

• Cambium: Small cells with thin walls that divide by mitosis

• Xylem: Wide tubular structures that transport water and minerals

• Cortex: Thin walled cells that strengthen the stem

• Pith: Large thin-walled cells that fill the centre of the stem

Distribution of Tissues in Dicotyledonous Roots

• Epidermis absorbs water and mineral ions from the soil often using root hairs

• Phloem transports sugars and other foods

• Xylem transports water and mineral ions up stem

• Cortex bulks out the root to strengthen it and increase surface area