exchange

Why can substances not diffuse directly across the cell membrane in multicellular organisms?

Cells are not in direct contact with the external environment, diffusion distances between cells and their environment are large, larger organisms have higher metabolic rates so they need more oxygen and glucose

How have multicellular organisms evolved to have specialised exchange surfaces?

A large surface area, thin walls for short diffusion distances between cells, an extensive blood supply which maintains steep concentration gradient, being surrounded by selectively permeable plasma membrane which controls what substances are exchanged

Two reasons why insects need efficient systems for exchanging gases?

To deliver oxygen to cells which allows aerobic respiration to occur to release energy for cellular processes, and to remove carbon dioxide from cells as this builds up as a waste product of respiration and reduces pH which can denature enzymes

2 adaptations of the tracheae in insects for gas exchange?

Reinforced with spirals of chitin to prevent it collapsing, multiple tracheae which increases surface area

5 Adaptations of tracheoles in insects to aid gas exchange?

Penetrate directly into tissues which reduces the gas diffusion distances, thin walls which reduces gas exchange distance, highly branched which maximises surface area, not reinforced with chitin which allows gas exchange to occur, fluid at the ends of the tracheoles which allows oxygen to dissolve to aid diffusion and reduces water loss

1 adaptation of the spiracles to allow for efficient gas exchange in insects?

They open and close which allows them to control gas exchange with the atmosphere and minimise water loss

6 stages of gas exchange in insects?

1. Air enters the tracheal system through open spiracles

2. Air moves into larger tracheae and diffuses into smaller tracheoles

3. Tracheoles branch throughout the body transporting air directly to cells

4. Oxygen dissolves in water in tracheal fluid and diffuses down its concentration gradient from tracheoles into body cells

5. Carbon dioxide diffuses down its concentration gradient out of body cells into tracheoles

6. Air is then carried back to the spiracles via tracheae and released from the body

How are the concentration gradients between the tissues and air in the tracheal system maintained ?

Cells using up oxygen for respiration which keeps oxygen concentration low in cells, cells producing carbon dioxide in respiration which keeps carbon dioxide concentration high in cells, continuous ventilation which supplies fresh air to the tracheal system via spiracles

What is 5 examples of additional ventilation mechanisms insects may have to drive air through the tracheal system?

Mechanical active ventilation, tracheal fluid, enlarged collapsible trachea or accessory sacs, wing muscles connected to sacs, vibration of thoracic muscles

How can lactate accumulate in tissues affect the rate of gas exchange?

Reduces the water potential in tracheal fluid at the ends of tracheoles, water leaves the tracheoles via osmosis, a higher surface area is exposed for gas exchange

Describe the structure of a gill?

Gills are covered by an operculum flap, they consist of stacked filaments containing gill lamellae which are surrounded by extensive blood vessels

5 adaptations of the gills for efficient gas exchange?

1. The lamellae provide a large surface area

2. The lamellae membranes are thin to minimise diffusion distance

3. The gills have a rich blood supply to maintain steep diffusion gradients

4. The countercurrent flow of blood and water creates even steeper concentration gradients

5. Overlapping filament tips increase resistance slowing water flow over gills and allowing more time for gas exchangeThe lamellae provide a large surface area

What happens in the countercurrent flow system?

Blood and water flow over the lamellae in opposite directions. This means that oxygen rich blood meets water that is at its most oxygen rich when it first moves across the gills, maximising diffusion of oxygen into the blood. Oxygen poor blood returning from body tissues meets oxygen reduced water that has had most of its oxygen removed still allowing diffusion of oxygen into the blood. This maintains a steep concentration gradients across the entire gills

Why is countercurrent flow more efficient at gas exchange than parallel flow?

Because parallel flow reduces the concentration gradient so less oxygen can be absorbed

5 important plant structures?

upper epidermis, spongy mesophyll, stomata, lower epidermis, vascular tissue (xylem and phloem)

3 adaptations of leaf structures for efficient gas exchange?

1. Air spaces which provide a network for gases to quickly diffuse in and out of the leaf and access photosynthesising cells

2. Mesophyll cells which are dispersed throughout the leaf provide a large surface area across which gases can diffuse

3. Stomata which open when conditions are suitable for photosynthesis allowing inward diffusion of carbon dioxide and outward diffusion of oxygen and close to minimise water loss

2 ways plants limit water loss?

They have a waterproof waxy cuticle on their leaves, they have guard cells that can close stomata when needed

What are xerophytes?

Plants adapted to living in dry environments with limited water availability

6 key adaptations of xerophytes to reduce water loss?

1. Thick waxy cuticle to reduce water loss through evaporation

2. Rolling or folding of leaves which encloses the stomata on the lower surface to reduce air flow and the evaporation of water

3. Hairs on leaves trap moisture air against the leaf surface to reduce the diffusion gradient of water vapour

4. Sunken stomata in pits-reduce air flow and evaporation of water

5. Small needle like leaves-reduce the surface area of which water can be lost

6. Water storage organs which conserve water for when it is in low supply

Why is the human gas exchange system located inside the body?

Because air is not dense enough to support and protect these delicate structures and the body would otherwise lose water and dry out

Describe the pathway of air?

Air first enters the trachea then travels into the two bronchi which lead into the lungs. The air then travels into smaller airways called bronchioles and then into the alveoli

Ciliated epithelium located in the airways what is it made of?

Goblet cells which produce and secrete mucus that traps dust and microbes and cilia which waft the mucus upward to the mouth so it can be swallowed

4 adaptations of the trachea?

1. Rings of cartilage keep the airway open

2. Smooth muscle can contract or relax to constrict or dilate the airway and change airflow

3. Elastic tissue contains elastic fibres with elastin that allows stretching and recoiling

4. Lined with ciliated epithelial cells and goblet cells,

4 adaptations of the bronchi?

1. Reinforced with cartilage to keep the airway open

2. Smooth muscle can contract or relax to constrict or dilate the airway and change airflow

3. Elastic tissue contains elastic fibres with elastin that allows stretching and recoiling

4. Lined with ciliated epithelial cells and goblet cells

4 adaptations of the bronchioles?

1. No cartilage so can change shape

2. Smooth muscle can contract or relax to constrict or dilate the airway and change airflow

3. Elastic tissue contains elastic fibres with elastin that allows stretching and recoiling

4. Simple squamous epithelium

8 adaptations of the alveoli for gas exchange?

1. Wall consists of one layer of squamous epithelial cells which allows rapid diffusion

2. Large surface area which increases rate of gas exchange

3. Partially permeable means only certain gases can move across the wall

4. Surrounded by dense network of capillaries which brings blood close to air for gas exchange

5. Ventilation of air maintains steep diffusion gradient

6. Elastic fibres which allow stretching recoiling

7. Collagen fibres contain strong collagen that prevents alveoli from bursting and limits overstretching

8. Moist inner surface- this allows gases to dissolve

5 adaptations of the pulmonary capillaries for gas exchange?

1. Thin walls maintain a short diffusion distance

2. Red blood cells pressed against capillary walls reduces diffusion distance

3. Large surface area increases diffusion speed

4. Movement of blood maintains steep diffusion gradient

5. Slow blood movement which allows more time for diffusion

What is ventilation?

The constant movement of air in and out of the lungs

Describe inspiration?

The external intercostal muscles contract while the internal intercostal muscles relax moving the ribcage up and out. The volume of the thoracic cavity increases. The diaphragm contracts and flattens further increasing the volume of the thoracic cavity. The lung pressure decreases below atmospheric pressure. Air flows into the lungs down the pressure gradient

Explain expiration?

The external intercostal muscles relax moving the ribcage down and in. The volume of the thoracic cavity decreases. The diaphragm relaxes and unflattens further decreasing the volume of the thoracic cavity. The lung pressure increases above atmospheric pressure. Air is forced out of the lungs down the pressure gradient

What is physical digestion?

The breakdown of large food pieces into smaller ones to increase the surface area for chemical digestion

What is chemical digestion?

When enzymes catalyse hydrolysis reactions that break bonds in large insoluble molecules to form smaller soluble molecules

8 key components involved in digestion?

Salivary glands, oesophagus, stomach, liver, pancreas, small intestine, large intestine, rectum

Describe starch digestion?

Salivary amylase breaks down starch into the disaccharide maltose in the mouth. Acid in stomach denatures salivary amylase. Pancreatic amylase continues starch digestion in small intestine. The epithelial cells in the ileum lining produce maltase to break down maltose into alpha glucose monomers

What happens in digestion of lipids?

Salts emulsify lipids into tiny droplets called micelles increasing the surface area of the lipids. Pancreatic lipase breaks down micelles into fatty acids and monoglycerides

What are the three types of proteases and how they digest proteins?

Endopeptidases- hydrolyse internal peptide bonds in the middle of proteins to form shorter polypeptides, increasing the number of ends for other proteases to work on

Exopeptidase- these hydrolyse peptide bonds at the ends of polypeptides to remove terminal amino acids or dipeptides

Dipeptidases- break down any remaining dipeptides into amino acids

5 adaptations of the ileum for absorption?

1. The walls are folded into finger like villi to increase surface area

2. Epithelial cells have microvilli to further increase surface area

3. It has thin walls which reduce diffusion distance

4. It has an extensive capillary network and a good blood supply to maintain steep diffusion gradients

5. Muscles in the ileum wall contract to mix content and bring new material into contact with villi

How are amino acids and monosaccharides absorbed into the blood?

Sodium ions are actively transported out of ileum epithelial cells. This sets up a sodium ion concentration gradient between the ileum lumen and the epithelial cells. Sodium ions are co transported with amino acids or monosaccharides from the lumen into the epithelial cells. Amino acids and monosaccharides move by facilitated diffusion from epithelial cells into the blood

How are triglycerides absorbed into the blood?

1. Micelles are broken down to release fatty acids and monoglycerides.

2. As they are both non-polar, fatty acids and monoglycerides can diffuse into the epithelial cells that line the ileum.

3. Triglycerides reform inside the cells’ endoplasmic reticulum.

4. Triglycerides are packaged into chylomicrons for transport.

5. Chylomicrons are released from the epithelial cells by exocytosis into lacteals, which are lymphatic vessels in the villi.

6. Chylomicrons are transported via lymph vessels in the lymphatic system to the blood.

What do micelles and bile salts do?

Micelles speed up transport aiding lipid digestion and bile salts emulsify fats