6.1 Exchange between organisms and their environment

Why is diffusion alone not an efficient means of exchange in large organisms?

- Could only meet the needs of relatively inactive organisms - Even if the exchange surface could supply enough of a substance, it would take too long for it to reach the cells near the middle of the organism.

Why is SA/volume ratio important in multi-cellular organisms?

- Exchange takes place only at the organism's surface - Materials absorbed are used by cells that mostly make up its volume - Therefore for exchange to be efficient the exchange surfaces of an organism must be large compared to its volume

How have larger organisms evolved to combat the issue of low SA/volume ratio with size?

- Flattened shape so no cell is ever far from the surface (e.g. Leaves + Flatworms) - Specialized exchange surfaces with large areas to increase the SA/volume ratio (e.g. Lungs in mammals + Gills in fish)

How are exchange surfaces specialized to allow effective transfer of materials both passively and actively?

- Large surface area relative to the volume of the particular organism (increases rate of exchange) - Very thin (Shortens diffusion pathway - materials cross exchange surface rapidly) - Selectively permeable (allow selected materials to cross) - Movement of environmental medium (maintains diffusion gradient) - Transport system (ensure movement of internal medium - e.g. blood - to maintain diffusion gradient -Located inside organism (as being thin, they can easily be damaged and dehydrated) with a means of moving the external medium over the surface e.g. ventilation of lungs in a mammal

Why is a mass transport system necessary in multi-cellular organisms?

- Majority of cells are far away from exchange surface - Diffusion alone cannot supply/remove tissue fluid with materials needed to keep its composition constant within a suitable metabolic rate - Therefore, in a mass transport system, once absorbed materials are rapidly distributed to the tissue fluid and waste products are returned to the exchange surface for waste removal.

Give four examples of things that need to be interchanged between an organism and its environment

- Respiratory gasses (e.g. Oxygen and Carbon Dioxide) - Nutrients (e.g. glucose, fatty acids, amino acids, vitamins, minerals) - Excretory products (e.g Urea and Carbon Dioxide) - Heat

Name three factors that affect the rate of diffusion of substances into a cell

- Surface area - Thickness of cell-surface membrane - Permeability of cell-surface membrane to the particular substance - Concentration gradient of substance between inside and outside of cell - Temperature

How does surface area to volume ratio change with an organism's size?

As organisms become larger, their volume increases at a much faster rate than their surface area

Where and how does exchange of materials between an organism's internal and external environments take place (Generally)

At exchange surfaces Involves crossing cell plasma membranes

What is Ficks Law

Diffusion = Surface area x Difference in concentration Length of diffusion pathway

How do you calculate the volume of a cube/cuboid

HEIGHT X WIDTH X LENGTH

How is an organism's metabolic rate related to its evolved exchange surface and transport system?

High metabolic rate Exchange more material (By diffusion/active transport) Require larger SA/Volume ratio (Ficks law) Reflected in Exchange surface + Transport system

How do exchanges take place? (excluding heat)

Passively - no metabolic energy required (e.g. Osmosis and Diffusion) Actively - metabolic energy required (e.g. by active transport)

In an organism, which two factors affect the amount of each material that will be exchanged? Why is this?

Size Metabolic rate - Organisms which are large have a higher metabolic rate because there are more cells and molecules undergoing chemical reactions - Organisms with a high metabolic rate exchange more materials - Therefore they require a larger SA to Volume ration (Ficks law - increases rate of diffusion)

Why is a complicated transport system not necessary in small organisms?

Small organisms have a surface area that is large enough, compared with their volume, to allow efficient exchange across their body surface.

How do you calculate Surface area to Volume ratio?

Surface area Volume

What is the immediate environment around cells in multi-cellular organisms?

Tissue Fluid

What is the function of a mass transport system?

To maintain the diffusion gradients that bring materials to and from the cell-surface membranes To maintain the stability of the external environment (tissue fluid) within a suitable metabolic rate In large organisms, to carry substances between the exchange surfaces and the rest of the body (and between parts of the body)