Scaling up Study Guide

Cells are dynamic units that constantly interact with their environment, taking in necessary nutrients and expelling waste products. This exchange of substances occurs across the cell membrane, a selectively permeable barrier that controls what enters and leaves the cell. There are three primary mechanisms by which substances are transported across cell membranes: diffusion, osmosis, and active transport. These processes differ in their energy requirements, the types of substances they move, and the concentration gradients they follow.

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. This passive process does not require the cell to expend energy. It is driven by the random motion of particles. Examples include the movement of oxygen into cells and carbon dioxide out of cells in respiration, or the absorption of digested food molecules from the small intestine into the bloodstream. Factors like temperature, surface area, concentration gradient, and diffusion distance affect the rate of diffusion.

Osmosis is a special type of diffusion that refers specifically to the net movement of water molecules across a selectively permeable membrane from an area of higher water potential (dilute solution) to an area of lower water potential (concentrated solution). Like diffusion, osmosis is a passive process and does not require energy. It is crucial for maintaining water balance in cells, for example, in plant roots absorbing water from the soil, or in kidney filtration. If animal cells are placed in pure water, they may burst due to water influx, and if placed in a very concentrated solution, they may shrivel and die due to water loss.

Active transport is the movement of particles against a concentration gradient, meaning from an area of lower concentration to an area of higher concentration. Unlike diffusion and osmosis, active transport requires energy, which is typically supplied by respiration. This process is vital for cells to absorb substances they need even when their concentration is low outside the cell, or to excrete waste against a gradient. For instance, active transport is used by root hair cells to absorb mineral ions from the soil and by cells in the small intestine to absorb glucose, even when the glucose concentration in the gut is lower than inside the cells.

Blood is a vital transport tissue in the body, made up of plasma, red blood cells, white blood cells, and platelets. Red blood cells and plasma are particularly adapted for their specific transport functions. Red blood cells are biconcave discs, which gives them a large surface area-to-volume ratio, essential for efficient oxygen diffusion. They also lack a nucleus and most other organelles, creating more space for haemoglobin, the protein responsible for binding and transporting oxygen. The biconcave shape also allows them to flex and squeeze through narrow capillaries. Plasma, the liquid component of blood, is primarily water (about 90%), which makes it an excellent solvent. This property enables plasma to transport a wide variety of substances, including dissolved nutrients (like glucose, amino acids, vitamins, and mineral ions) from the digestive system to body cells, waste products (such as carbon dioxide and urea) from cells to excretory organs (lungs and kidneys), hormones from endocrine glands to target organs, and heat around the body. The fluidity of plasma ensures that all these components can be moved efficiently throughout the circulatory system. This coordinated action of red blood cells and plasma ensures the continuous supply of necessary substances and removal of waste products, maintaining the body's internal environment.