A quite different specialized exchange surface is the root hairs of a flowering plant. Root hairs are very thin extensions of the cells that make up the outer layer, or epidermis, of a root. Each root hair is about 200-250 µm across. There may be thousands of them on each tiny branch of a root, so together they provide an enormous surface area that is in contact with the soil surrounding the root.
Soil is made up of particle of minerals and humus. In between these particles are spaces that are usually filled with air. Unless the soil is extremely dry, there is a thin layer of water coating each soil particle. The root hairs make contact with water, and absorb it by osmosis. The water moves in because there is a lower concentration of solutes in the water in the soil than there is inside the root hair cell. There water potential outside the root hair is therefore higher than the water potential inside it, and water moves passively down the water potential gradient into the cells.
Mineral ions are also absorbed from the soil into root hair cells. If there is a higher cell than inside it, then it can be taken up passively, by facilitated diffusion. More usually, however, the plant requires ions that are present in relatively low concentrations in the soil, while its cells already contain a higher concentration of that ion. In that case, the ions will be taken up by active transport, through carrier proteins, using energy to move them against their concentration gradient. Note that the cell wall provides absolutely no barrier to the diffusion of water molecules or mineral ions-they are all able to pass through it quite freely.
Summary
The plasma membrane controls exchange between the cell and its environment. Special transport proteins are sometimes involved.
Within cells, membranes allow compartmentalization and division of labour to occur, within membrane-bound organelles such as the nucleus, ER and Golgi apparatus.
Some chemical reactions take place on membranes as in photosynthesis and respiration. Membranes also contain receptor sites for hormones and neutrotransmitters;possess cell recognition markers, such as antigens;and may contain enzymes, as with microvilli on epithelial cells in the gut.
Diffusion is the net movement of molecules or ions from a region of their higher concentration to one of lower concentration. Oxygen and carbon dioxide cross membranes by diffusion through the phospholipids bilayer. Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins.
Water moves from regions of higher water potential to regions of lower water potential. When this takes place through a partiallyb permeable membrane this diffusion is called osmosis. Pure water has a water potential of zero. Adding solute reduces the water potential by an amount known as the solute potential, which has a negative value. Adding pressure to a solution increases the water potential by an amount known as the pressure potential, which has a positive value.
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Soil is made up of particle of minerals and humus. In between these particles are spaces that are usually filled with air. Unless the soil is extremely dry, there is a thin layer of water coating each soil particle. The root hairs make contact with water, and absorb it by osmosis. The water moves in because there is a lower concentration of solutes in the water in the soil than there is inside the root hair cell. There water potential outside the root hair is therefore higher than the water potential inside it, and water moves passively down the water potential gradient into the cells.
Mineral ions are also absorbed from the soil into root hair cells. If there is a higher cell than inside it, then it can be taken up passively, by facilitated diffusion. More usually, however, the plant requires ions that are present in relatively low concentrations in the soil, while its cells already contain a higher concentration of that ion. In that case, the ions will be taken up by active transport, through carrier proteins, using energy to move them against their concentration gradient. Note that the cell wall provides absolutely no barrier to the diffusion of water molecules or mineral ions-they are all able to pass through it quite freely.
Summary
The plasma membrane controls exchange between the cell and its environment. Special transport proteins are sometimes involved.
Within cells, membranes allow compartmentalization and division of labour to occur, within membrane-bound organelles such as the nucleus, ER and Golgi apparatus.
Some chemical reactions take place on membranes as in photosynthesis and respiration. Membranes also contain receptor sites for hormones and neutrotransmitters;possess cell recognition markers, such as antigens;and may contain enzymes, as with microvilli on epithelial cells in the gut.
Diffusion is the net movement of molecules or ions from a region of their higher concentration to one of lower concentration. Oxygen and carbon dioxide cross membranes by diffusion through the phospholipids bilayer. Diffusion of ions and larger polar molecules through membranes is allowed by transport proteins.
Water moves from regions of higher water potential to regions of lower water potential. When this takes place through a partiallyb permeable membrane this diffusion is called osmosis. Pure water has a water potential of zero. Adding solute reduces the water potential by an amount known as the solute potential, which has a negative value. Adding pressure to a solution increases the water potential by an amount known as the pressure potential, which has a positive value.
