Movement of Substances Across Cell Membranes

There are many ways in which substances can move across the cell membrane and in which then helps in the function of the body (cells, tissues and organs) overall. There are 3 processes that a substance can move across the membrane, theses are diffusion (simple and facilitated), osmosis and active transport. Both diffusion and osmosis are passive processes, which mean they do not need energy to occur, whereas active transport does require energy. All three processes are involved in many functions of the body, including photosynthesis and respiration and gas exchange in a fish.

The structure of the cell membrane is important in the movement of the substances. It has structure which is referred to as the ‘fluid mosaic model’. This is when the membrane consists of two layers of phospholipid molecules. Proteins are embedded in each thin layer of phospholipids, some span the membrane from one side to the other, and others appear on one face of the membrane only. The proteins that span the membrane, called intrinsic proteins, are the ones that are important in the transport of substances across the membrane. The phospholipids join together and make a phospholipid bilayer.

One process in which water can move across membranes is osmosis. This is a process that involves the passive moment of water molecules through a partially permeable membrane from an area of high water concentration to an area of low water concentration. It does not require any energy to take place as it works on a concentration gradient. For cells to survive, ion concentration needs to be the same on both sides of the membrane. If the cell does not pump out all of its extra ions to even out the concentrations, the water is going to move in. This can be very bad as the cells can swell up and explode.

Osmosis is a very useful process in the plant as it insures that the cells in the plant are turgid, swollen and hard, this turgidity is important as it makes the plant stand up to the sun light. Another process that substances can move across membranes is active transport, this process involves energy in the form of ATP to take place. This process moves substances against their concentration gradient. This happens when intrinsic protein molecules act as molecular pump. They allow the cell to use active transport to accumulate glucose or ions against their concentration gradient.

Animal and plant cells that specialise in absorption usually have abundant mitochondria which provide the ATP needed to power active transport. There are several vital processes that depend on active transport. Which includes, absorption of amino acids from the gut, absorption of mineral ion by plant roots, excretion of urea and hydrogen ions by mammalian kidney, exchange of sodium and potassium ions in nerve cells and loading of sugar from the leaf into the phloem in plants. There are two types of diffusion and the first is simple.

This is the net movement of particles from a region of high concentration to a region of low concentration. It occurs in cells through membranes but only if the pores are big enough. It is a passive process and so happens without any energy input from the organism. The rate at which it occurs depends on the difference between the concentration gradients, the distance across the membrane and the size of the molecules that are diffusing. Some cells, such as those in the human gut and inside plant leaves, have special adaptations to allow diffusion to happen quickly. These adaptations reduce the distance over which diffusion occurs.

The biconcave-disc shape of red blood cell gives it a much greater surface area, allowing maximum amount of oxygen to diffuse into it. The final type of movement is facilitated diffusion. This is the process for larger lipid-insoluble molecules, such as glucose. They get help from intrinsic proteins within the phospholipid membrane. These proteins are called carrier proteins, which provide a passage through. The protein recognises the ions and interacts with their shape and so opens and the molecule can move through. This process too is passive to moves in the way of the concentration gradient.

Also in nerve impulses these processes are very important. A resting potential gives a neurone the potential to transmit a nerve impulse, the resting potential is produced mainly by the active transport of sodium ions to the outside of the nerve fibre making the outside positive. The pump that does this consists of enzyme ATPase, which is a carrier protein the cell membrane. As sodium ions are pumped out of the cell, potassium ions are pumped in, but at a slower rate. This is part of the process that involves the movement by the processes above and without them the whole process could not occur.