This post is going to describe some of the ways molecules can cross the cell membrane. (For Eton students revising for Trials, diffusion and active transport are found in the F block syllabus, osmosis comes in E Block)
Diffusion is the simplest to understand. Diffusion does not even need a cell membrane to occur. In the example below the dye molecules will move randomly in the solution. As the dye starts in one place, these random movements will mean that slowly spread out until an equilibrium is reached. This movement of the dye from the region of high concentration to the low concentration is called diffusion.
When considering diffusion into a cell, if the cell membrane is permeable to a particular molecule then the random movements of the molecule will mean that there will be a net (overall) movement from the higher concentration to the lower concentration down the concentration gradient.
Key Points about diffusion:
- Always happens down a concentration gradient (from a high concentration to a lower one)
- Never requires any energy from the cell – it is a passive process
Active Transport is a process that will move molecules into a cell against the concentration gradient – i.e. from a low concentration to a high concentration. This “pumping” of the molecules against the gradient requires energy from the cell and of course this energy comes from respiration.
You can see from the diagram above that active transport is working against the concentration gradient, is using energy from inside the cell (actually a molecule made in mitochondria in respiration called ATP) and that a specific transport protein is involved in the cell membrane. This protein will have a binding-site that is specific for a particular molecule and the solute molecule to be transported will collide with the transport protein due to random movement. Energy from the cell can cause the transport protein to change shape such that the solute is released on the other side of the membrane.
Can you think of another area of the iGCSE syllabus which features collisions between a specific binding-site on a protein and a certain other molecule? Linking ideas is a key characteristic of the A* Biologist!
Osmosis is the hardest of these processes to understand properly, especially as an iGCSE student when you are often told an over-simplified account that does not make sense…. Let’s try to simplify it in a way that does make sense.
Firstly it is only water molecules that can move by osmosis into and out of cells – never anything else. Indeed osmosis is the only way water can cross a membrane – it never moves by diffusion or active transport.
Osmosis is a passive process – it never needs any energy from the cell’s respiration and the only energy involved is the kinetic energy of the water molecules.
Osmosis can only occur through a partially permeable membrane. All cell membranes are partially permeable and this means they let small molecule like water through but prevent the diffusion of the larger solute molecules.
The water molecules on both sides of the membrane in the diagram above will be moving around randomly. They will occasionally hit one of the pores in the membrane and so pass across the membrane. This movement will be happening from left to right and from right to left.
The presence of the sucrose (solute) in the solution on the right means that some of the water molecules on that side of the membrane are less able to move. This is because they are temporarily attracted to the solute molecules by weak hydrogen bonds. So their kinetic energy is reduced and this makes them less likely to randomly collide with the pores in the membrane. The presence of the solute on the right means that water molecules on the left on average are more likely to collide with the membrane than the water molecules on the right and this leads to an overall movement from left to right. This net movement of water molecules from the dilute solution to the more concentrated solution through the partially permeable membrane is called osmosis.
This diagram has the two solutions reversed so in which direction will osmosis happen here? Thats right from right to left. You can see the hydrogen bonds attracting water molecules to the solute – these are the ones that lower their kinetic energy overall.
You might even have been taught about osmosis with reference to the water potential of a solution. The water potential of a solution is just a measure of how much kinetic energy the water molecules in a solution possess. So a dilute solution will have a high water potential, a concentrated solution (with lots of dissolved solute) a lower water potential.
Osmosis is the
- net movement of water
- through a partially permeable membrane
- from a solution with a high water potential (a dilute solution) to a solution with a lower water potential (a concentrated solution)
- Oxygen diffuses from the air in the alveolus into the blood
- Carbon Dioxide diffuses from the air spaces in the leaf into the palisade mesophyll cells of the leaf
- Glucose diffuses from the blood into an actively-respiring muscle
- Nitrates are pumped from the soil into root hair cells by active transport
- In the kidney, glucose and other useful molecules are pumped from the nephron back into the blood by active transport.
- In nerve cells, sodium and potassium ions are pumped across the cell membrane to set up the gradients needed for a nerve impulse
- Water enters root hair cells from the soil by osmosis
- In the kidney, water is reabsorbed from the nephron by osmosis.
- In the large intestine, water is reabsorbed from the colon back into the blood by osmosis
There are many many more examples of each process, but this should be enough to be going on with…….
In the topic of sexual reproduction in plants, the final stage is often overlooked. I think it is helpful for students to think of this topic in several distinct stages.
- Flower Structure (hermaphrodite nature of most plants)
- Pollination (self v cross pollination; wind v insect pollinated flowers)
- Fertilisation (how does the pollen tube reach the egg cell to fertilise it?)
- Seed and Fruit formation (what forms what after fertilisation)
- Seed Dispersal (by animals, by wind, by water, by explosive means)
Once the seed has been dispersed there then follows a period of dormancy when nothing happens. In latitudes such as the UK this often is there to delay germination until the following spring when growing conditions become more favourable. The process of taking an inert seed and growing a new plant from it is called germination.
You don’t need to worry too much about the details of germination but there are a few vital parts of the process that GCSE candidates need to appreciate for A* marks. Firstly you should know the structure of a typical seed.
The seed coat or testa surrounds the seed and provides a tough waterproof container. Inside there are the embryonic plant (composed of a plumule and radicle), one or two seed leaves called cotyledons and a storage tissue called endosperm.
Germination starts when the seed starts to take up water by osmosis. There is an opening in the testa called the micropyle that allows water to move into the seed causing it to swell and thus rupture the seed coat to allow the embryo plant to emerge.
Water entering the seed also activates the embryo plant such that it starts to release digestive enzymes such as amylase. Amylase catalyses the digestion of starch into a simple sugar maltose. The endosperm and cotyledons contain energy stores in the form of starch, lipids and proteins and as these get broken down by the various enzymes, they provide the energy for the early growth of the seedling. The radicle emerges first and grows downwards (positive geotropism) and then the plumule or shoot grows upwards towards light (positive phototropism). Remember that throughout the early stages of this growth the energy required comes from stored food molecules in the seed. If you measure the mass of the plant during this phase, it would be decreasing. Only when the first leaves emerge above ground and the plant can start the process of photosynthesis will the overall mass start to increase.
The topic of plant transport can appear quite complicated but you will see from your past paper booklets that the questions examiners tend to set on it are much more straightforward.
The key piece of understanding is to realise that there are two transport systems in plants, learn their names and what they transport.
- Xylem vessels move water and mineral ions from the roots to the leaves.
- Phloem sieve tubes move sugars, notably sucrose, and amino acids around the plant. Both of these molecules are made in photosynthesis in the leaves and so can be transported from the leaves to the areas in the plant where they are needed.
Water is needed for photosynthesis of course in the leaves (remember that rain water cannot enter leaves directly because of the waxy cuticle on the surface of the leaf). All the water that is used in photosynthesis is absorbed in the roots from the soil and moved up the plant in the xylem vessels. Minerals such as nitrate, phosphate and magnesium ions are also required in the leaves for making amino acids, DNA and chlorophyll respectively. These minerals are moved up the plant along with the water in the xylem.
How does water enter the roots from the soil?
Water molecules can only enter root hair cells (and indeed can only cross any cell membrane) by one mechanism and that is OSMOSIS. If you understand the mechanism of osmosis that is great but don’t worry too much about it at this stage. You need to know that osmosis is a net movement of water from a dilute solution to a more concentrated solution across a partially permeable membrane.
How do mineral ions enter the roots from the soil?
Minerals are pumped into the root hair cells from the soil using ACTIVE TRANSPORT. This a process that uses energy from respiration in the cell to move ions against their concentration gradient (so from a lower concentration in the soil to a higher concentration inside the cell cytoplasm.)
What do we know about xylem vessels?
The cells that water and minerals are transported in are called xylem vessels. They have some interesting specialisations for this function. They are dead cells that are empty with no cytoplasm or nucleus. The end walls of these cells break down to provide a continuous unbroken column of water all the way up the plant. The cell walls of xylem vessels are thick and strengthened and waterproofed with a chemical called lignin.
What causes the water to move up the xylem?
Clearly it will take energy from somewhere to move water against gravity all the way up a plant from the roots to the leaves. The key question here is what provides the energy for this movement? There is no pumping of water up the plant and indeed the plant spends no energy at all on water movement. The answer is that it is the heat energy from the sun that evaporates water in the leaves that provides the energy for water movement. When you combine this with the fact that water molecules are “sticky” – they are attracted to their neighbours by a type of weak bond called a hydrogen bond – you can see that the water evaporating into the air spaces in the leaf can pull water molecules up the continuous column of water found in the xylem. The proper adjective for this stickiness is cohesive and you should know the name for the evaporation of water in the leaves (Transpiration)