Hi everyone, welcome to IGCSE Study Buddy where you can revise biology topics from the Cambridge IGCSE syllabus. This video summarizes topic 3, movement into and out of cells. So basically, in this chapter, we will learn how substances move into and out of cells.
There are three main ways in which this may take place. That's diffusion, osmosis and act... transport.
Let's take a look at diffusion first. Diffusion is quite often how molecules move in and out of our cells through the cell membrane. The cell membrane controls what substances enter and exit the cell. Molecules that our cells need such as glucose and proteins move into the cell for use in metabolic reactions and storage. Waste products that need to be disposed of from the cell such as carbon dioxide and lactic acid are transported out into the blood to be excreted from the body.
For example, this diagram demonstrates a cell surrounded by nutrients that's shown by the purple dots. We can see that on the left there are a lot more nutrients outside the cell than inside. By diffusion, the nutrients will move into the cell from higher to lower concentration until the number of nutrients inside and outside the cell are balanced. Therefore, diffusion is the net movement of particles from a region of their higher concentration to a region of their lower concentration that is down a concentration gradient as a result of their random movement. So it's the constant random movement of particles and their kinetic energy that allows diffusion to occur.
Now let's take a look at the factors that influence diffusion. They are surface area, temperature, concentration gradient and distance. The first factor is surface area. The larger the surface area, the higher the rate of diffusion. This is because more molecules at a given time will be diffusing.
The next factor temperature, the higher the temperature, the higher the rate of diffusion. This is because molecules are faster and have more kinetic energy with higher temperatures. Concentration gradients is another factor that affects diffusion.
The higher the concentration gradient, the higher the rate of diffusion. We are talking about the greater difference in their concentrations. For example, in the box on the left with the purple molecules, The difference in concentration is much higher than that of the box on the right with the green molecules. Therefore diffusion will take place much faster in the box on the left.
Distance is another factor. The shorter the distance, the higher the rate of diffusion. The shorter the distance the particles have to move, the quicker the process is going to be.
So once again, these are the factors. factors that influence diffusion. The next process we'll be looking at is osmosis. We must first understand the important role of water as a solvent in organisms. Substances dissolve in water so water acts as the medium in which substances are moved around the body.
Water is important for transport. Dissolved substances can be easily transported around organisms. Water is needed for digestion. Once the food in our body is digested, the nutrients need to be moved to cells all over the body. It's water that allows this to happen.
Water is also necessary for excretion. For example, Waste substances such as urea dissolve in water and this makes them easy to be removed from the body through urine. So water moves into and out of cells through the cell membrane which is partially permeable.
This process is called osmosis. Partially permeable means that it will allow only certain molecules or ions to pass through it. In the case of diffusion, we were talking about the movement of particles but osmosis is about the movement of water molecules.
When we are talking about water, we cannot use the term concentration anymore because a concentration shows the amount of substance dissolved in water. Because water cannot be dissolved in water, we need to use another term instead, water potential. For a very dilute solution, because it has a lot of water, it has a high water potential.
For a very concentrated solution, because it has less water, it has a low water potential. So let's define osmosis. Osmosis is the net movement of water molecules from a region of higher water potential or dilute solution to a region of lower water potential or concentrated solution through a partially permeable membrane. So this diagram could help us understand osmosis. The left-hand side of the beaker has less dissolved solutes.
So therefore, the solution is more dilute or less concentrated compared to the right-hand side. The solutes are too large to pass through the partially permeable membrane. and therefore cannot diffuse. However, water molecules can pass freely through the membrane.
The molecules will travel from the region of high water potential to low water potential, from the left hand side to the right hand side via osmosis. Let's investigate osmosis using materials such as dialysis tubing. So as you can see, a section of dialysis tubing filled with concentrated sucrose solution has been suspended in distilled water. Dialysis tubing is also known as whisking tubing and it is a non-living partially permeable membrane. The pores in this membrane are small enough to block the large molecules such as sucrose from moving across the membrane.
but allow smaller molecules such as water to pass through by osmosis. Water moves from a region of higher water potential, that is dilute solution, to a region of lower water potential, or concentrated solution, through a partially permeable membrane. Therefore, the water level outside the tubing will decrease as water moves into the tubing via osmosis. Now let's investigate the effects on plant tissues in different concentrations. Adding a cell into pure water or dilute solution, there will be a higher water potential outside the cell than inside the cell and therefore water will move into the cell.
As water enters the cells, they become turgid or swollen due to the high water content. An animal cell can burst if too much water enters. A plant cell has support from its cell wall and therefore will most likely maintain its turgidity without bursting.
Turgor pressure is the pressure on the cell wall from the cell membrane pushing upon it. Let's look at what will happen if we add the cell. into concentrated solution. There will be a higher water potential inside the cell than outside and therefore water will move out of the cell. As the water moves out, cells become flaccid or shrinks.
A plant cell can become plasmalized if too much water is lost. This is when the cytoplasm shrinks due to the loss of water. but the cell wall fails to shrink due to its tough structure. The cytoplasm eventually tears away from the cell wall.
Here's a picture comparing the plant cells when they are immersed in solutions of different concentrations. We must understand the importance of water potential and osmosis in the uptake and loss of water by organisms. Plants obtain water by osmosis through their roots. Osmosis takes place at the roots because of the difference in water potential between the soil and inside of the roots. The water is important because it transports minerals and nitrate ions.
The water also maintains the turgidity of the cell. This provides support and strength for the plant. If the plants lose more water than what they gain, their cells will become flaccid and the plant will wilt. The next process we'll be learning about is active transport.
Active transport is used in cases where diffusion or osmosis cannot be relied upon for example what if a cell wanted to absorb extra nutrients from outside the cell despite having a higher concentration of those nutrients inside the cell diffusion wouldn't work because the concentration gradient is going the opposite way these situations are encountered frequently in plant and villi epithelial cells. Active transport is the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration that is against a concentration gradient using energy from respiration. Active transport uses energy to oppose the concentration gradient and forcefully transport molecules against it. In the cell membranes of all cells, there are certain embedded protein molecules or protein carriers that carry out this process. The protein basically captures the molecules from one side of the cell and it changes shape in a way to transport the captured molecules to the other side of the cell.
Energy from respiration is required to alter the protein shape. Let's take a look at the comparison between the three processes that facilitates movement in and out of cells. In the case of diffusion and active transport, we are talking about the movement of particles, whereas osmosis involves the movement of water molecules. In diffusion, the particles move down a concentration gradient and in active transport, the particles move against a concentration gradient.
In osmosis, the water molecules move from a place of high water potential to a place of low water potential. In diffusion, the particles get energy from the kinetic energy of their random movement and in active transport, Particles need energy from respiration. In osmosis, the water molecules move across a partially permeable membrane.
So these are the main things to learn from chapter 3, movement into and out of cells. Hope you found it useful. Thank you for watching. And please don't forget to subscribe to IGCSE Study Buddy for more biology revision videos.
Bye!