Cellular Transport

Your cells need to take in substances that they need, such as oxygen and glucose, and they also need to get rid of waste products and chemicals that are needed elsewhere in the body. 
There are 3 main ways that substances can move into and out of the cell:
  • Diffusion
  • Osmosis
  • Active transport

We are going to look at each one in turn, and highlight some examples of where each form of movement is used.
Diffusion
  • The net (overall) movement of particles in a liquid or gas from an area of high concentration to an area of low concentration (down a concentration gradient)
  • Diffusion is a passive process (doesn't require energy)
  • It takes place because of random movement of the particles (due to their inherent kinetic energy)
There are 3 main things that affect the rate of diffusion:
  1. Temperature - the higher the temperature, the faster the rate (due to an increase in the speed of the random movements of the particles)
  2. Concentration gradient - the bigger the difference in concentrations of particles between the two areas, the faster the rate of diffusion
  3. Distance - the shorter the distance, the faster diffusion occurs
Diffusion in living organisms
In living organisms diffusion of dissolved substances such as oxygen, carbon dioxide and glucose occurs into and out of cells across the semi-permeable cell membrane. Semi-permeable means that only certain substances can cross the membrane.
Here are 3 common examples of where diffusion takes place in living organisms:
Picture
Diffusion of gases between alveolus and blood capillary


In the lungs oxygen passes from the alveoli into the blood stream, and carbon dioxide goes the other way.
Lungs are perfectly adapted for this exchange:
  • large surface area of alveoli
  • extensive blood supply
  • moist surface of alveoli
  • thin walls of alveoli and capillaries

For more info and to revise breathing and respiration, see: 
 Picture
Diffusion of soluble products of digestion in small intestine
Glucose and amino acids get absorbed across the lining of the small intestine into the blood stream. Again, it is highly adapted for diffusion:
  • large surface area due to folded lining of small intestine into villi
  • cells of the villi are themselves topped with finger-like projections called microvilli, increasing the SA further
  • rich blood supply
  • walls of intestine only 1 cell thick
For more info and to revise digestion, see:
 Picture
Diffusion of gases in the leaf
In the leaf, the stomata allow the passage of CO2 into the leaf, and oxygen and water vapour out.
  • most leaves are broad, giving them a large surface area
  • large air spaces increase SA for diffusion inside the leaf
  • leaves are thin










Make sure you are clear about how leaves are adapted for photosynthesis. Plants come up very often in exam papers!
You might have noticed some common features of diffusion surfaces. There are 4 common ones:
  • large surface area
  • short diffusion distance (often characterised by diffusion distance being only 1 or 2 cells thick)
  • moist surfaces
  • mechanisms to maintain a steep concentration gradient (blood flow in the case of alveoli and villi, wind to move gases away in the case of the leaf)
Osmosis
  • Osmosis is a special case of diffusion
  • The movement of water molecules from an area of high concentration of water to an area of low concentration of water (down a concentration gradient)
  • It occurs across a partially permeable membrane
  • It is a passive process (doesn't require energy)
Here the membrane is only permeable to water molecules. They will move from an area of high water (dilute solution) to an area of low water (concentrated solution). Thus it will move from the right to the left, and consequently you will see a raise in water level
Like diffusion, in the case of osmosis the water molecules move in order to try to regain a balance between the two sides
Here are some key words that you need to know:
Isotonic
Having the same concentration of solutes as another solution 
Hypertonic
A solution with a higher concentration of solute molecules than another solution
Hypotonic
A solution with a lower concentration of solute molecules than another solution
Osmosis in animals
The concentration inside body cells needs to remain constant in order to function properly. Changes in the concentration of solutes within the cell or of the solutions surrounding the cell can cause water to move into or out of the cell. 
  • If the cytoplasm becomes more concentrated, then the surrounding fluid is hypotonic and water will move INTO the cell by osmosis. This can cause the cell to swell and burst
  • If the cytoplasm becomes more dilute, then the surrounding fluid is hypertonic and water will move OUT OF the cell. This can cause the cell to shrivel and die

These examples illustrate the importance of maintaining a constant internal environment (more about this at a later date when you learn about homeostasis!)
Osmosis in plants
In plants, osmosis is important to support the stems and leaves, and it's through this process that water enters the plant cells. Water is stored in the cell vacuole, causing it to swell and press the cytoplasm against the cell walls. Plants need the fluid surrounding the cells to always be hypotonic, meaning water will always move into the cell.
  • When no more water can enter the cell, the pressure builds up causing the cell to become hard and rigid. This is known a turgor (the cell is turgid). Unlike the animal cell the plant cell doesn't burst due to the cell wall
  • If the solution surrounding the cell is hypertonic, water will move out of the cell by osmosis, and the cell will become flaccid. This causes wilting
  • If even more water is lost, the vacuole and cytoplasm shrink, and the cell membrane pulls away from the cell wall. This state is called plasmolysis (the cell is plasmolysed) and it results in cell death
Active Transport
  • The movement of substances from an area of low concentration to an area of high concentration (AGAINST a concentration gradient) or the movement of larger substances across a semi-permeable membrane
  • This is an active process and so requires energy
  • Requires carrier proteins in the cell membrane
ATP is what we know of as energy! It comes from cellular respiration - remember this. Cells that undergo a lot of active transport will therefore be packed full of mitochondria, the organelles within which respiration occurs.
See in the above example that the concentration of particles is higher inside the cell than outside, but the carrier protein moves them into the cell.
The fact that active transport allows cells to move substances into them AGAINST a concentration gradient means they can absorb substances such as ions and sugars from very dilute solutions.
Osmosis is living organisms
There are two common examples that can be used to illustrate the importance of active transport
Picture
Small intestine
Diffusion isn't the only way that the soluble products of digestion move from the gut into the blood. 
As the time since your meal increases, the amount of dissolved food molecules will be higher in the blood that the gut. The cells of the small intestine use active transport to move these last molecules into the bloodstream, meaning that none of your digested food is lost.
 Picture
Plant roots
Plants often absorb minerals and ions from the soil that are at a very low concentration, lower even than within the root hair cells themselves. They therefore need to use active transport. 
In a nutshell:
  • Diffusion and osmosis represent the movement of substances (water in the case of osmosis) from an area of high to low concentration, down a concentration gradient. They are passive, and do not require energy
  • Active transport is the movement of substances from low to high concentration, against a concentration gradient. As it's name suggests, it is an active process, requiring energy. Cells that undergo a lot of active transport have a lot of mitochondria to provide the energy