IB myp 4-5 Biology – Notes- All Topics
Topic :Metabolism-Biochemistry and Enzymes
Topic :Metabolism– Weightage : 21 %
All Questions for Topic : Nutrition,Digestion,Biochemistry and enzymes,Movement and transport,Diffusion,Osmosis,Gas exchange,Circulation,Transpiration and Translocation,Homeostasis
TRANSPORT
Mechanisms of transport across the plasma membrane
On left side , we have a typical cell membrane . For cells to function, they must be able to control the movement of substances into and out of the interior of the cell. Content of the cell is surrounded by a partially permeable membrane called the plasma membrane. This barrier is essential for transporting and regulating the materials that go in and out of the cell. This plasma membrane has selective permeability; that is, it allows some substances to cross it more easily than others.
There are different mechanisms by which molecules may be transported across the plasma membrane. Small molecules go across the plasma membrane by diffusion, osmosis, or active transport. Larger molecules across the membrane, usually by endocytosis and exocytosis.
Transport Rate Affecting Factors:
- Surface area to Volume ratio
- temperature and
- concentration gradient
Types of Transport:
Diffusion
Diffusion is the movement of particles from an area of higher concentration to lower concentration. Active transport is the opposite, but it requires energy.
Molecules (including ions) in a liquid or gas are constantly moving and tend to spread out into the available space. If we place a membrane separating pure water from a solution of dissolved dye in water, and assuming the membrane is permeable to the dye molecules, the dye molecules will move randomly. However, they will tend to diffuse to the side where there is just pure water until they are evenly spaced and reach equilibrium.
If there is a region of high concentration, and another of low concentration, there is a concentration gradient between these regions. A substance will diffuse from where it is more concentrated to where it is less concentrated, or down its concentration gradient.
Diffusion is the movement of particles within a gas or liquid from a region of high concentration to a low concentration (down a concentration gradient), until an equilibrium is reached.
In a cell, when diffusion takes place, we talk about passive transport, as the cell does not have to spend energy for diffusion to occur.
Osmosis
Osmosis is the movement of water particles from an area of higher water potential to lower water potential across a partially permeable membrane.
In Osmosis, equilibrium is equal concentration on both sides of a membrane. Even then, the particles keep moving, but the overall rate of diffusion does not change much
Osmosis a special case of passive transport
Osmosis is a form of diffusion in which only water molecules move. The tendency of water molecules to move from one place to another is measured as the water potential.
Osmosis is the diffusion of water molecules from a region of higher concentration of water to a region of lower concentration of water, down a water potential gradient, through a partially permeable membrane.
How Osmosis works:
- Water molecules are polar too. However, they are so small that they can diffuse across a membrane,
- when a cell is placed in water, the cell will give out water, because the concentration outside the cell is greater. Hence, the cell shrinks. This process is called plasmolysis. In hypotonic solutions, the cell will take in water, causing it to burst. This process is called Lysis. plant cells plasmolyse, but do not burst. This is because of its cell wall. The pressure on the Cell wall (given by the expanding cell Membrane) is called turgor pressure – the phenomenon is called turgidity. the plant becomes turgid.
- Cells are deliberately very small. If they were bigger, nutrients would not enter the cell fast enough, and heat and waste would not be released so quickly. A smaller cell size increases its “Surface Area :Volume Ratio”
- Here, the Surface Area has to do with the amount of nutrients/resources that can enter the cell at one time.
- The volume has to do with the amount of nutrients needed to run the cell.
- However, the SA:V Ratio should not be too high, else the excretion of waste becomes harder. If this happens, cells typically divide.
- Here, the shape of cells is taken as cubes for ease.
- Size 1 is ideal, while 6 is the maximum, after which a cell starts showing inefficiency
There are 3 types of solutions:
- Hypotonic: Solvent > Solute
- Isotonic: Solvent = Solute
- Hypertonic: Solvent< Solute
Brown mean motion is the random type of motion of gas particles.
Temperature affects particle movement. More temperature causes more kinetic energy, resulting in so. Mass affects particle movement. More mass causes more inertia. So a smaller mass allows for more speed of the particles. This goes for all types of transport.
Active transport is a method by which particles are moved against a concentration gradient.
As we have seen earlier, particles move from one place to another by diffusion, but only until equilibrium is reached. If there is no concentration gradient between the two places no diffusion will occur. Sometimes cells need substances which are found at lower concentration outside the cell.
In active transport, protein molecules (called carriers) in the plasma membrane pick up and carry particles across the membrane, in the opposite direction of diffusion (against a concentration gradient). The procedure is an energy-consuming process, as the substance must be moved against its natural tendency to diffuse.
This movement requires energy (ATP) and involves protein carriers in the plasma membrane.
moving large molecules across membranes
Some particles are too big to cross a membrane by diffusion or by active transport. In these cases, an infolding or extension of the cell membrane will engulf the molecule and form a vesicle to transport it.
Endocytosis and exocytosis are active processes involving the bulk transport of materials through membranes.
This can be either into the cell, endocytosis, or out of the cell, exocytosis.
