chapter 3

• Atoms/molecules/ions are always moving, with the higher the temperature, the faster they move, as they have more kinetic energy at high temperature.
• When particles can move freely, they tend to spread themselves out as evenly as they can, a process known as diffusion.
• A (named substance) will move through the protein against the concentration gradient (from low to high concentration) – 1 mark.
• Diffusion of substances through a membrane is affected by temperature, surface area, concentration gradient, distance travelled/thickness of membrane crossed, molecule size/type, membrane permeability, pressure, number of protein channels, and energy/number of mitochondria.
• Diffusion is the net movement of molecules and ions from a region of their higher concentration to a region of their lower concentration down a concentration gradient, as a result of their random movement.
• The energy for diffusion comes from the kinetic energy of random movement of molecules and ions.
• Living organisms can use diffusion to obtain many of their requirements and get rid of many of their waste products.
• Carbon dioxide diffuses from the air (high concentration) into the stomata of a leaf (low concentration) to be used in photosynthesis, this is an example of diffusion down its concentration gradient.
• Diffusion is also important for respiration in animals and plants, as cell membranes are freely permeable to O2 and CO2.
• Diffusion is increased when surface area is increased, temperature is increased, concentration gradient is increased, and distance travelled by the molecules is decreased.
• Diffusion is a passive process that does not require energy, substances move down a concentration gradient from high to low, it does not have to occur across a membrane, and it occurs with gases.
• Water can make up around 80% of the body in some living organisms, acts as a solvent for many substances, transports substances around the body, and is found inside and outside of every cell in the body.
• Various substances are dissolved in this water, concentration inside the cell may be different from outside the cell, creating a concentration gradient down which water and solutes will diffuse if they are able to pass through the membrane.
• In an opposite scenario, if an animal cell is placed in a solution of higher concentration than the cytoplasm, water will exit the cell, causing the cell to shrivel up and, if the cell is an RBC, it becomes "crenated".
• The definition of osmosis must include the movement/diffusion of water molecules from high water potential to low water potential across a partially permeable membrane.
• If the solutions are of different concentrations, then osmosis will occur.
• Plant cells are surrounded by a fully permeable cell wall and a partially permeable cell membrane, and when placed in pure water, they will absorb water, just like an animal cell.
• Cell membranes are partially permeable and choose which substances to allow to pass through and which to leave out.
• Cell membranes often separate two different solutions: the cytoplasm and the solution around the cell.
• By osmosis, water molecules move from high water potential to low water potential (down a water potential gradient) across a partially permeable membrane.
• A solution with a high concentration of water molecules is referred to as a dilute solution or has a high water potential.
• Osmosis is the net movement/diffusion of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
• The cytoplasm of animal cells contains many solutes (concentrated solution), and if an animal cell is placed in pure water, water molecules will diffuse from the dilute solution to the concentrated solution, causing the cell to swell and eventually burst.
• Exposing the cell membrane to high temperatures will damage it and prevent osmosis from taking place.
• If the concentration of solutes inside and outside the cell are the same, there is no net movement of water and no water potential gradient.
• Active transport is an energy-consuming process against the concentration gradient.
• The definition of active transport must include the movement of molecules or ions from low concentration to high concentration ( against a concentration gradient ), using energy from respiration, using proteins/carriers in the membrane.
• The cytoplasm swells and presses out against the cell wall, but the cell wall resists and presses back.
• In an opposite scenario, what happens if a plant cell is placed in a concentrated solution?
• Sometimes, cells need to take in substances which are only present in small quantities around them.
• The cell membrane will keep shrinking until it tears away from the cell wall, a cell like this is said to be plasmolysed.
• The change in shape required for active transport requires energy.
• Water causes the cytoplasm and vacuole to swell, but the strong cell wall prevents the plant cell from bursting.
• Plants that have no wood to support them can remain standing upright when their cells are turgid, and their leaves remain firm.
• Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall.
• Important examples of active transport include nitrate ion uptake by root hair cells and uptake of glucose by epithelial cells of villi in the small intestines and by kidney tubules.
• The force inside the cell that is pushing the cell membrane against the cell wall is known as the turgor pressure.
• Cells have high turgid pressure and resist increase in volume, meaning they will not absorb excess water.
• The energy for active transport is provided by respiration in the cell.
• The cell wall is inelastic, rigid, and inflexible.