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Movement of Substances. What you need to know. What is diffusion? What is osmosis? How is active transport different from diffusion? What causes the movement of substances? How does the permeability of the membranes affect the movement of substances?
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What you need to know • What is diffusion? • What is osmosis? • How is active transport different from diffusion? • What causes the movement of substances? • How does the permeability of the membranes affect the movement of substances? • How does the movement of substances affect cells?
Diffusion • Net (overall)movement of substances from a region of higher concentration to a region of lower concentration (down a concentration gradient) • Passive – no cellular energy required • Osmosis – a special type of diffusion
Molecules are equally spaced apart/equally distributed throughout the solution
Life example: Absorption of nutrients by the intestine • Diffusion of digested nutrients (food molecules) into fingerlike structures called villi in the small intestine
Life example 2: Exchange of gases in the lungs • Movement of O2 and CO2 between the air sacs (alveoli) and the blood capillaries surrounding them during gaseous exchange in the lungs
Active transport • Movement of substances from a region of lower concentration to a region of higher concentration (against a concentration gradient) • Require cellular energy • Involve transport proteins found on the cell surface membrane
Life example: Absorption of nutrients from the soil • Some essential nutrients are present in low concentration in the soil • They are taken up by the root hair cells via active transport
Osmosis • Net (overall) movement of water molecules from a region of higher water potential (lower concentration) to a region of lower water potential (higher concentration) • Occurs across a partially/semi- permeable membrane • Passive – no cellular energy required
What causes substances to move passively? • No energy involved • A concentration gradient drives the movement of substances • i.e. a DIFFERENCE in concentration between two regions • E.g., between inside the cell and the cell’s external environment
Permeability Membranes can be Permeable – let everything in and out e.g., cell wall Semi Permeable - let some things in and out e.g., cell surface membrane Impermeable – let nothing in and out e.g.,
Osmosis and its effects on cells • Depending on the concentration of the environment, cells can undergo physical changes • These changes differ slightly in animal and plant cells because of their structural differences
A cell’s external environment can be • Dilute (hypotonic): water molecules > solute molecules • concentrationE < concentrationi • Similar to the cell’s internal environment (isotonic): • concentrationE= concentrationi • Concentrated (hypertonic): water molecules < solute molecules • concentrationE> concentrationi
Osmosis and animal cells • Recap: Animal cells do not have a cell wall • Hypotonic: water enters the cell as water potential outside the cell is > water potential inside the cell • Isotonic: no net movement of water molecules since water potential inside and outside the cell is similar • Hypertonic : Water leaves the cell as water potential is inside the cell is > water potential outside the cell
Many animals that live in the sea have cytoplasm with a similar concentration to sea water • It is the function of our kidneys to make sure the fluid in our bodies that surrounds our cells (plasma) has a stable concentration
Osmosis and plant cells • Recap: Plant cells have a cell wall • A cell wall keeps the cell in shape despite changes in movement of water
Hypotonic: water molecules enter the cells • cells become turgid due to pressure (turgor pressure) exerted on cell wall • Isotonic: no net movement of water molecules • Hypertonic: water molecules leave the cells • Cells become flaccid/plasmolysed • Cytoplasm shrinks away from the cell wall • But cell retains its shape due to presence of a cell wall
Importance of turgor pressure • Turgor pressure gives plants their strength • If plants did not have this they would be wilted • Plants that don’t have wood such as lettuce and house plants rely on turgor pressure for strength
Osmosis and Food Preservation • Bacteria and Fungi are the most common causes of food spoilage • Both these cells are enclosed by a membrane • If a food is placed in a sugary or salty solution then any bacteria or fungi present will lose the water in their cells to the more concentrated solution outside • When this happens the cells will shrivel and die and the food will not become contaminated with loads of bacteria nad fungi
Examples • Fish and Meat (bacon) may be stored in a salty solution • Jams, marmalades and tinned fruits are stored in a sugary solution
Life example 1: Absorption of water by roots • Water moves from the soil into the roots and finally into the xylem via osmosis
Life example 3: Freshwater and marine fishes/animals Saltwater fishes Freshwater fishes
