BIOLOGY FORM 4 Chapter 3 - Movement of Substances Across the Plasma Membrane
Movement of Substances
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Transcript of Movement of Substances
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Movement of Substances
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What you need to know
1. What is diffusion?2. What is osmosis?3. How is active transport different from diffusion?4. What causes the movement of substances?5. How does the permeability of the membranes
affect the movement of substances?6. How does the movement of substances affect
cells?
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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
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Equilibrium
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Molecules are equally spaced apart/equally distributed throughout the solution
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Life example: Absorption of nutrients by the intestine
• Diffusion of digested nutrients (food molecules) into fingerlike structures called villi in the small intestine
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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
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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
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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
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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
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Comparison between the 3
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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
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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.,
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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
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A cell’s external environment can be
(a) Dilute (hypotonic): water molecules > solute molecules
• concentrationE < concentrationi
(b) Similar to the cell’s internal environment (isotonic):• concentrationE = concentrationi
(c) Concentrated (hypertonic): water molecules < solute molecules
• concentrationE > concentrationi
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Osmosis and animal cells
• Recap: Animal cells do not have a cell wall(a) Hypotonic: water enters the cell as water potential outside the
cell is > water potential inside the cell(b) Isotonic: no net movement of water molecules since water
potential inside and outside the cell is similar(c) Hypertonic : Water leaves the cell as water potential is inside
the cell is > water potential outside the cell
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What happens to animal cells in different environments?
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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
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Life example: Amoeba
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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
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1. Hypotonic: water molecules enter the cells– cells become turgid due to pressure (turgor
pressure) exerted on cell wall
2. Isotonic: no net movement of water molecules
3. 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
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Plasmolysed red onion cells
•
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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
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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
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Examples
Fish and Meat (bacon) may be stored in a salty solution
Jams, marmalades and tinned fruits are stored in a sugary solution
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Life example 1: Absorption of water by roots
• Water moves from the soil into the roots and finally into the xylem via osmosis
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Life example 2: Ultrafiltration by the kidneys
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Life example 3: Freshwater and marine fishes/animals
Saltwater fishes Freshwater fishes
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Experimental point of view