1.1- S2: Cell membranes and transport across cell membranes

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What are functions of plasma membranes?
partial permeability- control of which molecules can pass through membrane. Allow recognition of other cells/ cell communication (cell membrane receptors)
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What are the functions of membranes within cells?
around organelles- divide cell into compartments --> various functions more efficient e.g respiratory enzymes kept together in the mitochondria. Folded organelle membranes increase surface area- chemical reactions more efficient. vesicles- transport
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Describe the structure of the 'fluid mosaic model'
phospholipid molecules form a bilayer. cholesterol between phospholipid molecules. Proteins with a polysaccharide chain attached are glycoproteins. Lipids with a polysaccharide chain attached are glycolipids.
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Why is the model described as 'fluid'
The phospholipid molecules are constantly moving due to kinetic energy.
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What is the role of phospholipid molecules?
The phospholipid head is hydrophillic whereas the tail is hydrophobic (repelling water.) Therefore molecules arrange themselves into a bilayer. The centre is hydrophobic so water soluble substances (e.g. ions) are not allowed through-barrier
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What is the role of cholesterol in the membrane?
present in some eukaryotic cell membranes, fits between phospholipids, binding to the hydrophobic tails. They pack more closely together. Membrane is more rigid and the barrier is more complete so ions pass less easily.
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What is the role of proteins in the membrane?
Channel proteins allow small or charged particles through. Carrier proteins transport molecules and ions across the membrane by active transport and facilitated diffusion. They also act as receptors.
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What is the role of glycolipids and glycoproteins?
They stabilise the membrane by forming hydrogen bonds with surrounding water molecules. Act as receptors and sites where drugs, hormones and antigens bind.
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What is cell signalling?
Cells communicating with each other using (messenger) molecules, between cells or inside cells during processes. Messenger molecules bind to receptors on cell membranes.
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Explain how cell signalling works?
membrane-bound proteins on the cell membrane act as receptors for messenger molecules with specific shapes. Messenger molecules with a complementary shape can bind to these receptors. a target cell responds to a particular messenger molecule.
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Explain the example of glucagon
A hormone which binds to receptors on the target cells in the liver with a complementary shape to the hormone. Liver cells break down glycogen stores to glucose when blood sugar levels are low.
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Explain an example of drugs binding to cell membrane receptors
Histamine binds to receptors causing inflammation. Antihistamines block histamine receptors, preventing histamine binding to receptors on target cells, stopping inflammation.
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Explain how membranes are affected by low temperature (below 0°C)
Lack of kinetic energy > phospholipids move less and are packed closely> rigid membrane. Proteins in membrane denature and ice crystals form piercing the membrane> increases permeability of the membrane.
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Explain how membranes are affected by high temperature (above 45°C)
Phospholipids have more kinetic energy and water inside the cell expands putting pressure on the membrane > membrane becomes more permeable. Proteins denature and cannot function, making the membrane more permeable.
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Define diffusion
The passive movement of particles, with a NET MOVEMENT from an area of higher concentration, to an area of lower concentration, down a concentration gradient.
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Explain 4 factors affecting the rate of diffusion
concentration gradient: the greater the concentration gradient, the faster the rate of diffusion. Diffusion distance: shorter the distance, faster rate. Surface area: larger SA, faster rate. Temperature: increase, more kinetic energy, rate increases.
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Define osmosis
The diffusion of WATER MOLECULES across a partially permeable membrane, with a net movement of water molecules from an area of higher water potential to an area of lower water potential, down a water potential gradient.
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What is the unit for water potential, and what is the value of pure water?
kPa (kilopascals) water has the highest water potential but its value is 0 kPa.
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What is a hypotonic and hypertonic solution?
hypotonic- a lower concentration of solutes/high water potential than another solution. hypertonic- a higher concentration of solutes/lower water potential than another solution.
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How is an animal cell affected by the water potential of its surrounding solution?
Cells in hypotonic solutions could burst as there is a net movement of water into the cell. Cells in hypertonic solutions are crenated as there is a net movement of water molecules out of the cell. isotonic solutions- dynamic equilibrium.
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How is a plant cell affected by the water potential of its surrounding solution?
in hypotonic solutions become turgid-net movement of water into the cell > vacuole swells and the vacuole and cytoplasm push against the cell wall.In hypertonic solutions become flaccid (plasmolysis) as cytoplasm and membrane pull away from cell wall
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Which types of molecules can pass through the bilayer?
fat-soluble molecules can diffuse down a concentration gradient through the membrane (steroid hormones.) Carbon dioxide and oxygen molecules are small enough to diffuse through the bilayer, and some water molecules.
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Explain facilitated diffusion through a carrier protein.
(large molecules e.g glucose amino acids) large molecules attach to carrier protein with a specific shape, the protein changes shape, releasing the molecule onto the other side of the membrane.
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Explain facilitated diffusion through a channel protein.
(ions e.g calcium) They form pores in the membrane for ions to diffuse through down a concentration gradient. The proteins are shaped for specific ions and are sometimes gated, meaning then can open and close.
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What is active transport?
Using energy in the form of ATP to carry molecules across the membrane, against the concentration gradient. Uses carrier proteins (similar process to facilitated diffusion but against concentration gradient.)
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Explain endocytosis
When molecules are too large or bulk transport is required. ATP is used for the membrane to fuse, separate and 'pinch off' to form a vesicle. The ingested substance is then inside the cell. e.g phagocytes
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Explain exocytosis
substances produced by cell need to be released from cell (lipids, hormones and enzymes.) Vesicles containing these substances pinch off from sacs of Golgi apparatus, and move towards plasma membrane.Vesicles fuse with plasma membrane,release content
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