4.0 Cell Membranes And Transport Flashcards
What does the Fluid mosaic model mean
- Fluid: The lipids and proteins within the membrane are not fixed in place; they can move laterally within the layer, which is why the membrane is described as “fluid.” Move within their own monolayer.
- Mosaic: the patchwork of proteins that float in or on the fluid lipid bilayer. Proteins interspersed and scattered within membrane.Different shaped proteins produce a mosaic of patterns.
Fuild Moscaic model / membrane structure
- PHOSPHOLIPID BILAYER: The cell membrane is primarily composed of a double layer of phospholipids (roughly 7nm wide, js visible under an electron microscope at high mag.). These molecules have hydrophilic heads and hydrophobic tails. The tails face inward, shielded from water, while the hydrophilic heads face outward towards the aqueous environment.
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PROTEINS:
1. Integral Proteins: These span the entire lipid bilayer and are involved in functions such as transport, acting as channels or carriers for molecules to pass through the membrane.
2. Peripheral Proteins: These are attached to the exterior or interior surfaces of the membrane and are often involved in signaling or structural support. - CHOLESTEROL: In animal cell (Plant cells do not have cholesterol) membranes, cholesterol molecules are interspersed within the lipid bilayer. Helps to modulate the fluidity of the membrane.
- CARBOHYDRATES: These carb side chains are attached to proteins (glycoproteins) or lipids (glycolipids) on the extracellular surface of the membrane. They play a crucial role in cell recognition and communication.
Fluid mosaic model function
helps to explain:
- passive and active movement between cells and their surrounding
- Cell-to-cell interactions
- Cell signalling
General Function of Membrane
- creates an **enclosed space **which seperates the internal cell components from the external environment
- controls the exchange of material across them, also acts as interface for communication
- membranes are partially permeable (due to hydrophobic tails that do not allow polar molecules through)
- substances cross the membrane by diffusion, osmosis and active transport
General function of membrane/phosolipid bilayer within cell
- acts as intracellular membranes within the cell such as the nucleus, mitochondria and lysosomes
- can form compartments within cell providing basic structure of organelles allowing specialisation of processes within the cell
- eg lysosomes need a membrane to prevent them from breaking down other cellular components
Phospholipid Bilayer Structure
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Cholesterol:
1. molecules are interspersed within the phospholipid bilayer of the cell membrane
2. positioned with its hydroxyl (-OH) group near the hydrophilic heads of the phospholipids, while the rest of the cholesterol molecule (which is hydrophobic) is embedded within the hydrophobic core of the lipid bilayer. -
Glycolipid:
1. Glycolipids are found in the outer leaflet of the phospholipid bilayer, facing the extracellular environment
2. lipid section found within the inner layer of phospholipid bilayer (hydrophobic part), with carbohydrate side chain attached to lipid extending into external environment -
Glycoprotein:
1. proteins that have carbohydrate chains covalently attached to them. These proteins can be either integral or peripheral, but the carbohydrate portions always extend out of the cell, facing the extracellular space.
2. protein component embedded within or attached to the cell membrane
Phospholipids
- polar hydrophillic head (containing phosphate and glycerol) soluble in h20
- non-polar hydrophobic tail (hydrocarbon chain/fatty acid) insoluble in h20
- phospholipid monolayer forms when put in water, where row of heads face towards water and tails sticking up away from water
- micelle forms when shaken in water, sphere like shape forms
- individual phospholipid molecules can move within their own monolayers by diffusion
Cholesterol
- hydrophobic tails and hydrophillic heads
- fit between phospholipid molecules, orientated the same way
- absent in prokaryotic membranes
- regulates the fluidity of membrane
IMPERMEABILITY:
- increases mechanical strength and stability of membranes, otherwise cell would burst and break down
Fluidity of Membrane
CHOLESTEROL:
AT LOW TEMP:
- prevents them from packing too closely when temperature is low, prevents membrane from freezing
- chains are packed together less tightly, lower intermolecular forces, more fluid
AT HIGH TEMP:
- stops the membrane from becoming too fluid by binding to the hydrophobic tails of phospholipids, stabalising them and packing them more closely together
- chains are packed tightly, higher intermolecular forces, less fluid
HYDROCARBON CHAIN:
- The more unsaturated, the more double carbon bonds, the more fluid (vise versa)
- The longer the hydrocarbon chain the less fluid
- more kinks = more fluid
Glycolipids and Glycoproteins
- has carb side chain in external environment, allowing them to act as receptor molecules
- can bind with certain substances at the cell’s surface
- invovled in cell adhesion and stabilisation (as carb side chains can form H bonds with water molecules surrounding cell)
- can be involved in endocytosis
- act as signalling receptors for hormones and neurotransmitters (glycoproteins ONLY)
- some act as cell markers or antigens for cell-to-cell recognition
Proteins
TRANSPORT PROTEINS:
Provide hydrophillic channels for ions and polar molecules
INTRINSIC PROTEINS:
- found embedded within membrane, inner layer, outer layer or spanning the whole membrane (transmembrane proteins)
EXTRINSIC PROTEINS:
- found on the inner or outer surface of membrane, usually bolund to intrinsic proteins or phospholipids
CHANNEL PROTEINS:
- water-filled pores that allows charged substances through the membrane, fixed shape and can be gated to control ion exchange. Facillitated diffusion.
CARRIER PROTEINS:
- can change shape (conformational change), mainly involved in active transport by using ATP to carry ions against the conc. gradient.
- also involved in passive facillitated diffusion down the conc. gradient
Stability of Membrane
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BONDS PRESENT:
- Hydrophobic Interactions: between the fatty acid tails of phospholipids. These interactions prevent the phospholipids from dispersing into the aqueous environment, maintaining the structure of the bilayer
- Van der Waals Forces: contribute to the stability of the membrane by keeping the tails aligned and tightly packed
- hydrogen bonds that can be formed with the external environment -
CHOLESTEROL:
- modulate the fluidity of the membrane by fitting between the phospholipid molecules, cholesterol prevents the fatty acid chains from packing too closely in low temperatures (preventing rigidity) and restricts excessive movement in high temperatures (preventing the membrane from becoming too fluid).
- also adds to the overall structural integrity of the membrane, making it less permeable to very small water-soluble molecules and reducing the risk of the membrane breaking apart. -
FLEXIBILITY:
- The fluid nature of the phospholipid bilayer allows the membrane to bend and flex without breaking, which is important for processes such as cell division, vesicle formation, and movement.
Selective Permeability of Membrane
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STRUCTURE OF PHOSPHOLIPIDS:
- Amphipathic Nature: Phospholipids have a hydrophilic (water-attracting) “head” and two hydrophobic (water-repelling) “tails.”
- Bilayer Formation: In an aqueous environment, phospholipids arrange themselves into a bilayer this arrangement creates a hydrophobic core within the membrane. -
BARRIER TO POLAR & LARGE MOLECULES:
- Only small, non-polar molecules (e.g., oxygen, carbon dioxide) can diffuse through the bilayer without assistance.
- Transport Proteins: For polar and larger molecules to cross the membrane, they must pass through specific transport proteins (e.g., channels, carriers) embedded in the bilayer. This allows the membrane to selectively permit certain substances while excluding others. -
REGULATION OF TRANSPORT:
- The cell membrane uses various mechanisms like facilitated diffusion, active transport, and endocytosis/exocytosis to regulate the movement of substances across the membrane, contributing to its selective permeability
Process of Cell signalling
- A (ligand) signal arrives at a specific/target protein receptor at the cell surface membrane, signal recognition
- signal results in conformational change in the shape of the receptor, spanning the membrane passing the signal into the cell (signal transduction)
- changing the shape of the receptor allows it to interact with the G protein, which brings about the release of a 2nd messanger (a small molecule which diffuses through the cell relaying the message)
- 2nd messanger activates a cascade of enzyme catalysed reactions which brings about the required change
INVOLVES ATP
Significance of cell signalling
allows mutlicellular organisms to control and coordinate their bodies and respond to their environment
