cell membranes - 2b Flashcards
What is the basic structure of the cell membrane?
The fluid mosaic model — a phospholipid bilayer with proteins, cholesterol, glycolipids, and glycoproteins scattered throughout.
What are the properties of phospholipids in membranes?
Hydrophilic heads face out toward water
Hydrophobic tails face in, forming a barrier to water-soluble substances.
What does “fluid mosaic” refer to?
Fluid: phospholipids move within the layer
Mosaic: proteins scattered throughout like tiles.
What are intrinsic (integral) proteins?
Proteins embedded across the whole membrane; may function as channels or carriers.
What are extrinsic (peripheral) proteins?
Proteins found only on one side of the membrane, often for signalling or enzyme activity.
What is the role of cholesterol in cell membranes?
Maintains membrane stability and fluidity
Binds to phospholipid tails, restricting movement
Makes membrane less permeable to small molecules.
What are glycolipids and glycoproteins?
Glycolipids: carbohydrates attached to lipids
Glycoproteins: carbohydrates attached to proteins
Involved in cell recognition and signalling.
What types of transport occur across membranes?
Simple diffusion
Facilitated diffusion
Osmosis
Active transport
Co-transport.
What is simple diffusion?
Passive movement of small, non-polar molecules (e.g. O₂, CO₂) directly through the phospholipid bilayer down a concentration gradient.
What factors affect the rate of diffusion?
Concentration gradient
Thickness of membrane
Surface area
Temperature.
What is facilitated diffusion?
Passive movement of large or charged molecules (e.g. glucose, ions) via channel or carrier proteins down a concentration gradient.
What are channel proteins?
Water-filled pores that allow specific ions to pass through.
What are carrier proteins?
Bind to large molecules and change shape to move them across the membrane.
What is osmosis?
The diffusion of water from an area of higher water potential to lower water potential across a selectively permeable membrane.
What is water potential?
The potential (likelihood) of water molecules to move. Pure water = 0 kPa; adding solute lowers water potential (more negative).
What happens to an animal cell in a solution with higher water potential?
Water enters, cell swells and may burst.
What happens to an animal cell in a solution with lower water potential?
Water leaves, cell shrinks.
What happens to a plant cell in a solution with higher water potential?
Water enters, becomes turgid.
What happens to a plant cell in a solution with lower water potential?
Water leaves, becomes flaccid/plasmolysed.
What is active transport?
Movement of molecules against the concentration gradient, requiring ATP and carrier proteins.
How does ATP help in active transport?
ATP hydrolysis releases energy to change the shape of the carrier protein.
What is co-transport?
Movement of one substance along its gradient coupled with movement of another substance against its gradient using a co-transporter protein.
Example of co-transport?
Sodium-glucose transport in the small intestine:
Sodium ions move into the blood via active transport
Sodium moves back into epithelial cells from gut lumen via co-transporter, pulling glucose with it.
How can membrane permeability be investigated?
Use beetroot: pigment leaks out when membrane is disrupted
Measure color intensity with colorimeter.
What factors affect membrane permeability?
Temperature: High temps denature membrane proteins, increase fluidity
Ethanol/solvents: Disrupt phospholipid bilayer → more leakage.
