2.4 Transport across Membranes Flashcards
What are the roles of the cell-surface membrane?
- separates the internal cell environment from the external environment
- controls the exchange of material across two areas
- acts as an interface for communication
Membranes are …
partially/selectively permeable.
By which main processes can can substances cross membranes?
- diffusion
- osmosis
- active transport
The major component of the cell-surface membrane is the …
phospholipid bilayer.
What is the fluid mosaic model?
The fluid mosaic model explains how different components of the cell membrane are arrange.
- “fluid” because the components can move laterally, allowing diffusion
- “mosaic” because it is made of many parts working together like a mosaic made of many tiles
What are the main components of the cell surface membrane?
- phospholipids
- cholesterol
- glycolipids
- glycoproteins
- other proteins (e.g. transport proteins) → can be intrinsic or extrinsic
Phospholipids in the cell-surface membrane.
- forms a bilayer
- hydrophilic phosphate heads point out
- hydrophobic fatty acid tails point inwards
- Individual phospholipid molecules can move around within their own monolayers by diffusion
- forms the basic structure of the membrane
- acts as a barrier to most water-soluble substances due to hydrophobic fatty acid tails
- ensures water-soluble molecules such as sugars, amino acids and proteins cannot leak out of the cell and unwanted water-soluble molecules cannot get in
- can be chemically modified to act as signalling molecules by moving within the bilayer to activate enzymes or being hydrolysed and releasing smaller water-soluble molecules that bind to specific receptors in the cytoplasm
Cholesterol in the cell-surface membrane.
- regulates the fluidity of the membrane (more cholesterol = less fluid)
- sit in between the phospholipids, preventing them from packing too closely together when temperatures are low; this prevents membranes from freezing and fracturing
- stabilises the cell membrane at high temperatures
- contributes to the impermeability of the membrane to ions and increases mechanical strength and stability of membranes
- only in eukaryotic membranes
Glycolipids and glycoproteins in the cell-surface membrane.
- lipids or proteins with carbohydrate chains attached
- carbohydrate chains project out into whatever fluid is surrounding the cell (they are found on the outer phospholipid monolayer)
- location enables glycolipids and proteins to act as receptor molecules and bind to certain substances
- there are 3 main types of receptors:
1. signalling receptors for hormones and neurotransmitters
2. receptors involved in endocytosis
3. receptors involved in cell adhesion and stabilisation (carbohydrates can form hydrogen bonds with water molecules) - some act as cell markers or antigens, for cell-to-cell recognition
Proteins in the cell-surface membrane.
- embedded within the membrane → intrinsic
- most commonly, they span the entire membrane → transmembrane proteins
- transport proteins (e.g. channel/carrier) → create hydrophilic channels to allow ions and polar molecules to travel through the membrane
- transport proteins are specific to a particular ion or molecule
- help control which substances enter or leave
Describe how the movement of substances across the cell membranes is affected by membrane structure.
- Phospholipid bilayer allows the movement of non-polar, lipid soluble molecules
- Phospholipid bilayer prevents the movement of polar, water soluble molecules
- Carrier proteins allow active transport
- Carrier / channel proteins allow facilitated diffusion and co-transport
-
Shape and charge of channel / carrier determines which
substances move - More transport proteins = more movement
- Larger surface area = more diffusion/movement
- Cholesterol affects fluidity / permeability
What is diffusion?
The net movement of a substance from a region of its higher concentration to a region of its lower concentration.
Which factors effect diffusion?
- steepness of concentration gradient
- temperature
- surface area
- properties of molecules or ions
How does concentration gradient effect the rate of diffusion?
The steeper the concentration gradient, the faster the rate of diffusion.
How does temperature effect the rate of diffusion?
A higher temperature means molecules have more kinetic energy so they move faster resulting in a higher rate of diffusion.
How does surface area effect the rate of diffusion?
A larger surface area means more molecules can cross it at any one moment so there is a faster rate of diffusion.
How do the properties of molecules or ions effect the rate of diffusion?
Smaller molecules diffuse more quickly than large molecules as they require less energy to move. Uncharged and non-polar molecules can diffuse directly across the phospholipid bilayer.
What is facilitated diffusion?
A type of diffusion that occurs through transport proteins found on the plasma membrane.
What are the two types of proteins that enable facilitated diffusion?
- Channel proteins
- Carrier proteins
What is osmosis?
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane.
What is meant by water potential?
The tendency of water to move out of a solution.
Osmosis in plant cells.
- If plants are placed in a solution with a very high water potential water will enter the cell, making it turgid.
- If plants are placed in a solution with a very low water potential water will leave the cell, making it plasmolysed (this process is called plasmolysis)
Osmosis in animal cells.
- Hypertonic solution: water leaves the cells → shrivelled
- Isotonic solution: no net movement
- Hypotonic solution: water enters the cells → cells swell and may burst
What is active transport?
Active transport is the movement of molecules and ions through a cell membrane against the concentration gradient using energy from respiration.
What is co-transport?
Co-transport is the coupled movement of substances across a cell membrane via a carrier protein. (Involves both facilitated diffusion and active transport)
How are root hair cells adapted for exchange?
- shape increases it’s surface area so greater rate if osmosis
- thin wall for a short diffusion distance
- permanent vacuole containing cell sap which is more concentrated than soil water so maintains a steep water potential gradient
How are epithelial cells specialised for efficient diffusion?
- microvilli which increase the surface area
- each villus has a constant blood supply which maintains the concentration gradient
How are neurones and muscle cells adapted for facilitated diffusion?
- cell membranes contain many channel proteins for sodium, potassium and calcium ions
- K+ ions high concentration in the neuron
- Na+ ions high concentration outside the neuron
