Topic 2 - Cell membrane (Structure and function) Flashcards
MEMBRANE STUCTURE
What roles does a cell membrane have?
Controls what enters and exits the cell to maintain homeostasis
Structure of the plasma membrane
Made out of phospholipid molecules, layers of these molecules form a phospholipid bilayer
Phospholipids
Made of a phosphate head and a fatty acid tail
Hydrophilic head (polar)
Hydrophilic, water-loving
Hydrophobic tail (non-polar)
Hydrophobic, water-fearing, they can be saturated or unsaturated. The more unsaturated fatty acids the greater the fluidity of the membrane.
Role of proteins
They provide SUPPORT and allow cells to recognise other structures. They also allow cells to COMMUNICATE and assist with TRANSPORT of substances across the membrane.
Types of proteins
- Receptor : Receive signals
- Channel : Transport
- Carrier : Transpot
How can proteins be associated with the membrane (PIT)
Peripheral - surface (temporarily attached)
Integral - strength (permanently attached)
Transmembrane - move/cross through the membrane bilayer
Cholesterol role
Helps to keep the membrane flexible (provides fluidity)
Selectively permeable
Select only certain molecules to cross the membrane
What type of molecules are ‘permeable’ and ‘impermeable’
Permeable (Can cross membrane)
- Iodine, water, glucose
Impermeable (Can’t cross membrane)
- Starch, ions
MEMBRANE FUNCTION
What is the main difference between PASSIVE and ACTIVE transport across the cell membrane
Passive transport does not require energy, whereas active transport does
Define diffusion
The movement of molecules from an area of high concentration to an area of low concentration
Simple diffusion (5)
Movement of molecules across the (phospholipid bilayer)
- Hydrophobic molecules
- Small molecules
- Fat-soluble (non-polar)
- Down a concentration gradient (high to low)
- eg water, alcohol, gases (O2, CO2)
Facilitated diffusion (5)
Diffusion with the help of transport proteins (through channels)
- Hydrophilic molecules
- Charged ions
- Water-soluble
- Down a concentration gradient (high to low)
- eg glucose, amino acids
Similarities/Differences between the two types
- Similarities : Both move down a concentration gradient (High to low)
- Differences : Simple does not require proteins, whereas facilitated does.
Does diffusion require a membrane?
No, diffusion itself does not require a membrane. For example gas molecules are diffusing into the open air
TONICITY and OSMOSIS
What is osmosis
Movement of water from an area of low solute concentration to an area of high solute concentration (Passive)
Tonicity
Describes the concentration of solutes on either side of a membrane.
- Hypertonic
- Hypotonic
- Isotonic
Hypotonic
Water moves INTO the cell
- Solution has a lower concentration of solutes and a high concentration of water outside of the cell
- Therefore, the water will move into the cell and the cell may burst (lyse in animal cells) or become turgid (plant cells).
Hypertonic
Water moves OUT of cell
- Solution has a higher concentration of solutes outside of the cell
- Therefore the water will move out of the cell and the cell will shrink (animal cells) or plasmolyse (plant cells)
Isotonic
Cell has the same concentration of solutes on both sides of the membrane and there will be no NET MOVEMENT of water
Active transport
Active transport REQUIRES energy to transport substances across a membrane.
- Used to move molecules AGAINST a concentration gradient from an area of low solute to an area of high solute
Bulk transport
Movement of large molecules (Solids + Liquids)
- There are two types of bulk transport : Endocytosis (into cell) and Exocytosis (out of cell)
Endocytosis (into cell)
The cell membrane folds around the substance to form a VESICLE containing the 2 types of endocytosis
- Solid (Cell eating) = Phagocytosis
- Liquid (Cell drinking) = Pinocytosis
Exocytosis (out of cell)
Substances are enclosed in a membrane to form a vesicle at the golgi body
- Transports hormones, enzymes, or wastes out of the cell
