1.3 Flashcards
Describe the structure of the phospholipid bilayer of the plasma membrane
The cell membrane consists of two adjacent layers of phospholipids, which form a bilayer.
The fatty acid tails of phospholipids are exposed to the interior of the membrane and the phosphate heads are exposed to the exterior aqueous side.
What hold integral membrane proteins in the phospholipid bilayer
Regions of hydrophobic R groups allow strong, hydrophobic interactions that holds integral membrane proteins within the phospholipid bilayer.
The integral membrane proteins interact extensively with the hydrophobic region of membrane phospholipids.
Link between integral and transmembrane proteins
Some integral membrane proteins are transmembrane proteins
Describe peripheral membrane proteins
Peripheral membrane proteins have hydrophilic R groups on their surface and are bound to the surface of membranes, mainly by ionic and hydrogen bonds interactions
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What do many peripheral membrane proteins interact with?
Many peripheral membrane proteins interact with the surfaces of integral membrane proteins
Functions of peripheral membrane proteins
Peripheral/extrinsic proteins have fewer hydrophobic R-groups interacting with the phospholipids and are responsible for cell-cell interactions
What does the phospholipid bilayer act as?
The phospholipid bilayer acts as a barrier to ions, and most uncharged polar structures.
What transports large, charged and hydrophilic molecules across the cell membranes?
Transmembrane proteins - This process is called facilitated diffusion
What can transmembrane proteins act as
Transmembrane proteins can act as channels or transporters
How do you small molecules such as oxygen and carbon dioxide passed through the phospholipid bilayer
Diffusion
Describe, facilitated diffusion
Facilitated diffusion is the passive transport of substances across the membrane through specific transmembrane proteins
What do different cells have to perform specialised functions
To perform, specialised functions, different cell types have different channel and transporter proteins
Describe channels
Most channel proteins in animal and plant cells are highly selective.
Channels are multi-subunit proteins with the subunits arranged to form water filled pores that extend across the membrane
Describe how transport occurs across channels
Transport across the channels is always passive and specific to one type of ion or molecule (facilitated diffusion). Solute passage can be gated or ungated.
Describe Passage through an ungated channel protein
Passage through an ungated channel protein does not require a change to the conformation of the protein.
Movement of molecules as passive, and is specific to the ions or molecules that are being allowed to pass across.
Describe passage through a gated protein channel
Gated protein channels, change confirmation to allow or prevent diffusion.
Gated channels respond to a stimulus which causes them to open or close. The stimulus may be chemical (ligand gated) or electrical (voltage gated).
Describe how ligand gated channels and voltage gated channels are controlled
Ligand gated channels are controlled by the binding of signal molecules, and voltage gated channels are controlled by changes in ion concentration
Describe how transporter proteins work
Transporter proteins bind to the specific substance to be transported and undergo a conformational change to transfer the solute across the membrane.
Transporters alternate between two confirmations, so that the binding site for a solute is sequentially exposed on one side of the bilayer, then the other.
What are the types of transport?
Facilitated diffusion or active
What does the conformational change an active transport require?
Energy from the hydrolysis of ATP by ATPases
Describe how active transport works
Active transport uses pump proteins that transfer substances across the membrane against a concentration gradient.
Explain pumps involved in active transport
Pumps that mediate active transport transport of protein coupled to an energy source
What is required for active transport?
A source of metabolic energy is required for active transport
How is an electrochemical gradient formed
For a solute carrying a net charge, the concentration gradient and the electrical potential difference combine to form the electrochemical gradient that determines the transport of the solute.
How is a membrane potential created?
A membrane potential (an electrical potential difference) is created when there is a difference in electrical charge on the two sides of the membrane
What is the sodium potassium pump?
Na/K-ATPase (sodium potassium pump) it’s an enzyme found in the plasma membrane of all animal cells.
How do ion pumps establish and maintain ion gradients?
Ion pumps, such as the sodium potassium pump, use energy from the hydrolysis of ATP to establish and maintain ion gradients.
Describe how the sodium potassium pump works
The sodium potassium pump transports ions against a steep concentration gradient, using energy directly from ATP hydrolysis.
It actively transports sodium ions out of the cell and potassium ions into the cell (against their concentration gradient).
Describe the process of the sodium potassium pump
- The pump has high affinity for Na+ ions inside the cell and 3 Na+ ions bind to the binding sites.
- Transporter proteins hydrolyses ATP to ADP + Pi
- Protein attaches to Pi (phosphorylation) causing a confirmational change to the protein. Protein new has low affinity for Na+ ions which are released outside of the cell.
- New confirmation has a high affinity for K+ ions, and 2 K+ ions bind to the surface of the protein outside of the cell.
- This triggers dephosphorylation (release of phosphate group) of protein; protein revert to original confirmation with binding sites exposed to the interior of the cell.
- Protein now has low affinity for K+ ions inside the cell and are released into the cell. Protein has high affinity for Na+ ions which bind to the protein. The cycle begins again.
How is a concentration gradient and electrical gradient established in sodium potassium pumps?
For each ATP hydrolysed, 3 Na+ ions are transported out of the cell and 2 K+ ions are transported into the cell. This establishes both concentration gradient and an electrical gradient.
Where is the sodium potassium pump found?
The sodium potassium pump is found in most animal cells, accounted for a high proportion of the basal metabolic rate in many organisms.
What drives active transport of glucose in the small intestine?
In the small intestine, the sodium gradient created by the sodium potassium pump, drives the active transport of glucose.
What does the sodium potassium pump generate in intestinal epithelial cells
In intestinal epithelial cells the sodium potassium pump generates a sodium ion gradient across the plasma membrane
The transport of sodium ions and glucose in the small intestine
The glucose transporter responsible for this glucose symport transports sodium ions and glucose at the same time, and in the same direction
How does sodium ions enter the intestinal epithelial cells?
Sodium ions enter the cell down its concentration gradient; the simultaneous transport of glucose pumps glucose into the cell against its concentration gradient
