Chapter 5 - Plasma Membranes Flashcards
What makes up a membrane?
A phospho-lipid bilayer
What is compartmentalisation?
The formation of separate membrane bound areas in a cell
What is the plasma membrane?
The membrane that separates the cell from its environment
What is the structure of the p-l bilayer in terms of hydrophobia and phillia?
The hydrophilic phosphate heads form both the inner and outer layer, sandwiching the fatty acid tails of the phospholipids to form a hydrophobic core within the membrane
What do the hydrophilic heads mean for the membrane?
It can interact with water
What are intrinsic proteins?
Transmembrane proteins that are embedded through both layers of the membrane
What is the role of channel proteins?
They provide a hydrophilic channel for the PASSIVE movement (diffusion) of polar molecules and ions through the membrane
What are the roles of carrier proteins?
Important roles in both PASSIVE transport AND ACTIVE transport into cells- this often involves the shape of the protein changing
How are intrinsic proteins held in place?
By the hydrophobic R-groups on the external surface interacting with the hydrophobic core of the membrane
What are the roles of glycoproteins?
They play a role in cell adhesion and as receptors for chemical signals
What is the structure of glycoproteins?
- Embedded in cell-surface membrane
- They have attached carbohydrate chains
What are glycolipids?
Lipids with attached carbohydrate chains. They are antigens/cell markers
Give three types of intrinsic proteins?
- Channel proteins
- Carrier proteins
- Glycoproteins
What are extrinsic proteins?
They are proteins that are present on just one side of the bilayer at a time, and some move between layers
They have polar R-groups on their outer surface, which interact with the polar phospholipids or intrinsic proteins
Are all extrinsic proteins bound to one side of the bilayer?
No, some can move between layers
What is the structure of cholesterol in terms of hydro philia / phobia?
It is a lipid with one hydrophobic end and one hydrophilic end
What is the purpose of cholesterol?
It regulates the fluidity of the membrane
Where is cholesterol positioned, and how does this allow it to fulfil its role?
Between the phospholipid bilayer, with the hydrophilic end interacting with the hydrophilic heads, and the hydrophobic end interacting with the hydrophobic tails- this pulls them together, maintaining the structure of the membrane
What are membranes with protein channels known as in terms of permeability and why?
Selectively permeable, as the protein channels are often specific to one molecule or ion
What is facilitated diffusion?
When protein channels are required in order for diffusion through the membrane to occur
Give 3 factors that affect rate of facilitated diffusion?
- Membrane surface area
- Membrane thickness
- Number of channel proteins
What is the supplier of metabolic energy?
ATP
What does active transport require?
Energy and carrier proteins
Summarise the process of active transport through a membrane
- Molecules or ions to be transported bind to receptors in channel protein on the outside of the cell
- On the inside of the cell, ATP binds to receptors and is hydrolysed, forming ADP
- Phosphate molecule (from ADP) binds to carrier protein, which opens the floodgates to the inside of the cell, and the ions/molecules enter
- Phosphate molecule is released and goes back to ADP to form ATP, and the channel protein returns to its original shape
What is endocytosis?
The bulk transport of material INTO cells
What are the two types of endocytosis, and what is the difference?
- Phagocytosis for solids
- Pinocytosis for liquids
(the process is the same)
Summarise the process of endocytosis
- Material to be transported comes into contact with cell membrane
- Cell membrane invaginates more and more, enfolding the material
- Eventually the membrane fuses, forming a vesicle
- Vesicle containing material moves off into cell
What is exocytosis?
The bulk transport of material OUT OF cells
Summarise the process of exocytosis
- reverse of endocytosis*
1. Secretory vesicle containing material moves towards and fuses with cell membrane
2. Contents of cell are then released outside of the cell
Why is endo/exocytosis a form of active transport?
It requires ATP for
- changing the shape of cells to engulf the material
- the fusion of cell membranes as vesicles form, or as vesicles reach the cell surface membrane on exit
- the movement of vesicles along the cytoskeleton
What 3 types of molecules can be transported across cell membranes via passive diffusion?
- Oxygen
- Carbon dioxide
- Steroid hormones
What 4 types of molecules are transported across cell membranes via facilitated diffusion?
- Glucose
- Amino acids
- Water
- Selected ions
What is the highest possible value of water potential?
0 kPa
What happens when an animal cell is placed in a solution with a higher water potential than the cell cytoplasm?
Water will enter the cell by osmosis
What happens if too much water enters an animal cell and why?
The hydrostatic pressure will increase to a point where the cell will swell and eventually burst. This happens because animal cell-surface membranes cannot stretch much and cannot withstand the increased pressure.
What is it called when an animal cell bursts due to increased water pressure?
Cytolysis
What happens when an animal cell is placed in a solution with a lower water potential than the cell cytoplasm?
Water will leave the cell by osmosis
What is crenation?
Where water leaves the cell to the point where the cell shrinks in animal cells
What happens when a plant cell is placed in a solution with a higher water potential than the cell?
Water enters the cell by osmosis, and the increased hydrostatic pressure pushes the membrane against the cell wall
In plant cells, what is the pressure of the cell membrane against the cell wall due to hydrostatic pressure known as?
Turgor pressure
Why do plant cells not burst in higher water potential solutions?
Because as turgor pressure increases, it resists the entry of further water, and the cell is said to be turgid. This is made possible by the rigid cell walls of plant cells.
What happens when a plant cell is placed in a solution with a lower water potential than the cell?
Water leaves the cell by osmosis, causing a reduction in the volume of the cytoplasm
What does it mean for a plant cell to be plasmolysed?
When too much water leaves plant cells, the cell-surface membrane pulls away from the cell wall (plasmolysis)
