The Cell or Plasma Membrane
- The plasma membrane forms the boundary between the internal environment of the cell, the cytoplasm and its external environment.
- It is selectively permeable
- Permeable means that substances can pass through it.
- Selectively means that it chooses what goes in and what comes out.
- It is incredible flexible. Watch the clip below.
The internal and external environment of cells
Internal environments of cells are very different from their external environments.
Your textbook uses human cells as an example. It compares the 6 elements that make up 99% of the human body (Carbon, Oxygen, Hydrogen, Nitrogen, Calcium and Phosphorus) in terms of their conceptration inside human cells vs outside human cells. They did the same for the common ions (sodium, potassium, chloride and calcium).
The results were quite amazing!
Your textbook uses human cells as an example. It compares the 6 elements that make up 99% of the human body (Carbon, Oxygen, Hydrogen, Nitrogen, Calcium and Phosphorus) in terms of their conceptration inside human cells vs outside human cells. They did the same for the common ions (sodium, potassium, chloride and calcium).
The results were quite amazing!
Protecting the internal environment
The cell works constantly to keep the internal environment stable.
Unicellular vs Mulitcellar organisms:
- Plasma membrane regulates the internal environment.
- Complex chemical reactions occur including respiration and photosynthesis.
- Enzymes can only perform their tasks within narrow temperature and pH ranges.
- Hydrogen ion concentrations kept within strict limits to maintain suitable cytoplasmic pH.
- Toxic waste products are removed to ensure they do not interfere with chemical reactions in the cytoplasm.
Unicellular vs Mulitcellar organisms:
- Unicellular organism cell membranes need to work constantly to maintain a stable internal environment in an ever changing external environment.
- Multicellular organism cells are usually surrounded by extracellular fluid. This is any liquid surrounding cells.
- The contents of this fluid are kept constant by the workings of organs such as lungs, kidney, and liver.
The "Fluid Mosaic Model" of Plasma Membrane
PHOSPHOLIPID BILAYER:
The cell membrane is referred to as a phospholipid bilayer because it mostly consists of two layers of phospholipid molecules.
A Phospholipid molecule:
When two layers of these molecules are arranged with their hydrophilic heads out and hydrophobic tails together, water is unable to pass through.
The cell membrane is referred to as a phospholipid bilayer because it mostly consists of two layers of phospholipid molecules.
A Phospholipid molecule:
- has a phosphate group on its the head which is water soluble (water attracting)
- has a fatty acid tail which is water insoluble (water repelling).
- Thus, we say it has a hydrophilic head and a hydrophobic tail.
When two layers of these molecules are arranged with their hydrophilic heads out and hydrophobic tails together, water is unable to pass through.
CHOLESTEROL & PHYTOSTEROL:
- In animals, cholesterol is interspersed among the phospolipids in order to provide flexibility to the membrane.
- In plants and bacteria, this is flexibility achieved by phytosterol instead of cholesterol.
MEMBRANE PROTEINS:
Proteins are also embedded in the phospholipid bilayer.
Transport Proteins: Act as passageways to allow certain substances to move across the membrane
Receptor Proteins: Bind to external signal molecules like hormones which informing cells to carry out specific functions.
Recognition Proteins: Also called glycoproteins because they combine with a sugar molecule. The are unique to the individual prganism and also the body or cell to recognise the difference between foreign cells and the body's own "self" cells.
Transport Proteins: Act as passageways to allow certain substances to move across the membrane
Receptor Proteins: Bind to external signal molecules like hormones which informing cells to carry out specific functions.
Recognition Proteins: Also called glycoproteins because they combine with a sugar molecule. The are unique to the individual prganism and also the body or cell to recognise the difference between foreign cells and the body's own "self" cells.
You should now do...
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Read and complete "Scientific Literacy: Development of the cell membrane model" on p. 186
Question Set 8.1 p. 189 |