Cell Membrane Structure and Function

Study Notes

### Fluid Mosaic Model

The Fluid Mosaic Model describes cell membranes as consisting of protein molecules embedded in a fluid bilayer of phospholipids.

Membrane fluidity is influenced by the composition of fatty acid tails in phospholipids.
Longer chain fatty acids make the membrane less fluid.
The presence of cis-unsaturated and polyunsaturated fatty acids increases membrane fluidity.
Trans-unsaturated fatty acids decrease fluidity.
Cholesterol can affect membrane fluidity depending on temperature: at high temperatures, it reduces fluidity; at low temperatures, it maintains fluidity.

### Types of Membrane Transport

Passive transport does not require energy input and relies on the concentration gradient.
- Simple diffusion involves the movement of a substance across a membrane from an area of high concentration to an area of low concentration.
- Facilitated diffusion requires the assistance of transport proteins to move substances across the membrane.
Active transport requires energy input, often in the form of ATP, to move substances against their concentration gradient.
- Sodium-potassium pump is an example of active transport that moves sodium ions out of the cell and potassium ions into the cell against their concentration gradients, using the energy from ATP hydrolysis.
- Some active transport coupled with the movement of another molecule down its concentration gradient, known as cotransport.

### Membrane Proteins and Function

Recognition proteins are involved in cell-cell recognition, allowing cells to identify and interact with each other.
Receptor proteins bind to specific signaling molecules, triggering cellular responses.
Channel proteins form pores through the membrane, allowing the passage of specific ions or molecules.
Transport proteins facilitate the movement of molecules across the membrane.
Plasma proteins are not necessarily associated with the plasma membrane.

### Plant Cells in Hypotonic Solutions

When a plant cell is submerged in a hypotonic solution, the water potential of the solution is lower than the water potential inside the cell, causing water to move into the cell.
This influx of water causes the cell to become turgid, pushing the cell membrane against the cell wall, providing the plant with structural support.

### Factors Affecting Diffusion Rate

The rate of diffusion across a selectively permeable membrane is influenced by several factors:
- Concentration Gradient: The steeper the concentration gradient, the faster the rate of diffusion.
- Temperature: Higher temperatures increase the rate of diffusion.
- Molecular Size: Smaller molecules diffuse faster than larger ones.

### Endocytosis

Endocytosis is a process by which the cell takes in material from the external environment by forming vesicles.
- Phagocytosis is the process by which the cell engulfs large particles, such as bacteria or cell debris.
- Pinocytosis is the process by which the cell engulfs fluids and dissolved substances.
- Receptor-mediated endocytosis is a specialized type of endocytosis that involves the binding of specific ligands to receptors on the cell surface.

### Membrane Transport Summary

Simple Diffusion: movement of molecules down their concentration gradient, no energy required.
Facilitated Diffusion: movement of molecules down their concentration gradient with the help of transport proteins, no energy required.
Active Transport: movement of molecules against their concentration gradient, requires energy.
Bulk Flow: movement of liquids or gasses through a membrane or a tube.
Endocytosis: uptake of large molecules or particles into a cell by forming vesicles.
- Phagocytosis: uptake of solid particles.
- Pinocytosis: uptake of fluids.
- Receptor-Mediated Endocytosis: intake of specific molecules.

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