Cellular Level of Organization
Lesson focus - Name the main parts of a cell and their functions, distinguish cytoplasm from cytosol, explain selective permeability, and describe the processes that transport substances across the plasma membrane.
The Plasma Membrane
- A flexible outer surface made of a lipid bilayer
- Separates the internal from the external environment
- Has selective permeability to nutrients and electrically charged ions
Membrane Proteins
| Component | Function |
|---|---|
| Enzymes | Help catabolize reactions |
| Ion channels | Allow certain ions into and out of the cell |
| Identity markers | Determine blood type and allow the body to recognize self from non-self (MHC) |
| Carrier proteins | Allow transit of certain substances across the membrane |
Transport Across the Plasma Membrane
Diffusion is the mixing of particles in a solution that occurs because of the particles' kinetic energy. Particles move from areas of higher concentration to areas of lower concentration.
| Process | Energy requirement |
|---|---|
| Passive | Requires no cellular energy; substances move down a concentration or electrical gradient |
| Active | Uses cellular energy (ATP) to drive substances against a concentration or electrical gradient |
Electroneutrality - the cell maintains an electrically neutral charge to maintain homeostasis.
Factors Affecting Diffusion Rate
| Factor | Effect |
|---|---|
| Steepness of concentration gradient | Greater difference = greater rate |
| Temperature | Higher temperature = faster rate |
| Mass of diffusing substance | Larger mass = slower rate |
| Surface area | Larger area = faster rate |
| Diffusion distance | Greater distance = slower rate |
Passive Processes
Simple Diffusion
- Does not require the assistance of membrane proteins
- Osmosis - the net movement of a solvent (H₂O) through a selectively permeable membrane; occurs from areas of high pressure to low pressure
- The solute within the solution also exerts a force called osmotic pressure
Facilitated Diffusion
- Diffusion that occurs via ion channels or carrier-mediated channels
Clinical Application: Tonicity
| Solution | Description |
|---|---|
| Hypotonic | Lower concentration of solute than is found inside normal cells (↓ solute; ↑ solvent) |
| Isotonic | Solute concentration and water concentration are the same compared to body fluids (solute = solvent) |
| Hypertonic | Higher concentration of solute than is found inside normal cells (↑ solute; ↓ solvent) |
Quick tips: solute is typically salt, solvent is typically water, and water follows salt.
Cytoplasm
The cytoplasm is all of the cellular contents between the plasma membrane and the nucleus.
- Cytosol - the fluid portion of the cytoplasm
- Organelles - "little organs"
Organelles
| Organelle | Function |
|---|---|
| Cilia / flagella | Allow movement and attachment of organisms |
| Ribosomes | Sites of protein synthesis |
| Rough ER | A network of flattened sacs lined with ribosomes; responsible for protein synthesis |
| Smooth ER | Steroid and fatty acid synthesis; detoxifies potentially harmful substances |
| Golgi body complex | Packages and modifies proteins and other molecules; attaches or breaks apart specific molecules based on the cell's or surrounding tissue's needs |
| Lysosomes | Responsible for breaking down other molecules |
| Mitochondria | The "powerhouse of the cell"; generates the majority of the cell's ATP |
The endoplasmic reticulum (ER) is a network of membranes in the form of flattened sacs or tubules. Cristae are the ridges within the mitochondria and are the site of cellular respiration / the Krebs cycle.
Nucleus
The nucleus houses most of the cell's DNA.
Review
- The plasma membrane
- Transport across the plasma membrane
- Cytoplasm
- Nucleus