Answer
In eukaryotic cells, some organelles are considered membranous because they are surrounded by membranes that separate their internal contents from the surrounding cytoplasm. These membranes are lipid bilayers composed of phospholipids, similar to the cell's plasma membrane. Organelles enclosed in membranes are often involved in specialized functions within the cell. Here are some examples of membranous organelles, along with information about whether they are enclosed by a single or double membrane:
**Enclosed by a Single Membrane:**
1. **Endoplasmic Reticulum (ER):**
- **Structure**: The endoplasmic reticulum consists of a network of membranes.
- **Functions**: It plays a role in protein synthesis, lipid metabolism, and detoxification. The rough ER has ribosomes on its surface and is involved in protein synthesis, while the smooth ER is involved in lipid metabolism and detoxification.
2. **Golgi Apparatus (Golgi Complex):**
- **Structure**: The Golgi apparatus consists of a stack of flattened membranous sacs.
- **Functions**: It is involved in modifying, sorting, and packaging proteins and lipids for transport to various cellular destinations, including secretion from the cell.
3. **Lysosomes:**
- **Structure**: Lysosomes are membrane-bound vesicles.
- **Functions**: Lysosomes contain enzymes that break down cellular waste materials, cellular debris, and foreign invaders, such as bacteria. They play a role in cellular digestion.
4. **Vacuoles (in plant and fungal cells):**
- **Structure**: Vacuoles are large membrane-bound organelles.
- **Functions**: In plant cells, vacuoles store water, ions, and nutrients, and they help maintain turgor pressure. In fungal cells, vacuoles have various functions, including storage and maintaining cell structure.
**Enclosed by a Double Membrane:**
1. **Mitochondria:**
- **Structure**: Mitochondria have a double membrane structure with an outer membrane and an inner membrane, which forms folds called cristae.
- **Functions**: Mitochondria are the "powerhouses" of the cell, producing adenosine triphosphate (ATP) through cellular respiration. They are involved in energy production.
2. **Chloroplasts (in plant cells):**
- **Structure**: Chloroplasts also have a double membrane, with an outer and inner membrane.
- **Functions**: Chloroplasts are responsible for photosynthesis in plant cells, where they convert light energy into chemical energy (glucose) by using pigments like chlorophyll.
3. **Nucleus:**
- **Structure**: The nucleus is enclosed by a double membrane called the nuclear envelope.
- **Functions**: The nucleus contains genetic material (DNA) and serves as the control center of the cell, regulating gene expression and directing cellular activities.
These organelles are enclosed by membranes to create distinct compartments within the cell, allowing for specialized functions and the regulation of various cellular processes. The double membrane in some of these organelles adds an additional layer of compartmentalization and control.
Work Step by Step
In eukaryotic cells, some organelles are considered membranous because they are surrounded by membranes that separate their internal contents from the surrounding cytoplasm. These membranes are lipid bilayers composed of phospholipids, similar to the cell's plasma membrane. Organelles enclosed in membranes are often involved in specialized functions within the cell. Here are some examples of membranous organelles, along with information about whether they are enclosed by a single or double membrane:
**Enclosed by a Single Membrane:**
1. **Endoplasmic Reticulum (ER):**
- **Structure**: The endoplasmic reticulum consists of a network of membranes.
- **Functions**: It plays a role in protein synthesis, lipid metabolism, and detoxification. The rough ER has ribosomes on its surface and is involved in protein synthesis, while the smooth ER is involved in lipid metabolism and detoxification.
2. **Golgi Apparatus (Golgi Complex):**
- **Structure**: The Golgi apparatus consists of a stack of flattened membranous sacs.
- **Functions**: It is involved in modifying, sorting, and packaging proteins and lipids for transport to various cellular destinations, including secretion from the cell.
3. **Lysosomes:**
- **Structure**: Lysosomes are membrane-bound vesicles.
- **Functions**: Lysosomes contain enzymes that break down cellular waste materials, cellular debris, and foreign invaders, such as bacteria. They play a role in cellular digestion.
4. **Vacuoles (in plant and fungal cells):**
- **Structure**: Vacuoles are large membrane-bound organelles.
- **Functions**: In plant cells, vacuoles store water, ions, and nutrients, and they help maintain turgor pressure. In fungal cells, vacuoles have various functions, including storage and maintaining cell structure.
**Enclosed by a Double Membrane:**
1. **Mitochondria:**
- **Structure**: Mitochondria have a double membrane structure with an outer membrane and an inner membrane, which forms folds called cristae.
- **Functions**: Mitochondria are the "powerhouses" of the cell, producing adenosine triphosphate (ATP) through cellular respiration. They are involved in energy production.
2. **Chloroplasts (in plant cells):**
- **Structure**: Chloroplasts also have a double membrane, with an outer and inner membrane.
- **Functions**: Chloroplasts are responsible for photosynthesis in plant cells, where they convert light energy into chemical energy (glucose) by using pigments like chlorophyll.
3. **Nucleus:**
- **Structure**: The nucleus is enclosed by a double membrane called the nuclear envelope.
- **Functions**: The nucleus contains genetic material (DNA) and serves as the control center of the cell, regulating gene expression and directing cellular activities.
These organelles are enclosed by membranes to create distinct compartments within the cell, allowing for specialized functions and the regulation of various cellular processes. The double membrane in some of these organelles adds an additional layer of compartmentalization and control.
