Cell membrane and endoplasmic reticulum

This slide deals with the cell membrane and endoplasmic reticulum.

1. Cell membrane and endoplasmic reticulum Prakash Dhakal M. Sc. Public Health Microbiology Tribhuvan University, Nepal

2. Cell membrane The cell membrane (also known as the  plasma membrane  or  cytoplasmic membrane ) is a biological membrane that surrounds the cytoplasm of the cell. In animal cell the plasma membrane is the outer boundary of the cell while in plants and prokaryotes it is usually covered by a cell wall. Structure of cell membrane The currently accepted model for the structure of plasma membrane is Fluid mosaic model . This model was proposed by S.J Singer and G.L Nicholson (1972). According to the fluid mosaic model, the plasma membrane is mosaic components of phospholipids, cholesterol and proteins that move freely and fluidly in the membrane .

3. Structure of cell membrane

4. The principal components of the plasma membrane are lipids (phospholipids and cholesterol), proteins and carbohydrate groups that are attached to some of the lipids and proteins. Phospholipids The cell membrane consists primarily of a thin layer of  phospholipids that arrange to form two layers of phospholipids ( phospholipid bilayer ). It is amphipathic in nature, meaning that they have both hydrophilic and hydrophobic regions. Phosphate head is hydrophilic (water loving). The hydrophilic heads of phospholipids in a membrane bilayer face outward , contacting the aqueous (watery) fluid both inside and outside the cell. The hydrophobic or “water-fearing,” part of a phospholipid consists of its long fatty acid tails. It faces inside of the membrane.

5. Cholesterol , another lipid, is found alongside phospholipids in the core of the membrane. Proteins Proteins are the second major component of plasma membranes. There are two main categories of membrane proteins: integral and peripheral . Integral membrane proteins  are, as their name suggests, integrated into the membrane. Peripheral membrane proteins  are found on the outside and inside surfaces of membranes, attached either to integral proteins or to phospholipids.

6. Carbohydrates Carbohydrates are the third major component of plasma membranes. In general, they are found on the outside surface of cells and are bound either to proteins (forming  glycoproteins ) or to lipids (forming  glycolipids ). These carbohydrate chains can be either straight or branched. The proportions of proteins, lipids and carbohydrates in the plasma membrane vary between different types of cells . For a typical human cell, proteins account for about 50 percent of the composition by mass, lipids account for about 40 percent and the remaining 10 percent comes from carbohydrates.

7. Functions of cell membrane It separates the contents of the cell from its outside environment. Cell membrane supports the cell and helps in maintaining the shape of the cell. The lipid bilayer is semi-permeable, which allows only selected molecules to diffuse across the membrane. Internal organelles are also encased by cell membranes.

8. Endoplasmic reticulum (ER) Endoplasmic reticulum (ER) are found only in Eukaryotic cells . They forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known as  cisternae . Structure Structurally, the endoplasmic reticulum is a network of membranes called cisternae . The sac like structures are held together by the cytoskeleton. The phospholipid membrane encloses the cisternal space/lumen. ER are found throughout the cell and connected to the nucleus. There are two basic types of ER. They are smooth ER and rough ER . Both smooth and rough ER have the same types of membranes but they have different shapes.

9. Rough ER looks like sheets or disks of bumpy membrane while smooth looks more like tubes. Rough ER is called rough because it has ribosomes attached to its surface. Smooth ER acts as a storage organelle. It is important in the creation and storage of lipids and steroids. Rough ER is very important in the synthesis and packaging of proteins. The RER is attached to the nuclear envelope that surrounds the nucleus.

10. Functions It is mainly responsible for the transportation of proteins and carbohydrates to another organelle, which includes lysosomes , Golgi apparatus, plasma membrane etc. They provide increased surface area for cellular reactions. They help in the formation of nuclear membrane during cell division. They play a vital role in the synthesis of proteins, lipids, glycogen and other steroids like cholesterol, progesterone, testosterone etc.