Definition, Structure, Membrane and Cytoplasm

Definition of Cells

Cell is the smallest unit of living things structurally and functionally and can not be further divided into smaller parts. Structurally, it means that the cell is the smallest unit of living things, whereas functionally it means that the cell has the ability to be able to carry out various life processes such as overhaul, synthesis, respiration, etc.

Cell Structure

Cells have three main structures, namely the cell membrane, cytoplasm, and cell organelles.

Based on their structure, cells are divided into prokaryotic cells and eukaryotic cells. Following is the difference between prokaryotic and eukaryotic cells.

There are two types of cells including in eukaryotic cells, namely animal and plant cells. However, each of these cells has differences, namely:

Cell membrane

The cell membrane is the boundary between the outer environment and the inside of the cell. The cell membrane is selectively permeable, that is, it can only be passed by certain substances, such as glucose, amino acids, glycerol and ions. There are two types of molecular movement:

  1. Passive transport: movement of molecules or ions without using energy and occurs spontaneously from high to low concentrations. Example:
    • Diffusion: the transfer of molecules from high concentrations to low concentrations.
    • Osmosis: displacement of water molecules through a semipermeable membrane from a solution with a high water concentration to a solution with a low water concentration.
  2. Active transport: the movement of molecules or ions using energy. Examples: sodium / potassium pump, endocytosis, and exocytosis.

Cell membrane functions, namely:

  1. As a receptor.
  2. Protects cell contents from leaving the cell.
  3. Arrange the molecules in and out of cells.
  4. The process of biochemical occurrence, for example: oxidation reactions and respiration.

The cell membrane consists of a protein layer and a double lipid layer (lipid bilayer).

  1. The protein layer forms two kinds of layers, namely the peripheral or extrinsic protein layer (covering the outer lipid bilayer) and the integral or intrinsic protein layer (covering the inner lipid bilayer).
  2. The lipid layer is composed of phospholipids which consist of phosphate groups. The head is hydrophilic (likes water) and the tail is hydrophobic (doesn’t like water). Apart from phospholipids, the lipid layer also consists of glycolipids (containing carbohydrates) and sterols (containing alcohol).

cell membrane structure

Cell Membrane Structure
Image Source: Mader, 1998


The cytoplasm is the fluid inside the cell, where the cell organelles are located. The main components of the cytoplasm are:

  1. Gel-like liquid (cytosol).
  2. Substance deposits in the cytoplasm.
  3. Cell organelles.
  4. The cell skeleton (cytoskeleton) is composed of three types of fibers, namely microfilaments, microtubules and intermedia filaments.

Cell Organelles

1. Nucleus (Cell Nucleus)

The nucleus plays a role in controlling cell activity. The largest organelle in the cell has a diameter of about 10µm, is located in the center of the cell, and is round or oval in shape. Based on the number of nuclei, cells can be divided into:

  1. mononucleic cells (single nucleated), for example plant cells and animal cells.
  2. binucleate (double nucleus) cells, for example
  3. miltinuclear cells (many nuclei), for example Vaucheria and some types of fungi.

nuclear structure

Nucleus Structure:
Image Source: Raven & Johnson, 1996

2. Endoplasmic Reticulum

The reticulum comes from the word reticular, which means woven threads or nets. It lies rotating in the cytoplasm (endoplasm) so that the endoplasmic reticulum is a thread or mesh that surrounds the cytoplasm. Based on the presence of ribosomes, the endoplasmic reticulum can be divided into:

  1. Rough Endoplasmic Reticulum, the surface covered by ribosomes.
  2. Endoplasmic Reticulum Smooth, the surface is not covered by ribosomes.

RE functions are:

  1. In the Rough Endoplasmic Reticulum and Smooth Endoplasmic Reticulum functions to: synthesize fat and cholesterol, transport molecules from one part of the cell to another.
  2. The Rough Endoplasmic Reticulum functions to: accommodate proteins synthesized by ribosomes to be distributed to the Golgi apparatus.

endoplasmic reticulum cell organelle

Endoplasmic Reticulum
Source: Campbell, 2005

3. Ribosomes

  • Ribosomes are the smallest organelles in the cytoplasm.
  • There are two subunits that make up the ribosome, namely the small subunit and the large subunit.
  • Ribosomes are synthesized in the nucleus and function for synthesis protein.

4. The Golgi Complex

  • The Golgi complex is a secretory organelle.
  • The Golgi complex has functions including:
  1. Polysaccharide secretion site.
  2. form glycoproteins to be carried out of cells.
  3. Forms the plasma membrane.
  4. Forming a secretory pouch as a wrapper for substances to be released by cells.
  5. Forms acrosome in sperm, yolk in egg cell and lysosome.
  • Almost all plant and animal cells have a Golgi complex, but in plant cells this organ is called a dictiosome.

5. Lysosomes

  • The lysosome is a membrane in the form of a small bag with a diameter of 0.05 – 1.5 µm which contains hydrolytic enzymes (lysozyme).
  • The functions of lysosomes, namely to perform intracellular digestion, autophagy, exocytosis, autolysis, and destroy carcinogenic compounds.

6. Micro Agency

  • The micro body has a shape that resembles a lysosome, slightly rounded with a diameter of 0.3 – 1.5 µm which contains the enzymes catalase and oxidase.
  • There are two types of micro bodies, namely, peroxisomes and glyoxisomes.
  • Peroxisomes are found in animal cells, fungi, and leaves of higher plants. The function of peroxisomes, which helps in light absorption and respiration, protects cells from H.202, and plays a role in converting fat into carbohydrates.
  • Glyoxysomes are present in plant cells. Glyoxisome function, which plays a role in the metabolism of fatty acids and the site of the glyoxylate cycle.

7. Mitochondria

Mitochondria have an elliptical shape like a stick and are present in aerobic eukaryotic cells. The mitochondria are composed of cristae (protrusions of the expansion of the inner membrane) and the mitochondrial matrix (an inner chamber rich in cytochromic enzymes, DNA, RNA, and proteins). Mitochondrial functions, namely for food oxidation, cellular respiration, dehydrogenation, oxidative phosphorylation, and electron transfer systems.mitochondrial structure

Mitochondrial Structure
Image source: Raven & Johnson, 1996

8. Chloroplasts

Chloroplasts are only found in plant and algae cells. Inside the chloroplast there are chlorophyll (photosynthetic pigment) and stroma. Photosynthetic pigments function to capture sunlight which will be converted into chemical energy. The function of chloroplasts, namely as a place for photosynthesis to take place.

9. Centrioles

Centrioles consist of a single layer of cylinders in which there are nine microtubules. Every animal cell, some microorganisms, and lower plants have two centrioles in the cytoplasm. Centrioles play a role in the process of cell division.


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