Cell Activities: Cells are the basic structural and functional units of living tissue. Cellular functions include basic life processes such as protein and lipid (fat) synthesis, cell division and replication, respiration, metabolism, and ion transport as well as providing structural support for tissues, protecting the body against disease or injury, and serving as selective barriers to the passage of various materials into and out of the cell. The study of cellular functions and processes is referred to as cellular physiology.

Cell Functions

A cell is the structural and functional unit of life. Each cell contains smaller organelles that perform various functions such as metabolism, transportation and secretion of substances. Because some cells perform specific functions, they have especially the structure of the cell is modified. For example, red blood cells are the oxygen carriers in the body. They lack a nucleus to make more space for the oxygen-carrying pigment, haemoglobin. The various cell organelles float in a liquid called the cytoplasm.

1. Cells Provide Structure and Support

Like a classroom is made of bricks, every organism is made of cells. While some plant cells such as the collenchyma and sclerenchyma are specifically meant for structural support, all cells generally provide the structural basis of all organisms. For instance, skin is made up of a number of skin cells. Vascular plants have evolved a special tissue called the xylem, which is made of cells that provide structural support.

2. Facilitate Growth Through Mitosis

In complex organisms, tissues grow by the simple multiplication of cells. This takes place through the process of mitosis in which the parent cell breaks down to form two daughter cells identical to it. Mitosis is also the process through which simpler organisms reproduce and give rise to new organisms.

3. Cells Allow Passive and Active Transport

1. Cells import nutrients to use in the various chemical processes that go on inside them.
2. These processes produce waste that a cell needs to get rid of.
3. Small molecules such as oxygen, carbon dioxide and ethanol get across the cell membrane through the process of simple diffusion.
4. This is regulated with a concentration gradient across the cell membrane. This is known as passive transport.
5. However, larger molecules, such as proteins and polysaccharides, go in and out of a cell through the process of active transport in which the cell uses vesicles to excrete or absorb larger molecules.

4. Cells Produce Energy

An organism’s survival depends upon the thousands of chemical reactions that cells carry out relentlessly. For these reactions, cells require energy. Most plants get this energy through the process of photosynthesis, whereas animals get their energy through a mechanism called respiration.

Digestion in Cells

Digestion in cells takes place as follows:

1. A eukaryotic cell can move large amounts of material into itself by a process called endocytosis. In endocytosis, the plasma membrane invaginates (folds inward) taking inside itself a portion of extracellular fluid (ECF) containing dissolved or suspended nutrients. The plasma membrane then seals off the ingested fluid in a compartment called a vesicle.
2. Endocytosis includes two processes known as pinocytosis and phagocytosis. Pinocytosis, or “cell drinking,” refers to the continuous taking-in of very small droplets of ECF. It occurs in almost all cells of the human body.
3. Phagocytosis, by contrast, is carried out only occasionally, and only by specialized cells known as phagocytes. These cells can engulf large particles—usually bacteria or other microorganisms.
4. Once inside the cell, the ingested particle is sealed off in a vacuole.
5. Eventually, the vacuole fuses with a lysosome, which contains a digestive enzyme that destroys the contents of the vacuole.
6. Exocytosis is the reverse of endocytosis; it is a process in which the waste products of digestion or toxins from destroyed bacteria are released into the extracellular fluid.
7. The vesicle or vacuole containing the waste products moves toward the plasma membrane and fuses with it.
8. This fusion releases the contents of the vesicle outside the cell. Exocytosis is also used to carry hormones or neurotransmitters produced inside the cell to the outside.
9. In addition to transporting large molecules into the ECF, exocytosis also increases the total surface of the plasma membrane.

Signalling in Cells

Signalling in Cells takes place as follows:

1. Another important function of human cells is communication and response to signals from their environment. Many of these signals are chemicals in the ECF secreted by distant glands (endocrine signals); by nearby cells (paracrine signals); or even by themselves (autocrine signals).
2. The proteins that signal nearby cells or the cell itself are called cytokines. Cytokines include such substances as tumor necrosis factors and interferons.
3. The chemical signal binds to protein receptors on the cell’s surface that also serve to activate the cell’s response to the signal.
4. Chemical signals may trigger either immediate or longer-term responses in receptor cells. Immediate responses include changes in the electrical charge across the cell’s plasma membrane, changes in the cell’s metabolism, or movement toward or away from the chemical stimulus (chemotaxis).
5. Long Term responses to chemical signals include changes in the genetic material inside the cell’s nucleus.

Migration and Movement in Cells

Migration and movement in cells are as follows:

1. Cell migration refers to the capacity of cells to move within the body. In the early stages of embryo development, the rapidly dividing cells migrate to form different layers of tissue that eventually give rise to the brain and nervous system, the internal organs, the skin, and other specialized parts of the body. The pattern of cell migration is regulated by special proteins known as migration proteins.
2. In the adult, cell migration is critical to the proper functioning of the immune system and wound healing. When a person cuts their finger, for example, white blood cells divide rapidly and move in the direction of the injury to engulf any bacteria that enter, while cells in the skin proliferate and migrate to repair the wound itself.
3. Cells migrate in a specific direction in response to a chemical signal picked up by receptor proteins in the plasma membrane.
4. This orientation of a cell’s movement toward or away from a chemical signal is called chemotaxis. Movement toward the chemical is called positive chemotaxis; movement away from it is negative chemotaxis.
5. Cell locomotion itself is a cyclical process. When the cell receives a chemical signal, it first forms a protrusion at its front. The protrusion then attaches itself to the surface or substrate over which the cell is moving. Next, the cell body contracts and pulls itself forward toward the protrusion. The attachments at the rear of the cell are then released and the cycle is repeated.
6. Some human cells have hairlike processes that extend outward from the plasma membrane called cilia (if there are many of them per cell) or flagella (if there is only one). Both cilia and flagella move mucus or other fluids over the surface of the cell. For example, the tissue that lines the airway is covered with cilia that move mucus upward toward the throat and mouth. The human sperm has a single flagellum that enables it to swim upward to fertilize the ovum.

Metabolism in Cells

The metabolism in cells are as follows:

1. Metabolism refers to the sum total of the physical and chemical processes that produce and maintain living cells (anabolism) and the breaking down of complex molecules to provide energy for the cells (catabolism).
2. An important part of cellular metabolism is respiration, which is the exchange of oxygen and carbon dioxide between cells and the external environment.
3. In cellular respiration, the cells break down molecules of glucose to form carbon dioxide and water.
4. The energy released in this process is stored in the cells in the form of adenosine triphosphate or ATP.
5. Respiration has two phases: glycolysis, in which the glucose is broken down to pyruvic acid; and the complete oxidation of pyruvic acid to form water and carbon dioxide.
6. Glycolysis takes place in the cytosol of the cell. The pyruvic acid, however, is oxidized in the mitochondria through a process called chemiosmosis.
7. Pairs of hydrogen ions (protons), which carry a positive charge, accumulate on one side of the inner membrane of the mitochondrion.
8. The membrane becomes energized, which allows enzymes contained within it known as ATP synthases to make ATP from adenosine diphosphate (ADP) and a phosphate molecule.

Cell Division and the Cell Cycle

When eukaryotic cells divide, each daughter cell must receive a complete set of genes, a pair of centrioles, some mitochondria, some ribosomes, and a portion of the endoplasmic reticulum. Because each cell contains multiple ribosomes and mitochondria, dividing the organelles is simple. To divide the genetic material in the cell nucleus accurately, however, requires the cell to first duplicate each chromosome and then distribute one of each doubled chromosome to each daughter cell. The repeated process of doubling the genetic material and then halving it during cell division is called the cell cycle. The cycle consists of four phases:

1. G (which stands for gap)1: In this phase, the chromosomes are prepared for replication.
2. S (which stands for synthesis): In the S phase, the chromosomes are doubled and form two identical sister chromatids.
3. G (gap)2: In this phase, the cell prepares for mitosis.
4. M (which stands for mitosis): Mitosis is a process with five phases during which one copy of each duplicated chromosome is distributed to each daughter cell. The five phases of mitosis are:
   a. Prophase. In prophase, the nucleoli disappear from the nucleus and the chromatin condenses and forms chromosomes. The two centrosomes move to opposite ends of the cell and form the mitotic spindle.
   b. Prometaphase. The nuclear membrane (or envelope) disappears, which allows the spindle fibers to interact with the chromosomes. Protein structures called kinetochores appear at the centromere of each sister chromatid. The kinetochores attach to opposite poles of the spindle.
   c. Metaphase. In metaphase, all the paired structures in the cell are lined up midway between the spindle poles at the cell’s equator. This position is called the metaphase plate.
   d. Anaphase. In anaphase, each of the sister chromatids moves toward one of the poles of the dividing cell. The movement is caused by motor proteins in the kinetochores and the shortening of microtubules in the kinetochores.
   e. Telophase. In telophase, the nuclear membrane reforms around the nucleus in each daughter cell, the spindle breaks down, and the chromatin begins to uncoil and disperse within the nucleus.

A special method of cell division occurs during the maturation of sex cells (sperm and ova) known as meiosis. In meiosis, each daughter cell receives only half of the number of chromosomes found in the parent cell, as there is no S phase in meiosis.

Cell Death

The death in cells takes place as follows:

1. Cells may die from either direct injury or from a process of self-destruction called apoptosis. Cells that are damaged by mechanical trauma or toxic chemicals swell because the plasma membrane can no longer control the movement of ions and water into the cell. The cell contents then leak out, causing inflammation in nearby cells.
2. In apoptosis, which is also called programmed cell death or PCD, the cell essentially commits suicide.
3. Apoptosis is necessary to the body’s health as part of normal functioning (as when the lining of a woman’s uterus sloughs off during menstruation) or to destroy dangerous cells.
4. Dangerous cells include cells infected with viruses, cells with damaged DNA, and cancer cells.
5. There are three possible pathways to apoptosis. The first is triggered by signals within the cell itself.

Summary

The life of a cell depends on its environment and on the activities that take place within it. This section explains some of the processes that take place in cells and some of the ways cells respond to their environment. The functions of all cells depend on or require special molecules called enzymes.  Enzymes are proteins that help chemical reactions take place.
They help cells build products like proteins, make copies of DNA molecules, make energy available for cell work, and even break down certain molecules. Each enzyme is very specific in its action. Without enzymes, most chemical reactions that take place in cells would proceed very slowly, if at all. Enzymes enable those reactions to happen faster. Different enzymes exist in different parts of a cell. Enzymes on the surface of a cell help receptor proteins signal the cell when they detect certain molecules in the fluid environment. Enzymes in the cell cytoplasm allow the structural proteins of the cytoplasm to do their work, for instance, to contract, to change the cell shape, or to divide the cell. Enzymes in the nucleus of a cell allow the cell to copy its DNA. Enzymes in the mitochondria of a cell allow the cell to convert energy from food nutrients into ATP.

FAQs on Cellular Activities

Q.1. What are cellular processes?
Ans: Any process that is carried out at the cellular level, but not necessarily restricted to a single cell. For example, cell communication occurs among more than one cell but occurs at the cellular level.

Q.2. What are the different phases of the cell cycle?
Ans: There are 4 phases of cell division: G1, S, G2 and M.

Q.3. What are the five life functions of cells?
Ans: The life processes are metabolism, nutrition, transport, cellular respiration, synthesis, excretion, regulation, growth & development and reproduction.

Q.4. How does a cell perform its functions?
Ans: Cell organelles such as the endoplasmic reticulum and mitochondria assist cells to accomplish their duties. In brief, the cell’s cell organelles are required to fulfil certain functions. Some of these tasks may necessitate the use of mitochondrial energy.

Q.5. What is cell migration and why is it important?
Ans: Cell migration is essential for adult homeostatic activities like building an effective immune response and repairing damaged tissues. Some pathological processes, such as vascular disease, chronic inflammatory illnesses, and tumour growth and metastasis, can be aided by migration.

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