cell overview

This module is all about the cell as the unit of structure and function of living things. It discusses the relevance of size and shape to the function the cell performs; the difference between prokaryotic and eukaryotic cells and plant and animal cells. It describes the organelles in relation to the function they perform in the cell.

All living things are made up of cells. The cell is the unit of structure and function of life. It exhibits all the characteristics of living things. It is equipped with structures designed to perform the various life functions such nutrition, synthesis and respiration, steady state regulation, reproduction, growth, development and adaptation.

Englishman Robert Hooke is acknowledged as the first to report the cell. He was then studying under the microscope a thin section of a cork and noticed cube-like structures that reminded him of small cell rooms in the monastery – thus he called these structures cells. What Hooke actually saw were plant cell walls, the skeleton of dead plant cells.

Henceforth, interest on the cell prospered and more knowledge about it was known. However, the discovery and use of the electron microscope hastened the growth of information in cell biology. Likewise, new discoveries in biochemistry contributed to the in depth understanding of the functioning cell.

The Cell Theory The Cell Theory is one of the unifying principles in Biology. Although a lot of scientists worked on the cell, Matthias Schleiden, a botanist and Theodore Schwann, a zoologist, working independently of the other were acknowledged for the Cell Theory

The Cell Theory states all living things are made up of cells and cells arise from preexisting cells. This theory is a product of inductive reasoning - reaching a generalization based on many concurring observations. The ability of cells to come to divide to form new cells is the basis for all reproduction and for growth and repair of multicellular organisms.

Cell Size and Shape

In general, cells are small and microscopic. Most plant and animal cells are between 10 to 20 microns (1 micron = 10 –3mm). The bird’s egg is an exception because it is big enough to be seen by the naked eye.

Cell size is restricted by the relationship between cell volume and surface area. In smaller cells, the proportion of the cell surface area to volume is greater compared to larger cells. Materials can move faster in and out of smaller cells. Likewise, cell size is limited by its nuclear regulation. If cells are large, they have more than one nucleus to be able to regulate their activities. In the exceptionally big cell, like the bird’s egg cell, a lot of nutrients stored for the developing embryo. When the egg cell is fertilized and begins it activities of metabolism, it divides first many times before there is additional increase in volume.

Cell shape is highly diverse. Its shape is adapted to the function of it performs. The other factors that affect cell shapes are surface tension, pressure exerted by adjoining cells, rigidity of the cell membrane and viscosity of the protoplasm. Stellar nerve cell and spider-like bone cell are specialized to communicate with other cells. The filamentous skeletal muscle and spindle shaped smooth muscle are adapted to shorten and lengthen to bring about movement or contraction. The squamous, cuboidal and columnar epithelial cells with large surface area are adapted for covering more surfaces for protection. The oval and round blood cells are suitable for moving from one place to another.

Prokaryotes vs. Eukaryotes

Cell may vary in size, shape and organization but they all share common essential features. They all have an outer membrane, which separates the cell from its external environments. They all have the genetic material containing the hereditary information that directs cell activities and enables them to reproduce and pass on traits to the offspring. An in their protoplasm, the various important biological molecules responsible for energy production are found. Cells are broadly categorized as prokaryotes and eukaryotes. Both cells have protoplasm, which are colloid of organic and inorganic materials. However, they vary in the organization of the nuclear material. In eukaryotic cells, the DNA is associated with proteins in structures called chromosomes. These chromosomes are found in the nucleus surrounded by a nuclear membrane. In contrast to prokaryotes, only a single chromosome is found this is not contained in a membrane bound nucleus. The Prokaryotes

Modern prokaryotes include the bacteria and blue-green algae. The cell membrane is coated by a cell wall, which is secreted by the cell itself. The cytoplasm of the prokaryote contains a lot of very small organelles, the ribosomes. No membrane bound organelles are found in the cytoplasm of the prokaryotes. Lacking the membrane-bound organelles, the prokaryote is much simpler in structure. The DNA is in the nucleoid region, and no membrane separates the DNA from the rest of the cell. A prokaryote has large number of ribosomes, where proteins are synthesized. The border of the cell is the plasma membrane, which in some prokaryotes folds in to form

Lecture Notes in General Zoology by C.C.Divina/Chapter 3 2

structures called mesosomes. Outside the plasma membrane are fairly rigid cell wall and often an outer capsule, usually jelly-like. Some bacteria have flagella, locomotion organelles or pili, attachment structures or both. The Eukaryotic Cell Eukaryotes are larger cells and with more complex and membrane-bound organelles. Some cells have cell walls like plant and fungal cells which are not found in animal cells. All multicellular organisms are made up of eukaryotic cells.

Animal cells have a lot of sub-cellular organizations called organelles in the cytoplasm. These organelles are capable of performing some of life functions but not self-sufficient to neither live nor reproduce once removed from the cell. The organelles perform their specific functions to maintain the life and well being of the cell.

Inside the cell membrane is the protoplasm, a complex colloid where membranous structures like the RER, SER and Golgi bodies; formed organelles like the nucleus, mitochondrion, lysosome, centrioles, microtubules and microfilaments are found. The figure below shows the animal cell.

CELL MEMBRANE. The cell is limited and bounded by the cell membrane or otherwise

known as the plasmalemma. It regulates the passage of the materials into and out of the cells maintaining the structural and functional integrity of the cell.

The “fluid mosaic model” is the proposed

structure of the cell membrane, wherein two layers of phospholipids are formed with integral and peripheral proteins. This membrane is not static but rather in quasi fluid and very dynamic.

The cell membrane is semi-permeable

membrane. It has pores that would only allow molecules with smaller diameter than the pore to go in and out of the cells. The bigger molecules are kept outside these pores. Water is a small molecule and therefore could easily go in and out of the cell via these pores, however, bigger molecules like carbohydrates cannot pass through the poles. Moreover, the phospholipid layer serves as a barrier for entry or exit of molecules, which are not lipids soluble.

Materials are transported through the membrane from area of greater concentration to lesser

concentration through osmosis (diffusion through a semi-permeable membrane). Some molecules, like glucose, which cannot pass through the pores, may require carrier molecules to pass through the membrane, as in facilitated transport. In some instances when molecules have to move against the concentration gradient, from area of lower concentration to area of greater concentration, the carrier molecules in the membrane with expenditure of energy actively transport them.

Other molecules may enter the cell through endocytosis. Pinocytosis (drinking of the cell)

phagocytosis engulfing of materials


ENDOPLASMIC RETICULUM. Filling the space between the cell membrane and the internal nucleus is the cytoplasm is a membranous system similar to the cell membrane that forms intricate channels and is continuous with the outer layer of the nuclear envelope. This is the endoplasmic reticulum or otherwise called the ER. Two types of ER, are found in the cytoplasm, the smooth ER (SER) and the rough ER (RER).

The RER is characterized by the presence of

attached granules, the ribosomes and is involved in protein synthesis.

On the other hand, SER is agranulated and not associates with ribosomes. It is involved in synthesis of glycolipids, metabolism of carbohydrates and detoxification of drugs and other poisons. Enzymes of SER are important to the synthesis of fatty acids, phospholipids, steroids and other lipids. RIBOSOME. Ribosome is a granule found attached to the RER or free in the cytoplasm. It is chemically made up of conjugated RNA and protein. It has a complex structure of two sub-units with one smaller than the other. Ribosome is the site of protein synthesis

GOLGI APPARATUS. The Golgi body or Golgi apparatus is a stack of membranes originating from the endoplasmic reticulum. The membranes at the bottom are flattened and empty while the ones at the top are swollen. This structure is involved in the production and packaging of cellular secretions. It is the site of lipoprotein and glycoprotein production, and sometimes referred to as the chemical factory of the cell. The secretions produced in the Golgi bodies are in vesicles. These may remain inside the cell to be used or secreted outside through exocytosis.

LYSOSOME. Lysosome is ovoid or sometimes irregularly shaped body abundant in animal

cell. It originated from Golgi bodies and contains a lot of hydrolytic enzymes capable of breaking down proteins, carbohydrates, lipids and nucleic acids. It causes death or destruction of old and damaged cells through the process of autolysis. It is commonly referred to as the suicide bag of the cell.

Mitochondrion. Mitochondrion is a minute rod-

like organelle surrounded by two membranes. The outer membrane limits the organelle while the inner one is highly folded forming the cristae. Cristae increase the surface area of the mitochondrion for enzyme attachment.

The mitochondrion is involved in energy

production as it is the site of cellular respiration. It contains the necessary respiratory enzymes for the


chemical reactions of cellular respiration. Likewise, DNA is found in mitochondrion required for production of these enzymes. The energy produced in cellular respiration is stored in the form of ATP (adenosine triphosphate). ATP is the immediate source of energy of the cell for all its activities thus, the mitochondrion is called the powerhouse of the cell. MICROFILAMENT AND MICROTUBLE . The cytoskeleton is web-like structure in the cytoplasm of all eukaryotic cells and in which the organelles are suspended. This intracellular scaffolding acts as both muscle and skeleton for the cell; allows the cell its complex surface, and its organelles to move, and gives the cell its normal shape and holds its parts in proper spatial relationship to each other.

Microfilament is thin,

elongated rod found frequently in dense bodies under the cell membrane. Microtubule is a hollow, fluid elongated cylinder. Microfilament through contraction causes movement in the cell while the microtubules provide rigidity and structural reinforcements. Cilia and flagella are made up of microtubules.

CENTRIOLE. This is an animal cell structure found near the nucleus and consists of two bodies, it is made up of microtubules responsible for the formation of spindle fibers and astral rays during cellular reproduction.

NUCLEUS. The nucleus is the largest organelle in the cell. It is bounded by a nuclear envelope or nuclear membrane. This membrane is semi-permeable controlling the passage of materials in and out of the nucleus.

Inside the nucleus is a dense unbounded nucleolus. This is a concentration of RNA and is known as the pacemaker of the cell. The chromatin materials of the nucleus are found in the form of chromosomes. These are longs chains of DNA and nucleoproteins. These chromosomes are the gene carriers and control the cellular activities.

Extracellular Matrix of Animal Cells. Although animals lack walls, they have an elaborate extracellular matrix (ECM) or otherwise referred to as glycocallyx. The main components of ECM are glycoproteins like collagen, which forms strong fibers outside the cells. The ECM functions for support, adhesion, movement and development in animal cells.


Intercellular Junctions. Many plant and animal cells are integrated into one functional organism. Neighboring cells adhere, interact and communicate through the special patches of direct physical contact.

In animals, there are three main types of intercellular junctions, the tight junction, the desmosome and gap junction. The tight junction forms continuous belts around the cell, the membranes of neighboring cells are actually fused at a tight junction forming a seal that prevents leakage of extracellular fluids across a layer of cells. Desmosomes (anchoring junctions) function by fastening cells together into sheets. Gap junctions (communicating junctions) provide connection cytoplasmic channels between adjacent cells. Gap junctions are especially common in animal embryos where chemical communication between cells is essential for development Comparison of Plant and Animal Cells

The basic patterns of the plant and animal cells differ. Plant cells produce their own food because they have the green machinery in their cells– the chloroplasts and related storage organelles, the choromoplast and leucoplast and vacuoles. Furthermore, they are more rigid cells as they are bounded by cell walls. On the other hand, animal cells have lysosomes, not commonly found in plant cells. Instead other vesicular organelles like glyoxysomes and peroxisomes maybe found. Animal cells have centrioles that give rise to astral rays and spindle fiber during cell division. They vary more in shape as they do not have cell walls. Components of Prokaryotic, Plant and Animal Cells Component PROKARYOTE PLANT CELL ANIMAL CELL Cell Wall Present Present Absent Glycocallyx Absent Absent Present Plasma membrane Present Present Present Cytoskeleton Absent Present Present Nucleus Absent Present Present chromosomes Single Multiple Multiple Mitochondria Absent Present Present Plastids Absent Often Present Absent Ribosomes Present Present Present ER Absent Present Present Golgi Complex Absent Present Present Vacuoles Absent Present Present Lysosomes Absent Often Absent Present Cilia (9+2) Absent Absent in most Present in some Flagellum Often Present, Absent Present in some Centrioles Absent Absent in most Absent


ANIMAL CELL


Name _______________________________ Date __________________

Task 3.1 Size and Shapes of Cells

1. Observe the different animal cells above and take note of

their shapes. List down the shape and function of each cell seen in the figures. Relate the functions to the shapes of

cells. Shape Function

2. Are all cells microscopic? If your answer is no, what are some exceptions to this? Why should cells be small?


Name _________________________ Date _______________________

Task No. 3.2 Cellular Organelles

1. What cellular structures are common among all cells? Why are these organelles present in all

cells?

2. Differentiate prokaryotic from eukaryotic cell? Animal cell from plant cell? a. Prokaryotic cell vs. eukaryotic cell

b. Plant cell vs. animal cell

3. In a tabular form, summarize the different cellular structures and their functions.

Cell Structure Function


4. Why is the cell considered as the unit of life?

5. Presence and number of organelles vary in different cells because they have different

specialized functions. Which organelle(s) do you think are predominantly present in great number in the following cells. Cite the reasons for your answers.

a. heart muscle

b. liver cell

c. sperm cell

d. red blood cell

6. Research further on the cell membrane fluid mosaic model. Explain how this can explain why the cell membrane is semi-permeable. Relate the functions performed by cell membrane with the different structures present in them.


Some Review Questions True or False.

1. Cells are considered living. 2. Water is a source of energy for living things. 3. Deoxyribose nucleic acids are found in the nucleus, mitochondrion and chloroplast. 4. Glucose molecules pass through the cell membrane by facilitated diffusion. 5. The cell theory states that all living things are made up of cells and came from pre-existing cells. 6. All substances can get in and out of the cell. 7. Bigger cells generally have more nuclei. 8. The ribosomes, nucleus and cell membrane are found in all cells 9. Energy can be changed from one form to another but it cannot be created nor destroyed in the

cell. 10. ATP is the principal energy carrier in living systems.

Matching type: For Nos. 1-20. To which group do the following descriptions and examples belong? Choose form the answer pool below and write the letter of your choice.

A. plant cell B. animal cell C. prokaryotic cell D. eukaryotic cell E. all cells

___ with cell membrane ___with nucleus ___with ribosomes ___ with lysosome ___with chloroplast ___with cellulose wall

___ with chromosomes ___ with golgi bodies ___with mitochondria ___eukaryotic and undergoes photosynthesis ___threadlike sperm cell ___undrgoes cellular respiration ___has single chromosome ___no nucleus ___capable of reproduction ___no cell wall ___with large vacuole

Multiple Choice. Write the letter corresponding to your answer. Then give the reason for your choice

1. Which shape of the cell is adapted for moving and carrying a lot of materials (a. round b. stellar c. filamentous d. threadlike).

2. Which structures are present in all cells? (a. cell membrane, chromosome , ribosome b. cell membrane, nucleus, ribosome c. ribosome, chromosome, cell wall d. cell wall, nucleus , ribosome)

3. Which cell do you think would have more mitochondria? (a. liver cell b. skin cell d. sperm cell d. cardiac muscle cell)?

IV. Complete the following statements 1. The cell membrane is semi-permeable because _____________________. 2. The lysosome is the suicide bag of the cell because _________________. 3. The powerhouse of the cell is the mitochondrion because_____________. 4. The nucleus is called the director of the cell because ________________.


cell overview

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