The function of Cell Membrane:

All prokaryotic and eukaryotic cells have a thin and elastic cell membrane, covering the cytoplasm. The cell membrane functions as a semi-permeable barrier, allowing very few molecules across it while fencing the majority of chemicals inside the cell. In this way, the membrane maintains the internal composition of the cell to a constant or nearly constant level. In addition to this vital role, the cell membrane can also sense chemical messages and can identify materials and other cells.

Cell wall:

The cell wall is a non-living and strong component of the cell, located outside the plasma membrane. It provides shape, strength, protection, and support to the inner living matter (protoplasm) of the cell.

Structure of Cell wall:

Plant cells have a variety of chemicals in their cell walls. The outer layer of the plant cell wall is known as the primary wall and cellulose is the most common chemical in it. Some plant cells, for example, xylem cells, also have secondary walls on the inner side of the primary wall. It is much thicker and lignin and other chemicals are embedded in it in the walls of neighboring cells there are cytoplasmic connections called Plasmodesmata. Through these connections, cells transfer chemicals among each other.

Presence of Chitin in the Cell wall of fungi:

Fungi and many protists have cell walls although they do not contain cellulose. Their cell walls are made of a variety of chemicals. For example, chitin is present in the cell wall of fungi. Prokaryotes have a cell wall composed of peptidoglycan that is a single large polymer of amino acids and sugar.

Nucleus:

A prominent nucleus occurs in eukaryotic cells. In animal cells, it is present in the center while in mature plant cells, due to the formation of a large central vacuole, it is pushed to the side.

Structure of Nucleus:

The nucleus is bounded by a double membrane known as a nuclear envelope.

Nuclear Envelope:

The nuclear envelope contains many small pores that enable it to act as a differentially-permeable membrane.

Nucleoplasm:

Inside the nuclear envelope a granular matrix, the nucleoplasm, is present. The nucleoplasm contains one or two nucleoli (singular, nucleolus) and chromosomes.

Visibility of Nucleolus:

The nucleolus is usually visible as a dark spot and it is the site where ribosomal RNA is formed and assembled as ribosomes.

Chromosomes: One of the rods shaped bodies found in the nucleus of cells that contain genetic information (DNA) Chromosomes are visible only during cell division while during the interphase (non-dividing phase) of the cell they are in the form of fine thread-like structures known as chromatin Structure of DNA: Chromosomes are composed of Deoxyribonucleic acid (DNA) and histone proteins provide structural support to DNA for making the structure of the chromosome.

Messenger Ribonucleic Acid (mRNA):

DNA contains the message for the synthesis of specific proteins. According to this message, a molecule of messenger Ribonucleic Acid (mRNA) is synthesized. In this way, the message is handed over to mRNA, which carries it to ribosomes. Ribosomes manufacture specific proteins according to the message present on mRNA.

The function of DNA:

In this way, DNA controls all the activities of the cell and is also responsible for the transmission of characteristics to the next generation. That is why it is honored as hereditary material.

Main Function of Nucleus:

The nucleus is called the brain of the cell. The nucleus controls all the activities of the cell.

Note:

The prokaryotic cells do not contain a prominent nucleus rather their chromosome is made of DNA only and is submerged in the cytoplasm.

Endoplasmic Reticulum (ER):

The endoplasmic reticulum is a network of interconnected channels that extends from the cell membrane to the nuclear envelope. This network exists in two forms;

1. Rough Endoplasmic Reticulum (RER):

Rough Endoplasmic Reticulum (RER) is so-named because of its rough appearance due to the numerous ribosomes that are attached to it.

The function of Rough Endoplasmic Reticulum (RER):

It connects to the nuclear envelope through which the messenger RNA (mRNA) travels to the ribosomes. Due to the presence of ribosomes, RER serves a function in protein synthesis.

1. Smooth Endoplasmic Reticulum (SER):

The Smooth Endoplasmic Reticulum (SER) lacks ribosomes and is involved in lipid metabolism and the transport of materials from one part of the cell to the other. It also detoxifies the harmful chemicals that have entered the cell.

Golgi Apparatus:

An Italian physician named Camillo Golgi discovered a set of flattened sacs (cisternae) that are stacked over each other. Golgi named this set of cisternae as Golgi apparatus. It is also called the Golgi body or Golgi complex and is found in both plant and animal cells.

Functions of Golgi Apparatus:

It modifies molecules coming from rough ER and packs them into small membrane-bound sacs called Golgi vesicles. These sacs can be transported to various locations in the cell or its exterior, in the form of secretions.

Lysosomes:

In the mid-twentieth century, the Belgian scientist Christian Rene de Duve discovered lysosomes.

Formation of lysosomes:

These are single-membrane-bound organelles. Lysosomes contain strong digestive enzymes and work for the breakdown (digestion) of food and waste materials within the cell.

The function of lysosomes:

During its function, the lysosome fuses with the vacuole that contains the targeted material, and its enzymes break down the material.

Water balance problems in the animal cells:

OR

Animal cell in the hypotonic environment:

1. When an animal cell, such as the red blood cell, is placed in an isotonic solution, the volume of the cell remains constant because the rate at which water is entering the cell is equal to the rate at which it is moving out.
2. When a cell is placed in a hypotonic solution (which has a lower salt concentration than the cell) water enters and the cell swells and may rupture like an over-filled balloon.
3. An animal cell placed in a hypertonic solution (which has a higher salt concentration than the cell) will lose water and will shrink in size.
4. So in hypotonic environments (freshwater) animal cells must have ways to prevent the excessive entry of water and in hypertonic environments (seawater) they must have ways to prevent excessive loss of water.

Water balance problems in plant cells:

OR

Plant cell in the hypotonic environment:

1. Most plant cells live in a hypotonic environment because there is a low concentration of solutes in extracellular fluids than in their cells. As a result, water tends to move first inside the cell and then inside the vacuole.
2. When the vacuole increases in size the cytoplasm presses firmly against the interior of the cell wall, which expands a little.
3. Due to the strong cell wall, plant cell does not rupture but instead becomes rigid. The internal pressure of such a rigid cell is known as turgor pressure and this phenomenon is known as turgor.
4. In an isotonic environment the plant cells are flaccid (loose / not firm) because the net uptake of water is not enough to make it turgid.
5. In a hypertonic environment a plant cell loses water, causing the cytoplasm to shrink within the cell wall. The shrinking of cytoplasm is called plasmolysis.

Like mitochondria, the chloroplast is also bound by a double membrane. The outer membrane is smooth while the inner one gives rise to membranous sacs called thylakoids (the stack of thylakoids is known as a granum (plural = grana]) floating in a fluid termed the stroma.

The function of chloroplast:

Chloroplasts are the sites of photosynthesis in eukaryotes. They contain chlorophyll, the green pigment necessary for photosynthesis, and associated accessory pigments. These pigments are present in thylakoids of the grana of chloroplasts.

Chemical analysis reveals that the cell membrane is mainly composed of proteins and lipids with small quantities of carbohydrates. Electron microscopic examinations of cell membranes have led to the development of the fluid-mosaic model of cell membranes.

The fluid-mosaic model of cell membrane:

1. Lipids are aligned in such a way that they make a bilayer. It gives fluidity and elasticity to the cell membrane.
2. Proteins may be fully submerged in the lipid bilayer or some of them may "stick out into the interior and outside of the cell. These proteins function as gateways that allow certain molecules to cross into and out of the cell.
3. Small amounts of carbohydrates are also found in cell membranes. These are joined with proteins (in the form of glycoproteins) or with lipids in the form of glycolipids). Both these forms act as fingerprints of the cell.

Most plant cells live in a hypotonic environment because there is a low concentration of solutes in extracellular fluids than in their cells. As a result, water tends to move first inside the cell and then inside the vacuole. When the vacuole increases in size the cytoplasm presses firmly against the interior of the cell wall, which expands a little. Due to the strong cell wall, plant cell does not rupture but instead becomes rigid. The internal pressure of such a rigid cell is known as turgor pressure and this phenomenon is known as turgor. 

(a) Size and shape:

The function of Nerve cells:

Nerve cells long for the transmission of nerve impulses.

The function of Xylem cells:

Xylem cells are tube-like and have thick walls for the conduction of water and support.

The function of Red blood cells:

Red blood cells are round to accommodate globular hemoglobin.

(b) Surface area to the volume ratio:

The function of root hair cells:

Root hair cells have a large surface area for the volume ratio maximum absorption of water and salts.

(c) Presence or absence of organelles:

1. Ceils involved in making secretions have more complex ER and Golgi apparatus
2. Cells involved in photosynthesis have chloroplasts.

Difference Between Prokaryotic and Eukaryotic Cells:

1. The significant difference between prokaryotic and eukaryotic cell is that eukaryotic cell has a prominent nucleus and many membrane-bound organelles, which are not present in prokaryotic cell
2. The DNA of a prokaryotic cell floats in the cytoplasm near the center (this region is called nucleoid); the DNA of a eukaryotic cell is held within the nucleus.
3. There is a much higher level of intracellular division of labor in eukaryotic cells than in prokaryotic.

Some more differences between prokaryotic and eukaryotic cells: 

Cell size and surface area to volume Ratio:

Cells vary greatly in size.

Mycoplasmas:

The smallest cells are bacteria called mycoplasmas, with diameter between 0.1 µ to 1.0 µ m.

Bulkiest cells:

The bulkiest cells are bird eggs, and the longest cells are some muscle cells and nerve cells. Most cells lie between these extremes.

Note:

Most cells are small in size. Large cells have less surface area in relation to their volume while small cells of the same shape have more surface area.

The surface-to-volume relationship of cube-shaped cells:

The figure shows 1 large cell and 27 small cells. In both cases the total volume is the same:

Volume = 30 µ m 30 m x 30 µ m = 27,000 µ m³

Total surface areas:

In contrast to the total volume, the total surface areas are very different. Because the cubical shape has 6 sides, its surface area is 6 times the area of 1 side. The surface areas of the cubes are as follows:

Surface area of 1 large cube = 6 x (30 µ m x 30 µ m) = 5400 µ m²

Surface area of 1 small cube = 6 x (10 µ x 10 µ m) = 600 µ m² and

Surface area of 27 small cubes = 27 x 600 µ m = 16.200 µ m²

This relationship between cell size and surface area to volume ratio works to limit cell size.

As the size of a cell increases, cell volume increases more rapidly than its surface area.

Effect of cell size on surface area

Animal Tissues:

In the bodies of animals, there are four major categories of tissues: epithelial tissue, connective tissue, muscle tissue, and nervous tissue.

1. Epithelial tissue:

Epithelial tissue covers the outside of the body and lines organs and cavities. The cells in this type of tissue are very closely packed together and joined with little space between them.

The function of Epithelial tissue:

Epithelial tissue helps to protect organisms from microorganisms, injury, and fluid loss. These tissues are commonly classified on the base of the shape of the cells as well as the number of cell layers.

Types of Epithelial tissue and their function:

Some types include:

2. Connective Tissue

As the name implies, connective tissue serves a "connecting" function. It supports and binds other tissues. Unlike epithelial tissue, connective tissue typically has cells scattered throughout an extracellular matrix.

Types of Connective Tissue:

There are many types of this tissue.

3. Muscle Tissue:

Muscle tissue consists of bundles of long cells called muscle fibers. It is the most abundant tissue in a typical animal. The cells of this tissue can the contract

Voluntary Action Muscle Tissue:

The skeletal muscles are voluntary in action i.e. their contraction is under the control of our will.

 Involuntary Action Muscle Tissue:

The smooth and cardiac muscles are involuntary in action i.e. their contraction is not under the control of our will.

1. Nervous Tissue:

Animal's survival depends on its ability to respond appropriately to stimuli from its environment. This ability requires the transmission of information from one part of the body to another. Nervous tissue forms a communication system and performs this task.

Specification of nervous tissue:

This tissue is mainly composed of nerve cells, or neurons, which are specialized to conduct messages in the form of nerve impulses.

Location of nervous tissue:

Nervous tissue is found in the brain, spinal cord, and nerves.

Cell theory:

The cell theory, in its modern form, includes the following principles.

1. All organisms are composed of one or more cells, within which all life processes occur
2. Cells are the smallest living things, the basic unit of organization of all organisms Cells arise only by divisions in previously existing cells.

The function of leucoplasts:

Leucoplasts are the third type of plastids. They are colorless and store starch, proteins, and lipids. They are present in the cells of those parts where food is stored.

The function of chromoplasts:

The second type of plastids in plant cells are chromoplasts. They contain pigments associated with bright colors and are present in the cells of flower petals and fruits. Their function is to give colors to these parts and thus help in the pollination and dispersal of fruit.

Diffusion:

Diffusion is the net movement of a substance from an area of higher concentration to the area of lower concentration i.e. along a concentration gradient.

Facilitated Diffusion:

Many molecules do not diffuse freely across cell membranes because of their size or charge. Such molecules are taken into or out of the cells with the help of transport-proteins present in cell membranes. When one of these transport proteins makes it possible for a substance to move down its concentration gradient (from higher to lower concentration), the process is called facilitated diffusion. The rate of facilitated diffusion is higher than simple diffusion. Facilitated diffusion is a type of passive transport: Facilitated diffusion is also a sport because there is no expenditure of energy in this process. 

Hypertonic Solution:

A Hypertonic solution has relatively more solute.

Hypotonic Solution:

A Hypotonic solution has relatively less solute.