Lesson 1: The Cell Theory

According to the cell theory, life exists only in form of cells. The cell according to this theory is the structures and functional unit of all living organism.

The activity of the organism is dependent on the activities of the cells, individually and collectively. Furthermore, it asserts that all existing cells come from the reproduction of pre-existing cells.

The cell theory was first put forward by the German Zoologist, Theodor Schwann and Botanist Matthias Sheld on about 1838 — 39 based on the discovery of the cell in the cork of a plant by Robert Hooke (1665). This theory is generally accepted but it has been shown that there are entities such as viruses which do not have cell and exist at sub-cellular levels yet are still able to perform some functions of living forms.

However, this category are so small that this does not nullify the general validity of the general theory as regards the vast majorly of living thing, ranging from procaryes like bacteria and blue green algae to eukaryote including complex higher plants and animals.

Unit 1: Cell Structure and Functions of Cell Component
Cells whether plant or animal share certain common structural features and functions of components.

Cell wall
The cell wall is a rigid non-living layer that is found outside the cell membrane and surrounds the cell. Plants, bacteria and fungi all have cell walls. In plants, the wall is comprised of cellulose.

It consists of three layers that help support the plant. These layers include the middle lamella, the primary cell wall and the secondary cell wall.

Middle lamella: Separates one cell from another. It is a thin membranous layer on the outside of the cell and is made of a sticky substance called pectin.

Primary cell wall: Is on the inside of the middle lamella and is mainly composed of cellulose.

Secondary cell wall: Lies alongside the cell membrane. It is is made up of a thick and tough layer of cellulose which is held together by a hard, waterproof substance called lignin. It is only found in cells which provide mechanical support in plants.

The human body cannot break down the cellulose in cell walls because we do not produce the enzyme cellulase.

Unit 2: Functions of the Cell Wall
The main function of the wall is to protect the inner parts of the plant cell, it gives plant cells a more uniform and regular shape and provides support for the plant body.

The cell wall is completely permeable to water and mineral salts which allows distribution of nutrients throughout the plant.

The openings in the cell wall are called plasmodesmata which contain strands of cytoplasm that connect adjacent cells. This allows cells to interact with one another, allowing molecules to travel between plant cells.

Unit 3: Cell Membrane
The cell membrane, also called the plasma membrane, physically separates the intracellular space (inside the cell) from the extracellular environment (outside the cell). All plant and animal cells have cell membranes. The cell membrane surrounds and protects the cytoplasm. Cytoplasm is part of the protoplasm and is the living component of the cell.

The cell membrane is composed of a double layer (bilayer) of special lipids (fats) called phospholipids.

Phospholipids consist of a hydrophilic (water-loving) head and a hydrophobic (water-fearing) tail. The hydrophobic head of the phospholipid is polar (charged) and can therefore dissolve in water. The hydrophobic tail is non-polar (uncharged), and cannot dissolve in water.

The lipid bilayer forms spontaneously due to the properties of the phospholipid molecules. In an aqueous environment, the polar heads try to form hydrogen bonds with the water, while the non-polar tails try to escape from the water. The problem is solved by the formation of a bilayer because the hydrophilic heads can point outwards and from hydrogen bonds with water, and the hydrophobic tails point towards one another and are ‘protected’ from the water molecules

Some of the components of the eukaryotic cell which are readily seen under the electron microscope include the following:

Unit 4: Endoplasmic Reticulum
Endoplasmic Reticulum is a system used for transportation though the cytoplasm. They are membrane lined and connected to the nuclear membrane. Two types of ER are distinguishable.

The smooth ER, involved in the packaging and transport of lipids and steroid and which lack ribosomes on its wall. The rough endoplasmic reticulum on the other hand is involved in synthesis and packaging of proteins. They have ribosome on their walls which gives them the granular (so called rough) appearance.

Unit 5: Ribosomes
Ribosomes are the site for protein synthesis in all cells. They occur freely in the cell if attached to the ER as described above. They are usually made up of two different types of sub units made up of ribosomal RNA.

Unit 6: Mitochondrion
Mitochondrion (mitochondria) is a long rod or sausage shaped structure. It is. surrounded by two layers of membrane; the inner membrane is folded to form structure called cristae. The essence of folding is to increase the surface area available for reaction. The mitochondrion is the centre where cellular respiration takes place and ATP (Adenosine triphosphate (ATP) is an organic compound and hydrotrope that provides energy to drive many processes in living cells, e.g. muscle contraction, nerve impulse propagation, condensate dissolution, and chemical synthesis) is generated.

It occurs in a larger number in cells that are metabolically active for instance, the sperm cells which have to swim to the egg cells over relatively long distances.

Unit 7: Golgi bodies
Golgi bodies are sacs which are stacked together in the cell and concerned with packaging and ‘export’ of materials including enzymes and hormones from the cell. Such packaged materials are released in the form of vesicles from the cell membrane surface.

Unit 8: Lysosomes
Lysosomes are vesicles which contain enzymes that break down cell parts during autolysis or break down materials during digestion.

Unit 9: Vacuoles
Vacuoles – these share the same characteristic of plants because they are often very large, as such, they store many different-kinds of materials in the form of cell sap.

Unit 10: Chloroplasts
Chloroplasts are found only in plants cells and contain the green pigment chlorophyll which is the centre of photosynthesis.

Unit 11: Centroles
Centroles are found only in animal cells and are concerned with cells division. They are two rod like structure located close to the nucleus.

Scroll Down to Select Page 3 for the next topic – Lesson 2: Similarities and Differences between Plant and Animal Cells

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