The Cell Structure| Form One | High School Biology

- The bodies of living organisms are made up of small microscopic units called cells.
- A cell is the basic functional unit of an organism.

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Read More On

• The Cell - Introduction to the Cell
• Estimation of the Cell Size - Tissues and Organs in Plants and Animals

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- The cell membrane, also known as plasma membrane or plasmalemma consists of three layers when viewed under the electron microscope.
- The three layers are composed of one layer of phospholipid sandwiched between two protein layers.
- It is flexible and has pores. The cell membrane is important in that:

• It encloses the cell contents.
• It allows for selective movement of materials in and out of the cells. The pores allow materials particularly of small molecular size to move in and out of the cells.

- Cytoplasm consists of a fluid medium in which chemical reactions take place.
- It contains organelles and other inclusions such as starch, glycogen, fat droplets and many other dissolved substances.
- Cytoplasm is not static; it undergoes a movement known as cytoplasmic streaming.
- It provides a suitable medium for cellular reactions to take place.

- Mitochondrion is a sausage shaped organelle that provides sites for respiratory reactions that yield energy for the cell. Mitochondria is thus, referred to as the powerhouse of the cell.
- It is bound by two membranes. The inner membrane is greatly folded into cristae to increase surface area for respiration.

- The arrangement and number of mitochondria in a cell depends on the cell energy requirements. Cells that require large amounts of energy contain high amount of mitochondria.
- Such cells include muscle cell, sperm cell, apical meristem cells, and kidney cell.
- Mitochondria are self replicative that is they can divide to form new ones.

- Endoplasmic reticulum appears as a series of interconnected channels, running throughout the cytoplasm.
- Their membranes are continuous with the outer membrane of the nuclear membrane.
- Some endoplasmic reticula have granules called ribosomes on their surfaces and are referred to as rough or granular endoplasmic reticula.
- Endoplasmic reticula that are not associated with ribosomes are called smooth endoplasmic reticula.

- The rough endoplasmic reticulum transports proteins while the smooth endoplasmic reticulum transports lipids.
- Generally, endoplasmic reticula also act as storage areas for synthesized molecules such as enzymes. They also contribute to mechanical support.

- These are spherical in shape. While some are bound to the endoplasmic reticula, some ribosomes are scattered within the cytoplasm (free ribosomes). Their largest dimension is 25 nanometres.
- They are synthesised in the nucleolus.
- They form sites for protein synthesis.

- These are spherical sac-like organelles bound by a single membrane. They contain lytic enzymes which break down large molecules, destroy worn out organelles or even the entire cells.
- Lysosomes also play crucial role in digestion in unicellular organisms.

- The lysosomes are also vital in breakdown of bacteria and other harmful microbes that might have been ingested in food. This explains their high relative abundance in injured or infected cells.
- The membrane of the lysosomes are intact. This is important because if the enzymes leak out, they may destroy the whole cell.

- These are stacks of membrane bound tube like sacs. They are found close to the cell membrane.
- Golgi bodies perform the following functions;

• They package and transport glycoproteins.
• They are involved in secretion of synthesized proteins and carbohydrates.
• They manufacture lysosomes.

NB: Golgi bodies are abundant in cells that are active in secretion. For instance pancreatic cells which secrete enzymes and the nerve cells which secrete neurotransmitter substances.

- These are rod shaped structures located just outside the nuclear membrane.
- They take part in cell division and also in the formation of cilia and flagella in lower organisms.
- Plant cells lack centrioles.

- Chloroplasts are egg-shaped structures surrounded by two membranes and contain a gel-like stroma through which runs a system of membranes that are stacked together to form grana.
- The granum contains chlorophyll which traps light energy that is used during photosynthesis.
- It is in the chloroplasts that photosynthesis takes place.

- These are sacs that are filled with fluid called cell sap. Vacuoles vary in size.
- Animal cells contain small vacuoles which may be numerous in the cells while plant cells contain one large centrally placed vacuole.
- Sap vacuoles store sugars and salts thereby contributing to the osmotic properties of the cell. This influences how materials move in and out of the cell.
- In some unicellular organisms, food vacuole stores and digests food substances while the contractile vacuole excretes unwanted materials from the cell.

- This is the rigid outer cover of plant cells and some lower organisms.
- In plants it is composed of cellulose fibres.
- Cell wall is important in that:

• It gives plant cells their definite shape
• It provides mechanical support and protection against mechanical injury.
• The cell wall allows gases, water and other substances to pass through it.

Nucleus is a double membrane bound structure made up of a viscous fluid known as nucleoplasm in which nucleolus and chromatin materials are suspended. The nuclear membrane has minute pores, nuclear pores which allow materials to move in and out of the nucleus.

- Nucleus controls all the activities of the cell.
- Nucleolus is responsible for manufacture of ribosomes while chromatin contains hereditary materials.
- Nucleus generally takes a spherical or oval shape.

Comparison between Plant Cells and Animal Cells

Plants	Animals
Usually Large	Smaller in Size
Regular in shape	Irregular in shape
Has a cell wall	Has no cell wall
Usually has a large vacuole	Usually has no vacuole but when present, they are often temporary and small structures within the cytoplasm
Cytoplasm and Nucleus are usually located towards the periphery of the cell	Cytoplasm occupies most space in the cell with the nucleus usually centrally placed
Some have chloroplasts	Has no chloroplasts
Usually most store oils, starch and proteins	Store glycogen and fats
Has no centriole	Has centrioles