2.3 - Living Processes in Multicellular Organisms

Unlike unicellular organisms, multicellular organisms are made up of more than one type of cell. Different cells perform different specialized functions in order for normal body processes to be carried out. Here are some of the examples of different cells that can be found in the human body.

Figure 7 - This is an example of an intestinal epithelial cell. The presence of microvilli helps to enhance the absorption of nutrients during food digestion.

Figure 8 - Smooth muscle cell (top) can be found lining the wall of organs such as stomach, intestines, urinary bladder and uterus. It is mainly involved in involuntary muscle movements. The cardiac cell can be found in the wall of the heart. The contraction of cardiac muscle is also involuntary but strong and rhythmical.

Figure 9 - Red blood cells and platelets do not have any nucleus. Meanwhile, different white blood cells have different shape of nucleus which can be used for differentiation. For example, neutrophils have a characteristics multilobed nuclei (commonly 3 to 5 lobes).

Figure 10 - The presence of myelin sheath helps the electrical impulses to be transmitted faster along the nerve cells.

Figure 11 - Sperm is the male reproductive cell/gamete that is produced and stored in the testes.

Similarly, in plants, some cells undergo specialization to perform different functions essential for life.

Figure 12 - Mature xylem is made up of dead cells. Xylem tissues do not have organelles which makes the transport of water and mineral ions smooth.

Figure 13 - This is the cross-section of a leaf. All the cells are arranged in a way that can maximize the process of photosynthesis.

Figure 14 - Translocation refers to the movement of sucrose and amino acids (products of photosynthesis) in the plants.

Figure 15 - The function of root hair cell is to increase the surface area for water absorption from the environment.

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Cell Organization in Humans

Similar cells are arranged together to form a tissue. The cells work together as a unit to allow the tissue to be functional. In humans, tissues can be categorized into four main types which are epithelial tissue, muscle tissue, connective tissue and nerve tissue.

Epithelial tissue

• It can be found throughout the body
• Lines all body surfaces including hollow cavities and hollow organs
• Major tissues in glands
• Performs functions such as protection, secretion, absorption, excretion, diffusion and sensory reception.
• There are four types of epithelial tissues – simple cuboidal, simple squamous, simple columnar and stratified squamous.
• Examples include those lining the digestive and respiratory tracts.

Figure 16 - This is an example of simple columnar epithelium which can be found in the wall of small intestines.

Muscle tissue

• Soft tissue that makes up muscles in humans and animals
• Can be divided into smooth muscle , skeletal muscle and cardiac muscle (formed from the three basic muscle cells)
• Able to contract and relax to generate movement

Figure 17 - Cardiac muscles (top left) are found at the wall of the heart. Skeletal muscles (top right) are striated muscle tissue which is mainly involved in voluntary movements. Smooth muscles (bottom) are non-striated muscle tissues contracts slowly and automatically.

Connective tissue

• Connects and supports other tissues in the body
• Different types of connective tissues that can be found in the body namely cartilage , bone , adipose , blood , loose connective tissue as well as fibrous connective tissue

Figure 18 - One example of connective tissue is the blood. Blood is composed of 55% plasma and 45% blood components (red blood cells, white blood cells, platelets).

Figure 19 - This is the five basic levels of organization in humans.

Tissue Organization in Plants

The three main tissues that can be found in plants are parenchyma tissues , collenchyma** tissues**, sclerenchyma tissues and **vascular tissues (xylem and phloem)**. The figure below simplifies the classification of plant tissues.

Figure 20 - The tissue organization in plants.

Figure 21 - Parenchyma cells (top left) are thin-walled cells that perform functions such as photosynthesis, storage and secretion. Collenchyma cells (second right) have irregular walls which are thicker compared to parenchyma cells. They provide support in herbaceous plants. Sclerenchyma (bottom left) are dead tissues with very thick cell wall. Sclerenchyma cells are found in plants where strong support is needed such as wood and fibres.

Density of Cell Components and Specialized Cell Functions

Some of the cells are found in abundance due to their specific functions. The table below shows the density of some of the organelles in some cells. Note that in some people with certain disorders, it may be due to the dysfunctional, lack or total absence of a certain type of cell components. An example is Tay-Sachs disease where patients are deficient of an enzyme which is found in the lysosomes.

Organelles

Mitochondria

• Type of cells the organelles are found in abundance:
  • Adipose cells
  • Sperm cells
  • Muscle cells
  • Plant meristem cells
• Functions:
  • Fat cells need lots of mitochondria to store energy.
  • Sperm cells require energy to travel and fertilize the egg.
  • Muscle movement requires energy to do work, especially in flying animals and insects
  • Active division requires energy to produce new cells

Chloroplasts

• Type of cells the organelles are found in abundance:
  • Palisade mesophyll cells
  • Spongy mesophyll cells
• Functions:
  • Needed to maximize the process of photosynthesis for food production

RER and Golgi apparatus

• Type of cells the organelles are found in abundance:
  • Pancreatic cells
  • Goblet cells
  • Liver cells
• Functions:
  • Actively involved in the synthesis and secretion of digestive enzymes and proteins such as insulin and glucagon
  • Produce and secretes mucus
  • Synthesis of proteins for metabolic processes
  • Liver cells also have many smooth endoplasmic reticulum as they are actively involved in drugs and toxins detoxification.