Question

PART A: Answer all of the following parts:
a) Briefly explain where you would find the following structures in an animal cell, and what they do: cell membrane, nucleus, mitochondrion. (5 marks)
b) Plant cells typically have cell walls, large vacuoles and chloroplasts. Where are each of these located in the cell, and what is their function? (5 marks)
c) Which of the six structures listed in parts (a) and (b) would you find in a bacterial cell? Explain why the other structures do not occur in bacteria. (

Answer 1

Part A

a. Cell Membrane: Cell membrane is present on the outside of the animal cell. It acts as a barrier between the interior of cell and the extracellular environment. It encloses and protects the interior organelles of the cell. The cell membrane allows passage of certain molecules inside and out of the cell as it is a phospholipid bilayer containing proteins. It is also involved in cell signaling. It is in contact with the cytoskeleton. The proteins in the cell act as transport proteins, enzymes or receptors.

Nucleus: Nucleus is present in center of the cell. It stores the genetic material/DNA of the cell in form of chromosomes. It is required for replication, proteins synthesis, reproduction and metabolism. Thus, nucleus is the administrative center of the cell.

Mitochondrion: They are found distribute throughout the cytoplasm of the cell. They are the powerhouse of the cell as they provide ATP from cellular respiration. They have the electron transport chain which generates ATP. They also have mitochondrial DNA which codes for mitochondrial genes and few other genes.

b. Cell Wall: It is rigid structure found at the exterior of the cell, outside of cell membrane. It is composed of cellulose, hemicellulose and pectin. It provides shape to the plant cell. It is also involved in intercellular communication. It protects the plant cell from osmotic damage in hypotonic medium. It also provides strength to the cell. The cell wall acts as an osmotic barrier allowing only certain molecules to pass through. It regulates cell growth as well as cell volume. Proteins in cell wall can be used for secretion and transport. It stores glucose in form of cellulose. It is also involved in cell signaling.

Large Vacuoles: The large vacuoles are located in central area of the cell. The large vacuole helps to maintain shape of the cell. It is a membrane bound fluid filled sac. It stores water, which helps maintain cell shape. They may also store food within them. It also holds waste products which will prevent them from being toxic to the cell.

Chloroplasts: Chloroplasts are present throughout the cytoplasm of the cell. Chloroplasts contain chlorophyll pigment, which harvest sunlight. Photosynthesis occurs in the chloroplasts which convert carbon dioxide and water in glucose in presence of sunlight. This glucose is used for energy generation by the plant cell. They can also contain several enzymes in the stroma which is essential for vital functions.

c. Bacteria lack mitochondria, true nucleus and chloroplast as these are membrane bound organelles. They have cell wall and cell membrane. Large vacuoles may be present in some types of bacteria.

Bacterial cell lack nucleus, but have nucleoid which lacks the nuclear envelope. The bacterial cell DNA is not bound to histones and hence is compressed as chromosome. The cell also reproduces by binary fission. There is also no segregation of transcription and translation, as the bacterial mRNA does not undergo alternate splicing. Hence, transcription and translation undergo simultaneously. Bacteria are unicellular, and do not require organelles to be enclosed in membranes. A nuclear envelope in eukaryotes is usually continuous with the endoplasmic reticulum and Golgi apparatus.

Bacteria have a cell wall. However, it is composed of peptidoglycan layer, made up of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) and lipopolysaccharide. The peptidoglycan later may be thick in gram positive bacteria and thin in gram negative bacteria. The gram-negative bacteria also have a cell membrane composed of lipopolysaccharide. Gram positive bacteria have cell membrane made up of phospholipid.

Bacteria lack membrane bound organelles. Hence, they lack both chloroplast and mitochondria. Photosynthetic bacteria such as purple bacteria, cyanobacteria can undergo photosynthesis. However, bacteria lack chloroplast. Chlorophyll are membrane bound organelles, which are absent in bacteria. Photosynthetic bacteria have light harvesting pigments which allow them to carry out photosynthesis.

Mitochondria is also absent in bacteria. Energy generation (ATP) occurs in the plasma membrane instead.

Large vacuoles are found in filamentous Sulphur bacteria. However, they lack the large central vacuole that is present in plant cells. However, they are not as large as in plant cells. Cyanobacteria have large gas vacuoles that help them to float.