Cell Biology Cheat Sheet for NEET

This cheat sheet provides a high-yield overview of "Cell: Structure and Function" crucial for NEET aspirants. It covers the fundamental concepts, important organelles, their functions, and key differences between various cell types, essential for quick revision and exam success.

Key Concepts

The cell is the fundamental structural and functional unit of all living organisms. Organisms can be unicellular (e.g., bacteria, amoeba) or multicellular (e.g., plants, animals). Cells are broadly classified into prokaryotic and eukaryotic based on their internal organization. Prokaryotic cells lack a true nucleus and membrane-bound organelles, while eukaryotic cells possess a membrane-bound nucleus and various organelles. Understanding the specific roles of each organelle is vital. Cell theory, proposed by Schleiden and Schwann and later modified by Virchow, states that all living organisms are composed of cells and their products, and all cells arise from pre-existing cells.

Prokaryotic Cells

These are primitive cells lacking a well-defined nucleus and membrane-bound organelles. They typically have a cell wall, plasma membrane, cytoplasm, and ribosomes. Genetic material is a single, circular DNA molecule (nucleoid) and may include plasmids. Examples: Bacteria, blue-green algae (cyanobacteria), Mycoplasma, PPLO (Pleuro Pneumonia-like Organisms).

Eukaryotic Cells

These are more complex cells with a true nucleus and various membrane-bound organelles. They are found in protists, fungi, plants, and animals. They possess an elaborate internal compartmentalization, allowing for specialized functions within the cell.

Cell Organelles & Their Functions

• Cell Membrane (Plasma Membrane): Outer boundary, selectively permeable, composed of a lipid bilayer and proteins (Fluid Mosaic Model). Involved in transport, cell recognition, and signal transduction.
• Cell Wall: (Present in plants, fungi, bacteria) Provides structural support and protection. In plants, primarily cellulose; in fungi, chitin; in bacteria, peptidoglycan.
• Cytoplasm: Jelly-like substance filling the cell, site of many metabolic reactions. Contains cytosol and cell organelles.
• Nucleus: Control center of the cell, contains genetic material (DNA) organized into chromosomes. Enclosed by a nuclear envelope with pores.
  • Nucleolus: Site of ribosomal RNA (rRNA) synthesis.
• Endoplasmic Reticulum (ER): Network of interconnected membranes.
  • Rough ER (RER): Has ribosomes, involved in protein synthesis and modification for secretion/insertion into membranes.
  • Smooth ER (SER): Lacks ribosomes, involved in lipid synthesis, detoxification, and calcium storage.
• Ribosomes: Sites of protein synthesis (translation). Made of rRNA and proteins. Present in both prokaryotes (70S) and eukaryotes (80S), and within mitochondria/chloroplasts (70S).
• Golgi Apparatus (Golgi Complex): Stack of flattened sacs (cisternae). Modifies, sorts, and packages proteins and lipids synthesized in the ER. Forms lysosomes and peroxisomes.
• Lysosomes: "Suicidal bags," contain hydrolytic enzymes for intracellular digestion of waste materials and cellular debris.
• Vacuoles: Membrane-bound sacs. In plants, large central vacuole stores water, nutrients, and waste, and maintains turgor. In animals, smaller and temporary.
• Mitochondria: "Powerhouses of the cell," sites of aerobic respiration and ATP production. Double-membraned, with folds called cristae increasing surface area. Possess their own circular DNA and ribosomes.
• Plastids: (Present in plant cells and some protists)
  • Chloroplasts: Sites of photosynthesis, contain chlorophyll. Double-membraned, with stacks of thylakoids (grana). Possess their own circular DNA and ribosomes.
  • Chromoplasts: Contain non-photosynthetic pigments (e.g., carotenoids) giving color to fruits and flowers.
  • Leucoplasts: Colorless plastids for storage (e.g., amyloplasts for starch, elaioplasts for oil, aleuroplasts for protein).
• Cytoskeleton: Network of protein filaments (microtubules, microfilaments, intermediate filaments) providing structural support, cell shape, and enabling cell movement and transport.
• Cilia and Flagella: Hair-like appendages involved in locomotion and movement of substances. Structure is $9+2$ arrangement of microtubules (eukaryotic).
• Centrosome and Centrioles: (Mainly in animal cells) Centrosome contains two centrioles. Involved in cell division (spindle formation) and flagella/cilia basal body formation. $9+0$ arrangement of microtubules in centrioles.

Formulas & Results

Fluid Mosaic Model of Cell Membrane

$$ \text{Membrane Structure} = \text{Phospholipid Bilayer} + \text{Intrinsic Proteins} + \text{Extrinsic Proteins} + \text{Carbohydrates} $$ (Carbohydrates often form glycocalyx with lipids/proteins on outer surface)

Ribosome Sedimentation Coefficients

• Prokaryotic Ribosomes: $70\text{S} = 50\text{S} + 30\text{S}$ (S = Svedberg unit, not additive)
• Eukaryotic Ribosomes: $80\text{S} = 60\text{S} + 40\text{S}$ (S = Svedberg unit, not additive)
• Mitochondrial/Chloroplast Ribosomes: $70\text{S}$ (like prokaryotes)

Endosymbiotic Theory (explains origin of Mitochondria & Chloroplasts)

Suggests that these organelles evolved from free-living prokaryotes that were engulfed by ancestral eukaryotic cells. Evidence:

1. Possess their own circular DNA.
2. Have $70\text{S}$ ribosomes.
3. Reproduce by binary fission.
4. Double membrane structure (inner from prokaryote, outer from host).

Cilia and Flagella Microtubule Arrangement

• Eukaryotic Cilia/Flagella (axoneme): $(9+2)$ arrangement (9 doublet microtubules peripherally + 2 single microtubules centrally)
• Centrioles: $(9+0)$ arrangement (9 triplet microtubules peripherally + no central microtubules)

Worked Examples

Example 1: Differentiating Cell Types

Question: A student observes a cell under a microscope and identifies a cell wall, chloroplasts, a large central vacuole, and no centrioles. Is this a prokaryotic cell, an animal cell, or a plant cell? Justify your answer.

Solution: The presence of a cell wall, chloroplasts, and a large central vacuole are characteristic features of plant cells. Animal cells lack cell walls and chloroplasts, and typically have small, temporary vacuoles, if any. Prokaryotic cells lack chloroplasts and a nucleus, and their cell wall composition is different (peptidoglycan vs. cellulose in plants). Furthermore, the absence of centrioles (usually present in animal cells but absent in higher plants) reinforces the conclusion. Therefore, the observed cell is a plant cell.

Example 2: Organelle Function and Interdependence

Question: A cell is observed to be actively synthesizing and secreting digestive enzymes. Which organelles would be most prominent and functionally integrated in this cell, and what are their specific roles in this process?

Solution: For active synthesis and secretion of digestive enzymes (which are proteins), the following organelles would be highly prominent and functionally integrated:

1. Rough Endoplasmic Reticulum (RER): Ribosomes on the RER synthesize the protein enzymes. The enzymes enter the RER lumen for folding and initial modification.
2. Golgi Apparatus: The enzymes then bud off from the RER in vesicles and travel to the Golgi. Here, they undergo further processing, sorting, and packaging into secretory vesicles.
3. Vesicles: Secretory vesicles containing the enzymes bud off from the Golgi and move towards the plasma membrane.
4. Plasma Membrane: The vesicles fuse with the plasma membrane, releasing the digestive enzymes outside the cell via exocytosis. This sequence highlights the coordinated action of the endomembrane system.

Example 3: Understanding a Specific Organelle

Question: Why are mitochondria and chloroplasts often considered "semi-autonomous" organelles? List two pieces of evidence supporting this claim.

Solution: Mitochondria and chloroplasts are considered "semi-autonomous" because they possess some degree of independence from the nuclear control of the cell. They can synthesize some of their own proteins and replicate themselves, although they still rely on nuclear genes for many other functions and structural components.

Two pieces of evidence supporting their semi-autonomous nature are:

1. Presence of their own genetic material: Both mitochondria and chloroplasts contain their own circular DNA molecules, distinct from the nuclear DNA. This DNA encodes for some of their unique proteins and ribosomal RNA.
2. Presence of 70S ribosomes: Unlike the 80S ribosomes found in the cytoplasm of eukaryotic cells, mitochondria and chloroplasts possess smaller, 70S ribosomes, which are characteristic of prokaryotic cells. These ribosomes are capable of synthesizing proteins encoded by their organellar DNA.
3. Ability to self-replicate: They multiply by binary fission, a process similar to how bacteria reproduce, independent of the cell's nuclear division cycle.

Common Mistakes

• Confusing the functions of RER (protein synthesis for secretion/membranes) and SER (lipid synthesis, detoxification).
• Misidentifying the type of ribosomes (70S vs. 80S) in different compartments (cytoplasm, mitochondria, chloroplasts, prokaryotes).
• Forgetting that the cell wall is absent in animal cells and its composition varies between plants, fungi, and bacteria.
• Mixing up the $9+2$ arrangement of cilia/flagella with the $9+0$ arrangement of centrioles.
• Assuming all cell organelles are membrane-bound (e.g., ribosomes and nucleolus are not).
• Not understanding that Svedberg units are not additive for ribosomal subunits.
• Attributing all cellular functions solely to the nucleus; many metabolic processes occur in the cytoplasm.

Quick Revision Checklist

• Distinguish prokaryotic from eukaryotic cells based on key features.
• Identify all major eukaryotic cell organelles and recall their primary functions.
• Understand the structure and components of the cell membrane (Fluid Mosaic Model).
• Know the composition and function of the cell wall in different organisms.
• Trace the pathway of a secreted protein through the endomembrane system (RER $\rightarrow$ Golgi $\rightarrow$ Vesicle $\rightarrow$ Plasma Membrane).
• Recall the specific roles of mitochondria and chloroplasts, including their semi-autonomous nature.
• Understand the function and structure (microtubule arrangement) of cilia, flagella, and centrioles.
• Review the components and functions of the cytoskeleton.
• Differentiate between $70\text{S}$ and $80\text{S}$ ribosomes and their locations.
• Remember the basic tenets of Cell Theory.