The cell is the basic unit of life in all living organisms. It can exist as a single-celled organism, like bacteria, or as part of a multicellular organism, like plants and animals. Cell transportation refers to the various mechanisms by which substances move in and out of cells. The cell membrane is selectively permeable, allowing certain substances to pass through while blocking others.

1. Cell Structure and Functions
The cell is the basic unit of life in all living organisms. It can exist as a single-celled organism, like bacteria, or
as part of a multicellular organism, like plants and animals. Each cell is made up of various components, each
with specific functions that allow the cell to live, grow, and reproduce.
1. Cell Membrane (Plasma Membrane)
 Structure: A phospholipid bilayer with embedded proteins.
 Function:
o Selective Permeability: Controls the movement of substances in and out of the cell.
o Communication: Contains receptors that allow cells to receive signals from the environment.
o Support and Protection: Provides a boundary between the cell's internal environment and the
outside world.
2. Cytoplasm
 Structure: A jelly-like fluid made up of water, salts, and proteins that fills the cell.
 Function:
o Support: Holds the organelles in place.
o Metabolic Activity: Site of many biochemical reactions, including glycolysis.
3. Nucleus
 Structure: A membrane-bound organelle containing the cell's genetic material (DNA).
 Function:
o Storage of Genetic Information: DNA inside the nucleus contains the instructions for making
proteins.
o Control Center: Regulates all cell activities, including growth, metabolism, and reproduction.
4. Nuclear Membrane (Nuclear Envelope)
 Structure: A double membrane surrounding the nucleus with nuclear pores.
 Function:
o Selective Permeability: Regulates the passage of molecules between the nucleus and the
cytoplasm.
o Protection: Keeps the DNA inside the nucleus safe from damage.
5. Nucleolus
 Structure: A dense, spherical structure inside the nucleus.
 Function:
o Ribosome Production: Synthesizes ribosomal RNA (rRNA) and assembles ribosomes, which
are then transported out to the cytoplasm.
6. Ribosomes
 Structure: Tiny structures made of rRNA and proteins; can be found floating in the cytoplasm or
attached to the rough endoplasmic reticulum.
 Function:
o Protein Synthesis: Translate mRNA into amino acid sequences to build proteins.

2. 7. Endoplasmic Reticulum (ER)
 Rough Endoplasmic Reticulum (RER):
o Structure: Network of membranes studded with ribosomes.
o Function: Synthesizes and modifies proteins, which are then transported to other parts of the
cell.
 Smooth Endoplasmic Reticulum (SER):
o Structure: Network of membranes without ribosomes.
o Function: Synthesizes lipids, detoxifies harmful substances, and stores calcium ions.
8. Golgi Apparatus (Golgi Body)
 Structure: Stacks of flattened membrane-bound sacs.
 Function:
o Modification and Packaging: Modifies, sorts, and packages proteins and lipids for transport to
other parts of the cell or to the outside of the cell.
9. Mitochondria
 Structure: Double-membraned organelle with an inner membrane folded into cristae.
 Function:
o Energy Production: Generates ATP (adenosine triphosphate) through cellular respiration,
providing energy for the cell's activities.
10. Lysosomes
 Structure: Membrane-bound sacs containing digestive enzymes.
 Function:
o Digestion: Break down waste materials, cellular debris, and foreign invaders like bacteria.
o Autophagy: Recycle damaged organelles and other cell components.
11. Peroxisomes
 Structure: Small, membrane-bound organelles containing enzymes.
 Function:
o Detoxification: Break down fatty acids and detoxify harmful substances like hydrogen peroxide.
12. Cytoskeleton
 Structure: A network of protein fibers, including microfilaments, intermediate filaments, and
microtubules.
 Function:
o Structural Support: Maintains the shape of the cell.
o Movement: Facilitates movement of the cell and its components.
o Cell Division: Helps in the segregation of chromosomes during cell division.
13. Centrioles (in Animal Cells)
 Structure: Cylindrical structures made of microtubules.
 Function:
o Cell Division: Play a key role in organizing the mitotic spindle during cell division.

3. 14. Cilia and Flagella
 Structure: Hair-like structures (cilia) or a single long tail-like structure (flagella) made of microtubules.
 Function:
o Movement: Enable movement of the cell (e.g., sperm cell) or movement of substances across
the cell surface (e.g., in the respiratory tract).
15. Vacuoles
 Structure: Membrane-bound sacs, more prominent in plant cells.
 Function:
o Storage: Store nutrients, waste products, and other materials.
o Support in Plant Cells: Central vacuole maintains turgor pressure, helping the plant remain
upright.
16. Cell Wall (in Plant Cells)
 Structure: A rigid layer made of cellulose (in plants), chitin (in fungi), or other polysaccharides.
 Function:
o Support and Protection: Provides additional protection and maintains the shape of the cell.
o Prevents Over-Expansion: Helps prevent the cell from bursting when water enters.
17. Chloroplasts (in Plant Cells)
 Structure: Double-membraned organelle containing chlorophyll, with internal structures called
thylakoids.
 Function:
o Photosynthesis: Converts light energy into chemical energy (glucose) during photosynthesis.
Cell Transport: Mechanisms of Moving Substances
Across the Cell Membrane
Cell transportation refers to the various mechanisms by which substances move in and out of cells. The cell
membrane is selectively permeable, allowing certain substances to pass through while blocking others. Cell
transport is vital for maintaining homeostasis, acquiring nutrients, and removing waste.
1. Types of Cell Transport
Cell transport mechanisms can be broadly classified into two main types:
 Passive Transport: Movement of substances across the cell membrane without the use of energy
(ATP).
 Active Transport: Movement of substances against their concentration gradient, requiring energy
(ATP).
Passive Transport
In passive transport, substances move from an area of higher concentration to an area of lower concentration,
down their concentration gradient.

4. 2.1 Simple Diffusion
 Definition: Movement of small, nonpolar molecules (e.g., oxygen, carbon dioxide) directly across the
lipid bilayer of the cell membrane.
 Mechanism: Molecules move randomly until they are evenly distributed on both sides of the
membrane.
 Examples:
o Oxygen diffuses into cells where it is consumed in cellular respiration.
o Carbon dioxide diffuses out of cells as a waste product.
2.2 Facilitated Diffusion
 Definition: Movement of larger or polar molecules (e.g., glucose, ions) across the membrane through
specific transport proteins.
 Mechanism:
o Channel Proteins: Provide a hydrophilic passage for molecules to pass through the membrane.
o Carrier Proteins: Bind to a specific molecule, change shape, and transport the molecule across
the membrane.
 Examples:
o Glucose transport into cells via the glucose transporter (GLUT).
o Ion channels allowing the passage of ions like sodium (Na⁺) and potassium (K⁺).
2.3 Osmosis
 Definition: The diffusion of water molecules across a selectively permeable membrane from an area of
lower solute concentration to an area of higher solute concentration.
 Mechanism: Water moves to balance the solute concentrations on both sides of the membrane.
 Types of Solutions:
o Isotonic Solution: The concentration of solutes is the same inside and outside the cell; no net
water movement.
o Hypertonic Solution: The concentration of solutes is higher outside the cell; water moves out of
the cell, causing it to shrink.
o Hypotonic Solution: The concentration of solutes is lower outside the cell; water moves into the
cell, causing it to swell.
 Examples:
o Red blood cells shrinking in a hypertonic solution.
o Plant cells maintaining turgor pressure in a hypotonic environment.
2.4 Filtration
 Definition: Movement of water and solutes across the cell membrane due to hydrostatic pressure (force
exerted by a fluid).
 Mechanism: Driven by pressure gradients, substances move through small pores in the membrane.
 Examples:
o Filtration in the kidneys where blood pressure forces small molecules out of the blood into the
kidney tubules.
Active Transport
Active transport requires energy (usually in the form of ATP) to move substances against their concentration
gradient, from areas of lower concentration to higher concentration.

5. 3.1 Primary Active Transport
 Definition: Direct use of ATP to transport molecules across the membrane.
 Mechanism: Transport proteins (pumps) use energy from ATP hydrolysis to change shape and move
molecules.
 Examples:
o Sodium-Potassium Pump (Na⁺/K⁺ Pump): Pumps 3 Na⁺ ions out of the cell and 2 K⁺ ions into
the cell, crucial for maintaining cell membrane potential.
o Calcium Pump: Actively transports Ca²⁺ out of the cell or into organelles like the endoplasmic
reticulum.
3.2 Secondary Active Transport (Co-Transport)
 Definition: Indirect use of ATP; transport of one molecule against its concentration gradient is coupled
with the transport of another molecule down its gradient.
 Mechanism:
o Symport: Both molecules move in the same direction.
o Antiport: Molecules move in opposite directions.
 Examples:
o Glucose-Na⁺ Symporter: Uses the energy from Na⁺ moving down its gradient to transport
glucose into the cell against its gradient.
o Na⁺/Ca²⁺ Exchanger: Moves Na⁺ into the cell and Ca²⁺ out of the cell.