The cell is the basic structural and functional unit of life. Some organisms consist of just one cell; others have many cells working together. Although cells differ in shape, size and function, they share basic features such as a boundary, internal contents and genetic material.
Table of Contents
Cell Theory
The cell theory explains the relationship between cells and living organisms:
- All living organisms are made of cells and their products.
- The cell is the basic unit of structure and function in living organisms.
- New cells arise from pre-existing cells.
The first two ideas were developed by Matthias Schleiden and Theodor Schwann. Rudolf Virchow later explained that new cells come from existing cells.
Prokaryotic and Eukaryotic Cells
Cells are broadly classified as prokaryotic or eukaryotic.
| Feature | Prokaryotic cell | Eukaryotic cell |
|---|---|---|
| Nucleus | No membrane-bound nucleus; DNA lies in a nucleoid region | Membrane-bound nucleus present |
| Membrane-bound organelles | Absent | Present |
| Ribosomes | 70S | Usually 80S in the cytoplasm |
| Examples | Bacteria and archaea | Plants, animals, fungi and protists |
A prokaryotic cell may also have structures such as a cell wall, capsule, flagellum or pili. Its genetic material is not enclosed by a nuclear membrane.
Plant Cell and Animal Cell
Both plant and animal cells are eukaryotic. They contain a plasma membrane, cytoplasm, nucleus and several organelles. Their most familiar differences are:
| Feature | Typical plant cell | Typical animal cell |
|---|---|---|
| Cell wall | Present outside the plasma membrane | Absent |
| Plastids | Present, including chloroplasts in green parts | Absent |
| Vacuole | Usually one large central vacuole | Usually small vacuoles, if present |
| Centrioles | Generally absent in higher plant cells | Usually present |
Plasma Membrane and Cell Wall
Plasma Membrane
The plasma membrane forms the boundary of every cell. It is mainly a phospholipid bilayer with proteins. It is selectively permeable, helping control the movement of substances into and out of the cell.
Diffusion is the movement of particles from a region of higher concentration to lower concentration. Osmosis is the movement of water across a selectively permeable membrane. Cells can also use energy for active transport when substances must move against a concentration gradient.
Cell Wall
In plants, the cell wall lies outside the plasma membrane. It gives the cell shape, support and protection. The primary plant cell wall is mainly made of cellulose. Channels called plasmodesmata connect the cytoplasm of neighbouring plant cells.
Remember: The cell wall provides support, while the plasma membrane regulates much of the cell’s exchange with its surroundings.
Cell Organelles and Their Functions
Endoplasmic Reticulum (ER)
The ER is a network of membranes within the cell.
- Rough ER (RER) has ribosomes on its surface and helps produce proteins.
- Smooth ER (SER) lacks attached ribosomes and is involved in lipid synthesis and other functions.
Golgi Apparatus
The Golgi apparatus receives materials from the ER, modifies them, sorts them and packages them for use in the cell or for secretion. It also helps form lysosomes.
Lysosomes
Lysosomes are membrane-bound structures containing digestive enzymes. They break down materials taken into the cell and help recycle worn-out cell components.
Vacuoles
Vacuoles store substances such as water, ions and waste materials. In many plant cells, a large central vacuole helps maintain cell firmness, called turgor.
Mitochondria
Mitochondria are major sites of aerobic respiration and ATP production. They have two membranes; the inner membrane forms folds called cristae. Mitochondria also contain their own DNA and ribosomes.
Plastids
Plastids occur in plant cells and some other eukaryotes.
- Chloroplasts contain chlorophyll and carry out photosynthesis.
- Chromoplasts contain coloured pigments.
- Leucoplasts are colourless plastids associated with storage.
Chloroplasts contain internal membrane sacs called thylakoids. Stacks of thylakoids are called grana, and the surrounding fluid is the stroma.
Ribosomes
Ribosomes are the sites of protein synthesis. They are not surrounded by a membrane. They may be free in the cytoplasm or attached to rough ER.
Microbodies
Microbodies, including peroxisomes, are small membrane-bound structures containing enzymes that carry out particular chemical reactions.
Nucleus and Genetic Material
The nucleus is surrounded by a double membrane called the nuclear envelope. Nuclear pores allow materials to move between the nucleus and cytoplasm.
Inside the nucleus are chromatin and one or more nucleoli. Chromatin contains DNA associated with proteins; it condenses into visible chromosomes during cell division. The nucleolus helps produce components needed to make ribosomes.
Cytoskeleton, Cilia, Flagella and Centrioles
The cytoskeleton is a network of protein structures that helps maintain cell shape, positions cell components and supports movement.
Cilia are short projections and flagella are longer projections that can help cells or surrounding fluids move. In typical eukaryotic cilia and flagella, microtubules have a 9 + 2 arrangement.
Centrioles are cylindrical structures commonly found in animal cells. They help organise microtubules and are associated with spindle formation during cell division.
Quick Revision
- Plasma membrane: Controls exchange with the surroundings.
- Nucleus: Houses most of the cell’s genetic material.
- Ribosomes: Make proteins.
- RER: Helps make proteins; SER: Helps make lipids.
- Golgi apparatus: Modifies and packages materials.
- Lysosomes: Digest and recycle materials.
- Mitochondria: Produce much of the cell’s ATP.
- Chloroplasts: Carry out photosynthesis.
- Vacuole: Stores materials and helps maintain turgor in plants.
Check Your Understanding
- State the three main points of cell theory.
- What is the main difference between a prokaryotic and a eukaryotic cell?
- Which organelle is the site of protein synthesis?
- How do rough ER and smooth ER differ?
- Name two structures found in a typical plant cell but absent from an animal cell.
Answers: (1) Organisms are made of cells; cells are the basic unit of life; new cells arise from existing cells. (2) Eukaryotic cells have a membrane-bound nucleus; prokaryotic cells do not. (3) Ribosomes. (4) Rough ER has attached ribosomes and helps make proteins; smooth ER lacks attached ribosomes and helps make lipids. (5) Cell wall and plastids.
For all chapters, visit the CBSE Class 11 Biology – Complete Study Guide.
Frequently Asked Questions
What is the cell theory?
All living organisms are made of cells, the cell is the basic unit of life, and new cells arise from existing cells.
How do prokaryotic and eukaryotic cells differ?
Eukaryotes have a membrane-bound nucleus and organelles; prokaryotes lack them.
What is the plasma membrane?
It is a selectively permeable boundary that encloses the cell and regulates movement of substances.
What is the fluid mosaic model?
It describes a flexible phospholipid bilayer with proteins embedded in or associated with it.
What is the main component of a plant cell wall?
Cellulose is its major structural component.
What is the function of the nucleus?
The nucleus stores genetic material and helps control cellular activity.
What are ribosomes?
They are the sites of protein synthesis.
What is the rough endoplasmic reticulum?
It is a membrane network bearing ribosomes and involved in making and processing proteins.
What is the function of the smooth endoplasmic reticulum?
It helps make lipids and participates in other metabolic processes.
What does the Golgi apparatus do?
It modifies, sorts and packages materials for transport or secretion.
What are lysosomes?
They are membrane-bound compartments containing digestive enzymes.
Why are mitochondria important?
They generate much of a eukaryotic cell’s usable ATP through aerobic respiration.
What is a chloroplast?
It is a plastid in which photosynthesis takes place.
What is the role of the cytoskeleton?
It supports cell shape and contributes to movement, organisation and intracellular transport.
What are cilia and flagella?
They are hair-like cell projections involved in movement of a cell or material across its surface.