Cell Structure and Organisation Notes

Chapter 1: Cell Structure and Organisation

1.1 What Are Cells?

• Cells are the simplest structural and functional units of life.

• They function like chemical factories performing various processes to sustain life.

• Cells intake raw materials and execute chemical reactions to produce new molecules.

• These molecules are either utilized by the cell or transported to other parts of the body for different functions.

Studying the Parts of a Cell

• Most cells are too small to see with the naked eye.

• Light microscopes can magnify objects up to 1000x.

• Electron microscopes can magnify objects to more than $200,000x$.

• Robert Hooke and Antonie van Leeuwenhoek were inventors of microscopes.

• Cameras can be attached to microscopes to capture micrographs (pictures).

• Light micrographs can be color images, while electron micrographs are typically black and white but can be artificially colored.

Plant vs. Animal Cells

• Plant cells have a cell wall, chloroplasts, and a large central vacuole.

• Animal cells do not have a cell wall or chloroplasts, and they have small vacuoles.

Viewing Sections of a Cell

• Cells can be cut along their length or at right angles to their length to be observed from different perspectives.

Generalised Animal Cell

• Includes: ribosome, vacuole, cell membrane, mitochondrion, chromatin, nucleus, nuclear membrane, cytoplasm

Generalised Plant Cell

• Includes: nucleus, cell membrane, cytoplasm, cell wall, ribosome, mitochondrion, vacuole, chloroplast, nuclear membrane, chromatin, organelles

Organelles in an Animal Cell (Electron Microscope View)

• Includes: free ribosome, bound ribosome, rough endoplasmic reticulum (RER), cytoplasm, cell membrane, smooth endoplasmic reticulum (SER), mitochondrion, nucleus, vacuole, Golgi body, vesicle

Cell Composition: Protoplasm

• Each cell consists of protoplasm, a complex jelly-like substance.

• Protoplasm is where chemical activities occur for cell survival and growth.

• Composed of three parts: cell membrane, cytoplasm, and nucleus.

Cell Membrane

• Surrounds the cytoplasm.

• Made of lipids (lipid bilayer) and proteins.

• Partially permeable, allowing only small and soluble substances to pass through.

• Regulates the movement of substances in and out of the cell.

Cell Wall

• Encloses the entire plant cell, surrounding the cell membrane.

• Made of cellulose (a carbohydrate).

• Fully permeable, not controlling what enters or leaves the cell.

• Protects the cell from injury and gives it a fixed shape.

Cytoplasm

• Jelly-like substance inside the cell, enclosed by the cell membrane.

• Part of the protoplasm between the cell membrane and the nucleus.

• The site of most cellular activities.

• Composed of cytosol (liquid component) and specialized structures called organelles.

Organelles in the Cytoplasm

• Each organelle has a specific function.

• Examples: nucleus, endoplasmic reticulum (rough and smooth), ribosomes, Golgi body, mitochondria, chloroplasts, vacuoles.

Nucleus

• Stores genetic information in the form of chromosomes.

• Controls cell activities like growth and repair.

• Essential for cell division; cells without a nucleus cannot divide.

• Internal parts are visible only under an electron microscope.

Chromatin & Nuclear Membrane

• Chromatin: Long thread-like structure within the nucleus, made of proteins and DNA.

  • DNA carries hereditary information and instructions for cell activities.

  • During cell division, chromatin condenses into chromosomes (human cells contain 46 chromosomes).

• Nuclear Membrane: Separates the nucleus from the cytoplasm and is continuous with the rough endoplasmic reticulum (RER).

Endoplasmic Reticulum (ER)

• Two types: rough endoplasmic reticulum (RER) and smooth endoplasmic reticulum (SER).

Rough Endoplasmic Reticulum (RER)

• Network of flattened spaces lined with a membrane.

• Appears rough due to ribosomes on its surface.

• The outer surface is continuous with the nuclear membrane.

• Function: transports proteins made by ribosomes to the Golgi apparatus.

Ribosomes

• Small, round structures where protein synthesis occurs.

• Types: bound ribosomes (attached to RER) and free ribosomes (in the cytoplasm).

• Bound ribosomes produce proteins usually secreted out of the cell.

• Free ribosomes produce proteins used by the cell.

Smooth Endoplasmic Reticulum (SER)

• More tubular than RER, and lacks ribosomes.

• Functions:

  • Synthesizes substances like fats and steroids (e.g., sex hormones).

  • Converts harmful substances into harmless ones through detoxification.

Golgi Body (or Golgi Apparatus)

• Stack of flattened spaces surrounded by membranes.

• Vesicles fuse and pinch off from the Golgi body.

• Functions:

  • Chemically modifies substances from the ER.

  • Stores and packages substances in vesicles for secretion out of the cell.

Movement of Substances Made by ER

1. Vesicles containing substances from the ER are pinched off.

2. These vesicles fuse with the Golgi body and release their contents.

3. Substances may be modified inside the Golgi body.

4. Secretory vesicles containing modified substances pinch off from the Golgi body and move to the cell membrane.

5. The secretory vesicles fuse with the cell membrane, releasing their contents outside the cell.

Mitochondria

• Small, oval or sausage-shaped organelles.

• Sites of aerobic respiration.

• Food substances are broken down to release energy for cell activities like growth and reproduction.

Chloroplasts

• Oval structures found in plant cells.

• Contain chlorophyll (green pigment).

• Essential for photosynthesis: the process by which plants make food.

Vacuoles

• Fluid-filled spaces enclosed by a partially permeable membrane.

• Store substances within the cell.

• Plant cells: one large central vacuole containing cell sap (sugars, mineral salts, amino acids).

• Animal cells: numerous small, temporary vacuoles storing water and food substances.

Differences Between Plant and Animal Cells

1.2 How Are Cells Adapted to Their Functions?

Differentiation

• Cells in the human body perform specific functions.

• Cells develop or lose structures to perform these functions via differentiation.

• Definition: Differentiation is the process by which a cell becomes specialized for a specific function.

Red Blood Cell

Muscle Cell

Root Hair Cell