Edexcel iGCSE Biology-2.2 cell structures- Study Notes- New Syllabus

2.2 Cell Structures

📝 Introduction

Cells are the fundamental units of life and are responsible for carrying out all the processes necessary for an organism’s survival. Although cells may vary greatly in size, shape, and function, they all contain specialised internal structures known as organelles.

Each organelle performs a specific function that contributes to the overall activity of the cell. Just as different departments in a factory perform different jobs, different cell structures work together to ensure that the cell functions efficiently.

🏗️ Overview of Cell Structures

The main cell structures required by the iGCSE specification are:

• Nucleus
• Cytoplasm
• Cell Membrane
• Cell Wall
• Mitochondria
• Chloroplasts
• Ribosomes
• Vacuole

Each structure has a specialised role that contributes to the survival of the cell.

🧠 Nucleus

What is the Nucleus?

The nucleus is a large membrane-bound organelle found in most eukaryotic cells. It is often the most prominent structure within the cell and acts as the control centre of cellular activities.

Structure

• The nucleus is surrounded by a nuclear membrane (nuclear envelope) that separates its contents from the cytoplasm.
• Inside the nucleus is genetic material in the form of DNA.
• The DNA is organised into chromosomes and contains genes that determine the characteristics and activities of the cell.

Functions

Controls Cell Activities

• Growth
• Metabolism
• Reproduction
• Protein synthesis
• Cell division

Stores Genetic Information

• DNA contains instructions for making proteins and controlling cell behaviour.
• These instructions are passed from one generation to the next during reproduction.

Coordinates Protein Production

• The nucleus contains the genetic code needed for building proteins.
• Proteins produced according to these instructions determine how the cell functions.

Importance

Without a nucleus, a cell loses control over its activities and cannot function normally. The nucleus therefore acts as the cell’s information and control centre.

Important Point

• Present in plant, animal, fungal, and protoctist cells.
• Absent in bacteria.
• Absent in viruses.

💧 Cytoplasm

What is the Cytoplasm?

The cytoplasm is a jelly-like substance that fills most of the cell interior. It surrounds the organelles and provides a medium in which many cellular activities occur.

Structure

The cytoplasm consists mainly of:

• Water
• Dissolved nutrients
• Enzymes
• Salts
• Proteins

Organelles are suspended within the cytoplasm.

Functions

Site of Chemical Reactions

Many metabolic reactions occur in the cytoplasm, including:

• Protein synthesis
• Breakdown of nutrients
• Enzyme-controlled reactions

Supports Organelles

• The cytoplasm helps maintain the position of organelles inside the cell.

Transport Medium

• Dissolved substances can move through the cytoplasm to reach different parts of the cell.

Importance – Without cytoplasm, organelles would not be able to function together efficiently and essential biochemical reactions could not occur.

Important Point – The cytoplasm is the main site of chemical activity inside the cell.

🛡️ Cell Membrane

What is the Cell Membrane?

The cell membrane is a thin, flexible boundary surrounding the cell. It separates the cell contents from the external environment.

Structure

The membrane consists mainly of:

• Lipids
• Proteins

It is extremely thin and can only be seen clearly using an electron microscope.

Functions

Controls Movement of Substances

The membrane regulates what enters and leaves the cell.

Substances entering may include:

• Oxygen
• Water
• Glucose
• Mineral ions

Substances leaving may include:

• Carbon dioxide
• Waste products

Protection

• The membrane protects internal structures from changes in the external environment.

Communication

• Special proteins within the membrane help cells detect signals from their surroundings.

Partially Permeable Nature

• Some substances can pass through.
• Other substances cannot pass through.

This allows the cell to maintain a stable internal environment.

Importance – The ability to control the movement of substances is essential for maintaining life.

Important Point – The cell membrane is present in all living cells.

🧱 Cell Wall

What is the Cell Wall?

The cell wall is a rigid outer layer found outside the cell membrane in certain organisms. It provides additional support and protection.

Plant Cell Walls

Composition

Plant cell walls are made primarily of cellulose.

Cellulose fibres form a strong framework around the cell.

Functions

• Provides support
• Maintains shape
• Prevents bursting when water enters
• Strengthens plant tissues

Fungal Cell Walls

Fungal cell walls are made of chitin rather than cellulose.

Bacterial Cell Walls

Bacterial cell walls are made of substances different from both cellulose and chitin.

Importance

The cell wall allows plant cells to withstand internal water pressure without bursting.

This contributes to the rigidity and upright structure of plants.

Important Point – Animal cells do not possess a cell wall.

🔋 Mitochondria

What are Mitochondria?

Mitochondria are specialised organelles responsible for releasing energy from food. They are often called the powerhouses of the cell because they provide the energy required for cellular activities.

Structure

Mitochondria are surrounded by two membranes:

• Outer membrane
• Highly folded inner membrane

The folds increase the surface area available for respiration.

Functions

Site of Aerobic Respiration

Mitochondria carry out aerobic respiration.

During respiration:

• Glucose is broken down
• Oxygen is used
• Energy is released

ATP Production

The released energy is stored in ATP molecules which power cellular processes.

Importance

Energy released by mitochondria is needed for:

• Active transport
• Muscle contraction
• Growth
• Cell division
• Protein synthesis

Adaptations

Cells with high energy demands contain many mitochondria.

Examples include:

• Muscle cells
• Sperm cells
• Root hair cells

Important Point – The number of mitochondria usually reflects the energy requirements of a cell.

🌱 Chloroplasts

What are Chloroplasts?

Chloroplasts are specialised organelles found in plant cells and some protoctists. They are responsible for photosynthesis.

Structure

Chloroplasts contain the green pigment chlorophyll.

Chlorophyll absorbs light energy from the Sun.

Functions

Photosynthesis

Chloroplasts are the site where photosynthesis occurs.

During photosynthesis:

• Light energy is absorbed
• Carbon dioxide and water are used
• Glucose is produced
• Oxygen is released

Food Production

Photosynthesis allows plants to manufacture their own food.

Importance – The food produced in chloroplasts supports plant growth and ultimately provides energy for most food chains.

Distribution

Chloroplasts are most abundant in:

• Leaves
• Young green stems

They are absent from roots because roots do not receive sufficient light.

Important Point – Only cells capable of photosynthesis contain chloroplasts.

⚙️ Ribosomes

What are Ribosomes?

Ribosomes are extremely small organelles found in all living cells. They are among the smallest structures within the cell.

Structure

Ribosomes do not possess a surrounding membrane.

They may occur:

• Freely in the cytoplasm
• Attached to rough endoplasmic reticulum

Functions

Protein Synthesis

Ribosomes join amino acids together to form proteins.

This process is called protein synthesis.

Importance of Proteins

Proteins are needed for:

• Enzymes
• Hormones
• Cell structures
• Growth
• Tissue repair

Without ribosomes, cells would be unable to manufacture the proteins required for life.

Important Point

Although tiny, ribosomes are essential because every cell depends on proteins.

💦 Vacuole

What is a Vacuole?

A vacuole is a membrane-bound sac used mainly for storage. The size and importance of vacuoles vary between plant and animal cells.

Vacuole in Plant Cells

Structure

Plant cells usually contain a large permanent central vacuole.

It is filled with cell sap.

Cell Sap Contains

• Water
• Sugars
• Mineral ions
• Amino acids
• Dissolved substances

Functions

Storage

Stores useful substances required by the cell.

Maintains Turgor Pressure

Water inside the vacuole pushes against the cell wall.

This pressure is called turgor pressure.

Provides Support

Turgor pressure keeps stems and leaves firm and upright.

Without sufficient water, vacuoles shrink and plants wilt.

Vacuoles in Animal Cells

Animal cells may contain small temporary vacuoles.

These are much smaller than the large central vacuole found in plant cells.

Important Point – The large permanent vacuole is one of the most distinctive features of plant cells.

🔄 How Cell Structures Work Together

No organelle works independently. All cell structures cooperate to maintain life.

Example

• The nucleus provides instructions.
• Ribosomes produce proteins.
• Mitochondria supply energy.
• Cytoplasm provides the medium for reactions.
• The cell membrane controls substance movement.
• The vacuole stores materials.
• Chloroplasts manufacture food in plants.
• The cell wall provides support.

📊 Summary Table