Cell structure

All living organisms, from the smallest bacterium to the largest whale, are made up of fundamental units called cells. A cell is the smallest unit of life that can carry out all the processes necessary for living. Most cells are too small to be seen with the naked eye, so we use microscopes to study them. The study of cells is the foundation of biology, as everything an organism does is ultimately the result of the activities of its cells.

While all eukaryotic cells share some common features, there are key differences between animal and plant cells, largely due to their different ways of life. Plants are stationary and make their own food (autotrophs), while animals typically move and consume other organisms (heterotrophs). These differences are reflected in their cell structure.

Common features:

• Nucleus: Controls cell activities and contains genetic material.
• Cytoplasm: Jelly-like substance where chemical reactions occur.
• Cell membrane: Controls the movement of substances into and out of the cell.
• Mitochondria: Site of aerobic respiration to release energy.
• Ribosomes: Site of protein synthesis.

Features unique to plants (and some other organisms):

• Cellulose Cell Wall: A rigid outer layer that provides structural support and prevents the cell from bursting.
• Large Permanent Vacuole: Contains cell sap (a solution of sugars, salts, and amino acids) and helps maintain the cell's shape and turgidity.
• Chloroplasts: Contain the green pigment chlorophyll and are the site of photosynthesis.

Organelles are 'little organs' within the cell, each performing a specific job to keep the cell alive and functioning correctly. This is known as the division of labour within the cell.

• Nucleus: Often called the 'control centre'. It contains the cell's genetic material (DNA) in the form of chromosomes. It controls all the activities of the cell, such as growth and cell division.
• Cytoplasm: A jelly-like substance that fills the cell. It's where most of the cell's metabolic chemical reactions take place. Organelles are suspended within it.
• Cell Membrane: A partially permeable barrier that surrounds the cell. It controls which substances can enter and leave the cell, such as oxygen, nutrients, and waste products.
• Mitochondria (singular: mitochondrion): The 'powerhouses' of the cell. This is where aerobic respiration occurs, a process that releases energy from glucose for the cell to use.
• Ribosomes: Tiny structures, either free in the cytoplasm or attached to the endoplasmic reticulum. They are responsible for making proteins (protein synthesis).
• Cell Wall (Plants only): A fully permeable outer layer made of cellulose. It provides strength, support, and protection, preventing the plant cell from bursting when it takes in too much water.
• Chloroplasts (Plants only): Disc-shaped organelles containing chlorophyll. They are the site of photosynthesis, where light energy is used to make food (glucose).
• Large Permanent Vacuole (Plants only): A large, membrane-bound sac filled with cell sap. It helps to support the cell by pushing the cytoplasm against the cell wall, maintaining turgor pressure.

To calculate the size of microscopic objects, we use the relationship between the size of the image (what we see through the microscope or in a diagram), the actual size of the object, and the magnification. This can be remembered with the formula triangle 'IAM'.

Image Size (I) = Actual Size (A) × Magnification (M)

It is crucial to work in consistent units. In biology, we often use millimetres (mm) and micrometres (µm). Key conversion: 1 mm = 1000 µm To convert mm to µm, you multiply by 1000. To convert µm to mm, you divide by 1000.

In multicellular organisms, cells become specialised to perform a particular function. This process is called differentiation. The cell's structure is adapted to make it efficient at its job. For example:

• Root Hair Cells (Plants): Found on the surface of plant roots. They have a long, thin extension which dramatically increases the surface area for efficient absorption of water and mineral ions from the soil.
• Red Blood Cells (Animals): Transport oxygen. They have a biconcave disc shape to increase the surface area to volume ratio for diffusion. They contain haemoglobin to bind with oxygen and have no nucleus to maximise space for haemoglobin.
• Palisade Mesophyll Cells (Plants): The main site of photosynthesis. They are packed with chloroplasts, are rectangular and arranged in columns at the top of the leaf to maximise light absorption.
• Sperm Cells (Animals): Carry male genetic material to the egg. They have a tail (flagellum) for swimming and mitochondria to provide energy for movement. The acrosome in the head contains enzymes to digest the egg membrane.

In complex multicellular organisms, there is a division of labour. Cells are organised into a hierarchy of structures to perform tasks efficiently.

1. Cells: The basic building blocks (e.g., a muscle cell).
2. Tissues: A group of similar cells working together to perform a specific function (e.g., muscle tissue, which contracts).
3. Organs: A structure made of different tissues working together to perform a specific, complex function (e.g., the heart, made of muscle, nervous and connective tissues, pumps blood).
4. Organ Systems: A group of organs working together to perform a major function in the body (e.g., the circulatory system, including the heart, arteries and veins, transports substances around the body).
5. Organism: The entire living being, made up of many organ systems working together.

An example in a plant: Palisade cell → Palisade mesophyll tissue → Leaf (organ) → Shoot system → Plant (organism).