Animal Cell

INTRODUCTION

Cell is the basic unit of life.

  • There are 50- 100 trillion cells in the human body.

  • Every cells has different size and shape due to their unique function.


Major parts of include:

  • Nucleus

  • Cell membrane (Plasma Membrane)

  • Cytoplasm


NUCLEUS

Its primary function is to store and protect the cell's genetic material (DNA) and regulate gene expression, thereby directing key processes such as metabolism and cell division.

—Every type of cell that exists is categorized on the basis of the absence or presence of the nucleus within its cell (categorized either as a prokaryotic or eukaryotic cell).

Prokaryotic Cell— no nucleus, no membrane- bound organelles, smaller and simpler, have ribosomes (but smaller ones) and a cell wall.

  • e.g. bacteria and archaeia

Eukaryotic Cell— have a nucleus, have a membrane- bound organelles, larger and more complex.

  • e.g. animal cell


Main components of the Nucleus:

  • Nuclear envelope— A double-layered skin that wraps around the nucleus to protect it and separate it from the rest of the cell.

  • Nuclear pores— Tiny holes in the nuclear envelope that control what gets to go in and out of nucleus.

  • Nuclear lamina—A dense protein network made of lamin intermediate filaments that lines the inner nuclear membrane, providing mechanical support, structural organization, and a role in DNA replication.

  • Nucleoplasm — A thick, jelly-like fluid that fills up the inside of the nucleus, holding everything in place.

  • Chromatin — Long strands of DNA wrapped around proteins that holds the genetic information. It condenses into chromosomes during cell division.

  • Nucleolus — A membrane-free sub-region within the nucleus responsible for transcribing ribosomal RNA (rRNA) and assembling the subunits of ribosomes.


CELL MEMBRANE (PLASMA MEMBRANE)

It is a thin, selectively permeable barrier that surrounds every cell in order to protect it and give it shape. It controls what enters and what leaves (called selective permeability).


Structure - Fluid Mosaic Model

The membrane is mostly made of phospholipids

Phospholipid Bilayer (two layers of phospholipids) has each hydrophilic (water-loving) phosphate head and two hydrophobic (water-fearing) fatty acid tails.

- the bilayer is roughly 7-10nn thick


Proteins are embedded within or attached to this bilayer, and they carry out most of the membrane’s function.

  • Integral proteins span the entire bilayer, passing from one side to the other. These are often involved in transporting substances across the membrane.

  • Globular proteins have a rounded, ball-like shape and may act as enzymes, receptors, or transporters.

  • Alpha-helix proteins are integral proteins folded in coiled, spiral shape that commonly forms the core of transport channels.

  • Channel proteins are a special type of integral protein with a tunnel-like pore running through them. They allow specific small molecules or ions- such as water or sodium- to pass across the membrane without directly interacting with hydrophobic core.

  • Peripheral proteins are attached only to the surface of the membrane rather than embedded through it. They often help anchor the membrane to the cell’s internal framework or participate in cell signaling.


Cholesterol act as a stabilizer of the membrane. Keeping the membrane from getting too rigid when cold and too loose when hot.


Carbohydrates are attached to the outer surface of the membrane, where they form recognition markers. A carbohydrate attached to a protein is called glycoprotein and a carbohydrate attached to a lipid is called glycolipid.


Trasport through the plasma membrane occurs in 2 different ways:

  1. Passive transport - no energy needed, moves down the concentration gradient.


Mechanism

  • Simple diffusion - slip directly between the phospholipids.

  • Facilated diffusion - move through channel or carrier proteins, no energy needed.

  • Osmosis (water) - moves toward higher concentration, often through aquaporin channels.


  1. Active transport - requires energy (ATP), moves against gradient.

Mechanism

  • Pumps - carriers proteins use ATP to push substances against their concentration gradient.

  • Endocytosis - brings substances in and forming vesicle by using the process of phagocytosis (cell eating) for large solid particles and pinocytosis (cell drinking) for liquids.

  • Exocytosis - releases the substances out of the cell. A vesicle fuses with the plasma membrane, and its contents are released to the outside.


CYTOPLASM

A gel-like substance that fills the cell between the plasma membrane and the nucleus. It's everything inside the cell except the nucleus. It's where most of the cell's chemical reactions happen.

What it does

  • Holds the organelles in place and gives the cell its shape.


Three main parts of cytoplasm:

  1. Cytosol - the fluid portion. It's mostly water (about 70–80%) with dissolved salts, sugars, amino acids, and proteins.

  2. Organelles - the little "organs" of the cell suspended in the cytosol (mitochondria, ribosomes, endoplasmic reticulum, Golgi apparatus, etc.).

  3. Inclusions - temporary stored materials waiting to be used.


CELL ANATOMY

Functions of each structure:

Nucleus - the control center

  • Nucleolus — produce ribosomes.

  • Nuclear envelope — separates the nucleus from the rest.

  • Nuclear pores — small openings in the envelope that control what passes between the nucleus and the cytoplasm.

Ribosomes - makes proteins.

Endoplasmic Reticulum (ER) - a network of folded membranes that extends through the cytoplasm. It comes in two forms:

  • Rough ER — where proteins are attached then folds and modifies them into their proper shape.

  • Smooth ER — lacks ribosomes. It synthesizes lipids, detoxifies drugs and harmful chemicals, and stores calcium ions

Golgi Apparatus - it receives proteins and lipids from the ER in small transport vesicles, then modifies, sorts, and packages them.

Mitochondrion - the powerhouse. Supplies the ATP energy that powers every other activity in the cell.

Lysosome - it digests waste and recycles useful components.

Peroxisome - break down fatty acids and neutralize toxic substances, particularly hydrogen peroxide (H₂O₂).

Centrosome - a pair of centrioles surrounded by a centrosome matrix. During cell division, it organizes the microtubules of the cytoskeleton into the spindle fibers that separate chromosomes.

Cytoskeleton - a network of protein fibers running through the cytoplasm. It's made of three types:

  • Microfilaments — the thinnest fibers. They give the cell shape, enable movement (like muscle contraction and cell crawling), and help divide the cell during cytokinesis.

  • Microtubules — the thickest, hollow tubes. They act as tracks for moving organelles and vesicles, and they form the spindle during division.

  • Intermediate filaments — mid-sized fibers. They provide mechanical strength, anchoring organelles and the nucleus in place.

Microvilli - maximize the cell's surface area for absorption.

Plasma Membrane - the boundary

Cytosol - the fluid medium